Fine-grained Heat-affected Zone Research Articles

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

Insight into Type IV cracking in Grade 91 steel weldments

In this work, an insight into the premature Type IV cracking was undertaken to clarify its mechanisms in Grade 91 steel pipe weldments. High-resolution microscopy observations of the as-welded heat-affected zone (HAZ) reveal that the commonly recognized fine-grained region susceptible of cracking on the edge of HAZ belongs to the inter-critical HAZ (ICHAZ), rather than the fine-grained HAZ (FGHAZ). Instrumented indentation tests uncover that the ICHAZ is the weakest region across the weld, exhibiting the largest displacement and the lowest hardness in three thermal stages. Localized deformation of matrix grains and high stress triaxiality in the ICHAZ promoted nucleation of creep cavities along grain boundaries. This localized deformation was induced by the creep strength mismatch of matrix grains with different Cr concentrations. Cavity-free regions exhibit a relatively homogenous Cr distribution, whereas, an inhomogeneous Cr distribution is observed in the cavity-containing regions. It is believed that this local Cr inhomogeneity in the ICHAZ is caused by the partial dissolution of Cr-rich M23C6 carbides and an insufficient homogenization during rapid welding thermal cycles.

Microstructure and mechanical characterization of high strength low alloy steel welded joint by hybrid laser arc welding

Hybrid laser arc welding was developed to joint EQ70 steels. Welds with no visible defects were obtained. Investigations on microstructure and mechanical properties showed that: the welded metal (WM) consisted of acicular ferrite (AF), granular bainite (GB) and M-A constituents. The arc zone and laser zone possessed semblable HAZ microstructures. Coarse grained heat affected zone (CGHAZ) and fine grained HAZ (FGHAZ) were both composed of lath martensite (LM), GB and polygonal ferrite (PF). The intercritically HAZ (ICHAZ) consisted of martensite and M-A constituents. The microstructure of the subcritically HAZ (SCHAZ) was tempered sorbite. Partly cracked blocky M-A constituents were found at the grain boundary of intercritically reheated CGHAZ (ICCGHAZ). Some M-A constituents was slender and distributed in the grain of ICCGHAZ. The arc zone and laser zone had similar hardness and micro-shear strength distributions. The highest hardness and micro-shear strength of the backing and double-pass welded joints appeared near CGHAZs, reheated WM and ICCGHAZ. Slightly soften showed up at the SCHAZs. SCHAZ, CGHAZ and ICCGHAZ showed a reduction of micro-shear toughness. -40°C sub-size Charpy impact testing showed that FL specimens possessed the lowest impact values. The coarse CGHAZs and cracked blocky M-A constituents of ICCGHAZ may be the reason caused the toughness deterioration.

Effect of long-term exposure at 650 °C on microstructural and creep characteristics of T92/Super304H dissimilar welds

This study reports the effect of 650 °C exposure on microstructural evolution and creep behavior of post-weld heat-treated dissimilar welds between the alloys T92 and Super304H. The dissimilar welds were exposed for durations of 24 h, 100 h, 250 h, 500 h, and 1000 h to investigate the thermal stability of the microstructure. Creep tests were carried out at 650 °C and 120 MPa on the post-weld heat-treated and on 1000 h exposed dissimilar metal welds. The isothermal exposure degraded the microstructure of T92-HAZ, resulting in type IV failure at an accelerated rate under creep conditions. Further, accelerated recovery of martensitic laths and extensive precipitation of Laves phase on M23C6 carbides sitting at the grain boundaries of the fine-grained heat-affected zone (FGHAZ) during creep deformation were observed. The isothermal exposure on Super304H-HAZ showed no significant microstructural changes except precipitation of thick M23C6 carbides at the grain boundaries. These prominent microstructural changes in the HAZ of T92 lead to the accelerated void formation in the FGHAZ and are responsible for consequent premature failure.

Failure competition behavior of 9Cr/617 dissimilar welded joint during LCF test at elevated temperature

Failure competition behavior was found after low cycle fatigue (LCF) tests for the dissimilar welded joint between modified 9Cr steel and nickel-based alloy 617 at 600°C. The failure location shifted from base metal (BM) and fine grain heat affected zone (FGHAZ) on the 9Cr steel side to steel/Ni interface with the increase of the strain amplitude. At higher strain amplitude, strain concentration at the steel-Ni interface resulted in steel-Ni interface failure due to the differences in deformability. With the decreasing of the strain amplitude, the lower degree of transformation from lath to cell structure in coarse grain heat affected zone (CGHAZ) caused the competition between interface and FGHAZ failure. FGHAZ failure was governed by higher fraction of dynamic recrystallization (DRX) and formation of cell structure in FGHAZ. DRX was found promoting the formation of cell structure in FGHAZ, thus contributing to its lowest microhardness after LCF tests. As the strain amplitude further decreased, the reduction of DRX fraction made the formation of cell structure dominate the failure. The competing behavior between FGHAZ and BM led to the presence of case for BM failure.

Microstructure and Corrosion Resistance to H2S in the Welded Joints of X80 Pipeline Steel

The microstructure and corrosion resistance in H2S environments for various zones of X80 pipeline steel submerged arc welded joints were studied. The main microstructures in the base metal (BM), welded metal (WM), coarse-grained heat-affected zone (CGHAZ), and fine-grained heat-affected zone (FGHAZ) were mainly polygonal ferrite and granular bainite; acicular ferrite with fine grains; granular bainite, ferrite, and martensite/austenite constituents, respectively. The corrosion behavior differences resulted from the microstructure gradients. The results of the micro-morphologies of the corrosion product films and the electrochemical corrosion characteristics in H2S environments, including open circuit potential and electrochemical impedance spectroscopy, showed that the order of corrosion resistance was FGHAZ > BM > WM > CGHAZ.

Estimation of Residual Stresses by Nanoindentation in an Experimental High Strength Microalloyed Steel Subjected to Rapid Thermal Cycles

The effect of welding thermal cycles on the microstructure and residual stresses was studied in the welding zone of an experimental high strength microalloyed steel with an acicular ferrite and quasi-polygonal ferrite microstructure. Autogenous gas tungsten arc welding was performed on a plate of microalloyed steel to produce a weld bead neglecting the weld joint design and the selection of filler material. To obtain a high welding quality, the welding speed and distance between the electrode and plate was controlled with a small vehicle. To determine the residual stresses, nanoindentation tests were performed in each one of the welding zones and subzones of the heat-affected zone (HAZ). These results showed that all of the nanoindentations presented pile-up and sink-in, and thus, a modification was proposed to calculate the actual contact area and determine the residual stresses. Tensile residual stresses were presented in the coarse-grained HAZ (grain boundary ferrite, acicular ferrite, and bainite); fine-grained HAZ (polygonal ferrite); subcritical HAZ (acicular ferrite and quasi-polygonal ferrite); and base material (acicular ferrite and quasi-polygonal ferrite). The fusion zone (acicular ferrite and bainite) and intercritical HAZ (quasi-polygonal ferrite) had compressive residual stresses.

Effect of cathodic potential on stress corrosion cracking behavior of different heat-affected zone microstructures of E690 steel in artificial seawater

In this work, the stress corrosion cracking (SCC) behavior of E690 steel base metal (BM) and different heat-affected zone (HAZ) microstructures, i.e., coarse grain HAZ (CGHAZ), fine grain HAZ (FGHAZ), and intercritical HAZ (ICHAZ), was investigated at different cathodic potentials in artificial seawater by slow strain rate tensile tests, scanning electron microscopy and electron back-scattered diffraction measurements. The results show that the HAZ microstructures and BM exhibit different SCC susceptibilities: FGHAZ < ICHAZ < BM < CGHAZ, which are controlled by anodic dissolution (AD) at the open circuit potential. With the cathodic potential equaling to −750 mV, the SCC susceptibility of the four microstructures increases because of the synergistic effect of AD and weak hydrogen embrittlement (HE). At -850 mV, AD is inhibited, and the SCC susceptibility of BM decreases, while the SCC susceptibility of the HAZ microstructures increases. At a potential below -850 mV, the SCC susceptibility of the four microstructures gradually increases because of the augment of HE, and the SCC susceptibility of the HAZ microstructures is higher than that of BM. The distinction reveals that the HAZ microstructures have the greater HE susceptibility than BM.

Ion-induced irradiation hardening of the weld heat-affected zone in low alloy steel

The microstructural distribution in the heat-affected zone (HAZ) under the stainless overlay cladding of low alloy steel was investigated. The distribution of the coarse-grain (CG) HAZ and the fine-grain (FG) HAZ by metallurgical observation agreed well with the results of finite element analysis. Base metal contained mixed bainite with ferrite, whereas the CGHAZ and the FGHAZ contained mixed lower bainite with martensite and mixed upper bainite with ferrite, respectively. Ferrite fraction in FGHAZ was higher than those other areas. After ion irradiation with 0.5 dpa, irradiation hardening of HAZ was higher than that of base metal. The irradiation hardening increased with increasing volume fraction of solute clusters, although irradiation hardening at the FGHAZ was greater than that at the CGHAZ, which contained more clusters than the FGHAZ. This difference in irradiation hardening may be due to the differences in the amount of ferrite and others in the different microstructures.

Transmission Electron Microscopy Studies of Plasma Arc-Welded DP600 Dual-Phase Steel in Keyhole Mode

Microstructural characteristics of keyhole plasma arc-welded DP600 steel were analyzed using optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. The fusion zone (FZ) was observed to consist of allotriomorphic, Widmanstatten, and acicular ferrite along with bainite and martensite leading to enhancement in the hardness compared with the base metal. The coarse-grain HAZ consisted of bainite and martensite, while the fine-grain HAZ consisted of bainitic ferrite and tempered martensite. The sub-critical HAZ was found to consist of tempered martensite with reduced density of dislocations and carbide precipitation. This softening resulted in yield point phenomena and failure in the sub-critical HAZ during the transverse tensile test of welds. The non-isothermal tempering resulted in retarded cementite precipitation due to inadequate time for diffusion of carbon, which led to the lowest hardness of 168 HV0.5 in the sub-critical HAZ. The different features in the FZ and HAZ and their correlation with mechanical properties are discussed.

Characterization of elastoplastic properties of dissimilar weld joint of A302/Cr5Mo using the inversion analysis

In this work, the microstructure, macro-and micro-mechanical properties of dissimilar A302/Cr5Mo were investigated by metallographic experiments, tensile and nanoindentation tests under room temperature. Based on inversion analysis, the plastic parameters of micro-domains (small regions across weld joint) were estimated, and the comparison with the experimental results on parent metal (PM) yielded a good agreement. Therefore, by means of this inversion analysis method, elastoplastic properties of PM, welding metal(WM), fine grain heat affected zone(FGHAZ), and coarse grain heat affected zone(CGHAZ) were obtained, which could provide a straightforward way of determining plastic properties on different regions across welded joints.

Microstructure characterisation and microhardness of P92 steel heat treated at the transformation temperatures

Modern power plant components for the transport of superheated steam are manufactured from creep resistant steels, such as ASTM grade P92, that have microstructural stability and excellent mechanical properties at high temperature and pressure. Before application as pipes, tubes and connectors, P92 steel undergoes a heat treatment of austenitisation and tempering to achieve a tempered martensite microstructure, with carbides along the prior austenite grain boundaries and carbonitrides within the matrix. In the power plant, pipes and tubes are joined by welding, inducing residual stresses and inhomogeneous microstructures in the weld joint, which reduce creep resistance. As in most steels, the temperature gradient from the fusion line to the base metal leads to different microstructural zones in the weld joint. The fine-grained heat affected zone (FGHAZ), which forms at temperatures near the transformation region, acts as a weak zone for Type IV cracking during creep. The lower and upper critical transformation temperatures for the P92 steel in this work are 800°C-850°C (Ae1) and 900°C-950°C (Ae3) respectively. As-received samples were heated to temperatures of 830, 850, 900°C to simulate the fine grain heat affected zone, or the very high temperature of 1200°C to simulate the coarse grain heat affected zone (CGHAZ), held at that temperature for 90 minutes and then tempered at 730°C to 780°C for 120 minutes. The microstructures before and after heat treatment were observed by scanning electron microscopy and optical microscopy, and Vickers microhardness measurements were done. After tempering, coarse carbides were seen on the prior austenite grain boundaries (PAGBs) and nitrides were observed in the matrix. In general, the hardness increased with an increase in the austenitisation and tempering temperature combination.

Probing true creep-hardening interaction in weld simulated heat affected zone of P91 steels

The inducement of this paper was to discuss the novel perspective in establishing true creep-hardening interaction at room temperature by hardness and at high temperature by impression creep testing in two boron modified P91 steels. Different sub-heat affected zones (HAZs) were physically simulated. Their creep-strengths were governed by heat treatment, grain-size, hardness, boron addition, and microstructural stability. Post weld heat treatment resulted in reduction in hardness by 50% whereas, impression creep testing showed enhanced creep-strength (50–77%) and the narrow band of creep-damage (∼57%) in P91B steel than P91 steel. An increase in creep-rate with the increase in prior-austenite grain size showed grain boundary embrittlement in P91 steel. Whereas, with an increase in creep-rate and the corresponding decrease in prior-austenite grain size showed grain boundary and sub-grain boundary hardening in P91B steel. Furthermore, type-IV (fine-grain HAZ) and type-III (coarse-grain HAZ) failures were observed depending upon critical prior-austenite grain size of 17 μm in P91 and P91B samples respectively. Based on prior-austenite grain size and creep-strength, sub-HAZs were classified into different classes pursuant to the nature of creep-damage and discussed systematically. Grain boundary hardening was ineffective for both coarse + fine grain HAZ (CF) and B-CF due to the locking movement of fine-grains. However, the formation of soft ferrite grains weakens both coarse + inter-critical HAZ (CI) and B-CI. This study provided better understanding of failure mechanisms in P91 and P91B steels that results in the delay of failures of power plant components.

Microstructure of resistance spot weld ultra high strength steel Q&P980

1mm thick Q&P980 steel plate was resistance spot welded and the appearance and microstructure of welded joint were investigated. The results show that Q&P980 steel achieves a better connection, When welding current is 12.5 kA, welding time is 10cycle, and electrode pressure is 2.4 kN. The joint can be divided into Fusion Zone (FZ), Coarse Grained (CG) HAZ, Fine Grained (FG) HAZ, and Inter Critical (IC) HAZ, Base Material (BM). The FZ undergoes recrystallization, and the HAZ undergoes a phase change due to thermal cycling and plastic deformation. FZ, CGHAZ and FGHAZ are completely austenitized and lath martensite is formed. ICHAZ generates tempered martensite.

Investigation of local tensile strength and ductility properties of an X100 submerged arc seam weld

Tensile testing is the most common method to determine the mechanical properties of a weld joint, yet the local tensile properties can be different from the global properties when stress localization occurs. In the current study, Digital Image Correlation was used to explain the difference in local and global tensile behavior in different sections of a submerged arc specimen. Tensile tests are used to compare the base metal and weldment properties of a two-pass submerged-arc welded X100 pipe steel in terms of strength and ductility. The base metal exhibits superior tensile strength compared to specimens machined from the second pass (Pass 2) or the overlap weld region spanning between Passes 1 and 2, corresponding to the reheated heat affected zone. Both cross-weld tensile specimens fractured in the fine grained heat affected zone (FGHAZ). Although the overlapped tensile specimen achieved higher global strain compared to the Pass 2 weld specimen, it sustained lower local strain, which promoted fracture in the HAZ. In addition, the overlapped tensile specimen exhibited a higher degree of strain hardening. The martensite-austenite (MA) microconstituents in the FGHAZ of Pass 2 contained higher carbon than the MA in the overlapped FGHAZ, which led to increased MA hardness as measured by nanoindentation. A higher fraction of MA with a finer spacing in the FGHAZ of the overlapped HAZ zone provides higher strength and strain hardening but lower ductility. The tensile fracture was dominated by void formation from the hard MA/ferrite interfaces in all tensile specimens, and so an increased fraction of MA led to accelerated microvoid coalescence, and reduced ductility in the overlapped HAZ specimens. In-situ strain monitoring technology via Digital Image Correlation was used to identify the potential fracture location during tensile test. X-ray tomography clearly revealed microvoid formation and coalescences of voids.

Experimental Study of Welding Effect on S690Q High Strength Steel Butt Joints

ABSTRACTIn this study, welding effects on grade S690Q high strength steel butt joint are investigated experimentally in terms of microstructure, micro‐hardness, and tensile behaviour. Three butt joints with 16 mm thickness are fabricated with reheated, quenched and tempered S690Q high strength steel (HSS) plates by using Shielded Metal Arc Welding with different welding heat inputs. Post‐welding microstructure transformation is revealed in base material, coarse‐grained heat‐affected zone (CGHAZ), fine‐grained heat‐affected zone (FGHAZ) and tempering zone. It is found that the main microstructure of S690Q high strength steel is tempered martensite. After welding, the microstructure is transformed to granular bainite in CGHAZ and to ferrite &amp; cementite in FGHAZ. While, in tempering zone, some temper martensite decomposed to ferrite. Then, micro‐hardness tests performed for the butt joints confirmed the existence of a soft layer within the HAZ due to the microstructure transformation. The HAZ width increased when higher welding heat input is adopted. Subsequent coupon tensile tests showed that all the welded joint fractured within the HAZ. The tensile strength of the butt joints deteriorated with the increase of the welding heat input since the soft layer became bigger in HAZ.

Physical Simulation-Based Characterization of HAZ Properties in Steels. Part 2. Dual-Phase Steels

The development of high-strength weldable steels has diversified the range of design alternatives subjected to more and more severe operation conditions. An excellent combination of high work-hardening rate and ductility improved the ultimate tensile strength of dual-phase (DP) steels and made them lucrative for application in automotive industry. The paper focuses on HAZ hardening and softening of three automotive DP steels, which affect their mechanical properties and their crack susceptibility, respectively. A Gleeble 3500 thermomechanical simulator was used to simulate the welding thermal cycles for all HAZ subzones that correspond to TIG welding of three DP steels. Steel samples were heated to different peak temperatures (650, 775, 950, and 1350°C) and cooled at two cooling times of 5 and 30 s. The Rykalin 2D model was used for the numerical simulation of this process. The hardness and microstructure of the specimens were then tested and analyzed using Vicker’s hardness test and optical microscope, respectively. In the investigated cooling time range, the HAZ of the examined cold-rolled DP steels was more susceptible to softening than to hardening. The softening occurred in almost all HAZ sub-regions but was the most pronounced in the fine-grained and intercritical HAZ, where we recorded the highest decrease in hardness for all DP steels. With an increase in heat input and longer cooling time, softening occurred in CGHAZ of all three DP steels under study, which can be attributed to the formation of upper bainite.

Effect of microstructures on notch fatigue properties in ultra-high strength steel sheet welded joint

The fatigue properties of different regions, namely, the weld metal (WM), the fusion line (FL) and the fine grain heat affected zone (FGHAZ), in welded joints with different surface notch root radii r=0.1,0.2and0.3mm were investigated by using welded joints of 980 MPa grade ultra-high strength steel sheets with a thickness of 2.3 mm. The fatigue strength of the FGHAZ was higher than those of the WM and FL despite the lower hardness of the FGHAZ, while the effect of the microstructure became prominent at r≥0.2mm, where the stress concentration factor Kt≤2.24. The strain gauge measurement and FE analysis imply that the effect of the microstructure arose only within 50 μm from the surface of the notch. In addition, the critical distance approach was applied to evaluate the relationship between the effective stress range Δσeff and the number of cycles to faliure Nf. As a result, the critical distance L of WM, FL and FGHAZ were 37.2, 59.7 and 26.7 μm, respectively. The fracture surface and cross-secctional observation suggested that fatigue cracks of WM and FL initiated at twin or prior austenite grain boundaries, then propagated along martensite laths in the crystallographic propagation stage (Stage I). In contrast, the fatigue cracks of FGHAZ initiated inside ferrite grains, and then the fine grain boundaries and martensite-austenite of the FGHAZ blocked and branched the cracks. These results can explain the fatigue strength reversal phenomenon with respect to the hardness of the FGHAZ.

Effect of Heat Input and Undermatched Filler Wire on the Microstructure and Mechanical Properties of Dissimilar S700MC/S960QC High-Strength Steels

The effect of heat input on the microstructure and mechanical properties of dissimilar S700MC/S960QC high-strength steels (HSS) using undermatched filler material was evaluated. Experiments were performed using the gas metal arc welding process to weld three samples, which had three different heat input values (i.e., 15 kJ/cm, 7 kJ/cm, and 10 kJ/cm). The cooling continuous temperature (CCT) diagrams, macro-hardness values, microstructure formations, alloy element compositions, and tensile test analyses were performed with the aim of providing valuable information for improving the strength of the heat-affected zone (HAZ) of both materials. Micro-hardness measurement was conducted using the Vickers hardness test and microstructural evaluation by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The mechanical properties were characterized by tensile testing. Dissimilar welded samples (S700MC/S960QC) with a cooling rate of 10 °C/s (15 kJ/cm) showed a lower than average hardness (210 HV5) in the HAZ of S700MC than S960QC. This hardness was 18% lower compared to the value of the base material (BM). The best microstructure formation was obtained using a heat input of 10 kJ/cm, which led to the formation of bainite (B, 60% volume fraction), ferrite (F, 25% volume fraction), and retained austenite (RA, 10%) in the final microstructure of S700MC, and B (55%), martensite (M, 45%), and RA (10%), which developed at the end of the transformation of S960QC. The results showed the presence of 1.3 Ni, 0.4 Mo, and 1.6 Mn in the fine-grain heat-affected zone of S700MC. The formation of a higher carbide content at a lower cooling rate reduced both the hardness and strength.

Effect of microstructure on corrosion of welded joints of X80 steel in water saturated supercritical CO2

Purpose The purpose of this paper is to research whether microstructure varieties of zones at welded joints pose a tremendous effect on its corrosion in SC-CO2. Design/methodology/approach The interrelation between the corrosion and microstructure of base metal (BM), fine grain heat affected zone (FHAZ), coarse grain heat affected zone and weld metal (WM) in welded of X80 steel in water saturated supercritical CO2 was studied by using optical microscope, weight loss test, electrochemical measurements and surface analytical techniques. Findings The all subzones of X80 weld joints were attacked by SC-CO2 corrosion and showed flower-like corrosion scale spots consisted of granular FeCO3. The most severe corrosion appeared at WM due to lower proportion of ferrite to pearlite, but the slightest corrosion displayed at BM. GHAZ with larger grains and more polygonal ferrite exhibited more severe corrosion than that at BM. Due to its smaller grain, FHAZ displayed comparatively more severe corrosion to that at BM. Originality/value There exists close interrelation between the corrosion and microstructure of the welded carbon steel in water saturated supercritical CO2.