Fundamental studies of the metallurgical causes for reheat cracking in 1 1/4Cr-1/2Mo, 2 1/4Cr-1Mo and Cu precipitation hardenable steels and means of mitigating the potential problem

Cr-Mo and Cu-precipitation hardenable steels are considered to be susceptible to weld related reheat cracking. This study was instituted to determine the mechanisms for reheat cracking in these steels as well as to determine methods to successfully avoid reheat cracking. Two heats of 1 1/4Cr-1/2Mo, one being calcium treated: three heats of 2 1/4Cr-1Mo, one being a conventional grade of 2 1/4Cr-1Mo-Ca-treated, and the other two being modified (with 1/4V) grades, one of which is calcium treated were utilized. A heat of A710 steel was used to characterize its behavior. The reheat cracking susceptibility of the materials was first determined by Gleeble technique. Subsequently, a new simple and versatile test was developed, namely, the spiral notch test. The materials were evaluated by this new test and a good correlation was found between the two tests. The effects of multiple thermal cycles and PWHT on reheat cracking susceptibility was investigated. TEM/STEM studies were conducted to determine the carbide evolution kinetics in the CGHAZ during PWHT. A technique was developed to prepare specimens with "pristine" reheat cracks to study the prior austenite grain boundary segregation and its relation with reheat cracking susceptibility. From the results a distinct difference in carbide evolution and segregation pattern for reheat crack susceptible and non-susceptible heats was evident. The M₃C type carbides persisted for longer time in reheat crack sensitive heats and in resistant heats the M₃C type carbides transformed to M₂₃C₆ type carbides earlier during PWHT. The prior austenite grain boundaries were enriched in P in susceptible and in S in resistant materials. Although, how the carbide evolution kinetics and the trace element segregation were related in affecting the reheat cracking susceptibility was not fully defined, it was obvious that the two were interlinked. The activation energy calculations revealed that diffusion of P was the rate controlling step for reheat cracking. Thus, all the results point to the fact that P is the element responsible for reheat cracking. In the A710 Cu-precipitation hardenable steel which is extremely susceptible to reheat cracking, ε-Cu precipitated during tempering strengthened the grains with respect to the grain boundaries causing creep deformation to occur at grain boundaries resulting in reheat cracking. Whether P or other element segregation assisted the intergranular cracking is unknown.