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This work provides an enhanced understanding of the fundamentals of reheat cracking in the 1¼Cr-½Mo and 2¼Cr-1Mo steels widely used in the pressure vessel and petrochemical industries. The techniques employed to examine reheat cracks were essentially developed in this work to avoid the examination of cracks not related to the true reheat cracking mechanism (cracked samples used in earlier studies to determine the grain boundary segregating elements were produced by means not in concert with the reheat cracking mechanism). The examination of true and "pristine" reheat cracks was coupled with an extensive study of the carbide evolutionary sequences during PWHT. This information was interrelated to develop a more precise understanding of the reheat cracking mechanism in the Cr-Mo materials.The results show a distinct difference in carbide evolution and segregation pattern for reheat crack susceptible and nonsusceptible heats. Although the relation between the carbide evolution kinetics and the trace element segregation 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 P as the element responsible for reheat cracking. Publication of this document - WRC Bulletin No. 409 was sponsored by the Weldability Committee of the Welding Research Council, Inc.