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Study on effect of weld groove geometry on mechanical behavior and residual stresses variation in dissimilar welds of P92/SS304L steel for USC boilers

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Dak G. , Pandey C.

Archives of Civil and Mechanical Engineering

2022

tom Vol. 22, no. 3

art. no. e140

This research article reports the correlation between microstructure, mechanical properties, and residual stresses of dissimilar weld joints (DWJs) between P92 martensitic steel and 304L austenitic stainless steel (ASS). The groove geometry plays a vital role in DWJs. Thus the effect of groove geometry on mechanical and microstructural properties was also investigated. The V-shape and narrow shape groove profile were implemented for P92/304L SS DWJs. The microstructural characteristic, tensile strength, micro-hardness, Charpy impact toughness and residual stresses were evaluated for both the groove geometry in as-weld (AW) and post-weld heat treatment (PWHT) (760 °C, 2 h) state. Microstructural observations performed using an optical microscope (OM), and scanning electron microscope (SEM) showed that high temperature during the weld thermal cycle leads to the formation of the coarse grain heat-affected zone (CGHAZ), fine grain HAZ (FGHAZ), and inter-critical HAZ (ICHAZ) across the P92 HAZ. The ERNiFeCr-2 (Inconel 718) welding consumable wire (filler rod) of diameter 2.4 mm was used for this investigation. The ERNiFeCr-2 weld fusion zone showed a fully austenitic microstructure with the formation of the secondary phases due to the solidification segregation. The EDS and SEM area mapping results indicated that the secondary phases in the inter-dendritic region contain a higher amount of the Mo and Nb than the matrix region. The ultimate tensile strength (UTS) of the as-weld and PWHT tensile specimen of the P92/304L SS DWJs was 630 and 621 MPa, respectively, for V-groove geometry specimens and 620 and 629 MPa, respectively, for narrow groove geometry specimens. The tensile fracture was experienced at the interface between weld metal and 304L base metal, and the UTS value of DWJs was very close to the UTS of the 304L SS. The abrupt variation in the micro-hardness value of the CGHAZ (456HV0.5), FGHAZ (375HV0.5), and ICHAZ (221HV0.5) was noticed in the as-weld state due to their distinguish microstructure characteristics. After PWHT, the micro-hardness value of the CGHAZ (255HV0.5), FGHAZ (236HV0.5), and ICHAZ (207HV0.5) was below the maximum allowable value of 265HV0.5 for P92 material because of the tempering of the martensite. The Charpy impact test indicated that the ERNiFeCr-2 weld fusion zone has a low toughness value of 33 J (AW) and 25 J (PWHT) for V-groove design and 35 J (AW) and 28 J (PWHT) for narrow groove design than that of the P92 and 304L parent metal. The impact toughness of the ERNiFeCr-2 filler weld was below the minimum requirement of 47 J (EN ISO 3580:2017). The tensile residual stresses were generated in the weld fusion zone due to the volumetric contraction during the solidification. The residual stresses developed in the case of the narrow groove design were less than that for the V-groove design due to the less quantity of weld metal available for volumetric contraction in the case of the narrow groove geometry. From comparing mechanical and microstructural properties obtained for V-groove and narrow groove geometry DWJs, it was found that narrow groove design reduces the overall heat affected zone span, and it requires less welding time and less heat input.