奥氏体不锈钢激光焊接过程中残留液体金属的在线观察

激光焊接时较快的冷却速度,有可能促进凝固热裂纹的生成,而凝固热裂纹的生成与凝固温度区间内的残留液体金属行为直接相关.采用由高速摄像机和光学显微镜头组成的摄像装置,针对SUS304,SUS316,SUS310S等3种奥氏体不锈钢激光焊接熔池后端残留液体金属的凝固行为进行高速高倍在线观察,对不同焊接速度下的熔池凝固行为进行了分析,并对3种材料凝固前沿残留液体金属的存在范围进行了量化比较.结果表明,在线观察法可辨别的残留液体金属是固液相共存区间的一部分,在线观察得到的残留液体金属的存在范围与热裂纹试验获得的热裂纹敏感性有较好的对应关系.     

Prediction of solidification cracking in laser welds of type 310 stainless steels

The occurrence of solidification cracks in laser welds of type 310 stainless steels was predicted by numerical analyses of the solidification brittle range (ductility curve for cracking) and thermal strain in the weld metal. The solidification brittle range in laser welding was estimated from that in arc welding based on the numerical analyses of supercooling (for calculating dendrite tip temperature) and segregation (for calculating completely solidified temperature) during rapid solidification. The calculated solidification brittle range was reduced with an increase in the welding speed because of the enhanced supercooling and the inhibited solidification segregation. The thermal strain analysis by FEM suggested that solidification cracks would occur in SUS310S welds at laser travelling velocity of 60 mm/s applying the initial strain of 1.5%, while no solidification cracks in SUS310EHP welds at any laser travelling velocities applying the higher initial strain of 2.2%. The cantilever type cracking test in laser welding revealed that the predicted results of occurrence of solidification cracks were consistent with experimental ones.     

The effect of solidification behaviour of austenitic stainless steel on the solidification cracking susceptibility

0Introduction Oneofthemainproblemsinweldingausteniticstain lesssteelsishotcracking[1,2].Inweldingofsingle phase austeniticstainlesssteels,thetendencyofhotcrackingis moreserious[3].Inordertopreventhotcrackingofthis kindofmaterial,itwasattemptedtogaintwo p…     

Criteria for hot cracking evaluation in austenitic stainless steel welds using longitudinal varestraint and transvarestraint tests

Abstract

The longitudinal varestraint test (LVT) and transvarestraint test (TVT) are widely used for assessment of weld metal cracking susceptibility. The TVT is preferred over the LVT for study of weld metal cracking. However, few reports exist that discuss the relative merits of the two tests for evaluating cracking susceptibility. This investigation was carried out to compare weldability assessments using the two tests and the relevant criteria for weldability evaluation. Several stainless steels solidifying in the austenitic and ferritic solidification modes were tested. The study shows that the LVT can be used for evaluation of fusion zone cracking through a maximum cracking distance criterion. This parameter correlated well with the maximum crack length in the TVT, traditionally used to derive the brittleness temperature range (BTR). The study further indicates that the total crack length can be related to the BTR by considering the area density of cracking.     

Interface shape and microstructure controlled dissimilar friction stir lap welded steels

Abstract

When fusion welding is conducted on the dissimilar materials between a reduced activation ferritic/martensitic steel F82H steel and an austenite stainless steel SUS 316 steel, δ ferrite is generally formed and inevitably deteriorates the weld properties. In this study, dissimilar welding of F82H to SUS 316 steel was successfully achieved by friction stir lap welding technique. It revealed that the shape and microstructure of the joint interface can be controlled by controlling the welding temperature, in another word, by changing the applied load. By controlling the welding temperature at ～710°C, a sound dissimilar joint can be obtained with a smooth joint interface and no mixed microstructure, despite the relative overlapping position of the steel plates. All the dissimilar joints showed high shear tensile strength and fracture in the base metal of F82H steel plate, which has lower strength than the SUS 316 steel plate at room temperature.     

Experimental and numerical analysis of solidification cracking behaviour in fibre laser welding of 6013 aluminium alloy

Abstract

Experimental observation and numerical modelling were employed to investigate the solidification cracking behaviour during fibre laser welding of 6013 aluminium alloy. The solidification cracking initiation location and propagation path were studied using a high speed camera system and via metallurgical analysis. A three-dimensional thermomechanical finite element model of fibre laser welding of aluminium alloys was developed, which considered cylindrical volumetric heat source, temperature dependent material properties, solidification shrinkage and stress relaxation in the weld molten pool. The transient evolution and distribution of mechanical strain in the brittle temperature range (BTR) were analysed in detail to find the factors which drove the crack initiation and propagation. The results showed that the solidification cracking initiated near the fusion line and then propagated along the centreline of the weld, which was the result of the strain distribution characteristic in BTR.     

激光熔覆原位合成Mo2NiB2涂层工艺研究

目的选择和优化激光熔覆工艺参数,以制备三元硼化物金属基陶瓷Mo_2NiB_2涂层。方法通过激光熔覆原位合成法在碳钢表面制备了以Mo_2NiB_2为增强相的涂层,并采用金相显微镜、扫描电镜(SEM)及X射线衍射(XRD)对涂层组织进行分析。利用ANSYS计算熔覆过程的温度场,进而计算涂层凝固特征参数,即凝固形状控制因子。结果微观组织分析表明,在激光功率为2500 W、扫描速度为1.5 mm/s、预置厚度为1 mm时,可获得细密、均匀的组织,涂层中白色部分为生成的Mo_2NiB_2,灰色部分为Fe、Ni固溶体。涂层与基体结合部位以平面晶形式生长,然后以树枝晶的方式远离界面生长。温度场及凝固特征参数的计算表明,温度梯度达105℃/m数量级,形状控制因子达109(℃·s)/m~2数量级。结论激光功率的增加会使涂层中的枝晶组织趋于细密,形状因子K值大幅增大。凝固形状控制因子K为3×10~9~5×10~9(℃·s)/m~2时,凝固组织为平面晶,表现为"白亮带";K为7×10~9(℃·s)/m~2以上时,凝固组织为树枝晶;K为13×10~9(℃·s)/m~2时,树枝晶晶粒出现明显的细化现象。     

Repair welding of ductile cast iron by laser cladding process: microstructure and mechanical properties

Abstract

The aim of this research was to determine the feasibility of a newly developed process in the repair of cracked gas turbine casings made of ductile cast iron. This study investigated the microstructural characteristics, metallurgy and mechanical properties of the repair weldments produced using fibre laser cladding. Optical microscopy, scanning electron microscopy and element probe microanalysis were used to investigate the microstructure at the cladding weld interface. The mechanical properties of the cladded specimens were evaluated after laser cladding. Our results revealed that the weldability of ductile cast iron can be enhanced by performing laser surface pretreatment to sublimate graphite nodules. Microhardness at the interface of the laser cladded weldments depended largely on the range of the heat affected zone and the degree of phase complexity. Under tensile loading, failures were limited to the base metal region of the weldments. Test results demonstrate that the impact toughness of the interface between the fusion zone and the base metal can be enhanced through the application of post-cladding heat treatment.     

Evaluation of properties in laser welds of A6061‐T6 aluminium alloy

In order to clarify the effect of tip velocity on the weld solidification process of hot-work tool steel (SKD61) during welding, information about microstructure evolution was obtained by the combination of a liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. From the experimental results, it was found that the solidification mode was changed from FA mode (L → L+δ → L+δ+γ → L+γ → γ) to A mode (L → L+γ → γ) at high tip velocity. Moreover, the effect of tip velocity on the microstructure selection during solidification between the primary δ, ferrite and the primary γ, austenite was theoretically proven by the Kurz, Giovanola and Trivedi model. Therefore, it was understood that the solidification cracking susceptibility of hot-work tool steel (SKD61) weld metal was increased due to the δ to γ transition of the primary phase.     

Three-dimensional turbulent model of heat transfer and fluid flow in GTAW process

A two-equation K-ε turbulent fluid flow model is built to model the heat transfer and fluid flow in gas tungsten arc welding (GTAW) process of stainless steel SUS310 and SUS316.This model combines the buoyancy force,lorentz force and marangni force as the driving forces of the fluid flow in the weld pool.The material properties are functions of temperature in this model.The simulated results show that the molten metal flowing outward is mainly caused by the marangoni convection,which makes the weld pool become wider and shallower.The comparison of the weld pool shape of SUS310 and SUS316 shows that the slight differences of the value of thermal conductivity mainly attributes to the difference of the weld pool shape and the distinction of heat transport in laminar and turbulent model makes large diversity in the simulated results.     

Weldability of 17-4PH stainless steel in overaged heat treated condition

Abstract

Studies on the weldability of 17-4PH stainless steel, in the 621°C overaged condition, showed that Cr_eq/Ni_eq ratio higher than 1·5 resulted in primary ferritic mode of solidification in the weld metal. Post-weld aging treatment at 482°C enhanced the strength of the weld joint with corresponding reduction in impact toughness of the weld metal while post-weld aging at 621°C caused marginal reduction in strength of the weld joint with significant increase in impact toughness of the weld metal.     

316L不锈钢激光快速成形过程中熔覆层的热裂机理

采用微观测试分析方法，针对316L不锈钢粉末，深入研究了激光快速成形过程中熔覆层的开裂行为及其形成机理，研究结果表明，316L不锈钢激光熔覆层裂纹多发生在树枝晶的晶界，呈现出典型的沿晶开裂特征，裂纹断面上有明显的氧化彩色，扫描电镜照片显示裂纹断面上树枝晶的方向与轮廓清晰可见，树枝晶晶界相当圆滑，表明裂纹是在高温下产生的，熔覆层中的裂纹是凝固裂纹，属于热裂纹范畴，裂纹产生的主要原因是熔覆层组织在凝固温度区间晶界处的残余液相受到熔覆层中的拉伸应力作用所导致的液膜分离的结果。     

氮对316L不锈钢焊缝凝固模式和组织的影响

采用氮含量不同的三种焊丝分别对316L奥氏体不锈钢进行了TIG焊接,通过金相显微镜和扫描电镜对其焊缝微观组织进行了观察,对比分析了焊缝的凝固模式和焊缝组织的析出行为,研究了氮对焊缝凝固模式和组织的影响.结果表明,焊缝氮含量为0.018%时,焊缝的主要凝固模式为初生相为铁素体的FA模式,δ铁素体以蠕虫状或网状分布于枝晶轴上;氮含量增加到0.088%和0.16%时,焊缝的主要凝固模式转变为初生相为奥氏体的AF模式,δ铁素体以颗粒状分布于初生奥氏体枝晶间,其数目明显减少;焊缝奥氏体组织随着氮含量的增加有明显的粗化趋势.     

Fracture toughness and microstructure of 780 MPa class high-tensile strength steel plates: improvement of steel structural performance by laser beam welding for mid-thick section steel plate

Fracture toughness and microstructure of laser weld metal of 780 MPa class steels are investigated and compared with those of SM490A and SM570Q.

In SM490A and SM570Q, Charpy energy transition temperature of laser weld metals is 60–90°C higher than that of base metal, and upper bainite microstructures are observed in the laser weld metals.

In 780 MPa class steels, difference of Charpy energy transition temperatures between laser weld metal and base metal are only 10–30°C, and no upper bainite microstructures are observed in the laser weld metals. Hardness of the laser weld metals of 780 MPa class steels is identical to that of martensite microstructure. A superior toughness of the laser weld metal of 780 MPa class steels would be owing to the martensite microstructure resulted from a high carbon equivalent.     

Evaluation of weld metal hot cracking susceptibility in superaustenitic stainless steel

Solidification cracking susceptibilities of two types of superaustenitic stainless steel, 254SMO and SR50A, were evaluated by transverse Varestraint tests. The susceptibilities were compared with those of conventional austenitic stainless steel 316L, and factors influencing the difference of susceptibility were discussed. The comparison showed that 254SMO and SR50A are more sensitive to solidification cracking than 316L. In the transverse Varestraint tests, both total and maximum crack lengths are longer in the superaustenitic stainless steel. Because of the longer maximum crack length, the superaustenitic stainless steel also has a wider brittleness temperature range of cracking than 316L: about 178 °C for the superaustenitic stainless steel and 43 °C for 316L. It is believed that straight subgrain boundaries owing to the cellular dendritic solidification and segregations of sulfur and phosphorus in the subgrain boundaries of superaustenitic stainless steel make it more sensitive to solidification cracking. In addition to the solidification cracking, reheat cracking is also observed within the previous weld bead in the superaustenitic stainless steel because of fully austenitic solidification with significant segregations. This suggests that caution should be given to the occurrence of reheat cracking when superaustenitic stainless steel is multi pass welded.     

Enrichment characteristic of carbon atoms in solid-liquid zone of high carbon steel under different directional solidification rates

To reveal the formation mechanism and main influencing factors of C-segregation in high carbon steel under different solidification rates (40, 80, 160, 200 and 320 µm·s⁻¹), the enrichment characteristics of carbon atoms in the solid-liquid zone of Fe-0.61%C steel were studied using a zone melting liquid metal cooling apparatus and electron probe microanalysis. The relationships among micro-segregation of carbon atoms, solid-liquid interface morphology and solidification rate were fully discussed. The results show that large dendrite spacing and a slow-moving dendritic interface create less resistance and more time for the migration of interdendritic carbon atoms to liquid zone. This results in the continuous enrichment of carbon atoms in liquid zone, further expands the solid-liquid temperature range, prolongs the solidification time of molten steel, and causes the formation of carbon micro-segregation at the solidification end as the solidification rate is 40 µm·s⁻¹. Conversely, abundant and elongated secondary dendrite arms with small spacing seriously impede the diffusion of interdendritic carbon-rich molten steel to liquid zone. As a result, there is only obvious dendrite segregation, but little difference in the carbon content along the solidification direction as solidification rate exceeds 200 µm·s⁻¹.

     

A method of measuring the heat generation parameters in the resistance spot welding in single phase AC machines

In the fabrication of reactors (made of Cr–Mo steel modified with vanadium) operating in high hydrogen pressure and high-temperature service, internal cladding in austenitic stainless steel (typically made of AISI 347) is required in order to guarantee satisfactory corrosion resistance against the process fluids desulphurized hydrocarbons. Application of an electro-slag strip cladding (ESSC) process for internal weld overlay and clad restoring on heads, barrels and nozzles of 2¼Cr–1Mo–¼V reactors (with single-layer or double-layer technique) is described in the paper. The first part of the paper is focused on metallurgical aspects, cracking phenomena (hydrogen cracking, solidification cracking, under-clad cracking and hydrogen-induced disbonding) and cladding residual stresses referable to ESSC process. The second part of the paper is focused on operating aspects of the ESSC process and UT examination of weld overlay and clad restoring, optimized for the detection of planar defects under cladding.     

Dependence of strength and fracture behaviour on chemical compositions in spot-welded L-type joints

In order to achieve lighter and stronger car bodies by applying high strength steel sheets, one of the key technologies is enhancement of joint strength. In this study, we investigated dependence of strength and fracture behaviour on chemical compositions of steels in spot-welded L-type joints in detail. Consequently, the following experimental results were obtained. (1) Maximum load of the joint decreased with increase of carbon (C) and phosphorous (P). The maximum load was decreased by 0.4–0.7 kN with increase of 0.1% in C, with C content ranging from 0.03 to 0.5%, and 0.5 kN with increase of 0.01% in P, with P content ranging from 0 to 0.03%. (2) Fracture portion changed from the outside to the inside of weld metal with increase of C and P. (3) The fracture path was estimated to accord with the solidification segregation site in the weld metal in the case of a steel of 0.2% C, 0.03% P. (4) By implementation of an appropriate post heat during spot welding process for the steel of 0.2% C, 0.03% P, the degree of solidification segregation was clearly reduced and the maximum load of the joints was improved by 70%.     

An analytical algorithm to predict weldability of precipitation-strengthened nickel-base superalloys

A new mathematical-solidification-weldability model has been developed to obtain the optimum chemical composition in the γ-Ti–Al–Nb precipitation-strengthened nickel-base superalloys, in which, the best resistance to hot cracking be achieved. To solve the weldability model, SEM microphotographs and EDAX analysis extracted from microstructure of the welds and phase diagrams were used as model data inputs. The relation between the segregating elements versus the retained weld liquid, and as well as, solidification path of welds (L → γ) were attained. The model is able to predict the brittleness temperature range (BTR) and the extent of weld mushy zone as a direct criterion of weldability determination. For the weld type A (alloy Inconel 718), formation of secondary phases or eutectics is too probable. In case of weld type B (alloy Udimet 500), formation of secondary phases enriched in Ti in the interdendritic locations can be anticipated. The BTRs and the size of mushy zones for welds type A, B and C (65 wt% alloy 500 + 35 wt% alloy 718) have been calculated. The solidification-weldability model showed that weld type C with dilution level of 65% and the ratio Nb/(Ti + Al) = 0.53 has the best weldability among the dilutions studied and can be offered as a new-weldable nickel-base superalloy.