Weld overlay cladding of high strength low alloy steel with austenitic stainless steel – Structure and properties

The present work aims at studying structure–property correlations in a weld overlay clad high strength low alloy steel with austenitic stainless steel of American Institute for Steel and Iron (AISI) 347 grade. Optical microscopy studies revealed that the interface between the two steels was nearly flat. The base plate had ferrite plus bainite microstructure adjacent to the interface and tempered bainite/martensite structure away from the interface. Grain coarsening and decarburization were observed near the interface. The stainless steel exhibited austenite dendritic structure. Tensile strength, notch-tensile strength and charpy impact energy of the base plate were found to be higher than those for the interface. The microhardness was observed to be maximum on the clad layer near interface. The shear bond strength of the weld overlay-interface was higher than the shear strength of the base plate. Fractography was carried out using scanning electron microscope on tensile, notch-tensile and shear bond test specimens of the interface as well as shear test specimens of the base plate. It revealed the presence of predominantly dimpled rupture. Charpy impact specimens of the interface failed in mixed mode while impact specimens of the base plate failed in ductile mode. Electron probe microanalysis across the bond interface indicated linear change in concentrations of Cr, Ni, Mn, Cu, Mo, Nb and Si between the levels appropriate to the clad layer and base metal.     

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR AND FRACTURE TOUGHNESS OF A DUPLEX STAINLESS STEEL

Abstract— The mechanical behaviour of the duplex stainless steel AISI 329 has been investigated for ageing times up to 15,000 h at 475, 425, 375, 325 and 275°C. The study has concentrated on changes in the monotonic stress-strain behaviour and fracture toughness as a function of ageing temperature and time. It is shown that the tensile behaviour of the steel changes strongly due to ageing. A large increase in yield strength and reductions in ductility and fracture toughness are observed. The deformation hardening behaviour of the aged steel is explained by using a model based on a modified rule of mixtures. Finally it is shown that the higher toughness of aged duplex stainless steels, in comparison with ferritic stainless steels aged under the same ageing conditions, may be associated with the increase in crack growth resistance induced by ductile ligaments of austenite which bridge the crack faces.     

不锈钢/铝合金异种金属激光-MIG熔钎焊工艺研究

分别采用激光-MIG复合焊和单MIG焊,实现了2mm厚的304不锈钢和6061铝合金对接接头的熔钎焊,对比了不同焊接热源对接头显微组织、界面层化合物及力学性能的影响。结果表明,采用激光-MIG复合焊可以获得性能良好的不锈钢-铝对接接头。激光-MIG复合焊接头的界面层化合物为FeAl_2和Fe_4Al_(13),厚度约为5μm;而单MIG焊接头的界面层化合物厚度约为3μm,主要为Fe_4Al_(13)。激光-MIG复合焊接头的抗拉强度为105MPa,比单MIG焊接头提高了10.8MPa,达到铝合金母材的33.9%。接头试样拉伸断裂均起裂于钎焊界面处,并向余高处扩展,且由脆性断裂转变为韧性断裂。     

Influence of grain refining elements on mechanical properties of AISI 430 ferritic stainless steel weldments – Taguchi approach

The effect of grain refining elements such as copper, titanium and aluminum on transverse tensile strength, ductility, impact toughness, microhardness and austenite content of AISI 430 ferritic stainless steel welds through Gas Tungsten Arc Welding (GTAW) process in as-welded condition was studied. Taguchi method was used to optimize the weight percentage of copper, titanium and aluminum for maximizing the mechanical properties and austenite content in the weld region of ferritic stainless steel welds. Based on Taguchi orthogonal array the regression equations were developed for predicting the mechanical properties of ferritic stainless steel welds within the range of grain refining elements. The observed mechanical properties and austenite content have been correlated with microstructure and fracture features.     

不锈钢母材及其焊缝金属材料单拉本构关系研究

为研究国产双相型S22053和奥氏体S30408不锈钢母材及其焊缝金属材料的单拉本构关系和破坏模式,对4组共12个材性试样进行了单向拉伸试验,并对其破坏截面进行了电镜扫描。基于试验曲线,利用修正的Ramberg-Osgood （R-O）模型对材料本构关系参数进行了拟合,进而对不锈钢母材和焊缝金属的单拉本构关系进行了对比分析。结果表明:不锈钢母材和焊缝金属材料均发生韧性破坏,其本构关系都表现出明显的非线性;焊缝金属材料的屈服强度和极限强度均高于不锈钢母材,而延性低于母材;修正的R-O模型与母材和焊缝金属材料试验曲线吻合良好;在焊缝连接承载力精确分析中,应考虑焊缝金属材料与母材本构关系的差异,分别使用相应的本构关系模型。     

Fracture toughness correlation with microstructure and other mechanical properties in near-eutectoid steel

The variation of yield strength and fracture toughness was investigated for four different heat treatments attempted on specimens of a near-eutectoid steel. The aim of this study was to optimize the microstructure for simultaneous improvements in strength and toughness. Further, the fracture toughness deduced through empirical relations from tensile and charpy impact tests was compared with those measured directly according to ASTM Designation: E 399. Among the four different heat treatments attempted in this study, the plane strain condition was valid in the fracture toughness tests for (i) normalized and (ii) hardened and tempered (500°C for 1 h) treatments only. The latter of the two heat treatments resulted in simultaneous improvement of strength and plane strain fracture toughness. The finely-dispersed carbides seem to arrest the crack propagation and also increase the strength. The pearlitic microstructure of the former leads to easy crack propagation along cementite platelets and/or cementite/ferrite interfaces. The nature of variation of empirically determined toughness values from tensile tests for different heat treatments is similar to that measured directly through fracture toughness tests, although the two sets of values do not match quantitatively. On the other hand, the toughness data deduced from charpy impact test is in close agreement with that evaluated directly from fracture toughness tests.     

Effect of hydrogenation on mechanical properties and tensile fracture mechanism of a high-nitrogen austenitic steel

Austenitic stainless steels are frequently used for hydrogen applications due to their high ductility at low temperatures and lower hydrogen environment embrittlement compared to ferritic steels. We study the effect of electrochemical hydrogen saturation up to 40 h on tensile behavior and fracture mechanisms in high-nitrogen austenitic 17Cr–24Mn–1.3V–0.2C–1.3N steel. Hydrogen saturation weakly influences the characteristic of stress–strain curves, but decreases steel ductility, yield stress, and ultimate tensile stress. Hydrogenation provides a change in steel fracture peculiarities—a hydrogen-assisted thin brittle surface layer of ≈5 μm and ductile subsurface layer of 50–150 μm in width in hydrogen-saturated specimens. The subsurface layer shows ductile transgranular fracture with elongated dimples and flat facets. The central parts of fracture surfaces for hydrogenated specimens show ductile fracture mode similar to hydrogen-free state, but they include numerous secondary cracks both for central part and for transition zone between ductile central part and subsurface layer associated with highest hydrogen saturation. The possible reasons of decrease in hydrogen-associated ductility and change in fracture character are discussed.     

Influence of hot rolling and heat treatment on structure and properties of HSLA steel explosively clad with austenitic stainless steel

Abstract

The present work aims at studying structure–property correlations in an explosively clad HSLA steel with austenitic stainless steel of AISI 304L grade. The clad plate was subjected to hot rolling followed by a quenching and tempering treatment to achieve better mechanical properties in the base plate. Optical microscopy studies revealed that the interface between the two steels was wavy in the as clad plate and the waviness decreased substantially due to hot rolling. Subsequent heat treatment has not shown any significant effect either. The base plate had tempered martensite/bainite structure in as clad or heat treated conditions and ferrite-pearlite-bainite structure in hot rolled condition. The grains were finer and elongated near the interface. The stainless steel exhibited equiaxed grain structure in as clad, hot rolled or heat treated plates. Tensile properties and charpy impact energy of the base plate were lowered due to hot rolling and then increased substantially due to heat treatment. The microhardness was observed to be a maximum at the bond interface for all three conditions studied. The shear bond strength was the highest in the as clad condition and decreased for the rolled as well as heat treated conditions. Scanning electron microscopy fractography on shear bond specimens revealed the presence of predominantly equiaxed dimples with few regions of rubbed fracture. Quantitative electron probe microanalysis across the bond interface indicated linear change in concentrations of nickel, chromium and manganese between the levels appropriate to the clad layer and base metal.     

激光快速成形304不锈钢性能及微观组织研究

目的 满足实际生产需求,提高304不锈钢的抗拉强度。方法 在304不锈钢粉末中添加不同质量分数的Ni60AA粉末,采用激光束对粉末进行快速成形,得到不同的试样。通过金相显微镜对不锈钢试样的显微组织进行观察,利用拉力试验机对试样进行抗拉强度测试。结果 随着添加Ni60AA粉末含量的增加,板材试样的抗拉强度呈现出先增大后减小的趋势,当Ni60AA粉末的质量分数为10%时,试样抗拉强度最大,为754~771MPa。结论 添加Ni60AA粉末后,激光快速成形的304不锈钢板材试样微观组织中有部分镍化合物析出,形成强化相,304不锈钢试样的抗拉强度得到很大提高。     

同时确定钢纤维高强混凝土的断裂韧度及拉伸强度

考虑钢纤维高强混凝土试件细观非均质性对宏观断裂的影响机制,将钢纤维掺量、长度、直径及钢纤维抗拉强度等细观层面的钢纤维特征参数,引入钢纤维高强混凝土宏观断裂模型的虚拟裂缝扩展量的具体计算公式,从而发展了考虑钢纤维特性的可同时确定钢纤维高强混凝土的断裂韧度与拉伸强度的模型及方法。采用变化参数为钢纤维掺量和混凝土水灰比的三点弯曲试件,基于所提模型,同时确定了钢纤维高强混凝土的断裂韧度与拉伸强度,确定值与试验拉伸强度值以及尺寸效应模型计算的断裂韧度吻合良好。基于测试数据离散性为钢纤维高强混凝土固有属性的事实,采用确定的断裂韧度及拉伸强度,建立起钢纤维高强混凝土塑性——准脆性——线弹性不同结构断裂模式的±20%全曲线,其可涵盖实验室条件下的所有试验数据。该文所提模型及方法适用于钢纤维高强混凝土及高强混凝土,可为钢纤维高强混凝土等复合材料真实断裂韧度与拉伸强度的确定,及个性化结构断裂破坏的预测等关键科技问题提供依据。     

Effects of aging at 700°C on ductile fracture of AISI 316 stainless steel

Abstract

The micromechanisms of ductile fracture have been studied in a commercial AISI 316 austenitic stainless steel. Tensile, Charpy impact, and ductile fracture toughness testing have been performed on unaged material and samples aged at 700°C for times up to 4380 h. Examination of the specimens after testing has shown that the microstructural changes occurring at grain boundaries are responsible for the observed losses of ductility and crack growth resistance. The relative magnitude of the observed changes in mechanical properties has been explained using a simple model to describe the ductile fracture process.

MST/1001     

不锈钢晶间腐蚀弯曲评价方法的影响因素

用嵌含有GTN延性损伤模型的ABAQUS有限元法,模拟研究了不锈钢晶间腐蚀弯曲评价方法中材料力学性能、弯曲角度和压头直径对弯曲试样塑性应变分布、延性损伤和裂纹起裂的影响规律,分析了其对晶间腐蚀弯曲评价结果的影响。结果表明：随着试样弯曲角度的增大和弯曲压头直径的减小,试样拉伸面的塑性应变增加,试样越容易产生弯曲开裂;在晶间腐蚀弯曲评价标准中,当固定弯曲角度和压头直径时,对于塑性、韧性和抗断裂综合力学性能较低的不锈钢材料,在弯曲过程中材料本身会发生开裂;因此,需要考虑材料力学性能对晶间腐蚀弯曲评价结果的影响;对于该研究中的典型的奥氏体不锈钢材料,当其弯曲断裂应变低于0．51左右时,在弯曲过程中材料本身会发生开裂,不宜用弯曲方法来评价其晶间腐蚀敏感性。     

Fracture toughness of M2 and H13 alloy tool steels

Abstract

The influence of microstructural variations on the fracture toughness of two tool steels having compositions (wt-%) lC–4Cr–5Mo–2V–6W (AISI M2 high-speed steel) and 0·35C–5Cr–1·5Mo;amp;#x2013;1V (AISI H13 hot-work steel) was investigated. In the as-hardened condition, the H13 steel has a higher fracture toughness than M2 steel, and the latter steel is harder. In the tempered condition, the H13 steel is again softer and has a higher fracture toughness than M2. There is a decrease in fracture toughness and an increase in hardness when the austenitizing temperature is above I050°C for M2 steel and above 1100°C for H13 steel, in both the as hardened and hardened and tempered conditions. The fracture toughness of both steels was enhanced by reducing the grain size and increasing the overall carbide volume in the matrix. The steel samples of average grain diameter ≥40μm exhibit 2–3 MN m ^?3/2 lower fracture toughness than samples of average grain diameter ≤15 μm. A high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. Tempering improved the fracture toughness of M2 and H13 steels. The present results are explained using observations of changes in the microstructure and the modes of fracture.

MST/468     

316L/X65复合管弧焊工艺研究

采用手工钨极氩弧焊和电弧焊实现了316L/X65双合金复合管的对接焊接。为了研究复合管的焊接工艺,对焊接接头进行了硬度、拉伸和冲击测试,利用光学显微镜、扫描电镜和能谱仪分析了接头的微观结构和成分特点,并对接头进行了电化学实验。结果表明:复合管焊缝由碳钢层、扩散层、过渡层和不锈钢层组成,碳钢层主要为针状铁素体,扩散层出现了马氏体组织,过渡层和不锈钢层焊缝中铁素体呈骨架或蠕虫状分布在奥氏体晶界;本实验焊接工艺下,覆层未受到碳钢层的稀释,化学成分与母材基本一致,但耐蚀性略有降低;焊接接头各项力学性能良好。     

Estimating interface bonding strength in clad sheets based on tensile test results

The bond strengths of the three-layered aluminum/stainless steel/aluminum composite clad sheets produced by cold roll bonding were estimated by the tensile test results. The developed procedure was based on the drops in flow stress after the maximum point of the stress–strain curves, which are related to debonding of interfaces and signify the effect of bond strength on the tensile behavior. The shape of the stress–strain curve of a specimen heat treated at 600 °C to form a brittle intermetallic layer at the roll bonding interface was found to be analogous to those obtained from the tensile test of conventional one-layer specimens, which was considered to be a good evidence of interface debonding during tensile test of roll bonded sheets that show sharp flow stress drops. An important contribution of this work is correlating the tension test to peel test results by simple calculations based on the principles of mechanics of materials.     

Micromechanics modelling of ductile fracture in tensile specimens using computational cells

This study extends the computational cell framework to model ductile fracture behaviour in tensile specimens. In the computational cell model, ductile damage occurs through void growth and coalescence (by cell extinction) within a thin layer of material located well inside the fracture process zone for the ductile process. Laboratory testing of a high strength structural steel provides the experimental stress–strain data for round bar and circumferentially notched tensile specimens to calibrate the cell model parameters for the material. Numerical simulations employing the micromechanics model reproduce the essential features of the ductile behaviour for the tensile specimens, including the development of intense necking and void growth in the centre of the specimen cross section. The resulting methodology enables the detailed study of ductile failure in small‐scale tensile specimens.     

基于微观机制的复杂应力状态下钢材韧性断裂行为研究

韧性断裂是钢材最常见的破坏形式,研究钢材韧性断裂机理并准确预测钢材韧性断裂行为具有重要的理论意义和工程实用价值。基于微观机制的断裂预测方法对研究钢材韧性断裂行为有较好的适用性。该文基于体胞模型空穴演化机理改进了现有的韧性断裂模型,校核了Q345钢材断裂模型参数。此外,在韧性断裂模型中引入损伤因子,以考虑应力状态在加载过程中的变化,使断裂模型能准确描述每一加载时刻的累积损伤值。文末采用Fortran语言将断裂模型编写USDFLD子程序,并将其植入有限元程序ABAQUS,对一组十字型刚节点试件单轴拉伸试验进行数值模拟。结果表明,该断裂模型在拉-剪复合应力状态下具有良好的预测精度,且能够准确捕捉钢材断裂起始位置及裂缝扩展路径。该文改进的韧性断裂模型也可用于其它韧性金属材料断裂预测分析。     

Local mechanical properties of radial friction welded supermartensitic stainless steel pipes

In this work, supermartensitic stainless steel pipes were radial friction (RF) welded and their microstructures and local mechanical properties (hardness, fracture toughness and micro-tensile strength) were characterized in the as-welded condition. Defect-free RF welds were produced with a matching consumable ring (CR) under optimized welding conditions. The formation of a fine structure consisting of a mixture of virgin martensite and some stable austenite retained in the CR region was observed. On the other hand, the presence of virgin martensite plus δ-ferrite was found in the microstructures of the heat affected zone (HAZ) and thermo-mechanically affected zone (TMAZ). A ductile fracture was detected in the CR and weld interface regions at −40 °C. Moreover, both the CR and weld interface regions showed higher hardness and strength values than those of the base material (overmatching), without presenting significant losses in ductility and fracture toughness, which was attributed directly to the fine transformed microstructure of the weld region.     

Ductile-to-brittle transition in tensile failure due to shear-affected zone with a stress-concentration source: a comparative study on punched-plate tensile-failure characteristics of precipitation-hardened and dual-phase steels

The effect of shear-affected zone (SAZ), with a stress-concentration source induced by the punching process, on tensile properties was investigated. Tests using honed specimens (which have the same shapes and stress-concentration without any SAZ) and smooth specimens were conducted to compare the effect with that of the punched specimens. Dual-phase steel, which has a high work-hardening ability and low yield strength, and precipitation-hardened steel, which has a low work-hardening ability and high yield strength, were used in the tests. Materials with two tensile strength grades were prepared from both types of steel. Only the precipitation-hardened steel with higher strength grade punched specimen showed a brittle fracture with extremely short fracture-elongation, whereas the other specimens showed a ductile fracture. The fracture surface analysis revealed that cracks initiated in the maximum shear stress plane of the SAZ under tensile loading at first. We call the crack “shear crack.” The steel which showed brittle fracture used in this study easily exhibited plastic-strain localization compared with the other steels. If the shear crack is sharp, then the transition from ductile to brittle failure tends to occur. Furthermore, the strength characteristics of the punched specimen depend on the crack length dependency of the strength resistance and the failure phenomenon of the original material.     

Tensile and Impact Properties of Shielded Metal Arc Welded AISI 409M Ferritic Stainless Steel Joints

The present study is concerned with the effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphology of the joints fabricated by austenitic stainless steel, ferritic stainless steel and duplex stainless steel filler metals were evaluated and the results were reported. From this investigation, it is found that the joints fabricated by duplex stainless steel filler metal showed higher tensile strength and hardness compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Joints fabricated by austenitic stainless steel filler metal exhibited higher ductility and impact toughness compared with the joints fabricated by ferritic stainless steel and duplex stainless steel filler metals.