Coarse-grained Heat-affected Zone Research Articles

Effects of Ti and Cu Addition on Inclusion Modification and Corrosion Behavior in Simulated Coarse-Grained Heat-Affected Zone of Low-Alloy Steels.

In this paper, the effects of Ti and Cu addition on inclusion modification and corrosion behavior in the simulated coarse-grained heat-affected zone (CGHAZ) of low-alloy steels were investigated by using in-situ scanning vibration electrode technique (SVET), scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), and electrochemical workstation. The results demonstrated that the complex inclusions formed in Cu-bearing steel were (Ti, Al, Mn)-Ox-MnS, which was similar to that in base steel. Hence, localized corrosion was initiated by the dissolution of MnS. However, the main inclusions in Ti-bearing steels were modified into TiN-Al2O3/TiN, and the localized corrosion was initiated by the dissolution of high deformation region at inclusion/matrix interface. With increased interface density of inclusions in steels, the corrosion rate increased in the following order: Base steel ≈ Cu-bearing steel < Ti-bearing steel. Owing to the existence of Cu-enriched rust layer, the Cu-bearing steel shows a similar corrosion resistance with base steel.

Investigating the heat input effect of the GTAW process upon the microstructure and HAZ extension of HSLA-100 steel weld joints using thermal cycles

In the current research, the welding of HSLA-100 steel was accomplished with three different heat inputs using the GTAW process. The microstructure of various joint zones was investigated utilizing an optical microscope and scanning electron microscope. For the purpose of assessing the impact of thermal cycles upon the extension and alterations in the heat-affected zone, the thermocouples were placed at distinct distances from the fusion line. The results indicated that the weld metal microstructure comprised of acicular ferrite and a number of polygonal and quasi-polygonal Ferrite with M/A constituent. An epitaxial growth was observed in the fusion line in all heat inputs. It was determined that the heat-affected zone microstructure consisted of two regions, namely a coarse-grained heat-affected zone with a granular and lath bainite microstructure and a fine-grained heat-affected zone with the prevailing microstructure of granular Bainite. The results indicated that increasing the heat input leads to a decrease in the amount of acicular ferrite and M/A constituent volume fraction in the weld metal and an increase in lath bainite in coarse-grained zone. It was found that by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm, the grain size increases from 29.2μm to 76.5 μm and from 15.7μm to 32.5μm in coarse-grained and fine-grained zone, respectively. The thermal cycles evaluation pertinent to the implanting of a thermocouple disclosed that the heat-affected zone width is extended from 3.6 mm to 4.5mm by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm.

High cycle fatigue behaviors of API X65 pipeline steel welded joints in air and H2S solution environment

To investigate the mutual effect of hydrogen, microstructures and stress concentration on the fatigue failure, fatigue behaviors of X65 steel welded joints in both air and saturated H2S solution were investigated at high cycle regime. The experimental result demonstrates that due to lower dislocation density observed by electron backscattered diffraction (EBSD), the fine grain heat affected zone (FGHAZ) is prone to induce cyclic strain localization and further lead to fatigue crack propagating along the FGHAZ in air. Furthermore, the quasi-cleavage with brittle-like fatigue striations and secondary crack on the fracture surface in saturated H2S solution is attributed to hydrogen embrittlement. Moreover, compared with base metal (BM) and FGHAZ, the weld metal (WM) and coarse grain heat affected zone (CGHAZ) are composed of bainite and martensite/austenite (M/A) phase, and more sensitive to hydrogen. Therefore, the fatigue crack is prone to grow along the interface between WM and CGHAZ under the normal applied stress.

Investigation of CGHAZ Simulations of Low-Carbon Bainitic Steels

The effect of ten different combinations with various amounts of niobium (0-0.6 wt.%) and chromium (1-4 wt.%) on weldability and mechanical properties of thermomechanically rolled and direct-quenched low-carbon (0.035 wt.%) microalloyed bainitic steel were investigated. Two compositions were alloyed with boron to increase the hardenability, and two with titanium to improve the toughness properties in heat affected zone. The target of the study was to produce steel with 700 MPa yield strength combined with good impact toughness. Coarse grained heat affected zone (CGHAZ) simulations were performed using the Gleeble 3800 thermomechanical simulator to evaluate the weldability of the investigated steels using cooling time from 800 °C to 500 °C (t8/5) of 5 s and 15 s to simulate different heat inputs in actual welding procedure. Microstructures were characterized using light optical microscopy, and hardness profiles of simulated heat affected zones were determined as well as Charpy-V impact toughness at-40 °C and-60 °C. Shorter t8/5 time (5 s) produced generally better impact toughness properties compared to longer t8/5 -time (15 s). Steels with 4 % Cr had the highest impact energies. Generally, more softening occurred with longer t8/5-time (15 s). However, Cr and Nb alloying decreased the amount of softening in the CGHAZ region, especially with longer t8/5 -time. These results indicate that even with higher t8/5 -time, it is possible to achieve strength properties equivalent to the base material in the CGHAZ region by Cr and Nb alloying.

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.

The measurement of SH-CCT curve and analysis on microstructure and performance of heat-affected zone of Q690 high-strength bridge steel

The heat-affected zone continuous cooling transition curve (SH-CCT) of the experimental steel is drawn by using the Gleeble-3500 thermal simulation testing machine. The influences of different cooling rates and peak temperatures on the microstructure and mechanical properties of HAZ were studied. The results show as the cooling rate increases, the experimental steel undergoes LB + GB and LM + LB + GB phase transformations successively, and the micro-hardness gradually increases. When the heat input is 20kJ/cm, the microstructure of inter-critical heat-affected zone (ICHAZ) is F + B, the microstructure of fine grain heat-affected zone (FGHAZ) and the coarse grain heat-affected zone (CGHAZ) is LM + LB + GB. As the peak temperature increases, the micro-hardness first increases and then decreases, the -40°C impact absorbing energy gradually decreases. This conclusion can provide a reference for further study on the microstructure and mechanical properties of HAZ of Q690 bridge steel.

The critical influence of La content on the microstructure-toughness relationship in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The present study was envisaged to investigate the role of La content on the particle, microstructure and toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels. Three steels with La content of 0.016 wt.%, 0.046 wt.% and 0.093 wt.% were prepared and simulated in a 100 kJ/cm heat input welding thermal cycle. Subsequently, the particle and microstructure of selected specimens were characterized and the impact absorb energy was measured at −20 °C. The results indicated that the La2O2S inclusions in 0.016 wt.%-La steel were gradually modified to LaS-LaP in 0.046 wt.%-La steel and to LaP in 0.093 wt.%-La steel. A higher fraction of acicular ferrite was obtained in the simulated CGHAZ of 0.016 wt.%-La steel, since the inclusion of La2O2S was more powerful to induce the formation of acicular ferrite. Furthermore, the fraction of M-A constituents in the simulated CGHAZ increased with increasing La content. The impact toughness in the simulated CGHAZ of 0.016 wt.%-La steel was the highest, owing to the high fraction of the fine-grained acicular ferrite and low fraction of M-A constituent.

Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone and Microstructure Evolution of G115 Novel Martensitic Heat-Resistant Steel

Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone and Microstructure Evolution of G115 Novel Martensitic Heat-Resistant Steel

Study on microstructure-toughness relationship in heat affected zone of EQ70 steel by laser-arc hybrid welding

The microstructures and toughness of the laser-arc hybrid welded joint of EQ70 steel were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD), micro-shear test and Charpy impact test. The micro-shear toughness reduction and impact toughness reduction both occurred when the micro-zones traversed the coarse grained HAZ (CGHAZ) and inter-critically reheated coarse grained HAZ (ICCGHAZ). The CGHAZ were composed of lath martensite (LM), granular bainite (GB) and polygonal ferrite (PF). Necklace M-A constituents were distributed at the ICCGHAZ grain boundaries. The All-Euler maps showed that high angle boundaries (HABs) above 45°can notably block the micro-cracks propagation. The block width was the effective grain size controlling the toughness. The increase of block width in CGHAZ and ICCGHAZ, together with the necklace grain boundary M-A constituents, both contribute to the toughness reduction.

Fracture toughness of coarse-grain heat affected zone of quenched and tempered CrMo steels with internal hydrogen: Fracture micromechanisms

The determination of fracture behaviour of welds in presence of internal hydrogen is essential for the integrity assessment of CrMo vessels and pipes working under high hydrogen pressures. Homogeneous coarse-grain tempered bainitic/martensitic microstructures were obtained in 42CrMo4 and 2.25Cr1Mo steels by means of laboratory heat treatments in order to reproduce the coarse grain heat affected zone (CGHAZs) of real welds. Afterwards, the fracture toughness behaviour of the simulated CGHAZs was assessed under low displacement rate J-fracture tests, using pre-charged (in hydrogen gas) CT specimens. The hydrogen embrittlement (HE) experienced by the CGHAZs of both steels was considerably greater than in the base steels. The greater hardness, i.e. higher dislocation density, characteristic of the CGHAZ microstructures, explains the more strongly trapped hydrogen, and hence the sharper drop in the fracture toughness, also associated to an increase of intergranular fracture micromechanisms. Scanning electron microscopy examination of the fracture surfaces revealed the action of hydrogen-enhanced localized plasticity (HELP) mediated hydrogen-enhanced decohesion (HEDE) micromechanism, in which HELP took place first, providing enough hydrogen in the process zone to modify the local resistance, and eventually weakening the cohesion of the internal interfaces, HEDE.

Direct Observation of Austenite and Pearlite Formation in Thermally Simulated Coarse Grain Heat-Affected Zone of Pearlite Railway Steel

In the present study, coarse grain heat affected zone (CGHAZ) of the pearlite railway steel was thermally simulated at different cooling rates ranging from 1 to 4 °C/s to in–situ observe the austenite and pearlite formation by high temperature laser scanning confocal microscopy (HT–LSCM). During heating, austenite nucleation was started at 811 °C at fast heating rate of 9.1 °C/s. Pearlite was completely decomposed into austenite in 21 s at 1005 °C. At peak temperature of 1300 °C, austenite was composed of two microstructural compositions, i.e., high-carbon austenite and low-carbon austenite. Homogenization of austenite and formation of grain boundaries were continued even during cooling when the temperature was above the pearlite transformation. During cooling, Low-carbon austenite has increased the pearlite transformation by providing additional nucleation sites. By increasing the cooling rate from 1 to 4 °C/s, the pearlite transformation temperature was reducing, pearlite growth rate was increasing, and pearlite interlamellar spacing was becoming narrow, respectively. Mathematical interpretation of pearlite growth rate was developed that gives true changing behavior of pearlite growth rate with the varying cooling rate. Present study provides direct observation and unique quantitative information of austenite and pearlite formation in CGHAZ in pearlite railway steel.

On the Microstructural Strengthening and Toughening of Heat-Affected Zone in a Low-Carbon High-Strength Cu-Bearing Steel

In this article, the influence of simulated thermal cycles for the heat-affected zone (HAZ) on the microstructural evolution and mechanical properties in a low-carbon high-strength Cu-bearing steel was investigated by microstructural characterization and mechanical tests. The results showed that the microstructure of the coarse-grained heat-affected zone (CGHAZ) and the fine-grained heat-affected zone (FGHAZ) was mainly comprised of lath martensite, and a mixed microstructure consisting of intercritical ferrite, tempered martensite and retained austenite occurred in the intercritically heat-affected zone (ICHAZ) and the subcritically heat-affected zone (SCHAZ). Also, 8–11% retained austenite and more or less Cu precipitates were observed in the simulated HAZs except for CGHAZ. Charpy impact test indicated that the optimum toughness was obtained in FGHAZ, which was not only associated with grain refinement, but also correlated with deformation-induced transformation of the retained austenite, variant configuration as interleaved type and a relatively weak variant selection. The toughness of ICHAZ and SCHAZ exhibited a slight downtrend due to the presence of Cu precipitates. The CGHAZ has the lowest toughness in the simulated HAZs, which was attributed to grain coarsening and heavy variant selection. In addition, the contribution of Cu precipitates to yield strength in simulated HAZs was estimated based on Russell–Brown model. It demonstrated an inverse variation trend to toughness.

CO2 corrosion behavior of simulated heat-affected zones for X80 pipeline steel

The CO2 corrosion behavior of the simulated heat-affected zones (HAZs) of X80 pipeline steel in 3.5 wt% NaCl aqueous solutions was surveyed using electrochemical measurements and welding thermal simulation technology combined with microstructural analysis. The results revealed that changes in the corrosion resistance of the HAZ sub-regions were associated with variations in the microstructural sensitivity to the CO2 corrosion behavior in the NaCl aqueous solutions. The weakening of the corrosion resistance in the coarse-grained HAZ was attributed to the larger grain size and coarsening martensite/austenite constituents in the matrix. The corrosion products on the metal surface can lead to a minor reduction in the corrosion rate. In addition, increasing H+ concentration and higher residual stress can destroy the dynamic balance on the metal surface, thereby eventually damaging the corrosion product film.

Continuous cooling transformation behaviour and toughness of heat-affected zones in an X80 line pipe steel

A continuous cooling transformation (CCT) diagram was developed using thermal simulation techniques to replicate the coarse-grain heat-affected zone (CGHAZ) of an X80 line pipe steel. Specimens were heated to a peak temperature of 1350 °C with a 1 s hold time, followed by cooling at several rates between 7.6 and 356 °C/s. The thermo-mechanical specimen design was compatible with sub-size instrumented Charpy impact testing, which allowed the toughness to be evaluated. Following dilatometry measurements during continuous cooling, specimens were prepared and examined using optical and scanning-electron microscopy. In addition, the initial CGHAZ microstructure (upper bainite) was reheated to a peak temperature of 850 °C for 1 s (double cycled) and cooled at three different cooling rates (10, 5 and 2 °C/s). This allowed the influence of intercritical reheating, which occurs in multi-pass welds to be investigated in terms of microstructure, tensile strength and notch toughness. It was observed that the microstructure consisted of effective grain (matrix) and untempered MA when cooling at 10 or 5 °C/s. However, when reheating was followed by a cooling rate of 2 °C/s, the microstructure consisted of a combination of effective grain and a more benign distribution of tempered MA, which provided better low temperature impact toughness.

FEM-thermodynamic simulation methodology to predict the influence of t8/5 on the coarse grain heat-affected zone of a Cr-Mo low-alloy steel pipe

Abstract This work aimed to develop a microstructure-predicting methodology (MPM) using coupled physical-FEM-thermodynamic simulation to predict the influence of cooling time from 800 to 500 °C (t8/5 – 15, 30, 80, and 210 s) on the microstructure and mechanical behavior of the coarse grain heat-affected zone (CGHAZ) of a Cr-Mo low alloy steel welded joint. The MPM was experimentally validated. The MPM certainly predicted that the increase of t8/5 caused a remarkable change in CGHAZ microstructure (martensite and bainite to ferrite with aligned and non-aligned martensite-austenite-carbide M-A-C) and also decreases the Vickers microhardness. The MPM and experimental data show that the use of welding parameters that increases t8/5 (reducing the cooling rate) can jeopardize the welded joint behavior. Besides, the MPM proved to be a suitable tool for estimating the microstructure and microhardness of the CGHAZ.

Evaluation of Microstructure, Microhardness, and X-Ray Elastic Constants for Simulated Heat-Affected Zones in Q1N Steel

Reliable assessment of residual stresses in critical structures using x-ray diffraction techniques requires determination of an accurate x-ray elastic constant (XEC) to accurately calculate residual stress state from measured crystallographic elastic strain. Experimental derivation of the XEC is the most reliable approach. This study examines whether experimental XECs derived for quenched and tempered Q1N steel are still accurate to quantify stress in the heat-affected zones (HAZ) of welded joints. A GleebleTM thermal simulator is used to generate coarse-grained, fine-grained and intercritical HAZ microstructure in Q&T Q1N steel. Thermal simulation conditions (grip separation, thermal cycles and cooling rate) were optimized to produce the desired coarse-grained HAZ microstructure at a peak temperature of 1300 °C in large Q1N steel samples. While thermal conditions produced HAZ microstructure and microhardness values that were characteristic of real weld HAZs, there is no measurable effect on bulk elastic properties or the XECs. The bulk elastic modulus has an average value of 204.0 ± 1.3 GPa for the base metal and the selected peak temperatures used for the three different HAZ regions. The XECs have an average value of 176.6 ± 3.6 GPa for the {211} crystallographic planes of bcc Fe for all samples. Results instil confidence in use of base metal XECs for residual stress analysis within actual or real weld HAZ of Q&T Q1N steel on submarine pressure hulls.

SPT analysis of hydrogen embrittlement in CrMoV welds

In this paper Small Punch Tests (SPT) are used to determine the mechanical properties of the heat affected zone (HAZ) of CrMoV welded joints and to evaluate the embrittlement caused by the presence of internal hydrogen. Hydrogen permeation tests were carried out to determine the effective diffusion coefficients of the different regions of welded joints, whose hydrogen contents were measured using a hydrogen analyser.The paper demonstrates that SPTs can be used to estimate the tensile properties and the toughness of small areas, such as the subzones of a welded joint which are not thick enough for machining standard specimens. In cases where it is not possible to determine the fracture toughness of brittle regions using standard SPT samples, tests using longitudinally notched SPT samples were conducted. The normalized energy at failure, W/t2, was selected as the most appropriate SPT parameter to estimate the fracture toughness regardless of the fracture behaviour of the material (ductile or brittle). It was seen that the fracture toughness of the fine grained HAZ (FG-HAZ) is similar to that of the Base Metal (BM), while the fracture toughness of the coarse grained HAZ (CG-HAZ) is more similar to that of the Weld Metal (WM). The effect of hydrogen on BM and FG-HAZ is low, with embrittlement indexes of about 20%. However, a strong effect of hydrogen was observed in CG-HAZs and WMs, with embrittlement indexes of 60–80%. and a clear change from ductile to brittle behaviour in their fracture surfaces.

In Situ Observation and Growth Kinetics of Bainite Laths in the Coarse-Grained Heat-Affected Zone of 2.25Cr-1Mo Heat-Resistant Steel During Simulated Welding

Austenite → bainite transformation has been documented in situ by confocal scanning laser microscope, and bainite growth kinetics has been quantified in the coarse-grained heat-affected zone of 2.25Cr-1Mo steel. Four typical nucleation sites of bainite laths have been identified. In addition, growth rate of nine distinctive bainite laths varies from 9 to 200 μm/s, depending on the degree of supercooling and nucleation sites of bainite laths.

The significant impact of introducing nanosize precipitates and decreased effective grain size on retention of high toughness of simulated heat affected zone (HAZ)

Abstract The study of toughening mechanism associated with the heat-affected zone (HAZ) continues to be of significant interest for improving the low temperature (−40 °C) toughness. In this study, the effect of different heat input on microstructure, hardness and low temperature impact toughness of coarse-grained HAZ (CGHAZ) in a high-strength low-alloy (HSLA) plate steel was explored by using welding thermal simulations. After different welding thermal cycles with heat input of 60, 90, 120, and 180 kJ cm−1, average effective grain size of 5.7, 5.9, 6.2, and 6.7 ± 0.2 μm, were respectively obtained. Although the effective grain size marginally increased with the increase of heat input, the values were remarkably smaller than the prior austenite grain size of 54, 93, 104, and 118 μm. Compared to the as-processed HSLA steel, impact toughness of steel welded with a heat input of 60 kJ cm−1 decreased dramatically from 318 ± 20 to 31 ± 8J·cm−2. But interestingly, impact toughness was increased to a peak value of 325 ± 20 J cm−2 when the heat input was increased to 180 kJ cm−1. Toughness was enhanced by grain refinement (D = 6.7 ± 0.2 μm) and through the nano-size precipitates (d = 10 nm), which were induced by medium heat input of 180 kJ cm−1. The values of precipitation strengthening were 203 MPa and 90 MPa while the critical stress values of crack propagation were 32 GPa and 10 GPa for the steels welded at 180 and 270 kJ cm−1, respectively. A critical analysis of fracture toughness and yield strength in terms of theoretical predictions is presented.

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.