The effect of microstructures on fatigue crack growth in Q345 steel welded joint

It is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R‐ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R‐ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R‐ratio increasing. To the low R‐ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R‐ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse‐grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack‐tip stress shielding and then resulted in higher crack growth resistance.     

Fracture toughness of a welded duplex stainless steel

The present work includes fracture toughness testing on 30 and 50 mm thick duplex stainless steel 2205 (22% Cr, 5.5% Ni, 3% Mo, 0.15% N). Base metal and submerged arc weldments (SAW) at subzero temperatures have been tested using full size three point bending. The evaluation of the results has been carried out using J-integral calculations and correlations to Charpy impact values. It is shown that the temperature dependence of the fracture toughness can be described by a transition temperature curve, the master curve. The reference temperature was evaluated to −143 and −101 °C for base and weld material respectively.     

Joining phenomena and tensile strength of friction welded joint between pure titanium and low carbon steel

This paper describes the effect of the friction welding condition on the joining phenomena and the tensile strength of friction welded joint between pure titanium (P-Ti) and low carbon steel (LCS). The adjacent region of the weld interface at the P-Ti side was intensely upsetting with accompanied large deformation of itself when the joint had sparkle at both applied friction pressures of 30 and 90 MPa, although that of the LCS side was hardly upset. The temperature of the whole weld interface at a friction pressure of 30 MPa reached to 1150 K or over at a friction time of 3.0 s or longer. However, the half radius and centreline portion temperatures of the weld interface at a friction pressure of 90 MPa was not reached to 1150 K, although the periphery portion of that was reached to its temperature. The central portion of the weld interface at a friction pressure of 90 MPa was deformed to a convex shape from the viewpoint of the P-Ti side, although that of 30 MPa remained almost flat after when the friction torque reached the initial peak. When the joint was made at a friction pressure of 30 MPa, a friction time of 3.0 s or longer, and a forge pressure of 270 MPa or higher, it achieved 100% joint efficiency and the P-Ti base metal fracture with no crack at the weld interface. However, many joints at friction times of 1.2 and 1.5 s fractured at the weld interface, although those achieved 100% joint efficiency, because whole weld interface temperature was below 1150 K. On the other hand, many joints at a friction pressure of 90 MPa with high forge pressure also fractured at the weld interface, although those achieved 100% joint efficiency, because the weld interface temperature at the half radius and periphery portions was below 1150 K. Those joints did not have the intermetallic compound layer at the weld interface. The difference of the fractured portion of the joint in both applied friction pressures was due to the difference between the maximum temperature at the weld interface during the friction process and the deformation amount of the LCS side caused by applied forge pressure. To obtain 100% joint efficiency with the P-Ti base metal fracture with no crack at the weld interface, the joint should be made with high forge pressure, low friction pressure, and with opportune friction time at which the temperature at whole weld interface reached around 1150 K.     

Effects of strength mis-matching on the fracture behavior of nuclear pressure steel A508-III welded joint

In this paper, according to the nuclear pressure steel A508-III, the effect of strength mis-matching on the fracture behavior was analyzed by fracture mechanics test and the crack tip stress field of three-point bend specimen was analyzed by using finite element analysis method (FEM). The fracture of heat-affected zone (HAZ) was emphasized especially. The results of FEM show that if the under-matching weld was used, the opening stress and stress triaxiality in the vicinity of crack tip would increase for weld-crack specimen, and would reduce for HAZ-crack specimen. This tendency was confirmed by the test results.     

高强度热冲压钢板强韧性工艺优化研究

为改善强韧性,本文基于热冲压高强度钢板强度、塑性和韧性指标,选取加热温度、保温时间和开始淬火温度为设计因子,引入Kahn试验获得高强度热冲压硼钢撕裂强度和单位面积裂纹形核功来表征材料断裂韧性,进行多指标综合评分的L9(34)正交试验设计,以研究不同淬火工艺参数对热冲压高强钢强韧性的影响规律.结果表明:在加热温度为920~950℃、保温时间1 min、开始淬火温度为650~700℃条件下,热冲压硼钢SPFH具有优良的成形性能和强韧化指标.采用优化后工艺进行典型车身结构件热冲压试验,其撕裂强度、单位面积裂纹形核功和强韧比分别提升10.91%、20.32%和22.17%,在保证强度的基础上韧性得到了大幅度提高.     

The influence of reaction layer on the strength of aluminum/steel joint welded by resistance spot welding

We investigated the influence of interfacial reaction layer on the strength of resistance spot welded aluminum/steel joint by analyzing fracture surface and observing interfacial structure. The results revealed that the interfacial strength was approximately 6.509 MPa in the case that the reaction layer thickness exceeds 1.5 µm.     

Thickness effects on the fatigue strength of welded steel cruciforms

Over 100 fatigue tests were conducted on high strength welded steel (HSLA-80) cruciforms of different thickness. Tests were conducted under both constant and random amplitude axial loads to characterize thickness effects on fatigue strength. Specimens were similar in size, except for the thickness which was varied between four nominal values. Examination of both experimental and analytical results (obtained using linear cumulative damage and Rayleigh approximation) indicates thicker specimens exhibit lower fatigue lives under both constant and random amplitude loadings. These results, when compared with the commonly used ‘fourth root rule' thickness correction formula, indicate the latter to be generally conservative, particularly at low stress levels.     

X90钢级管线钢预精焊接头的性能研究

为综合研究X90管线钢的焊接性,选用国内某钢厂轧制的X90管线钢卷板,利用预精焊工艺制备试验钢管4根,采用金相分析、扫描电镜(SEM)断口分析、夏比V型缺口冲击试验、拉伸、弯曲、硬度等试验,研究了焊接接头各个区域的组织和性能.试验结果表明:内外焊缝区组织均为针状铁素体,热影响区(HAZ)粗晶区晶粒粗化严重,主要组织为粒状贝氏体和贝氏体铁素体,在原奥氏体晶界和贝氏体板条内部存在块状或条状的(马氏体-奥氏体)M-A组元;HAZ冲击功离散性较大,出现了单值较低(45 J)的试样,SEM断口分析呈现典型的解理断裂特征;焊接接头抗拉强度805~815 MPa,断裂位置均在HAZ;焊接接头反弯试样易在HAZ出现裂纹和脆断现象;HAZ硬度在220~250 HV之间,较母材下降30 HV左右.HAZ是X90预精焊钢管焊接接头的薄弱环节,为提高X90管线钢的焊接稳定性,应重点研究精焊内外热循环双热影响亚区的组织转变和脆化机理.     

Characterization of weld strength and impact toughness in the multi-pass welding of super-duplex stainless steel UNS 32750

This paper investigates the weldability, metallurgical and mechanical properties of the UNS 32750 super-duplex stainless steels joints by Gas Tungsten Arc Welding (GTAW) employing ER2553 and ERNiCrMo-4 filler metals. Impact and tensile studies envisaged that the weldments employing ER2553 exhibited superior mechanical properties compared to ERNiCrMo-4 weldments. Microstructure studies performed using optical and SEM analysis clearly exhibited the different forms of austenite including widmanstatten austenite on the weld zone employing ER2553 filler. Also the presented results clearly reported the effect of filler metals on strength and toughness during the multi-pass welding. This research article addressed the improvement of tensile and impact strength using appropriate filler wire without obtaining any deleterious phases.     

Experimental investigations for mechanical joint strength following ultrasonically welded pin osteosynthesis

The purpose of this study was to determine whether fixation of cranial bone segments using ultrasonically welded pin osteosynthesis showed differences in mechanical stability as compared to fixation of cranial bone segments using screw osteosynthesis. Right and left cranial bone segments from each of 16 young sheep were obtained by craniotomy and re-fixed: on the right with a mesh plate and pins, and on the left with a mesh plate and screws. All osteosynthesis materials consisted of PDLLA, fully amorphous polyactid. A total of 167 cranial bone / mesh plate segments from 16 animals were investigated; 84 segments were pin-fixed and 83 segments were screw-fixed. The implantation time of the re-fixed segments ranged from 1 day to 196 days. The mechanical methods chosen for simulation of stress on the bone segment bonds were two bending tests (horizontal and vertical directions) and a tensile test. The values obtained in the mechanical tests indicate differences in the bond strength between the pin- and screw- fixation methods over the length of in vivo implantation time. The mechanical stability of the ultrasonically welded pin osteosynthesis bonds over the screw osteosynthesis bonds proved to be statistically significant. The implication of these findings should also be relevant in the field of medicine.     

Characterization of fracture toughness of high strength low alloy steel weldments using stretch zone width measurements

Most of the industrial applications involving use of high-strength low-alloy steels require good weldability. Thus it is important to characterize the properties of the welded steels, especially the heat affected zone. Attempts have been made to characterize the fracture toughness of the HAZ by the use of theJ-integral and the crack opening displacement. In the present study, the effect of addition of titanium, vanadium and niobium, as well as combinations of these, on the fracture toughness of the heat affected zone of welded steel plates is examined. Six compositions were used in this study. Three-point bending specimens as well as tensile specimens were prepared. The fracture surfaces were examined in a scanning electron microscope to determine the fracture mode as well as the extent of the stretch zone as the crack blunts. Calculation of the fracture toughness parameter,J _lc, is carried out through a quantitative stereofractographic analysis of the stretch zone at the crack tip. The results show that there is a marked increase inJ _lc due to the addition of the various alloying elements. Generally, the addition of niobium and titanium alone produce the highestJ _lc due to the extent of grain refinement that these elements produce.     

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K_IC. To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.     

不锈钢激光-MAG复合焊接头微区性能研究

激光复合焊可用于不锈钢焊接,但激光复合焊接头热影响区小,组织变化梯度大.研究接头微区性能可以确定接头薄弱环节,为焊接接头的工艺评定和断裂分析提供理论依据.为此,本文对4 mm厚SUS301L-HT不锈钢进行激光-MAG复合焊接,采用维氏硬度、微型剪切和微拉伸等试验,研究了焊接接头焊缝、热影响区及母材的微区力学性能,并结合金相、断口扫描等分析了各微区力学性能的差异.结果表明：焊缝区域组织主要为柱状奥氏体树枝晶+少量的δ铁素体;母材的剪切强度和抗拉强度最高,分别为560和1 066 MPa,其次为复合焊接头热影响区,焊缝区域最差,接头硬度分布规律与各微区强度变化趋势一致;运用数学方法,得出了接头微拉伸强度与微型剪切强度、硬度之间关系的经验公式.接头各微区剪切断口和拉伸断口SEM分析呈现典型的韧性断裂特征.     

Influence of crack depth and strength matching on deformation and plastic zone for welded joint specimen

By using the 2-dimensional elasto-plastic finite element method, the effects of crack depth and strength matching properties of welds on the deformation and plastic zone have been studied for three point bend specimens. The results indicate that, in the loading process of a welded joint specimen, the influence of crack depth and strength matching properties on the deformation parameters such as loading point displacement, crack mouth opening displacement, plastic rotational factor, and development of plastic zone is evident. The boundary between shallow crack and deep crack specimens is also influenced by the strength matching of welded joints. Moreover, the crack depth and strength matching of the welded joints have an important effect on the fracture-resistant capability of the specimens. The fracture-resistant capabilities of shallow crack specimens and overmatched joint specimens are better than those of deep crack specimens and undermatched joint specimens, respectively.     

Weld metal impact toughness of electron beam welded 9% Ni steel

9% Ni steel was electron beam welded and the impact toughness of its weld metal was investigated. The improvement of weld metal toughness was studied by an increase of Ni content in the weld metal and its mechanism was discussed. The weld metal impact toughness was as low as 25–55 J at –196°C. However, it increased remarkably by an increase of Ni and showed 118 J at the same temperature when Ni content was 16.6%. As Ni lowers the martensite start temperature, martensite formed in weld metal with a high Ni content has more chance of auto-tempering because of its slow cooling rate at a low temperature range. Therefore, the improvement is believed to be due to the formation of auto-tempered martensite and retained austenite during the cooling cycle after welding     

Influence of silicon on strength and toughness of 5 wt-%Cr secondary hardening steel

Abstract

The effects of manganese and silicon on the mechanical properties of the 5 wt-%Cr secondary hardening steel H-11 have been investigated. This steel normally contains about 0·5 wt-%Mn and 1wt-%Si. Two other steels were also considered, both identical to H-11, except that one contained no manganese and the other neither manganese nor silicon. The room temperature hardness and impact toughness were determined for the three steels for tempering temperatures ranging from 200 to 600°C. The results indicate that manganese has no influence on the tempering response or toughness of H-11, but that silicon significantly influences both.

MST/506     

Modelling of ductile fracture initiation in strength mismatched welded joint

In this paper, the effect of strength mismatch and width of the welded joints on the stress–strain distribution in the crack tip region has been discussed. The single-edge notched bend (SENB) specimens (precrack length a₀/W = 0.32) were experimentally and numerically analysed. The model of local approach to fracture, proposed by Gurson, Tvergaard and Needleman, was used. High-strength low-alloyed (HSLA) steel was used as a base metal in quenched and tempered condition. The flux-cored arc-welding process in shielding gas was used. Two different fillers were selected to make over- and undermatched weld metal. The experimental analysis of fracture behaviour of the over- and undermatched welded joints was followed by numerical computations of void volume fraction in front of the crack tip. The critical void volume fraction, f_c, used in prediction of the crack growth initiation on the SENB specimen had been previously determined on a round smooth specimen. Three widths of weld metal were considered: 6, 12 and 18 mm. A comparison of the crack tip opening displacement (CTOD) values corresponding to crack initiation in the SENB specimens is given, as determined both experimentally and using the GTN model.     

Effect of galvanizing on the strength of welded joints between steel and aluminum

The effect of aluminum additions on the properties of zinc coatings produced on steel by electroplating or hot dipping and the influence of such coatings on the strength of welded joints between thin steel and aluminum sheet were studied.