The Influence of Carbon, Chromium and Molybdenum Contents on the Tempering Behavior of High Strength 14 Cobalt/10 Nickel Secondary Hardening Steels

摘要

For the past 25 years new ultra high strength steels have largely been extensions of the approach developed by Speich and coworkers. HY180 was the first commercially available alloy based on the principles outlined by Speich and coworkers, which suggested that HY180 achieves excellent toughness after tempering at 510 deg C, in part, because of the absence of coarse intra-lath cementite. To achieve higher strengths in this system the approach has been, and will presumably continue to be, to increase the carbon content. AF1410 and AerMet 100 have carbon levels of 0.16 and 0.24 wt.percent respectively, compared to 0.10 wt.percent carbon in HY180. However, there has been little reported work concerning the effect of amounts of strong carbide forming elements on the secondary hardening response at higher carbon levels. Further, it is generally believed that coarse particles of both inter-lath and intra-lath cementite, which are more predominant at higher carbon levels, could be detrimental to the toughness of alloys of this type and it is unclear if the amount of coarse cementite could be altered by varying the amounts of strong carbide forming elements used to obtain the secondary hardening response. In this work the effects of molybdenum and chromium contents on tempering response and cementite precipitation have been investigated as a function of carbon content. The carbon content varies from 0.16 to 0.25 wt.percent, and the molybdenum and chromium levels vary from 1.25 to 1.75 wt.percent and from 2.0 to 2.5 wt.percent, respectively. The cobalt and nickel contents have been held constant at 14 and 10 wt.percent, respectively. Increasing the molybdenum content was found to increase the peak hardness, while increased chromium levels led to an increased rate of overaging. The extent of cementite precipitation appeared to be controlled by the carbon level of the alloy.