Intercritical Heat-affected Zone Research Articles

Galvanic Corrosion Behavior of the X80 Steel Welded Joint

Wire beam electrode techniques and classical electrochemical techniques were used to investigate the effect of the area ratio of each part of the welded joint on the galvanic corrosion behavior. The results showed that the order of the corrosion current density of the different regions in the simulated X80 steel welded joint was as follows: coarse-grained heat-affected zone > fine-grained heat-affected zone > intercritical heat-affected zone > base metal > weld metal. As the area ratio of weld metal increased, the galvanic potential shifted positively, the maximum anode galvanic current density increased and the main anode galvanic effect increased. On the other hand, as the area ratio of the base metal and the heat-affected zone decreased, the coupled potential shifted negatively and the maximum anode galvanic current density decreased. The galvanic corrosion intensity of the simulated X80 steel welded joint increased as the area ratio of the weld metal decreased and increased as the area ratio of the heat-affected zone and base metal increased. To enhance corrosion resistance, it is advisable to choose a shape with a larger groove to increase the WM area in the welded joint. Additionally, selecting a welding method with lower heat input and a higher energy density can help reduce the HAZ area in the welded joint.

Influence of microstructural morphologies on liquid metal embrittlement of Zn-coated dual-phase steels

Dual-phase (DP, ferrite+martensite) steels are crucial for automotive components, predominantly spot-welded using thin Zn-coated cold-rolled sheets. However, the effect of martensite morphologies, namely fine-lamellar (LD) or coarse-globular (GD) shapes, on Zn-assisted liquid metal embrittlement (LME) remains underexplored. This study investigates LME mechanisms during resistance spot welding of Zn-coated DP steel sheets with LD or GD microstructures. Analysis focuses on cracks within the weld shoulder zone, categorized into sub-critical HAZ (SCHAZ), inter-critical HAZ (ICHAZ), and upper-critical HAZ (UCHAZ). The LME susceptibility index, calculated as total crack length divided by total contact length, increased in the order of LD-SCHAZ, LD-ICHAZ, GD-ICHAZ, and GD-SCHAZ. These results indicated that the microstructure of the GD sheet itself was very vulnerable to the cracking because the initial microstructure was almost retained in the SCHAZ. Cracking analysis in the LD-SCHAZ and GD-SCHAZ suggested that martensite grains near ferrite/ferrite boundaries impeded crack growth by deflecting propagation, and that the LD sheet showed higher crack resistance due to unfavorably oriented ferrite/martensite boundaries.

Characterization of As-Welded Microstructure in a P91 Steel

The microstructure of the heat affected zone (HAZ) and fusion zone (FZ) in the as-welded condition of a single-pass weld performed by the flux-cored arc welding (FCAW) process was studied in a P91 steel using microhardness measurements, field-emission scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The evolution of precipitates in each region of the single-pass weld was analyzed, and particular attention was paid to the presence of retained austenite (RA). Experimental observations indicated that M23 C6 carbide played a major role in the thermal cycle associated with the weldment. On one hand, the dissolution of M23 C6 led to the precipitation of spherical NbCN in the finegrained HAZ (FGHAZ) near the coarse-grained HAZ (CGHAZ). On the other hand, the total or partial dissolution of M23 C6 carbides changed the chemical composition of the initially formed austenite. In the regions where the M23 C6 carbides were completely dissolved (i.e., the CGHAZ and FZ), retained austenite was observed as a thin, continuous film along primary austenite grains and lath boundaries. Instead, a shell of retained austenite was observed around some partially dissolved M23 C6 of the intercritical HAZ (ICHAZ) and FGHAZ.

Microstructure and deformation behavior of rapid cooling friction stir welded Q&P1180 steel joint

In this work, Q&P1180 steels were butt welded by friction stir welded (FSW) and rapid cooling FSW (RCFSW), aiming to investigate the influence of rapid cooling on the microstructure and deformation behavior of joints. The results showed that for FSW and RCFSW joints, stir zone (SZ) composed of martensite phase, and inter-critical heat affected zone (ICHAZ) composed of ferrite/martensite dual-phase microstructure, and subcritical heat affected zone (SCHAZ) consisted of ferrite, martensite, carbide and residual austenite phases. Compared with FSW joint, the grain of HAZ was refined, the contents of residual austenite and ferrite increased, while the content of cementite decreased in RCFSW joint. The tensile strength and elongation of FSW joint were 1292 MPa and 7.5 %, respectively. Compared to FSW joint, the tensile strength of RCFSW joint slightly decreased (1237 MPa), while the elongation greatly increased to 13.5 %. During tensile deformation, the strain was mainly concentrated in HAZ of FSW joint, while the strain transferred from HAZ to basal material (BM) of RCFSW joints, and finally fractured in BM.

Micro-Alloying Effect on the Inter-Critical Grain Coarsened Heat Affected Zone of a S355 Steel Welded Joint

The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro-alloyed steels (HSLA). Following repeated heating cycles in in with temperature ranging Ac1 /Ac3, the ICHAZ will face with an evident toughness and fatigue behavior reduction especially due to martensite-austenite constituent (MA) formation. Microalloying in high strength steels causes the generation of some phases in the matrix able to increase the mechanical properties of the joint. In this paper we report an investigation related to 1000 ppm vanadium addition in the welded joint of a structural S355 steel. The inter-critical zone of ta double pass welded joint is here reproduced by dilatometer, with second peak temperature ranging 720°C-790°C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.

Effects of Cr additions on Corrosion and Erosion-Corrosion Behaviors in the Weld Heat-Affected Zone of Hadfield Steel in Brine Environments

The corrosion and erosion-corrosion behaviors in the weld heat-affected zone (HAZ) of Hadfield steels with varying Cr contents (1, 2, and 3 wt%) were examined. Various experimental methods, including electrochemical polarization, impedance, and weight loss measurements, were utilized. Two types of isothermal heat treatments were conducted in a box furnace to simulate the intercritical HAZ, known to be the most vulnerable region in terms of mechanical properties and environmental stabilities, and large-scale samples for the erosion-corrosion experiment were fabricated. The results showed that increasing the Cr content improved the resistance to corrosion and erosion-corrosion, but there was an inflection point where adding more Cr had the opposite effect. Up to 2 wt%, a higher resistance was exhibited owing to the formation of a thin and protective oxide scale enriched with Cr that adhered to the steel surface. On the other hand, adding 3 wt% of Cr resulted in decreased resistance. This was due to the formation of coarse M<sub>7</sub>C<sub>3</sub> (M: Cr) precipitated along the grain boundary, which caused the development of a thick and unstable oxide scale that detached locally. Based on these findings, it is essential to optimize the Cr content to ensure a high resistance to corrosion and erosion-corrosion in the HAZ of Hadfield steel.

Study on Microelectrochemical Inhomogeneity of an SA508-309L/308L Overlay Welded Joint.

The corrosion behavior of the dissimilar metal welded joint (DMWJ) is highly dependent on its heterogeneous microstructures. However, directly measuring the electrochemical properties of microstructures in different heat-affected zones (HAZs) is a formidable challenge, because traditional bulk electrochemistry can only offer an average signal. Herein, the microelectrochemical properties of an SA508-309L/308L DMWJ were measured in 3.5 wt % NaCl solution using lithography and capillary techniques. Specifically, high-throughput microelectrochemical tests, including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP), were conducted on 168 spots (Φ 12 μm). Results revealed five typical EIS responses and seven varieties of PDP curves (different magnitudes of the current density). The maps of thermodynamic and kinetic metrics, such as polarization resistance derived from EIS, corrosion potentials, and corrosion currents extracted from potentiodynamic polarization curves, demonstrated good consistency. The uniform corrosion tendency of the SA508 HAZ subregions during the immersion tests is basically consistent with its Ecorr_avg order of subcritical HAZ (C5, -371 mV) < intercritical HAZ (C4, -546 mV) < fine-grained HAZ2 (C3, -579 mV) < fine-grained HAZ1 (C2, -593 mV). The random presence of inclusions leads to highly heterogeneous microelectrochemical properties of the DMWJ, thereby causing localized corrosion to occur preferentially. Moreover, the macroscopic corrosion behavior is affected by the corrosion products, which display a protective effect that modifies the local electrochemical activity of the SA508 HAZ. The combination of microelectrochemical properties allows for a more comprehensive understanding of the macroscopic corrosion behavior of metals and the galvanic effect between the heterogeneous microstructures.

Microstructure characteristics and mechanical performances of the dissimilar welded joint of Q&P980 and Q&P1180 steels subjected to the paint-baking heat treatment

The dissimilar welded joints of Q&P980 and Q&P1180 steels with and without paint-baking heat treatment were investigated and their microstructures were characterized using confocal laser scanning microscope (CLSM), electron backscatter diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) in order to understand the change of mechanical performances of the welded joints. The two Q&P steels have slight differences in compositions, but the obviously varying fraction of tempered martensite and thus the difference in the tensile strength. Prior to paint-baking heat treatment, the dissimilar welds exhibit a lower martensite fraction of the intercritical heat-affected zone (ICHAZ) in the Q&P980 side compared to the Q&P1180 side, and it is not affected by the paint-baking treatment. After the treatment, the occurrence of carbides and the coarsening of martensite in the whole welded joint, as well as the decomposition of retained austenite in the ICHAZ, are observed. The paint-baking treatment also causes the decreasing microhardness values of all regions. Especially, the significantly decreasing microhardness in the fusion zone compared to other heat-affected zones results in a partial pull-out failure mode compared to a full pull-out failure mode without paint-baking treatment. The change in failure mode and the softening of the martensitic matrix are found to be responsible for the increase in the peak load and the energy absorption of the dissimilar welded joint of Q&P980 and Q&P1180 steels.

Microstructural characterization of simulated and actual constituent regions of P91 steel weldment

In the present study simulated and the actual microstructures of the coarse grain, fine grain and intercritical heat-affected zones of P91 steel weldment are characterized using electron backscattered diffraction (EBSD) technique. Both the simulated and actual heat-affected zone (HAZ) microstructures showed profound substructural recovery after post-weld heat treatment/tempering. The subgrain size distributions between the simulated and the actual HAZs are comparable. The actual HAZ microstructures exhibited greater low angle grain boundaries and local strain distributions compared to the simulated HAZ microstructures due to the complex microstructural geometry of the actual weldment. The similarity in the variations of grain size, precipitate size and hardness of the simulated and actual HAZs validates the simulation of HAZ microstructures through furnace heat treatment.

Effect of PWHT on metallurgical and mechanical characterization of dissimilar welded joint of P91 and P92 steels

P91 and P92 steels are widely used in nuclear and thermal power plant’s high-temperature (650–700 °C) operating components. The welding procedure, base plate composition, filler material composition, and post-weld heat treatment (PWHT) significantly affect the metallurgical characterization of weldments at room temperature. This work focuses on joining P91 and P92 steel by the TIG welding with filler ER90S-B9. The study analyzes the materialization of coarse-grained HAZ (CGHAZ), inter-critical HAZ (ICHAZ), and fine grained HAZ (FGHAZ) under welded and PWHT. PWHT was conducted at 840 °C for 2, 4 and 6h. An optical microscopic (OM) scanning electron microscope (SEM) was employed to characterize the welded and PWHT samples. It was perceived that the tensile strength of PWHT increased as the heat treatment times increased from 2 to 6 h, while the hardness value decreased accordingly. The maximum tensile strength (594.71 MPa) was observed in PWHT_6 h, while the minimum tensile strength (537.52 MPa) was perceived in the as-welded sample. The PWHT_6h reveals the minimum hardness values (271.12 HV) at the fusion zone among all the conditions. The WFZ consisted of an untampered lath martensitic grain structure, demonstrating an average hardness value of 314.15 HV in the PWHT_2h condition. The hardness value in CGHAZ samples for as-welded, PWHT_2h, PWHT_4h, and PWHT_6h was observed at 456.85, 436.81, 368.63, and 327.41 HV, respectively.

Deformation behavior, microstructure evolution, and rupture mechanism of the novel G115 steel welded joint during creep

To promote the wide application of the G115 steel in ultra-supercritical (USC) power plants, creep deformation, microstructure evolution and rupture mechanism of the G115 welded joint were systematically investigated. The fabricated G115 welded joint exhibits superior creep strength to the P92 or MARBN steels. The deformation behaviors are analyzed by the Norton's power law, proving that the deformation mechanism is closely associated with the stress levels. During short-term creep under higher stress, the weld metal (WM) and heat-affected zone (HAZ) exhibit outstanding microstructure stability, and the rupture occurs within the base metal (BM) due to the lath fracture. While, during long-term creep under lower stress, the brittle Type IV cracking occurs within the inter-critical heat-affected zone (ICHAZ) or fine-grained heat-affected zone (FGHAZ) due to the formed micro-voids along boundaries, obviously deteriorating the creep strength. The weld strength reduction factors are calculated under different stresses, and prove that synergistic action of back stress and dislocation motion can enhance the creep resistance of the G115 welded joint. Additionally, the allowable service stress of G115 weldments should be designed according to the creep strength of the welded joints under expected performance life.

The relative effect of boron and nitrogen concentrations on microstructure and creep resistance of weld simulated heat-affected zones of modified 9Cr-1Mo steel

To comprehend the combined impact of B and N concentrations on modified 9Cr-1Mo steel's heat affected zone (HAZ) creep resistance, five different steels with varying B (0-100 ppm) and N (20-500 ppm) concentrations were subjected to two types of weld thermal cycles (WTC), namely coarse-grained HAZ simulation and inter-critical HAZ simulation with peak temperatures of 1175 °C and 850 °C, respectively. Prior to and after creep testing (at 650 °C, 120 MPa stress), the microstructures and precipitates in the WTC simulated samples were studied using optical and electron microscopy, electron backscattered diffraction, and auger electron spectroscopy techniques. As expected, simulated HAZs of high B-containing steels (70-100 ppm B) showed superior creep resistance compared to low B steels (0-25 ppm B) due to the B stabilizing effect of M23C6 precipitates (primarily (Fe,Cr)23(B,C)6) though the rupture time for the simulated HAZs are lower than that of the base metals tested at the same test temperatures and stress levels. An interesting finding is that, despite the fact that 70 ppm B steel (with 108 ppm N) demonstrated the best creep resistance in normalized and tempered condition, 100 ppm B steel (with just 20 ppm N) obtained the best creep resistance of simulated HAZ. Large difference in the rupture lives between simulated ICHAZ and simulated CGHAZ specimens reveals the effect of triaxility in reducing the rupture lives in simulated CGHAZ specimens. This is also applicable to actual weld joints.

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.

Physical Simulation and Numerical Simulation of Flash Butt Welding for Innovative Dual Phase Steel DP590: A Comparative Study.

In this study, the microstructure and performance of newly designed dual-phase steel (DP590) after joining by flash butt welding (FBW) for vehicle wheel rims was analysed and compared by two simulations, i.e., physical simulation and numerical simulation, due to the high acceptance of these two methodologies. Physical simulation is regarded as a thermal-mechanical solution conducted by the Gleeble 3500 simulator and which can distribute the heat-affected zone (HAZ) of the obtained weld joint into four typical HAZs. These are coarse-grained HAZ, fine-grained HAZ, inter-critical HAZ and sub-critical HAZ. A combination of ferrite and tempered martensite leads to the softening behaviour at the sub-critical HAZ of DP590, which is verified to be the weakest area, and influences the final performance due to ~9% reduction of hardness and tensile strength. The numerical simulation, relying on finite element method (FEM) analysis, can distinguish the temperature distribution, which helps us to understand the relationship between the temperature distribution and real microstructure/performance. Based on this study, the combination of physical and numerical simulations can be used to optimise the flash butt welding parameters (flash and butt processes) from the points of temperature distribution (varied areas), microstructure and performance, which are guidelines for the investigation of flash butt welding for innovative materials.

Stress corrosion cracking of X80 steel heat-affected zone in a near-neutral pH solution containing Bacillus cereus

Bacillus cereus (B. cereus) is observed to have varying effects on the stress corrosion cracking (SCC) sensitivity of different microstructures in the simulated heat-affected zone (HAZ) of X80 steel. At open circuit potential (OCP), the SCC sensitivity of different microstructures increased from 3.40–7.49% in an abiotic medium to 10.22–15.17% in a biotic medium. At −0.9 V (SCE), it increased from 22.81–26.51% to 35.76–39.60%. The increment in SCC sensitivity upon exposure to B. cereus was highest in the coarse-grained HAZ (7.68 and 16.79% at OCP and −0.9 V, respectively), followed by the intercritical and fine-grained HAZs. Owing to differences in the phase composition, grain boundary type, dislocation density, and surface volta potential, the initial adhesion number and position of B. cereus in the microstructure of the HAZ were differed, resulting in different sensitivities to SCC.

Comparative Study of Typical Electromagnetic Heating Methods in Welded Pipe: Induction Heating Versus Contact Heating

Abstract Typical electromagnetic heating techniques include induction heating (IH) and contact heating (CH). The study of these techniques in welding has always been a controversial topic owing to the lack of straightforward quantitative analysis of heat transfer characteristics. Therefore, to explore the comparative nature of heat transfer in CH and IH, high-frequency welding of ferromagnetic materials was studied. Our results revealed quantitative differences between the two heating methods in terms of heating rate and intercritical heat-affected zone (ICHAZ). Compared with IH, CH resulted in a higher active current accumulating at the V-shaped angle of a welded pipe and a greater heating rate for a thick-walled pipe, because of variable magnetic flux leakage in IH. Given identical electrical input, the area of the heat-affected zone (HAZ) in the case of CH was found to be &amp;lt;1/2 of that in the case of IH, which is the root cause of why the area of performance optimization in localized heat treatment of a contact welded pipe is much smaller than that in the case of induction welding. Our study also reveals that IH resulted in a smaller temperature gradient and more uniform temperature distribution than CH, which is the main reason why heating quality in induction welding is superior to that of contact welding. Such comparison between IH and CH in terms of welding quality and energy utilization helps reveal the intrinsic rules of high-frequency electromagnetic heating and provide scientific data and a rationale for promotion of electromagnetic heating in more applications.

Microstructure and Mechanical Properties of Resistance Element Welded Joints for DP780 Steel and 6061 Aluminum Alloy

DP780 steels and 6061‐T6 aluminum alloys are joined using the resistance element welding (REW) process with a flat Q235 rivet. The macrostructure, microstructure, and mechanical properties of the REW joints are investigated. The nugget zone and upper‐critical heat‐affected zone (UGHAZ) microstructure exhibit lath martensite. The inter‐critical heat‐affected zone (ICHAZ) microstructure consists of ferrite and martensite. Two types of intermetallic compound (IMC) layers are formed at the steel/Al interface. A tongue‐like Fe2Al5 layer is found adjacent to the steel side, while a needle‐like Fe4Al13 layer is found adjacent to the aluminum alloy side. The welding current has a significant influence on the nugget size, peak load, and energy absorption of joints. The fracture behavior analysis of the REW joints is performed using the in situ digital image correlation (DIC) method. There are two failure modes of the REW joint under different welding currents: pull‐out failure (POF) and the base material pull‐out failure (BPF). At a welding current of 17 kA, the maximum tensile‐shear value of the REW joints is 6.27 kN with a nugget diameter of 5.37 mm. The failure mode of the REW joints is mainly BPF mode that exhibits superior lap‐shear performance.

Effect of microstructure on the mechanical properties and corrosion resistance of a welded joint of 620-grade marine steel

The relationship between the microstructure and the mechanical and corrosion properties of a welded joint of 620-grade marine steel was studied using metallographic microscopy, scanning electron microscopy, an energy dispersive spectrometer, transmission electron microscopy, and microhardness and tensile tests. The results showed that the strength and hardness of the weld center area (WMmid) were higher than those of the inner and outer welding surface region (WMin and WMout) because the volume fraction of the martensite-austenite (MA) constituents (21.6%) was higher than that in WMin and WMout (18.0% and 14.3%, respectively). There were numerous MnO-Al2O3-SiO2-TiO2-type inclusions located at the bottom of dimples in the fracture surface; however, the MA constituents took precedence over this kind of inclusion in inducing pitting corrosion. In contrast, pitting corrosion can be initiated by Al2O3-MgO-CaO-CaS inclusions in the heat-affected zone (HAZ) and base metal (BM). The corrosion resistance of the welded joint was in the order of weld metal &amp;gt; HAZ &amp;gt; BM. The WMmid with smaller dendrite spacing and a larger size of MA constituents had better corrosion resistance compared with the WMin and WMout. The corrosion resistance of the HAZ decreased in the sequence of coarse grain HAZ, fine grain HAZ, and intercritical HAZ.

Creep-fatigue interaction behavior of simulated microstructures and the actual weldment of P91 steel

The paper presents first of its kind study on the creep-fatigue interaction (CFI) behavior of simulated coarse grain (CG), fine grain (FG) and inter-critical (IC) heat-affected zones (HAZs) of P91 steel weldment. Total axial strain-controlled fatigue tests were conducted at 550 °C using trapezoidal waveform and introducing 30-min hold at peak tensile strain of ±0.6%. Irrespective of the initial microstructure, the alloy exhibited continuous cyclic softening throughout the deformation under CFI condition. Strain localization together with interlinkage of subsurface creep cavities caused enhanced crack propagation that resulted in the lowest fatigue life in the case of ICHAZ. The actual weldment failed in the interface between FGHAZ and ICHAZ. The mode of failure was found to be largely governed by Type IV cracking.

The analysis of internal stresses on a welded joint in Grade 91 steel under creep test loading: Synchrotron X-ray diffraction measurements and modelling

The analysis and understanding of creep damage of Grade 91 steel welded joints is an important topic in the energy industry. Creep tests on welded joints were carried out at 600∘C, 100MPa and then interrupted at 0%, 10%, 30%, 50%, 80% of the expected life and after failure. Creep damage is characterised by cavity bands located exclusively in the core of the sample in the InterCritical Heat Affected Zone (ICHAZ). These samples were tested using in situ synchrotron X-ray Diffraction (XRD) along the welded joint under creep conditions for the different creep times. The experimental results show a significant evolution of strain and creep damage characteristics on the welded joint, with a local maximum at the Heat Affected Zone (HAZ). Subsequently, a finite element creep strain analysis was performed for comparison with the experimental results.