An assessment of creep deformation and fracture behavior of 2.25Cr-1Mo similar and dissimilar weld joints

Abstract

The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated in the as-welded, postweld heat-treated, and creeptested conditions. The heat-affected zone (HAZ) of similar weld joint consisted of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prioraustenitic-grain (FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo steel, the dissimilar weld joint displayed a definite INCONEL/ 2.25Cr-1Mo weld interface structure present either as a sharp line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint, at stresses Ն150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near to the weld interface. The failure occurred by extensive intergranular creep cavity formation.

Review of Dissimilar Metal Welding for the NGNP Helical-Coil Steam Generator

“…It has recently been suggested 13 that martensite formation can be avoided in DMWs by careful control over the welding parameters so that the cooling rate is low enough to avoid martensite formation within the concentration gradient of the PMZ. Unfortunately, no detailed microstructural information was provided to support this claim, and this is not consistent with the large body of experimental evidence 8,9,12,14,15,16,17,18,19 that clearly shows martensite will always form under the combination of composition and cooling rate associated with practical welding conditions. …”

“…Thus, thermal cycling can intensify this factor. Recent experimental studies 8,34 have confirmed that most of the strain that occurs during creep deformation is associated with the localized strain near the interface. Figure 23 shows results from an instrumented creep test on a weld involving 2.25Cr-1Mo steel and Inconel 82 filler metal where the extension was measured globally (i.e., within the entire gage length) and locally along the interface.…”

“…At elevated temperatures, C will diffuse down the chemical potential gradient. In view of this, it is important to note that Cr lowers the chemical potential of C 8,21 , and the Cr level increases from the ferritic side to the austenitic side of the interface. Thus, the C and Cr concentration gradients produce a rather sharp C chemical potential gradient across the interface.…”

“…Laha et al 8 recently investigated the creep strength of welds between 2.25Cr-1Mo steel and alloy 800H at 550°C over stresses from 90 to 250 MPa. Comparison was made between 2.25Cr-1Mo similar welds and base metal.…”

“…Transmission electron micrographs of carbon replicas showing carbides that form in a weld between 2.25Cr-1Mo steel and 309 stainless steel after a PWHT of 690˚C for 30 hours. 12 Figure 13 8 shows the variations in microhardness across the interfacial region of a weld made between 2.25Cr-1Mo steel and alloy 800H in the as-welded condition after application of a PWHT at 700°C for one hour, and after creep testing at 150 MPa/550°C (with a corresponding failure time of approximately 1,000 hours). This weld was made with Inconel 182 filler metal.…”