A Method for Improving Hydrogen Sulfide Accelerated Cracking Resistance of Low Alloy Steels Available to Purchase

The influence of a new heat treatment, "Intercritical Hardening", on mechanical properties, microstructure and sulfide cracking behavior of C-75, 4140, and 4340 steels is described. This treatment involves heating above the lower critical $Ac1$ temperature. Heating above the $Ac1$ temperature introduced untempered martensite and drastically lowered sulfide cracking resistance. Newly formed martensite was then eliminated by tempering below the $Ac1$ temperature. Tempering below $Ac1$ after intercritical hardening increased strength and ductility and dramatically improved sulfide cracking resistance. Highest strength consistent with immunity to sulfide cracking was developed by the 4340 steel due to the fact that nickel lowered the initial temperature required to develop fresh martensite.