Elucidating the Effect of Step Cooling Heat Treatment on the Properties of 2.25 Cr-1.0 Mo Steel Welded with a Combination of GMAW Techniques Incorporating Metal-Cored Wires.

Subhash Das,Jay Vora,Vivek Patel,Khaled Giasin,Joel Andersson,Danil Yurievich Pimenov

doi:10.3390/ma14206033

Abstract

The prospect of using metal-cored wires instead of solid wires during gas metal arc welding (GMAW) of 2.25 Cr–1.0 Mo steels embraces several challenges. The in-service requirements for the equipment made up of these steels are stringent. The major challenge faced by the manufacturers is temper embrittlement. In the current study, the temper embrittlement susceptibility of the welded joint was ascertained by subjecting it to step cooling heat treatment. A 25 mm thick 2.25 Cr–1.0 Mo weld joint was prepared using a combination of the regulated metal deposition (RMD) and GMAW processes incorporating metal-cored wires. After welding the plates were exposed to post-weld heat treatment followed by a rigorous step cooling heat treatment prescribed by API standards. The temper embrittlement susceptibility of the weld joint was ascertained by Bruscato X-factor as well as by formulating ductile-to-brittle transition temperature (DBTT) curves by carrying out the impact toughness testing at various temperatures. Detailed microscopy and hardness studies were also carried out. It was established from the study that the X-factor value for the welded joint was 15.4. The DBTT for the weld joint was found to occur at −37 °C which was well below 10 °C. Optical microscopy and scanning electron microscopy indicated the presence of carbides and the energy dispersive X-ray spectrometry studies indicated the presence of chromium and manganese-rich carbides along with the presence of sulfur near the grain boundaries. This study establishes a base for the usage of metal-cored wires particularly in high temperature and pressure application of Cr–Mo steels.

Highlights

2.25 Cr–1.0 Mo steels find their application in the fabrication of equipment used for processing under higher temperatures and pressures
Root passes of the weld groove were carried out using the regulated metal deposition (RMD) process whereas subsequent passes were completed by the gas metal arc welding (GMAW) process incorporating metal-cored wires for both processes
Due to constraints related to cost, the tramp elements in the metal-cored wires cannot be reduced below a certain limit and this increases the X-factor of the weldment subjected to step cooling heat treatment (SCHT)

Summary

Introduction

2.25 Cr–1.0 Mo steels find their application in the fabrication of equipment used for processing under higher temperatures and pressures. This is because the mechanical and metallurgical properties of these steels ensure a safe working environment [1,2,3,4]. This equipment is subjected to severe varied cycles of temperature and pressure during their operation. There are several mechanisms for embrittlement out of which the two major phenomena reported by researchers are migration of impurity elements and changes in size and shape of carbides formed at the grain boundaries [10,11,12]

Methods

Results

Conclusion