Influence of alloying on hydrogen-assisted cracking and diffusible hydrogen content in Cr-Mo steel welds

Study of hydrogen-assisted cracking and measurement of diffusible hydrogen content in different Cr-Mo steel welds shows that under identical conditions, susceptibility to cracking increased and diffusible hydrogen content decrease with increase in alloy content. Hydrogen permeation studies show that hydrogen diffusivity decreases and solubility increases with increase in alloy content. Thus decrease in diffusible hydrogen content with increase in alloying is attributed to increase in apparent solubility and decrease in apparent diffusivity of hydrogen. Analysis of the results indicates that variation of diffusible hydrogen content and apparent diffusivity of hydrogen with alloy content can be represented as a function of carbon equivalent CE₁ originally proposed to predict the hardness in the heat-affected zone of alloy steel welds.     

Metallurgical failure analysis of a horse trailer: A criminal investigation

Following the deaths of three people involved in a vehicular collision with an ejected horse, criminal charges were brought against the horse’s owner. Failure analysis was performed on the horse’s trailer to determine its roadworthiness. The failure mode of the hinge between the door and frame became the focus of the investigation. The analysis concentrated on proving whether or not the hinge was broken prior to the collision; that the trailer was not roadworthy; and whether the trailer was capable of safely transporting two horses on a public freeway. Analysis using optical microscopy and the scanning electron microscope indicated the hinge was significantly corroded and mechanically deformed. Additionally, there was evidence of paint on an alleged fresh fracture surface. The results were presented at the trial of the defendant.     

不锈钢焊接凝固裂纹的三维数值模拟

焊接凝固裂纹的产生与否取决于两个方面的因素,材料凝固裂纹阻力和凝固裂纹驱动力,凝固裂纹阻力可以通过实验方法来获得,而凝固裂纹驱动力主要采用有限元计算方法得到.本文通过对SUS310不锈钢,在裂纹敏感温度区间内,焊缝金属动态应力应变场演变过程三维模拟计算的研究,得到了10 mm厚板的凝固裂纹驱动力曲线.利用动态单元再生技术,解决了焊接凝固过程的三个因素-焊接熔池的变形、初始温度的变化和凝固收缩的影响.并且模拟计算得到的驱动力与实验测量的凝固裂纹阻力曲线进行了比较,预测了焊缝金属的凝固裂纹.     

Effect of tempering treatment on hydrogen-induced cracking in high-strength steel

The fracture characteristics of high-strength steel ASTM A-490 under a hydrogen environment were investigated, with special emphasis placed on changes in fracture characteristics due to a tempering treatment at temperatures from 200 to 400 °C. A mechanical test was performed on cathodically charged specimens subjected to a constant load. Experimental analyses show that tempering treatment in the range from 200 to 400 °C does not alter the essential nature of delayed fracture due to crack growth. However, the role of intergranular (IG) cracking becomes prominent in the subcritical crack growth period with an increase in the tempering temperature to 400 °C. This development of IG cracks in the subcritical crack growth period is uniquely dependent on the tempering treatment performed in the tempering range from 250 to 400 °C. Furthermore, an increase in the fraction of the IG facet in the subcritical crack growth area is dependent on the increase in the stress intensity at the crack tip in those specimens tempered at 300 and 400 °C.     

Experimental investigation of the hot cracking mechanism in welds on the microscopic scale

The results of the accurate experimental observations on binary Al-Si alloys are presented, which clearly demonstrate that the solidification cracking is a result of the accumulation of macroscopic tensile displacement in aβmicroscopic intergranular liquid film of segregates at the final stage of the weld metal solidification. The reconstructed mechanism of crack initiation provides a clear phenomenological interrelation between the cracking susceptibility, parameters of the welding process and properties of the base and filler material. The correspondent numerical model takes into account the effects of displacement accumulation as well as the influence of thermo-dynamical and thermo-mechanical properties of the welded material. It is successfully applied for development of technological means for elimination of the solidification cracking during welding of aluminium alloys AA6056, such as a multi-beam welding.     

A study on low magnetic permeability gas tungsten arc weldment of AISI 316LN stainless steel for application in electron accelerator

Low magnetic permeability is an important criterion in selection of the material of construction of beam pipes and vacuum chambers of electron accelerators for safeguarding against distortion of the magnetic field. In the modified design of new 20 MeV/30 mA Injector Microtron for the existing synchrotron radiation sources Indus-1 and Indus-2, AISI 316 LN stainless steel has been identified as the material of construction of its vacuum chamber. Welding of AISI 316LN stainless steel with conventional filler alloys like ER316L and ER317L of AWS A5.9 produces duplex weld metal with 3–8% ferro-magnetic delta ferrite to avoid solidification cracking. The results of the study has demonstrated that GTAW of AISI 316LN SS with high Mn adaptation of W 18 16 5 N L filler produced a crack free non-magnetic weld with acceptable mechanical properties. Moreover, AISI 316LN stainless steel is not required to be solution annealed after the final forming operation for obtaining a low magnetic permeability, thereby avoiding solution annealing of large vacuum chamber in vacuum/controlled atmosphere furnace and associated problems of distortion. Besides Injector Microtron, the study also provides useful input for design of future indigenous accelerators with vacuum chambers of austenitic stainless steel.     

The effect of hydrogen induced cracking on the integrity of steel components

The occurrence of hydrogen embrittlement is a much researched phenomenon, known to cause mechanical property degradation and catastrophic failures. The ductility loss brought about by hydrogen ingress is encountered even in unstressed bodies where such cracking is termed hydrogen induced cracking (HIC) and is in phenomenological contrast to catastrophic failures encountered by stressed bodies subjected to hydrogen producing environments. This article will discuss HIC in some detail. This form of cracking is especially detrimental and often observed in oil country tubular goods (OCTG) which are subjected to sour gas. Consequently, the significance of HIC is most appreciated by oil companies at various stages of oil extraction, transportation and storage. In this article the chemical and metallurgical genesis of HIC, its harmful impact on material and component integrity are discussed. It has been noted that MnS inclusions are extremely harmful to this form of cracking. Similarly, centreline segregation in the ingot stage and deoxidation practices during steelmaking were found to affect HIC. Some case studies of HIC obtained from literature are presented. The variables affecting the propensity to HIC are provided in brief. Suitable measures to reduce or eliminate HIC in steels are also discussed.     

Hydrogen induced cracking of a tappet adjusting screw

The electroplated tappet adjusting screws used in diesel engines failed during initial bend testing. The analysis of the failure showed that the fracture was nucleated from the subsurface of the screw. The fracture surface was intergranular at the ID and OD region and microvoid coalesence in the center. The improper baking after electroplating of the screw led to H₂-induced blistering/cracking. The high strength of the threaded region of the adjusting screw increased the failure propensity.     

Air conditioner failure investigation-intergranular cracking in a pure copper condenser tube

A failure of a 7 1/2-year-old commercial rooftop air conditioning system was investigated. The systematic failure analysis procedure and eventual discovery of a leak in a pure copper condenser coil are discussed. Severe denting of several tubes, including the tube that leaked, was discovered in the area of the steel tube sheet. A metallurgical examination, including scanning electron microscopy, revealed intergranular cracking, a process that is common among some copper alloys suffering from stress-corrosion cracking but is rarely observed in pure copper. Research of available literature is presented, and the conditions under which pure copper may exhibit intergranular cracking are discussed. Also presented are vibration measurements of the air conditioner taken during operation, which rule out operating loads as the cause of the tube denting and eventual cracking. The cause of the denting remains unknown. However, the denting probably occurred during manufacturing, and the high stresses associated with the dents, in the presence of corrosion, resulted in intergranular cracking and eventual leaking.     

Progress in failure analysis of constrained shrinkage cracking in reinforced concretes

Shrinkage-induced cracking in constrained concrete slabs is an important failure mode for concrete structures. Several empirical relations have been reported in earlier studies. This paper develops a mechanics-based relationship for shrinkage-induced cracking through a shear-lag model that can be used to analyze the stress field, crack width, and the final crack spacing for cracked concrete slabs. The present model prediction is observed to be close to the empirical relation based on long-term observations.     

煤矿机械用Q690高强钢抗裂性研究

对煤矿机械用Q690高强钢,采用其配套焊丝(GHS75M)进行抗裂性试验,并测定了焊丝熔敷金属扩散氢逸出特性,研究了焊缝金属残余扩散氢对试验用钢抗裂性的影响.结果表明:该钢具有一定的冷裂倾向.试验焊材熔敷金属扩散氢含量为1.7 mL/100 g,初期扩散氢逸出速率较大,在焊缝冷却过程中扩散氢可以较快逸出.残余扩散氢含量主要与从峰值温度到100℃温度区间的冷却时间t100有关,单道焊焊缝金属冷速快,t100较小,残余扩散氢较多,而多层多道焊增加了t100,且提高HDr100容许值,残余扩散氢含量很低.该钢采用焊后热处理是保守的工艺.     

12Cr1MoV联箱与支管焊缝裂纹分析

为研究某厂联箱与支管运行1.4×105h后发生开裂的原因,采用宏观渗透及光学显微镜、扫描电镜、能谱仪、显微硬度计等测试方法对失效管进行检测.研究表明:焊后存在焊接缺陷,大量的裂纹垂直于焊缝,加速裂纹的扩展,改变应力分布状态;部分夹杂物沿直线分布,使材料受力具有方向敏感性,导致材料开裂失效;断口表面主要是Fe的氧化物,焊件长期过热,外表面形成不均匀氧化层,产生应力集中,致使焊缝从管壁外侧开裂;母材硬度超标.优化焊接工艺方案,焊前预热焊后回火,消除管件外壁的氧化层及减少设备停机次数均能有效防止同类焊接结构开裂失效.     

不锈钢母材及其焊缝金属材料单拉本构关系研究

对12个奥氏体型及12个双相型不锈钢正面角焊缝和侧面角焊缝连接试件进行了单调拉伸试验,考察了不同焊接工艺对角焊缝连接力学性能的影响。结果表明：采用氩弧焊焊接工艺的不锈钢角焊缝试件破坏面与电弧焊焊接工艺的试件破坏面形状相差较大,后者破坏面更加光滑;同时由于受到复杂应力的作用,正面角焊缝试件的真实破坏角度并不为相关规范规定的理论值45°;对于奥氏体型不锈钢角焊缝,氩弧焊试件与电弧焊试件的强度比分别为1.03 (正面角焊缝试件)及1.13 (侧面角焊缝试件),相对变形量之比为1.46及1.11;而对于双相型不锈钢角焊缝,两者的强度比分别为1.12和1.04,相对变形量之比为1.66及1.45;氩弧焊试件表现出了更好的力学性能。对于两种不锈钢材料,正面角焊缝强度均远大于侧面角焊缝的强度,建议在工程设计和相关规范的编制/修订中考虑正面角焊缝强度提高的影响。     

Finite element analysis of residual stress in the welded zone of a high strength steel

The distribution of the residual stress in the weld joint of HQ130 grade high strength steel was investigated by means of finite element method (FEM) using ANSYS software. Welding was carried out using gas shielded arc welding with a heat input of 16 kJ/cm. The FEM analysis on the weld joint reveals that there is a stress gradient around the fusion zone of weld joint. The instantaneous residual stress on the weld surface goes up to 800 ∼ 1000 MPa and it is 500 ∼ 600 MPa, below the weld. The stress gradient near the fusion zone is higher than any other location in the surrounding area. This is attributed as one of the significant reasons for the development of cold cracks at the fusion zone in the high strength steel. In order to avoid such welding cracks, the thermal stress in the weld joint has to be minimized by controlling the weld heat input.     

插销法研究自保护焊U75V钢轨的冷裂敏感性

采用插销试验法研究了JDHS-38#自保护药芯焊丝焊接U75V钢轨的冷裂敏感性,评估了预热温度、线能量、焊道数3个因素对U75V钢轨临界断裂应力的影响.结果表明,现场对钢轨施焊时,采取250℃的预热温度,14 kJ/cm左右的线能量,双层焊等措施可以使钢轨接头热影响区少产生甚至不产生焊接冷裂纹,使其临界断裂应力达646 MPa,与抗拉强度相当.若采用其他工艺参数,临界断裂应力较低,且当拘束拉伸应力高于抗拉强度时,于热影响区发生氢致断裂.焊接接头过热区组织主要为马氏体、贝氏体,断口显微形貌主要为河流花样、泥状花样及爪状花样.     

Numerical simulation of periodic cracking mechanism in microscopic surface films during roll bonding processes

The microscopic surface films existing on the top of metallic layers play an important role in the process of joining by plastic deformation. The bond formation during cold welding processes is basically associated with the fracturing of surface films to produce intimate metallic contacts. The present paper aims at providing a numerical model to describe the cracking pattern of brittle surface films bonded to the ductile substrates. A microscale finite element model is developed which takes into account the fracturing mechanisms of thin surface films in roll bonding processes. The presented model is calibrated by using the existing experimental data for an aluminum alloy covered by a thin layer of oxide film. The model is also validated against a well‐known analytical model for periodic cracking. The distribution of stresses within the fractured surface film demonstrates that the generated cracks in the surface film have essentially a periodic pattern. Moreover, it is shown that the crack spacing is highly dependent on the properties of the surface film. Finally, the obtained results for the roll bonding show that a crack density saturation takes place at the entry of the roll bite where a small surface expansion is applied to the rolled samples.     

酸洗-冷轧联合机组闪光焊接头断带原因分析

采用宏观分析、力学性能测试、金相检验和断口分析等手段对某酸洗-冷轧机组闪光焊低碳Stb35L钢薄板接头在冷轧过程中频繁出现断带的原因进行了分析。结果表明:闪光焊接头发生开裂和断带的主要原因是其热轧原料钢板的头尾板形较差,而闪光焊前又未对其进行校平或剪切,从而导致接头局部出现焊缝搭接错边现象;在冷轧过程中,焊缝搭接处产生折叠和缺口裂纹,裂纹不断扩展最终导致断带;另焊缝中存在未挤压干净的链状氧化物是导致接头开裂和断带的另一个原因。     

Metallurgical investigation of a failure in 316L stainless steel orthopaedic implant

An orthopaedic implant (nail for shinbone) made of 316L stainless steel (SS) that failed prematurely was examined to determine the root cause for the fracture. Detailed scanning electron microscopy was carried out to conclusively establish the evidence(s). Based on the results of extensive fracture surface analysis as well as the background information provided on the implant, it was determined that the implant (stainless steel nail) failed by the mechanism of predominantly ductile fracture facilitated by the presence of non-metallic inclusions.     

Evaluation of stress corrosion cracking and hydrogen embrittlement in an API grade steel

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using aqueous extracts from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46 steel were obtained at different electrode potentials (E_corr, 100 mV below E_corr and 300 mV below E_corr) with 9 × 10⁻⁶ s⁻¹ and 9 × 10⁻⁷ s⁻¹ strain rate. In addition, the hydrogen permeation tests were carried out in order to evaluate the susceptibility of hydrogen penetrates into theses steels. The results demonstrated the incidence of cracking and their dependence on the potential imposed. In that case, cracking occurred by stress corrosion cracking (SCC) and the hydrogen embrittlement (HE) had an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility.     

Catastrophic failure of a fan in a diesel engine cooler

The fan used to cool a diesel engine fractured catastrophically after approximately 100 h of operation. The fan failed at a spider, which was resistance spot welded to a shim placed between two circular spiders of 3 mm thickness. The detailed analysis of the fracture indicated that the premature failure of the fan was due to inadequate bonding between the sheets at the weld nugget. The fracture was initiated from the nugget-plate interface. The inadequate penetration and lack of fusion between the steel sheets during resistance spot welding led to poor weld strength and the fracture during operation. The propensity to crack initiation and failure was accentuated by improper cleaning of the surfaces prior to welding and to inadequate nugget-to-sheet edge distance.