HAZ Toughness Research Articles

Tempering Procedure Effects On the Impact Toughness in Simulated Haz of Grade 22 Steel

Abstract Welding of thick low alloy steel components without elevated preheat imposes rapid cooling rates that may promote martensite formation in the CGHAZ. Freshly formed martensite contains a distorted and supersaturated matrix with high dislocation densities. As a result, impact toughness properties of these regions are inherently low and post weld tempering procedures are typically required. Conventional PWHT involves extended isothermal holds at elevated temperature which allows for considerable softening of the martensitic structure but has been found to have less effect on improving impact toughness. In contrast, short-term tempering processes with rapid heating and cooling rates and short exposures to elevated temperatures have been shown to greatly improve impact toughness through a shift in the kinetics of martensite tempering that produces high number densities of refined carbides. Temper bead welding (TBW) was developed for in-service repair of thick steel components where PWHT is impractical or not possible. With TBW, the heat-affected-zone is tempered by one or more rapid re-heat cycles from overlapping weld beads or weld layers. This study compares the CVN impact toughness and fracture morphology of the CGHAZ in the as-welded condition, with conventional PWHT, with a single TBW reheat, and with multiple TBW reheats for an SA-387 Grade 22 alloy steel.

Process characteristics of Q890D joint using hybrid fiber laser-arc hybrid welding

This article conducted a study on the process characteristics, structure, and properties of the Q890D joint. The results indicated that three-layer welding methods can obtain a joint free of pores and defects. The base metal was composed of a large amount of polygonal ferrite and dispersed granular carbides, while the weld metal consisted of lath martensite. The results also revealed that the average hardness of WM was 417.24 HV (20.7% higher than BM). The tensile performance of welded joints was better than that of BM. There were no obvious cracks on the surface of the bending specimens. Compared with BM, the impact strength and toughness of WM had been improved, since the impact toughness of HAZ was highly dispersed.

Improvement of microstructure and mechanical properties for underwater wet 16Mn/304L dissimilar steel welded joints assisted by presetting butter layer

Underwater wet flux-cored arc welding (FCAW) is a simple and economic method to join dissimilar steel structures widely used in offshore applications. The available research on underwater wet welding of dissimilar steel joints is inadequate. In this paper, a butter layer was applied to eliminate the formation of hardened martensite at heat affected zone of 16Mn side and to reduce the stress concentration at the fusion boundary. The influence of ERNiCrMo-3 butter layer on the microstructure, elemental distribution, and mechanical performance of underwater wet 16Mn/304 L dissimilar steel welded joints was investigated. The grain boundary type and grain growth orientation across the fusion boundary were particularly investigated. The use of the ERNiCrMo-3 butter layer helped to eliminate the formation of hardened martensite resulting from the rapid cooling rate. Type I boundary and type II grain boundary were high angle grain boundaries, which was found at the Ni-based weld metal/ferritic steel (butter layer/ferritic steel) interfaces. The impact toughness of HAZ at 16Mn side was improved by the use of butter layer because of the absence of martensite. The butter layer was accompanied by an increase in ultimate tensile strength in the orders of ~80 MPa. The expected research results can enrich the underwater welding/repair technology of dissimilar steel structures and guide the future design of dissimilar steel welded joints.

Preliminary investigation on underwater wet welding of Inconel 625 alloy: microstructure, mechanical properties and corrosion resistance

This paper aimed to investigate the feasibility of underwater wet welding for Inconel 625 alloy using Ni-based filler. The microstructure, elemental distribution, and properties of underwater wet welded Inconel 625 alloy joints were analyzed. The grain boundary type and grain growth orientation across the fusion boundary were particularly investigated. A continuous and smooth weld with no visible defects was obtained. The nickel based welds was fully austenite structure with extensive grain boundary migration. The heat-affected zone experienced obvious grain growth. A compositional transition zone with a width of 50 μm was observed across the fusion boundary. Apparent epitaxial growth across the fusion boundaries was found and the grain boundaries roughly perpendicular to the fusion boundaries are was mainly high angle grain boundaries. The density of low Σ CSL grain boundary in Inconel 625 base metal was higher than that in Ni-based weld metal. The ultimate tensile strength of underwater wet welded joints reached up to 83% that of Inconel 625 alloy. The impact toughness of HAZ was deteriorated because of the segregation of carbide and Laves phase in grain boundary. The corrosion resistance of Ni-based welds maintained at a high level based on the corrosion current density, although it was inferior to that of Inconel 625 alloy. The expected research results could enrich the underwater welding/repair technology of Ni-based alloy structures.

Favorable Impact Toughness of High Heat Input Coarse-Grained HAZ in an Ultra-Low Carbon High-Strength Microalloyed Steel

Favorable Impact Toughness of High Heat Input Coarse-Grained HAZ in an Ultra-Low Carbon High-Strength Microalloyed Steel

Microstructure and Impact Toughness of the Coarse-Grain HAZ Physically Simulated with Different Heat Inputs in a 690 MPa Ultra-low Carbon High-Strength Microalloyed Steel

Microstructure and Impact Toughness of the Coarse-Grain HAZ Physically Simulated with Different Heat Inputs in a 690 MPa Ultra-low Carbon High-Strength Microalloyed Steel

Morphology and crystallography of microstructures in Mg-deoxidized offshore engineering steels after simulated welding thermal cycles

ABSTRACT The morphology and crystallography of microstructures in heat-affected zone of offshore engineering steels with 0.0002 and 0.0042 wt% Mg were studied. With increasing Mg content from 0.0002 to 0.0042 wt%, the prior austenite grain (PAG) size increases from 92 to 163 μm, which decreases the PAG boundary density and increases the bainite transformation temperature. So the bainite nucleation sites and driving force reduce, affecting bainites with morphology and crystallography. For morphology, the sympathetic nucleation reduces and widening growth rate of bainitic laths increases, coarsening microstructures. For crystallography, the variant selection is intensified, coarsening the unevenly distributed Bain zones with specific variants and reducing the high-angle grain boundary (HAGB) density from 1.4 to 0.6 μm–1. The coarsened microstructure with lower crack resistance increases the ductile-brittle transition temperature, contributing to the ductile fracture transforming to cleavage fracture and the HAZ toughness at –40 °C decreasing from 201 to 58 J.

Effect of Welding Heat Input on the Microstructure and Impact Toughness of HAZ in 420 MPa-Grade Offshore Engineering Steel

In the present work, the effect of the welding heat input on the microstructure, martensite–austenite (M–A) constituents, and impact toughness of the coarse-grained heat-affected zone (CGHAZ) in offshore engineering steel with Ca deoxidation is studied. With the heat input increased from 50 to 100 kJ/cm, the HAZ toughness decreased rapidly, while the measured microhardness decreases steadily. The grain sizes are increased from 52 to 132 μm, and the width of bainite lath increased from 0.4 to 2 μm. The area fraction of lath bainite (LB) decreased, while the area fraction of granular bainite (GB) increased. The average width of M–A constituents grows from 0.3 to 0.6 μm, and the average length grows from from 0.5 to 0.9 μm. Its area fraction is increased from 5.3 to 8.6% and then decreased to 6.1%, and its number density decreased from 0.7 to 0.2 μm−2. The morphologies of M–A constituents change from dot-like to slender and blocky, which are deleterious to impact toughness. The fracture mechanism changes from ductile to quasicleavage and cleavage as the heat input is increased. As the M–A constituents are always found as the cleavage initiation, they should be responsible for the decrease in HAZ toughness when the heat input is above 100 kJ/cm.

Effect of post weld heat treatment on mechanical properties of MIG welded mild steel

ABSTRACT In railway locomotive, fabricated bogie frame structures are susceptible to crack when made up of un-heat-treated and heat-treated structure. Therefore, variation in behaviour of the samples made of the same material in both the conditions, that is, PWHT and un-heat-treated is important to quantify and analyse. In the present work, specimens from IS 2062 E410 are prepared as per standard weld procedure specification using MIG welding and half of the welded samples are heat-treated following a predefined heat treatment procedure. The samples are tested to find out their mechanical properties like hardness, toughness, tensile strength and microstructure of HAZ. Refined grain structure and significant improvement in mechanical properties of the specimen are observed in the samples undergone post-weld heat treatment.

Effect of Mg Addition on the Microstructure and Properties of a Heat-Affected Zone in Submerged Arc Welding of an Al-Killed Low Carbon Steel.

To reveal the effect of Mg treatment on the microstructure evolution behavior in the actual steel welding process, the microstructure and properties of Al-deoxidized high-strength ship plate steel with Mg addition were analyzed after double-side submerged arc welding. It was found that the Al–Mg–O + MnS inclusion formed under 26 ppm Mg treatment could promote acicular ferrite (AF) nucleation in the coarse-grained heat-affected zone (CGHAZ) and inhibit the formation of widmanstätten ferrite and coarse grain boundary ferrite. In the fine-grained heat-affected zone (FGHAZ) and intercritical heat-affected zone (ICHAZ), polygonal ferrite and pearlite were dominant. Al–Mg–O+MnS cannot play a role in inducing AF, but the grain size of ferrite was refined by Mg addition. The impact toughness in HAZ of the Mg-added steel was higher than that of Mg-free steel. With the heat-input rising from 29.55 to 44.11 kJ/cm, it remained relatively stable in Mg-treated steel. From the fusion line to the base metal, the micro-hardness of the fusion zone, CGHAZ, ICHAZ and FGHAZ decreased to some extent after Mg addition, which means the cold cracking tendency in the welding weak zone could be reduced. Finally, the mechanisms of Mg-containing inclusion-induced AF were also systematically discussed.

On the effect of Qnet on compressive residual stresses and ductile toughness of dissimilar HAZ in robotic GMAW joints between A572-50 and A36 steels

Different heat input values applied in robotic dissimilar welded joints of A36 carbon and A572-50 alloyed steels were evaluated by X-ray residual stresses, Vickers hardness, impact energy, microstructural analysis, and fractography. A welding process using pulsed metal arc for ten combinations involving welding current, arc voltage, welding speed, and power was performed by welding robot. Residual stress measurements in thickness direction at the HAZ within both steels indicated that five weldings achieved compressive stresses, resulting in the highest value (171 MPa) in the HAZ of A572 steel for joint number 3. Greater absorbed energy levels were achieved for the HAZ of A572 steel, followed by weld bead ER70S6 and the HAZ of A36 steel, which was linked to formation of a major amount of fine needles of acicular ferrite, giving the largest energy (160 J) for the HAZ of A572 steel of trial 31. Moreover, three joints showed ductile type failure with microhollow morphology of different sizes, including welding 3 within both kinds of dissimilar HAZ in A36 and A572 steels. Trial 3 was the best showing superficial compressive residual stresses within both kinds of HAZ, intermediate microhardness, and larger Charpy energy associated with acicular ferrite and ductile failure as a consequence of higher heat (0.615 kJ/mm). Welding 27 was better with complete compressive residual stresses in the HAZ of A36 steel, major absorbed energy, and fragile fracture by mechanism of transgranular cleavage within both types of HAZs.

Effects of Al, Si and N Content on the Formation of M-A Constituent and HAZ Toughness of Ti-Containing Low-Carbon Steel

The present work investigated the effects of Al, Si, and N content on the impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Ti-containing low-carbon steel. Simulated CGHAZ of differing Al, Si, and N contents were prepared, and Charpy impact toughness was determined. The results were interpreted in terms of microstructure, especially martensite-austenite (M-A) constituent. All elements accelerated ferrite transformation in CGHAZ but at the same time increased the amount of M-A constituent, thereby deteriorating CGHAZ toughness. It is believed that Al, Si, and free N that is uncombined with Ti retard the decomposition of austenite into pearlite and increase the carbon content in the last transforming austenite, thus increasing the amount of M-A constituent. Regardless of the amount of ferrite in CGHAZ, its toughness decreased linearly with an increase of M-A constituent in this experiment, indicating that HAZ toughness is predominantly affected by the presence of M-A constituent. When a comparison of the effectiveness is made between Al and Si, it showed that a decrease in Si content is more effective in reducing M-A constituents.

Deterioration of HAZ toughness by residual Sn and Determination of its Allowable Content for Electric Furnace Steels

One promising direction in steelmaking to respond to global climate change issue is to increase the use of electric furnace manufacturing methods using zero-carbon electricity, which have received more and more attention recently. Considering the replacement from blast furnace steel, the temper embrittlement problem induced by some tramp elements, which is unique behavior for electric furnace methods, must be resolved.Sn is considered as a tramp element that cannot be removed in steel refining process, and it has been reported that the toughness of the base metal and the welded heat affected zone can be significantly degraded. In the case of electric furnace steels with degraded toughness, segregation of Sn is observed at grain boundaries and the resulting fracture mode shifts to intergranular fracture mode. In this study, in order to quantify the allowable amount of Sn added to thick steel plates that is used for large-heat input welding, experiments were conducted on specimens with different amounts of Sn and heat-treated for different holding times at 400℃, which is a typical embrittlement temperature and investigated the effect of the amount of Sn on the change in low-temperature fracture toughness. As a result of fracture toughness tests, clear evidence of temper embrittlement was observed. Apart from previous famous study by McMahon, there was no intergranular fracture in fracture surface but transgranular and cleavage fracture was observed. Therefore, the mechanism by which Sn tends to cause cleavage fracture in the weld heat affected zone was proposed. Based on our newly proposed mechanism, a model was developed to estimate the transition temperature change for various conditions of tempering time, tempering temperature and Sn content.Finally, the authors propose allowable maximum content of Sn based on the model for the establishment of the rational ore sorting system from scraps in near future society.

Prediction of HAZ width and toughness of HY85 steel using artificial neural network

ABSTRACT Physical weld simulation of single pass welding of HY 85 steel was performed using Gleeble® 3800 thermo-mechanical simulator at a peak temperature of 1300ºCwith heating rate of 200ºCs−1, and cooled with cooling rates ranging from 0.5 to 120ºCs−1 and corresponding heat inputs ranging from 1137 to 4.7 kJ/cm. The impact toughness of coarse grain heat-affected (CGHAZ) zone of simulated HAZ specimens was determined at a temperature of −50°C and related with their microstructures. Back propagation neural network (BPN) model was used to model heat-affected zone width and impact toughness of simulated heat-affected zone specimens, using heat input and cooling rate as input parameters. Predicted data of HAZ width and impact toughness is compared with the experimental data and it is observed that the back propagation neural network model with 4 hidden neurons adequately predicts the impact toughness and HAZ width. Calculated relative percentage error for HAZ width is ±3.15, whereas for impact toughness it is ±7.93%.

Effect of different welding energy on microstructure and toughness of HAZ of low carbon bainitic steel

Low carbon bainitic steel (LCBS) with excellent combined properties is the first choice for materials of pipeline transiting geological disaster. However, welding will worsen its toughness. In this paper, three kinds of welding heat input were designed to study the relationship between the toughness and microstructure in the coarse grain zone of welding heat affect zone (CGHAZ) of LCBS. The evolution characteristics of the microstructure of LCBS and the CGHAZ, and impact fracture were investigated by optical microscopy (OM), scanning electron microscopy (SEM). The results indicated that microstructure of LCBS consists mainly of bainite ferrite (BF) and granular bainite (GB). Heat input for 22 kJ/cm, the original austenite grains become coarsening, the microstructure is a small amount of quasi polygonal ferrite (QF) and polygonal ferrite (PF), which exhibits low Charpy impact toughness. However, for heat input of 19 kJ/cm, the degree of grain coarsening is small and distribution of martensite-austenite (M-A) constituents is the chain. The statistics of image software show that with the increase of heat input (16–22 kJ/cm), the average grain size of original austenite is basically the same (25 um), which is mainly due to Nb solute drag restraint the growth of austenite grain.

Mechanism of Improving Heat-Affected Zone Toughness of Steel Plate with Mg Deoxidation after High-Heat-Input Welding

In the present study, the mechanism of improving HAZ toughness of steel plate with Mg deoxidation after the simulated welding with the heat input of 400 kJ/cm was investigated through in situ observation, characterization with SEM-EDS and TEM-EDS, and thermodynamic calculation. It was found that intragranular acicular ferrite (IAF) and polygonal ferrite (PF) contributed to the improvements of HAZ toughness in steels with Mg deoxidation. With the increase of Mg content in steel, the oxide in micron size inclusion was firstly changed to MgO-Ti2O3, then to MgO with the further increase of Mg content in steel. The formation of nanoscale TiN particles was promoted more obviously with the higher Mg content in the steel. The growth rates of austenite grains at the high-temperature stage (1400~1250 °C) during the HAZ thermal cycle of steels with conventional Al deoxidation and Mg deoxidation containing 0.0027 and 0.0099 wt% Mg were 10.55, 0.89, 0.01 μm/s, respectively. It was indicated that nanoscale TiN particles formed in steel with Mg deoxidation were effective to inhibit the growth of austenite grain. The excellent HAZ toughness of steel plates after welding with a heat input of 400 kJ/cm could be obtained by control of the Mg content in steel to selectively promote the formation of IAF or retard the growth of austenite grain.

Effects of local-brittle-zone (LBZ) microstructures on crack initiation and propagation in three Mo-added high-strength low-alloy (HSLA) steels

Fracture toughness of intercritically-reheated coarse-grained HAZ (ICRCGHAZ), i.e., local brittle zone, is severely reduced at low temperatures by the presence of undesirable microstructural features such as coarsened prior austenite grains, bainitic microstructures including bainitic ferrite (BF), granular bainite (GB), and acicular ferrite (AF), and martensite-austenite constituent (MA). In this study, effects of the microstructural features on critical CTOD and crack initiation and propagation process were investigated in the ICRCGHAZ of three Mo-added high-strength low-alloy (HSLA) steels. The Mo addition caused the increased volume fractions of GB, BF, and MA and the resultant deterioration of critical CTOD. According to the observation of cracking process, most of hard MAs, particularly MAs located along prior austenite grain boundaries (PAGBs), provided initiation sites of {100} cleavage cracks. In addition, a large hardness difference between PAGB-MA and matrix led to an easy MA/matrix interfacial separation and an increase in crack initiation sites, thereby resulting in very low critical CTOD in the high-Mo-containing steel having the highest fraction of MA. The detailed microscopic observation of cracking process at MAs and macroscopic MA/matrix hardness difference provide plausible ideas for reliably evaluating critical CTOD and for designing HSLA steels in consideration of ICRCGHAZ microstructures.

The Effect of Peak Temperature on HAZ Microstructure and Toughness of V- and Nb-Bearing Microalloyed Steels

Welding heat cycle processes of V- and Nb-bearing microalloyed steels at different peak temperatures (1350, 1100, 950, 800 and 650 °C) were simulated by Gleeble 3500. The microstructure and toughness of two steels were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and low-temperature impact toughness test (− 40 °C). The results indicated that the microstructure of both V and Nb steels consisted of the lath bainite (LB), granular bainite (GB), polygonal ferrite (PF) and a small amount of martensite austenite (MA) islands at 1350 °C, and acicular ferrite (AF) was also observed in V steel. Ferrite (F) phase became uniform, and hard phase LB decreased with the decrease in peak temperature. Impact tests indicated that both the two steels exhibited the best impact toughness (32.23 and 30.90 J) at the peak temperature of 950 °C. The impact toughness of V steel was better than of Nb steel all the time, which was consistent with the analysis of microfracture.

Novel Hot-Rolled Structural Plate Steels with Yield Strength of 700 Mpa and Excellent Usability

Hot rolled structural plate steels with yield strength of 700 MPa are an excellent choice for a variety of demanding applications that require excellent toughness and welding properties. SSAB has developed novel hot-rolled plate steels that are produced using precision controlled rolling and an innovative cooling and tempering strategy that ensures invariable mechanical properties in the width and the length directions of the plate. The recently developed steel meets or exceeds the requirements of EN 10025-6 for the S690QL grade. The minimum yield strength (ReH) is between 630 MPa and 690 MPa depending on plate the thickness, and the interval of tensile strength is 760 - 940 MPa, while the minimum elongation at fracture is 14 %. Further, an impact energy of 69 J at-40 °C on transverse V-notch specimen is guaranteed. The novel grades represent superior bendability and surface quality, weldability with excellent HAZ strength and toughness with very low CET and CEV values, exceptional consistency of properties within a plate guaranteed by close tolerances. In addition to the excellent formability, the novel hot rolled steels exhibit greatly improved toughness properties which provides for good resistance to fracture. These outstanding properties are achieved through carefully controlled manufacturing processes. In the present study, a sophisticated recrystallization based hot rolling process optimization method is presented. With the final aim to improve the impact toughness of the novel steel, recrystallization analyses and modelling of austenite grain size development through the rolling pass schedule is performed.

Phase transformation and impact toughness in HAZ of micro alloyed X80 line pipe steel

In the present work, phase transformation behavior and structure-property correlation in X-80 steel was analyzed at three distinct austenitizing peak temperatures (1250 °C, 1030 °C and 950 °C). Dilation studies and simulations of Charpy size samples were carried out using Gleeble 3800 thermal simulator. Dilation studies indicated that the dissolution of Nb rich carbides occurred at 1020 °C. During cooling from 1250 °C, mixed microstructure of bainitic ferrite and martensite formed at fast (200 °C s−1 −80 °C s−1) and medium (50 °C s−1–40 °C s−1) cooling rates. For slow cooling rates (20 °C s−1–0.5 °C s−1), ferrite/pearlite microstructure was obtained. For lower peak temperatures 1030 °C and 950 °C, mixed microstructure of bainitic ferrite and martensite was observed for fast cooling rates. Whereas, ferrite/pearlite with small fraction of bainitic ferrite nucleated in the microstructure at medium and slow cooling rates. Highest area fraction of M-A (8%) was observed at medium cooling rates (50 °C s−1–40 °C s−1) for 1250 °C peak temperature. Charpy size samples simulated for a 1250 °C peak temperature, exhibited the minimum notch toughness at 12.5 °C s−1, due to deleterious effect of granular bainite and M-A constituents. Simulated samples at lower peak temperatures 1030 °C and 950 °C exhibited higher notch toughness due to a mixed microstructure of bainitic ferrite and ferrite-pearlite.