For the last 10-15 years, margins between the tempering temperature and the post-weld heat treatment
(PWHT) temperature of Cr-Mo steels have frequently been specified by end users or process licensors.
This trend seems to have been caused by a belief in the oil refining and petrochemical industries that the tempering temperature of Cr-Mo steels must be above the PWHT temperature. However, regardless of whether the tempering temperature is above or below the PWHT temperature, the mechanical properties of these steels after two or more heat treatment cycles depend exclusively on the heat treat temperature and accumulated time at temperature. This paper presents experiments illuminating the effects of tempering temperatures above and below the PWHT temperature on the mechanical properties of Cr-Mo steels, thereby verifying that the tempering temperature may be below the PWHT temperature without having any adverse effect on mechanical properties.
Key words: Cr-Mo steels, Tempering Temperature, PWHT Temperature, Larson-Miller Parameter