Segregation Modeling and Mitigation in Ultra-High-Strength Steels

Author(s):

Heather Murdoch, Michael Rupinen, and Daniel Field

Abstract:

The composition of a segregated region as predicted from Scheil solidification modeling is used to determine potential effects of elemental segregation on both carbide and austenite stability in ultra-high-strength steels. This modeling approach captured both the increase in M6C and fresh martensite, which were experimentally observed in segregation bands for a family of nickel–cobalt steels. Strong partitioning of molybdenum (Mo) and tungsten (W) promotes the formation of M6C, and the austenite composition can also be enriched in both these and additional alloying elements, which impacts the austenite → martensite transformation. Modeling of the martensite start (MS) temperature for the segregated compositions showed a depression of the MS with increased Mo and W content in solution, which would lead to an incomplete transformation. However, the increased formation of M6C with increasing alloy content actually reduced the effect on the MS temperature as the enriched elements are drawn to the carbide instead of remaining in austenite, offering a possible tool to balance the competing segregation issues.