Reheat Cracking in Welds of 1¼Cr-½Mo Steel Pressure Vessel During Fabrication

Abstract

Abstract 1¼Cr–½Mo steel has been widely used in fluid catalytic cracking (FCC) reactors, continuous catalyst regeneration (CCR) reactors and desulfurization reactors, as these reactors operate at approximately 500 °C (932 °F). The first part of this paper presents a case history of reheat cracking in the weld metal of an FCC reactor made of 1¼Cr–½Mo steel during fabrication. The indication of cracking was first found in the weld metal by MT after postweld heat treatment at 691 °C (1,275 °F). Both flux cored arc welding (FCAW) and submerged arc welding (SAW) processes were employed to fabricate the reactor. However, the cracking was only found in the FCAW welds. Metallurgical investigation revealed that the cracking was intergranular, and chemical composition analysis showed that the cracked weld metal contained a high amount of boron. To evaluate the effect of boron on reheat cracking, a reheat cracking test using a C-ring type specimen was conducted. The test results showed that boron increased the susceptibility to reheat cracking of the weld metal. The last half of this paper is a review of the literature which focuses on the effects of boron on reheat cracking in 1¼Cr–½Mo steels. It was found that even 5–10 ppm of boron may significantly increase susceptibility to reheat cracking. The mechanism of how boron induces intergranular cracking was also discussed. Finally, this paper presents recommendations to reduce the risk of reheat cracking in 1¼Cr–½Mo steel weld metal.