Structure-property changes during hardening and tempering of new ultra high strength medium carbon low alloy steel

Abstract

A medium carbon low alloy steel, electroslag refined, modified AFNOR 15CDV6, has been developed for satellite launch vehicle and related applications. Conventionally processed (without electroslag refining) mostly bainitic AFNOR 15CDV6 (with 0.15 wt-% carbon and similar to3.5 wt-% other alloying elements) has a yield strength of similar to800 MPa. Electroslag refining, coupled with increased carbon (0.29 wt-% carbon, but no change in percentage of other alloying elements), increased the yield strength to about 1300-1400 Wa, without sacrificing ductility. The microstructure of the modified grade was martensitic. Martensite in the as hardened state was mostly in the form of laths, although similar to20% plate martensite was also observed. Until 150degreesC tempering, no noticeable loss of tetragonality was observed, while the unit cell parameter cla ratio dropped to almost 1 after 300degreesC tempering. The interesting observation at 150degreesC tempering was the predominant presence of fine rodlike. carbide, which may also explain the increased yield strength. Tempering above 150degreesC converted the epsilon carbide to cementite, relatively thicker precipitates of similar morphology. At higher tempering temperatures, no evidence of spheroidisation of cementites was noted. The highest tempering temperatures of 500 and 600degreesC resulted in two marked changes in the microstructure: the appearance Of M23C6 type (Cr, Fe and Mo bearing) carbides, and the appearance of, in some regions of the microstructure at least, a relatively 'recovered' lath structure. Misorientation among adjacent laths, nearly constant at 8-9degrees until 450degreesC tempering, increased noticeably, to 13 and 16degrees, after the respective tempering temperatures of 500 and 600degreesC