Session: MF-24-01 Matls & Fabrication for Refining-Toughness Issues in Alloys at Elevated Temperatures

Paper Number: 122714

122714 - De-Embrittlement of 1¼cr-½Mo Steels 

It is known that the impact toughness of embrittled Cr-Mo steel after long-term exposure can be restored by heat treatment.  This phenomenon is, therefore, worth considering when performing fitness-for-service assessments of pressure vessel and piping because repair welding followed by post-weld heat treatment (PWHT) often raises concerns regarding possible further degradation of the mechanical properties.  In this study, heat treatment at various temperatures ranging between 560 °C and 720 °C (1,040 °F and 1,328 °F) were performed on embrittled 1.25Cr-0.5Mo steels which were taken from pressure vessels in FCC and CCR units after long-term operation.  The service temperatures of these vessels ranged between 500 °C and 540 °C (932 °F and 1,004 °F).  It was found that heat treatment at or above 560 °C (1,040 °F) improved the impact toughness of the steels.  It should be noted that this temperature was lower than that recommended in API RP571, “Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.”  Microstructural evolution caused by the heat treatment was also investigated using transmission electron microscopy (TEM).  In addition to intragranular and intergranular chromium carbides (M₂₃C₆ or M₆C), needle-like carbides (M₂C) were observed in the ferrite matrix of the embrittled steels.  These carbides remained unchanged after heat treatment, and therefore, the toughness improvement was not considered to be related to the carbides.

Presenting Author: Kazuki Suda JGC Corporation

Presenting Author Biography: Kazuki Suda received his Master's degree in Materials engineering. Has more than 5 years experience of materials. Worked as a materials engineer in JGC Corporation.

Authors:

Kazuki Suda JGC Corporation
Mikihiro Sakata JGC Corporation