Characterization of the Interface of an Alloy 625 Overlay on Steels Using Nanoindentation
- PDF / 13,672,225 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 102 Downloads / 162 Views
JMEPEG https://doi.org/10.1007/s11665-018-3444-1
Characterization of the Interface of an Alloy 625 Overlay on Steels Using Nanoindentation Tao Dai and John Lippold (Submitted December 12, 2017; in revised form March 9, 2018) Industry standards require postweld heat treatment (PWHT) to reduce the heat-affected zone hardness of steels such as F22 (2.25Cr-1Mo) and AISI 8630 overlaid (clad) with Alloy 625 weld metal. PWHT results in carbon diffusion and accumulation at the interface between the steel and overlay. The accumulation of carbon in a planar solidification growth zone adjacent to the fusion boundary results in high hardness and the potential for hydrogen-assisted cracking. The planar growth zone (PGZ) is so narrow that normal Vickers hardness testing cannot fully reveal the hardness distribution in this zone. This study focused on the application of nanoindentation to characterize the hardness in the narrow microstructural regions adjacent to the fusion boundary. The development of nanohardness maps revealed that the PGZ is not necessarily the region that exhibits peak hardness after PWHT. The highest hardness values were associated with clusters of M7C3 carbides in specific subregions in the PGZ and also in the partially-mixed zone adjacent to the fusion boundary or in steel ‘‘swirl’’ structures. It was also confirmed in this study that nanohardness has a linear correlation with Vickers hardness values. The results presented here provide new insight into the role of carbon diffusion during PWHT and its effect on interface embrittlement associated with Alloy 625 overlays on steel. Keywords
alloy 625 overlay, joining, microscopy, nanoindentation, optical metallography, postweld heat treatment, steel, welding
1. Introduction Overlaying (or cladding) low-carbon alloy steels with corrosion-resistant alloys (CRAs) such as Ni-based alloys or stainless steels is widely used in the oil and gas industry to provide corrosion protection. In certain situations, the fusion boundary of the dissimilar metal weld overlay is exposed to the production fluid, and the ‘‘sour’’ environments introduce the risk of sulfide stress cracking. In order to prevent this form of cracking, the heataffected zone (HAZ) hardness must be controlled below 250 VHN according to National Association of Corrosion Engineers (NACE) Standard MR0175 and the International Standards Organization (ISO) 15,156 (Ref 1). PWHT is commonly applied to reduce the HAZ hardness, but this heat treatment results in carbon diffusion from the HAZ into the overlay. Previous research has shown that carbon diffusion and pileup can result in very high hardness in the planar solidification growth zone adjacent to the fusion boundary (Ref 2). The planar growth zone (PGZ) is normally very narrow (1050 lm) when examined in cross section which makes it difficult to determine the precise hardness using the Vickers hardness testing method, because the impression created by Vickers hardness testing with 100 g load possibly samples larger microstructural zones than narrow PGZ. A low-an
Data Loading...
