Intercritical Heat-affected Zone Research Articles

A Physical Simulation of Heat Affected Zone of Creep Resistant Steel: Microstructure and Mechanical Properties Analysis

The structural integrity of ASTM P92 steel welded joints is of concern due to the Type IV cracking associated with the heat-affected zone (HAZ). Two HAZ treatments were done on P92 steel using a Gleeble® 3500 thermo-mechanical simulator at 900°C (intercritical HAZ) and 950°C (fine-grained HAZ). Followed by post-weld heat treatment (PWHT) of the samples in two sets:: a conventional PWHT at 760°C for 2 hours followed by air cooling; or re-austenitisation at 1050°C for 40 minutes then air cooling, followed PWHT. After conventional PWHT, the HAZ simulated at 950°C had the lowest toughness (108J) than the base metal (130J). After the heat treatments, samples that underwent a 900°C HAZ simulation had higher Charpy toughness (improved from 130-208 ± 6J) and lower hardness (decreased from 234.4-222.3HV0.5) than those at 950°C. The microstructures had lath martensite with differing precipitate densities depending on the thermal treatment. Keywords: P92 steel, heat affected zone, post-weld heat treatment, precipitates

Effect of beam wobbling on microstructure and hardness during laser welding of X70 pipeline steel

Laser welding with beam wobbling was employed to weld X70 pipeline steel. The influence of beam wobbling on weld surface morphology, fusion zone microstructure, and microhardness was investigated. The formation of spatter was suppressed by beam wobbling using a beam with higher energy density. At a welding speed of 1.0 m min−1, the weld metal produced with a circular laser wobble pattern contained more martensite but less ferrite, compared to that of static spot welded joint. The presence of more martensite led to the increase in fusion zone hardness. A softened region was found in the inter-critical heat-affected zone. The hardness of the softened region could be improved by increasing the welding speed (from 1.0 to 1.5 m min−1).

Effects of V–N Microalloying on Microstructure and Property in the Welding Heat Affected Zone of Constructional Steel

Shielded metal arc welding and welding thermal simulation experiment were carried out for constructional steel containing 0% V and 0.10% V, and the microstructure, precipitation feature, microhardness HV0.2, and −20 °C impact value in the welding heat affected zone (HAZ) were investigated. The results showed that in the coarse-grained heat affected zone (CGHAZ), V and N were completely dissolved in the matrix of steel containing 0.10% V to promote the growth of prior austenite grains, meanwhile the fraction of high angle grain boundaries (HAGBs) decreased, thereby leading to the mean −20 °C impact value decreases from 87 J to 18 J. In the grain refined heat affected zone (GRHAZ), V(C, N) precipitates experience re-dissolution and re-precipitation at grain boundaries, V–N microalloying changes the microstructure from lath bainite + granular bainite + small amount of polygonal ferrite to polygonal ferrite + pearlite + martensite, thereby leading to the mean microhardness decreases from 335 HV0.2 to 207 HV0.2, and the mean −20 °C impact value decreased from 117 J to 103 J. In the intercritical heat affected zone (ICHAZ), V(C, N) precipitates experience re-dissolution, re-precipitation, and growth, causing the formation of micro-sized V(C, N) precipitates, thereby leading to the mean −20 °C impact value decreases from 93 J to 62 J.

Evolution of Microstructure in Welding Heat-Affected Zone of G115 Steel with the Different Content of Boron.

Welding thermal simulation was performed to investigate the effects of boron content (0, 60, and 130 ppm), welding peak temperature (Tp), and cooling time from 800 to 500 °C (t8/5) on the microstructure, carbide, subgrain, and microhardness of heat-affected zone (HAZ) in G115 steel. According to the experimental results, the microstructure of coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), inter-critical HAZ (ICHAZ), and sub-critically HAZ (SCHAZ) was martensite, martensite containing a small amount of undissolved carbide, martensite, and over-tempered martensite, tempered martensite, respectively. The presence of B element improved the thermal stability of M23C6 carbide, thereby resulting in a greater amount of undissolved carbides with a larger diameter in the materials with higher B content under the same Tp. Element B is effective in improving Ac1 and Ac3 for the material. Besides, compared with the material without and containing 60 ppm B, the Ac1 and Ac3 of the material containing 130 ppm B increased by 95 and 108 °C, 69 and 77 °C, respectively. Meanwhile, the FGHAZ area of the material containing 130 ppm B was significantly lower than the material without or containing 60 ppm B, indicating that element B can significantly reduce the formation range of FGHAZ. The alloy content in austenite of ICHAZ of materials without or containing 60 ppm B increased, compared with CGHAZ, its Ms and Mf declined by 50 and 7 °C, 46 and 7 °C, respectively. In contrast, the alloy content in austenite of the material with 130 ppm B content decreases, its Ms and Mf was 37 °C and 32 °C higher than CGHAZ, respectively. The microhardness of HAZ was ranked in descending order as CGHAZ, FGHAZ, ICHAZ, and SCHAZ. Differently, the microhardness of CGHAZ and FGHAZ showed an increasing trend with the rise of B content but exhibited a decreasing trend with the rise of t8/5.

On the changes in the low-cycle-fatigue life and cracking mechanism of P91 cross-weld specimens at elevated temperatures

A significant reduction in the fatigue life of cross-weld P91 specimens was observed in tests at 538 and 566 °C compared to that at 25 °C. The remarkable reduction of the fatigue strength was directly correlated to the shift of the fatigue cracking locations from the BM region at 25 °C to the interface between the intercritical heat-affected zone (ICHAZ) and BM at elevated temperatures. The low-cycle fatigue (LCF) resistance deterioration mechanisms were attributed to synergic interactions between a high rate of microstructure deterioration accompanied by severe plastic deformation and lower strain hardenability in the ICHAZ region at elevated temperature.

Insight into the creep-fatigue interaction and remaining creep damage mechanisms in different micro-regions of 9%Cr steel welded joints

The deformation and damage mechanisms of welded joints have gained high interest for their widespread use in power plants. The inter-critical heat-affected zone (ICHAZ) is generally identified as the weakest region in the whole welded joint. However, the micro-regions' nanoindentation and creep results in the present work reveal that the weakest position moves to base material after fatigue loading. It is found from electron backscatter diffraction (EBSD) analysis that the early saturated grain growth in the ICHAZ contribute to the movement of the weakest region. It is important to note that compared with the creep strength in the as-received condition, prior fatigue loading enhances the creep deformation. It is also observed from three-dimensional X-ray tests that large fatigue cycles and long hold time of fatigue loading accelerate the nucleation of creep voids. Finally, a fatigue damage parameter based on the nanoindentation microhardness is proposed, which is validated to be capable of capturing the effect of various loadings on the subsequent creep life and creep fracture location. These results benefit the understanding of the creep behaviour of welded joints after fatigue damage and provide a guide for the design and damage evaluation of welded components.

Influence of chemical composition and thermomechanical treatment of low-carbon steels on the microstructure and mechanical properties of their laser welded joints

The paper reports microstructures (revealed by transmission electron microscopy) in various zones of laser welds of the X70 and X80 low-carbon steels with different initial microstructures, as well as chemical and phase compositions. In the X70 steels with 0.13% C, the microstructure refinement has been achieved through helical rolling at temperatures of 920 °C and 850 °C (designated as X70-920 and X70-850, respectively). For all studied cases, both initial steel microstructures and phase compositions have determined the formation of different microstructures with various microhardness levels in the weld metal and heat-affected zones. For the X70-850 steel with a more dispersed and homogeneous microstructure (dF = 3.3 μm), a lower microhardness level of 340 HV has been observed in the weld metal, compared with the X70-920 one (dF = 5.5 μm, 370 HV). The reason has been the formation of both bainite and martensite laths in the X70-850 weld metal, while only lath and lamellar martensite has formed in the X70-920 one. For the X80 steel (0.55% C), lowering the carbon content and additional microalloying with chromium, molybdenum and nickel have enabled to decrease the microhardness level down to 295 HV in the weld metal due to the degenerate upper bainite formation and the carbon level reduction in martensite. The dispersed and homogeneous initial microstructures of the X70-850 and X80 steels has provided the formation of granular ferrite-bainite microstructures in the intercritical heat-affected zone. They have possessed a lower proportion of residual austenite regions and small sizes of twinned martensite areas. The welded X80 steel specimen has been characterized by higher ductile properties compared to both X70 ones.

Microstructural characterization of the physical simulated and welded heat-affected zone of 9% Ni steel pipe

The microstructure, Vickers microhardness, and prior austenite grain size (PAGS) of the heat-affected zone (HAZ) in a circumferential welded joint of a 9% Ni steel pipe were compared with those of a physically simulated (Gleeble machine) HAZ to validate a developed methodology for evaluating the HAZ via finite element method simulation. The welding of the cap pass was computationally simulated; subsequently, the thermal cycles of the HAZ subzones were reproduced (thermomechanical simulator), and the HAZ (physically simulated and of the welded joint) microstructures were compared. The HAZs in these conditions showed similar microstructures, microhardnesses, and PAGSs, thereby validating the developed methodology. The subcritical (SCHAZ), intercritical (ICHAZ), fine-grain (FGHAZ), and coarse-grain HAZs (CGHAZ) exhibited base metal unaltered microstructure, dual-phase microstructures (martensite and ferrite), refined martensite, and coarse martensite with coalesced bainite, respectively. The physically simulated HAZ was investigated using X-ray diffraction spectroscopy and transmission electron microscopy, and it exhibited globular and thin-film austenite morphologies, with minimum and maximum contents in the CGHAZ and ICHAZ, respectively. The developed computational and physical simulation methodology can be reliably reproduced and can evaluate the HAZ microstructure of the 9% Ni steel welded joint.

An investigation on laser/DP-MIG hybrid weldability of aluminum to Al-Si coated boron steel

Laser/double pulse-MIG (laser/DP-MIG) hybrid welding-brazing of 5052 aluminum alloy to Al-Si coated boron steel was successfully conducted in a lap configuration with an Al-5Si filler. The heat input plays a significant role in determining the welding performance. The satisfactory appearance and excellent brazed interface with a maximum thickness of 4.1 μm was achieved at the optimum heat input of 161.8 J/mm, and the maximum joint strength reached to 168.8 MPa in excess of 75% aluminum resistance. Three kinds of intermetallic compounds (IMC) phases were nicely identified: τ5-Fe2Al8Si, θ-Fe(Al,Si)3 and η-Fe2(Al,Si)5. Microstructure evolution of brazed joint showed that microstructure of upper-critical HAZ (UCHAZ) consisted of martensite whereas that of inter-critical HAZ (ICHAZ) consisted of ferrite and martensite. The two failure modes (interfacial failure (IF) and HAZ failure (HF)) was associated with the interfacial characteristic and porosity percentage. IF mode was a typical intergranular fracture, and its cracks initiated from notch of weld joint and then redirected toward the region of pore concentration. HF mode was a mixture of major ductile fracture and minor cleavage fracture, and its cracks might initiate from notch of weld joint and then propagated along the IMC throughout the whole interface.

Microstructure evolution and dynamic mechanical behavior of laser welded dissimilar joint between QP1180 and TRIP780

The present work investigates dissimilar fiber laser joints between QP1180 and TRIP780 steels. The scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) characterizations were conducted to study the detailed microstructures of each region of the joints. The result showed that a large volume fraction of tempered martensite (TM) is responsible for the softened zone in the inter-critical heat-affected zone at the QP1180 side of the joints. The digital image correlation (DIC) technique demonstrated that the softened zone triggered by TM in the heat-affected zone (HAZ) at the QP1180 side has little effect on the tensile mechanical properties because of the higher hardness than at the TRIP780 side. The dimples on the fracture surface and higher elongation of the tensile samples show good ductility, which is due to the TRIP effect. The formability of dissimilar weld joints was characterized by the Erichsen cupping test. The Erichsen values varied at different punch pole locations due to the softening zone and non-uniform microstructure on the two sides of HAZ.

A novel approach to envisage effects of boron in P91 steels through Gleeble weld-HAZ simulation and impression-creep

This paper induced a novel methodology for the characterization of creep behavior of weld heat-affected zone (HAZ) for boron-free P91 (PM) and boron modified P91B (B-PM) steels. Gleeble-3800 thermo-mechanical simulator replicated specimens, representing coarse-grain HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and inter-critical HAZ (ICHAZ). Short-term impression creep tests were conducted at 625°C/270-410MPa on PM/B-PM and their simulated HAZs after being subjected to post-weld heat treatment (PWHT) of 760°C/3 h. Microstructural characterization and local strain analyses were accomplished by electron back-scattered diffraction. Simulated microstructures of P91B-FG/ICHAZ after PWHT exhibited lath martensitic structure and large prior-austenite grain size as regards P91-FG/ICHAZ, correspondingly. Average values of local microstructural strain from local average misorientation were relatively high in B-PM and P91B-ICHAZ than PM and P91-ICHAZ, respectively. Similar observations were found for P91-CG/FGHAZ with their counterparts. Stress dependent steady-state creep-rate (SSCR) followed power-law for all specimens except PM. The minimum and maximum ranges of SSCR for P91B specimens were observed to be in a narrower range than P91 specimens. The value of stress exponent for all specimens was evaluated, and corresponding mechanisms were discussed. The analyses of microstructures and corresponding impression creep behavior of P91/P91B samples suggested that modification of 100 ppm boron to P91 steel improved creep-rupture ductility that delayed type IV failure at outer HAZ of P91 steel weldments.

Gas metal arc welding of XPF800 steel for automotive industry: Microstructural evaluation and mechanical properties investigation

In this study, the effect of gas metal arc welding on the mechanical and microstructure properties of hot-rolled XPF800 steel newly produced by TATA Steel has been investigated. This steel finds its role in the automotive industry as chassis and seating applications. The microstructure transformation during gas metal arc welding has been analyzed using scanning electron microscope, optical microscope, and energy dispersive X-ray spectrometry. Tensile, Charpy impact, and microhardness tests have been implemented to determine the mechanical properties of welded samples. Acceptable welded joints have been obtained using heat input in the range of 0.28–0.46 kJ/mm. It has been found that the base metal hardness of the welded sample is 320 HV0.1. On account of the heat-affected zone softening, the intercritical heat-affected zone hardness values have diminished ∼20% compared to base metal.

Welding-induced yield strength heterogeneity in X100Q: Modelling and characterisation

Yield strength (YS) heterogeneity in X100Q high strength low alloy (HSLA) steel welded joints is a key factor which influences fatigue life performance. The effects of welding-induced microstructural changes are investigated using a mechanistic YS model, which enables understanding of the key strengthening mechanisms which give rise to YS heterogeneity in X100Q. The model is used to predict YS in parent metal, physically-simulated fine-grain heat-affected zone material, and inter-critical heat-affected zone material, incorporating cooling rate effects. The model successfully predicts the measured welding-induced YS heterogeneity, with high-angle boundary (HAB) and precipitate strengthening mechanisms as dominant factors.

The Effect of Long‐Term Ageing on Microstructural Properties and Laves Phase Precipitation of Welded P91 and P92 Steels

Creep resistant 9% Cr steels are frequently used in civil engineering projects of supercritical coal‐fired power plants operating at temperatures of superheated steam around 610 °C. This work focus on analysis of microstructure degradation and degradation of mechanical properties of orbital narrow‐gap welding of P91 and P92 steels. Nowadays welding technology of orbital narrow‐gap using tungsten inert gas is not limited to welding of high‐pressure steam pipelines. This method is highly productive and efficient and leads to an increase in productivity, quality, and repeatability of welding process. It replaces the original method of manual welding with coated electrode into a standard bevel, which is a time‐consuming process that costs a high amount of weld metal. Welds are degraded at 650 °C/30 000 h without applied stress and the effect of degradation is evaluated across the weld based on the observed changes in microstructure and hardness. Attention is focused mainly on precipitation and coarsening of Laves phase in the main areas of heat affected zone (HAZ): coarse grain HAZ, fine grain HAZ, and intercritical HAZ. The results show that frequency and rate of Laves phase coarsening in the fine grain and coarse grain HAZ are higher than in the base material.

Effect of nickel-based filler metal types on creep properties of dissimilar metal welds between Inconel 617B and 10% Cr martensitic steel

Creep rupture behaviors of dissimilar metal welds (DMWs) between Inconel 617B and 10%Cr martensitic steel with two different filler metals Inconel 617 and 82 were comparatively investigated at 600 °C to study the effect of nickel-based filler metal types on creep properties. Both the crept DMWs ruptured on the steel side prematurely compared to steel base metal (BM). The failure mode of DMW shifts from interfacial failure at weld metal (WM)/steel interface to Type Ⅳ crack in the intercritical heat-affected zone (ICHAZ) of steel with longer rupture life at the same creep condition by replacing Inconel 617 filler metal with Inconel 82. Type Ⅳ crack was related to matrix softening and fine prior austenite grain boundaries (PAGBs) scarcely decorated by precipitates in the ICHAZ. Interfacial failure was controlled by creep voids nucleation, growth and coalescence at the interface, while oxide notch only triggered crack initiation. The failure mode shift is because Inconel 82 filler metal compared to Inconel 617 can achieve slighter creep deformation concentration in the HAZ closely adjacent to interface and avoid interfacial failure of the DMW due to the lower creep strength gradient across the interface it induced. Based on this study, creep strength compatibility should be considered when selecting nickel-based filler metals for similar DMWs to obtain prolonged creep life.

Experimental investigation on microstructure, mechanical properties, and residual stresses of dissimilar welded joint of martensitic P92 and AISI 304L austenitic stainless steel

In this study, microstructural characteristics and mechanical properties of martensitic steel P92 and AISI 304L austenitic stainless steel (ASS) dissimilar metal weld (DMW) have been examined. The welding was performed using the multipass gas tungsten arc welding (GTAW) process. The ERNiCrMo-3 (Inconel 625) filler metal was used due to its excellent compatibility with the P92 and AISI 304L ASS metals. The microstructure characterization performed with an optical microscope and scanning electron microscope revealed the presence of an unmixed zone in the form of beach, peninsula, and island at the base metal and weld fusion zone interface. Secondary phases enriched with Nb and Mo, and migrated grain boundaries (MGBs) were also observed at the weld fusion zone. The microstructural characterization at the P92 side revealed the presence of a soft ferrite zone and delta ferrite at the weld and P92 interface. The different peak temperature experienced by the P92 metal during the welding process results into the formation of three distinct zones with diverse mechanical and microstructural properties namely coarse grain heat affected zone (CGHAZ), fine grain heat affected zone (FGHAZ), and inter-critical heat affected zone (ICHAZ). The post weld heat treatment (PWHT) at 760 °C for 2 h followed by air cooling was performed to homogenize this heterogeneous microstructure formed toward the P92 side. It was observed that this PWHT does not have any significant effect on weld fusion zone and 304L SS heat affected zone (HAZ) microstructure. But, PWHT modifies the microstructure of P92 side HAZ. The tensile test, Vickers micro-hardness test, and charpy impact test were carried out to determine the mechanical properties of the DMW. The tensile strength was found as 587.709 MPa in the as-welded condition and 594.515 MPa in PWHT condition. The tensile test results indicated that specimens were failed from 304L stainless steel (SS) base metal (BM) in as-welded and PWHT condition. The charpy impact toughness test results showed that the toughness of the weld joint was very low in as-welded and PWHT condition (47J) compared to the P92 (190J) and SS 304L (285J) BM. From the microhardness examination, it was observed that the ICHAZ was the weakest zone among all the other regions due to its lower micro-hardness value compared to the other regions. After welding process, residual stresses were measured in as-welded and after PWHT condition at the center of the weld and at the HAZ of P92 and SS 304L (through the thickness of the plate) using experimental-based deep hole-drilling (DHD) strain-gauge method. The results showed that both circumferential (hoop stress) and axial (longitudinal stress) welding residual stresses were decreased after PWHT.

Damage behaviour in different micro-regions of P92 steel weldment under sequential low cycle fatigue and creep loading

Understanding the evolution of microstructure and micro-region properties of weldments during cyclic loading is a challenge for the reliability assessment of high-temperature components. This work is devoted to quantitatively evaluating the variation of micro-region properties and corresponding responsible microstructural features of P92 steel weldments under interrupted fatigue tests and subsequent creep fracture. To achieve this target, high-resolution characterization techniques, including electron backscatter diffraction (EBSD) and nanoindentation tester were used. The results revealed that the microstructures in the inter-critical heat affected zone (ICHAZ) and adjacent regions, fine-grained HAZ (FGHAZ) and parent metal (PM), show significant sensitivity to fatigue loading. Notably, the grain size in the ICHAZ reaches saturation after fatigue cycles of 10% fatigue lifetime, and the weld metal (WM) keeps unchanged throughout the fatigue loading. After subsequent creep fracture, only the WM presents an increasing trend in the average grain size. Nanoindentation tests uncover that the reductions of the elastic modulus and microhardness in each region also present three cyclic softening behaviour during the fatigue process due to the evolution of martensite lath structure. However, due to nucleation and growth of cavities accelerated by carbide precipitation, relatively low elastic modulus and microhardness were observed after subsequent creep loading.

Assessment of Heat-Affected Zone Softening of Hot-Press-Formed Steel over 2.0 GPa Tensile Strength with Bead-On-Plate Laser Welding

High-strength hot-press-formed (HPF) steels with a fully martensitic microstructure are being widely used in the fabrication of automotive body structure, and 2.0 GPa-strength HPF steel has recently been commercially launched. However, heat-affected zone (HAZ) softening is unavoidable in welding martensitic steel. In this study, the HAZ softening characteristic of 2.0 GPa HPF steel was investigated by applying a high-brightness laser welding process, wherein the heat input was controlled by varying the welding speed. Microstructural evaluation and hardness test results showed that the base metal with a fully martensitic microstructure was changed to the same type of fully martensitic microstructure in the weld metal, while relatively soft microstructures of tempered martensite and ferrite phase were partially formed in the intercritical HAZ (ICHAZ) and subcritical HAZ (SCHAZ) areas. In the tensile test, the joint strength was 10–20% lower than that of the base metal, and the fracture initiation was estimated at the ICHAZ/SCHAZ boundary, where the lowest hardness was confirmed by the nanoindentation technique.

Microstructural evolution of P92 steel in IN740H/P92 dissimilar weld joints during creep deformation

In this study, we report the microstructural development of heat-affected zone in P92 heat-resistant steel during creep deformation in cross-weld specimens with dissimilar weld joints between IN740H/P92 materials. Different specimens were prepared via the interruption of creep testing at 50, 70, and 90% of the measured creep-rupture life and were carefully examined and compared. To explain the deformation behavior, finite element analysis was performed in conjunction with softening behavior measured by electron back-scattered diffraction and hardness mapping. The FGHAZ II region defined in this study could be related to the generally known inter-critical heat-affected zone, which showed major changes in microstructure during creep deformation due to the accelerated softening by dynamic recrystallization as well as to the development of a triaxial stress state. As a result, cavity formation and growth was observed only in the FGHAZ II region, which ultimately led to fracture.

Microstructural aspects of hydrogen stress cracking in seawater for low carbon steel welds produced by flux-cored arc welding

This study investigated the role of weld microstructures on hydrogen stress cracking (HSC) in low carbon steels. HSC behaviours were compared for base metal (BM) and transverse-weld joints (WJs) using in-situ slow strain rate testing (SSRT) with hydrogen charging. The HSC resistivity of transverse WJs was inferior to that of BM. The transverse WJs were fractured in the BM during SSRT in hydrogen-free condition. The in-situ SSRT changed the fracture location to the inter-critical heat affected zone (ICHAZ) for transverse WJs and granular bainite in ICHAZ acted as the initiation site of HSC in transverse welds.