Influence of alloying elements

Materials
Influence of alloying elements
Below are brief descriptions of most of the elements in stainless steels. Some of them promote the formation of
ferrite (F) and some of austenite (A). The contents of sulphur, phosphorous and cerium are so low in stainless
steels that they do not influence on the structure.
Alloying element Description and influence
Carbon, C (A) Most stainless steels have low carbon contents, max. 0.020 – 0.08%. Those with max.
0.030% C are called ELC steels. Low carbon content inhibits the formation of chromium
carbides and the resulting risk of intergranular-corrosion attacks. Low carbon also improves
weldability.
By convention, and as requested by standards, high-temperature grades often have higher C
contents because this promotes creep strength. With modern metallurgical methods it is no
longer necessary to increase carbon content; instead nitrogen can be added to maintain high
strength.
In martensitic stainless steels, C is an alloying element, and the content is usually between
0.15 and 1.2%. The high C content makes these steels hardenable.
C is used in spring steels to improve the tensile strength.
Chromium, Cr
(F)
Chromium is the main alloying element in stainless steels. In contents exceeding about 11%,
a stable, passive oxide film is formed on the surface – and reformed spontaneously. By
increasing the Cr content, up to max. 30%, the corrosion resistance increases. This is true for
wet corrosion as well as high-temperature corrosion.
Cr addition does not change the structure of pure iron, which is ferritic. Ferritic chromium
steels therefore have physical properties similar to those of carbon steel. And duplex
(austenitic-ferritic) steels come, in that respect, between ferritic and austenitic steels.
The negative effect of Cr is the risk of formation of the intermetallic phase sigma (
is hard and brittle.
Nickel, Ni (A) If sufficient nickel, at least 8%, is added to a chromium steel, the structure usually becomes
austenitic, which results in changed mechanical and physical properties. Ni helps the
formation of the passive Cr-oxide film.
Lower additions of Ni give a mixed structure of ferrite and austenite, i.e. duplex stainless
steels.
Increasing Ni has great influence on the resistance to stress corrosion cracking (SCC). It is
also beneficial under other special wet-corrosive conditions and mostly to high-temperature
corrosion. However, under certain high-temperature conditions, Ni is directly harmful.
See also section on Material Safety Data Sheet.
Molybdenum, Mo
(F)
Molybdenum greatly improves the general-corrosion resistance of stainless steels in most
media. Above all, Mo improves resistance to pitting and crevice corrosion. However, under
certain wet-corrosive and high-temperature conditions, Mo is a disadvantage. Mo promotes
the formation of sigma-phase.
Mo is beneficial for strength at elevated temperatures.
In conventional stainless steels the Mo content is 2 – 3%. In special steels, up to about 6%
Mo is added. This, however, makes the steels resistant to hot working, which has a direct
limitation on the size range of seamless tubes.
Titanium, Ti, and
niobium, Nb (F)
These two elements easily combine with carbon into stable Ti or Nb carbides. Ti/Nb then
obstruct the formation of Cr carbides (known as sensitisation = Cr depletion) in the region
adjacent to welds and the resulting risk of intergranular corrosion. Ti and Nb are known as
stabilising elements and are normally used in steel grades with relatively high carbon content
(> 0.05%) – so-called Ti or Nb stabilised steels. Stabilised Sandvik grades are 6R35 and
5R75 (both Ti) and 8R40 and 8R41 (both Nb). The Ti alloyed steels are commonly used in
Germany, whereas the Nb-alloyed variants are preferably used in the USA.
In the USA, niobium is also known as columbium (Cb). Nb is often used together with
tantalum, Ta, which is another stabilising element.
The addition to steel is min. 5 x %C for Ti and min. 10 x %C for Nb.
Copper, Cu (A) Improves resistance to corrosion in sulphuric acid. Around 1% Cu is added to some special
grades, e.g. 2RK65, 2RK66 and Sanicro 28.
Nitrogen, N (A) Nitrogen, like Ni, is a strong austenite former and is used to complement Ni in the N-alloyed
steels. N is added in contents of up to about 0.2 – 0.3%, which improves the strength and
corrosion resistance of austenitic and duplex steels. N is very important for the weldability of
duplex stainless steels, due to its ability to give rapid reforming of austenite during the
cooling of the weld. N reduces the tendency for formation of sigma-phase.
N is used in spring steels to improve the tensile strength.
N-alloyed Sandvik grades are the austenitic steels Sandvik 3R19, 3R69, 2RE69, 254 SMO,
9X1R51, 11RM10, 11R51, 253 MA and 353 MA, the duplex steels SAF 2304, SAF 2205,
SAF 2507 and the ferritic steel 4C54.

Silicon, Si (F) Si is used as a deoxidising agent in the melting of steel and, as a result, principally all steels
contain a small percentage of Si. Si has a positive effect on the resistance to high-temperature
corrosion. Si increases the tendency for formation of sigma phase and gives an increased risk
for hot cracking during welding.
Manganese, Mn
(A)
Manganese is, like Si, present in all steels. It promotes the formation of austenite. It easily
combines with sulphur to form sulphides (= inclusion particles), which can be both negative
and positive. They are harmful in conditions where there is a risk of pitting. Their positive
effect is in machining, where small and well distributed sulphides "lubricate" the cutting tool.
In Sandvik 11RM10 / 11RM20, Mn is added to increase the workhardening rate and improve
the wear resistance.
In certain high-temperature steels a Mn content above the normal level (max. 2%) is used to
obtain high creep strength. This is the case with e.g. Esshete 1250, which contains 6% Mn.
Sulphur, S Sulphur is mostly regarded as an unwanted impurity and is therefore, as described under the
section Steel melting and casting, normally reduced to a low level in the AOD converter. The
normal content in stainless steels according to the various production standards is max.
0.030% or lower, otherwise there is a risk of cracking when hot working and welding. In our
steel melting, however, we obtain considerably lower values. In steels for machining the S
content is somewhat higher to help form sulphides, see above under "Manganese".
Phosphorous, P Phosphorous is an unwanted impurity, which cannot be reduced in the steel-melting process.
Therefore the content of P must be low already in the raw materials used. The normal content
in stainless steels is max. 0.040% or lower.
Aluminium, Al
(F)
Aluminium improves the oxidation resistance at high temperatures. It is added to the Sandvik
grade Sanicro 31HT. In Sandvik 9RU10 Al is added to form aluminium carbonitrides to give
a precipitation hardening effect.
Cerium, Ce Cerium is a so-called rare earth metal (REM). It is added, together with other REMs, in the
grades Sandvik 253 MA and 353 MA to improve the oxidation resistance at high
temperatures. It is an unwanted element in all welding consumables for MIG welding as it
causes a very unstable arc.
Cobalt, Co Cobalt is an element of great interest to the nuclear industry, where a low Co content is
essential. Our stock-standard tube and pipe in grades Sandvik 3R12 and 3R60 can easily meet
a requirement of max. 0.2%. In many cases a max. Co content of 0.1% can be offered.
Lead, Pb Lead is used in free-cutting steels where there are extra high demands on machinability, such
as for Sandvik 20AP.
Facts
 ELC = Extra Low Carbon, max. 0.030%.
 Ferrite-forming elements: Cr, Mo, Si, Ti, Nb.
 Austenite-forming elements: C, N, Mn, Ni, Cu.
 Stabilisation with Ti or Nb in order to avoid formation of Cr carbides.
 Sensitisation occurs when Cr carbides are formed in the region close to a weld (or by heat
treatment). Results in Cr depletion and sensitivity to attacks by intergranular corrosion.
Alloying elements
The influence of alloying elements, as described in literature, might not be valid for steel in the context of blades,
in particular for pattern-welded steel. Special care should be used for cutting performance and endurance, since
these depend mainly on the angle of the cutting edge as well as on the size of the carbides. Cutting tools cover a
wide field of operation of which blades represent only a fraction. Even among blades a differentiation of the
purpose is essential for optimization.
Such a differentiation is also required for the weldability of steels. In general good weldability concerns
transformations in the microstructure of the heat-influence zone and the resulting side effects like welding
fissures and stresses, grain growth, etc. forge-welding cannot be compared with the modern welding techniques
since these are normally not performed at temperatures above Ac3 or before the final heat treatment, eliminating
the influence of the welding temperature.
Chrome (Cr)
Chrome reduces the cooling speed required for a martensitic hardening, as a result of which the harden- and
annealability increase. Chrome narrows the area of gamma-crystal in the iron-carbon equilibrium diagram. With
increasing chrome content the forge-weldability decreases. Above 1-2% mokume-gane techniques have to be
used instead, making chrome-containing steels only to a limited extent suitable for pattern welding. At about
12% Cr steel become stainless. Chrome-containing steels are bright after etching.
Carbon (C)
Carbon is the most essential alloying element of steel. It is substantially responsible for the hardening of steel.
The influence of carbon is discussed in more detail in the section "fundamentals".
Manganese (Mn)
Manganese expands the gamma-area significantly. The cooling rates required for hardening are strongly reduced,

thus increasing the hardness penetration depth. Smaller sections, like in blades, will air-harden. Steels containing
above 12% manganese are austenitic at room temperatures. A content of 4 to 10% manganese will cause steels to
harden martensitic even when slow-cooling. Due to the poor workability, these steels are normally not produced.
Manganese acts deoxidizing and strongly sulfur-binding. Normally the etched surface of manganese-steels is
dark.
Molybdenum (Mo)
Molybdenum decreases required cooling rates and supports the formation of a fine microstructure, increasing the
weldability. The gamma-section is narrowed, forgeability decreases with increasing molybdenum content. It is a
strong carbide creator, increasing the mechanical strength and yield point. Molybdenum is often used in high-
speed steel to improve the wear resistance.
Nitrogen (N)
Nitrogen forms nitrides, giving steel a hard surface layer when nitriting. Nitrogen atoms are a replacement for
carbon in steel. The nitrogen atom is slightly smaller than the carbon atom, causing less deformation of the
martensitic primary crystal-cell. The risk of aging due to segregation is increased, pronouncing the effect of
blue-brittleness as well.
Titanium (Ti)
Titanium is a strong deoxidizer, is a strong nitrogen and carbon binder, builds up sulfides and narrows the
gamma-section significantly. Titanium acts as a grain-refiner but tends to banded segregations at higher contents.
Vanadium (V)
Vanadium narrows the gamma section and is a very strong carbon binder. Vanadium in smaller quantities
replaces iron as a substitution element and is grain-refining. With increasing vanadium content or incorrect heat
treatment, the present vanadium will act as a very strong carbon binder, the surrounding area can be depleted of
carbon and might not be hardenable any more. Vanadium-carbides are in general extremely large (50-70 µm)
and very hard (about 2800 HV), thus increasing the resistance to wear and elevated temperatures, making
vanadium an essential alloying element with high speed steels.
Tungsten (W)
Tungsten-alloyed steels are increasingly prone to red-shortness (tending to crack during forging at higher
temperatures) and show an increased oxidation. They can be forge-welded with caution. Tungsten is an
extremely strong carbide-binder, forming very hard and small carbides, thus hindering grain growth and
improving the toughness, also at elevated temperatures. Tungsten-alloyed steels are mainly used for high speed
steels and elevated temperature speed steels. They are also applied to tools requiring a fine cutting edge.
Steel-Harming Alloying Elements
Arsenic (As)
Arsenic strongly promotes segregations whose elimination by annealing is difficult to impossible. Toughness,
weldability and tempering brittleness are negatively influenced. The gamma-section is cut-off, the melting
temperature lowered. Arsenic steel has been used as a solder when forge-welding. The influence on the gamma-
section leads to carbon being pushed away from the arsenic zones.
Phosphorus (P)
Phosphorus-alloyed steels tend to primary segregations and, due to the gamma-section cut-off, secondary
segregations. The diffusion speed of phosphorus is low; hence, these segregations are difficult to be eliminated
in the alpha- and gamma-crystal. The segregations increase the tempering- and cold-brittleness, the steels
become red-short and tent to brittle failure.
Oxygen (O)
Oxygen decreases the impact strength and the ageing brittleness. Oxygen entering steel while forging causes red-
shortness, damaging the steel permanently.
Sulfur (S)
Sulfur leads to extremely many segregations and consequently causes red-shortness and decreases the welding
point. The addition of manganese binds the sulfur to manganese-sulfide. Sulfur from the forge coals diffuses into
the surface of the steel bar complicating the forge-weld by causing local melting at welding temperature. Free-
cutting steels are often sulfur enriched (up to 0.4%) to improve the machinability, leading to short breaking chips.
Silicon (Si)
Silicon gets into the steel while smelting. Steel containing more than 0.4% Si is called silicon steel. Silicon acts
deoxidizing and eases graphite segregation. The gamma-section is narrowed while increasing the elastic limit,
making silicon a well-suited alloying element for spring steel.

Effects of Alloying Additions to Steel
Element Influence Uses
Carbon
Most important alloying element. Is essential to the
formation of cementite and other carbides, bainite and
iron-carbon martensite. Within limits increasing the
carbon content increases the strength and hardness of
a steel while reducing its toughness and ductility.
Added to construction steels to increase
strength, hardness and hardenability.
Nickel
Stabilises gamma phase by raising A4 and lowering
A3. Refines grains in steels and some non-ferrous
alloys. Strengthens ferrite by solid solution.
Unfortunatly is a powerful graphitiser.
Can take into solid solution larger proportions of
important elements such as chromium, molybdenum
and tungsten than can iron.
Used up to help refine grain size. Used in
large amounts in stainless and heat-
resisting steels.
Nickel based alloys can offer corrosion
resistance in more aggressive
environments and nickel is used as the
basis of complex superalloys for high
temperature service.
Manganese
Deoxidises the melt. Greatly increases the
hadenability of steels. Stabilises gamma phase. Forms
stable carbides.
High manganese (Hadfield) steel contains
12.5% Mn and is austenitic but hardens
on abrasion.
Silicon
De-oxidises melt. Helps casting fluidity. Improves
oxidation resistance at higher temperatures.
Up to 0.3% in steels for sandcasting, up
to 1% in heat resisting steels.
Chromium
Stabilises alpha phase by raising A3 and depressing
A4. Forms hard stable carbides. Strengthens ferrite by
solid solution. In amounts above 13% it imparts
stainless properties. Unfortunately increases grain
growth.
Small amounts in constructional and tool
steels. About 1.5% in ball and roller
bearings. Larger amounts in Stainless and
heat-resisting steels.
Molybdenum
Strong carbide-stabilising influence. Raises high
temperature creep strength of some alloys. Slows
tempering response.
When added to stainless steels it greatly improves the
pitting and crevice corrosion resistance. There are
limits to the proportion that can be taken into an iron
based matrix. However up to almost 30% can be
incorporated into nickel based alloys which provides
excellent corrosion resistance in many aqueous
environments.
Reduces 'temper brittleness' in nickel-
chromium steels. Increases red-hardness
of tool steels. Now used to replace some
tungsten in high-speed steels.
Vanadium
Strong carbide forming tendency. Stabilises martensite
and increases hardenability. Restrains grain growth.
Improves resistance to softening at elevated
temperatures after hardening.
Used to retain high temperature hardness,
eg in dies for hot-forging and die casting
dies. Increasingly used in high speed
steels.
Tungsten
Stabilises alpha phase and forms stable, very hard
carbides, which improves creep resistance and renders
transformations very sluggish, hence hardened steels
resist tempering influences.
Used in high-speed steels and other tool
and die steels, particularly those for use at
high temperatures.
Used in a few stainless steels, in
combination with molybdenum. to
improve pitting and crevice corrosion
resistance. It is also used in some high
temperature nickel based alloys and in
some high temperature austenitic
stainless steels.
Cobalt
Has similar corrosion resistance to that of Nickel, but
higher cost means that it is not normally used for such
applications.
Provides matrix - strengthening characteristics to
stainless and nickel based alloys designed for high
temperature applications.
Slows the transformation of martensite, hence
increases 'red hardness' which is useful in tool steels.
Used in super high speed steels and
maraging steels, permanent magnet steels
and alloys.
Niobium
In low alloy steels it acts as a carbide former and
improves creep resistance.
Used to stabilise stainless steels.

In stainless steels it combines with carbon, stabilising
the steel and reducing the susceptibility to
intergranular corrosion
Titanium
In stainless steels combines with excess carbon
reducing the risk of intergranular corrosion.
Used in stabilised stainless steels.
In nickel based alloys it is used with
aluminium to promote age hardening.
Reference:
'Metals Handbook', ASM, 2nd Desk Edition, 1998, ISBN: 0-87170-654-7.
'The Alloy Tree', by J C M Farrar, CRC Press and Woodhead Publishing, 2004, ISBN: 1 85573 766 3.
Influence of Alloying Elements on Steel Microstructure
Abstract:
It is a long-standing tradition to discuss the various alloying elements in terms of the properties they confer on
steel. For example, the rule was that Chromium (Cr) makes steel hard whereas Nickel (Ni) and Manganese (Mn)
make it tough. In saying this, one had certain types of steel in mind and transferred the properties of particular
steel to the alloying element that was thought to have the greatest influence on the steel under consideration. This
method of reasoning can give false impressions and the following examples will illustrate this point.
It is a long-standing tradition to discuss the various alloying elements in terms of the properties they confer on
steel. For example, the rule was that Chromium (Cr) makes steel hard whereas Nickel (Ni) and Manganese (Mn)
make it tough. In saying this, one had certain types of steel in mind and transferred the properties of particular
steel to the alloying element that was thought to have the greatest influence on the steel under consideration. This
method of reasoning can give false impressions and the following examples will illustrate this point.
When we say that Cr makes steel hard and wear-resisting we probably associate this with the 2% C, 12% Cr
tool steel grade, which on hardening does in fact become very hard and hard-wearing. But if, on the other hand,
we choose a steel containing 0,10% C and 12% Cr, the hardness obtained on hardening is very modest.
It is quite true that Mn increases steel toughness if we have in mind the 13% manganese steel, so-called Hadfield
steel. In concentrations between l% and 5%, however, Mn can produce a variable effect on the properties of the
steel it is alloyed with. The toughness may either increase or decrease.
A property of great importance is the ability of alloying elements to promote the formation of a certain phase or
to stabilize it. These elements are grouped as austenite-forming, ferrite-forming, carbide-forming and nitride-
forming elements.
Austenite-forming elements
The elements C, Ni and Mn are the most important ones in this group. Sufficiently large amounts of Ni or Mn
render a steel austenitic even at room temperature. An example of this is the so-called Hadfield steel which
contains 13% Mn, 1,2% Cr and l% C. In this steel both the Mn and C take part in stabilizing the austenite.
Another example is austenitic stainless steel containing 18% Cr and 8% Ni.
The equilibrium diagram for iron-nickel, Figure 1, shows how the range of stability of austenite increases with
increasing Ni-content.
Figure 1. Fe-Ni equilibrium diagram
An alloy containing 10% Ni becomes wholly austenitic if heated to 700°C. On cooling, transformation from g to
a takes place in the temperature range 700-300°C.

Ferrite-forming elements
The most important elements in this group are Cr, Si, Mo, W and Al. The range of stability of ferrite in iron-
chromium alloys is shown in Figure 2. Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at
all temperatures up to incipient melting. Another instance of ferritic steel is one that is used as transformer sheet
material. This is a low-carbon steel containing about 3% Si.
Figure 2. Cr-Fe equilibrium diagram
Multi-alloyed steels
The great majority of steels contain at least three components. The constitution of such steels can be deduced
from ternary phase diagrams (3 components). The interpretation of these diagrams is relatively difficult and they
are of limited value to people dealing with practical heat treatment since they represent equilibrium conditions
only. Furthermore, since most alloys contain more than three components it is necessary to look for other ways
of assessing the effect produced by the alloying elements on the structural transformations occurring during heat
treatment.
One approach that is quite good is the use of Schaeffler diagrams (see Figure 3). Here the austenite formers are
set out along the ordinate and the ferrite formers along the abscissa. The original diagram contained only Ni and
Cr but the modified diagram includes other elements and gives them coefficients that reduce them to the
equivalents of Ni or Cr respectively. The diagram holds good for the rates of cooling which result from welding.
Figure 3. Modified Schaeffler diagram
A 12% Cr steel containing 0,3% C is martensitic, the 0,3% C gives the steel a nickel equivalent of 9. An 18/8
steel (18% Cr, 8% Ni) is austenitic if it contains 0-0,5% C and 2% Mn. The Ni content of such steels is usually
kept between 9% and 10%.
Hadfield steel with 13% Mn (mentioned above) is austenitic due to its high carbon content. Should this be
reduced to about 0,20% the steel becomes martensitic.
Carbide-forming elements
Several ferrite formers also function as carbide formers. The majority of carbide formers are also ferrite formers
with respect to Fe. The affinity of the elements in the line below for carbon increases from left to right.
Cr, W, Mo, V, Ti, Nb, Ta, Zr.
Some carbides may be referred to as special carbides, i.e. non-iron-containing carbides, such as Cr7C3 W2C, VC,
Mo2C. Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C.
High-speed and hot-work tool steels normally contain three types of carbides, which are usually designated M6C,
M23C6 and MC. The letter M represents collectively all the metal atoms. Thus M6C represents Fe4W2C or
Fe4Mo2C; M23C6 represents Cr23C6 and MC represents VC or V4C3.

Carbide stabilizers
The stability of the carbides is dependent on the presence of other elements in the steel. How stable the carbides
are depends on how the element is partitioned between the cementite and the matrix. The ratio of the percentage,
by weight, of the element contained in each of the two phases is called the partition coefficient K. The following
values are given for K:
Al Cu P Si Co Ni W Mo Mn Cr Ti Nb Ta
0 0 0 0 0,2 0,3 2 8 11,4 28 Increasing
Note that Mn, which by itself is a very weak carbide former, is a relatively potent carbide stabilizer. In practice,
Cr is the alloying element most commonly used as a carbide stabilizer.
Malleable cast iron (i.e. white cast iron that is rendered soft by a graphitizing heat treatment called
malleablizing) must not contain any Cr. Steel containing only Si or Ni is susceptible to graphitization, but this is
most simply prevented by alloying with Cr.
Nitride-forming elements
All carbide formers are also nitride formers. Nitrogen may be introduced into the surface of the steel by nitriding.
By measuring the hardness of various nitrided alloy steels it is possible to investigate the tendency of the
different alloying elements to form hard nitrides or to increase the hardness of the steel by a mechanism known
as precipitation hardening.
The results obtained by such investigations are shown in Figure 4, from which it can be seen that very high
hardnesses result from alloying a steel with Al or Ti in amounts of about 1,5%.
Figure 4. Effect of alloying element additions on hardness after nitriding
Base composition: 0,25% C, 0,30% Si, 0,70% Mn
On nitriding the base material in Figure 4, hardness of about 400 HV is obtained and according to the diagram
the hardness is unchanged if the steel is alloyed with Ni since this element is not a nitride former and hence does
not contribute to any hardness increase.
The Effects of Alloying Elements on Iron-Carbon Alloys
Abstract:
The simplest version of analyzes the effects of alloying elements on iron-carbon alloys would require analysis of
a large number of ternary alloy diagrams over a wide temperature range. However, Wever pointed out that iron
binary equilibrium systems fall into four main categories: open and closed γ-field systems, and expanded and
contracted γ-field systems.
The form of the diagram depends to some degree on the electronic structure of the alloying elements which is
reflected in their relative positions in the periodic classification.
The simplest version of analyzes the effects of alloying elements on iron-carbon alloys would require analysis of
a large number of ternary alloy diagrams over a wide temperature range. However, Wever pointed out that iron
binary equilibrium systems fall into four main categories (Fig. 1): open and closed γ-field systems, and expanded
and contracted γ-field systems. This approach indicates that alloying elements can influence the equilibrium
diagram in two ways:
 by expanding the γ-field, and encouraging the formation of austenite over wider compositional limits.
These elements are called γ-stabilizers.
 by contracting the γ-field, and encouraging the formation of ferrite over wider compositional limits.
These elements are called α-stabilizers.
The form of the diagram depends to some degree on the electronic structure of the alloying elements which is
reflected in their relative positions in the periodic classification.

Figure 1. Classification of iron alloy phase diagrams: a. open γ-field; b. expanded γ-field; c. closed γ-field
(Wever, Archiv, Eisenhüttenwesen, 1928-9, 2, 193)
Class 1: open γ-field. To this group belong the important steel alloying elements nickel and manganese, as well
as cobalt and the inert metals ruthenium, rhodium, palladium, osmium, iridium and platinum. Both nickel and
manganese, if added in sufficiently high concentration, completely eliminate the bcc α-iron phase and replace it,
down to room temperature, with the γ-phase. So nickel and manganese depress the phase transformation from γ
to α to lower temperatures (Fig. 1a), i.e. both Ac1 and Ac3 are lowered. It is also easier to obtain metastable
austenite by quenching from the γ-region to room temperature, consequently nickel and manganese are useful
elements in the formulation of austenitic steels.
Class 2: expanded γ-field. Carbon and nitrogen are the most important elements in this group. The γ-phase field
is expanded, but its range of existence is cut short by compound formation (Fig.1b). Copper, zinc and gold have
a similar influence. The expansion of the γ-field by carbon, and nitrogen, underlies the whole of the heat
treatment of steels, by allowing formation of a homogeneous solid solution (austenite) containing up to 2.0 wt %
of carbon or 2.8 wt % of nitrogen.
Class 3: closed γ-field. Many elements restrict the formation of γ-iron, causing the γ-area of the diagram to
contract to a small area referred to as the gamma loop (Fig. 1c). This means that the relevant elements are
encouraging the formation of bcc iron (ferrite), and one result is that the δ- and γ-phase fields become continuous.
Alloys in which this has taken place are, therefore, not amenable to the normal heat treatments involving cooling
through the γ/α-phase transformation. Silicon, aluminium, beryllium and phosphorus fall into this category,
together with the strong carbide forming elements, titanium, vanadium, molybdenum and chromium.
Class 4: contracted y-field. Boron is the most significant element of this group, together with the carbide
forming elements tantalum, niobium and zirconium. The γ-loop is strongly contracted, but is accompanied by
compound formation (Fig. 1d).
The distribution of alloying elements in steels. Although only binary systems have been considered so far,
when carbon is included to make ternary systems the same general principles usually apply. For a fixed carbon
content, as the alloying clement is added the y-field is either expanded or contracted depending on the particular
solute.
With an element such as silicon the γ-field is restricted and there is a corresponding enlargement of the α-field. If
vanadium is added, the γ-field is contracted and there will be vanadium carbide in equilibrium with ferrite over
much of the ferrite field. Nickel does not form a carbide and expands the γ-field. Normally elements with
opposing tendencies will cancel each other out at the appropriate combinations, but in some cases anomalies
occur. For example, chromium added to nickel in a steel in concentrations around 18% helps to stabilize the γ-
phase, as shown by 18Cr8Ni austenitic steels.
One convenient way of illustrating quantitatively the effect of an alloying element on the γ-phase field of the Fe-
C system is to project on to the Fe-C plane of the ternary system the γ-phase field boundaries for increasing
concentration of a particular alloying element. For more precise and extensive information, it is necessary to
consider series of isothermal sections in true ternary systems Fe-C-X, but even in some of the more familiar
systems the full information is not available, partly because the acquisition of accurate data can be a difficult and
very time-consuming process.
Recently the introduction of computer-based methods has permitted the synthesis of extensive thermochemical
and phase equilibria data, and its presentation in the form, for example, of isothermal sections over a wide range
of temperatures.

If only steels in which the austenite transforms to ferrite and carbide on slow cooling are considered, the alloying
elements can be divided into three categories:
 elements which enter only the ferrite phase
 elements which form stable carbides and also enter the ferrite phase
 elements which enter only the carbide phase.
In the first category there are elements such as nickel, copper, phosphorus and silicon which, in transformable
steels, are normally found in solid solution in the ferrite phase, their solubility in cementite or in alloy carbides
being quite low.
The majority of alloying elements used in steels fall into the second category, in so far as they are carbide
formers and as such, at low concentrations, go into solid solution in cementite, but will also form solid solutions
in ferrite. At higher concentrations most will form alloy carbides, which are thermodynamically more stable than
cementite.
Typical examples are manganese, chromium, molybdenum, vanadium, titanium, tungsten and niobium.
Manganese carbide is not found in steels, but instead manganese enters readily into solid solution in Fe3C. The
carbide-forming elements are usually present greatly in excess of the amounts needed in the carbide phase,
which are determined primarily by the carbon content of the steel. The remainder enters into solid solution in the
ferrite with the non-carbide forming elements nickel and silicon. Some of these elements, notably titanium,
tungsten, and molybdenum, produce substantial solid solution hardening of ferrite.
In the third category there are a few elements which enter predominantly the carbide phase. Nitrogen is the most
important element and it forms carbo-nitrides with iron and many alloying elements. However, in the presence of
certain very strong nitride forming elements, e.g. titanium and aluminum, separate alloy nitride phases can occur.
While ternary phase diagrams, Fe-C-X, can be particularly helpful in understanding the phases which can exist
in simple steels, isothermal sections for a number of temperatures are needed before an adequate picture of the
equilibrium phases can be built up. For more complex steels the task is formidable and equilibrium diagrams can
only give a rough guide to the structures likely to be encountered. It is, however, possible to construct
pseudobinary diagrams for groups of steels, which give an overall view of the equilibrium phases likely to be
encountered at a particular temperature.
Structural changes resulting from alloying additions. The addition to iron-carbon alloys of elements such as
nickel, silicon, manganese, which do not form carbides in competition with cementite, does not basically alter
the microstructures formed after transformation. However, in the case of strong carbide-forming elements such
as molybdenum, chromium and tungsten, cementite will be replaced by the appropriate alloy carbides, often at
relatively low alloying element concentrations. Still stronger carbide forming elements such as niobium, titanium
and vanadium are capable of forming alloy carbides, preferentially at alloying concentrations less than 0.1 wt%.
It would, therefore, be expected that the microstructures of steels containing these elements would be radically
altered. It has been shown how the difference in solubility of carbon in austenite and ferrite leads to the familiar
ferrite/cementite aggregates in plain carbon steels. This means that, because the solubility of cementite in
austenite is much greater than in ferrite, it is possible to redistribute the cementite by holding the steel in the
austenite region to take it into solution, and then allowing transformation to take place to ferrite and cementite.
Examining the possible alloy carbides, and nitrides, in the same way, shows that all the familiar ones are much
less soluble in austenite than is cementite.
Chromium and molybdenum carbides are not included, but they are substantially more soluble in austenite than
the other carbides. Detailed consideration of such data, together with practical knowledge of alloy steel behavior,
indicates that, for niobium and titanium, concentrations of greater than about 0.25 wt % will form excess alloy
carbides which cannot be dissolved in austenite at the highest solution temperatures. With vanadium the limit is
higher at 1-2%, and with molybdenum up to about 5%. Chromium has a much higher limit before complete
solution of chromium carbide in austenite becomes difficult. This argument assumes that sufficient carbon is
present in the steel to combine with the alloying element. If not, the excess metallic element will go into solid
solution both in the austenite and the ferrite.
In general, the fibrous morphology represents a closer approach to an equilibrium structure so it is more
predominant in steels which have transformed slowly. In contrast, the interphase precipitation and dislocation
nucleated structures occur more readily in rapidly transforming steels, where there is a high driving force, for
example, in microalloyed steels.
The clearest analogy with pearlite is found when the alloy carbide in lath morphology forms nodules in
association with ferrite. These pearlitic nodules are often encountered at temperatures just below Ac1, in steels
which transform relatively slowly.
For example, these structures are obtained in chromium steels with between 4% and 12% chromium and the
crystallography is analogous to that of cementitic pearlite. It is, however, different in detail because of the
different crystal structures of the possible carbides. The structures observed are relatively coarse, but finer than
pearlite formed under equivalent conditions, because of the need for the partition of the alloying element, e.g.
chromium between the carbide and the ferrite. To achieve this, the interlamellar spacing must be substantially
finer than in the equivalent iron-carbon case.
Interphase precipitation. Interphase precipitation has been shown to nucleate periodically at the γ/α interface
during the transformation. The precipitate particles form in bands which are closely parallel to the interface, and
which follow the general direction of the interface even when it changes direction sharply. A further

characteristic is the frequent development of only one of the possible Widmanstätten variants, for example VC
plates in a particular region are all only of one variant of the habit, i.e. that in which the plates are most nearly
parallel to the interface.
The extremely fine scale of this phenomenon in vanadium steels, which also occurs in Ti and Nb steels, is due to
the rapid rate at which the γ/α transformation takes place. At the higher transformation temperatures, the slower
rate of reaction leads to coarser structures. Similarly, if the reaction is slowed down by addition of further
alloying elements, e.g. Ni and Mn, the precipitate dispersion coarsens.
The scale of the dispersion also varies from steel to steel, being coarsest in chromium, tungsten and molybdenum
steels where the reaction is relatively slow, and much finer in steels in which vanadium, niobium and titanium
are the dominant alloying elements and the transformation is rapid.
Transformation diagrams for alloy steels. The transformation of austenite below the eutectoid temperature can
best be presented in an isothermal transformation diagram, in which the beginning and end of transformation is
plotted as a function of temperature and time. Such curves are known as time-temperature-transformation, or
TTT curves, and form one of the important sources of quantitative information for the heat treatment of steels.
In the simple case of a eutectoid plain carbon steel, the curve is roughly C-shaped with the pearlite reaction
occurring down to the nose of the curve and a little beyond. At lower temperatures bainite and martensite are
formed. The diagrams become more complex for hypo- and hyper-eutectoid alloys as the ferrite or cementite
reactions have also to be represented by additional lines.
CHEMICAL ELEMENTS USED IN STEEL
Iron (Fe)
Iron is the single most important element in steel and comprises roughly
95% of the steel matrix. Other non-structural elements are listed below.
Carbon (C) Increasing the amount of carbon increases the strength and lowers the
ductility; current structural steels typically have carbon ranging
from .05% to .25%.
Manganese (Mn) Manganese has effects similar to those of carbon. It is usually used in
amounts varying from .5 to 1.7% and is critical to the production process
because of the way it combines with oxygen and sulfur.
Chromium (Cr) Chromium is primarily used to increase corrosion resistance. In
weathering steels, like ASTM A588, the chromium content varies from .1
to .9%.
Copper (Cu) Copper is also used for corrosion resistance. It is found in amounts not
less than .2% for electric arc furnace (EAF) steel and about .02 to .03%
for basic oxygen furnace (BOF) steel.
Silicon (Si) Silicon is one of the two most important de-oxidizers of steel, meaning
that it is very effective in removing oxygen from the steel during the
pouring and solidification process. Typical content is from.1 to .4%.
Aluminum (Al) Aluminum is the other de-oxidizer used to remove oxygen from steel
(killed). It is also used for grain refinement.
Columbium (Cb) Columbium is used to enhance the strength of steel and is one of key
elements in various HSLA grades. It has effects similar to those of
manganese and vanadium and is often used in combination with
vanadium. Due to weldability requirements, columbium is unused in
amounts less than .05%, such as in A572, for example.
Molybdenum (Mo) This element especially increases the strength of steel at elevated
temperatures, as well as providing corrosion resistance. Molybdenum is
particularly applicable for certain types of A588 and A514 steel. In the
latter, molybdenum content may be as high as .65%.
Nickel (Ni) Nickel is a powerful anti-corrosion agent and also is one of the most
important elements for improving the fracture toughness of steel. Nickel
contents vary between .25 and 1.5%, depending on the specifics of the
steel.
Vanadium (V) Vanadium aids in the development of a tough, fine-grained steel structure.
Vanadium is an important alloying element in HSLA steels, such as A572
and A588.

Sulfur (S) & Phosphorus (P) Both elements have detrimental effects on steel strength, but especially
ductility and weldability of steel. Sulfur promotes segregation in the steel
matrix. Sulfur and phosphorus are both restricted to no more than .04
to.05%.
Effects of Elements on Steel
Steels are among the most commonly used alloys. The complexity of steel alloys is fairly significant. Not all
effects of the varying elements are included. The following text gives an overview of some of the effects of
various alloying elements. Additional research should be performed prior to making any design or engineering
conclusions.
Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel. Hardness and
tensile strength increases as carbon content increases up to about 0.85% C as shown in the figure
above. Ductility and weldability decrease with increasing carbon.
Manganese is generally beneficial to surface quality especially in resulfurized steels. Manganese contributes to
strength and hardness, but less than carbon. The increase in strength is dependent upon the carbon
content. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese
has a significant effect on the hardenability of steel.
Phosphorus increases strength and hardness and decreases ductility and notch impact toughness of steel. The
adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon
steels. Phosphorous levels are normally controlled to low levels. Higher phosphorus is specified in low-carbon
free-machining steels to improve machinability.
Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability
decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur
levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added
to improve machinability.
Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in
increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface
quality.
Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge welding,
but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface quality. Copper
is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20%. Weathering steels
are sold having greater than 0.20% Copper.
Lead is virtually insoluble in liquid or solid steel. However, lead is sometimes added to carbon and alloy steels
by means of mechanical dispersion during pouring to improve the machinability.
Boron is added to fully killed steel to improve hardenability. Boron-treated steels are produced to a range of
0.0005 to 0.003%. Whenever boron is substituted in part for other alloys, it should be done only with
hardenability in mind because the lowered alloy content may be harmful for some applications.
Boron is a potent alloying element in steel. A very small amount of boron (about 0.001%) has a strong effect on
hardenability. Boron steels are generally produced within a range of 0.0005 to 0.003%. Boron is most effective
in lower carbon steels.
Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase
hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used
with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures,
chromium contributes increased strength. Chromium is a strong carbide former. Complex chromium-iron
carbides go into solution in austenite slowly; therefore, sufficient heating time must be allowed for prior to
quenching.
Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in
ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of
steels.
Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the
tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures.
Aluminum is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and
is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior to
quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are
difficult to dissolve into solution in austenite.
Zirconium can be added to killed high-strength low-alloy steels to achieve improvements in inclusion
characteristics. Zirconium causes sulfide inclusions to be globular rather than elongated thus improving
toughness and ductility in transverse bending.
Niobium (Columbium) increases the yield strength and, to a lesser degree, the tensile strength of carbon steel.
The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can
also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the
transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain
microstructure having improved strength and toughness.

Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve
improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than
elongated thus improving toughness and ductility in transverse bending.
Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of
Vanadium can significantly increase the strength of steels. Vanadium is one of the primary contributors to
precipitation strengthening in microalloyed steels. When thermomechanical processing is properly controlled
the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition
temperature also increases when vanadium is added.
All microalloy steels contain small concentrations of one or more strong carbide and nitride forming
elements. Vanadium, niobium, and titanium combine preferentially with carbon and/or nitrogen to form a fine
dispersion of precipitated particles in the steel matrix.
Effects of Alloying Elements in Steel
Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the
finished melt. Listed below is a summary of the effects various alloying elements in steel.
Carbon
The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and
strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.
Manganese
Manganese is added to steel to improve hot working properties and increase strength, toughness and
hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for
nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)
Chromium
Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is
added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion
resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels,
chromium can increase the response to heat treatment, thus improving hardenability and strength.
Nickel
Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important
class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the
tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low
temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low
temperatures when added in smaller amounts to alloy steels.
Molybdenum
Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion
especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high
temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels
to decay in service or in heat treatment.
Titanium
The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to
for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence
of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon
combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as
inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.
Phosphorus
Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small
amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic
stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during
welding.
Sulphur
When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is
reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As
manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not
present, the weak spots at the grain boundaries are greatly reduced during hot working.
Selenium
Selenium is added to improve machinability.
Niobium (Columbium)
Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for
titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.
Nitrogen
Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an
austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless
steels.
Silicon

Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small
percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon
killed steels are somewhat harder and stiffer than aluminium killed steels.
Cobalt
Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any
stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This
problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.
Tantalum
Chemically similar to niobium and has similar effects.
Copper
Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to
produce precipitation hardening properties.
Aluminium to Zirconium of steel elements
Aluminium (Al) is added to steel as a deoxidizer. Added to control grain size aluminium can control austenite
grain growth in reheated steels.
Carbon (C) is the most important element in the majority of steel, affecting hardness and strength by heat
treatment. The ductility and weldability decreases with increasing carbon content.
Cobalt (Co) can be used up to 10% content in some high speed steels. It becomes radioactive when exposed to
nuclear radiation therefore for radioactive applications it must not be present in steel.
Copper (Cu) can be present in stainless steels for precipitation hardening properties. Used in "weathering" steels.
Chromium (Cr) is added to steel to increase corrosion and oxidation resistance. It also increases hardenability
and combined with high carbon improves wear and abrasion resistance
Manganese (Mn) contributes to strength and hardness with variable carbon content. It is an austenite forming
element in some steels and has a significant effect on hardenability.
Molybdenum (Mo) is added to nickel chrome alloy steels to improve strength and hardness and also in
chromium nickel austenitic steels it improves corrosion resistance. Molybdenum is used in some high speed steel
grades.
Nickel (Ni) is an important element which increases hardenability, tensile and impact values of steels. Added to
high chromium stainless steels in amounts of over 8% it produces austenitic structures which gives high
temperature strengths and resistance to oxidation and corrosion.
Niobium (Nb) stabilises carbon in the same way as titanium and strengthens steels for high temperature service.
Nitrogen (N) is added to stainless steel to improve the austenitic stability with increased yield strength.
Phosphorous (P) is normally controlled to low levels but higher phosphorous can be used to improve
machineability.
Silicon (Si) is a principal deoxidiser in steel, used in silicon manganese, corrosion and heat resisting steels.
Sulphur (S) is often added to improve machineability, but does decrease ductility and notch impact toughness.
Tungsten (W) is a major element in high speed and some tool steels. In the heat treated condition it retains
hardness at elevated temperatures and is particularly useful for cutting tools.
Vanadium (V) helps improve fatigue stress and wear resistance when used with other alloying elements.
Zirconium (Zr) can be added to high strength low alloy steels, affecting inclusion improvement, giving
toughness and ductility in bending modes.
Alloying elements
This might be a good time to talk about the effects of alloying elements in steel and what they do to it. Bear in
mind that I am talking about LOW alloy steels, that is, no individual element at more than 2% of the steel
(with the balance Fe).
Mn=Manganese. manganese is present in all modern steels to some degree. The primary reason for this is to tie
up the sulphur that may be present. Sulphur in steel not containing manganese will end up in the austenite
grain boundaries as a compound FeS or iron sulfide. Iron sulfide is liquid at temperatures that are in the forging
range, and if present, lubricates the austenite grains at their boundaries and makes the steel come apart in a
situation referred to as "hot short". If the Mn is above about 1%, and increasing with the percentage, the
hardenability of the steel is dramatically improved. Mn is generally only considered an "alloying element" if it is
above 1%. The classic example is O-2. O-2 is essentially 1090, but with 1.6% Mn. This renders it oil hardening,
in rather thick sections, by delaying the decomposition of the austenite as the temperature drops, and gives more
time to get the piece cooled and still not form pearlite.
Cr=chromium. Chromium is a useful alloying element in low alloy steels not because of any "stainless" quality,
but because it improves the hardenability of the steel quite a bit. It is also a carbide forming element, which can
help keep the austenite grain size small by "pinning" the grain boundaries of the austenite. This is only true until
the time/temperature combination to dissolve all of the carbides is reached, then the austenite grains are free to
grow.
Ni=nickel. nickel is useful as an alloying element because it increases the strength of the ferrite phase by
entering into solid solution in the ferrite. It is not a carbide forming element, and in fact will reject carbon and
render it graphite if it is present in large enough amounts in steel or cast iron with high carbon contents. It does

help in creating some of the very high strength alloys, but none of these are over .7%C, and the only one I know
of that is that high is L-6 tool steel, with most of the others being at or under .4%C.
Mo=molybdenum. A strong carbide former, it is also very helpful in increasing the hardenability of low alloy
steels. In higher amounts, it has many of the same effects as tungsten, imparting hot hardness to the steel, and is
the major alloying element in the "M" series of high speed steels (M-1, M-2, M42, etc.)
V=vanadium. vanadium is usually only present in quite small amounts in low alloy steels, typically under .25%.
At these levels, it serves mostly as a carbide former to inhibit grain growth in the austenite when the steel is
heated. Vanadium carbides are VERY persistent and difficult to dissolve into the austenite solution, which is
why it is so effective at keeping the grain size small.
In my mind, as a bladesmith, the ideal knife steel would be something about 1.1%C, .25V, and about .4Mn, and
that is all. This would be a shallow hardening steel, quite similar to W-2, which is something that I have never,
ever, seen anywhere in a form that could be positively identified as such by the people that actually made the
steel. A close second would be the steel we were discussing that led to this, the "carbon V" which is also quite
similar, with the addition of a little Cr, which would make it deeper hardening, not a bad thing to have. 52100 is
about as close to this as we can get with readily available alloys, so far as I know. It is very good steel, but one
really should have temperature controls to do the heat treating on it, it is picky about being over-heated, it does
not like it, not even a little bit.
Alloying Elements and Properties of Steel
Here are a few major alloying elements for steel and what they can do.
This list is based on the "Materials in Action Series; Structural Materials"
Element
Influence on
Ferrite
Influence on
Hardenability
Tendency to
form hard
Carbides
Major Functions
Manganese
Mn
Powerful
solution
strengthener
Moderate
increase
Middle
1. Takes care of Sulphur
(S).
2. Cheap increase of
hardenability.
Silicon
Si
Hardens, but
reduces ductility
Moderate
increase
-
1. Deoxidation of liquid
steel.
2. Improves oxidation
resistance.
3. Strengthens low alloy
steel.
4. Increases electrical
resistivity (important for
transformer cores).
Chromium
Cr
Strengthens a
little
Provides
corrosion
resistance
Moderate
increase
Strong
1. Corrosion resistance.
2. Hardenability.
3. Abrasion resistance
(needs high C, too).
4. Strength + oxidation
resistance at high T.
Titanium
Ti
Age hardening
possible
Very strong
increase
Extremely
strong
1. Forms hard carbides.
2. Prevents local depletion
of C carbon in stainless
steels due to Cr-carbide
formation
Vanadium
V
Moderate solid
solution
hardening
Very strong
increase
Very strong
1. Restricts grain
coarsening of austenite.
2. Increases hardenability.
3. Delays softening during
tempering.
Nickel Strengthens Mild - 1. Improves strength and

Ni improvement
stabilizes
austenite
toughness at subzero T.
2. Together with Cr
provides austenitic steel.
Molybdenum
Mo
Age hardening
possible
Strong increase Very strong
1. Increase hardenability.
2. Prevent embrittlement of
certain Ni/Cr steels.
3. Keeps strength at higher
T.
4. Restricts austenite grain
growth.
5. Improves corrosion
resistance of stainless
steels.
6. Provides carbides with
high abrasion resistance.
Cobalt
Co
Strengthens in
solid solution
Decreases
slightly
Like Fe
1. Contributes hardness at
moderately high T.
The list could go on for a while, of course. It includes some properties not much discussed before, for
example:
Behavior at low and/or high temperatures.
Properties like wear (or abrasion) resistance or corrosion resistance (note that stainless steel, while
oxidation resistant, might nevertheless corrode in some other chemical environment.
Making steel in the first place (look for "liquid").
Counteracting the effects of other elements.
Keeping the structure from unwanted changes ("grain growth")
Alloy Steels
General Remarks
Writing these modules, I found it surprisingly hard to find data or good metallographic pictures for the plain
carbon steel of the preceding chapter.
Well, there is a simple reason for that: There is practically no such thing as plain carbon steel - and
probably never has been.
Steel practically always contains other elements besides carbon, too, which were added intentionally or
unintentionally.
Unintentional elements are in particular Sulfur (S) and Phosphorous (P); but also Sn, As and Sb.
All these elements tend to diffuse to grain boundaries, where they might segregate; reducing the
cohesive strength - the steel becomes brittle. If it does not happen right away, it might happen after some
temper treatment; we have (undesired) temper brittleness.
By now you should be sensitive to words like "tend" and "might", which indicate that things are not so
simple and easy.
Phosphorous, for example, is not always harmful. In properly treated
steel, it might be beneficial, too, as shown below.
Since the "bog iron" (German: "Raseneisenerz"), used for millennia to
make iron and steel, contained relatively large amounts of P, it "might"
have been crucial for the early smiths to keep the Phosphorous from
segregating to grain boundaries. What bog iron looks like is shown on the
right - we all have seen stones like that, but possibly not recognized what
it was.
However, if you were lucky, some other elements contained in your iron
"might" have helped in this respect and you may never have noticed that

you had a problem.
But generally, some elements, in particular Sulfur S (and P), are almost
always bad news, and not easy to avoid.
But fortunately, Manganese (Mn) is also quite ubiquitous - and it sort of
"soaks up" the Sulfur (by forming immobile sulfides).
We thus have a first reason for adding something else: To neutralize bad
effects of unwanted, but hard to avoid trace impurities. But this, while
being quite important, is nevertheless only a minor point for making
alloyed steels, sort of a fringe benefit.
So small wonder that you will always find 0.5 % -1 %or so of Mn in
practically any alloy steel (and "plain carbon", too).
The major reasons for adding all kinds of elements to carbon steel are:
1. Improved strength while maintaining good ductility and in particular workability ("Verarbeitbarkeit").
The key words in this context are solution strengthening and precipitation hardening ("Mischkristall-
und Ausscheidungshärtung") while maintaining weldability ("Schweißbarkeit").
2. Improved hardenability. The key is to enable martensite formation even at relatively low cooling
rates so that it can occur in the interior of massive steel pieces , too. In German, hardenability is called
"Härtungstiefe" (= "hardening depth"), which gives a better impression of what is meant: Even regions
deep in the bulk, which by necessity cool down more slowly than surface near region, become "hard",
i.e. experience martensite formation.
3. Improved corrosion resistance. The key word is "stainless steel", resulting from rather large
additions of Chromium (Cr).
4. Stabilized austenite at low temperatures. In other words, we get (nonmagnetic) austenitic steel (with
an fcc lattice) at room temperature (and somewhat below). It is almost, but not quite the same thing as
point 3. from above.
In addition, we should not forget that properties like weldability, and pedestrian concerns like money, are
also part of the alloying game
All the obviously desirable features from above can be achieved to some extent by adding a suitable amount
of the right elements.
To make things complicated, most elements do several things from the list above, and a combination of
two elements usually does not just produce the sum of the individual properties, but something new.
In addition, improving one property by adding a certain element might easily produce problems with
some other properties. You many have to compromise.
And not to forget: As we have seen by alloying Iron with just Carbon: Many variations of properties are
possible with just one element!
In discussing, not to mention making alloyed steels, a certain amount of alchemy is in evidence, even
today. And new discoveries and new steels will certainly come forth in the future, too.
The link provides a short list of some alloying elements and what they are used for.
It is entirely impossible to touch all bases here. Let's just give the four categories from above a cursory
glance and make a basic distinction at the beginning:
We distinguish between
 Low-alloy steels: We only add less than about 2 weight % of the major alloying element(s)
(and usually keep the carbon concentration low)
 High-alloy steels: We add a lot more than 2 weight % and possibly as much as 20 weight %.
In between is "medium-alloy", but that already goes to far in this context.
Improved Strength and Good Workability
Here we are generally talking low-alloy steels, in particular with a rather low carbon concentration. The
general idea is to avoid martensite formation, which is bad for welding and shaping, but still have good
strength properties.
If you want to shape a piece of material by any method (for car bodies you just press some sheet metal
in a form), you must have some "workability"; in other words, you need some plastic deformation, i.e.
ductility. Think of pure martensite as being like glass, and you get the idea.
Weldability is a particular important part of "workability"; another one would be "hot pressability"
(Heißpreßbarkeit")or "drawability" ("Ausziehbarkeit; Tiefziehbarkeit"). Just consider how you would

make a car body, if those two properties are non-existent, and you have a good idea of how important
"workability" is for mass production!
We clearly need strength (= "hardness") without martensite formation,
This leaves us with all the basic mechanisms discussed in chapter 8 for strengthening.
We thus add suitable elements to obtain:
 Solution hardening. Except for nitrogen, which dissolves as an interstitial like carbon, all other
suitable elements will always be of the substitutional solid solution type.
 Precipitation hardening. Either by forming finely dispersed hard and small carbides of the
alloying elements, or by influencing the cementite formation to occur in fine particles, or by
producing precipitates of compounds of the alloying elements (e.g. borides, or intermetallic
phases), or by all of the above.
 Grain size reduction. You may produce small grains (i.e. from a martensitic transformation
which rips on i "apart" in many grains), and/or keep small grains small by keepig grain
boundaries from moving (i.e. grains from growing) by precipitating suitable elements there
(without making the grain boundary brittle, of course). This will always lead to hardening, too.
It only remains to check the "easy" elements of the
periodic table under all kinds of conditions. Let's do
that for solution hardening first.
What we find is that Carbon and Nitrogen have by
far the biggest direct effect on the yield strength
(owing to their being interstitials), and that
Phosphorous in solution is very good, too, (but,
remember, very bad if segregated in grain
boundaries).
Then we have Silicon (Si), Manganese (Mn)
Titanium (Ti) and copper (Cu, not shown) and
some others as still pretty good solution hardener.
Cu, however, has drawbacks (including its prize),
and Si causes problems here, too (also it is much in
use for other purposes).
This leaves Mn, Ti, and to some extent Ni and
Vanadium (V) as alloying elements (we also had
Mn to neutralize spurious S, if you remember)
Complex - but not difficult. We had much the same
picture before for Copper.
In essence, we understand that part of steel
alloying.
Precipitation hardening can be more efficient than solution hardening, and indeed, very small amounts of
Boron (B; 0.005 %), or about 0.1 % of Niobium (Nb) or Vandium (V) will produce considerable increases
in strength.
Always provided that the heat treatment was right, the grain size is small, and so on and so forth.
Just on example: Niobiumcarbide particle of about 1 nm size will increase the yield stress from about 20
MPa to 200 MPa, at a concentration of about 0.1 weight % Nb, while "huge" particles with about 10
nm diameter have practically no effect anymore!
We understand that immediately, by looking at the mechanism of precipitation hardening. If a 1 nm
particle can stop a dislocation completely, a 10 nm particle can do no more - but we have 1000 times
fewer 10 nm particles at a fixed solute concentration.
We also understand why these micro-alloyed steels are rather recent developments: Try to optimize
such a steel if you don't know what happens, can`t see your precipitates anyway, and can't measure their
size and other properties for some quantitative data. In other words, with no knowledge about
deformation and dislocations, just optical microscopes, and without the whole bag of microanalytical
tools, you are simple blind and the best you can to is go by trial and error following up some guesses.
Anyway, with some basic understanding and giving proper care to their needs, micro-alloyed steels may
have much better strength than "mild" carbon steels, with all other properties (exept the prize) being
comparable.
To some extent, micro-alloyed steels are the steel industry's answer to the Al car body challenge from Audi,
because they allow to maintain the easy manufacture and strength of a steel car body, while considerably

reducing the weight (the sheet metal can be thinner).
Of course, you may now ask yourself a simple
question:
Hardening mechanisms of maraging
steel
What do I always add carbon, if I can get all
kinds of hardening mechanisms from other
elements, too?
Good thinking. Take carbon-free Iron, add
sizeable amounts of elements like Ni and Co,
and rather small amounts of, for example, Al, Si,
Mo, or Ti. This gives you some solution
hardening if nothing else happens.
Keep out P, S and so on, make sure the grain
size is very small and the grain boundaries not
embrittled by segregation of the wrong elements.
Upon cooling down this alloy, some relatively
soft martensite will form (No carbon!). This is
when you shape your piece of steel in the form it
is supposed to have when it is finished.
After that, you do some tempering, just right, to
now form lots of very small intermetallic
precipitates between the major elements and the
minor elements.
This puts some precipitation hardening on top of everything else and you end up with "maraging steel"
(short for martensitic aging), being fantastically strong while still ductile - and being rather expensive.
The picture above shows the total effect with an increase of the tensile strength to a fantastic 1500 MPa!
Even larger values have been achieved while still keeping a maximum elongation of 6 % - 8 % before
fracture!
A maraging steel is what you use for landing gear of Jumbo jets, for ultra centrifuges (needed for
making atomic bombs) or for golf clubs (needed for hitting little balls). Interestingly, if you enter
"maraging steel" into Google, you will find either golf club advertisement, or stern warnings concerning
trade restriction, but very little useful information. We have a real high-tech material here!
Improved Hardenability
Shaping a sword, a car body, or whatever by banging, pressing, stamping, rolling or drawing a piece of some
rather soft steel into the desired shape, and then making it hard by heating and quenching, is actually a great
way of getting strong (= hard) products with comparably little effort.
So we want to keep this old-fashioned hardening method, known for millennia for plain carbon steel, but
we also want to make the result less sensitive to the cooling rate. Remember, with plain carbon steel,
you only get hard martensite in those parts of your work piece that cool down with cooling rates of
about 1000 K/s.
There is no way to achieve this kind of cooling rate with anything thicker than a few mm! Therefore the
only option left is to alloy the right elements to our plain carbon steel, hoping that this will lower the
austenite - martensite transformation temperature. This then might produce the good hardenability we
are after - which, remember, is not just a large hardness value, but hardness as deep as possible into the
bulk of a massive sample.
This brings us to an old piece of wisdom concerning of what is better: Being practical, or being
theoretical? If you don't have a good theory here, you do not even know if that feat is possible at all.
Even if you trust your luck, you have no idea of how much of what you should add. Good luck and all
the time in the world to you practitioner!
Well, the truth is that we know a lot about alloying and hardenability, but we really do not have a "final"
theory yet, and a lot of what is known about hardenability did come from an empirically established data
base.
Thanks to Walter Jominy (the Chief Metallurgist for Chrysler Corporation sometime before the war),
there is at least a simple but accurate test to assess the hardenability of a given sample.
Just take a standard size sample, heat it to some high temperature, and then spritz water (at defined
conditions, of course) at one end as shown below. The cooling rate will be different from one end to the
other of the sample, and all you do after it has cooled down completely, is to measure the hardness along

its length.
What you might find is shown to the right of the test set-up.
Plain carbon steel with sufficient carbon (e.g. 0.8 % ) may become very hard in the region where the
cooling rate was very high, but the bulk of the sample remains "soft" (red curve), while very mild steel
with little carbon (e.g. 0.3 %) just shows some hardening (green curve).
Now add some Cr, V, Mn, Ni, or Mo (or some other suitable elements), and if you do everything right,
you may obtain the blue curves - steels with good hardenability and adjustable hardness.
All you have to do now is to check what happened to the other 10 or so properties of supreme interest
(ductility, weldabiliy, fracture toughness, corrosion resistance, ....).
If you are extremely lucky (and after 10 - 20 years of work), you may find a new kind of steel with
properties just right for your purpose and better than anything else available so far.
Austenitic and Stainless Steels
We all know it: Iron and steel rusts! What we probably do not know:
Relatively pure iron ("wrought iron") rusts far less then steel.
In Delhi is a 1600 year old huge iron pillar (7 m tall, 6 tons in weight;
see picture on the right) that does not rust. It was forged together from
many pieces of wrought iron with low carbon content. Its "secret" has
recently been unraveled: The relatively large amounts of P in the iron
and in slag particles within the iron, catalyzed the formation of -
FeOOH ("Misawite") and a layer of crystalline phosphates that
together form a stable protective layer.
In the "Württembergisches Landesmuseuum" (which we encounter in "sword" conncections, too) and in
many others, iron bars in the typical double-pyramid shape of the Celts as shown above are on display.
If you ever visit these museeums (the display above is from the museeum in Heidelberg), don't miss this
part; you will experience some surprise: These wrought iron bars, about 2000 years old, look like new.
There is hardly a trace of rust.
But these are the exceptions to the rule: Iron and steel rusts! In the museeums mentioned above, you can
also see the evidence for this fact: Most steel objects like swords are just lumps of rust.
In general, this is easy to understand: Since metals have too many electrons by definition, and Oxygen has

too few, in air metal-oxides will form. The noble metals are just the (rather easy to understand) exception to
the rule.
The oxidation of a metal exposed to air will go on as long as oxygen can meet metal, i.e. as long as
either one can diffuse through the oxide layers formed.
Stainless metals, obviously not decomposing into oxides foreever, thus can only exist if the
unavoidable oxide layer formed in air will be impenetrable to oxygen as soon as a certain (small)
thickness has been reached.
This is not all that difficult to achieve, after all, metals (and other reactive elements) like Al, Si, Pb, Cu,
Cr, ... are quite stable in air (at room temperature), and, as we have seen, even some relatively pure iron
does not rust.
Iron, plain carbon steel, and many alloy steels, however, do generally not form a stable oxide - they rust!
And sooner or later our car body, sword, or cooking pot is just a piece of ugly iron(hydro)oxide.
And there is nothing particular systematic that you can do. The method of choice, of course, is to paint
the object, or more generally, to apply a protective coating, e.g. paint, Zn, Cd, or Cr, or if money is of
no consequence, Au. But this will only help for some time if the object is mechanically stressed (i.e.
used) because than the thin protective layer will sooner or later been worn off or develop cracks - rusting
just starts later, as we all know.
The alternative is to alloy a sufficient amount of typically Cr, so that the surface always is covered with a
stable Cr2O3 layer.
The minimum amount of Cr you must add is 13 % (a number that can actually be calculated), but up to
25 % or so are used.
But now you have high alloy steel; and while it may not easily corrode, its properties may also be quite
different from plain carbon steel.
Staying simple, you can get stainless steel by only alloying pure Fe (no carbon) with Cr and nothing else.
But even then you will get something new: Fe - Cr alloys stay ferritic (i.e.. in the bcc phase) at all
temperatures - they do not form fcc austenite at all. Well, no reason why they should, considering that
this is no longer Fe with a little bit of something.
The problem, however, is that now you have no possibility of using some kind of martensitic
transformation for hardening.
So if you also want strength, weldability and so on, you start a whole new game of going through the
periodic table in search of proper additional alloying elements.
Adding some Carbon again will help; 0.6 % is already enough to produce some martensite and thus
hardenability. Simple Fe - Cr - C stainless steels, quenched and tempered, are indeed used for, e.g., ball
bearings, kitchen knives or surgical instruments.
We now have stainless steel, with a bcc lattice at room temperature (lossely still called "ferrite"), it is
also "ferro"-magnetic (try your kitchen ware). But we can do more than that with high alloy steel
containing a lot of Cr.
Besides having sufficient Cr, add some Ni (say 10 %) and the ubiquitious Manganese (about 1%).
What you will obtain is a steel that is still austenitic at room temperature (i.e a fcc and non-magnetic)).
It is not the stable phase at room temperature, but the transformation temperature is lowered and never
takes place for normal cooling rates.
This is mainly a result of the Ni addition; the transformation temperature goes rapidly down with
increasing Ni concentration (from 914 o
C at 0 % Ni to 720 o
C for 8% Ni, or to 600 o
C for 15 % Ni.
Austenitic steels are materials quite different from regular steel.
Not only are they stable in corrosive environments (thanks to stable Cr2O3 on their surface) and non-
magnetic.
They are relatively tough but still more ductile than regular steel and thus are easy to work with because
they can be pressed or drawn. They also have better creep properties (we will learn what that is in
chapter 10) A certain problem is that they work harden very rapidly, which makes them difficult to
machine.
You will find a lot of austenitic steel around you. Your kitchen sink

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