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Microstructural evolution and stress relaxation cracking mechanism for Super304H austenitic stainless steel weld metal

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Journal of Materials Science & Technology 2022年第5期100卷 82-90页

作者： Xiaopeng Xiao Dianzhong Li Yiyi Li Shanping Lu Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China School of Materials Science and Engineering University of Science and Technology of ChinaShenyang 110016China Key Laboratory of Nuclear Materials and Safety Assessment Insititute of Metal ResearchChinese Academy of SciencesShenyang 110016China

The pre-compressed CT technique was used to quantitatively investigate the formation of stress relaxation cracks under different tensile residual stresses and aging time in Super304H austenitic stainless steel weld metal.The statistical results revealed that intergranular cracks could occur within 2000 h under 650℃ when the residual stress was applied with greater than 18 KN pre-compression force.Detailed grain interior and boundary analyses showed that the growth of intragranular Cu-rich particles could induce a strong grain interior,and the intergranular Nb(C,N)carbides were one of the causes to crack under short-term aging time.For long-term aging time conditions,the intergranular M_(23)C_(6)carbides were more susceptible to crack than intergranular Nb(C,N)carbides.Finally,the mechanism responsible for stress relaxation cracking formation was carefully illustrated for the weld metals after short-term aging and long-term aging,respectively.

关键词： Stress relaxation cracking Super304H steel weld metal Aging treatment Residual stress Precipitates

Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si‑Bearing 15Cr–9Ni–Nb Austenitic Stainless Steel weld metal

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Acta metallurgica Sinica(English Letters) 2023年第4期36卷 637-649页

作者： Yakui Chen Dong Wu Dianzhong Li Yiyi Li Shanping Lu Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China School of Materials Science and Engineering University of Science and Technology of ChinaShenyang 110016China CAS Key Laboratory of Nuclear Materials and Safety Assessment Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China

Two 15Cr–9Ni–Nb austenitic stainless steel weld metals with 2.5%Si and 3.5%Si(namely 2.5Si and 3.5Si samples,respectively)were designed and prepared through tungsten inert gas(TIG)welding and then hold at 800℃ or 900℃ for 3 h for stabilization.The microstructure and mechanical properties were investigated both for the as-welded and after-stabilization heat treatment(SHT)weld metals.There are 3.0–4.0%martensite and 2.5–3.5%δferrite in the 2.5Si as-welded weld metal and 6.0–7.0%δferrite in the 3.5Si as-welded weld metal.After SHT,a large amount of martensite formed in the 2.5Si weld metal,andδ→γtransition occurred during the SHT process both for the 2.5Si and 3.5Si weld metals.There were a large amount of coarse NbC and few nanoscale NbC in the as-welded weld metal.During the SHT,a large amount of nanoscale NbC formed in the matrix,while a large number of G phases formed at the austenite grain boundaries and theδ/γinterfaces.The decrease in solid solution C andδferrite content led to the decline of the yield strength of the weld metal after SHT.The martensite formed in 2.5Si weld metal after SHT had less effect on strength because of its low carbon content.The G phases formed during the SHT reduced the impact energy of the weld metal because it promoted the intergranular fracture,while theδ→γtransition reduced the amount of theδ/γinterfaces and avoided the intergranular fracture,which was beneficial for the impact toughness of the weld metals.

关键词： Austenitic stainless steel weld metal Stabilization heat treatment Martensite NbC δ→γtransition

Precipitation Behavior and Mechanical Properties of a 16Cr-25Ni Superaustenitic Stainless Steel weld metal During Post-weld Heat Treatment

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Acta metallurgica Sinica(English Letters) 2022年第4期35卷 577-590页

作者： Wenbin Tian Dong Wu Yiyi Li Shanping Lu Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China School of Materials Science and Engineering University of Science and Technology of ChinaShenyang 110016China CAS Key Laboratory of Nuclear Materials and Safety Assessment Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China

A 16Cr-25Ni superaustenitic stainless steel weld metal for austenitic stainless steel/ferrite heat-resistance steel dissimilar metal weld was designed and prepared through tungsten inert-gas welding.The precipitate evolution and its correlation with mechanical properties were investigated during post-weld heat treatment(PWHT)at 690℃ for up to 12 h.The primary precipitates in the as-welded weld metal were identified as Mo-rich M6C carbides in the interdendritic region and semicontinuous fine-sized M23C6 carbides along grain boundary.After PWHT,three types of precipitates coexisted in the interdendritic region:primary M6C carbides,newly precipitated Mo-rich M2X carbonitrides and some of the secondary M23C6 carbides.Additionally,mass secondary M23C6 carbides formed and coarsened along grain boundary.No undesirable intermetallic phases formed during the whole period.The M2X and interdendritic M23C6 improved the strength of the weld metal after PWHT,but the elongation and impact toughness degraded,which were mainly owing to the intergranular M23C6 carbides that changed the fracture mode from ductile transgranular mode to mixed mode of transgranular and intergranular fracture.Meanwhile,the coarsening of M2X carbonitrides may lead to the elongation loss during 8 h to 12 h.Evolution of impact toughness was also related to the M2X carbonitrides,which made the crack easier to propagate compared with austenitic matrix and contributed to the decline of impact toughness.However,due to the sluggish precipitation of M2X carbonitrides with longer holding time,the decreasing trend became slow from 4 to 12 h.The results showed that PWHT should be controlled within 8 h to obtain better combination of strength and ductility.

关键词： Superaustenitic stainless steel weld metal Post-weld heat treatment Precipitate evolution Mechanical properties

Characterization of the structural details of residual austenite in the weld metal of a 9Cr1MoNbV welded rotor

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International Journal of Minerals,metallurgy and Materials 2014年第6期21卷 563-568页

作者： Xia Liu Hui-jun Ji Peng Liu Peng Wang Feng-gui Lu Yu-lai Gao Department of Mechanical Engineering Tsinghua University Shanghai Turbine Plant of Shanghai Electric Power Generation Equipment Co. Ltd. School of Materials Science and Engineering Shanghai University School of Materials Science and Engineering Shanghai Jiao Tong University

The existence of residual austenite in weld metal plays an important role in determining the properties and dimensional accuracy of welded rotors. An effective corrosive agent and the metallographic etching process were developed to clearly reveal the characteristics of residual austenite in the weld metal of a 9Cr1MoNbV welded rotor. Moreover, the details of the distribution, shape, length, length-to-width ratio, and the content of residual austenite were systematically characterized using the Image-Pro Plus image analysis software. The results revealed that the area fraction of residual austenite was approximately 6.3% in the observed weld seam; the average area, length, and length-to-width ratio of dispersed residual austenite were quantitatively evaluated to be （5.5 ± 0.1）μm2, （5.0 ± 0.1）μm, and （2.2 ± 0.1）, re-spectively. The newly developed corrosive agent and etching method offer an appropriate approach to characterize residual austenite in the weld metal of welded rotors in detail.

关键词： steel research austenite characterization weld metal color metallography

Effects of Hot Bending Parameters on Microstructure and Mechanical Properties of weld metal for X80 Hot Bends

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Journal of Iron and Steel Research(International) 2014年第12期21卷 1129-1135页

作者： Xu WANG Bo LIAO Da-yong WU Xiu-lin HAN Yuan-sheng ZHANG Fu-ren XIAO Key Laboratory of Metastable Materials Science and Technology College of Materials Science and EngineeringYanshan University CNPC Bohai Petroleum Equipment Manufacture Co. Ltd. China Petroleum Technology and Development Corporation

Mechanical properties of weld metal are the key factors affecting the quality of heavy-wall X80 hot induction bends. The effects of bending parameters on the mechanical properties of weld metal for hot bends were investigated by simulation conducted on a Gleeble 3500 thermal simulator. Continuous cooling transformation （CCT） dia- grams of the weld metal were also constructed. The influences of hot bending parameters （such as reheating temperature, cooling rate, and tempering temperature） on the microstructure and mechanical properties of weld metal were also analyzed. Results show that the strength of all weld metal specimens is higher than the value indicated in the technical specification and increases with the increase of reheating temperature, cooling rate, and tempering tempera ture. The impact toughness is apparently related to the variation of reheating temperature, cooling rate, and tempering temperature.

关键词： hot induction bend X80 steel weld metal heat treatment microstructure

A PROBABILISTIC ASSESSMENT OF TEMPERATURE EFFECTSON THE LOW CYCLE FATIGUE BEHAVIOUR OF 1Cr_(18)Ni_9Ti STEEL weld metal

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Acta metallurgica Sinica(English Letters) 1998年第6期11卷 477-484页

作者： Q.Gao Y.X.Zhao L.X.Cai Southwest Jiaotong Univ Chengdu China

Based on the test results obtained from the single-step test and the incremental-step test at room temperature and 240℃, a probabilistic assessment of temperature effects on the cyclic stress-strum response and the fatigue life of 1Cr18Ni9Ti steel weld metal is performed. In orber to assess the temperature effect on cyclic stress amplitude where there is a scatter of the material cyclic constitution, a probabilistic assessment approach on the basis of probabilistic modified Ramberg-Osgood relations is introduced.The investigation shows that the cyclic stress amplitude and the scatter of cyclic stress amplitude data are decreased at 240℃. Similarly, from the consideration of the fatigue life scatter a probabilistic assessment of temperature effect on the fatigue life is suggested on the basis of probabilistic Langer S-N relations. The investigation shows that the crack initiation life is increased and the scatter of crack initiation life data is decreased at 240℃.

关键词： 1Cr_(18)Ni_9Ti stainless steel weld metal low cycle fatigue temperature effect probabilistic assessment

Size Evolution of the Surface Short Fatigue Cracks of 1Cr18Ni9Ti weld metal

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Journal of Materials Science & Technology 2003年第2期19卷 129-132页

作者： Yongxiang ZHAO Institute of Mechanical Engineering Southwest Jiaotong UniversityBeiyuan 32-2-06#Chengdu 610031China

Size evolution of the surface short fatigue cracks of lCrl8Ni'9Ti weld metal was investigated. A local viewpoint is applied to be agreement with a so-called 'effectively short fatigue crack criterion'. Attention was paid to the dominant effectively short fatigue crack (DESFC) initiation zone and the zones ahead of the DESFC tips. The results revealed that the evolutionary size shows a significant character of microstructural short crack (MSC) and physical short crack (PSC) stages. In the MSC stage, fatigue damage is due to mainly the initiation and irregular growth of the effectively short fatigue cracks (ESFCs). In the PSC stage, the damage is conversely due to mainly the DESFC growth and partially, the growth of the ESFCs and the coalescence of the ESFCs themselves with the DESFC. The process involves from a non-ordered/chaotic state in the initiation of MSC stage, gradually to an independently random state at the transition point between the MSC and PSC stages and then, to an ordered/history-dependent random state. Interactive effect of the collective cracks is stronger and shows an increase in the MSC stage, it reaches a maximum value at the transition point and then, tends to a decrease in the PSC stage. The DESFC acts as a result of the interactive cracks and thus, is deemed suitable to describe the behaviour of collective cracks.

关键词： Short fatigue crack Damage evolution ICrl8Ni9Ti steel weld metal

Kinetics of Acicular Ferrite Transformation in weld metal

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Journal of Iron and Steel Research(International) 2001年第1期8卷 34-36页

作者： TIAN Zhi-ling Central Iron and Steel Research Institute Beijing 100081 China Petroleum University Shandong 257061 China Tianjin University Tianjin 300072 China

Acicular Ferrite(AF) is beneficial in welds because it provides a tough and strong microstructure.Therefore,much works have been done in the study of the effect of alloying elements and welding conditions aimed at obtaining more AF in the weld metal.However,the knowledge about the nature of AF is limited due to the lack of kinetics researches.In this paper,the kinetic curves of AF transformation in an X65 weld metal were established by means of welding thermal simulation test.The results showed that AF transformation is a typical diffusion process of nucleation and growth with incubation period.AF transformation in weld metal obeys the relationship described by JMA Equation.The effect of welding heat input was also studied.

关键词： weld metal acicular ferrite kinetics pipe steel

Microstructure and Mechanical Properties of GMAW weld metal of 890MPa Class Steel

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Journal of Iron and Steel Research(International) 2014年第5期21卷 539-544页

作者： Yun PENG Xing-na PENG Xiao-mu ZHANG Zhi-ling TIAN Tao WANG State Key Laboratory of Advanced Steel Processes and Products Central Iron and Steel Research Institute Beijing Institute of Structure and Environment Engineering

The relationship between microstructure and mechanical properties of gas metal arc weld metal with strength over 890 MPa is discussed. The microstructure of the weld metals is characterized with OM, SEM, TEM and EBSD. The microstructure of the weld metals is mainly composed of martensite and bainite. Experimental results show that the microstructure with predominant fine lath bainite possesses good toughness of 77 .l, while its yield strength is less than 800 MPa. The microstructure of coarse lath martensite and bainite has the lowest toughness of 43 J and its yield strength is 820 MPa. The mixed microstructure with fine martensite, bainite and retained austenite films bears good combination of toughness and yield strength （62 J and 880 MPa, respectively）. It is concluded that fine effective grain size and ductile phase of austenite films are two main factors to achieve good mechanical properties.

关键词： high strength steel weld metal microstructure mechanical property