Weld Coarse Grained Heat Affected Zone Research Articles

Role of heat inputs on microstructure and mechanical properties in coarse-grained heat-affected zone of bainitic steel

This study aims to find out the effect of welding heat inputs on microstructure and mechanical properties relationship of naval high strength steel. The physical weld thermal cycles with four different heat inputs between 10 to 200 kJ/cm were experimented using the thermomechanical simulator Gleeble®3800. The weld coarse-grained heat-affected zone (CGHAZ) microstructure was characterized in detail by using optical microscopy, field emission scanning electron microscope (FESEM), and transmission scanning electron microscope(TEM) facilities and mechanical properties such as impact toughness, hardness and tensile strength were obtained using Charpy impact, microhardness, and tensile testing facilities. It was often noticed that the impact toughness decreased with increasing heat inputs attributed to the grain and bainite lath coarsening and the formation of new phases with different morphologies in CGHAZ regions. At lower heat input, the microstructure consisted of a fully martensite structure, while at higher heat inputs consisted of granular bainite and bainitic ferrite; however, in medium-range heat inputs, it consisted of bainite and martensite. The different phases were confirmed through TEM images. The mechanical properties were found to be decreased with an increase in heat input. It was due to the formation of coarse bainite lath, granular bainite, and hard M/A constituents. The weld structure properties were established in detail.

Effect of heat input on impact toughness in transition temperature region of weld CGHAZ of a HY 85 steel

Physical weld simulated coarse grain heat affected zone (CGHAZ) specimens of HY 85 steel with heat inputs 15, 22, 50kJ/cm were prepared by thermo mechanical simulator. Impact toughness was determined at test temperatures from room temperature to −196°C. Ductile to brittle transition temperature (DBTT) were determined by two methods (i) average impact toughness value of upper and lower shelf energy and (ii) fracture appearance transition temperature (FATT) from the SEM fractographs. Impact toughness and hardness values have been found to decrease with increase in heat input. This decrease is attributed to slower cooling rate, increase in prior austenite grain size and increase in width of bainitic ferrite lath. DBTT was determined for base metal and weld CGHAZ specimens based on upper and lower shelf energy criterion. FATT method based on SEM fractographs is applicable only for base metal but not for weld CGHAZ specimens because they were fully brittle in character. Best impact toughness and hardness for heat input 22kJ/cm observed are 66J and 322 VHN respectively.

Continuous cooling transformation behavior in the weld coarse grained heat affected zone and mechanical properties of Nb-microalloyed and HY85 steels

Continuous cooling transformation (CCT) diagrams were determined for weld simulated coarse grained heat affected zone (CGHAZ) of Nb-microalloyed and HY 85 steels using a Gleeble®3800 thermo-mechanical simulator. Samples were heated to a peak temperature of 1573K (1300°C) and cooled at different cooling rates from 0.5K/s to 120K/s. Microstructures, hardness, and impact toughness at 223K were determined corresponding to simulated samples of weld CGHAZ thermal cycles. Transformation temperatures AC3, AC1, Bs, Bf, Ms, and Mf were also determined using dilatometric analysis. At slow cooling rates pro-eutectoid ferrite and pearlite were observed in Nb-microalloyed steel and a mixture of bainitic ferrite and granular bainite was observed in HY 85 steel but no pro-eutectoid ferrite was observed. At moderate cooling rates, only bainite was observed in both the steels. At high cooling rates, lath martensite was observed for both the steels. A strong dependence of prior austenite grain size on mechanical properties has been found. A prior austenite grain size of approximately 30μm (80K/s) has shown impact toughness more than 100J and 198J for Nb-microalloyed steel and HY85 steel samples respectively. Based on microstructures, hardness, impact toughness, and transformation temperatures, CCT diagrams for the weld CGHAZ region were determined.

Effect of thermo-mechanical cycling on the microstructure and toughness in the weld CGHAZ of a novel high strength low carbon steel

We have studied the microstructural evolution in the simulated coarse grain heat affected zone (CGHAZ) of novel low carbon microalloyed steel with yield strength of 1000MPa using electron microscopy, while the crystallographic characteristics were studied by electron backscatter diffraction (EBSD). The deterioration in low temperature toughness after the simulated welding cycle was attributed to the formation of coarse blocky M–A constituent. However, the lower bainite microstructure of the matrix was beneficial for low temperature impact toughness because of similarity in the crystal structure of variants of lath martensite, which were present in high proportion at the high angle grain boundaries. A high fraction of small M–A constituent also influenced impact toughness.

Study on Microstructure and Mechanical Properties of Thick Low-Alloy Quench and Tempered Steel Welded Joint

Abstract Low-alloy quench and tempered (Q&T) steel plates having yield strength of a minimum of 670 MPa are used extensively for the fabrication of impellers, penstocks, excavators, dumpers, and raw material handling devices, where welding is an important and probably most economic and productive process of joining. Thick low-alloy Q&T steel plates having a relatively high carbon equivalent (∼0.6) are frequently susceptible to a crack-sensitive microstructure leading to cold cracking. Thus, the welding parameters need to be carefully selected for these grades of steel to obtain the desired microstructure and properties. The present study deals with the characterization of microstructure evolution and mechanical properties of thick (38 mm) welded low-alloy quench and tempered steel plates (A 517 Grade F by ASTM) with the shielded metal arc welding process using a low hydrogen covered electrode (E 11018 M). The microstructural characterization was done for top (P1), middle (P2), and bottom (P3) regions of the weldment by optical microscope and transmission electron microscope to understand the microstructural features. The micrographs of the weld and coarse-grain heat-affected zone (CGHAZ) of the three regions mainly show variation in lath martensite and different morphologies of ferrite. Interestingly, the weld metal and CGHAZ in the middle (P2) region show coarse grains compared to the other two regions where grain size is more or less the same and finer. Furthermore, among the three regions, the P3 region, in general, has comparatively higher hardness and toughness. The high cycle fatigue test under two load ratios (i.e., R = 0.1 and −1) conducted only on specimens extracted from the P3 region, because of its better properties, shows better fatigue performance of weld joint under R = 0.1.

Physical Simulation on Microstructure and Properties for Weld HAZ of X100 Pipeline Steel

In this paper, the simulation heat affected zone (HAZ) continuous cooling transformation (SH-CCT) diagram of X100 pipeline steel was drew by using Gleeble 3500-HS thermal/mechanics simulation test machine, and the microstructure and properties of the HAZ of single-pass/double-pass welding were simulated and studied. The results show that the hardening trend of X100 pipeline steel weld coarse grain HAZ (CGHAZ) is significant when using the small welding heat input. To prevent the hardening of weld CGHAZ and reduce the cold cracking sensitivity, moderate heat input welding specification (between 12 and 18 kJ/cm) and weld preheating process (between 100 and 150°C) should be adopted. When appearing obvious embrittlement, the microstructure of double-pass weld intercritically reheated coarse grain HAZ (ICCGHAZ) of X100 pipeline steel is mainly upper bainite, the precipitation and the chain-like distribution of quantity more martensite-austenite (M-A) constituents in austenite grain boundaries is one of the main reasons leading to toughness deterioration. In conclusion, optimization control measures on the microstructure and properties of X100 pipeline steel weld HAZ are presented by above comprehensive analysis.

Tensile Deformation Behavior and Phase Transformation in the Weld Coarse-Grained Heat-Affected Zone of Metastable High-Nitrogen Fe-18Cr-10Mn-N Stainless Steel

The tensile deformation behavior and phase transformation in the weld coarse-grained heat-affected zone (CGHAZ) of a metastable high-nitrogen austenitic stainless steel was explored through tensile tests, nanoindentation experiments, and transmission electron microscopy analysis. True stress–strain response during tensile test was found to be seriously affected by δ-ferrite fraction, which depends on peak temperature of the CGHAZs. The strain-induced martensitic transformation (SIMT) occurred in base steel, whereas the SIMT disappeared and deformation twinning occurred predominantly in the CGHAZs. The relationship among true stress–strain response, nanoindentation hardness, and deformed microstructures was carefully investigated and discussed in terms of changes of stacking fault energy.

Behavior of Cu precipitates during thermo-mechanical cycling in the weld CGHAZ of Cu-containing HSLA steel

The behavior of Cu precipitates in the weld coarse-grained heat-affected zone (CGHAZ) of 1.5 wt% Cucontaining high strength low-alloy (HSLA) steel subjected to a thermal/thermo-mechanical welding cycle and post-weld heat-treatment (PWHT) was evaluated. The welding cycles of CGHAZ and PWHT were simulated by the Gleeble system. The hardness was measured at each PWHT condition, and observation and analysis of the Cu particles were performed using HRTEM. The hardness decreased initially with an increasing PWHT time at 550 °C. However, the peak hardness resulting from precipitation of Cu particles occurred after about 60 min at 550 °C. In the thermo-mechanical cycle, the peak hardness appeared at an earlier PWHT time. These results correspond with the HRTEM observations in which Cu particles were observed at an earlier PWHT time and larger Cu particles were observed at the same given PWHT time after the thermo-mechanical cycle. Thus, this paper confirms that the kinetics of Cu precipitation can be accelerated by an intrinsic mechanical cycle during the welding cycle.

Effect of thermo-mechanical cycling on the microstructure and strength of lath martensite in the weld CGHAZ of HSLA steel

This work has investigated the effect of thermo-mechanical cycling on the microstructure and strength of lath martensite in the weld Coarse Grained Heat Affected Zone (CGHAZ). The weld CGHAZ was simulated by Gleeble simulator according to the thermal cycle calculated by Rosenthal equation. The results show clearly that deformation promotes the dislocation density during cooling and thus decreases the width of lath martensite in the weld CGHAZ. In addition, the fraction of particle increased in the deformed weld HAZ and this is because the precipitation kinetics is accelerated by deformation. Meanwhile, lath martensite strength in deformed CGHAZ was a higher than undeformed CGHAZ, which is due to the contribution of grain boundary strengthening and precipitation strengthening.

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt 8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt 8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.

Behavior of (Ti, Nb)(C, N) complex particle during thermomechanical cycling in the weld CGHAZ of a microalloyed steel

This study aims to investigate the effect of deformation (stress and strain) on the coarsening kinetics of a (Ti, Nb)(C, N) complex particle during isothermal and continuous thermomechanical cycles in the weld coarse-grained heat-affected zone (CGHAZ) of a Ti + Nb microalloyed steel. Results in isothermal coarsening tests showed that the coarsening rate was enhanced by deformation under both compressive and tensile stress: it was enhanced mainly owing to the increased diffusivity of solute atoms. Then the compressive stress was found to be more effective than tensile stress. Interestingly, despite the increase in the coarsening rate, the mean particle size in the weld CGHAZ, after continuous thermomechanical cycle, decreased more than it did in the simple thermal cycle. This was because of enhancement of the precipitation of fine particles by increased precipitation kinetics under deformation.

Effect of Cu and B addition on tempering behavior in the weld CGHAZ of high strength low alloy plate steel

To understand the effect of Cu and B on mechanical properties of the weld coarse-grained heat-affected zone (CGHAZ) of high strength low alloy plate steels, six alloys including different Cu and B content were prepared. The CGHAZ and tempered conditions were simulated by a Gleeble simulator and mechanical properties were measured by Vickers hardness test and charpy impact test. Toughness of as-welded specimens was deteriorated with comparing to base steels and decreased with increasing Cu + B content. Meanwhile, Vickers hardness showed an opposite tendency, which is due to formation of martensite in the weld CGHAZ. For as-tempered conditions, Vickers hardness at first decreased with increasing tempering temperature and then was reinforced at around 550 °C with an unexpected decrease of toughness, which was mainly due to the precipitation of Cu particles and carbides, such as (Ti, Nb)C and (Mo, Mn) 2C.

Toughness investigation on simulated weld HAZs of SQV-2A pressure vessel steel

In order to get detailed information about weld HAZs toughness of SQV-2A steel and determine the optimum welding and heat treatment parameters, the toughness of simulated CGHAZs (coarse grained heat affected zone) and CGHAZs (intercritically reheated CGHAZ) were systematically investigated. The influence of tempering thermal cycles on weld ICCGHAZs toughness was clarified. The effect of post weld heat treatments (PWHT) on weld CGHAZs toughness was also determined. The results showed that high toughness (absorbed energy >200 J) of weld HAZs could be achieved by selecting the optimum welding and PWHT parameters (cooling time Δ t 8/5: 6–40 s, PWHT: 893 K, 3.6–7.2 ks). Tempering thermal cycles with peak temperature of above 573 K could remarkably improve the toughness of deteriorated ICCGHAZs and reduce the hardness, when cooling time Δ t 8/5(2) of the reheating thermal cycle was 6 s, which implies that welding of SQV-2A without PWHT is possible, provided that low heat input welding is adopted and welding procedure is correctly arranged. Metallography and fractography revealed that M–A constituents in weld HAZs played an important role in controlling weld HAZ toughness.

Decomposition behaviour of M‐A constituent and recovery of toughness deterioration of HAZ toughness in 780 and 980 MPa class HSLA steels welded with high heat inputs and its improvement

Summary This paper describes an investigation of the decomposition of M‐A constituent by PWHT (post weld heat treatment) together with its effect on the toughness of the weld CGHAZ (coarse‐grained heat affected zone) of 780 and 980 MPa class HSLA steels. For all steels tested, the area fraction of the M‐A constituent decreases with an increasing PWHT temperature and time, but the toughness dependence of the PWHT parameters of the 980 MPa class steels is quite different from that of the 780 MPa class steels. The toughness of the weld CGHAZ of the 780 MPa class steels is increased as the M‐A constituent is decomposed into ferrite and cementite by the PWHT. The weld CGHAZ of the 980 MPa class steels, however, shows embrittlement at PWHT temperatures above 823 K, though the toughness increases with the PWHT time at temperature below 623 K. The foregoing results suggest that the toughness of HSLA steels impaired by high heat input welding can be significantly improved by PWHT at a temperature around 623 K.

８０及び１００キロ級高張力鋼大入熱ＨＡＺ部の靭性劣化とその改善に関する研究（第２報） 島状マルテンサイトの金属学的特性に関する検討

The internal microstructure, chemical composition, and hardness of M-A (Martensite-Austenite constituent) formed in weld CGHAZ (Coarse Grained Heat Affected Zone) of 780-980 MPa class HSLA steels have been investigated in order to reveal metallographic characteristics of the M-A constituent. The martensite was classified into lath and twin type. The massive M-A constituent had higher C contents, and included more twin type martensite and retained austenite than the elongated M-A. The cementite was found to be classified into a coarse rod type and a dendritic or fine needle type. The coarse rod type is considered to be precipitated directly from the austenite. The dendritic or fine needle type is probably precipitated by the self tempering of martensite.The C content of the M-A constituent was increased with Δt8/5 (cooling time from 1073 to 773 K) and reached 1.3-2.2% for Δt8/5=100-1000s.The hardness of the M-A constituent was increased with C content. The hardness of the massive M-A constituent (Hv-950) was generally higher than that of the elongated M-A constituent (Hv-700). The hardness and C content of the M-A constituent can be reduced significantly by a post weld heat treatment at temperatures from 623 to 773 K.

８０及び１００キロ級高張力鋼大入熱溶接ＨＡＺ部の靭性劣化とその改善に関する研究（第３報） 溶接後熱処理による島状マルテンサイトの分解挙動及び靭性の改善に関する検討

The decomposition of M-A constituent by PWHT (Post Weld Heat Treatment) and its effects on the toughness have been investigated for the weld CGHAZ (Coarse Grained Heat Affected Zone) of HSLA steels of 780-980 MPa classes.As the PWHT temperature and time were increased, the area of the M-A constituent was decreased for all the steels employed, but dependence of the toughness of the 980 MPa class steels on PWHT parameters was quite different from that of the 780 MPa class steels. For the weld CGHAZ of 780 MPa class steel, the toughness was increased, as the M-A constituent was decomposed into ferrite and cementite by the PWHT. For the weld CGHAZ of 980 MPa class steel, however, embrittlement occurred at PWHT temperatures above 823 K though the toughness was increased with PWHT time at temperature below 623 K.From these results, it can be concluded that the toughness of HSLA steels impaired by the high-heat-input welding can be improved significantly by PWHT at temperature about 623 K.