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The fracture toughness K 1C of Cr -Mo engineering steels and susceptibility to temper embrittlement can be assessed by several different methods. The method employed in the present research for determination of K 1C was use of Charpy V notch impact data combined with application of the Rolfe -Barsom -Novak equation in the upper shelf region of a 3Cr -1Mo steel. For this purpose, specimens from an aged hydrocracker reactor were cut at the interface region between the stainless steel weld overlay and the base metal, such that the root of the V notch was located at the interface. All specimens were de-embrittled, and some were then subjected to a step cooling operation. Hot tensile tests were additionally carried out on the material. Assessment shows that the step cooling operation increases the transition temperature by about 65 -100°C, while K 1C in the interface region is decreased by 55%.