Subcritical Heat-affected Zone Research Articles

Suppression of softening in heat affected zone by Mo addition in friction stir welded martensitic steel

The drastic softening in the heat affected zone (HAZ) of friction stir welded 0.2C–2Si–3Mn martensitic steel occurred due to the martensite tempering in the subcritical temperature range just below A 1 temperature in HAZ. It was revealed that the softening in the subcritical HAZ was successfully suppressed by Mo addition. The optimum condition was found to be 3 wt-% Mo addition with the solution temperature of 1120°C, where densely distributed nanometer-sized Mo2C were confirmed by transmission electron microscopy and atom probe tomography in the subcritical HAZ. These fine Mo2C precipitates gave rise to an increase in the hardness of approximately 110 HV compared with a no-Mo-added sample, which was in good agreement with the predicted precipitation strengthening value based on the Ashby–Orowan model.

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.

Microstructure, Mechanical Properties, and Corrosion Behavior of Laser‐Welded Dissimilar Joints Between DP980 and QP980 Steel

Herein, fiber laser welding experiments are performed to evaluate the microstructure, mechanical properties, and corrosion behavior of dissimilar/similar joints between DP980 and QP980 steel. Results show that the fusion zone (FZ) of all joints consists of lath martensite with high microhardness (≈480 HV); the retained austenite content in the subcritical heat‐affected zone (HAZ) in the QP980 side of the dissimilar weld joint (WJ) is less than base metals (BMs). Lower hardness in the subcritical HAZ is found compared with BMs in all joints. The microstructure of softening zones is tempered martensite, and the reduction in hardness is about 8.5−10.8% of BMs, which has a great influence on the tensile fracture position of WJs. All joints fracture at softening zones with a typical feature of ductile fracture, and the joint efficiencies are above 96%. The electrochemical corrosion test reveals that the welded samples exhibit a higher corrosion rate compared with the BMs due to the increase in martensite content and area fraction of grain boundary.

Microstructure and microhardness evolution of thermal simulated HAZ of Q&P980 steel

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. Ac1 and Ac3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450–460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

HAZ Characterization and Mechanical Properties of QP980-DP980 Laser Welded Joints

The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding. The weld zone (WZ) was fully martensitic structure, and heat-affected zone (HAZ) contained newly-formed martensite and partially tempered martensite (TM) in both steels. The super-critical HAZ of the QP980 side had higher microhardness (~ 549.5 Hv) than that of the WZ due to the finer martensite. A softened zone was present in HAZ of QP980 and DP980, the dropped microhardness of softened zone of the QP980 and DP980 was Δ 21.8 Hv and Δ 40.9 Hv, respectively. Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain, leading to the formation of low angle grain boundaries (LAGBs). Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs, which led to significant dislocation interaction and formation of cracks. The electron back-scattered diffraction (EBSD) results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing, resulting in the failure of joints located at the sub-critical HAZ of DP980 side. The QP980-DP980 dissimilar steel joints presented higher elongation (~ 11.21%) and ultimate tensile strength (~ 1011.53 MPa) than that of DP980-DP980 similar steel joints, because during the tensile process of the QP980-DP980 dissimilar steel joint (~ 8.2% and 991.38 MPa), the strain concentration firstly occurred on the excellent QP980 BM. Moreover, Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value (~ 5.92 mm) and the peak punch force (~ 28.4 kN) due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.

Microstructure and formability performance of fiber laser welded 1.2 GPa grade hot-rolled TRIP steel joints

Laser welding of 1.2 GPa hot-rolled transformation induced plasticity (TRIP) steels with thickness of 1.9 mm was carried out to study its microstructure, mechanical properties and deformation behavior. Defect-free joints with complete penetration were prepared under the applied welding parameters. It was found that the fusion zone (FZ) consisted of full martensite of high microhardness (489 HV). Compared with the base material (BM) (436 HV), the subcritical heat affected zone (SCHAZ) contained a narrow softening zone with 62 HV microhardness drop due to tempering of martensite. The laser welded joint achieved 96.6% joint efficiency comparing to the BM. The formability of welded specimen depended on the weld position. As the weld offset increased to 6 mm, the formability approached 95.7% of the BM. The deformation was small in FZ and was primarily concentrated in BM. The martensite in FZ effectively restrained transverse deformation in welded joint, such that main cracks occurred and propagated in the BM. Stretch-flange-formability in terms of hole-expansion ratio of weld specimens was degraded compared with the BM (33.3%). Premature failure appeared at joint with low punch force because complete martensite exhibited low ductility in the FZ.

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.

Microstructure and Mechanical Performance of Resistance Spot Welded Martensitic Advanced High Strength Steel

Resistance spot welded 1.2 mm (t)-thick 1400 MPa martensitic steel (MS1400) samples are fabricated and their microstructure, mechanical properties are investigated thoroughly. The mechanical performance and failure modes exhibit a strong dependence on weld-nugget size. The pull-out failure mode for MS1400 steel resistance spot welds does not follow the conventional weld-nugget size recommendation criteria of 4t0.5. Significant softening was observed due to dual phase microstructure of ferrite and martensite in the inter-critical heat affected zone (HAZ) and tempered martensite (TM) structure in sub-critical HAZ. However, the upper-critical HAZ exhibits obvious higher hardness than the nugget zone (NZ). In addition, the mechanical properties show that the cross-tension strength (CTS) is about one quarter of the tension-shear strength (TSS) of MS1400 weld joints, whilst the absorbed energy of cross-tension and tension-shear are almost identical.

Mechanical properties and failure behavior of resistance spot welded third-generation advanced high strength steels

Abstract Third generation advanced high strength steels are candidates for application in lightweight automotive components due to their remarkable combination of high strength and high ductility. Adoption of these steels is accompanied by challenges to better understand the mechanical performance of their resistance spot welds. In this work, the resistance spot weldability of two third-generation advanced high strength steels, designated herein as 3G-980 and 3G-1180, was investigated. The weldability windows of the steels were developed to determine process robustness and acceptable welding zones for different welding times. The mechanical properties of the spot welds were evaluated by conducting lap-shear and cross tension tests for different nugget sizes. The investigated steels exhibited both interfacial and pullout/partial-pullout failure modes during mechanical testing and the transition points between the failure modes were identified as a function of the welding condition. The optimal weld strengths of the 3G-980 and 3G-1180 samples were 28.7 kN and 30.7 kN for lap-shear and 10.2 kN and 9.4 kN for cross tension specimens respectively. Interestingly, it was shown that even interfacial failure of the joints can demonstrate load-bearing capacities close to the optimized condition in lap-shear testing. Also, in contrast to 3G-980, microhardness measurement of different weldment regions revealed a local drop in the hardness of the sub-critical heat-affected zone for 3G-1180. These measurements were correlated to the crack initiation region and propagation paths of interrupted lap-shear specimens for both steels.

Sulfide stress cracking susceptibility of the heat-affected zone of an 9% Ni steel welded joint

A 9% Ni steel (ASTM A333 Gr. 8) was selected for the first time for use in CO2 injection units (CO2-IUs) in the Brazilian pre-salt; however, because of the presence of H2S, it is susceptible to sulfide stress cracking (SSC), especially in welded joints. In this regard, the susceptibility to SSC of the subzones of the heat-affected zone (HAZ) of a 9% Ni steel (BM) welded pipe joint was evaluated. The thermal cycles of the HAZ were simulated numerically and physically. HAZ subzones were evaluated through optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers microhardness, and slow strain rate testing. The BM and subcritical HAZ showed similar microstructures (fine ferrite and carbide/retained austenite (γret) aggregates) and behaviors in aggressive environment, with plastic elongation ratios (REs) of 0.15 and 0.17, respectively. The intercritical HAZ (ICHAZ; fine ferrite and martensite) had the highest microhardness and γret content, with a decrease in RE (0.04); fine-grain HAZ (FGHAZ; martensite and bainite) showed the smallest RE (0.01) and brittle fracture. The coarse-grain HAZ (CGHAZ; coarse martensite and coalesced bainite) showed the lowest plastic region and brittle fracture, with RE of 0.05 and the highest time-to-failure ratio. The high microhardness of the ICHAZ, FGHAZ, and CGHAZ (>250 HV, NACE 0175 / ISO 15156) made them more susceptible to SSC than the BM. These results lead to the conclusion that both the 9% Ni steel and its welded joints are prone to SSC, and the use of this steel in CO2-IUs presents an operational failure risk.

Double Pulse Resistance Spot Welding of Dual Phase Steel: Parametric Study on Microstructure, Failure Mode and Low Dynamic Tensile Shear Properties.

Resistance spot welding (RSW) of dual phase (DP) steels is a challenging task due to formation of brittle martensitic structure in the fusion zone (FZ), resulting in a low energy capacity of the joint during high-rate loading. In the present study, in situ postweld heat treatment (PWHT) was carried out by employing a double pulse welding scheme with the aim of improving the mechanical performance of DP590 steel resistance spot weld joint. Taguchi method was used to optimize in situ PWHT parameters to obtain maximum peak load and failure energy. Experiments were designed based on orthogonal array (OA) L16. Mechanical performance was evaluated in terms of peak load and failure energy after performing low dynamic tensile shear (TS) test. Microstructural characterization was carried out using a scanning electron microscope (SEM). The results show that improvements of 17 and 86% in peak load and failure energy, respectively, were achieved in double-pulse welding (DPW) at optimum conditions compared to traditional single-pulse welding (SPW). The improvement in mechanical performance resulted from (i) enlargement of the FZ and (ii) improved weld toughness due to tempering of martensite in the FZ and subcritical heat affected zone (SCHAZ). These factors are influenced by heat input, which in turn depends upon in situ PWHT parameters.

A study on HAZ behaviour in 800 MPa cold rolled and hot rolled steel weld

Abstract In the automotive sector, the demand for advanced high strength steels (AHSS) is increasing day by day. Based on the application, cold rolled and hot rolled steels are used for various components in a vehicle body. Typically, cold-rolled grades with dual-phase, DP780 steel is used in the form of welded blanks. Hot rolled grades with Ti-Nb microalloy content, like HS800 steel are used in as long members. Welding is an important step to be considered in the design of materials for mass production as required in the automobile sector. In this investigation, Pulsed Gas Metal Arc Welding (P-GMAW) is carried out on both steels with a solid filler wire of strength 800 MPa. Static tensile tests indicated that failure in both steels welds occurred in the heat-affected zone region. The crack initiation and propagation behaviour were compared in both steels. In DP780 steels, the presence of the acicular ferrite and acicular martensite resisted the crack initiation and propagation in the weld region whereas, the chaotic nature of the acicular ferrite in HS800 steel. Microstructural studies revealed that the reason for heat affected zone (HAZ) failure in HS800 steel is due to the presence of hard TiN particles with a size of more than 1 μm which causes decohesion in the matrix. In DP780 steel failure is due to the presence of tempered martensite in the subcritical heat affected zone (SCHAZ). This study divulges the influence of filler wire chemistry, dilution, and welding parameters on cold rolled and hot rolled AHSS steels used in the automotive industry.

Microstructure and softening of advanced high-strength steel QP1180 lap joints welded with CMT

QP1180 lap joints are welded by the cold-metal-transfer method. The microstructure and softening of the welded joints are investigated. The results show that the fusion zone is composed of acicular ferrite and as hard as the base metal. The hardness of the supercritical heat-affected-zone (HAZ), where martensite is the main phase, is much higher than that of the base metal. Softening takes place in part of the intercritical HAZ and the whole subcritical HAZ. In intercritical HAZs, fresh martensite forms, while the size and amount of austenite are similar to those in the base metal. The hardness in the subcritical HAZ decreases due to the tempering effect. Austenite starts to decompose in the subcritical HAZ, where the hardness drops from 380 to 390 HV. However, in the softest place of the subcritical HAZ, the austenite phase almost disappears, and the hardness drops to approximately 320 HV.

Effect of Peak Temperature on Microstructure and Mechanical Properties of Thermally Simulated Welding Heat-Affected Zones for 09MnNiDR Steel

The microstructures and mechanical properties of various heat-affected zones (HAZs) for 09MnNiDR pressure vessel steel are systematically analyzed. The results show that at − 70 °C, the thermally simulated subcritical heat-affected zone (SCHAZ) reaches the impact toughness of 270 J, which is the highest among various HAZs. Owing to the appearance of martensite–austenite (M–A) constituents distributed along the grain boundary in the thermally simulated critical heat-affected zone and fine-grained heat-affected zone, the impact toughness sharply decreases compared with that of the SCHAZ. The impact toughness of the coarse-grained heat-affected zone (CGHAZ) reaches 20 J, which makes the CGHAZ the weakest zone. The microstructure of the CGHAZ is composed of relatively coarse bainite and ferrite. Moreover, the proportion of high-angle grain boundaries (HAGBs) in CGHAZ is the lowest, at only 22%. Coarse grain size, microstructural transformation, and different proportions of HAGBs significantly affect the impact toughness in each zone of HAZs.

Occurrence of sub-critical heat affected zone liquid metal embrittlement in joining of advanced high strength steel

Advanced high strength steels (AHSS) used in automotive structural components are commonly protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement (LME) cracking during welding. In this study, the effect of angular welding electrode misalignment as recommended by industry was investigated and resulted in LME cracking in the sub-critical heat affected zone (SCHAZ) which is a region where LME has not been previously observed. The largest misalignment case of 6° resulted in bending of the work piece and an angled weld nugget/heat affected zone (HAZ), causing LME cracks in the lower temperature HAZ regions. SEM and EBSD analysis characterized the LME crack area as the SCHAZ and below the Ac 1 transformation temperature. Furthermore, dilatometry analysis showed the Ac 1 temperature to be approximately 660 °C, meaning LME occurred below the traditional 700 °C minimum threshold. In addition to this novel observation of LME before transformation and at low temperature, it was also noted that only misalignment beyond 2° promoted LME cracking. The low angle 2° case was observed to be statistically the same using a Student’s t -test (α = 0.05) on 6 welded samples for LME crack severity compared to a standard 0° case.

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

Microstructure gradient characteristics and mechanical properties of friction stir welded high strength QP980 steel

Friction stir welding (FSW) was carried out on 1.2 mm thick QP980 steel plate and W-Re stirring tool was used for FSW. This study focused on the microstructure gradient characteristics and mechanical properties of QP980 FSW joints. The FSW joints without defect could be obtained under welding parameter of 400 rev min−1–400 mm min−1. The martensite was the mainly microstructure in the stir zone (SZ) and the microhardness of the SZ was increased to about 480 HV. The martensite of the subcritical heat affected zone (SCHAZ) underwent tempering transformation and decomposition, small amount of carbides were precipitated from the martensite, so the microhardness of the SCHAZ was slightly decreased. The microstructure in the SZ had the highest dislocation density and the highest fraction of low angle boundaries. The tensile test results indicated that the joints were broken in the base metal (BM), which indicated that the FSW joints had excellent performance.

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 Post-Weld Heat Treatment on the Microstructure of Plasma Arc Welded DP600 Steel

The effect of isothermal post-weld heat treatment on keyhole plasma arc welded DP600 steel was investigated using optical microscopy, scanning electron microscopy and analytical transmission electron microscopy. The isothermal heat treatment had no effect on the allotriomorphic, Widmanstatten and acicular ferrite in the fusion zone (FZ). The martensite lath structure showed complete recovery in FZ, base metal (BM) and heat-affected zone (HAZ) with the formation of coarse cementite particles of up to 200 nm size in subcritical HAZ. The coarse grain HAZ showed little change in bainite and had tempered martensite and transformed retained austenite. The fine grain HAZ consisted of bainite, tempered martensite and coarsened preexisting cementite. The recovered BM had tempered martensite and reduced dislocation density. Hardness was found to be the lowest in subcritical HAZ, which is attributed to complete tempering of martensite, formation of large cementite particles and disappearance of dislocations.

Softening Behavior of Electron Beam Welded 22SiMn2TiB Steel

Softening behavior occurs in heat-affected zone of many types of high strength steels causing the deterioration of mechanical properties. In this study, the softening behavior of 22SiMn2TiB joint was revealed by experimental and numerical analysis. The microstructure and hardness of the joint were tested and analyzed. The numerical simulations of the temperature field were carried out to obtain the thermal cycle. Based on the continuous cooling transformation diagram, the phase transformation was investigated. The results indicated that the microhardness in the HAZ was significantly decreased, and its minimum value was 273 HV which was 63.5% of base metal. The softening behavior occurs in intermediate critical heat-affected zone and the subcritical heat-affected zone, and its peak temperature is between 853.7 °C (Ac3) and 450 °C. When 22SiMn2TiB steel was heated above 732.3 °C (Ac1), martensite begins to transform into austenite. With different cooling rates, austenite may transform into martensite, bainite and ferrite, and the volume fraction of soft phases could increase resulting in the softening. When the peak temperature is between 732.3 and 450 °C, the recovery phenomenon and the decomposition of martensite caused softening with the decrease in the dislocation density and martensite.