Study on laser welded heat-affected zone in new ultralow carbon bainitic steel

800 MPa grade ultralow carbon bainitic （NULCB） steel is the recently developed new generation steel, which was produced by thermo mechanical controlled processing ＆ relaxation-precipitation controlling transformation （TMCP＆RPC） tech- nique. The microstructure and the mechanical properties of the heat-affected zone （HAZ） in NULCB steel under laser welding conditions were investigated by using a Gleeble-1500 thermal simulator. The experimental results indicate that the simplex microstructure in the HAZ is granular bainite that consists of bainite-ferrite （BF） lath and M-A constituent when the cooling time from 800 to 500℃ （t8/5） is 0.3-30 s, and the M-A constituent consists of twinned martensite and residual austenite. As t8/5 increases, the hardness and tensile strength of HAZ decreases, but they are higher than that of the base metal, indicating the absence of softened zone after laser welding. The impact toughness of HAZ increases at first and then decreases when t8/5 increases. The impact energy of HAZ is much higher than that of the base metal when t8/5 is between 3 and 15 s. It indicates that excellent low temperature toughness can be obtained under appropriate laser welding conditions.

Microstructure evolution of HAZ in the multi-pass underwater wet welded joints

The microstructure evolution of heat-affected zone （HAZ） in the multi-pass underwater welded joints is investigated in the present work, and it can be characterized by four zones, marked as Region A, B, C and D, respectively. The microstructure in Region A is the mixture of lath martensite and residual austenite. Besides this structure, lots of acicular ferrite appears in Region B as well, and the grain size becomes larger. Lath ferrite combined with granular bainite is observed in Region C. In Region D, tempered sorbite, granular bainite and fine acicular ferrite are found within the grains, while massive ferrite and Widmanstatten structure are distributed along the grain boundary： The thermal effect of multi-pass welding makes the microstructure and property more and more uniform in the ttAZ, and the largest hardness 356 HV in the HAZ appears in Region A.

Liquation cracking in the heat-affected zone of IN939 superalloy tungsten inert gas weldments

The main aim of this study was to investigate liquation cracking in the heat-affected zone(HAZ)of the IN939 superalloy upon tungsten inert gas welding.A solid solution and age-hardenable filler metals were further studied.On the pre-weld heat-treated samples,upon solving the secondaryγ′particles in the matrix,primaryγ′particles in the base metal grew to"ogdoadically diced cubes"of about 2μm in side lengths.The pre-weld heat treatment reduced the hardness of the base metal to about HV 310.Microstructural studies using optical and fieldemission scanning electron microscopy revealed that the IN939 alloy was susceptible to liquation cracking in the HAZ.The constitutional melting of the secondary,eutectic,and Zr-rich phases promoted the liquation cracking in the HAZ.The microstructure of the weld fusion zones showed the presence of fine spheroidalγ′particles with sizes of about 0.2μm after the post-weld heat treatment,which increased the hardness of the weld pools to about HV 350 and 380 for the Hastelloy X and IN718 filler metals,respectively.Application of a suitable solid solution filler metal could partially reduce the liquation cracking in the HAZ of IN939 alloy.

Effects of strength matching and crack depth on the fracture parameters for welded joints

Fracture parameters of welded joints with different strength matching and crack depth in weld metal are investigated by using the methods of elastoplastic finite element analysis and three point bend specimen test. The results show that for shallow crack, the plastic zone turns large in loading process, and the fracture toughness turns high. The extent of the plastic zone of overmatched joint is larger than that of undermatched joint because it will extends to parent metal from the weld metal in loading process for the same CTOD value. The plastic zone of undermatched joint is restricted within the weld, and the size of that is small. Overmatched joint shows the fracture behaviour of shallow crack may more easily than the undermatched joint, while the two sorts of joint specimens have the same crack depth. Therefore, the fracture-resistant capability of overmatched weld is better than that of undermatched weld. when the toughness of weld metals is similar for both overmatched and undermatched joints.

High-temperature creep properties of fine grained heat-affected zone in P92 weldment

The simulated fine grained heat-affected zone (FGHAZ) specimens for P92 welded joints were prepared by heat treatment, then the creep tests were carried out at 650 ℃ under the applied stress of 90-120 MPa to investigate high-temperature creep behavior of FGHAZ. The results show that the creep property of FGHAZ is much inferior to that of the base metal, which exhibits the much higher steady creep rate and shorter time to creep fracture. The power law equation can describe the steady creep rate dependence on applied stress, indicating that the stress exponent n of FGHAZ is distinguished between two regions with n=15.1 at high stresses (more than 100 MPa) and n=8.64 at lower stresses. Based on Monkman-Grant equation, the relationship between the secondary creep rate and time to rupture is obtained to evaluate the creep life of FGHAZ with the applied stress above 100 MPa.

Effect of Zr-Ti combined deoxidation on impact toughness of coarse-grained heat-affected zone with high heat input welding

In this study, the effects of Zr-Ti combined deoxidation and AI deoxidation on the impact toughness of coarse- grained heat-affected zone in high-strength low-alloy steels were investigated. More fine oxides were formed in the Zr-Ti-killed steel than in Al-killed steel. It was also found that more acicular ferrite grains were formed in the coarse-grained heat-affected zone in the Zr-Ti-killed steel than in Al-killed steel. The impact toughness of coarse-grained heat-affected zone of Zr-Ti-kiUed steel was higher than that of Al-killed steel. The good impact toughness was attributable to the pinning effect of fine oxides and the formation of acicular ferrite grains on fine oxides.

Microstructure and embrittlement of the fine-grained heat-affected zone of ASTM4130 steel

The mechanical properties and microstructure features of the fine-grained heat-affected zone（FGHAZ） of ASTM4130 steel was investigated by optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,and welding thermal simulation test.It is found that serious embrittlement occurs in the FGHAZ with an 81.37% decrease of toughness,compared with that of the base metal.Microstructure analysis reveals that the FGHAZ is mainly composed of acicular,equiaxed ferrite,granular ferrite,martensite,and martensite-austenite（M-A） constituent.The FGHAZ embrittlement is mainly induced by granular ferrite because of carbides located at its boundaries and sub-boundaries.Meanwhile,the existence of martensite and M-A constituent,which distribute in a discontinuous network,is also detrimental to the mechanical properties.

Ratcheting Behavior of Weld Joints Under Uniaxial Cyclic Loading Using Miniature Specimen

Because of the size limitation of weld joints in different regions, the traditional standard bar specimen is not suitable to investigate the mechanical properties of weld joints. In this study, miniature specimens were extracted from specific regions(base metal, weld metal, and three heat-affected zones) of API X80 and X70 weld joints. Uniaxial tensile tests were conducted to obtain the mechanical properties of different regions, and then uniaxial ratcheting tests were conducted to investigate the ratcheting behaviors of the different regions under the same peak and nominal stresses. Under both the tensile tests and ratcheting tests, the weld joints exhibit heterogeneous results, such as different mechanical properties and ratcheting behaviors, which were region dependent. Furthermore, the yield strength and yield-to-tensile strength ratio contribute differently to the ratcheting response.

Structure-property relationships in heat-affected zone of gas-shielded arc-welded V-N microalloyed steel

The structure-property relationship in heat-affected zone (HAZ)of a low-carbon steel bearing V-N subjected to gasshielded arc welding was explored.The microstructural characteristics of base metal (BM),coarse-grained HAZ (CGHAZ),fine-grained HAZ,and intercritical HAZ were significantly different.The effect of grain-refinement strengthening and transformation hardening on HAZ contributed to equivalent hardness of 260.8-278.5 HV in comparison with BM hardness of 272.0 HV.Moreover,excellent impact toughness at -20 ℃ was obtained because of high resistance to crack propagation by high-misorientation boundaries,leading to impact fracture consisting of dimples.In CGHAZ,free N was partly fixed by V(C,N)precipitates,such that the deterioration effect of N on toughness was considered to be nearly eliminated.In comparison with CGHAZ,weld metal contained higher fraction of acicular ferrite with fine plates,while the impact toughness was inferior because of the detrimental influence of coarse inclusions from the welding wire.The nanoscale V(C,N)precipitates in CGHAZ had weak effect on toughness because of small size.

蜂窝与K418B高温合金电阻定位焊工艺研究

目的获得蜂窝电阻定位焊的最佳工艺参数并提高焊接接头的力学性能。方法通过正交试验法,对15mm×10mm×4.2mm的GH3536蜂窝和15mm×10mm×2.5mm的K418B基板进行系列电阻焊实验,主要的焊接工艺控制参数包括焊接电流、焊接时间和焊接压力。对焊接接头进行了抗拉强度测试,系统分析了工艺参数对接头力学性能的影响;通过光学显微镜和扫描电镜(SEM)对拉伸试样的断口形貌和失效形式进行了观测;采用电镜配套的能谱(EDS)探头对焊接接头的界面元素进行了分析。结果GH3536蜂窝与K418B基板定位焊系列接头的最高平均抗拉载荷为123.76N。接头界面处的K418B基板为细小等轴晶组织。基板与蜂窝之间存在宽度约2μm的界面层,其成分与基板相近。接头断口焊合区面积随电流的增大而增大,且在高电流下焊合区焊痕呈蝴蝶状分布。结论在电阻定位焊工艺参数中,焊接电流对接头强度的影响最为显著,其次为焊接压力,焊接时间的影响程度相对较弱。得到的最优参数组合如下:焊接时间为2 ms,焊接电流为4.5kA,焊接压力为17.5N。在该参数下能够获得最高的接头平均抗拉载荷(123.76N)。

焊接工艺参数对旋转摩擦焊铝-铜接头弯曲性能的影响

对1070纯铝和T2纯铜进行了旋转摩擦焊接。研究了摩擦焊工艺参数对铝-铜接头弯曲性能的影响。采用电子探针显微分析仪检测了铝-铜接头界面的微观形貌和元素分布。结果表明:较短时间(2 s)摩擦焊制备的铝-铜接头(1号试样)弯曲试验时断裂于铝-铜界面,为脆性断裂,随着摩擦时间的增加,铝-铜接头弯曲试验时断裂于铝侧,为韧性断裂;以2~5 s的摩擦时间制备的铝-铜接头界面均未发现有明显的金属间化合物,但发生了成分变化;较短时间摩擦焊制备的1号试样成分变化区宽约1.6μm,较长时间(5 s)摩擦焊制备的4号试样成分变化区宽约2.7μm;较宽的成分变化区能提高铝-铜接头界面的结合强度,从而提高铝-铜接头的弯曲性能,较窄的成分变化区是弯曲试验过程中铝-铜接头界面断裂的主要原因。

旋转摩擦焊铝-铜接头的显微组织与力学性能

利用旋转摩擦焊工艺实现了1070纯铝与T2纯铜的连接。采用光学显微镜、扫描电子显微镜和电子探针显微分析仪对接头组织和铝-铜界面进行观察分析,并对相同工艺旋转摩擦焊的两块铝-铜接头进行锻压和弯曲试验。结果显示:1号和2号试样在弯曲过程中分别断裂于铝-铜界面和铝侧母材。两种试样的铝-铜界面均未观察到明显的金属间化合物。1号和2号试样铝-铜界面处分别存在宽约1.8和2.7μm的元素过渡区。较宽的元素过渡区可提高铝-铜界面结合强度,较窄的元素过渡区是1号试样在弯曲过程中界面断裂的主要原因。因此,应严格控制旋转摩擦焊的工艺稳定性,以提高焊接接头的弯曲性能。

某电厂低压加热器管板与壳体T型焊接接头开裂失效分析

某电厂低压加热器服役5年后,管板和壳体的T型焊缝部位发生严重开裂;通过宏观检查、断口形貌分析、金相组织分析、能谱分析(EDS)、化学成分分析和硬度检测等检验检测,对焊缝开裂原因进行了分析。分析表明:由于焊接热影响区(HAZ)存在对碱应力腐蚀开裂敏感的淬硬马氏体组织,且焊接接头未进行消应力热处理,在碱性环境中,管板焊接热影响区发生了碱脆,支裂纹在焊缝中扩展穿透造成设备失效。对T型焊接接头失效的原因进行分析,对类似场合换热器设计、制造和使用具有借鉴意义。

基于内聚力模型的气波机振荡管304对接钎焊接头拉伸强度研究

气波机核心部件振荡管采用钎焊连接,钎焊接头拉伸性能对气波机结构强度具有重要影响。为了对钎焊接头界面裂纹扩展过程进行探究,笔者针对气波机振荡管钎焊结构,制备304对接钎焊接头试样进行拉伸试验,并对拉伸断口进行形貌分析,断口处表现为韧性断裂;基于内聚力模型,开发了VUMAT子程序,进行了有限元模拟分析。模拟断裂载荷为5 039 N,试验值为5 242 N,2者误差为3.9%,验证了模拟的准确性,结果表明内聚力模型可以较好地模拟气波机振荡管钎焊接头的裂纹萌生与扩展过程。在此基础上研究焊缝缺陷对钎焊接头断裂载荷的影响,发现缺陷处内聚力单元损伤速率加快,引起断裂载荷显著下降。

激光-MAG和MAG焊接Weldox960钢接头的组织与性能对比

分别采用激光-活性气体保护电弧焊(Laser-MAG焊)和MAG焊对Weldox960钢板进行焊接,对比分析了两种工艺焊接接头的显微组织及性能。结果表明:Laser-MAG焊接接头热影响区粗晶区的平均晶粒尺寸较小,为48μm,而MAG焊接接头的为95μm,两种焊接接头的显微组织基本相同,焊缝区组织为细小粒状贝氏体,细晶区组织为细小板条马氏体和粒状贝氏体,部分相变区组织为铁素体+粒状贝氏体;Laser-MAG焊接接头的热影响区宽度小于MAG焊接接头的,软化带也较窄,其拉伸性能和冲击性能均优于MAG焊接接头的,条件疲劳极限为320 MPa,高于MAG焊接接头的。

钢轨固定闪光焊接头及母材拉伸性能分析

测试了型式检验不同成套工艺下的钢轨固定闪光焊接头及其母材的拉伸性能,分析了拉伸试样的断裂原因。结果表明:热处理钢轨各部位拉伸强度(Rm和Rp0.2)波动较大,其中轨头最高,但是经过固定焊后的接头各部位拉伸强度趋于一致。钢轨及其固定焊接头的屈强比在0.58~0.63。采用固定焊成型的接头具有优异的拉伸强度,热轧钢轨接头抗拉强度可达到母材的99%~126%,热处理钢轨接头抗拉强度可达到92%~95%。焊后接头的韧性普遍下降明显,接头的断后延伸率(A)占到母材的47%~86%,表明焊接生产工艺尚有优化空间。统计发现,接头拉伸试样一般断在焊缝中心或焊后热处理软化区,如果焊缝没有缺陷,拉伸试样基本都断在软化区,灰斑和MnS夹杂可显著弱化焊缝的拉伸性能。母材的MnS偏析降低了钢轨的焊接性。建议钢厂严格控制钢轨母材轨腰MnS偏析,提高钢轨焊接性;同时建议焊轨厂持续优化生产工艺,减少焊缝灰斑,提高焊接接头的韧塑性。

Structure Character of M-A Constituent in CGHAZ of New Ultra-Low Carbon Bainitic Steel under Laser Welding Conditions

800 MPa grade new ultra-low carbon bainitic （NULCB） steel is the recently developed new generation steel. The microstructure in the coarse-grained heat affected zone （CGHAZ） of NULCB steel under laser welding conditions was investigated by thermal simulation. The influence of the cooling time from 800℃ to 500℃.t8/5 （0.3-30 s）, on the microstructure of the CGHAZ was discussed. The experimental results indicate that the microstructnre of the CGHAZ is only the granular bainite which consists of bainitic ferrite （BF） lath and M-A constituent while t8/5 is 0.3-30 s. The M-A constituent consists of twinned martensite and residual austenite, and the change of the volume fraction of the residual austenite in the M-A constituent is very small when t8/5 is between 0.3 and 30 s. The morphology of the M-A constituent obviously changes with the variation of t8/5.As t8/5 increases, tile average width, gross and shape parameter of the M-A constituent increase, while the line density of the M-A constituent decreases.

Microstructure and Mechanical Properties of Intercritical Heat-affected Zone of X80 Pipeline Steel in Simulated In-Service Welding

The intercritical heat-affected zone（ICHAZ） of X80 pipeline steel was simulated by using the Gleeble-3500thermal/mechanical simulator according to the thermal cycle of in-service welding.The microstructures of ICHAZ with different cooling rates were examined,and the hardness,the toughness and corresponding fractography were investigated.Results show that untransformed bainite and ferrite as well as retransformed fine bainite and martensite–austenite（M–A）constituents constitute the microstructure of ICHAZ.The two different morphologies of M–A constituents are stringer and block.Second phase particles which mainly composed of Ti,Nb,C,Fe and Cu coarsened in ICHAZ.Compared with normal welding condition,the toughness of ICHAZ is poor when the cooling time is short under in-service welding condition because of the large area fraction and size of M–A constituents that connect into chains and distribute at the grain boundaries.The Vickers hardness of ICHAZ that decreases with the increase in the cooling time is independent with the area fraction of M–A constituents.

Research and engineering application of the arc welding pulsed controller technology

The tenacity of heat-affected zone （HAZ） will decline and the size of grains will increase, because of the overheating on HAZ when submerged are welding （SAW） is ased to thick plate with high heat input. The shaping will worsen when SAW is used to thin plate with high current at high speed. A new SAW technology, the pulsed direct current （DC） automatic SAW, will be put forward in this paper in order to overcome the above shortcomings. And a pulsed controller with micro-controller unit （MCU） as the core, nixie tube （NT） and keyboard as the man-machine conversation interface is developed. The main functions of the pulsed controller include the output of pulsed welding current and the working with twinwire. The research has widely prospects in application with significant meanings in theory and practical engineering.

Expulsion characterization in resistance spot welding by means of a hardness mapping technique

Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a short welding time. Expulsion has a significant impact on the final yield strength of the weld, thus the detection and characterization of expulsion events is significant for the quality assurance of resistance spot welds. In this study, hardness mapping, using a scanning hardness machine, was used as a quality assurance technique for re- sistance spot welding. Hardness tests were conducted on a resistance spot welded sample to prepare a hardness map. The test results showed good correlation between the hardness map and metallographic cross sections. The technique also provided further fundamental understand- ing of the resistance spot welding process, especially regarding the occurrence of expulsion in the nugget.