Alloying elements other than carbon can improve hardenability by forming secondary phases that impede the movement of dislocations. A reduced austenite grain size decreases hardenability by promoting easier migration of phase boundaries. Case hardening enhances surface hardness and fatigue life by introducing compressive residual stresses through carburization or nitriding, which increase carbon content in a thin surface layer. Copper-rich copper-tin alloys can be precipitation hardened when the composition is between 15-25% Sn, as a Cu5Sn6 intermetallic phase will form and strengthen the alloy. The heat treatment involves heating to dissolve the phase, then aging to cause it to re-precipitate in a finer distribution.

1. Briefly explain: why the presence of alloying elements other than carbon can improve the
hardenability of the alloy; and why a reduction in the austenite grain size will decrease the
hardenability of the alloy. Case hardening is a technique whereby both surface hardness and
fatigue life are enhanced for steel alloys. Explain why these two properties can be improved by
case hardening. Cu-rich Cu-Sn alloys are precipitation hardenable. After consulting the binary
phase diagram shown below, do the following: Specify the range of compositions over which
these alloys may be precipitation hardened. Determine the composition (in both wt.percentage
and at.percentage) of the second phase that can harden the alloy. Briefly describe the heat-
treatment procedures (in terms of temperatures) that would be used to precipitation harden an
alloy having a composition that lies in the middle of the range given for Part (a). Sn Mass
percentage Cu Cu Phase Diagram of the Sn-Cu System
Solution
The improvement of fatigue properties results from inceaesed hardness within the case as wellas
the desired residual compressive stress , the formation of of whch attends the carburizing or
nitriding process.
During carburisation, the component is heated in a carbon-releasing medium to a temperature
where the steel is completely austenitic. Carbon