Granular Bainite Research Articles

Microstructure and mechanical properties of heat affected zone of laser-MAG hybrid welded low carbon bainitic steel joints

In the study, laser-MAG hybrid welding of low carbon bainitic steel was conducted. The weld formation, microstructure and mechanical properties were investigated. The thermal simulation technology was used to investigate the microstructure evolution of the coarse grain heat affected zone (CGHAZ) under different t8/5 conditions. Meanwhile, the simulated heat affected zone continuous cooling transformation (SHCCT) diagram of low carbon bainitic steel was developed and the optimized heat input range (6–18 kJ/cm) was obtained. The microstructure in the CGHAZ were lath bainites (LB) with a little granular bainites (GB), and M-A constituents. The average tensile strength of the joint was 714 MPa, reached 92.5% of the base metal. The impact energy of the HAZ was 33.8 J, which was 66.0% of the base metal. The reduction of impact toughness in the HAZ attributed to easier propagation of crack leaded by the M-A constituents and parallel slats within LB.

Effect of Welding Heat Input on Microstructure and Impact Toughness in the Simulated CGHAZ of Low Carbon Mo-V-Ti-N-B Steel

Welding thermal cycles with heat inputs ranging from 25 to 75 kJ/cm were performed on a Gleeble 3500. The impact energy improved significantly (from 10 to 112 J), whereas the simulated coarse-grain heat-affected zone (CGHAZ) microstructure changed from lath bainite ferrite (LBF) and granular bainite ferrite (GBF) + martensite/austenite (M/A) to acicular ferrite (AF) + polygonal ferrite (PF) + M/A as the heat input increased. Simultaneously, the mean coarse precipitate sizes and the degree of V(C,N) enrichment on the precipitate surface increased, which provided favorable conditions for intragranular ferrite nucleation. The Ar3 of CGHAZ increased from 593 °C to 793 °C with increasing heat inputs; the longer high-temperature residence time inhibited the bainite transformation and promoted the ferrite transformation. As a result, acicular ferrite increased and bainite decreased in the CGHAZ. The CGHAZ microstructure was refined for the acicular ferrite segmentation of the prior austenite, and the microstructure mean equivalent diameter (MED) in the CGHAZ decreased from 7.6 µm to 4.2 µm; the densities of grain boundaries higher than 15° increased from 20.3% to 45.5% and significantly increased the impact toughness. The correlation of heat input, microstructure, and impact toughness was investigated in detail. These results may provide new ideas for the development of high welding heat input multiphase steels.

Effects of Fillet Weld Size and Sleeve Material Strength on the Residual Stress Distribution and Structural Safety While Implementing the New Sleeve Repair Process.

The process optimization and structural safety improvement of the in-service repair welding of the X80 pipeline are very important. In this paper, the temperature, microstructure, and stress distribution were analyzed using the combination of TMM (thermal-metallurgical-mechanical) simulations and the corresponding verification experiments. The effects of the sleeve material strength and the fillet weld size were discussed. The results showed that the fillet weld zone was mainly composed of ferrite and bainite when the material of the sleeve pipe was Q345B. Furthermore, the sleeve pipe’s HAZ (heat affected zone) was dominated by lath martensite, lath bainite, and granular bainite. Moreover, granular bainite and a small amount of ferrite were found in the HAZ of the X80 pipe. It was found that, as the fillet weld size increased, the welding residual stress distribution became more uniform. The hoop stress at weld toe reduced from ~860 MPa of case A to ~680 MPa of case E, and the axial stress at weld toe reduced from ~440 MPa of case A to ~380 MPa of case E. From the viewpoint of welding residual stress, fillet weld size was suggested to be larger than 1.4T. The stress concentration and the stress distribution showed a correlation with the cracking behavior. Weld re-solidification ripples on the weld surface and weld ripples between welding passes or near the weld toe could cause stress concentration and the corresponding crack initiation. Furthermore, when the material of the sleeve pipe changed from Q345B to X80, the high-level tensile stress zone was found to be enlarged. The hoop stress at weld toe increased from ~750 to ~800 MPa, and the axial stress at weld toe increased from ~500 to ~600 MPa. After implementing the new sleeve repair welding process where X80 replaces the material of sleeve pipe, the cracking risk in sleeve pipe will improve. From the perspective of the welding residual stress, it was concluded that the fillet weld size reduction and the sleeve material strength improvement are harmful to in-service welded structures’ safety and integrity.

Effect of Ferritic Morphology on Yield Strength of CGHAZ in a Low Carbon Mo-V-N-Ti-B Steel

For investigating the impact of ferritic morphology on yield strength (YS) of the high-heat-input welding induced coarse-grained heat-affected zone (CGHAZ) of a low carbon Mo-V-N-Ti-B steel, a group of particular welding heat inputs were designed to obtain different ferritic microstructures in CGHAZ. The tensile properties were estimated from typical samples with ferritic microstructures. The mixed microstructures dominated by the intragranular polygonal ferrite (IGPF), the intragranular acicular ferrite (IGAF), and the granular bainite (GB) were obtained at the heat inputs of 35, 65, 85 and 120 kJ/cm, respectively. When the main microstructure changed from IGPF to IGAF and GB, YS increased first and then decreased. The microstructure consisting mainly of IGAF possessed the maximum YS. As the main microstructure changed from IGPF to IGAF and GB, the contribution of grain refinement strengthening to YS was estimated to be elevated remarkably. This means the strength of CGHAZ in a low-carbon steel subjected to the high-heat-input welding could be enhanced by promoting the fine-grained AF and GB formation.

Effect of Powder Feeding Rate on the Forming Quality of Alloy by Laser Melting Deposition

12CrNi2 alloy steel was prepared by Laser Melting Deposition (LMD) technology, and the effect of powder feeding rate on surface quality, internal defects, microstructure, and microhardness of the single track and manufactured part were investigated. The results show that the metallurgical bonding of the single track deteriorates, the surface quality of the manufactured part is improved, the average microhardness of the manufactured part increases, and the number of pores first decreases and then increases with the increase of powder feeding rate. At the lower powder feeding rate, the manufactured parts have larger pore defects, while at the higher powder feeding rate, the manufactured parts have poor fusion defects. The main phase composition of the manufactured parts is ferrite(F), granular bainite (GB), and pearlite(P), and the manufactured part has finer grains at the higher powder feeding rate.

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.

Notch tensile properties of Magnetically Impelled Arc Butt (MIAB) welded T11 steel tubes

The Magnetically Impelled Arc Butt (MIAB) welding process uses a rotating arc as its heat source and is known as an efficient method for tube welding. Interaction of external magnetic field and induced magnetic flux results in the creation of Lorentz force along peripheral edges of tubes. The circumferential arc rotation helps in the uniform heating of faying surfaces. After sufficient melting, faying surfaces are pressed against each other for weld formation. This paper aims at studying the notch tensile properties of T11 tubes welded by MIAB welding. In MIAB welded T11 steel tubes various Thermo-Mechanically Affected Zones (TMAZ) are formed as an effect of heat and pressure during welding. Tensile properties of all these TMAZs are different due to varying microstructures. Therefore notched tension testing was carried out to compare the properties of various zones in welded T11 tubes. All TMAZs show higher notch ductility with Notch Strength Ratio (NSR) greater than one. In comparison to the base metal, all TMAZs except TMAZ III display no loss in notch strength. The presence of granular bainite in TMAZ III causes lower notch strength with a Normalized Notch strength (NTSN) ratio less than unity.

Effect of Ti content on the inclusions, microstructure and fracture mechanism of X100 pipeline steel laser-MAG hybrid welds

Laser-MAG hybrid welding has a wide range of application prospects for pipeline steel welding, because it combines the advantages of arc welding and laser welding. However, the brittle microstructure of bainite or martensite in hybrid welding results in low impact toughness, and thus it cannot be used in pipeline laying. Hence, in this study, we induced acicular ferrite (AF) into laser-MAG hybrid welds, because of its good impact toughness. We prepared X100 steel welds with different Ti content using laser-MAG hybrid welding. Then, the microstructure evolution, impact toughness and fracture mechanism of the welds with different Ti content were investigated. The results illustrated that with increasing Ti content, the outer layer inclusion composition evolved from mostly Al2O3 to Ti2O3 and finally to TiC. Thus, Ti2O3 induced AF nucleation by forming an Mn-depleted zone, which caused the volume fraction of AF to initially increase and then decrease. The impact toughness also changed accordingly (16.3 → 21.9→8.7J) and the cracks nucleated and propagated in granular bainite during impact fracture. When they encountered AF, the cracks deflected and even stopped propagating. Therefore, by controlling the Ti content in the laser-MAG hybrid weld, the impact energy at −40 °C was higher than in the base metal. In addition, the higher quantity of AF with a high effective grain boundary density (0.57/μm) and low kernel average misorientation value could effectively hinder crack propagation and consequently improve impact toughness.

Effect of cooling rate on the microstructure and mechanical properties of a high copper high nickel low carbon steel

A new composition steel was designed by adding 1wt.% Cu and 2wt.% Ni elements on the basis of low-carbon Ti-bearing microalloyed steel. The continuous cooling transition curve (CCT) is obtained by the phase transition curves, microstructure and hardness in the Gleeble3800 thermal simulation test machine. In order to further study the law of transformation and meet the needs of industrialized production, different cooling tests were carried out through industrialized trial production, and the corresponding organization and mechanical properties were analyzed. The results show that when the cooling rate increases, the microstructure transformation appears as granular bainite (GB) → granular bainite + lath bainite (GB+LB) → lath bainite (LB) → lath martensite (M). The mechanical properties increase with the increase of cooling rate, and the yield strength increases in a positive correlation trend. The results show that the steel can obtain good and stable comprehensive mechanical properties in a large cooling rate range without heat treatment. The best mechanical properties with a good combination of yield strength and impact performance are obtained under ultra-fast cooling conditions.

Study on nonuniform microstructure and strength-toughness mechanism of thick-walled hot induction seamless bend

This paper investigated the continuous transformation cooling curve of thick-walled seamless pipe steel, microstructure nonuniformity of thick-walled hot induction seamless bend along wall thickness direction caused by hot induction bending, and their influences on the mechanical properties. The differences in microstructure, dislocation, precipitate characteristic, local misorientation angle, and grain boundary distribution were investigated acocording to scanning electron microscopy, optical microscopy, transmission electron microscopy, x-ray diffraction, and Electron back-scattering diffraction. The tensile tests and low-temperature impact tests were used to determine the strength and impact toughness of thick-walled hot induction seamless bend. The results demonstrate that thick-walled seamless pipe steel can obtain bainite structure in a wide range of cooling rate, which is beneficial to the adjustment of bending process. The nonuniform microstructure is mainly caused by the difference in heating temperature and cooling rate. The structure in the center of wall thickness is dominated by granular bainite, showing the excellent combination of strength and toughness. Among all the samples, dislocation strengthening and fine-grain strengthening are the two most important strengthening mechanisms. However, for the outer surface samples, precipitation strengthening mechanism also plays an significant role, which is attributed to the high heating temperature that causes a large amount of carbonitrides to dissolve and a large amount of precipitation during the subsequent tempering process. The high density dislocations and the large average local misorientation result in dislocation pile-up and strain concentration, which promotes crack initiation and reduces toughness. Large average misorientation and high density high-angle grain boundaries help to consume more energy in crack propagation and improve impact toughness.

Microstructure evolution during continuous cooling process in Ti microalloyed steel

The phase transformation law and microstructure evolution of austenite in Ti microalloyed steel under different continuous cooling rates (0.1-30°C/s) after a two-stage deformation were studied based on the Gleeble-3800 Thermo-Mechanical Simulator. The continuous cooling transformation (CCT) curve was obtained by thermal expansion method combining with microstructures. The results showed that the Ac1 and Ac3 of the steel were 835°C and 902°C, respectively. With the increase of cooling rate, the austenite phase transition temperature decreased, from 778.5-712.2°C at 0.1°C/s to 633.7-488.7°C/s at 30°C/s. Accordingly, the austenite transformation structure changed from polygonal ferrite and pearlite to granular bainite. In addition, with the increase of cooling rate, the ferrite transformation was inhibited, and the transformation region of granular bainite was increased.

Crystallographic insights into the role of nickel on hardenability of wear-resistant steels

Two wear-resistant steel plates with different Ni content were studied to reveal the significant role of Ni on hardenability from the perspective of crystallography. From the surface to the quarter, and then to the center of the plates, the 0.44Ni steel exhibited higher hardness compared with the 0.02Ni steel, indicating that the 0.44Ni steel was of higher hardenability. The microstructure at the center changed from granular bainite to lath bainite with increased Ni content, together with an enhanced density of block boundaries. Ni improves the hardenability by weakening the variant selection through inducing larger transformation driving force and more self-accommodation of transformation strain to obtain a high density of block boundaries.

Wire‐Arc Additive Manufacturing Using Ni1Cu Weathering Steel

AbstractA part consisting of Ni1Cu weathering steel is prepared by wire‐arc additive manufacturing (WAAM). Micro‐ and crystalline structures of this sample are analyzed spectro‐ and microscopically. Mechanical properties of the Ni1Cu steel (including microhardness, tensile strength, and impact resistance) are also assessed. Ni1Cu weathering steel possesses excellent forming quality and almost no defects when used for WAAM. The main features of the steel microstructure are blocks and needles, consisting of ferrite and granular bainite. The microhardness values in the transverse and longitudinal directions are uniform and equal to 169 and 177 HV0.2, respectively. Ni1Cu steel also demonstrates excellent mechanical properties with the transverse yield strength, tensile strength, elongation, and impact value of Charpy impact equal to 379 MPa, 517 MPa, 35.5%, and 147 J at −20 °C, respectively. The longitudinal direction values are equal to 376 MPa, 510 MPa, 31.5%, and 130 J at −20 °C, respectively. The Ni1Cu steel corrosion potential and current are equal to −719 mV and 5.8 × 10−3 mA cm−1, respectively.

Controlled Rolling of X80 Pipeline Steel in the Austenite Recrystallization Temperature Region and Its Effect on the Microstructure and Mechanical properties

In recent years, the improvement in the strength and toughness of X80 pipeline steel by only controlled rolling in the nonrecrystallization temperature region has been limited, it is difficult to meet higher application demands. Therefore, to further improve the effect of controlled rolling on the strength and toughness, the controlled rolling of X80 pipeline steel in the austenite recrystallization temperature region and its effect on the microstructure and mechanical properties after final rolling in the nonrecrystalization temperature region are investigated by thermal simulation experiments and hot‐rolling experiments. An increase in the last‐pass rough‐rolling deformation in the austenite recrystallization temperature region refines the austenite grain size after rough rolling and finish rolling, making the austenite grain size smaller and more uniform and improving the strength and low‐temperature impact toughness. The mechanism of austenite evolution based on the last‐pass deformation in the austenite recrystallization temperature region is revealed. The finer and more uniform austenite grain size accelerates the refinement of the granular bainite microstructure at room temperature. The main strengthening mechanisms are dislocation strengthening and grain refinement strengthening. The increase in toughness is due to the presence of a high percentage of high‐angle grain boundaries, which significantly impede crack propagation.

Influence of Post-Weld Heat Treatment on Microstructure and Toughness Properties of 13MnNiMoR High Strength Low Alloy Steel Weld Joint.

Weld and base metals require hot or cold working during the steel equipment manufacturing process. As a result, the components should be subjected to a normalizing heat treatment in order to recover their mechanical properties. In this study, the submerged-arc welding of the high strength low alloy (HSLA) thick steel plate(13MnNiMoR) is adapted for the vessel head under the normalizing and tempering heat treatment. The findings showed that the material toughness decreases after heating to simulate a vessel head forming process. The stamping process is carried out under the conditions of 980 °C for one hour, normalizing at 920 °C for 1 h and tempering between 600–660 °C for 2 h, respectively. The martensite-austenite (M-A) constituent is distributed in granular bainite and the boundary of austenite in island constituent. Therefore, it was deemed to be the most detrimental to Charpy-V impact toughness. Between normalizing and tempering, intercritical normalizing at 740 °C was added. As a result of the ferrite with fine particles M-A constituent, the toughness increases significantly.

Studying the influence of the interpass temperature on the heat-affected zone of an API 5L X65 steel welded pipe joint through computational and physical simulations

Welding costs can be reduced by increasing the interpass temperature (IT). However, this can adversely impact the mechanical properties of the heat-affected zone (HAZ) of the welded joint; therefore, it is necessary to evaluate the influence of higher ITs on the HAZ. The influence of the IT on coarse-grained HAZ (CGHAZ) and intercritically reheated coarse-grained HAZ (ICCGHAZ) of an API 5 L X65 steel pipe joint welded with gas metal arc was investigated. A microstructure-predicting method (MPM) was built from numerical and thermodynamic simulations to examine the CGHAZ and ICCGHAZ. The results were validated using optical and scanning electron microscopy as well as dilatometry. Vickers microhardness and Charpy V-notch (CVN) impact tests were performed on physically simulated specimens. For all ITs considered herein, granular and lath bainite were observed in the CGHAZ; and bainite, ferrite, and Martensite–Austenite–Carbides (M-A-C) were observed in the ICCGHAZ. Increasing the IT did not significantly change the hardness of the CGHAZ and ICCGHAZ, but it decreased the CVN impact energy of these zones owing to presence of granular bainite and massive M–A. Contrary to that of the ICCGHAZ, the impact energy of the CGHAZ was greater than the DNVGL-ST-F101 standard specified limit (45 J). Based on the MPM and experimental results, it can be concluded that increasing the IT leads to embrittlement of the HAZ.

Production of Niobium Carbide Layers on High‐Strength Bainitic Steels by Thermochemical Treatment of Thermoreactive Diffusion Followed by Austempering

The development of third‐generation advanced high‐strength steels aims for the production of low‐alloy steels with a high strength‐to‐weight ratio through innovative heat treatments. Thermochemical treatments that eliminate reheating steps are an excellent alternative for the surface hardening of this type of alloy. Herein, the incorporation of a thermoreactive diffusion (TRD) thermochemical treatment along with austempering treatment to obtain carbide‐coated high‐strength bainitic steels is evaluated. For this purpose, samples of bainitic steels containing ≈1.5 wt% silicon are immersed in an austenitizing bath composed of ferro‐niobium and sodium borate for 2, 4, and 6 h at 1100 °C. Subsequently, they are held isothermally in a 360 °C salt bath for 1 h. After treatment, the samples are characterized by optical microscopy, scanning electron microscopy, X‐ray diffraction, dispersive energy spectroscopy, Knoop microhardness, and microadhesive wear tests. Microstructural analysis shows the formation of niobium carbide layers at the surface of a substrate composed of granular and plate bainite, characteristic of austempered high‐silicon steels. These layers show thickness in the range of 4−9 μm, hardness from 2333 to 2599 HK, presenting hardness up to 6 times higher than the substrates, and wear resistance up to 8 times greater.

Effects of different cooling rates on the microstructure, crystallographic features, and hydrogen induced cracking of API X80 pipeline steel

Hydrogen-Induced Cracking (HIC) is a primary failure mechanism of pipeline-welded joints in the absence of external loading in the oil and gas exploration industries. Three different cooling rates after austenitization were used to simulate in the laboratory different regions of the heat-affected zone (HAZ) formed when welding an API X80 pipeline steel specially designed to enhance the HIC resistance. The samples were characterized with regard to microstructure and crystallography as well as HIC resistance. The HIC resistance test used NACE TM0284-2011 methodology. The microstructure and its homogeneity varied as a function of cooling rates. Samples containing inclusions and segregation zone from the segregation bands of specimens showed reduced HIC resistance, while specimens containing only acicular ferrite and granular bainite coupled with the absence of segregation zone showed significant improvement in HIC resistance. The best HIC resistance results came from samples presenting fine acicular ferrite consisting of fine interlocking plates, with divergent crystallographic orientations, preventing the formation of localized strain distribution inside the grain and at grain boundaries. It was also found that a large proportion of medium-angle boundaries prevent microcrack initiation and the transgranular mode of crack propagation.

Effects of multiple welding thermal cycles on hydrogen permeation parameters of X80 steel

Hydrogen permeation tests were conducted in 1.2 MPa H2 on welding thermal simulated specimens to determine the hydrogen permeation parameters in each subzone of the girth welded joints. Optical microscope, electron back-scattered diffraction, and X-ray diffraction were applied to analyze the corresponding microstructure changes. The results show that the hydrogen diffusion coefficient D is mainly determined by the final heat-treated peak temperature, and the key microstructural affecting factors of hydrogen permeation will change with this temperature. The order of D from high to low is acicular ferrite, granular bainite, lath bainite, polygonal ferrite, and M-A constituent.

Application of Artificial Neural Network to the Prediction of Tensile Properties in High-Strength Low-Carbon Bainitic Steels

An artificial neural network (ANN) model was designed to predict the tensile properties in high-strength, low-carbon bainitic steels with a focus on the fraction of constituents such as PF (polygonal ferrite), AF (acicular ferrite), GB (granular bainite), and BF (bainitic ferrite). The input parameters of the model were the fraction of constituents, while the output parameters of the model were composed of the yield strength, yield-to-tensile ratio, and uniform elongation. The ANN model to predict the tensile properties exhibited a higher accuracy than the multi linear regression (MLR) model. According to the average index of the relative importance for the input parameters, the yield strength, yield-to-tensile ratio, and uniform elongation could be effectively improved by increasing the fraction of AF, bainitic microstructures (AF, GB, and BF), and PF, respectively, in terms of the work hardening and dislocation slip behavior depending on their microstructural characteristics such as grain size and dislocation density. The ANN model is expected to provide a clearer understanding of the complex relationships between constituent fraction and tensile properties in high-strength, low-carbon bainitic steels.