Hydrogen-Assisted Cracking Susceptibility of modified 9Cr-1 Mo steel and its weld metal

Abstract

Modified 9Cr-1 Mo steel, being air hardenable is highly susceptible to hydrogen-assisted cracking (HAC). As welding is carried out using matching consumables, the weld metal is also equally susceptible to HAC. Preheating and post heating are generally employed to avoid HAC during fabrication. Susceptibility of indigenously developed modified 9Cr-1 Mo steel plate and shielded metal arc welding (SMAW) electrode has been evaluated using the implant test and the Gap — Bead on Plate (G-BOP) test. For both these tests the same base metal plate and consumable were employed. The variation of Lower Critical Stress (stress below which no cracking takes place, LCS) as a function of preheat temperature was studied using the implant test. Preheat temperature that would prevent cracking in the weld metal was determined from the G-BOP test. Results of the implant test showed that LCS without any preheat was only 185 MPa, considerably lower than the YS of fully hardened steel of ∼ 1 000 MPa. With increase in preheat temperature, LCS increased to up to 267 MPa for a preheat of 250 °C. The microstructural examination of the tested implant specimens revealed that though cracks initiated at the helical notch in the specimen, at high stress levels they propagated predominantly through the weld metal, and at low stress levels through the HAZ. Results of the G-BOP test, designed specifically to evaluate the weld metal susceptibility to HAC showed that the weld metal cracked soon after welding, when prepared without any preheating of the base plate. The weld metal did not crack for a preheat of 200 °C. Being a self-restraint test, in the tests conducted at a preheat of 150 °C, crack stopped in the midway of the weld metal with the release of stresses caused by cracking. Based on results obtained from both these tests, this paper discusses the HAC cracking in both the weld metal and the HAZ of modified 9Cr-1 Mo steel.