Laser surface melting and subsequent treatment of 440C stainless steel

Surface treatments of 440C martensitic stainless steel is complicated by the alloy microstructure which contains large primary carbides and finer secondary carbides. Surface melting, followed by rapid solidification, breaks up the carbides resulting in a more homogeneous microstructure which was used as a starting point for further processing. Laser surface melting of the 440C samples produced a primary austenite structure with negligible carbide precipitation. Emersion in liquid nitrogen and elevated temperature treatments carried out in an attempt to stimulate carbide precipitation failed to produce significant hardening. Surface melting followed by a double temper treatment with air cooling between the tempers did result in surface hardening. Rapid nucleation and growth of carbide particles was observed along the grain boundaries, attributed to a high diffusion rate for carbon. The tempering time required to produce maximum hardening was found to depend on the heat input in surface melting. The laser surface melted material was also subjected to a high temperature soak, followed by water quench and a tempering treatment. Microstructural examination showed the surface melted region to contain a network of fine carbides, whereas the base metal contained a wide range of carbide sizes including large primary carbides. The significance of applying amore » laser surface melting treatment to the 440C steel prior to the standard quench and temper operation is the elimination of the large carbides in the surface microstructure. This has the potential to improve the service performance of the material in many applications.« less