Tempering effect on the fusion boundary region of alloy 625 weld overlay on 8630 steel

Abstract

The NACE standard MR0175 requires that the HAZ hardness of AISI 8630 steel overlaid with Ni-base Alloy 625 cannot exceed 250 VHN, requiring a postweld heat treatment (PWHT) to decrease the as-welded HAZ hardness. However, PWHT results in the diffusion of carbon from the steel into the overlay and may lead to interface embrittlement. To understand the effect of PWHT on the microstructure and hardness of the regions near fusion boundary, a wide range of PWHT conditions were investigated. The regions of interest include the coarse-grain HAZ (CGHAZ), the partially-mixed zone, the “swirl” structure, the planar growth zone (PGZ), and the Alloy 625 weld metal. The microstructure and hardness were evaluated using optical metallography, SEM, Vickers hardness testing, and nanoindentation. With an increase of the Hollomon-Jaffe parameter (HJP), the CGHAZ hardness decreases, the PGZ hardness increases, and the weld metal hardness also increases but at a much lower rate. At HJP > 19,300, carbide precipitation was observed in the PGZ. Following PWHT, the partially-mixed zone was found to contain fresh martensite since the PWHT temperature was above the A3 temperature of this region. Reducing the cooling rate from the PWHT temperature did not eliminate the untempered martensite in this region. Finally, the behavior of the 8630/625 overlay is compared and contrasted to the F22/625 overlay behavior which was reported previously. This work builds a foundation for future studies on hydrogen-assisted cracking and sulfide stress cracking in Alloy 625 overlay on steels used in the oil and gas industry.