Simulated Heat Affected Zone Specimens Research Articles

Effect of Post-Weld Heat Treatment Temperature on the Mechanical Properties and Microstructure of Weld Heat-Affected Zone of 2.25Cr-1Mo Steel

The effect of post-weld heat treatment (PWHT) temperature on the mechanical properties of the heat-affected zone (HAZ) of 2.25Cr-1Mo steels was investigated. SA-387 Gr. 22 Cl. 2 steel was employed, and the specimens taken from the steel were thermally cycled using Gleeble to simulate the coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and intercritical HAZ (ICHAZ). Before the PWHT, the simulated HAZs exhibited poor impact toughness as compared with the base metal. The base metal and simulated HAZ specimens were heat treated in a furnace at 610, 650, 690, and 730℃ for 8 h; their impact toughness improved significantly irrespective of the PWHT temperature. The microstructural changes owing to the PWHT were observed using scanning electron microscopy and correlated with the mechanical properties.

Investigation on microstructure and impact toughness of simulated heat affected zone of high strength low alloy steels by laser-arc hybrid welding

The microstructure and impact toughness of the simulated heat affected zone (HAZ) of a high strength low alloy steels by laser-arc hybrid welding (LAHW) were investigated in this paper. Homogeneous simulated HAZ specimens with different grain sizes were prepared using the welding thermal simulating technique. Instrumented impact test was conducted to investigate the toughness of the simulated HAZ. Multi-scale sub-structure characterization of the simulated HAZ specimens was implemented by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD). The instrumented impact test found that the peak temperature T M mainly affected the crack propagation process, the crack propagation energy E p decreased as the increase of T M , while the crack initiation energy E i barely changed, the decrease of crack stable energy E a led to the change of E p . The multi-scale sub-structure characterization showed that the prior austenite grain size (PAGS), packet width, block width both increased with the rise of T M . The block width was similar to the facet size in the crack unstable propagation zone of the simulated HAZ specimens. The block was the microstructure unit controlling the crack propagation process of the LAHW simulated HAZ specimens.

Local microstructure and mechanical characteristics of HAZ and tensile behavior of laser welded QP980 joints

Microstructural evolution of the heat-affected zone (HAZ) seriously affects the mechanical properties of the welded joints. In this paper, QP980 steels were butt-welded by fiber laser. The thermocouple and Gleeble1500 system were used to achieve the thermal simulated HAZ specimens of laser welded joints, which solved the problem that it was difficult to evaluate the mechanical properties due to the narrow HAZ of the laser welding. Large amounts of granular carbides were precipitated in the block martensite of the sub-critical HAZ (SCHAZ). Carbides were not found in lath martensite, but there were dislocation tangles in the lath martensite of the SCHAZ. In addition, the number of dislocations in the ferrite of the SCHAZ was significantly reduced. The SCHAZ had the lowest strength but the highest elongation owing to the tempering. The fine-grain strengthening resulted in the highest tensile strength in the fine-grained HAZ (FGHAZ). The tensile fracture morphologies indicated a brittle fracture mechanism in coarse-grained HAZ (CGHAZ) and FGHAZ, and exhibited a ductile fracture mechanism in the inter-critical HAZ (ICHAZ) and SCHAZ. According to the overview of the joint and the local mechanical characteristics of HAZ, the numerical model of the tensile test was performed by ABAQUS. And the simulation results showed good agreement with the experimental results, which both illuminated the constraint effect of SCHAZ and made the strain concentration transferred from SCHAZ to the BM with the increase of stress.

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

AbstractTo clarify the effect of microstructural changes on the fatigue property of the weld heat‐affected zone (HAZ), low‐ to high‐cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S‐N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S‐N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased.

Effect of Post-Weld Heat Treatment Temperature on the Mechanical Properties and Microstructure of Weld Heat-Affected Zone of Low-Alloy Steel for Nuclear Reactor Pressure Vessel

The effect of post-weld heat treatment (PWHT) temperature on the mechanical properties of the heat-affected zone (HAZ) of P-No. 3 low-alloy steel was investigated by taking into account the changes in microstructure. SA-508 Gr. 3 Cl. 1 forged steel was employed, and the specimens taken from the steel were thermally cycled using a Gleeble simulator to simulate coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), intercritical HAZ (ICHAZ), and subcritical HAZ (SCHAZ). The respective base metal and simulated HAZ specimens were heat treated in a furnace at 610, 650, 690, and 730°C for 8 hours. Before the heat treatment, the ICHAZ, FGHAZ and CGHAZ exhibited high hardness and poor impact toughness. However, when the PWHT was applied, the hardness decreased and the impact toughness improved; the impact toughness was highly dependent on the PWHT temperature. The impact toughness of the ICHAZ and FGHAZ increased significantly when the PWHT temperature was 650°C or below, while that of CGHAZ improved significantly when the PWHT temperature was 650°C or higher. The microstructural changes due to the PWHT were observed by scanning electron microscopy and correlated with the mechanical properties.r composed of iron-based composition system had similar characteristics to the material to be repaired. Key words: Low-alloy steel, Heat-affected zone, Post-weld heat treatment, Impact toughness

Microstructural Characteristics and m23c6 Precipitate Behavior of the Course-Grained Heat-Affected Zone of T23 Steel without Post-Weld Heat Treatment

The microstructural characteristics of a simulated heat-affected zone (HAZ) in SA213-T23 (2.25Cr-1.6W steel) used for boiler tubes employed in thermal power plants were investigated using nital, alkaline sodium picrate, and Murakami’s etchants. In order to investigate the microstructure formation process of the HAZ in the welding process, simulated HAZ specimens were fabricated at intervals of 100 °C for peak temperatures between 950 and 1350 °C, and the microstructural features and precipitate behavior at various peak temperatures were observed. The alkaline-sodium-picrate-etched microstructures exhibited a black dot or band, which was not observed in the natal-etched microstructure. As the temperature increased from 950 to 1350 °C, the black dot and band became wider and thicker. Experimental analyses using an electron probe micro-analyzer, electron backscatter diffraction, and transmission electron microscopy revealed the appearance of austenite in the black dot region at a peak temperature of 950 °C; its amount increased up to a peak temperature of 1050 °C and thereafter decreased as the peak temperature further increased. The amount of M23C6 decreased with an increase in peak temperature. Based on these results, we investigated the behaviors of austenite and M23C6 as functions of the peak temperature.

Cyclic plasticity and low cycle fatigue damage characterisation of thermally simulated X100Q heat affected zone

This paper presents the cyclic plasticity and low cycle fatigue (LCF) damage characterisation of thermally simulated heat affected zone (HAZ) for API 5L X100Q weldments. Microstructures representative of the HAZ for two cooling rates are generated using a Gleeble thermomechanical simulator for manufacture of strain-controlled cyclic plasticity test specimens. The simulated HAZ specimens are subjected to a strain controlled test programme which examines the cyclic effects of strain-range and the tensile response at room temperature. A modified version of the Chaboche rate independent plasticity model, which accounts for early stage damage is implemented to characterise the cyclic plasticity response, including isotropic and kinematic hardening effects. The constitutive parameters are fitted to experimental data using an optimisation procedure developed within a MATLAB code. The measured response of the simulated HAZ specimens is compared to that of the X100Q parent material (PM), and the simulated HAZ is shown to share the early stage fatigue damage behaviour of the PM, but exhibits significantly a higher yield and cyclic strength.

Primerjava mikrostrukture toplotno vplivanega področja varjenega in simuliranih vzorcev

The high-strength steel grade S690QL and a filler welding wire Mn3Ni1CrMo were the materials chosen for welding a V-shaped butt weld. In order to prevent the weld’s cold cracking, a multi-pass welding technique was applied. A metallographic investigation revealed microstructure variations in different areas of the weld’s heat-affected zone. A reverse-engineering approach was used to test a dilatometer’s capabilities to simulate different HAZ microstructures. Hollow steel-cylinder specimens were subjected to several weld thermal cycles in order to generate similar microstructures as in the real weld’s HAZ. The microstructures of the as-welded and simulated heat-affected zone specimens were investigated. Good agreement was found between the dilatometer-simulated HAZ microstructures and those in a real HAZ weld.

WES 2808 for brittle fracture assessment of steel components under seismic conditions Part-IV: Change in mechanical properties and fracture toughness of steel weld HAZ by pre-strain

Brittle fracture of the welded steel structures during an earthquake is reported to be caused mainly by dynamic and cyclic large strain. However, effect of pre-strain to fracture toughness of the welded joint, especially heat affected zone (HAZ) was not reported enough. In this paper, effect of pre-strain in HAZ tensile properties and critical CTOD was studied by using the simulated HAZ specimens of 490MPa class steel. Change in tensile properties of simulated HAZ by pre-strain was as well as that of base metal. The amount of change in strength of simulated HAZ was possible to describe by the same numerical formula as the base metal. In addition, the amount of temperature shift at critical CTOD of 0.1mm of simulated HAZ by the pre-strain was as well as that of base metal which has the same amount of the change in flow stress by the pre-strain.

Influence of Boron and Nitrogen on the Heat Affected Zone of Modified 9Cr-1Mo Steel:Gleeble Simulation Study

Boron is reported to improve Type IV cracking resistance of 9Cr-3W-3Co-V-Nb steel, with this improvement being attributed to the presence of uniform microstructure in the heat affected zone (HAZ) and its thermal stability during high temperature service. Similar results have been observed recently in modified 9Cr-lMo steel; hence, the present work aims to understand the role of boron and nitrogen in the microstructural evolution of the HAZ in this steel. For this purpose, using a Gleeble thermal-mechanical simulator, MAZ microstructures were simulated in normalized (1100°C/lh) and tempered (760°C/3h) modified 9Cr-lMo steels with different boron and nitrogen contents. The Gleeble simulation was carried out by subjecting the specimens to heating cycle at different peak temperatures in the range 875 to 1200°C using a heating rate of 45°C.s -1 . The effect of heating rate on the transformation temperatures was also studied using dilatometry. The simulated HAZ specimens were characterized using optical and scanning electron microscope, x-ray diffraction (XRD) and hardness measurements. Microstructural examination of the Gleeble-simulated specimens shows that typical lath martensitic structure is present in all specimens of the boron-containing steel. However, in the 910°C simulated specimens of the boron-free steel lath martensite is absent and fine prior austenite grains are present. It is also observed that hardness increases marginally with increase in peak simulation temperature in the boron-containing steel. To understand the microstructural evolution in the HAZ, the crystallite size, lattice strain and dislocation density were estimated by analysis of XRD data. While variations in crystallite size are marginal, the lattice strain and dislocation density vary with peak simulation temperature. Further detailed microstructural analyses have clearly shown the stability of microstructure in the HAZ depends on the boron and nitrogen contents in the steel. The present paper presents and discuss the results of this experimental investigation.

Integrity Assessment of Grade 92 Welded Joint under Creep Condition

Integrity of tempered martensitic grade 92 (9Cr-1.8W-0.5Mo-VNb) steel welded joint has been assessed by carrying out creep test at 923K over a wide stress range. The failure location in the joint was found to shift from base metal to heat affected zone (HAZ) of the weld joint with decrease in applied stress. The HAZ of the weld joint consisted of coarse prior-austenite grain (CGHAZ), fine prior-austenite grain (FGHAZ) and intercritical (ICHAZ) regions in a sequence from the fusion boundary to unaffected base metal. The failure was associated with localized creep deformation coupled with creep cavitation. To have a better insight to the creep failure behaviour of the joint, the different microstructural constituents of HAZ of the joint has been simulated. Creep and tensile tests were carried out on the simulated HAZs specimens. The steel suffered degradation in creep rupture strength on subjecting heating in the intercritical (between Ac1 and Ac3) (ICHAZ) as well as at temperature just above the Ac3 (FCHAZ). The extensive creep cavitation has been observed in the simulated FGHAZ of the joint. Similarly, localized extensive creep cavitation and precipitation of tungsten rich Laves phase has been observed in the FGHAZ of the weld joint. This caused the type IV failure in the FGHAZ of the weld joint. The integrity of grade 92 steel joint under creep condition is governed by the above microstructural modifications of the steel in HAZ by weld thermal cycle.

Analysis of the Creep Behavior of P92 Steel Welded Joint

Different regions of heat-affected zone (HAZ) were simulated by heat treatment to investigate the mechanisms of the Type IV fracture of P92 (9Cr-2W) steel weldments. Creep deformation of simulated HAZ specimens with uniform microstructures was investigated and compared with those of the base metal (BM) and the weld metal (WM) specimens. The results show that the creep strain rate of the fine-grained HAZ (FGHAZ) is much higher than that of the BM, WM, the coarse-grained HAZ (CGHAZ), and the inter-critical HAZ (ICHAZ). According to the metallurgical investigation of stress-rupture, the FGHAZ and the ICHAZ have the most severely cavitated zones. During creep process, carbides become coarser, and form on grain boundaries again, leading to the deterioration of creep property and the decline of creep strength. In addition, the crack grows along the FGHAZ adjacent to the BM in the creep crack growth test (CCG) of HAZ.

Suppression of Type IV fracture and improvement of creep strength of 9Cr steel welded joints by boron addition

The microstructure and creep strength of simulated heat-affected zone (HAZ) specimens and welded joints have been investigated for advanced 9–12% Cr steels in order to make clear the mechanisms responsible for Type IV fracture and to improve the creep strength of welded joints at elevated temperature. Creep and creep rupture tests were carried out at 650 °C (923 K) for up to about 2×10 4 h. The creep rupture time of simulated HAZ specimens has its minimum after heating to A C3 temperature, which produces fine-grained martensitic microstructure with high density of dislocations and large M 23C 6 carbides but no lath structure. The welded joints were fractured in fine-grained HAZ at low stresses, indicating Type IV fracture, which resulted in shorter creep life than those of base metal. Reducing the width of HAZ by means of EB welding is effective for the extension of creep life but the brittle Type IV fracture appears at low-stress and long-time conditions. The addition of about 100 ppm boron combined with minimized nitrogen of 10–20 ppm suppresses the formation of fine-grained region in HAZ and hence suppresses the Type IV fracture.

Microstructure and creep strength of welds in advanced ferritic power plant steels

The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 104 h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.

９％Ｃｒ系耐熱鋼のＨＡＺ組織とクリープ特性に及ぼす熱処理の影響

Type IV creep cracking in the heat affected zone (HAZ) of welded joint has been one of the serious problems in 9 to 12%Cr ferritic steels during long time service. We have already shown that the specimens heated to peak temperature close to Ac3 do not have clear lath martensitic microstructure and that the coarsening of precipitates, M23C6, in these specimens is observed after post weld heat treatment (PWHT). In this study, a coarse grained steel is produced by the higher temperature normalizing heat treatment. The average grain size of coarse grained steel was 250μm and that of the Ac3 simulated HAZ specimens was 8μm, compared to 30μm and 6μm respectively for a normal grain size steel. The creep life, microstructural changes and distribution of precipitates of simulated HAZ specimens for coarse grained steel are investigated and compared with those for the normal grain size steel. The creep rupture time of the simulated HAZ specimens for coarse grained steel is found to be longer than that for the normal grain size steel, because the regions of large precipitates in the coarse grained steel, which are considered to have low creep resistance, are relatively smaller than those for normal grain size steel.

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.

Degradation of Creep Strength in Welded Joint of 9%Cr Steel

The degradation of creep strength in the heat affected zone (HAZ) of welded joint has been investigated for a tungsten-strengthened 9Cr steel, 9Cr-0.5Mo-1.8W-VNb. The creep test was carried out for the simulated HAZ specimens and the welded joint at 923K. The creep rupture strength of the welded joint is almost the same as that of the base metal at high stresses but it decreases rapidly and then it becomes almost the same as that of the Ac3 simulated HAZ specimen at low stresses. The creep fracture of the welded joint occurs at the fine-grained zone of HAZ, corresponding to the Ac3 heating, at low stresses. The fine-grained zone of HAZ contains higher density of dislocations than the base metal. The recovery of higher density of dislocations and the sparse distribution of large M23C6 carbides promote the formation of coarse subgrains near prior austenite grain boundaries. This results in the concentration of creep deformation in the coarse subgrains, which accelerates eventual creep fracture.

Ｗ強化高Ｃｒフェライト鋼溶接継手の組織とクリープ強度

On an increasing demand to reduce CO2 emissions and save energy, high strength ferritic heat resisting steels of 9-12%Cr have been developed with a view to elevating metal temperatures of thermal power plants. Type IV creep cracking in heat affected zone (HAZ) of welded joints is a serious problem for these high strengthened steels. The present paper investigates the microstructures and creep properties of welded joints for W strengthened 11Cr-0.4Mo-2W-CuVNb steel. Most of the welded joint specimens were ruptured at HAZ and creep lives decreased than that of base metals. The comparison of creep properties of simulated HAZ specimens showed that fine grains produced by heating around Ac3 temperature was responsible for such degradation of creep strength. The growth of precipitates on grain boundaries and recovery of dislocation structures during creep was faster for fine-grained HAZ. The electron beam (EB) welded joints indicated two times longer creep life than gas tungsten arc (GTA) welded joints, however the brittle Type IV fracture was occurred even in the EB welded joints for long-term creep test. The FEM creep analysis by using the creep data of simulated HAZ specimens could explain the experimental results for creep properties of welded joints.

Quantification of Creep Cavitation in Welded Joint and Evaluation of Material Characteristics of Simulated Heat-Affected Zone in X 20 CrMoV 12 1 Steel

X 20 CrMoV 12 1(DIN 17 175) steel has been used for components subjected to high temperature in power plants and chemical and petroleum industries. Therefore, extensive studies have been made on this steel. However, these studies focused mainly on the base metal, and few studies on the welded joint have been reported. Actually, a large number of failures have occurred at the welded joint, so there is increasing need to investigate the characteristics of X 20 CrMoV 12 1 weldment. In this study, the interrupted and creep rupture tests were carried out and quantification of the creep damage was attempted for the X 20 CrMoV 12 1 welded joint. The interrupted and creep rupture tests were performed at four conditions\M650-60, 600-100, 600-120, and 575-150(|SDC-MPa)\Mon the X 20 CrMoV 12 1 welded joint specimens, respectively. It was revealed from the experimental results that creep damage mechanism of a welded joint was mainly creep cavitation, and that the intensively damaged area by creep cavitations was the transition region from fine-grained heat-affected zone (HAZ) to unaffected base metal, namely intercritical HAZ. For both the interrupted and ruptured specimens, quantification of creep damage was attempted by evaluating cavitated area fraction, average diameter, and the number of cavities with creep life fraction. In addition, on the basis of the heat input during the welding, microstructure, microhardness, and grain size of the actual intercritical HAZ, simulated HAZ was made in order to evaluate its material properties. For the simulated HAZ specimens, tensile, charpy impact, and creep rupture tests were carried out. As a result, yield, tensile strength, and elongation of simulated HAZ were similar to those of base metal, respectively, and impact property of simulated HAZ was slightly above base metal. Also, it was found that creep strength of simulated HAZ was inferior to that of the base metal.

Fracture toughness of cold worked and simulated heat affected structural steel

Deterioration of fracture toughness due to both cold working and welding are studied to investigate the combined effect which is seen in the welded part of cold worked members. It is known that fracture toughness is degraded in cold-worked steels by strain aging and is also degraded in the heat affected zone (HAZ). In this experimental study, CTOD tests are carried out using simulated HAZ specimens which are pre-strained before heat treatment. Although multi-pass welding produces a complex distribution of microstructures in HAZ, two types of thermal cycles are used to simulate the HAZ. One is to simulate intercritically reheated coarse grain HAZ (IRCG HAZ), the most degraded part in the local brittle zone. Another is to simulate subcritical HAZ (SC HAZ), the least degraded part in HAZ. In these two types of simulated microstructures, the effects of cold working are discussed based on fracture mechanics. It can be observed that the fracture toughness in IRCG HAZ is fairly low and the variation of fracture toughness with increasing pre-strain does not exist in that particular region. The fracture toughness in SC HAZ is not so crucial as seen in IRCG HAZ, but is strongly affected by pre-strain before heat treatment.