Structure–property relationships in intercritical heat affected zone of low-carbon microalloyed steels

Abstract

The conditions for martensite formation in the intercritical heat affected zone (HAZ) of two low-carbon microalloyed steels have been investigated using optical and transmission electron microscopy. Based on Charpy V-notch testing of a large number of thermally cycled specimens, it is concluded that embrittlement within the intercritical HAZ of such steels is closely related to the development of twinned martensite during the weld cooling cycle. The reduced HAZ toughness probably arises from the associated stress concentrations developed in the surrounding ferrite matrix, which give rise to the initiation of brittle fracture in the ferrite.

MST/634     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

BT20钛合金激光焊接接头的断裂韧性研究

对BT20钛合金及其激光焊接接头的断裂韧性进行了研究.同时分析了合金及激光焊接接头的硬度分布及显微组织.断裂实验表明,除了一个焊接接头紧凑拉伸(CT)试样是脆性启裂外,其它CT试样均在裂纹延性启裂并缓慢扩展后,发生脆性失稳断裂.母材的断裂韧性明显高于焊接接头,轧制方向对母材断裂韧性的影响不明显.焊接热影响区的断裂韧性介于母材和焊缝金属之间.本研究采用的焊后热处理没有改善焊接接头的断裂韧性,还有进一步恶化的趋势.添加活性剂对焊缝金属的断裂韧性没有明显作用,但对延性裂纹扩展长度有所改善.     

Application of local approach to inhomogeneous welds. Influence of crack position and strength mismatch

In steel welds there is often a large variation in fracture toughness and mechanical properties between the weld metal, base material and the various heat affected zone (HAZ) microstructures. The stress field in front of a crack in a weldment can be noticeably affected by the strength mismatch between the weld metal, HAZ and the base material. The crack position relative to the various microstructures will clearly influence the strength mismatch effect. In this paper the influence of crack tip positioning on the fracture performance of strength mismatched steel welds has been studied both experimentally and by FEM analysis. For a mismatched weld with local brittle zones small changes in crack tip location can give considerable changes in the fracture performance of a CTOD specimen. A high degree of strength mismatch increases the effect of crack positioning. Weld metal overmatch increases the stress level in the heat affected zone due to material constraint and thereby reduces the cleavage fracture resistance of the weldment when the coarse grained HAZ (CGHAZ) controls the fracture. The detrimental effect of high overmatch is most pronounced for specimens with notch position at fusion line and a short distance into the brittle CGHAZ. The Weibull stress has been shown to be a suitable fracture parameter in the case where one microstructure clearly controls the cleavage fracture and the calculation of the Weibull stress therefore can be limited to this zone.     

Effects of weld strength undermatch on fracture toughness of HAZ notched weldments in a HSLA steel

In the present work the effects of weld strength undermatch on fracture toughness of heat affected zone (HAZ) have been studied. In the investigation a high strength low alloyed steel (HSLA) with 800 MPa strength class was used, and the undermatched welded joints were made with two weld strength mismatch levels. Three-point bending test specimens with crack depth to specimen width ratio a/W ranging from 0.05 to 0.5 were extracted from the welded joints. The test results show that strength mismatching gives an obvious influence on the fracture toughness of coarse grained HAZ for the undermatched joints. The lower the weld strength mismatching, the higher the fracture toughness of the HAZ. In addition the tendency of fracture toughness change with crack depths is much the same as in previous studies on base metals or weld metals, that is, fracture toughness of the HAZ is increased with reduction of crack depths. From the measured results it shows that the macroscopically mechanical heterogeneity of the welds may have more important influence on the fracture toughness of the HAZ than the meso-heterogeneity in the reheated coarse grained HAZ. Furthermore, numerical verification indicates that the stress triaxiality at crack tip may be the essential reason for the change of fracture toughness of HAZ. It is also shown that the yield strength of HAZ determined by the limit load in the three-point bend test represents the combinative effects of HAZ and its surrounding materials. This revised version was published online in August 2006 with corrections to the Cover Date.     

Abnormal manifestation of the high-temperature degradation of the weld metal of a low-alloy steel welded joint

We have shown that hardness, impact toughness, mechanical properties in tension, and the local parameters of fracture mechanics (static and cyclic crack resistance) are sensitive to the operating degradation of weld metal of steam pipelines of thermal power plants made of 15Kh1M1F steel. The simultaneous decrease in the resistance to brittle and plastic fracture (hardness, strength, and impact toughness) represents a phenomenon of the operating degradation of weld metal. We have established a specific correlation between the characteristics of plasticity and other mechanical parameters of operated metal: the increase in °₅ of operated weld metal is in good agreement with the decrease in its strength, whereas the reduction of Ψ correlates with the lowering of resistance to brittle fracture. Electrolytic hydrogenation decreases the characteristics of strength and plasticity of operated weld metal much stronger than in the initial state. The absence of ferritic edgings on the boundaries of primary austenitic grains makes for a low resistance to brittle fracture, and the change in acicular ferrite deteriorates the mechanical properties. The ductile fracture of nonoperated metal is replaced by brittle intercrystalline failure in operated metal. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 73–79, January–February, 2007.     

Out-of-plane constraint effect on local fracture resistance of a dissimilar metal welded joint

An experimental investigation on effect and mechanism of out-of-plane constraint induced by specimen thickness on local fracture resistance of two cracks (A508 heat-affected-zone (HAZ) crack and A508/Alloy52Mb interface crack) located at the weakest region in an Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel in nuclear power plants has been carried out. The results show with increasing out-of-plane constraint (specimen thickness), the fracture mechanism of the two cracks changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture, and the corresponding crack growth resistance decreases. The crack growth path in the specimens with different out-of-plane constraints deviates to low-strength material side, and is mainly controlled by local strength mismatch. For accurate and reliable safety design and failure assessment of the DMWJ structures, it needs to consider the constraint effect on local fracture resistance. The new safety design and failure assessment methods incorporating both in-plane and out-of-plane constraint effects need to be developed for the DMWJ structures.     

Microstructure-fracture toughness correlation in weld joints of Cr-Mo steel

The strength-toughness-microstructure relationship in relation to the micromechanics of a fracture process has been investigated in the weld joints of two alloys: 0.5 Mo and 2.25 Cr-1 Mo steels. These alloys are extensively used to fabricate super-heater tubes, boilers, piping, gas lines, etc., by welding. The applications require high temperature and pressure to be maintained during service. The crack initiation toughness and tearing resistance were evaluated using crack tip opening displacement/J-integral parameters at different temperatures. Quantitative analysis of micro-structure and fracture surfaces was used to study the micromechanics of fracture process in the heat-affected zone (HAZ) of the alloys. Molybdenum steel exhibited a higher percentage of ferrite and lower martensite content, while the other steel showed aligned carbide as the major constituent. The higher hardness and strength values in the HAZ and welding zone (WZ) of Cr-Mo steel, compared to molybdenum steel, may be attributed to the higher amount of martensite phase in the alloy. The higher initiation toughness at 200° C in both the alloys was reflected in the larger dimple size, compared to the size observed at room temperature. A tendency for void sheet formation was noticed in both alloys. Acicular ferrite and martensite appeared to be the most influential constituents affecting tearing resistance and initiation toughness.     

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.     

An experimental investigation of in-plane constraint effect on local fracture resistance of a dissimilar metal welded joint

In this paper, an experimental investigation on effect and mechanism of in-plane constraint induced by crack depth on local fracture resistance of two cracks (A508 heat-affected-zone (HAZ) crack and A508/Alloy52Mb interface crack) located at the weakest region in an Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel in nuclear power plants has been carried out. The results show that the local fracture resistance of the two cracks is sensitive to in-plane constraint. With increasing in-plane constraint (crack depth a/W), the fracture mechanism of the two cracks changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture, and the corresponding crack growth resistance decreases. The crack growth path in the specimens with different in-plane constraints deviates to low-strength material side, and is mainly controlled by local strength mismatch, rather than toughness mismatch. For accurate and reliable safety design and failure assessment of the DMWJ structures, it needs to consider the effects of in-plane constraint on fracture mechanism and local fracture resistance. The new safety design and failure assessment methods incorporating constraint effect need to be developed for the DMWJ structures.     

Ductile tearing assessment of high-strength steel X-joints under in-plane bending

This study reports an experimental investigation of a fatigue-cracked, pre-notched circular hollow section X-joints fabricated from high strength steels (with the yield strength higher than 800 MPa) subjected to brace in-plane bending. The circular hollow section X-joint entails a prefabricated V-notch near the weld toe at the crown position. The experimental procedure applies a fatigue pre-cracking cyclic load followed by a monotonic brace in-plane bending, which leads to brittle through-thickness crack propagation after some amount of ductile tearing. The ductile tearing assessment, integrating the fracture resistance curve obtained from the small-scale fracture specimens and the crack extension in the large-scale tubular joint, predicts closely the load level at which unstable crack extension takes place. The generic level 2A curve outlined in the BS7910 provides an un-conservative estimate on the failure load of the X-joint specimen. The parametric numerical investigation reveals that the strength definition for the cracked joints imposes a significant effect on the shape of the failure assessment curve.     

Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel

The purpose of this study was to evaluate microstructural and mechanical change of DP780 steel after tungsten inert gas (TIG) welding and the influence of notch locations on the fatigue crack growth (FCG) behavior. The tempering of martensite in the sub-critical heat affected zone (HAZ) resulted in a lower hardness (~ 220 HV) compared to the base material (~ 270 HV), failure was found to originate in the soft HAZ during tensile test. The fusion zone (FZ) consisted of martensite and some acicular ferrite. The joint showed a superior tensile strength with a joint efficiency of 94.6%. The crack growth path of HAZ gradually deviated towards BM due to the asymmetrical plastic zone at the crack tip. The FCG rate of the crack transverse to the weld was fluctuant. The Paris model can describe the FCG rate of homogeneous material rather well, but it cannot precisely represent the FCG rate of heterogeneous material. The fatigue fracture surface showed that the stable expanding region was mainly characterized by typical fatigue striations in conjunction with secondary cracks; the rapid expanding region contained quasi-cleavage morphology and dimples. However, ductile fracture mechanism predominated with an increasing stress intensity factor range (ΔK). The final unstable failure fractograph was subtotal dimples.     

Dissimilar Resistance Spot Welding of Q&P and TWIP Steel Sheets

Dissimilar resistance spot welding of twinning induced plasticity (TWIP) and quenching and partitioning (Q&P) steel grades has been investigated by evaluating the effects of clamping force, welding current, and welding time on the microstructure, shear tension strength, and fracture of welded samples. The spot welding of TWIP and Q&P steels promotes the occurrence of an asymmetrical weld nugget with a greater dilution of TWIP steel because of its lower melting temperature and thermal conductivity. As a result, weld nuggets exhibit an austenitic microstructure. TWIP steel undergoes a grain coarsening in the HAZ, whereas Q&P steel undergoes some phase transformations. Welded samples tend to exhibit higher shear tension strength as they are joined at the highest welding current, even though an improper clamping force can promote excessive metal expulsion, thereby reducing the mechanical strength of the welded joints. Shear tension welded samples failed through interfacial fracture with partial thickness fracture mode for a low welding current, while partial thickness with button pull fractures were observed when a high welding current was used. The weld spots predominantly failed at the TWIP side. However, as TWIP steel can work harden significantly in the more resistant welded joints, the failures occur, instead, at the Q&P side.     

Role of butter layer in low-cycle fatigue behavior of modified 9Cr and CrMoV dissimilar rotor welded joint

The present work aims at studying the role of butter layer (BL) in low-cycle fatigue (LCF) behavior of modified 9Cr steel and CrMoV steel dissimilar welded joint. The significant difference of the chemical composition of base metals (BMs) makes it a challenge to achieve sound welded joint. Therefore, buttering was considered to obtain a transition layer between the dissimilar steels. The LCF tests of two kinds of specimens without and with butter layer were performed applying strain-controlled cyclic load with different axial strain amplitudes. The test results indicated that the number of cycles at higher strain amplitudes of welded joint without butter layer was greatly higher than that of the joint with butter layer, while the fatigue lifetime to crack initiation (2N_f) became closer to each other at low and middle strain amplitudes. The failure was in the tempered heat affected zone (HAZ) at the CrMoV side for specimens without BL, while the fracture occurred at the tempered HAZ in the BL for specimens with BL. The microstructure details of BM, BL, HAZ and weld metals (WMs) were revealed by optical microscopy (OM). It was found that the tempered martensite was major microstructure for welded joint and much more carbides were observed in tempered HAZ than other parts due to the repeated tempering. Microhardness test indicated a softest zone existing tempered HAZ of BL and also there was a softer zone in tempered HAZ at the CrMoV side due to repeated tempering during welding and post weld heat treatment (PWHT). And scanning electron microscopy (SEM) was applied to observe the fractography. It was indicated that the fatigue crack initiation occurred from the specimen surface and all specimens were ductile–brittle mixed fractures. It is deemed that the softening behavior in BL caused by twice tempering correspondingly decreased the LCF lifetime at higher strain amplitudes. So suitable welding parameters and heat treatment processes became a key measure to ensure LCF property without losing other properties for welded joint with BL.     

Failure mechanisms in rephosphorised sheet steel spotwelds

Abstract

The influence of phosphorus and sulphur on the microstructures and cross-tension strength of spotwelds in sheet steels has been studied. In steels containing 0·06%C and 0·33%Mn (wt-%), the addition of phosphorus had two effects on the microstructure. An addition of 0·10%P increased the hardenability, resulting in a martensitic weld and heat affected zone. A further increase to 0.15%P resulted in the stabilisation of ferrite in the weld and fusion zones. The ferrite along grain boundaries in the fusion zone aided the initiation of cleavage and decreased the cross-tension strength. The addition of sulphur to a steel containing 0·15%P decreased the cross-tension strength. This effect was associated with a change from ductile to brittle behaviour at the interface between the sheets in the heat affected zone, caused by increased wetting of the interface and grain boundaries by oxysulphides as the sulphur content was increased.

MST/920     

Evaluation of temperature‐sensitive fatigue crack propagation of a high‐speed railway wheel rim material

The fatigue crack growth rate (FCGR) of ER8C high‐speed railway wheel rim material was tested at various service temperatures. The temperature sensitivity of fatigue crack propagation was evaluated, and the effect of temperature on the crack propagation mechanism was analyzed. The obtained results indicate a fatigue ductile‐to‐brittle transition (FDBT) point at ?20°C for the ER8C wheel rim materials. A reverse relationship was found between FCGR and temperature for the near threshold and Paris regimes when the temperature was below the FDBT point. However, no evident changing rule was found when the temperature was above this transition point. An evident fatigue crack propagation mode transition was found from lamellar tearing to intergranular cracks, which was related to the FDBT for the near‐threshold regime.     

Zur Zähigkeit von Vergütungsstählen

On Toughness of Quenched and Tempered Steels Toughness as consumed fracture energy is dependent on fracture mechanism. Grain size and loading conditions influence the transition from ductile dimple fracture to brittle cleavage fracture. In quenched and tempered steels packet size and particle distribution are of importance as well as brittle intergranular fracture modes by grain boundary segregation of impurities in ferrite (temper embrittlement) or precipitates in austenite. Anisotropy of toughness arises from banded structures.     

Effect of aging on high temperature brittle intergranular fracture in austenitic stainless steels

Abstract

The present paper describes the effect of aging on crack growth at 550–750°C in a series of 316 and 347 based stainless steels. Crack initiation parameters and crack growth rates have been measured, and detailed fractography and microstructural characterisation carried out. The study shows that the high temperature brittle intergranular fracture mechanism operates in these alloys, as expected from incidences of cracking in austenitic stainless steels used in power plant. High temperature brittle intergranular fracture leads to lower crack tip opening displacements at initiation, and slightly higher crack growth rates than ductile intergranular failure. Susceptibility to high temperature brittle intergranular fracture is enhanced by aging. This increased susceptibility is explained in terms of the increased hardness, the reduction in dissolved C, and grain boundary precipitation. The effects of temperature, composition, and loading mode on the behaviour of the aged alloys are determined.

MST/3100     

基于断裂力学的高强度钢材梁柱节点受力性能分析

为了研究国产Q460C高强度结构钢材梁柱节点的断裂行为,该文基于断裂力学理论,计算了Q460C高强度钢材焊缝及热影响区材料的断裂韧性,并且采用三维有限元断裂模型,以I型裂纹尖端应力强度因子K_I和J积分为定量的评价指标,分析了焊缝及热影响区不同长度的裂纹对梁柱节点断裂韧性的需求。弹性分析表明,K_I沿梁翼缘宽度方向呈W形分布,最大值出现在翼缘中心,且与名义弯曲应力呈线性关系,而焊根裂纹的断裂韧性需求比热影响区裂纹更高。弹塑性分析表明,J_I最大值出现在翼缘的边缘,热影响区裂纹的断裂韧性需求比焊根裂纹更高。研究结果表明,Q460C高强度钢材梁柱节点的断裂由焊根裂纹控制,断裂承载力与梁全截面塑性承载力相近,临界转角小于0.02rad,因此建议通过改善焊接工艺或局部构造来保证节点拥有足够的转动能力。     

Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment

Local brittle zones (LBZ) cause the unstable fracture behaviour of weld metals. This threatens the safe service of welded structures and makes structural assessment procedures difficult. Therefore, the unstable fracture behaviour of an overmatched high strength low alloyed steel weldment was experimentally investigated. It showed that any interaction between two adjacent weld metal matrix and soft weld metal inclusions produces LBZ, causing local unstable fracture behaviour. The formation of a low hardness region is attributed to the multipass welding reheating process between Ac₁ and the self-tempering temperature. The presence of partly solid metallic inclusions with a high content of alloying elements and pro-eutectoid ferrite microstructure were found to be additional causes for the local unstable fracture behaviour of the weld metal. Local strength mis-match induced the yielding and strain hardening in the soft weld metal inclusions, contributing significantly to unstable fracture behaviour. Thus, a significantly different scatter of experimental results can be obtained. In the cases of specimens with through-the-thickness crack, not only is the scatter significantly lower, but the toughness itself.