Enhanced Low-Temperature Tensile Properties of Fe-14Cr-(4∼9)Ni Weld Metal by Retained Austenite

Strain-induced martensite transformation (SIMT) has been widely investigated for several decades. This research confirms that SIMT can effectively enhance the uniform elongation and strength-ductility balance (Ref 1-10). By utilizing this benefit, TRIP steels have been developed. However, the improvement of material behavior is not available at all temperatures. Studies (Ref 1,11) on Fe-29%Ni-0.26%C, Fe-31%Ni, and Fe-15%Cr13%Ni alloys demonstrated that the elongation exhibits maximum values between Ms and Md (the start temperature of thermallyinduced martensite transformation and strain-induced martensite transformation, respectively; Md > Ms), and beyond the tem per a ture range the effect of SIMT is minor. The critical factor for maximum elongation is the transformation manner rather than total amount of transformed martensite, in which martensite should be continuously formed with an increase in strain, not in bursts (Ref 11), that is, austenite ought to have strong mechanical stability. The fact that ductility increases with increasing the mechanical stability of retained austenite has also been confirmed in conventional Fe-Mn-Si TRIP steels (Ref 2-5). In addition to temperature, mechanical stability of retained austenite is also strongly related with carbon content and other elements (Ref 34,6-9,). Increasing carbon content is favorable to enhance the elongation of steels (Ref 5), and thus conventional TRIP steels usually have higher carbon content, greater than 0.1%. Fe-14Cr-(4~9)Ni weld metal of 980MPa grade is being devel oped for matching high strength base metal. Its high strength is designed result from martensite. As to its low-temperature tensile properties, we hope to incorporate the design concept of convention al TRIP steels, i.e., using the effect of SIMT of retained austen ite, to improve them. However, there are several char ac ter is tics differing from conventional TRIP steels as follows: (a) to ensure weldability, carbon content of weld metal must be restrict ed, usually <0.05%; (b) to maintain high strength, initial amount of retained austenite is low. Therefore, the effect of SIMT is much weaker than in conventional TRIP steels. Moreover, since the new weld metal is usually designed to serve below 313K (less than Ms), beyond the aforementioned optimum temperature range, whether the beneficial effect of low carbon retained austen ite is available is uncertain. In the present study, we focus on the developed Fe-14Cr-(4~9) Ni weld metal, and investigate the effect of SIMT of retained austenite on its low-temperature tensile properties.