Mitigation of Weldability Issue and Residual Stresses in Dissimilar Welded Joints of Ultra-Super Critical Power Plants

The growing demand for electricity can be fulfilled by either installing more and more number of power plants or by improving the output of existing power plants by increasing its efficiency. The installing of more number of power plants leads to the overall increase in generation of greenhouse gases like CO2, CH4, and NOx. The efficiency of the power plant depends on the operating conditions, i.e., temperature and pressure. The increase in operating temperature by 10 oC results in an increase in the efficiency of the power plants by 0.5 %. To operate the power plant at elevated temperature and pressure, the demand for the material with high creep rupture strength, high thermal conductivity, and high corrosion resistance increase significantly. The most commonly used material in power plants operating at high temperatures and high pressure are creep strength enhanced ferritic (CSEF) steel, austenitic stainless steel (ASS) and nickel-based super alloys. CSEF P91 steel was developed to sustain at extreme operating conditions of ultra-supercritical (USC) power plants, and later, P92 was developed to achieve better mechanical properties, higher creeprupture strength and high operating temperature with the reduction in wall thickness as compared to P91 steel. The most common application of P92 material in power plants includes high pressure and high-temperature steam piping, headers, and water-wall tubing. The other most commonly used material in the power plants is austenitic stainless steel, i.e., SS 304L. The austenitic grade stainless steel offers high resistance to corrosion due to the high wt. % chromium and nickel content (18–20 and 8–12, respectively). Due to the low carbon content, the 304L is less sensitive to the sensitization problem and offers excellent weldability. The joining of these dissimilar materials is frequently required in the power generation industry. The different chemical composition, mechanical, physical and metallurgical properties of the CSEF P92 and 304L austenitic grade stainless steel leads to the problems such as carbon migration and high welding induced residual stresses at the weld. The nickel-based filler wire is frequently used due to its intermediate physical and mechanical properties. The gas tungsten arc welding (GTAW) process was used to join the P92 and 304L with various filler metals like ERNiCrMo-3 (INCONEL 625), ERNiCr-3 (INCONEL 82), ERNiFeCr-2 (INCONEL 718) and Thermanit MTS 616 (P92 filler). The X-ray radiographic examination and macrostructure analysis confirmed the decent quality of the P92/304L weld joint. From the comparison of the V-groove and narrow groove geometry, it was found that the narrow groove requires less filler metal than the conventional Vgroove geometry. Also, a narrow groove geometry significantly reduces heat affected zone (HAZ) width due to less heat input. The overall welding induced residual stress was also less for narrow groove geometry. After welding, a substantial change in the microstructure and mechanical properties of the P92 HAZ was noticed. However, the microstructure and mechanical properties of the 304L remain unaffected. The post-weld heat treatment (PWHT) is required to eliminate the heterogeneous microstructure during the dissimilar welding. The microstructure of the weld fusion zone confirmed the formation of secondary phases due to the segregation of various alloying elements at the end of the solidification. The energy-dispersive X-ray spectroscopy (EDS) result showed that these secondary phases were enriched with molybdenum (Mo), chromium (Cr), and niobium (Nb). Micro-hardness study revealed that hardness of P92 coarse grain heat affected zone (CGHAZ), and fine grain heat affected zone (FGHAZ) was higher than P92 base metal after welding due to the formation of untempered martensite. After PWHT micro-hardness of CGHAZ, FGHAZ was decreased due to the tempering effect. The inter-critical heat affected zone (ICHAZ) was the weakest region before and after PWHT. The tensile study reveals that 304L was the weakest link in P92/304L dissimilar weld joint. The specimens were broken from the 304L in as-welded and PWHT condition. Both the P92 steel and weld fusion zone was safe in as-welded condition and PWHT condition. The impact toughness value of the ERNiCr-3 weld region was observed to be higher than the other nickel- based filler metal like ERNiCrMo-3, ERNiFeCr-2, and ERNiCrMoCo-1 (INCONEL 617). It is because of the high nickel (Ni) and low iron (Fe) content in the ERNiCr-3 filler metal. The tensile strength was evaluated at 450 oC, 550 oC, 650 oC, 750 oC, and 850 oC temperature to examine the performance of the dissimilar weld joint at elevated temperatures. After high-temperature tensile strength, the creep performance of the P92/304L dissimilar weld joint has also been investigated at 650 oC in the stress range of 80-200 MPa. All the creep specimens failed from the P92 steel base metal region during the creep test at different conditions. The coarsening of various precipitates such as M23C6 carbides, MX carbonitrides, and formation of laves phase influence the mechanical property, and creep-rupture strength of dissimilar weld joint.

Citation

Roshanlal , Dak Gaurav kumar (2019). Mitigation of Weldability Issue and Residual Stresses in Dissimilar Welded Joints of Ultra-Super Critical Power Plants (Doctor's thesis). Indian Institute of Tehcnology, Jodhpur

URI

https://ir.iitj.ac.in/handle/123456789/224

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Doctoral Theses

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