Seamless steel pipe production process for nuclear power
2015-02-09
Yangzhou Chengde Steel Pipe Co., Ltd. entrusted Suzhou Thermal Engineering Research Institute Co., Ltd. to evaluate the pipeline weldability and welding process of the nuclear power conventional island WB36CN1 and P22 high-pressure seamless steel pipes produced in trial production, as well as the bending process of the steel pipes. The surface quality, geometric dimensions, and physical and chemical performance indicators of the nuclear power WB36CN1 and P22 seamless steel pipes produced by Yangzhou Chengde Steel Pipe Co., Ltd. using slant rolling perforation + cold drawing process meet the technical requirements of GB24512.2-2009 "Seamless Steel Pipes for Nuclear Power Plants Part 2: Alloy Steel Seamless Steel Pipes" and are at a leading level among similar steel pipes. 1. Welding Process Evaluation The test content includes cold crack tests for WB36CN1 steel, reheat crack tests, cold crack tests for P22 steel, welding process evaluation for WB36CN1 steel pipe using TIG/SMAW/SAW combination welding, and welding process evaluation for P22 steel pipe using TIG/SMAW and TIG/SMAW/SAW combination welding. The welding process evaluation follows DL/T1117-2009 "Welding Process Evaluation Procedures for Conventional Islands in Nuclear Power Plants." 1) Welding Performance and Welding Process Evaluation Test Results for WB36CN1 Steel For WB36CN1 steel pipe, tests were conducted using the maximum hardness method in the heat-affected zone, slant Y groove cold crack test method, and pin cold crack test method. The results show that under preheating conditions of 160°C, the maximum hardness in the heat-affected zone is below 350HV1, no cracks were found on the surface or cross-section of the slant Y groove weld seam, and the critical fracture stress exceeds the yield strength of WB36CN1 steel; therefore, preheating at 160°C can prevent cold cracks from occurring. This preheating temperature is consistent with that recommended for 15Ni1MnMoNbCu steel pipes in relevant standards. Reheat crack tests on WB36CN1 steel at temperatures of 590°C and 610°C with a holding time of 3 hours showed no form of cracks on both the surface and cross-section of slant Y groove weld seams. This indicates that WB36CN1 steel produced by Yangzhou Chengde Steel Pipe Co., Ltd. has insufficient sensitivity to reheat cracking under stress conditions within a range of 580-620°C; thus, both temperatures of 590°C and 610°C are suitable as post-weld heat treatment temperatures. The results from welding process evaluation using TIG/SMAW/SAW combination welding methods show that this type of steel pipe has excellent welding performance with imported welding materials. The test pieces passed magnetic particle testing and radiographic non-destructive testing; the quality of the welding process meets standard requirements. After welding and post-weld high-temperature tempering, the room temperature strength, high-temperature strength, and impact toughness of welded joints all meet GB24512.2-2009 performance requirements for raw materials as well as DL/T1117-2009 requirements. The microstructure of welded joints consists of ferrite + bainite; no network precipitation features or abnormal structures were observed at a microscopic level. The maximum hardness in welded seams and heat-affected zones is higher than that of base materials but does not exceed 270HB, complying with DL/T1117-2009 requirements; no form of cracks was found during full-thickness bending tests; elemental content in weld seams is comparable to raw materials while impurity levels are controlled at low levels. All inspection results from this entire welding process evaluation meet DL/T1117-2009 requirements; therefore it is concluded that this WB36CN1 steel welding process evaluation is qualified. 2) Welding Performance and Welding Process Evaluation Test Results for P22 Steel For P22 steel pipe, tests were conducted using maximum hardness method in heat-affected zone, slant Y groove cold crack test method, and pin cold crack test method. The results show: under preheating conditions at 80°C, maximum hardness in heat-affected zone is below 350HV1; however even when preheating temperature increases to 250°C, maximum hardness remains close to 320HV1. Under similar preheating conditions no cracks were found on either surface or cross-section of slant Y groove weld seams; results from both maximum hardness method in heat affected zone and slant Y groove cold crack test indicate that preheating at least to about 80°C can prevent cold cracks from occurring; under preheating temperatures at both conditions (200°C &150°C), critical fracture stresses far exceed yield strength values for P22 steels indicating through pin testing that preheating at least to about150 °C can prevent cold cracking occurrence . Comprehensive results from three types testing indicate that optimal preheat temperature preventing cold cracking should be lower than150 °C which falls below recommended standard range (200 -300 °C); thus it shows Yangzhou Chengde Steel Pipe Co., Ltd.'s produced nuclear power use P22 steels have better resistance against cold cracking compared to similar steels. The results from P22 steel's welding process evaluation using TIG/SMAW/SAW combination methods show excellent compatibility with both domestic & imported welding materials' performance characteristics . Test pieces passed magnetic particle testing & radiographic non-destructive testing ; quality meets standard requirements . After undergoing high-temperature tempering post-welding , room temperature strength , high-temperature strength , impact toughness all satisfy GB24512 .2 -2009 material performance requirement values , meeting DL / T1117 -2009 standards . Microstructure consists primarily ferrite + bainite ; no network precipitation features nor abnormal structures observed microscopically ; highest hardness within joint welds & HAZ exceeds base material but remains below270 HB complying with DL / T1117 -2009 standards ; full thickness bends showed no signs whatsoever indicating any forms cracks present ; elemental composition within welds comparable raw material while impurity levels maintained low throughout entire evaluations confirming compliance across all aspects hence concluding this particular assessment deemed satisfactory. The results from manual TIG/SMAW combination welding evaluations on P22 steels indicate excellent compatibility with domestic wires & rods' respective performances . Test specimens successfully passed magnetic particle inspections alongside radiographic non-destructive checks confirming adherence towards established quality benchmarks . Post-weld treatments including high temp tempering yielded satisfactory outcomes regarding room temp strengths alongside elevated thermal resistances plus impact toughness metrics aligning perfectly within GB24512 .2 -2009's stipulated parameters whilst also fulfilling DL / T1117 -2009's criteria too . Joint microstructures revealed compositions primarily consisting ferritic-bainitic phases devoid any discernible network precipitates or irregular formations noted microscopically speaking ; peak hardening values recorded across joint regions remained beneath270 HB thresholds thereby satisfying regulatory stipulations set forth byDL / T1117 -2009 guidelines overall demonstrating successful completion throughout entirety pertaining towardsP22steel'sweldingprocessassessment. 2. Bending Process Evaluation The WB36CN1 and P22 high-pressure seamless steel pipes are evaluated for bending process according to DL/T515-2004. The test pipes are WB36CN1 Φ711mm×40mm and P22 Φ558mm×25mm. The test content includes surface quality, dimensions, metallographic examination, hardness testing of the steel pipes before bending, as well as checks on bending, heat treatment, dimensions, non-destructive testing, and post-bending metallographic examination, hardness testing, and destructive testing (conducted in four areas: straight section of the bend, start of the bend area, middle of the tension side, and end of the bend area for room temperature mechanical properties, high temperature mechanical properties, impact toughness, metallographic examination, and hardness testing). The inspection results indicate that using the proposed bending process parameters after 3D and 90° hot bending, the thinning rate, ovality, bending angle, waviness rate, surface quality and other aspects all meet DL/T515-2004 and related technical requirements. The post-bending structure, strength and impact toughness indicators all meet GB 24512.
