Granular Bainite Microstructure Research Articles

Effect of prior microstructure on carbide precipitation in a Cr-Mo-V pressure vessel steel at 650 °C

Cr-Mo-V steels are key materials used for constructing thick reactor pressure vessels (RPV) owing to an impressive combination of their mechanical properties and weldability. Conventionally, these steels are subjected to a quality heat treatment that comprises austenitization, quenching and tempering. Owing to its large thickness, an RPV experiences widely different cooling rates across its thickness during quenching, leading to considerable variation in the microstructure from the surface to the centre of the vessel. In this study, a Cr-Mo-V steel was austenitized at 950 °C for 1 h and cooled at two rates (water quenching and 10 °C/min), representative of the cooling rates at the surface and centre of a typical RPV, to study the effect of prior microstructure on the carbide precipitation during tempering of the steel at 650 °C. Electron microscopy of the as-cooled samples revealed the presence of martensitic microstructure in water quenched sample and predominantly granular bainitic microstructure in slow cooled sample. Synchrotron studies revealed the presence of retained austenite containing ∼0.8% carbon in the slow-cooled sample. Tempering of the steel at 650 °C for 200 h precipitated M7C3 and MC (VC) carbides in the water-quenched sample, while the slow cooled samples precipitated M23C6 as an additional constituent. This difference in the carbide precipitation in the two samples has been attributed to the excess carbon in the martensite/austenite (M/A) regions in slow cooled samples and explained on the basis of carbide precipitation sequence predicted using Thermo-kinetic simulations. This analysis has shown that M23C6 and M7C3 carbides in the slow cooled sample precipitate simultaneously as critical radii of their nucleation as well as their incubation time become comparable when the matrix contains more than 0.25% C. The present study has demonstrated that the microstructure prior to tempering may significantly affect the precipitation sequence of carbides during tempering.

Role of M-A constituents in bainitic microstructure on crack propagation behavior in the ICCGHAZ of HSLA steels for offshore applications

The objective of the study is to understand the effect of martensite-austenite (MA) constituents along with the neighbouring bainitic matrix and its effect on low-temperature impact toughness in the thermo-mechanically simulated inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of offshore High Strength Low Alloy (HSLA) steels. ICCGHAZ was simulated using a Gleeble 3500 thermomechanical simulator between the inter-critical temperature near Ac1 (lower ICT) and at a temperature 50 °C higher (upper ICT) for two different steels (steel A and steel B) with predominant microstructures composed of granular bainite (GB) and bainitic ferrite (BF), respectively. MA constituents were observed in the form of necklaces along prior austenitic grain boundaries (PAGB) of simulated ICCGHAZ for both steel A and steel B. Steel A combrised of discrete island MA constituents inside the GB grains and steel B consists of lath shaped MA constituents inside BF grains. Fractographical analysis showed that secondary cracks were initiated at the MA constituents along PAGB and propagated linearly into the GB/BF grains in the ICCGHAZ. The cracks initiated at the bulky MA at grain boundaries of steel A moved towards the pre-cracked or de-bonded discrete MA inside the grains and propagated through the interlinking of microcracks. In contrast, for steel B a notable percentage of cracks were initiated through the necklace-like MA at the grain boundaries and propagated through the BF lath. Pre-cracking of the slender MA present at the lath boundaries facilitated crack propagation in steel B. Furthermore, increasing the inter-critical temperature improved the impact toughness of steel A with a predominantly GB microstructure, whereas impact toughness improvement was limited in steel B with a BF microstructure.

Crystallography of transformation products with different cooling rates in low-carbon alloy steel and its effect on low-temperature toughness uniformity of heavy plates

The crystallography of transformation products with different cooling rates has been investigated in a typical low-carbon alloy steel, and its effect on low-temperature impact toughness uniformity in the thickness direction of heavy steel plates has been elucidated. The results show that both the full lath martenite (LM) microstructure transformed at the cooling rate of 45 K s−1 and granular bainite (GB) + lath bainite (LB) + LM microstructure transformed at the cooling rates of 3∼6 K s−1 have higher density of high-angle grain boundaries (HAGBs). The density of HAGBs in the full GB microstructure transformed at the cooling rate of 0.5 K s−1 is the lowest, and that in the LB + LM microstructure transformed at the cooling rates of 15∼22 K s−1 is the middle. In the GB + LB + LM microstructure, the division of austenite by multi-stage transformation reduces the LB and LM transformation temperatures, contributing to a weakening of variant selection and a tendency to close-packed plane grouping. Therefore, the GB + LB + LM microstructure achieves the refinement degree similar to the full LM microstructure, but slightly higher than that of the LB + LM microstructure. For the 60 mm thick heavy steel plate with ultra-fast cooling, the cooling rate decreases from 41 K s−1 at the subsurface to 14 K s−1 at the 1/4-thickness and then to 4 K s−1 at the 1/2-thickness. The full LM microstructure, LB + LM microstructure, and GB + LB + LM microstructure are obtained in sequence, resulting in a more uniform density of HAGBs. Consequently, the relatively uniform impact toughness at 143 K in the thickness direction is achieved with a small difference of ∼30 J.

Microstructural inhomogeneity and associated hetero mechanical behavior in advanced bainitic weathering steel weldments

A new filler metal was developed to join the new generation weathering steel Q690qNH. Microstructural evolutions and the associated mechanical behavior of weldments were comparatively studied in hot rolled and tempered thick Q690qNH plates with initial microstructure of lath bainite and granular bainite respectively. Phase transformations in the HAZ promote the work hardening capacity of the weld jointed tempered Q690qNH (Q690T) plate and enable the joints to exhibit a different mechanical response upon the tensile loading in comparison to the weld jointed hot rolled Q690qNH (Q690R) plate. Prior austenite reconstruction was carried out to understand the crack nucleation behavior in HAZ. The plastic incompatibility between martensitic-austenitic (M-A) and bainitic ferrite leads to the nucleation of plastic micro-voids from the prior austenite grain boundary or M-A boundary. The fracture of Q690R joint is caused by coalescing micro-voids in HAZ with a strength reduction of 10%–14% that caused by the welding process. The continuous yielding behavior regarding the retained austinite in HAZ results in a competition of the plastic deformation in each subzone of the Q690T joint, of which the catastrophic fracture is dominated by the initial microstructure comprising the ferrite and the granular bainite.

Correlation between welding structure and corrosion behavior of high strength low alloy (HSLA) ship steel in Cl--containing solutions

The high-strength low-alloy (HSLA) steel industry is in great demand due to the continued growth of the ship and marine sectors. The microstructure of HSLA steel is greatly influenced by both the heat input during welding and the variations in salinity resulting from changes in climate. This paper presents the development of HSLA ship steel using a vacuum electric arc furnace and heat treatment techniques. Electrochemical corrosion tests were conducted on the substrate, heat-affected zone, and fused zone of HSLA ship steel. The results showed that the heat treatment process can improve the microstructure homogeneity of the HSLA ship steel, eliminate chemical composition segregation, refine the grain, and promote the formation of the microstructure of pearlite and ferrite. The welded interface structure was uniform and free from defects or inclusions. The metallographic microstructure of the fused zone showed a massive distribution of grain boundary ferrite, martensite, and granular bainite, and that of the heat-affected zone displayed superheated characteristics consisting of pearlite and acicular ferrite. The interlocking microstructure of the base material enhanced performance in a 4.2 % NaCl solution. The corrosion resistance of the heat-affected zone and fused zones of HSLA ship steel deteriorated. Grain boundary ferrite may be related to the inferior corrosion resistance of the heat-affected zone, while the coarse microstructure of granular bainite led to the serious corrosion resistance in the fused zone of HSLA ship steel.

Influence of Martensite/Bainite Dual Phase-Content on the Mechanical Properties of EA4T High-Speed Axle Steel.

In this work, we have investigated the effect of martensite/bainite dual phase content on the mechanical properties of EA4T high-speed axle steel. For evaluation and control of the strength, ductility, and toughness of steel, the microstructure of lath martensite (LM) and granular bainite (GB) was clarified through an optical microscope (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Besides, the tensile fracture morphology was studied by scanning electron microscopy (SEM). For this purpose, this study conducted a quantitative analysis of the LM and GB fractions using the Pro Imaging software-2018 of OM. The remarkable effect of the LM/GB structure on mechanical properties is discussed. The results have shown that by increasing the volume fraction of the GB structure, the LM structure is refined and its microhardness and strength are improved. Meanwhile, the micro strength of LM follows the Hall-Petch relationship with the lath martensite packet size. Subsequently, the mechanical property prediction model of EA4T steel based on the LM/GB content was established by regression analysis of all experiment dates. When the LM fraction in the steel is about 40-70%, a superior combination of strength, ductility, and toughness can be obtained in EA4T steel.

Effect of Surface Hardness and Hydrogen Sulfide Partial Pressure on Sulfide Stress Cracking Behavior in Low Alloy Linepipe Steel

TMCP (thermo-mechanical controlled process) linepipes have long been used in severe sour environments, but recently sulfide stress cracking (SSC) caused by local hard zones has become a concern. In order to clarify the hardness threshold that leads to SSC, four-point bend (4PB) SSC tests as specified in NACE TM0316 were conducted under several H2S partial pressure conditions. At H2S partial pressures of 1 bar and higher, the surface hardness threshold (at 0.25 mm from the surface) observed in 4PB SSC specimens without SSC cracking was approximately correlated to a maximum acceptable hardness level of 250 HV0.1. A stable low surface hardness of 250 or less HV0.1 was achieved by suppressing hard lath bainite (LB) and obtaining a soft granular bainite (GB) microstructure, resulting in a superior SSC-resistant property. It was found that a SSC crack propagated when the surface hardness increased due to an increasing volume fraction of the LB microstructure. Under a 16 bar H2S partial pressure condition, the crack growth rate increased in the sour environment, and hydrogen embrittlement by H2S was promoted. However, in the 4PB SSC test at 16 bar, the stress concentration and the transition to a crack were suppressed because the shape of localized corrosion was semicircular due to low localized corrosivity. This can be the reason why the SSC susceptibility was similar to 1 bar condition, especially in the 4PB SSC test using the samples with lower surface hardness level of 250 or less HV0.1.

Hydrogen embrittlement of high-strength marine steel as a weld joint in artificial seawater under cathodic polarization

Hydrogen embrittlement (HE) behavior of E690 high-strength steel as a weld joint was investigated in artificial seawater. The most vulnerable location and microstructure to HE was verified, and the underlying mechanism was clarified. Results showed that the weld metal with granular bainitic microstructure leads to premature fracture of the entire weld joint under slow strain rate tensile testing with in-situ hydrogen charging. Dislocation slip bands (DSBs) could be preferentially developed in this zone, and HE initiation was induced via dislocation–hydrogen interaction because the weld metal had the lowest local strength among the weld joint. Martensite/austenite (M/A) constituents and carbides could also contribute to crack initiation and propagation through microvoid coalescence and decohesion.

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.

Effect of TMCP rolling schedules on the microstructure and performance of X70 steel

Four laboratory rolling schedules were devised and produced from a thick-wall API X70 steel plate. The microstructure and crystallographic texture were characterized followed by an evaluation of the performance of the rolled steels by investigating the mechanical properties and SCC susceptibility. High pH Stress Corrosion Cracking (SCC) is a damaging form of environmental corrosion that can cause rapid failure of buried gas transmission pipelines. It is a key question to ascertain if the manufacturing procedure has any bearing on the resistance of line pipe steels to SCC. Finish rolling above the recrystallization lower temperature, RLT (Tnr), resulted in the formation of large, equiaxed grains of upper bainite and granular bainite. Despite resulting in the highest Yield Strength (YS) and Ultimate Tensile Strength (UTS), the steel manufactured through this process had the highest SCC susceptibility. Finish rolling in the two-phase region increased the polygonal ferrite formation and deteriorated mechanical properties of the rolled steels. Finish rolling above the Ar3, combined with severe rough rolling, produced the lowest SCC susceptibility, with a primarily granular bainite microstructure and a high intensity of {332}<113> texture. Rolling strategies that balanced strength and ductility were shown to positively affect SCC performance.

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.

Preparation of TA15 powder reinforced 45CrNiMoY alloy steel with high mechanical property by pre-laid laser cladding technology

Based on the principle of in-situ precipitation strengthening, TA15 powder (Ti-6.5Al-1Mo-1V-1.5Zr) was used as an additive to improve the mechanical properties of 45CrNiMoY alloy steel by pre-laid laser cladding (PLLC) technology. Effects of the content of TA15 powders (0–1.5 wt%) on the microstructure, precipitation mechanism and mechanical property of PLLC 45CrNiMoY alloy steel samples were investigated. Results indicate that TA15 alloy has a significant effect on the microstructure of 45CrNiMoY alloy steel through metallurgical reaction in the laser molten pool. With the increase of TA15 powder content, the granular bainite (GB) microstructure were remarkably refined, the mount of the acicular ferrite (AF) was increased, and in-situ precipitates such as Ti-Y2O3-Zr (TYZ) composite oxide, TiC and ZrC carbide were produced. The 45CrNiMoY alloy steel sample with 1.0 wt% TA15 alloy addition has the best combination of tensile strength (1158 ± 9.7 MPa) and elongation (9.8 ± 0.4%), its microhardness was improved by 25.6% compared with the sample without TA15 powder. The TYZ composite oxides did not only serve as the AF nucleation sites but also spheroidize the detrimental manganese sulfide phase leading to a good elongation of the 45CrNiMoY alloy steel, the TiC and ZrC carbides played a key role in the improvement of the microhardness and tensile strength. The proper addition of TA15 powder can effectively improve the microstructure and mechanical properties of the PLLC 45CrNiMoY alloy steel.

Effect of vanadium on microstructure and mechanical properties of bainitic forging steel

This investigation attempts to thoroughly explore the effect of microalloying element V on the microstructural characteristics and mechanical properties of 20Mn2SiCrS bainitic forging steel. The results show that the cooling rate range of bainitic transformation was broadened with a 0.13% V addition and mainly a granular bainite microstructure was obtained after post-forging continuous cooling. The addition of V could refine the martensite/austenite (M/A) constituent and inhibit the formation of pro-eutectoid ferrite. Physical-chemical phase analysis revealed that only ~8.5% of the added V was in the V(C,N) precipitate in the as-forged condition, which resulted in relatively small precipitation strengthening (~40 MPa) compared with that in ferritic-pearlitic medium carbon forging steel. The addition of V can enhance the overall mechanical properties of the steel and it is concluded that the primary role of V in the as-forged bainitic forging steel is mainly to promote the formation of bainite and to refine the microstructure especially the M/A constituents as well as to resist softening of the M/A during the post-forging continuous cooling or during subsequent tempering when tempering treatment is necessary.

Study on the continuous cooling transformation behavior of heavy thickness X80 pipeline steel

In this paper, the continuous cooling transformation behavior of heavy thickness X80 pipeline steel after two steps deformation was studied by Gleeble1500 thermal simulator. The continuous cooling transformation (CCT) curve of the sample was set up according to the thermal dilation curve and microstructure. The results show that during the continuous cooling process, ferrite and degenerated pearlite in the X80 are obtained in the high temperature phase transformation region. The bainite in the X80 was obtained in the medium temperature transformation region, and when the cooling rate is less than 20°C/s, the micro structure is granular bainite. When the cooling rate is between 20∼30C/s, the mixed microstructure of granular bainite and lath bainite were obtained. According to the CCT curve, in the production, the start cooling temperature and cooling rate should be controlled properly to get expected microstructure so as to ensure the property requirements of the steel plate.

Effect of increased nitrogen content on continuous cooling transition of γ→α in hot-deformed low-C Mo–V–Ti steels

The two-stage controlled rolling and cooling at 0.5–50 °C/s of low-carbon Mo–V–Ti steels with the increasing nitrogen content from 0.0032 to 0.0081 and 0.0123 wt.% were simulated through a Gleeble 3500 system. The continuous cooling transition (CCT) of γ→α in each steel was estimated via microstructure characterization and CCT diagram. The results indicated that CCT diagram for each steel was divided into three regions of γ→ferrite, γ→pearlite and γ→bainite, and the increasing N content elevated all the starting temperatures for γ→α. Consequently, the polygonal ferrite (PF) and pearlite formed in each steel cooled at 1 °C/s and, however, the increasing N content led to slightly coarser ferrite grain and pearlite colony. With the increased cooling rate to 10 and 30 °C/s, a mixed microstructure of acicular ferrite (AF), granular bainite (GB) and lath bainite (LB) formed in 32N steel and in contrast, the mixture of PF+AF+GB in 81N and 123N steels. The increasing N content promoted (Ti,V)(C,N) precipitation, enhanced the intragranular PF/AF nucleation, increased martensite/austenite constituent and depressed LB. In addition, the mechanisms dominating the effect of increasing N on this CCT of γ→α were discussed.

Microstructure-based prediction of yield ratio and uniform elongation in high-strength bainitic steels using multiple linear regression analysis

Microstructures of high-strength bainitic steels were quantitatively analyzed using EBSD analysis based on transformation behavior and morphological characteristics. According to multiple linear regression analysis, reliable empirical equations to predict yield ratio and uniform elongation were proposed. From the results, the fraction of the granular bainite and polygonal ferrite microstructure were estimated to be the most influential factors on the yield ratio and uniform elongation of high-strength bainitic steels, respectively.

Variation in Morphology and Kinetics of Granular Bainite with Welding Thermal Cycles in High-Nb Fire-Resistant Steel: Experiments and Theoretical Calculations

The microstructure and kinetics of granular bainite (GB) were investigated in novel fire-resistant steel containing high niobium and subjected to simulated welding thermal cycles. The dilatometric data were analyzed to determine the activation energy and Avrami exponents. It was observed that austenite transformed to GB over a wide range cooling rates of 0.5-100 K/s. Theoretical calculations indicated that Avrami exponent (n) decreased with increased transformed volume fraction ( $$f_{b}$$ ) and cooling rate. At a moderate cooling rate of 20-6 K/s, bainitic ferrite as sheaves formed from austenite and the corresponding Avrami exponent was 2 3. The local activation energy for austenite transformation to bainitic ferrite decreased from ~ 198 to ~ 182 kJ/mol with increasing $$f_{b}$$ from 0.05 to 0.95, while the local activation energy for austenite transformation to martensite decreased from ~ 80 to ~ 71 kJ/mol with increasing $$f_{b}$$ from 0.05 to 0.95.

Tensile and impact behaviour of thermo mechanically treated and micro-alloyed medium carbon steel bar

In the present study, a comparison between thermo mechanically treated (TMT) and direct air cooled micro-alloyed hot rolled (MAHR) steel bar has been made in terms of microstructure, texture and mechanical properties. Tensile and Charpy impact tests were performed as a part of the mechanical investigation. Tensile strength and impact toughness values of TMT bar were higher than those of MAHR bar. It was also noticed that TMT bar exhibited better resistance to both ductile and brittle fracture at high and low temperatures compared to MAHR steel. This can be corroborated owing to the presence of fine tempered martensite at the periphery, a mixed microstructure of granular bainite, ferrite, and degenerated pearlite as an intermediate layer and a mixture of ferrite and pearlite at the core of the TMT bar. On the other hand, a coarse ferrite-pearlite phase mixture was present throughout the cross section for MAHR steel and sounded responsible for lower strength and impact toughness properties. Additionally, fractions of higher angle grain boundaries (HAGBs) in the core and smaller martensite packet around the rim section were also contributed beneficially to resist the cleavage crack propagation in TMT rebar. The higher intensity of gamma fiber of MAHR rebar has contributed lowest ductile-brittle transition temperature. Numerous cleavage cracks which were initiated from inclusions or carbide particles were also found responsible for crack propagation during both ductile and brittle types of fracture.

The relationship between low-temperature toughness and secondary crack in low-carbon bainitic weld metals

This paper aims to reveal the relationship among low-temperature toughness, microstructure and secondary crack from both microstructural and crystallographic perspectives. Instrumented Charpy tests were carried out in the upper platform, transition temperature range, as well as at lower temperatures. Quantitative fractographic analysis, microstructural characterization and secondary crack observation were conducted on half of the impact specimen. Crystallographic features were identified in the secondary crack proximity, e.g. variant selection, orientation relationship, cleavage plane of the corresponding crack matrix. Some results in literature are confirmed. Nickel addition is beneficial to hindering crack propagation as a result of microstructure improvement. The most effective barrier for crack propagation is high-angle misorientation boundaries between Bain packets and the boundaries of ferrite matrix with the microstructure of granular bainite. Additionally, high-angle misorientation boundaries between closed-packed plane packets arrest cracks in the mixed microstructure of fine lath bainite and intricate acicular ferrite. Most preferred crack propagation planes are {110} and {100} in our low carbon bainitic weld metals, with the decreasing possibility of {112} and {123}.

In-situ observation of microstructural evolution in reheated low carbon bainite weld metals with various Ni contents

Microstructural evolution in weld metals was in-situ observed through utilizing a laser scanning confocal microscope at two cooling rates. The specimens with various nickel contents were adopted for the observation. In the specimen with low fraction of Ni (≤2 wt. %), granular bainite microstructure (i. e. broad surface relief) transformation from intragranular nucleation site was in-situ observed, while, lath bainite microstructure originating from grain boundary of austenite was in-situ observed for specimens with high mass percentage of Ni (≥ 4 wt. %). With increasing nickel content, the transformation temperature dropped. The prior austenite grain size was initially depressed and subsequently coarsened dramatically with the addition of Ni. The microstructure difference was ascribed to various nucleation sites and growth direction in the matrix. On account of those observations, not only the chemical component, cooling rate and microstructure were systematically correlated, but also the microstructural evolution was definite.