Microhardness Of Heat Affected Zone Research Articles

Thermal Cycle Simulation of Heat Affected Zone in the Welded Mild Steel

The aim of this work was to study the microstructure evolution of simulated heat affected zone in mild steel using thermal cycle simulation and it was compared to the heat affected zone in the real welded joint. The optical microscopy, micro-hardness measurements, X-ray diffraction were used as characterization techniques. The microstructures and mechanical properties of the simulated heat affected zone were also determined. The use of the thermal cycle simulation technique revealed in detail the different microstructures in the heat affected zone (HAZ) of the welded joint. Some similarities were observed between the heat affected zone obtained by the thermal cycle simulation technique and the heat affected zone observed in the real welded joint. The thermal cycle simulation technique revealed more details about the microstructure and mechanical behavior of the heat-affected zone. The HAZ microhardness values were lowest hardness in the welded joint. Moreover, the lowest hardness value is obtained for the HAZ simulated at 850°C.

Surface roughness and microhardness improvement of laser cladding stainless-steel 316 by laser polishing based on multiple remelting

Laser polishing (LP) has emerged as a promising alternative to traditional post-treatment technologies. Nonetheless, the LP process encounters challenges when dealing with surfaces of high roughness, since the excessive thermal input could induce undesirable effects, including the formation of mid-frequency waviness, expansion of heat-affected zone (HAZ), etc. In this study, we investigated the surface roughness and the characters of the remelted layer and HAZ of polishing the laser-cladding 316 stainless steel layer deposited on the surface of a 45# steel cylinder by a 1064 nm hundred-watt pulsed laser and a 1080 nm thousand-watt CW laser respectively. It is observed that the combination of low power density and extended interaction time failed to produce minimum roughness by the pulse laser, contradicting existing LP theories. To address this, we developed a multi-physics model to elucidate the underlying mechanisms and propose an LP scheme based on multiple remelting cycles with the pulsed laser. The Sa roughness is reduced from 20.958 μm to 0.242 μm, decreasing 98.8 %. The processing quality is equivalent to the surface of the high-power CW laser (Sa of 0.283 μm), while the remelted layer and HAZ are significantly smaller. The pulse laser polishing based on multiple remelting increased the surface microhardness by 22 % to 560 HV, while the CW laser remelting reduced it by 33 % to 309 HV. And the HAZ microhardness all decreased in all cases. This study can provide a theoretical basis and solution for the surface finishing of additive manufacturing parts.

Effect of intrinsic thermal cycles on the microstructure and mechanical properties of heat affect zones in laser cladding coatings on full-scale rails

Laser cladding (LC) wear-resistant alloy on the surface of pearlite rails has been proven to be able to improve their wear resistance significantly. However, the rapid cooling rate in LC will result in the formation of martensite structures with high hardness but low toughness in heat affected zone (HAZ), making the rails' service safety endangered. In this paper, a novel method was proposed to eliminate the un-tempered martensite in U71Mn rail based on the in-situ gradient tempering (IGT) effect induced by the intrinsic thermal cycles in multilayer LC. The influence mechanisms of interlayer and inter-track thermal cycles on the evolutions of microstructure and mechanical properties of HAZs were studied systematically. The results illustrate that the martensite in HAZs can be gradually tempered within the optimal IGT temperature range, and the tempering processing mechanisms are as follows: firstly, the ε-carbides precipitate from HAZs and then dissolve to form granular cementites. Then, the cementites form colonies of lathlike particles with similar orientations. Finally, a composite structure consisting of fine-tempered sorbite and granular cementites forms, in which the lamellar spacing of sorbite is <1/5 of that of the original pearlite, and the proportion of the high angle grain boundaries has increased by 2.2 times. During the tempering process, the microhardness of HAZs gradually decreases, accompanied by the increase of the bending properties therein. The ultimate bending strength and fracture strain of the specimens with fully tempered HAZs are enhanced to be about 1.6 and 5.5 times as much as those of the specimens with fully quenched HAZs, even much better than the substrate.

Heterogeneous deformation behavior of hybrid manufactured high strength titanium alloy: Coordinate deformation and stress concentration

Hybrid manufactured titanium alloys have a broad application prospect in aero-engine for designable mechanical properties and high benefits. However, in recent studies, titanium alloys prepared by hybrid manufacturing always broke in heat affected zone (HAZ) during tensile test. In order to further study this failure behavior and clarify the failure mechanism caused by microstructure of hybrid manufactured samples, a hybrid manufactured Ti–5Al–2Sn–2Zr–4Mo–4Cr sample was fabricated by wrought and laser directed energy disposition technology in this experiment. Microstructure formation mechanism of the hybrid manufactured sample was analyzed, and the tensile behavior was observed from both macro and micro perspectives by digital image correlation (DIC) measurement and in-situ tensile test. Sample can be divided into four zones according to microstructure characteristics, respectively additive manufacturing zone (AMZ), initially deposited zone (IDZ), heat affected zone (HAZ), and wrought zone (WZ). Microhardness of HAZ is the lowest because primary α phases coarsened and secondary α phases dissolved under cyclic heat effects during the deposition process. Because of the large temperature gradient at early deposition, IDZ has fine α phases and the highest microhardness. The heterogeneous deformation behavior observed macroscopically by DIC shows the stress concentration first occurs in HAZ. In-situ tensile test shows that coordinated deformation among grains and stress concentration at α/β phase interfaces are key factors of crack initiation and failure. The interaction forces between grains with different deformability are caused by coordinated deformation. The α phases favorable to deformation rotate towards tensile axis, while stress concentration occurs at phase interfaces of α phases unfavorable to deformation and then produces micro holes.

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.

Effect of Preheating on Martensitic Transformation in the Laser Beam Welded AH36 Steel Joint: A Numerical Study

In this work, the effects of preheating temperatures on martensitic transformations in a laser beam-welded AH36 steel joint were observed using a numerical study. In the same weld, the martensitic contents increased slightly from the upper area, the middle area to the lower area, and simulated martensite contents in the fusion zone were slightly lower than that in the HAZ (Heat Affected Zone). Under different preheating temperatures, simulated martensitic contents decrease with the increase of the preheating temperature. According to the simulated results, the average cooling rate and the CCT (Continuous Cooling Transformation) diagram were drawn to analyze the relationships between preheating temperatures and martensitic transformations. Simulated martensitic contents agreed well with the experimental metallographic microstructures. Moreover, the measured microhardness was reduced with the increasing preheating temperature, and measured microhardness in HAZ was higher than that in the fusion zone. The accuracy of the simulation results was further confirmed. The main significance of this work is to provide a numerical model to design martensitic contents in order to control the performances of the weld, avoiding many tests.

Weakening modes for the heat affected zone in Inconel 625 superalloy welded joints under different high temperature rupture conditions

Inconel 625 superalloy (IN625) was generally welded as the base metal to fabricate the rotors in steam turbine with temperature over 700 °C. In this case the weakening modes for the heat affected zone (HAZ) in IN625 welded joints during the stress rupture test are systematically investigated in the present study. The stress rupture test results show a dependence of rupture time on stress lg (t) = −0.017 σ + 5.85, which can be achieved to predict the rupture life with different stress levels. Besides, all the samples are deemed as intergranular fracture with columnar grain morphology appeared on the fracture surface. The microhardness distribution reflects that all the specimens ruptured in the HAZ. It is deemed that the chain-like particles can weaken the binding force of the continuous grains and therefore cause the resultant fracture in the region with comparative low microhardness of HAZ. With the decrease of the applied stress, the formation of acicular δ phase (Ni3Nb) can reduce the deformation capacity of the HAZ by consuming the precipitate strengthened element Nb. Hence, the micro-voids tend to initiate along the grain boundaries. The coarse Laves phase particles with hard and brittle features become the dominated factor with the further increased rupture time. The micro-voids initiate and converge around the Laves phase under the stress to form micro-cracks, eventually cause the fracture of the ruptured specimens.

The heat affected zone of X20Cr13 martensitic stainless steel after multiple repair welding: Microstructure and mechanical properties assessment

Multiple repair welding of X20Cr13 martensitic stainless steel was performed using gas metal arc welding. The groove for repair welding was designed such that the heat affected zone (HAZ) of each repair welding sequence remained the HAZ of the previous repair (i.e. cyclic weld removal and repair welding was done). After post weld heat treatment, microstructure and mechanical properties of the HAZ were investigated during 0–9 repairs. Refinement of primary austenite grains in the HAZ was observed (from 2.8 ASTM grain size in the zero-repair sample to 6.3 after the ninth repair), increasing the impact toughness (from 34J to 63J, respectively). Moreover, HAZ micro-hardness decreased as repair welding continued due to sequential disintegration of the chromium carbides network at grain/sub-grain boundaries, promoting ductile fracture behavior. It was finally concluded that multiple repair welding has no detrimental effects on microstructure and mechanical properties of X20Cr13 stainless steel and increases HAZ impact toughness and promotes ductile fracture.

Microstructure and mechanical properties of friction stir welded joint of TRIP steel

Friction stir welding (FSW), as a new type of solid-solid welding method, was utilized to join transformation-induced plasticity (TRIP) steel. This method can avoid the solidification structural defects caused by fusion welding technology and improve the welding quality. In this study, the microstructural transformation and deformation behavior of TRIP steel during FSW were evaluated by optical microscopy, electron backscatter diffraction, and room-temperature tensile test. The results show that after FSW, martensite transformation occurred in the stir zone (SZ) and thermomechanically affected zone (TMAZ), and the retained austenite content decreased. The grain refinement and ratio of low-angle boundaries (LABs) increased. In heat-affected zone (HAZ), the retained austenite was thermally spheroidized and decomposed, the grains were coarsened, and the ratio of LABs decreased. No obvious preferred grain orientation was observed in each region of joint, and the texture strength decreased by phase transformation and grain rotation. After the tensile deformation of FSW joint, the retained austenite disappeared, the grains were refined, and the proportion of LABs increased in the base metal (BM) and HAZ. The content of retained austenite in SZ remained unchanged, but decreased in TMAZ. The grains were refined, and the ratio of LABs did not change significantly in SZ and TMAZ. FSW increased the microhardness of SZ and TMAZ by 218 HV and 93 HV, respectively, while the microhardness of HAZ decreased by 4 HV. The yield strength, tensile strength, and elongation of joint were 563 MPa, 942 MPa, and 12.8 %, respectively, reaching 96%, 104%, and 59%of BM, respectively. The tensile fracture exhibited ductile fracture characteristics. In addition, the joint exhibited strain nonuniformity during the tensile deformation, and the order of deformation degree was as follows: HAZ > TMAZ > SZ. Therefore, HAZ became a weak zone of joint mechanical properties. Continuous dynamic recrystallization and discontinuous dynamic recrystallization were the main mechanisms of grain refinement, and the martensite transformation conformed to the Kurdjumov-Sachs orientation relationship.

Enhanced Impact Toughness of Heat Affected Zone in Gas Shield Arc Weld Joint of Low‐C Medium‐Mn High Strength Steel by Post‐Weld Heat Treatment

In the present study, two different post weld heat treatments (PWHT) are carried out on gas shield arc welded low‐C medium‐Mn high strength steel to explore the effect of PWHT on microstructure, microhardness, and low temperature toughness of heat affected zone (HAZ). The results show that when the weld joint is PWHT at 630 °C for 30 min, the microhardness of HAZ is homogeneous and almost identical to base metal (BM). The microstructure of HAZ comprises of tempered martensite and strip‐shaped reversed austenite. The impact energy of fusion line (FL), coarse‐grained HAZ (CGHAZ), and fine grained HAZ (FGHAZ) at −40 °C is enhanced by 29 J (from 42 to 71 J), 42 J (from 68 to 110 J), and 33 J (from 140 to 173 J), respectively, compared to the weld joint, which is PWHT at 200 °C for 120 min. The formation of reversed austenite with certain stability in the HAZ is the main toughening mechanism when the weld joint is PWHT at 630 °C. Reversed austenite enhances the impact energy significantly through the transformation‐induced plasticity (TRIP) effect, which releases the stress concentration, improves the crack initiation energy, and effectively prevents the propagation of cleavage crack.

Effect of processing parameters of laser on microstructure and properties of cladding 42CrMo steel

Due to the abrasion of sprocket wheel socket and high cost, the failure 42CrMo steel is remanufactured using the laser cladding technology with paraxial powder feed. The remanufacturing was carried out through an 8-kW CO2 laser device with rectangular spot of 3 × 24 mm and the self-developed Fe-based powder. The effect of processing parameters on macromorphology, microstructure, and microhardness of cladding layer and heat-affected zone (HAZ) was investigated. The results showed that the quality of cladding layer is improved with the increase of laser energy density (P/A). Under the same energy density, the quality of cladding layer is superior when decreasing the scanning speed. The microhardness of heat-affected zone which is 1.5–2.5 mm in width ranges from 210 to 450 HV. The HAZ is composed of completely quenched zone, incompletely quenched zone, and tempered zone. The microhardness of cladding layer which is approximately 1.8 mm in thickness is between 210 and 450 HV and is double even tripled as substrates. From the fusion line to surface, the microstructure of cladding layer is plane crystal, cellular crystal, and dendrite crystal, in turn. With the increase of laser energy density, the microhardness of HAZ and cladding layer increases; meanwhile, the width of HAZ shows a fluctuant rising trend. The microhardness of coatings was improved on account of the martensite, fine grains, solid solution, and precipitated phases. Especially, the latter two are decisive factors under the higher laser energy.

Microstructure and Mechanical Properties of Laser Repaired TC4 Titanium Alloy

Laser Additive Manufacturing (LAM) was employed to fabricate repaired specimens with wrought TC4 as the substrate and TC4 powders with low oxygen (O≤0.13wt%) as the cladding materials. The microstructure and mechanical properties of TC4 specimens fabricated by LAM and wrought billet were investigated comparatively. The results show that the macrostructure of the laser repaired specimen can be divided into three domains, including wrought substrate zone (SZ), heat affected zone (HAZ) and laser deposited zone (LDZ). The LDZ microhardness is equal to that of the SZ basically. And the HAZ microhardness is higher than that of both the LDZ and SZ slightly. The results of room temperature tensile test show that the strength and ductility of the wrought specimen are slightly higher than that of the laser repaired specimens. Meanwhile, the strength of the laser repaired specimen with repair ratio of 40% (i.e. area fraction of the LDZ on the transverse section of tensile specimen within gauge part is 40%)is slightly lower than that of 50% repaired specimen, but the ductility is higher than the latter. Therefore it is favorable to match the strength and ductility of the wrought substrate with the LDZ with low oxygen TC4 powders as the cladding materials, so as to improve the comprehensive properties of laser repaired TC4 titanium alloy. The wrought specimen tensile fracture presents a typical ductile characteristic, and the repaired specimen shows a complex fractograph. From the LDZ to the SZ, the tensile fracture presents a successive transformation from cleavage step to dimple fracture. It can be seen that there is a good corresponding relationship between the fracture morphology and the microstructure of the tensile specimens.

Effect of rotation speed on microstructure and mechanical properties of refill friction stir spot welded 6061-T6 aluminum alloy

Refill friction stir spot welding (RFSSW) has been used to weld 6061-T6 aluminum alloy, and keyhole free joints were successfully obtained. Effect of tool rotation speed on microstructure and mechanical properties of joint was investigated. The joint was divided into four zones, i.e., the base material (BM), heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and stir zone (SZ) according to microstructural evolution. Defect was not found on the surface of welded joint, but inner defects of partial bonding, bonding ligament, hook, and voids were observed within the welded joint. The microhardness of HAZ and TMAZ was lower than that of the BM, while the maximum microhardness was obtained in the SZ of welded joints. With increasing tool rotation speed from 1100 to 1700 rpm, microhardness decreased with increasing grain size in the weld. The maximum tensile shear failure load of 7522 N was obtained for the joint under tool rotation speed of 1500 rpm. Two different failure modes of plug fracture and tensile-shear mixed fracture were observed during the tensile shear tests.

Effect of duty cycle on microstructure and mechanical properties of pulsed GTAW lap joint of Invar

0.7mm and 1.0mm Invar sheets in vertical position were lap-jointed by pulsed gas tungsten arc welding (pulsed GTAW). The macro morphology, microstructure and mechanical properties were studied. The feature size La (the largest weld width of lower sheet), P (the largest weld penetration of lower sheet) and R (the shortest distance from the cutting-edge of the uncombined sheets to the weld surface) are directly proportional to duty cycle. The fusion zone (FZ) is completely composed of columnar grains in which two forms of substructure are observed. Series of layer lines, which spread roughly parallel to the partially melted zone (PMZ), periodically appear in the FZ. No apparent precipitation, phase transformation and welding defects such as hot cracks are found in heat affected zone (HAZ) and FZ. Microhardness tests reveal that the microhardness of HAZ can reach more than 85% of base metal (BM), and the microhardness of FZ can reach more than 90% of BM. The effective bonding area has dominant influence on the tensile shear force in this study and the maximum tensile shear force is 4425N.

Geometrical and microstructural characteristics of the TIG-CMT hybrid welding in 6061 aluminum alloy cladding

A tungsten inert gas (TIG)-cold metal transfer (CMT) hybrid welding process is proposed. Compared with the conventional TIG-metal inert gas/metal active gas (MIG/MAG) hybrid welding method, the characteristic of TIG-CMT welding process is that there is no interaction between the two arcs. The addition of TIG can improve the wettability of molten metal. The microstructure in hybrid joint is coarser than that in conventional CMT joint, and the microhardness in weld metal (WM) of hybrid joint is higher than that of conventional CMT joint. In TIG-CMT hybrid welding process, the TIG current plays an important role in changing contact angle and the dilution of weld bead. Fine equiaxed grains have been observed in partially melted zone (PMZ) and coarsened equiaxed grains exist in heat-affected zone (HAZ). The width of PMZ increases with the increase of heat input. More heat input can also lead to the coarser columnar dendrite. In all weld joints, the microhardness in HAZ remains at a low level, and the microhardness in PMZ always has a sharp decrease from WM to HAZ. The outcome of this work shows that the TIG-CMT hybrid process is suitable for the multi-passes welding of aluminum alloy.

Welding cooling rate effects on microstructure of an API 5l x100 steel

The aim of this work was to investigate the influence of five different cooling rates on properties of weld metal (WM) and heat affected zone (HAZ) of an API 5L X100 steel welded by GMAW. Bead on pipe (BOP) welds were made on 100x300 mm sections of a 15.8 mm thickness and 580 mm outside diameter API 5L X100 pipe, through five preheat temperatures (-30, 34, 100, 150 and 200 °C), aiming to obtain different cooling rates. Thermocouples were used to obtain the cooling time from 800 to 500 ºC (Δt8/5) during welding. The microstructures of base metal (BM) and HAZ were analyzed by light microscopy and scanning electron microscopy (SEM). Hardness measurements were made on the BOP welds in order to identify maximum and minimum values. As expected, due to the different cooling rates during welding, the distinct preheat temperatures have influenced the weld properties. Maximum and minimum HAZ microhardness, and weld metal hardness decrease as the preheat temperature increases. HAZ area, grain size of fine grained HAZ and coarse grained HAZ increase with increasing preheat temperature.

Characterization of weldability, microstructure, and corrosion resistance of dissimilar welded joint between Mo2FeB2-based cermets and 316L stainless steel

Abstract

Friction Stir Welding of Austenitic Stainless Steel: A Study on Microstructure and Effect of Parameters on Tensile Strength

Friction stir welding was performed on austenitic stainless steel plates on an indigenously retrofitted vertical milling machine. Machine was retrofitted with a robust work fixture capable of measuring welding force and a tool adapter. AISI-304 equivalent grade stainless steel was welded by FSW using tungsten carbide tools with tapered cylindrical (conical) pin. The experiments were performed using L4 Orthogonal Array of Taguchi design. The tensile tests revealed failure of test specimens outside the weld region. The results of ANOVA showed the order of importance in which the parameters have affected the UTS in terms of percent contributions i.e. welding speed with (56.83% contribution), shoulder diameter (27.44% contribution) and tool rpm (15.73% contribution). The microstructure study was performed to investigate microstructure evolution in Stir Zone (SZ), Thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ).The microstructure characterization in three distinct FSW zones showed: (i) Fine grain structure with a circular and non-uniformly distributed darkly etched band formation in SZ, (ii) Finer grains in TMAZ, and (iii) Refinement of grains in HAZ Micro-hardness distribution across the weldline was also obtained. The highest hardness was seen to occur in the TMAZ on advancing side.

Interfacial Microstructural Characterization of Ti/Al Joints by Gas Tungsten Arc Welding

Ti/Al dissimilar materials were welded by gas tungsten arc welding with filler wires. Microstructure in the Ti/Al joint was studied with optical microscope, scanning electron microscope, energy-dispersive spectrometer, etc. Results show that various Ti–Al intermetallic compounds are formed along the interface. According to the degree of Ti alloy melted, interface is divided into different reaction layers. Under the effect of martensite transformation, microhardness in heat-affected zone is higher than it in titanium base metal. Element silicon enriched in the interface. The dispersive distribution of precipitated phase improved joint strength.

Effect of Welding Heat Input on Microstructure and Mechanical Properties of HSLA Steel Joint

Gas tungsten arc welding of HSLA steel was conducted with different welding heat inputs, and the influences of welding heat input on the microstructure, Vickers hardness, and impact toughness of heat-affected zone (HAZ) in prepared joints were systematically investigated. The microstructure in HAZ with low welding heat input mainly consisted of martensite, and the microhardness of coarse grain HAZ was measured higher than that of fine grain zone. The results show that increasing the welding heat input could suppress the formation of martensite and reduce the microhardness of HAZ. However, the impact toughness of HAZ was not monotonously improved with the increase of welding heat input. It is deemed that the coarsened grain, the formation of upper bainite, and the non-uniform distribution of carbides and inclusions in HAZ could degrade the impact toughness. Tests demonstrate that the optimum comprehensive properties of HAZ were obtained when the welding heat input was 0.67 kJ/mm.