1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
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IJMET
©IAEME
METALLURGICAL BEHAVIOUR OF AISI 304 STEEL BUTT WELDS
UNDER SEGREGATION
Rati Saluja1,
1*
K.M. Moeed2
Department of Mechanical Engineering, Goel Institute of Technology & Management, Lucknow,
UP, India
2
Department of Mechanical Engineering, Integral University, Lucknow, UP, India
ABSTRACT
Correlation is to integrate depictions of fundamental microscopic phenomena of
solidification, cooling, grain growth, segregation, metallurgical developments and its significance on
mechanical properties for AISI 304 steel. When formulating the susceptibility of to cracking,
segregation is not generally accountable, despite being a significant factor. Segregation is originated
by incomplete mixing in the weld pool which promotes the dendrite envelope motion governed by
the growth of dendrite tips. The growth model for dendrite tips can be obtained by solute transport
near the tip progressed by tip stability. Due to high segregations of chromium and molybdenum,
secondary ferrite is known to be susceptible to the formation of intermetallic phases such as sulfur or
carbon. But segregation of nickel, chromium and molybdenum can be lowered with the use of
nitrogen in the shielding gas. Competing factors add to the intricacy of the phenomenon, hence the
main objective of this scanty literature is concerned with metallurgical behavior of this class of
engineering materials under the influence of segregation.
Keywords: Austenitic, Dendrites, Ferrite, Segregation, Supercooling
1. INTRODUCTION
Selection of Type 304 steel as material of choice for a variety of engineering applications
such as structural steel parts cannot be traced only to its good corrosion resistant and formability
characteristics [1]. It is highly preferred due to its good welding characteristics, low cost, easy
availability in the market places this class of materials at the forefront of metallurgical technology
[2].
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME
The AISI 304 steel has atleast of eight elements, and the use of binary and ternary phase
systems to value higher order systems, which can lead to significant error, while predicting
microstructures [3]. Furthermore, the σ phase usually precipitates in Grade 304 steels at 800℃ and
that it has a strong effect on the properties of stainless steels [4]. σ phase has significant influence on
material properties, it causes chromium impoverishment near the precipitates and reduces the
corrosion resistance [5]. It has also been demonstrated that the σ phase does not affect the
mechanical properties of stainless steels at 1000℃ because it dissolves in the matrix at this
temperature [6]. Grade 304 steel series of austenitic stainless steels usually solidify during welding
as a mixture of austenite and ferrite. The ferrite almost fully transforms to austenite on cooling, but
there could be retention of a few percent of δ ferrite in the weld metal which leads to cracking
processed by segregation [7]. This arises from larger solidification temperature range during primary
austenitic solidification. The inhomogeneity is affected also by considerably slower diffusion of
alloying elements in austenite than in ferrite [8].
Weld pool macro segregation occur by lack of weld pool mixing especially in welding of
dissimilar metals, or some special types of rapidly solidified power metallurgy alloys. It also occurs
during incomplete weld pool mixing in single pass welding (greater extent) and even in multipass
welding [9]. When comparing the segregation behaviour for different solidification modes it has
been demonstrated that microsegregation is stronger in welds having primary austenitic solidification
[10].
To this end, the fundamental study of the microstructural development of Grade 304 steel
presented here provides a baseline for comparison when the more complicated material behavior
under segregation. The work presented in this paper represents the first step in understanding the
complex solidification behavior of 304 steel, which are routinely used as parent metals for various
fabrications in metallurgical, mechanical, chemical, automobile and nuclear industries process. The
data gathered here will serve as the foundation for the continuing investigation of the intricate
material behavior under rapid solidification conditions that can potentially be induced during various
arc welding processes.
2. WELDING METALLURGY OF GRADE 304 STEEL WELDS
2.1 Solidification and cooling
Under rapid solidification, a great number of welds having duplex microstructures are
replaced by single phase austenitic or single phase ferritic microstructures in arc welding. According
to Schaeffler diagram, in welds predicted to have less than 8% ferrite will transform to fully
austenitic with cooling rates of as high as 200000 K/s [10]. At high ferrite content approximately
more than 30% ferrite, arc welds predicted to have may become fully ferritic with these very high
cooling rates. Massive transformation to austenite in these welds may, however, abruptly decrease
the ferrite content [11,12].
The primary solidifying phase incorporates more solute than it does closer to equilibrium
conditions promotes which further restrain the formation of second phase ferrite in AF-mode and
second phase austenite in FA-mode [13]. Due to rapid solidification and retarded diffusion in FAwelds solid state transformation from ferrite to austenite is repressed [10, 14]. The difference
between microstructure of fusion zone resulting from Type A, AF and F mode solidification can be
easily distinguished as shown in figure 1 [15].
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3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME
Fig. 1 Microstructure of fusion zone resulting from Type A, AF and F mode solidification [15]
The growth of primary austenite dendrites or cells enhances as solidification rate and dendrite
tip undercooling increases [16]. As an optical micrograph shows the change
in dendrite morphology from cellular to dendritic as the growth velocity increases toward the center
of spot weld towards top after the spot weld arc is extinguished.
Fig. 2 Micrograph of dendrite morphology from cellular to dendritic [14]
2.2. Microstructure Development and its effects on mechanical properties
The welding metal contains a skeletal and lathy ferrite type dendrite structure as shown in
figure 3. It shows the morphology of the base material, which evidence the austenitic grain
microstructure of an AISI 304 stainless steel [17]. The lathy ferrite microstructure can emerge due to
greater ferrite contents and/or a characteristic cooling time after the welding procedure steel [18].
Fig. 3 Micrographs of the microstructures at the Fusion zone [19]
In AISI 304 steel, the ferrite-promoting element segregation specially chromium promotes
formation of ferrite stringers during solidification [19]. The final microstructure consists of γ
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4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME
dendrites with a Molybednum concentration gradient and interdendritic α phase [20]. The presence
of Molybdenum rich liquid present near the termination of solidification wets the surface of the
solidified dendritic structure promotes formation of cracking [21]. As austenite is more susceptible to
this phenomenon than ferrite due to the lower solubility of tramp elements in austenite as in steel that
form low melting point solids [22]. However, this is not an option, as a fully austenitic structure must
be maintained to conserve the magnetic and corrosion resistance properties of Grade 304 steel.
The change in scale morphology can be recognized to an edge effect phenomenon occurring
during the cooling of the AISI 304 austenite stainless steel. Fig. 5 (a), (b) and (c) shows the stereo
micrograph of WM, HAZ and FZ which could be discriminated easily. In welding zone, a small
amount chromium carbide precipitation is formed. Whereas the microstructure analysis revealed that
the ferrite to austenite formation contribute in increasing the weldment strength, whereby at
weldment area ferrite form occurred due high temperature during welding. This makes the structure
strength at weldment area weaker compare to HAZ and base metal [23].
Fig. 4 Micrographs of the Microstructures of PM, HAZ and Weldment [24].
2.3. Segregation of Alloying Elements
Solute undercooling predicts that solute is rejected from the solidifying metal into the liquid
ahead of the solid liquid interface, resulting in a depression of the liquidus temperature of the liquid
ahead of the interface due to solute buildup [25]. Due to continuous reduction in temperature till the
termination of solidification, the dendrites that had initially formed continue to grow into the
increasingly solute enriched liquid results a composition gradient within the dendritic substructure of
the material, generally referred to as coring [12]. But Researchers suggests that segregation of nickel,
chromium and molybdenum can be reduced with the use of nitrogen in the shielding gas. The
segregation of nickel remains, reasonably weak [26].Development of dendrites in a nickel-based
superalloy single-crystal weld has been demonstrated in figure 5.
Fig. 5 Scanning-electron micrograph showing the development of dendrites in a nickel-based
superalloy single-crystal weld [14]
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5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME
In the case of primary austenitic solidification and the segregation ratio above one indicates
interdendritic segregation for all major alloying elements [27]. Correspondingly,
the concentration of chromium and preferably molybdenum, at the dendrite cores decreases than in
the bulk composition. This indicates that the segregation tendency of chromium and molybdenum
reduces and that of nickel increases with respect to primary austenitic solidification [20]. In primary
ferritic solidification the value of segregation ratio for all other elements except for nickel is lower
than in primary austenitic mode [8]. At the end of solidification the last melt droplets can once again
solidify as ferrite, known as secondary ferrite. Due to high segregations of chromium and
molybdenum, secondary ferrite is known to be susceptible to the formation of intermetallic phases
such as S or C [9].
Therefore, during cooling, the segregation profiles developed in austenitic solidification
are not homogenised to the same extent as in ferritic solidification [28].
Fig. 6 Regions of fine scale dendritic, facetted and eutectic structure between primary dendrites [29]
Macrosegregation arising from dendritic solidification is defined as compositional
inhomogeneity on a scale larger than that of the dendrite arm spacings [29]. With a planar
solidification front, this would, indeed, be a fair first-approximation, but less so with cellular growth
and downright misleading with dendritic growth. The more removed is the topography of the
solidification front from planar, the more the enriched liquid is held within that texture [30]. In so
doing, it has also essentially removed the solute field ahead of the dendrite tips under the standard
range of solidification conditions [31] and, hence, has removed the mechanism for 'normal'
segregation. It is well known that the inhomogeneity of AISI 304 steel welds is greatly influenced by
microsegregation as well as by the partitioning of alloying elements between ferrite and austenite
during solid state transformation of delta ferrite to austenite.
This has an effect on the final degree of inhomogeneity especially in primary ferritic
solidification [22]. The enrichment of the interdendritic liquid in the mushy zone by especially
silicon reduces liquid density for the majority of steels, which consequently reverses the predicted
circulation currents. But steel compositions with addition of molybdenum and tungsten, may result in
residual liquid of increasing density and the inverse channel geometry in channel segregation [32].
For example, comparison of core and interdendritic (ID) manganese contents is shown in table.
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6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 36-43, © IAEME
TABLE 1
Comparison of core (D) and Interdendritic (ID) Manganese contents [33]
Composition
Average
Cooling rate
o
C/s
C
Mn
Liquid/
Solid
Solid
0.21
1.60
0.25
0.28
0.40
0.40
1.58
1.58
0.30
4.15
0.31
4.45
Quench
Temperature
o
C/s
Spacing,
µm
EPMA- Mn
Measurements
Wt. %
2D, λ1
1D, λ1
,Static
1D,λ2,
Coarsening
λ1
λ2
D
ID
D
ID
D
ID
D
ID
1225
350
140
1.44
1.98
2.06
1.23
2.21
1.41
1.84
1.44
1.80
1200
1305
280
184
80
25
1.31
1.30
2.18
2.19
2.67
1.17
1.27
2.14
3.13
1.47
1.35
1.71
1.87
4. CONCLUSIONS
AISI 304 steel transforms as alpha ferrite at the end of the solidification, which leads to
segregation followed by cracking. Evidence is presented to suggest for a range 304 steel series,
solidification, cooling rates, and secondary dendrite arm spacings under the influence of segregation
reveal the following.
1. During the solidification both micro segregation and macro segregation occurs in AISI 304
steel weldments due to presence of low alloys.
2. Microsegregation appears to be associated with the cracking in fisheyes. A small gas pore,
probably hydrogen, was found at the fraction initiation sites of each fish eye in the weld.
3. Macro segregation is found due to presence of manganese excessively in the composition
which results due variations in welding parameters under rapid solidification.
4. The solidus temperature can be reduced significantly with independent increases in either the
secondary dendrite arm spacing or cooling rate.
5. The solute-element concentration, especially phosphorus and sulfur, has a significant effect
on the solidus temperature the LIT and ZDT, due to their enhanced segregation near the final
stage of solidification.
6. Due to presence of Molybdenum rich liquid present near the termination of solidification in
the final microstructure consists of γ dendrites with a Molybdenum concentration gradient
and interdendritic α phase which results cracking.
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