Hardness Of Heat Affected Zone Research Articles

An In-Depth Investigation of the Effects of Tungsten Inert Gas Welding Process Parameters on Hardness and Corrosion Resistance of 2205 DSS Weldments: New Design of Experiment Parametric Studies and Optimization

The aim of this work is to examine and analyze, using response surface methodology, how the TIG welding process parameters of welding current (WC), welding speed (WS), and N2 with argon as shielding gas affect the hardness and corrosion resistance of 2205 DSS weldments. Due to the equal amounts of ferrite and austenite phases and alloying elements, duplex stainless steel DSS offers good mechanical properties and corrosion resistance. The mechanical characteristics and resistance to corrosion of the weld zone, as well as the heat-affected zone of the DSS, are, however, disturbed as a result of the welding process since it changes the distribution of these two phases and also the alloy is thermally disturbed. Therefore, in this work, an in-depth investigation of the effects of the above-mentioned parameters on the DSS quality has been performed. Results showed that increasing welding current while decreasing welding speed, which corresponds to the highest heat input, led to lower critical pitting potential and weld zone hardness but higher heat-affected zone hardness. The same results were obtained for decreasing welding current while increasing welding speed, which correspond to the lowest heat input. However, the addition of a small percent (%) of N2 to argon shielding gas resulted in increasing the critical pitting potential and decreasing the hardness in welds and heat-affected zones. Numerically, the RSM planned experimental results showed that an optimum welding current of 175A, welding speed of 170 mm/min, and 10% N2 with argon as shielding gas maximized the critical pitting potential up to 318 mV and optimized the hardness of the weld and heat-affected zone to about base metal hardness of 288 and 286 HV, respectively.

Investigation of the effects of beam oscillations in electron beam–welded S1100M TMCP steel

The development of thermomechanically controlled processed (TMCP) high-strength steel (HSS) has significantly contributed to designing and developing the intricate structural components. It has broader applications in the cranes and lifting process industry (base frame, crane jibs, and crane columns), trailers, agricultural and forestry machinery, earth-moving equipment, etc. However, the development of new-grade steels with higher tensile strength led to higher requirements for welded joints, and the associated weldability issues have inspired detailed studies on electron beam welding (EBW) with different beam oscillations. Beam oscillation application with EBW processes improves the welding efficiency, weld quality, weld geometry, keyhole, etc., affecting the welded joints mechanical and microstructural properties. Thus, the present study investigates the impact and comparison of various beam oscillations on the microstructural and mechanical properties of EB-welded S1100M steel. The influence of welding parameters on the microstructure of welded joints was analyzed using a scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). The analysis focused on evaluation of grain sizes, morphologies, distributions, and crystallographic orientations of different phase constituents in fusion zone (FZ) and heat-affected zone (HAZ). The mechanical properties were analyzed using hardness, tensile, and Charpy V-notch impact tests. The texture in the FZ is typically random, while the HAZ typically exhibits a strong rolling texture. In general, the cooling rate in EBW is very fast, possibly resulting in a fine-grained structure and reduced formation of coarse second-phase particles in the weld zone. The elliptical beam oscillation showed the highest hardness in HAZ 450 HV10. Elliptical beam oscillation slightly improves the welded joint’s tensile strength, and the impact test showed mixed fracture behavior.

Experimental investigation of the impact of GMAW welding parameters on the mechanical properties of AISI 316L/ER 316L using quaternary shielding gas

In this study, the parameters of Metal Inert Gas (MIG) and Metal Active Gas (MAG) were investigated of AISI 316L/ER 316L. A quaternary shielding gas mixture consisting of Argon (Ar), Helium (He), Carbon Dioxide (CO2), and Nitrogen (N2) was chosen. The Taguchi orthogonal array (OA-L9) methodology was employed to explore optimal welding settings, including arc current (120A, 160A, 200A), wire feed rate (3, 3.5, 4 m min−1), and shielding gas combination (G1, G2, G3). The findings highlighted the importance of shielding gas in influencing the ultimate tensile strength (UTS), elongation percentage (EL%), and material toughness of welding joints. Notably, the highest UTS (515.77 MPa), EL% (20.85%), and material toughness (133J) were achieved by the specific group gas combination shown as G1. It is recommended to configure welding parameters to an arc current of 160A, a wire feed rate of 4 m min−1, and the G1 gas combination. Welded specimens using a G1 gas mixture showcased the best UTS and EL%. Additionally, it was found that the fusion zone (FZ) and heat-affected zone (HAZ) hardness are most profoundly influenced by the choice of gas combination (G2), resulting in the best hardness values of 253.79 HV and 239.68 HV, respectively. The optimal parameters for achieving the desired material hardness were precisely identified as (120A, 3 m min−1, G2). These insights offer a pathway to enhance welding performance and, in turn, elevate the quality and efficiency of industrial applications.

In Situ Study on Fracture Behavior of Thick‐Section Friction Stir‐Welded AA 5083 Alloy

Friction stir welding (FSW) produces a gradient microstructure with distinct metallurgical zones. The present study focuses on characterizing the fracture behavior during uniaxial loading across thick‐section FSW joint in nonheat‐treatable 5083 aluminum alloy. The hardness variation is linked to microstructural features across the weld zones through correlative microscopy. Notably, the heat‐affected zone (HAZ) exhibits a lower hardness of 75 HV, while the thermomechanically affected zone (TMAZ) and stir zones (SZ) display a hardness of around 79HV. A marginal difference in hardness in HAZ prompts strain localization in HAZ on the advancing side of the weld, resulting in offset in fracture from the SZ, which is captured through in situ monitoring of the tensile test. Further, the postdeformation analysis reveals crack propagation along the maximum shear stress plane due to the coalescence of microvoids resulting from the fracturing and debonding of second‐phase particles. Overall, this study provides valuable insight for optimizing the FSW process of AA 5083 alloy for thick‐section applications.

Specification of the optimal gas metal arc welding (GMAW) parameters to enhance the mild steel strength (MS1018)

Arc fusion processes, such as high-efficiency gas metal arc welding (GMAW), have become increasingly prevalent in various industries. To optimize the GMAW parameters and enhance the strength of MS 1018, Monika and Chauhan conducted experiments using the low carbon steel filler wire (ER 70 S6). They used a Taguchi L9 OA (orthogonal array) to generate test data focusing on 3 GMAW parameters such as gas flow rate, arc voltage, and welding current. The performance characteristics considered were tensile strength (TS), weld zone hardness (WZH), and heat-affected zone hardness (HAZH). The effect of GMAW parameters investigated using the S/N (signal-to-noise ratio) transformation of individual test data, which is valid to account for variations observed in repeated tests. By conducting a thorough analysis of variance (ANOVA) on this transformed data, they were able to identify the optimal GMAW parameters for achieving high TS, WZH, and HAZH. Given that HAZH exhibited higher values than WZH, it was evident that TS had the most significant impact on the GMAW process. Consequently, the researchers established empirical relationships for TS, WZH, and HAZH in terms of the GMAW parameters. Their test data aligned reasonably well with the expected range of performance indicators, supporting the validity of their findings. Overall, Monika and Chauhan’s modified Taguchi approach, based on the L9 OA, allowed them to efficiently conduct a limited number of tests while obtaining comprehensive information on the optimal GMAW parameters for enhancing the strength of MS 1018.

Wire-EDM cutting strategies of WC-Co hardmetals: Effect of number of EDM pass on surface integrity

In this study, the effect of the number of wire-EDM passes on the surface integrity of WC-Co hardmetals was investigated. As-received doughnut-shaped WC-25 wt%Co samples were wire EDM’ed with one, two, three and five passes. The resulting surface conditions were investigated in terms of macroscopic and microscopic examinations. Surface roughness and hardness measurements were carried out. Detailed SEM investigations were coupled with EDS analyses. Based on the experimental findings, a detailed cutting strategy was offered for WC-Co hardmetals depending on the expectations of surface conditions, production route, cost and productivity. If there is at least one subsequent process including material removal, wire-EDM with one pass is suggested to decrease cost and increase productivity. However, heat-affected zone (HAZ), recast layer and lower surface hardness region should be removed at subsequent production steps. If there are no production steps to remove HAZ, recast layer and lower surface hardness region, the number of wire-EDM passes should be maximised.

Microstructure and Properties of Nonlinear Lap Joint of 6061 Aluminum Alloy by Friction Stir Welding

The microstructure and properties of non-linear friction stir welded lap joints of the AA6061-T6 aluminum alloy were investigated, with a particular focus on the influence of corner curvature on the formability and mechanical properties of the joints. The research results indicate that for the 6061-T6 aluminum alloy lap joint friction stir welding with a smaller radius (R < 7 mm), there is a more severe accumulation of welding material. When the radius exceeds 7 mm, good macroscopic joint formation can be achieved. Various regions at the joint corners are composed of α-Al and intermetallic precipitations β phases. The microstructure of the heat-affected zone (HAZ) appeared relatively coarse, the weld nugget zone (WNZ) had the finest grain, and partial dissolution of the β phase occurred. The grain size in the middle WNZ at the corner was larger than at the ends, and the grain size on the inner side of the corner was larger than on the outer side. The hardness distribution of the joint exhibited a “W” shape, with the lowest hardness in the inner HAZ. When R ≤ 7, with an increase in R, the shear strength of the friction stir welded joints increased, and then the change became relatively small. The maximum shear strength of the joint was 101.32 ± 6.89 MPa at R = 7, and the fracture mode was primarily a ductile mixed fracture.

Effect of water salinity on properties of multipass underwater wet manual metal arc welded joints

The weldability of steel under the water is limited due to the influence of the environment. Water causes limited visibility, presence of the residual stresses, increasing the cooling rate, and increasing the diffusible hydrogen content in deposited metal, leading to the formation of brittle microstructures in heat-affected zone (HAZ). The paper presents the results of mechanical properties testing of S420G2+M steel welded joints made with covered electrodes in the water with salinity values: 0‰, 7.5‰, and 35‰. The non-destructive tests: ultrasonic (UT) and radiographic (RT), and destructive tests: Vickers HV10 measurements, Charpy impact, and bending and metallographic macro- and microscopic tests were performed. Moreover, the diffusible hydrogen content in deposited metal by the mercury method was measured. It was observed that water salinity has an influence on the stability of the welding arc and the properties of joints. In particular, it was found that increasing the water salinity provides to: decreasing the hardness of HAZ (from 211 HV10 to 193 HV10), increasing the impact strength (from 82.5 to 101.3 J/cm2). Additionally, a slight increase in the diffusible hydrogen content in deposited metal (up to 65 ml/100g) was observed. The number of microcracks in the joints decreased with increasing the water salinity.

Mechanical Properties Of Welding API 5l X-65 Cladded UNS N08825 Material Due To The Effect of Welding Repair

Abstract This study evaluate the mechanical characteristics and microstructure of API 5L X-65-clad UNS N08825 during repair welding. Gas tungsten arc welding (GTAW) and shielded metal arc welding were used in combination to carry out the welding and the repair welding (SMAW). Filler metals ER NiCrMo3 and E NiCrMo3 were used throughout the GTAW and SMAW welding processes. The first specimen served as the primary focus, after which the weld areas of the remaining four specimens were ground, re-beveled, and then re-welded using the same parameters. It performed tensile testing, bending tests, Impact Charpy tests, macro photography tests, and Vickers hardness tests. Repairs weld carried out, the heat-affected zone's hardness in the capping region increased. The findings of the tensile test demonstrate that the tensile strength is not considerably impacted by the welding repair at any particular welding repair location. The biggest decline in the number is caused by the welding repair at PTR 2, which is 0.83 percent. The heat affected area (FL) value decreased by 10.44 percent according to the charpy impact test results. The PTR 1 and PTR 2 areas in the weld metal area had the lowest charpy impact test results, respectively, of 88 J and 258 J.

Influence of multi-layer laser cladding depositions and rail curvature on residual stress in light rail components

Rapid deterioration of critical rail components due to wear and fatigue is a significant challenge faced by the railway industry. Laser cladding has been applied to straight and curved light rail substrates which are particularly prone to these methods of degradation using a Stellite 21 hardfacing alloy. The influence of multi-layer cladding depositions, grinding-based finishing procedures and substrate rail grade on residual stress was analyzed using non-destructive neutron diffraction techniques. As cladding is a thermal process, microstructural changes from the heat inputs can result in a high internal stress state which reduces the loading capacity whilst a high hardness increases the likelihood of brittle failure. Stellite laser cladding depositions were found to result in low tensile residual stresses within the cladding layer, which become compressive in the heat affected zone (HAZ). Repeated thermal inputs from multi-layer cladding depositions did not negatively impact the hardness or microstructure after double layer laser cladding and increased the residual stress to 100 MPa at the cladding surface after grinding. Laser cladding on a curved rail substrate also produced increased internal stress after cladding due to higher strains resulting from the rail geometry but remained below the yield limit of the cladding and substrate material. These outcomes were compared to current literature indicating this critical combination of low internal strain and a cladding and HAZ hardness complementing the substrate material is difficult to achieve. Therefore, ensuring laser cladding is compatible with a variety of light rail components is essential in developing viable maintenance techniques to recondition critical railway infrastructure to avoid disruptive replacement procedures.

Interface characterization and intermetallic compounds formation mechanism of V-4Cr-4Ti/316SS laser welded joints

In fusion reactors, the efficient connection of V-4Cr-4Ti and 316 stainless steel (316SS) is critical for heat transfer between blankets and heat exchangers. However, the presence of brittle intermetallic compounds (IMCs) can degrade the properties of welded joints, so it is necessary to investigate the formation and their mechanism of IMCs. In this study, the morphology, microstructure, hardness of the welded joint between V-4Cr-4Ti and 316SS, were analyzed after laser welding with a Cu interlayer. No significant defects (e.g., pores, macrocracks, or microcracks) were observed in the welded joints after welding, indicating good weldability. Three interfaces are formed according to the analysis of the morphology: V-4Cr-4Ti and the heat affected zone (HAZ), the HAZ and the weld, and the weld and 316SS. According to the microstructural analysis, the CuTi2 with a structure of face-centered cubic (FCC) is formed at the interface between the IMC layer and the weld, while TiFe with a structure of body-centered cubic (BCC) is formed at the weld and 316SS interface. The σ phase is formed at all three characteristic interfaces. The results of nanoindentation test show that the hardness and elastic modulus of HAZ and weld are higher than that of base metal, which may be due to the formation of IMCs. In addition, the sequence and mechanism of IMCs formation in V-4Cr-4Ti/316SS laser welded joints with Cu interlayers are discussed.

Microstructural characteristics and mechanical properties of rotary friction-welded dissimilar AISI 431 steel/AISI 1018 steel joints

Abstract The main objective of this study was to analyze the microstructural characteristics and strength performance of dissimilar AISI 431 steel/AISI 1018 steel joints developed using rotary friction welding. The microstructural characteristics of different regions of dissimilar rod-to-plate joints were analyzed using optical microscopy. The tensile properties and microhardness of dissimilar rod-to-plate joints were evaluated to assess the joint performance. The microhardness distribution across the cross-sectional region of dissimilar rod-to-plate joints was recorded and correlated with the tensile failure. Scanning electron microscopy was used to analyze the fractured region of dissimilar rod-to-plate tensile specimens. Results showed that the dissimilar AISI 431 steel/AISI 1018 steel joints steel exhibited a tensile strength of 650 MPa, a yield strength of 452 MPa, and a % elongation of 18%. The microhardness of the weld interface (WI) was higher up to 515 HV0.5. The grain growth and resulting lower hardness in heat-affected zone (HAZ) are mainly responsible for the failure of the joints in HAZ only. The superior tensile properties and greater interface hardness of dissimilar AISI 431 steel/AISI 1018 steel joints are correlated with the evolution of finer grain microstructure in the WI zone.

Interfacial microstructure and mechanical properties of forging and SLM hybrid manufacturing Ti-6Al-4V parts

Ti-6Al-4V alloy composite samples and tensile parts were formed by SLM with forged Ti-6Al-4V alloy rolled plate as the base material. The microstructure analysis, tensile property test, microhardness test and fracture morphology observation were carried out on the bonding surface. The results show that the forging region and forming region of SLM form a close metallurgical bond, no cracks appear. The vertical and horizontal mixed tensile samples were prepared. The fracture position of vertically mixed tensile samples occurs at the side of the matrix, indicating that the bonding surface strength is higher than that of the matrix. The tensile strength of the horizontal mixed tensile sample reaches 1144.1 ± 0.5 MPa. The tensile strength of the vertical mixed tensile sample reaches 856.6 ± 0.4 MPa. The microhardness in SLM zone is the highest at room temperature, between 340 ± 5HV0.5 and 360 ± 5HV0.5, that in forging zone is between 300 ± 5HV0.5 and 350 ± 5HV0.5. The hardness of heat-affected zone is 330 ± 5HV 0.5.

Tensile shear fracture load bearing capability, softening of HAZ and microstructural characteristics of resistance spot welded DP-1000 steel joints

Abstract The main objective of this investigation is to enhance the tensile shear fracture load (TSFL) bearing capability and minimize softening in heat affected zone (HAZ) of resistance spot welded DP-1000 steel spot joints for automotive applications. The lap tensile and cross tensile shear fracture load tests (LTSFL and CTSFL) were conducted. The process parameters were optimized using numerical and graphical optimization techniques to maximize the TSFL capability of spot joints. The microstructure of spot joints was studied using optical microscopy and correlated to TSFL and hardness of spot joints. Results showed that DP-1000 steel spot joints made using the welding power of 70 W, welding time of 1.0 s, electrode pressure of 4.25 MPa showed maximum LTSFL of 22 kN and CTSFL of 9.1 kN. The parametric optimization showed 46.66, 45.77, 22.33 and 9.87% increase in LTSFL, CTSFL, nugget zone hardness and HAZ hardness of DP-1000 steel spot joints. The higher TSFL and nugget hardness of spot joints is mainly attributed to the evolution of finer martensitic sandwiching ferrite phases in nugget zone and lower softening of HAZ than other joints. Welding power showed significant influence on TSFL of spot joints followed by welding time and electrode pressure.

Elemental partitioning as a route to decrease HAZ hardness

In the present study, in order to find out a valuable method to decrease the hardness of heat-affected zone (HAZ), elemental partitioning was introduced before welding simulation to realize elemental redistribution. The results demonstrate that about 10% reduction of HAZ hardness can be achieved by introduction of elemental partitioning. During elemental partitioning, the content of retained austenite increases with increasing of elemental partitioned time. Further research reveals that the value of geometrically necessary dislocation density decreases with elemental partitioning. It is found that the alloying elemental contents in the matrix after elemental partitioning are lower than that without elemental partitioning. In addition, the differences of grain size distribution and elemental distribution resulted in elemental partitioning are studied and compared to explain the reason for the reduction in hardness.

Microstructure, Corrosion Resistance, and Hardness of Simulated Heat-Affected Zone of Duplex UNS S32205 and Superduplex UNS S32750 Stainless Steels

Microstructure, Corrosion Resistance, and Hardness of Simulated Heat-Affected Zone of Duplex UNS S32205 and Superduplex UNS S32750 Stainless Steels

Effect of Peak Temperatures on Microstructure and Properties of Welding Coarse Grain Heat-Affected Zone of SA508 Gr.4N Steel

The effect of peak temperatures on the grain growth, microstructure transformation and Vickers hardness of the weld coarse grain heat-affected zone (CGHAZ) of SA508 Gr.4N steel for nuclear reactor pressure vessel was studied. The results show that different t8/5 time has no obvious effect on grain growth at the same peak temperature, but different peak temperatures have a great influence. The higher the peak temperature, the more obvious the effect on grain growth. The grain size distribution at different peak temperatures and t8/5 time conforms to the Gaussian normal distribution curve, and a grain growth model suitable for CGHAZ of SA508 Gr.4N steel has been established. The room temperature martensite structure can be obtained by treatment at different peak temperatures and t8/5 time. At the same peak temperature, there is no significant difference in martensite structure at different t8/5 time, and the size of martensite lath only increases with the increasing peak temperatures. At different peak temperatures and t8/5 time, grain size has less effect on the Vickers hardness, which is closely related to the room temperature microstructure of the material.

Characterizing local distribution of microstructural features and its correlation with microhardness in resistance spot welded ultra-low-carbon steel: Experimental and finite element characterization

This study investigated the influence of microstructural features on the heterogeneous distribution of hardness in resistance spot welded (RSWed) 340BH steel. The microstructure and texture across the weld region was investigated using optical microscopic and electron backscattered diffraction (EBSD) techniques. Finite element analysis (FEA) was used to simulate the RSW process, which helped in predicting the nugget dimensions as well as the cooling rate in different weld regions. Rapid cooling in the fusion zone (FZ) led to the formation of martensite with typical packet and block substructures. Numerical reconstruction of the prior austenite grain (PAG) was employed to understand the orientation relationship between martensite and austenite. The results showed that child laths obey a Kurdjumov-Sachs orientation relationship (K-S OR) with respect to the austenite matrix with a slight deviation (~3° angular deviation) from a conventional K-S OR. Moreover, microhardness was measured across the weld zone. An increase in the hardness was observed throughout the weld zones. The hardness increase in the heat-affected zone (HAZ) was due to a finer (smaller) grain size and an increase in the dislocation density, whereas in the FZ, the improvement in hardness was attributed to the phase change during the solidification process. The microhardness of martensite formed during RSW was strongly dependent on the block size according to the Hall-Petch relationship. • A heterogeneous distribution of microstructure and hardness in RSWed 340BH steel was characterized using EBSD and FEA. • Lower ZHP values and fast kinetics of dynamic recrystallization resulted in fine structure of ferrite grains in ICHAZ. • FZ hardness was mainly driven by martensite formation, while grain size distribution and dislocation density were mainly responsible for higher hardness in HAZ. • The block size in the martensite substructure followed Hall-Petch type relationship with microhardness of lath martensite.

Parametric mathematical modeling and 3D response surface analysis for rod to plate friction welding of AISI 1020 steel/AISI 1018 steel

PurposeRotary friction welding (RFW) was used to solve the issues in fusion welding of rod to plate joints of low carbon steel (AISI 1020 steel/AISI 1018 steel) such solidification cracking, wider heat affected zone (HAZ), lower HAZ hardness, high residual stresses and distortion. The main objective of this investigation is to develop parametric mathematic models (PMMs), 3D response surface analysis to predict tensile strength (TS) and weld interface hardness (WIH) of rod to plate joints and correlate microstructure with TS and WIH of rod to plate joints.Design/methodology/approachThe three-factor x five-level central composite design (CCD) consisting fewer experiments was employed for designing experimental matrix. The tensile and microhardness tests were performed to evaluate mechanical performance of joints. The PMMs of TS and WIH of rod to plate joints were developed using polynomial regression equations incorporating the RFW parameters. The 3D response surfaces were developed using response surface methodology (RSM) to optimize RFW parameters for joining AISI 1020/AISI 1018 rod to plate.FindingsThe joints made using friction pressure/friction time (FRNP/FRNT) of 3.71 MPa/s, forging pressure/forging time (FRGP/FRGT) of 3.71 MPa/s and rotational speed (RTSP) of 19.99 rps exhibited higher TS and WIH of 452 MPa and 252 HV0.5. The PMMs accurately predicted TS and WIH of rod to plate joints at less than 1.5% error and 95% confidence. The RTSP revealed greater effect on TS and WIH of rod to plate joints followed by FRGP/FRGT and FRNP/FRNT. The superior TS and WIH of joints developed using optimized process parameters is correlated to the evolution of finer bainitic microstructure in weld interface due to the dynamic recrystallization of grains ensued by optimum frictional heating and plastic deformation.Originality/valueThe PMMs were developed for predicting TS and WIH of joints. The RFW parameters were optimized to enhance TS and WIH of joints. Low carbon steel rod to plates joints were developed using RFW for automotive applications without fusion welding defects. The microstructural features of low strength and high strength rod to plate joints were correlated to the TS and WIH of rod to plate joints.

Chemical composition and weld cooling time effects on heat-affected zone hardness of line pipe steels

In the past few years, the pipeline industry in North America has faced a reported increase of pipeline girth weld failures predominantly in grade X70 pipe welded with cellulosic consumables. Weld strength matching and HAZ softening were two factors believed to have contributed to the concentration of strain during, or soon after, construction which ultimately lead to an overload failure of the girth welds. The work reported in this paper aimed to establish and clarify the correlation between steel chemical composition and the HAZ hardness in typical production girth welds and a range of weld thermal simulations. The first stage was to calibrate the influence of weld cooling rate of both thermal simulations and real welds in steels over a wide range of chemical composition, typical of commercial high strength API pipe. Second, the influence of Pcm, carbon content, Mo content and cooling time in terms of minimum and maximum hardness were investigated. The comparison between simulated HAZ's and real welds showed that thermal simulation produces samples that adequately duplicate the real condition in terms of maximum and minimum hardness and can accurately assess both hardening & softening which occur in different regions of the HAZ. The simulated samples provided increased areas of different HAZ regions in order to more accurately identify the range of hardness values across the HAZ which was then correlated to real weld hardness profiles. It is concluded that ultimate hardness in the HAZ is directly related to steel chemical composition in terms of Pcm and weld cooling rate. It is however apparent that the relative percentage change in hardness from the original parent pipe hardness (also called % HAZ softening), which is important in determining the strain capacity of the weld joint, is principally dependent upon steel processing, especially the degree and severity of thermomechanical processing. This is evident by the fact that same API pipe grades have a wide range in chemical composition and most importantly Pcm, and the response to field welding conditions can be quite different depending on weld heat input, pipe wall thickness and preheat, all of which define the weld cooling rate.