Analysis of intercritical heat treatment of cast steels

Abstract

The effects of intercritical heat treatment (ICHT) on the microstructure and mechanical properties of C-Mn, 8630 type, and high strength low alloy (HSLA) cast steels have been investigated. A review of the cast steel literature indicates that intercritical heat treatment has been reported to result in improvements in low temperature toughness when used instead of standard quench and temper or normalize and temper heat treatments. However, it is not clear if this reported benefit of intercritical heat treatment can be achieved without corresponding decreases in strength and hardness.

Heat treatment on C-Mn and 8630 type case steels have shown that improvements in low temperature toughness due to intercritical heat treatment are due primarily to the significantly lower yield strength levels that occurred after heat treatment—even when the tempering temperature was adjusted to result in the same final tempered hardness values. Although the duplex ferrite/tempered martensite microstructures resulting from intercritical heat treatment had improved toughness, the ferrite present in the microstructure, in general, caused significant reductions in the yield strength.

Intercritical heat treatment of HSLA cast steels (microalloyed with small V and/or Nb additions) also resulted in dramatic improvements in toughness but without a corresponding large drop in yield strength. Although similar ferrite/tempered martensite microstructures were produced, the addition of microalloys caused the precipitation of vanadium and/or niobium carbonitrides during intercritical heat treatment and upon tempering that strengthened the ferrite grains considerably. Therefore, although intercritical heat treatment is generally of little benefit to the mechanical properties of conventional cast steels, intercritical heat treatment of HSLA cast steels can result in an improved combination of strength and toughness.