Effect of tandem submerged arc welding process and parameters of Gleeble simulator thermal cycles on properties of the intercritically reheated heat affected zone

The effects of real and Gleeble simulated double pass thermal cycles on the properties of the intercritically reheated coarse grained heat affected zones in X80 microalloyed pipeline steel has been investigated. The Gleeble simulated process involved heating the X80 steel specimens to the first peak temperature of 1400 °C and then reheating to the second peak temperature of 800 °C, with different cooling rates. The size and area fraction of martensite/austenite (M/A) constituents were obtained by a combination of field emission scanning electron microscopes and image analysis software. In addition, misorientation was characterized by electron back-scatter diffraction analysis. It is clear that the intercritically thermal cycles have a significant effect on morphology of M/A constituents. The M/A constituent’s size, such as mean diameter and length, are important factors influencing Charpy impact properties of thermally simulated intercritically reheated heat affected zones. The simulated thermal cycles of the intercritically reheated region in the high heat input tandem submerged arc welding processes, showed extremely poor Charpy impact absorbed energy. The intercritical reheated thermal cycles with lower heat input value showed higher Charpy impact absorbed energy due to a decrease in the prior-austenite grain and M/A particle size.     

In situ studies of nonlinear d-spacing changes in titanium and steel alloys

We traced element-partitioning behavior by measuring d-spacing changes in situ during phase transformations of titanium and steel alloys using a hybrid in situ observation system. For titanium alloy, nonlinear changes are clearly evident during heating and cooling and are related to element partitioning and microstructural evolution. For low-carbon-low-alloy steel, nonlinear changes are evident during cooling and are related to bainite and carbon-enriched austenite formation.     

Microstructures and mechanical properties of friction stir welded dissimilar copper/brass joints

In this study, an experimental investigation has been carried out on microstructure and mechanical properties of friction stir welded copper/brass dissimilar joints. Effect of axial tool force to welding quality has been investigated under obtained optimal tool rotation rate and tool traverse speed conditions. The tool for the dissimilar copper/brass friction stir welding manufactured from X155CrMoV12–1 cold work tool steel with material number of 1.2379. The friction stir welding quality was investigated by welding surface inspections, microstructural studies, micro hardness measurements and tensile tests. The experimental studies have shown that constant axial tool force during pre‐heating and during welding process are very important. As a result, by using 2.5–3 kN of axial tool force during pre‐heating and 5.5 kN of axial tool force during welding process, copper/brass dissimilar joints with well appearance and higher mechanical strength can be obtained.     

Investigation on decomposition behavior of austenite under continuous cooling in vanadium microalloyed steel (30MSV6)

In the present study, investigations are focused on microstructural evolution and the resulting hardness during continuous cooling transformation (CCT) in a commercial vanadium microalloyed steel (30MSV6). Furthermore, the effects of cooling rate and austenite grain size (AGS) on CCT behavior of the steel have been studied by employing high-resolution dilatometry. Quantitative metallography accompanied with scanning electron microscopy (SEM) has efficiently confirmed the dilatometric measurements of transformation kinetics and austenite decomposition products. A semi-empirical model has been proposed for prediction of microstructural development during austenite decomposition of the steel and the resultant hardness. The model consists of 8 sub-models including ferrite transformation start temperature, ferrite growth, pearlite start temperature, pearlite growth, bainite start temperature, bainite growth, martensite start temperature and hardness. The transformed fractions of ferrite, pearlite and bainite have been described using semi-empirical Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach in combination with Scheil's equation of additivity. The JMAK rate parameter for bainite has been formulated using a diffusion-controlled model. Predictions of the proposed model were found to be in close agreement with the experimental measurements.     

A study on austenite decomposition during continuous cooling of a low carbon steel

In this work, a model based on the finite element method and assumption of second order phase transformation has been developed to predict temperature history and austenite decomposition kinetics during continuous cooling of a low carbon steel. In order to accurately assess the temperature field and transformation rate the effects of various factors such as work hardening role on the kinetics of transformation, interconnection between austenite phase change and thermo-physical properties of the steel, and initial austenite grain size have been considered in the model. To verify the results of the modelling, time-temperature histories during cooling of a low carbon steel has been determined and microstructural studies have been performed. The comparison between the predictions and the experimental results indicates reliability of the proposed model.     

Thermomechanical Analysis of Preheat Effect on Grade P91 Steel During GTA Welding

Effect of preheat on the thermal cycles, residual stress, and distortion during autogenous GTA welding of Grade P91 steel has been analyzed using FEM. Phase transformation effect on the residual stresses is also analyzed. Thermomechanical analysis has shown that preheat reduced the peak temperatures, cooling rate, and the distortion values. The phase transformation involving martensite during cooling resulted in typical “M” shaped residual stress profile with a reduction in peak tensile residual stress value and a wider distribution of residual stress. There is good agreement between the predicted and measured thermal cycles, residual stresses, and distortion of the P91 steel weld joint.     

CSP生产低碳钢的组织演变和析出物研究

为了阐明EAF-LF-CSP工艺生产的低碳钢组织细化机理,在薄板坯和不同道次变形后的同一轧件上取样,利用金相、SEM、TEM、XEDS等技术研究了连轧过程中显微组织演变和钢中第二相析出物.结果表明:与普通连铸板坯相比薄板坯的凝固组织更加细小;随轧制道次增加,薄板坯表面和心部的组织差异逐渐减小,轧后室温组织细化;CSP生产的低碳钢中存在大量纳米尺寸的氧化物和硫化物,起到细化晶粒的作用.CSP生产中采用快速冷却和凝固工艺、单道次大压下连轧工艺和层流冷却工艺,是成品组织细化的主要原因.     

Microstructural evolution of tool steels after Nd-YAG laser repair welding

The present paper is aimed at investigating the microstructural behaviour of tool steels after repair welding or refurbishing by a pulsed Nd-YAG precision laser. The 1.2311 (40CrMnMo7), 1.2083 (X42Cr13) and 1.2343 (X38CrMoV5-1) steels were selected for experimental investigations to cover a wide range of steel grades, commonly used in tooling industry.Laser repair welding condition was simulated by preparing small deposits in one or more passes on steel samples having several reference geometries. Investigations on microstructural properties, microhardness evolution and on defect formation were carried out. The effects of different laser welding parameters were also considered.The study allowed to state several fundamental information on tool behaviour during repair welding in order to gain a deeper insight into this process, routinely considered in industrial practice but often neglected in scientific research works on welding metallurgy.     

Effects of cryogenic cooling on surface layer alterations in machining of AISI 52100 steels

Abstract

Microstructural phase transformations, commonly known as white layer formation in hard turned steel components, have in recent times become an interesting research topic in machining as they are related to the surface integrity and functional performance of components. Three main theories have been proposed to justify the mechanisms of white layer formation: (1) rapid heating and quenching; (2) severe plastic deformation; and (3) surface reaction with the environment. Coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on the machined surface alterations during machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial work material hardness, tool shape, cutting speed and feedrate. Optical and scanning electron microscopes (SEM) were used to analyse the affected layer in the machined subsurface, while X-ray diffraction technique was utilised to investigate the microstructural phase composition. The experimental results prove that the microstructural phase changes are heavily influenced by the cutting process parameters and the use of cryogenic cooling, in some cases leading to the total removal of martensite.     

Characterization of solid-phase welds between Ti---6A1---2Sn---4Zr---2Mo---0.1Si and Ti---13.5A1---21.5Nb titanium aluminide

Dissimilar-alloy welds have been produced between Ti---6Al---2Sn---Z4---2Mo---0.1Si (wt. %) and Ti---13.5Al---21.5 NB (wt.%) titanium aluminde using the three different solid-phase welding processes that create significantly different thermo-mechanical conditions at the weld interface. Exposure to supertransus temperatures, appreciable deformation and rapid cooling of the weld interface region during linear-friction welding promote dynamic recrystallization of beta grains and beta decomposition to fine martensitic products. In contrast, diffusion welding at temperatures below the base metal beta transus temperatures and at relatively low pressures minimizes deformation and microstructural variations in the weld interface region relative to the unaffected base metal. During capacitor-discharge resistance spot welding, extremely rapid heating of the weld interface region to near-solidus temperatures, and subsequent rapid cooling, result in the formation of a metastable, ordered-beta microstructure in the Ti---s13.5Al---21.6Nb and fine alpha-prime martensite in the Ti---6Al---2Sn---4Zr---2Mo-0.1Si.     

Manual metal arc weld modelling: Part 1 Effect of process parameters on dimensions of weld bead and heat-affected zone

Abstract

The properties of a weldment are determined largely by the size and distribution of microstructural regions within the weld metal and heat-affected zone (HAZ). It has been appreciated for some time that these properties may be controlled by adjusting the parameters of the welding process, although until recently this was done mainly qualitatively. Means of achieving more quantitative control are now beginning to be applied. In this paper measurements of the sizes of the weld bead and HAZ are presented for single manual metal arc weld beads. The process variables investigated were electrode type, gauge size, welding position, polarity, welding current, preheat, and welding velocity. Functional relationships between the process variables and the size of the weld bead and HAZ are determined. The generality of these relationships is examined by analysing measurements made previously on other materials. The results are discussed in terms of the theory of the welding process and, as shown in two accompanying papers, they can be used to develop models of multipass manual metal arc welding on which a practical welding procedure may be based.

MST/193a     

Embrittlement of a superduplex stainless steel in the range of 550–700 °C

Duplex stainless steels are very attractive materials, combining high mechanical properties with improved corrosion resistance. However, these steels present technical limitations because they can experience embrittlement as a consequence of thermal cycles. Moreover, the higher level of alloying elements (Cr and Mo) in the new duplex generation, called superduplex, accelerates the precipitation kinetics of harmful intermetallic phases, which are the responsible of embrittlement. This fact raises the question of the influence of these cycles on the actual performance of duplex components and structures and on its possible failure during service.

In order to investigate this question, heat treatments in the range of 550–700 °C, with different exposure times and cooling rates, have been made on a superduplex stainless steel type EN 1.4507. The evolution of mechanical properties has been followed by means of hardness measurements, impact and fracture toughness tests, whereas microstructural changes have been identified by using optical and transmission electron microscopy analysis. A correlation between the degree of embrittlement and the different types of precipitates has been established.     

Characterization of failure modes for different welding processes of AISI/SAE 304 stainless steels

Weld joints manufactured with a welding electrode type 308L and by three different arc welding processes shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and flux cored arc welding (FCAW) in a AISI/SAE 304 were studied in order to compare the failure mechanisms associated with their mechanical and microstructural properties. Chemical compositions were analyzed by optical emission spectroscopy and the ferrite numbers (FN) of the welds were also identified. Relevant microstructural characteristics of the different processes were analyzed by microscopy techniques. Finally, fatigue tests were performed to study the variations in the mechanical properties of each process and to analyze their most probable failure modes by means of a fractographic study, in which the characteristic morphologies of each one (nucleation, propagation, final fracture) were identified by means of optical stereoscopy and scanning electron microscopy (SEM). Three different fracture modes were found at the welding joints that showed correlations with microstructural changes produced during the welding process. The first failure mode displayed that the nucleation of the crack was at the weld root. The second failure mode was generated at the heat affected zone (HAZ), where the crack nucleated due to a variation in the grain size produced by the process and then further propagated through the edge of the weld. The third failure mode appeared due to the presence of exogenous inclusions generated by the welding process, which acted as stress concentrators in the weld and produce the initiation and further propagation of the crack. Lastly, some welding processes presented a combination of the previous failure modes and consequently multiple sites of crack nucleation.     

Microstructural evolution of aluminum alloy during friction stir welding under different tool rotation rates and cooling conditions

The microstructural evolution during friction stir welding (FSW) has long been studied only using one single welding parameter. Conclusions were usually made based on the final microstructure observation and hence were one-sided. In this study, we used the “take-action” technique to freeze the microstructure of an Al-Mg-Si alloy during FSW, and then systematically investigated the microstructures along the material flow path under different tool rotation rates and cooling conditions. A universal characteristic of the microstructural evolution including four stages was identified, i.e. dynamic recovery (DRV), dislocation multiplication, new grain formation and grain growth. However, the dynamic recrystallization (DRX) mechanisms in FSW depended on the welding condition. For the air cooling condition, the DRX mechanisms were related to continuous DRX associated with subgrain rotation and geometric DRX at high and low rotation rates, respectively. Under the water cooling condition, we found a new DRX mechanism associated with the progressive lattice rotation resulting from the pinning of the second-phase particles. Based on the analyses of the influencing factors of grain refinement, it was clearly demonstrated that the delay of DRV and DRX was the efficient method to refine the grains during FSW. Besides, ultra-high strain rate and a short duration at high temperatures were the key factors to produce an ultrafine-grained material.     

Influence of peak temperature during in-service welding of API X70 pipeline steels on microstructure and fracture energy of the reheated coarse grain heat-affected zones

In this investigation, thermal simulated specimens were used to investigate the effect of second peak temperature during in-service welding on characteristic fracture energy and microstructure feature of the subcritically (SC), intercritically (IC), supercritically (SCR), and unaltered (UA) reheated coarse grain heat-affected zones (CGHAZs). The API X70 high-strength pipeline micro-alloyed steel was subjected to processing during in-service welding by applying double thermal cycle shielded metal arc welding process with heat input of 9.3 kJ/cm and thermal cycles to simulate microstructure of reheated CGHAZs. This consisted of first thermal cycle with a peak temperature of 1350 °C, then reheating to different second peak temperatures of 600, 800, 1000, and 1200 °C with a constant cooling rate of 60 °C/s. Toughness of the simulated reheated CGHAZs were assessed using Charpy impact testing at −20 °C, and the corresponding fractographs, optical micrographs, and electron micrographs have been examined. It is found that accelerating cooling rate during in-service welding has an improving effect on the microstructure of CGHAZs. Owing to small heat-input and accelerating cooling, the grain size in reheated CGHAZs is relatively small and the brittle microphases are eliminated or minimized. The Charpy impact results show that the CGHAZ fracture energy is improved after the second thermal cycle. The SC CGHAZ showed higher absorbed impact energy and the IR CGHAZ had less absorbed energy, but the phenomenon of embrittlement in IR CGHAZ is not serious. Therefore, it can be concluded that the fracture energy of CGHAZ and IR CGHAZ can be improved by accelerating cooling with appropriate cooling rate.     

Stress Corrosion Cracking of Welded 2205 Duplex Stainless Steel in Sulfide-containing Caustic Solution

Duplex stainless steel (DSS) grades are used in pulp mills for their superior properties and resistance to general corrosion. However, stress corrosion cracking (SCC) of DSS equipment has been experienced in different pulp mills. The susceptibility of DSS grades to SCC can be mainly attributed to the various heating processes involved during the manufacturing of industrial equipments, especially welding. It is generally understood that heating cycles during welding may affect the dual microstructure (ferrite/austenite ratio) of the steel, making it more prone to cracking in aggressive environments such as chlorides and caustics and further exposure to high temperatures. Welded 2205 DSS failed in white liquor (mainly NaOH + Na₂S) was examined for SCC crack morphology and microstructure. Heat-treated 2205 DSS samples were tested in simulated white liquor to see the effect of microstructure on SCC susceptibility. Austenite is more susceptible to SCC than ferrite, but the SCC susceptibility primarily depends on the composition of the alloy and the chemistry of the exposure environment.     

Tetragonal to monoclinic transformation and microstructural evolution in ZrO2–9.7 mol% MgO during cyclic heating and cooling

The tetragonal (t) to monoclinic (m) transformation behaviour and its relationship to microstructural evolution were investigated by means of dilatometry and transmission electron microscopy for ZrO2–9.7 mol% MgO during cyclic heating and cooling between room temperature and 1490 K. In the as-sintered specimens, fine oblate ellipsoidal t-phase precipitates, 20–50 nm in diameter and 100–200 nm long, were distributed in the cubic (c)-phase matrix. They were below a critical size for transformation and exhibited no transformation in the first three cycles. In the fourth and further cycles, transformation occurred in two distinct stages. A low-temperature stage appeared at 850–1000 K on heating and at 400–700 K on cooling, while a high-temperature stage appeared at 1350–1400 K on heating and at 1000–1200 K on cooling. With the increasing number of cycles, at first the size of low-temperature stages increased and then decreased above ten cycles accompanying the development of the high-temperature stage. During cyclic heating and cooling, coarsening of ellipsoidal precipitates and decomposition of c- and t-phases occurred. As a result of the decomposition, MgO particles and a new m-phase containing a very low concentration of MgO were produced. The coarsened ellipsoidal t-phase precipitates were responsible for the low-temperature stage. The new m- or t-phase containing very low MgO produced by the decomposition was responsible for the high-temperature stage.     

Phase transformation and mechanical behaviour of thermo-mechanically controlled processed high-strength multiphase steel

A novel low-alloy high-strength steel [Fe–0.20C–1.65Mn–1.40Si–1.50Al–1.30Cu–1.05Ni–1.07Co (wt%)] has been thermo-mechanically processed with a finish rolling temperature of 850 °C, followed by air cooling and water quenching in order to obtain a good combination of strength and ductility. Phase transformations of the above steel at different cooling rates have been studied and continuous cooling transformation (CCT) diagram has been constructed using data, obtained from dilatometric study. The phase field of CCT diagram indicates microstructure changes from a mixture of ferrite and bainite to fully martensite accompanied with the enhancement of hardness with increasing cooling rate. The microstructural investigation at lower cooling rate (≤5 °C/s) suggests the possibility of achieving pearlite-free microstructure by direct air cooling from the austenite region. Directly air-cooled steel has demonstrated primarily ferrite–bainite microstructure, which shows attractive tensile strength (>1050 MPa) and ductility (>15 %). On the other hand, directly water-quenched steels reveal predominantly lath martensitic microstructure with high dislocation density which exhibits higher tensile strength (>1600 MPa) and lower ductility (~12 %). The multiple stages of strain hardening behaviour of the investigated steel under different cooling conditions have been examined with respect to microstructural evolution.     

Relation between tensile properties and microstructure in type 316 stainless steel SA weld metal

Tensile properties of a thick, multipass, submerged-arc (SA) weld-deposited type 316 are investigated by tests at room temperature and at 400 ° C and by microstructural and compositional analyses. The as-deposited metal, which shows a lower yield strength, a comparable ultimate tensile strength and a lower total elongation compared to the (solution-annealed) parent metal, is characterized by systematic variations in tensile properties across its thickness, with the highest strength and the lowest ductility in the weld centre. These variations are related to material variability (mainly changes in dislocation density) within the weld metal due to local dissimilarities in thermal and mechanical histories during welding.     

Cr7C3 / Ni3Al 复合材料制备过程中碳化铬的溶解再析出行为

将真空常压烧结方法制得的Cr₃C₂-Ni₂Al 复合焊丝堆焊于碳钢表面。分析表明, 在堆焊过程中, 利用氩弧物理热和Ni-Al 反应热, Ni 与Al 化合反应生成Ni₃Al 金属间化合物, Cr₃C₂ 则发生分解, 除少部分[ C]与[Cr ]固溶于Ni₃Al 基体中外, 大部分反应析出更稳定的Cr₇C₃ 相, 其尺寸取决于堆焊层中不同区域的冷却环境,较为均匀地分布于Ni₃Al 基体中。由于Cr 在Ni₃Al 中的固溶度远大于C , 加之Cr₃C₂ 转化为Cr₇C₃ 也造成部分富余的C , 结果造成在该Ni₃Al 表面强化材料焊层中形成石墨相, 其密度轻、熔点高, 主要偏聚于焊层表层。Cr₇C₃ / Ni₃Al 复合材料的室温、高温硬度远高于传统高温耐磨材料Stellite 合金, 该材料有望成为一种新型的高温耐磨表面强化材料。