Properties Of Heat Affected Zone Research Articles

Weld zone representation during the formability prediction of friction stir welded blanks with similar thickness sheets

During formability prediction of friction stir welded (FSW) blanks, it is necessary to incorporate nugget zone and heat-affected zone (HAZ) properties separately for better accuracy. In general, it is observed that the weld zone is represented as a single entity, in which only global weld zone properties are implemented. In this work, it is proposed that there is a domain of weld conditions in which the single-zone assumption (nugget and HAZ are modelled without incorporating their individual properties) breaks down and the double-zone assumption (nugget zone and HAZ are represented separately) should be followed. During limiting dome height simulations, a representative base material is considered from the literature with changing nugget zone and HAZ properties. The punch displacement at failure and maximum load are compared between FSW blanks modelled with single- and double-zone assumptions. It is observed that modelling FSW blanks with double-zone assumption plays a vital role for accurate formability prediction. The single-zone assumption is not valid for all the FSW conditions. Both in transverse and longitudinal weld orientations, there are possibilities of multiple domains in which the double-zone assumption is valid. It is found that there exists a relationship between the thickness distribution and availability of single-zone or double-zone models. Whenever the thickness distribution of a particular double zone is closer to the reference case modelled with single zone, it is sufficient to model that weld zone case with a single zone. Double-zone representation is required in cases when the thickness distribution is very different. The domains for weld zone modelling in FSW blanks are sensitive to failure location. Put simply, it is advisable to model HAZ separately, when failure occurs in it. But this is not always the case. It depends on a compromise between the accuracy and availability of HAZ properties.

Influence of different cooling rates on the microstructure of the HAZ and welding CCT diagram of CLAM steel

The evolution of microstructure in the weld HAZ (Heat Affected Zone) under welding thermal cycle was simulated by Gleeble-1500. The microstructures and properties of HAZ at different cooling rates were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and microhardness test. The results showed that when the cooling rates ranged from 3600 K/min to 60 K/min, fully martensitic transformations were achieved, and the cooling rates had little influence on the microstructure and microhardness of martensite. The maximum cooling rate to form ferrite was 60 K/min. When the cooling rates ranged from 60 K/min to 1 K/min, the microhardness of the HAZ began to decrease due to the presence of ferrite, and the lath sizes grew up to 0.13–0.15 μm. The critical cooling rate to gain fully ferrite was found to be slower than 1 K/min. The welding CCT diagram of CLAM steel was constructed and it showed that there were ferrite and martensite transformation regions only. The diagram made it possible to predict the microstructures and properties of HAZ and became an important tool to evaluate weldability of CLAM steel.

Improvement in steel weldability for large diameter thick-walled gas pipelines by optimizing chemical composition

The problem is considered of improving impact strength and brittle failure resistance of heat-affected zone (HAZ) metal during welding of thick-walled (30–40 mm) gas pipelines. It is shown that the best HAZ properties are obtained in the case of forming low-carbon bainite in this zone with slow post-welding cooling rates. The carbon content is reduced in pipe steel for this purpose; solid solution elements are added, which increase austenite stability, and microalloying with niobium and titanium is carried out in order to retard grain growth by carbonitride particles.

Effects of alloying elements on the thermal fatigue properties of the ferritic stainless steel weld HAZ

In the present study, the effects of the addition of Nb, W, and V alloying elements on the thermal fatigue properties of the ferritic stainless steel weld heat affected zone (HAZ) were investigated. Simulation of the weld HAZ and thermal fatigue testing were carried out using a metal thermal cycle simulator (MTCS) under a complete constraint force in the static jig. The fatigue life during cyclic heating and cooling in the temperature range of 200 to 900°C was strongly related not only to precipitation and coarsening of MX type precipitates and Laves phase but also to the Nb content in the solid solution and grain coarsening. The thermal fatigue life increased with higher Nb and W contents due to more (Nb,Ti)C precipitates and a less coarsened Laves phase, respectively. Contrary to the results with Nb and W additions, there were no obvious effects on grain size and precipitation behavior associated with V addition.

The Oxide Inclusion and Heat-Affected-Zone Toughness of Low Carbon Steels

The relationship between the oxide inclusions and the Heat-affected-zone (HAZ) toughness of microalloying steels has been investigated. The low carbon steels are smelted with special oxide introduction technique and the properties of HAZ has been studied with thermo-simulation. The optical microscope and SEM were used to analyze the size, composition and distribution of the inclusions, the mechanical properties after thermo-simulation was also analyzed. The results show that the inclusions in steel are mainly Ti and Al oxide with MnS, these complex inclusions are well distributed and the size is less than 3 micron. Microstructure of HAZ consists of intragranular acicular ferrite (IAF), intergranular ferrite and small amount of lath bainite while the cooling time during the phase formation is short. After the thermo simulation with the cooling time between 800°C and 500°C (t8/5) increasing the toughness of HAZ decreased and the size of prior austenite grain increased. Inclusions which located near the prior austenite grain boundary couldn’t induce the nucleation of IAF, only the ones inside the prior austenite grain can promote IAF’s growth.

The Potential of HAZ Property Improvement through Control of Grain Boundary Character in a Wrought Ni-Based Superalloy

The effects of grain boundary serration on grain coarsening and liquation behavior in simulated weld heat-affected-zone (HAZ) of a wrought Ni-based superalloy Alloy 263 have been investigated. Recently, the present authors have found that grain boundary serration occurs in the absence of adjacent coarse γ' particles or M23C6 carbides when a specimen is direct-aged with a combination of slow cooling from solution treatment temperature to aging temperature. This serration leads to a change in grain boundary character as special boundary based on the crystallographic analysis demonstrating that the grain boundaries tend to serrate to have specific segments approaching to one {111} low-index plane at a boundary. The present study was initiated to investigate the interdependence of the serration and HAZ property with a consideration of this serration as a potential for the use of a damage-tolerant microstructure. It was found that the serrated grain boundaries suppress effectively grain coarsening, and are highly resistant to liquation cracking in HAZ due to their lower tendency to be wetted and penetrated by the liquid phase. These results reflect closely a significant decrease in interfacial energy as well as grain boundary configuration by the serration.

Correlation between microstructure, hardness and strength in HAZ of dissimilar welds of rotor steels

Correlation between microstructure, hardness and strength was investigated in heat affected zone (HAZ) of dissimilar welding joints of newly developed rotor steels in terms of both the traditional micro-hardness testing and the nanoindentation technique. Relationships between micro-hardness and nano-hardness were obtained for HAZ of welds, where the mechanical properties were microstructure dependent. Lath width of the tempered martensites was selected as the characteristic microstructure size for correlating the micro-hardness. The detailed strength distribution in HAZ and the correlation with the characteristic microstructure size were discussed. A good agreement was observed for the correlated strength and the experimental results in both weld metal (WM) and base metal (BM), and therefore gave us confidence for the application of the proposed relationship between the hardness and the mechanical properties in HAZ of the dissimilar weld.

肉盛溶接による熱影響部の材質変化を考慮した原子炉圧力容器の溶接残留応力評価

In order to evaluate residual stress distributions in a reactor pressure vessel (RPV) due to weld-overlay cladding and post-weld heat-treatment (PWHT), thermal-elastic-plastic-creep analysis considering the changes of material properties in heat-affected zone (HAZ) has been performed. By considering the material property changes in HAZ, stress distributions obtained from the analysis agree well with the experimental results measured from weld-overlay cladded plate. Applying the analysis method, residual stress distributions for a vessel model caused by weld-overlay cladding, PWHT, hydrostatic test, operational load and transient loads during pressurized thermal shock (PTS) events have been computed. Effects of the residual stresses on the structural integrity of RPVs have been evaluated by comparing the difference in stress distributions and the values of stress intensity factor for a postulated crack during PTS events.

Influence of process parameters on microstructural evolution and mechanical properties in friction stirred Al-2024 (T3) alloy

Commercial Al-2024 (T3) alloy was friction stir welded at various heat index (HI) values using bead on plate approach. Quantitative analysis using electron microscopy and differential scanning calorimetry (DSC) revealed a complex variation in the precipitation evolution of Guinier-Preston-Bagaryatskii (GPB) zone and Al2CuMg (S phase) precipitates in nugget and heat affected zone (HAZ). Differential scanning calorimetry data also suggested formation energy of GPB zone equal to −236 J g−1. Tensile properties attained a maximum for HI values close to 3·94 in the nugget region, which was attributed to corresponding minimisation of the volume fraction of coarse S phase to the profit of GPB zone. The precipitation of fine S phase precipitates resulted in higher tensile properties in HAZ as compared to those in nugget for all HI values. Experimental data were used to determine major strengthening mechanisms by using constitutive relationships.

Study of the Simulated Heat Affected Zone of Creep Resistant 9–12% Advanced Chromium Steel

Global warming leads to research-development attempts worldwide with a view to reduce CO2 emission. Therefore, in the energy supply sector, a special focus has been directed to further development of steels that can endure the ultra-supercritical steam conditions. Our article presents the basic study on weldability of the advanced 9–12% chromium steel applying the thermomechanical simulation of the heat affected zone (HAZ). The changes of microstructures and material properties of HAZ before and after postweld heat treatment (PWHT) have been analyzed and compared by light microscopy, scanning electron microscopy (SEM), hardness measurements, and impact toughness testing. Microstructures at representative points during typical welding cycle and PWHT were studied in details.

トランプエレメントと窒素を多く含む電気炉鋼材の大入熱溶接HAZ靭性について

Steel scrap is raw materials of the recycled steels refined in electric arc furnaces, and this recycle process is good for keeping our living environment. Authors focus in the toughness of the recycled steels that contain much tramp elements and nitrogen. Chemical compositions of specimens are 0.17C-0.20Si-1.15Mn,Cu : 0.08–0.50mass%,Ti : 0.006–0.031mass%. This report contains properties of HAZs (heat affected zones) in thermal cycled mother steels, and these HAZs were estimated a heat input of 45-600 kJ/cm. The following experimental results are obtained. TiN particles fasten on γ grain boundaries, and improve the toughness of HAZs. A grain boundary ferrite is restrained by Cu, and ferrite grains become fine, too.

Mechanical Properties of Arc Welding Heat-Affected Zone of High Nitrogen Steel

Abstract The engineering application of high nitrogen steel (HNS) needs to obtain welded joint with good comprehensive properties. In order to acknowledge the influence of welding thermal cycle on the heat-affected zone (HAZ) of high nitrogen steel (HNS), the microstructure and mechanical properties of HAZ were investigated by using thermo-simulation technique. Experiment results indicate that the microstructure in the HAZ of HNS is composed of austenite and δ-ferrite. Carbide precipitation (Cr 23 C 6 ) occurs in the HAZ where the peak temperature of welding thermal cycle is about 800°C, and nitride (Cr 2 N) is not examined. The parameters of welding thermal cycle can affect the amount of δ-ferrite in the HAZ. Under the experimental conditions, the amount of the δ-ferrite in the coarse-grained heat-affected zone (CGHAZ) increases when the cooling rate increases, and it changes slightly while the peak temperature is lower than 1150°C. The hardness of the CGHAZ increases when the cooling rate increases, and that of the HAZ decreases while the peak temperature decreases. The results show that the hardness of the HAZ is higher than that of the base metal, indicating no softening of the HAZ under appropriate welding conditions. The impact toughness of CGHAZ is improved with the increase of cooling rate, whereas two brittle zones exist in the HAZ.

Microstructure and Mechanical Properties of Heat-affected Zone of High Nitrogen Steel Simulated for Laser Welding Conditions

In order to obtain some information on the weldability of high nitrogen steel (HNS), the microstructure and mechanical properties of the heat-affected zone (HAZ) of HNS under laser welding conditions were investigated by using thermo-simulation technique. The experimental results indicate that the microstructure in the simulated HAZ of HNS consists of austenite and d-ferrite that occurs in the grain boundary of austenite. The hardness of the simulated coarse-grained heat-affected zone (CGHAZ) increases when the cooling rate increases, and that of the simulated HAZ decreases while the peak temperature decreases. The results also show that the hardness of the simulated HAZ is higher than that of the base metal, indicating no softening of the HAZ under appropriate welding conditions. The impact toughness of simulated CGHAZ increases at first and then decreases with the increase of the cooling rate, whereas two brittle zones exist in the HAZ.

Welding-induced mechanical properties in austenitic stainless steels before and after neutron irradiation

The effects of neutron irradiation on the mechanical properties of welded joints made of austenitic stainless steels have been investigated. The materials are welded AISI 304 and AISI 347, so-called test weld materials, irradiated with neutrons at 573 K to doses of 0.3 and 1.0 dpa. In addition, an AISI 304 from a decommissioned pressurised water reactor, so-called in-service material, which had accumulated a maximum dose of 0.35 dpa at about 573 K, was investigated. The mechanical properties of heat-affected zones and base materials were analysed before and after irradiation. Tensile parameters were determined at room temperature and at 573 K, for all materials and irradiation conditions. In the test weld materials it is found that radiation hardening is lower and loss of ductility is higher in the heat-affected zone than in the base material. In the in-service material radiation hardening is about the same in heat-affected zone and base material. After irradiation, deformation takes place by stacking faults and twins, at both room temperature and high temperature, contrary to unirradiated materials, where deformation takes place by twinning at room temperature and by dislocation cells at high temperature. No defect free channels are observed.

Physical Simulation and Metallurgical Evaluation of Heat-Affected Zone during Laser Welding of Ultrafine Grain Steel

Two major challenges in relation to laser welding are abrupt change in metallurgical aspects and actual assessment of the mechanical properties due partly to very narrow heat affected zone (HAZ) and partly to high mechanical properties gradient. The rapid thermal cycle of laser welding imposed on the HAZ was physically simulated using a Gleeble™ dynamic simulator equipped with a special isothermal quenching device (ISO-Q™), and a relatively large volume of HAZ with a homogeneous microstructure was obtained. The thermal cycles were determined from actual laser welding followed by laser tempering. Estimations of microstructure and mechanical properties of the simulated HAZs of an ultrafine grain steel imposed by laser welding with or without post-weld laser tempering were performed. The results indicate that the simulated HAZs, depending on the thermal history, are composed of lathy martensite with different pocket size and dislocation density. The impact toughness of as-welded HAZ is improved in contrast to the base material, but is further degraded by a following laser tempering, which, however, alleviates the abrupt change in hardness of as-welded HAZ.

Weld crack assessments in API X65 pipeline: failure assessment diagrams with variations in representative mechanical properties

Applying accurate material properties to failure assessment diagrams (FAD) for flaw assessment has been problematic, particularly for welded joints in structures such as natural gas pipeline. In a study of API X65-graded natural gas pipeline, we evaluated material properties such as tensile properties and fracture toughness for the base metal, weld metal and heat-affected zone (HAZ), and investigated the influence on flaw assessment of variations in material properties of three regions. In particular, microtensile tests and crack-tip-opening-displacement (CTOD) tests made it possible to construct an HAZ-focused FAD reflecting HAZ properties. It was found that, when crack-like flaws exist in the HAZ, the HAZ-focused FAD yields a more accurate assessment than FADs constructed according to current codes.

Creep rupture properties of HAZs of a high Cr ferritic steel simulated by a weld simulator

In this work, creep rupture properties of the heat affected zones (HAZs) of P122 steel, simulated by employing a weld simulator are studied and the results are compared with those of HAZs simulated by heat treatment. Microstructures corresponding to intercritical HAZ, fine-grained HAZ and coarse-grained HAZ (CGHAZ) were produced using a weld simulator in the middle of specimens. These specimens were then subjected to heat treatment equivalent to post weld heat treatment and the creep specimens machined from them were tested at 923 K at three stress levels. Results showed that, irrespective of the peak temperature of simulation, fracture always occurred in that part of the specimens heated to the temperature close to Ac 3 during simulation, which was found to have a fine grain structure and minimum creep strength. However, in specimens with temperatures of simulation above Ac 3, creep cavities were observed and for those with a 1473 K simulation temperature, fracture occurred with very low ductility as observed in actual weld joints. Stress and strain distributions within the specimen during creep deformation were analyzed using the finite element method. The results indicate that creep deformation and fracture are influenced by the multiaxial stress state produced in the specimen due to the existence of different microstructures varying significantly in their creep properties. Results also suggest that the creep tests using specimens with CGHAZ microstructures simulated at the centre can successfully produce a creep fracture similar to the Type IV fracture that is observed in ferritic steel weld joints.

Experimental and Numerical Prediction of Collapse of Flatbar Stiffeners in Aluminum Panels

Flatbar stiffeners are attractive in stiffened plates from a fabrication point of view. Flatbars are susceptible to torsional buckling and collapse. Current approaches to assess torsional buckling strength are uncertain. For instance, formulations for torsional buckling of stiffeners in steel panels are still debated. Compared to steel structures, the strength of aluminum structures is influenced not only by the residual stresses and initial imperfections, but also reductions in mechanical properties in heat-affected zones (HAZ). In the present paper, an experimental and numerical investigation of the torsional buckling strength of longitudinal stiffeners in aluminum (aluminum alloy 5083-H116 and 6082-T6) panels subjected to axial compression is conducted. The experimental results are presented and compared with numerical predictions by using finite element method code ABAQUS, considering the influence of the HAZ and actual initial deflections. The effect of the HAZ on the torsional buckling strength of t...

Creep Fracture Analysis of W Strengthened High Cr Steel Weldment

Type IV cracking in heat affected zone (HAZ) of weldment is a problem for advanced high Cr ferritic steels. The present paper investigates the creep properties and microstructures of W strengthened P122 steel weldment at 923K. From the investigation of creep properties of simulated HAZ, it is clarified that heating up to around Ac3 during welding minimized the grain size and creep strength. Most of the welded joint specimens were type IV fractured in fine-grained HAZ and resulted in shorter creep lives than those of the base metals. Electron beam welded joint with very narrow HAZ also showed the brittle type IV fracture due to the formation of creep voids and cracks. The growth of intergranular precipitates was faster for pine-grained HAZ. On the basis of experimental results, the FEM code that simulates type IV crack growth behavior has been developed. The vacancy diffusion under multi-axial stress condition in HAZ of weldment is analyzed.

Microstructure Anticipation in Steel Heat Affected Zone by the CCT Diagrams / Gefügevorhersage in der Wärmeeinflusszone mittels ZTU-Diagrammen

The microstructure and properties anticipation of heat affected zone (HAZ) in steel is presented. A methodology is based on continuous cooling transformation (CCT) diagrams application. Investigations are performed on the Q&T low alloy Ni-Cr-Mo steel (HY100). Two CCT diagrams are made on a weld simulator. One of them is made by shock heating to 1350°C for simulation of coarse grained zone (CGHAZ) in HAZ and another by slow heating to 880°C for simulation of grain refined zone (GRHAZ). Both diagrams are supplemented with microstructure examinations, toughness and hardness measurements. The notch impact toughness of weld simulated specimens is determined by instrumented Charpy V test with recording total fracture absorbed energy, crack initiation and crack propagation energies. Based on microstructure analysis by the optical microscope and measured propagation energy, the bainite appearance is defined. From these data, bainite morphology is classified as fine and coarse acicular or globular bainite. Verification of the suggested methodology was performed by comparison of structure and hardness with the results of investigations of GMA welded joints. The reliability and limitations of weld HAZ microstructure and properties anticipation by the methodology supported on the two CCT diagrams are discussed.