Prior Austenite Grain Boundaries Research Articles

Effect of carbon segregation at prior austenite grain boundary on hydrogen-related crack propagation behavior in 3Mn-0.2C martensitic steels

The present study aimed at strengthening prior austenite grain boundary (PAGB) cohesive energy using carbon segregation and investigated the effect of carbon segregation at PAGB on the microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steels. At the low hydrogen content (below 0.2 wt. ppm), the fracture initiation toughness (JIC) and tearing modulus (TR), corresponding to crack growth resistance, were significantly improved by carbon segregation. In contrast, JIC and TR did not change by carbon segregation at the high hydrogen content (above 0.5 wt. ppm). Considering the non-linear relationship between the toughness properties and the PAGB cohesive energy, the experimentally evaluated toughness properties (JIC and TR) and the GB cohesive energy previously calculated by first-principles calculations were semi-quantitatively consistent even at the high hydrogen content. The microstructure observation confirmed that the plastic deformation associated with crack propagation, such as the local ductile fracture of uncracked ligaments and the formation of dislocation cell structures / nano-voids, played an important role in the non-linear relationship between the toughness properties and PAGB cohesive energy.

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.

Effect of prior austenite grain size on austenite reversion, bainite transformation and mechanical properties of Fe-2Mn-0.2C steel

To elucidate the response of austenite reversion and subsequent bainite transformation behavior to the prior austenite grain (PAG) size, the transformation kinetics and amount of reverted austenite in the intercritical region, the subsequent bainite transformation kinetics, and the tensile properties were investigated by dilatometers, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and the mechanical tests. The results show that, when annealed at 730 °C, fine-grained specimens exhibit faster reverted austenite transformation rate than coarse-grained specimens due to the presence of more PAG boundaries. As the intercritical temperature rises to 760 °C, more intragranular globular austenite forms in the coarse-grained specimen, which accelerates the reverted austenite transformation rate. Compared to fine-grained specimens, coarse-grained specimens exhibit faster bainite transformation and higher amount of bainitic ferrite. The microstructure after austempering consists of intercritical ferrite, bainitic ferrite, RA and fresh martensite. With increasing the austempering time, more bainitic ferrite is formed and the amount of RA and fresh martensite decreases. With increasing the austempering time, the increase in the amount of bainitic ferrite leads to an increase in the yield strength, while the decrease in the amount of RA leads to a decrease in the elongation. Moreover, compared with coarse-grained specimens, the finer microstructure in fine-grained specimens improves yield and tensile strengths, while higher amount of RA contributes to larger uniform and total elongations.

Revealing the mechanism of crack initiation and propagation in CGHAZ of S690QL welded joints through low-temperature impact tests with gradient setting of pendulum angle

As the most brittle areas of welded S690QL high-strength steel structures, coarse-grained heat-affected zone (CGHAZ) was crucial for the structural integrity of components. This paper designed an innovative low-temperature impact testing method with a gradient setting of pendulum angle to investigate the micro-mechanism of crack initiation and propagation in the CGHAZ of S690QL welded joints. The results reveal that continuous blocky M−A constituents were distributed along prior austenite grain boundaries (PAGBs), leading to stress concentration and crack initiation. During low-temperature impact processes, cracks propagate in a transgranular mode along {100} or {110} crystal planes. Due to the interaction between compressive stress and the crack tip, regions with high kernel average misorientation (KAM) such as PAGBs can divert the crack. The coarsening of prior austenite grains in the CGHAZ leads to a diminished hindering effect of PAGBs on crack propagation. The findings elucidate the complex relationship between microstructure and impact toughness of CGHAZ, providing insights into crack behavior and mechanisms in CGHAZ.

Coarsening behavior of M23C6 and Laves phase precipitates during long-term aging in a 10% Cr steel with low nitrogen and high boron contents

The coarsening evolution of M23C6 and Laves phase particles in the 10 % Cr steel with 0.008 % B and 0.003 % N during long-term thermal aging at 650 °C for 40,000 h was examined. Particles located on the prior austenite grain boundaries (PAGBs) were compared with those located on the lath/block boundaries. A 5 times slower coarsening of M23C6 on the lath/block boundaries than on PAGBs was attributed to a smaller interfacial energy. After 10,000 h aging, Laves phase particles tended to disappear on PAGBs, whereas larger particles grew on lath boundaries, resulting in a 2 times higher coarsening rate than on PAGBs.

Local crack arrestability and deformation microstructure evolution of hydrogen-related fracture in martensitic steel

Intergranular cracks in the uncharged specimen were arrested at low-angle prior austenite grain boundary (PAGB) segments of several micrometers. In contrast, even small, low-angle PAGB segments with sub-micrometer sizes impeded the propagation of hydrogen-related intergranular crack. At the hydrogen-related quasi-cleavage crack tip, the crystallographic orientation changed abruptly, and deformation microstructures developed, including the formation of low-energy dislocation structures. A certain degree of crack growth resistance (intrinsic crack growth resistance) in the hydrogen-related fractures could be attributed to the intense localized plastic works involved in the arrest of intergranular cracks and the propagation of quasi-cleavage cracks.

Effect of a Gradient Temperature Rolling Process on the Microstructure and Mechanical Properties of the Center of Ultra-Heavy Plates

As there is a small amount of deformation in the center during the rolling process of ultra-heavy plates, it is extremely easy to cause poor mechanical properties in the center. Increasing the deformation in the center is the most feasible method to eliminate the deformation effects in the cross-section of ultra-heavy plates. In this study, the gradient temperature rolling (GTR) process is compared with the traditional uniform temperature rolling (UTR) process. It is found that the GTR process can significantly increase the deformation in the center and thereby refine the grains.. The room temperature tensile test and instrumented Charpy impact test are used to test the strength at room temperature and impact energy at low temperature. Combined with the obtained impact load/energy displacement curve, the deformation and damage process under impact load are analyzed. The microstructure morphology and impact fracture obtained by different rolling processes in the center are analyzed by experimental methods such as OM, SEM, EBSD, etc. The prior austenite grain (PAG) boundary morphology is analyzed and the densities of grain boundaries are statistically quantified. The results showed that the strength, plasticity, and low-temperature toughness of the GTR process are improved compared to the UTR process, with increased dislocation density in the center microstructure, the density of PAG boundaries, and the density of packet boundaries. The size of the PAG in the center is refined by ~49%, the density of PAG boundaries increased by ~140%, the density of high-angle packet boundaries increased by ~39%, and the density of low-angle packet boundaries increased by ~49%. The crack propagation in the instrumented Charpy impact test of the GTR process showed stable expansion, indicating a ductile fracture compared to the semi-brittle fracture of the UTR process. The densities of PAG boundaries and high-angle packet boundaries are the most important factors affecting the strength and low-temperature toughness.

Prior austenite grain measurement: A direct comparison of EBSD reconstruction, thermal etching and chemical etching

It is well known that the prior austenite grain (PAG) microstructure of steels has a significant impact on their microstructure evolution and mechanical properties. Many PAG measuring techniques have been developed over many decades, with the effectiveness of each varying alloy-to-alloy. Some of the more common techniques, specifically for bainitic and martensitic grades, are thermal etching and picric acid etching, which both involve the preferential etching of PAG boundaries in order to reveal the austenitic microstructure. More recently, parent grain reconstruction techniques using EBSD (electron backscatter diffraction) mapping have shown good promise in recreating PAG microstructures from BCC-FCC orientation relationships, and can now be deployed at speed with the advent of rapid detectors (1000s of pixels per second). This study aims to compare the accuracy and relative advantages/disadvantages of picric acid etching, thermal etching and EBSD reconstruction methods. A TESCAN and NewTec In-Situ Testing (TANIST) capability was used to directly observe and measure the true high-temperature PAG structure during the austenitisation of SA-540 B24 low alloy steel. These high-temperature PAG measurements were then compared to measurements from the same area obtained using thermal etching, picric acid etching and EBSD parent grain reconstructions after quenching to room temperature. Reconstructing the parent austenite grains from EBSD data resulted in the closest measurement of PAG size. PAG boundaries were delineated well by thermal etching but surface effects (such as surface relief and ghost traces) created complexities when identifying the exact position of boundaries. Picric acid etching, which produced the least accurate measurement, was found to reveal PAG boundaries well, however it was limited by its ability to sufficiently etch annealing twin boundaries and its susceptibility to microsegregational effects. EBSD reconstruction and thermal etching were more consistent at reconstructing/revealing these boundaries, although inaccuracies with the techniques were still observed.

Effect of microstructure on hydrogen embrittlement and hydrogen-induced cracking behaviour of a high-strength pipeline steel weldment

Hydrogen embrittlement (HE) and hydrogen-induced cracking (HIC) behaviour of a X65 steel pipeline weldment were investigated using slow strain rate tensile (SSRT) testing of specimens that were specifically extracted from different zones of the weldment (i.e., weld metal (WM), heat-affected zone (HAZ), and base metal (BM)). The WM was found to be the most susceptible zone to HE and HIC, while BM the least. Analysis of microstructure, fracture surface, secondary crack formation, and mechanical behaviour revealed that the high HE susceptibility of WM is correlated to microstructural features including Ti-rich inclusions, martensite/austenite (M/A) constituents, and prior austenite grain boundaries (PAGBs).

Continuous Heating/Cooling Transformation Kinetics of a Novel CrNiMoV Steel for Large Structural Parts

Continuous heating transformation (CHT) and continuous cooling transformation (CCT) characteristics as well as phase transformation kinetics of a novel Cr–Ni–Mo–V steel for large structural parts are studied using a Gleeble‐3500 thermal–mechanical simulator. CCT curves are obtained. The results show that carbides in initial structure can be completely dissolved during CHT as the heating rates are not greater than 0.2 °C s−1. The final microstructure exhibits obvious sensitivity to cooling rate. It contains martensite with or without bainite. Besides, kinetic models of austenitization are established, and it is confirmed that diffusion is the major mechanism of austenitic transformation in steel during CHT. Meanwhile, bainitic phase transformations kinetic model is developed based on the J–M–A model. The fitted Avrami indexes n combined with transmission electron microscope reveal that the bainite nucleation sites of the steel are mainly located at the prior austenite grain boundaries, and the growth of bainite grains mainly follows a 2D mode. The kinetics of martensitic transformation is better modeled based on the K–M equation. A deep understanding of kinetics and microstructure characteristics during CHT and CCT is of great significance to regulate the final performance of the novel steel under development.

Initiation of high-temperature fatigue cracks at the most tip of fractured grain boundaries

Two 2.2Cr heat resistant steel samples that had been fatigue tested respectively at 550 °C and 650 °C are analyzed on the atomic scale to reveal the different crack initiation mechanisms. Cracks formed along prior austenite grain boundaries (PAGBs) are selected and manufactured into foil specimens with the aid of focused ion beam (FIB). Hence the interior crack progressing routes are illustrated and the atomic structure particularly at the most tip of cracks are characterized by using the high resolution TEM. It is revealed that the cracks mainly progress within a spinel phase (Cr2MnO4) while not its interface with neighbouring martensitic matrix. Moreover, geometric phase analysis (GPA) and lattice misfit at the most tip of cracks further manifest a elastic dominated and a plastic dominated mechanism corresponding to the two specimens, respectively. Cracking of the 650 °C specimen is mainly attributes to the accumulation of plastic strain that caused by the viscoelastic transformation of PAGBs, which causes intriguing periodic grain boundary sliding (GBS) waves ahead of the crack tip.

Mesoscale quantification of grain boundary character and local plasticity in hydrogen-assisted intergranular and intergranular-like cracking paths in tempered martensitic steel

The effects of the misorientation of prior austenite grain boundary (PAGB) segments on the local plasticity evolution in intergranular (IG) and IG-like fractures were investigated using a tempered martensitic steel. When a diffusible hydrogen content of 6.3 mass ppm was introduced, IG fractures with smooth fracture surfaces occurred in the elastic regime of the tensile test. According to the grain orientation spread (GOS) distribution, local plasticity did not contribute to the mesoscopic (block size) scale, irrespective of the PAGB segment misorientation. With a relatively low hydrogen content (4.0 mass ppm), an IG-like feature appeared on the fracture surface, which included plasticity-related traces such as tear ridges or nanovoids. The GOS values near the IG-like cracks were higher than those near the IG cracks and exhibited significant misorientation dependence. Specifically, at the low-angle PAGB segments, the GOS values of the grains neighboring the IG-like cracks increased with a decreasing misorientation. On the other hand, most of the cracked high-angle PAGB segments exhibited nearly constant GOS values. Remarkably, a Σ3 PAGB segment exhibited an abnormally high GOS value. These facts and corresponding microstructural observation results indicate that the low angle and Σ3 PAGB segments allow crack-tip blunting before crack growth.

Dual grain size-effects on hydrogen-assisted fatigue crack growth in 1 GPa-class medium-carbon martensitic steel

The influence of prior austenite grain (PAG) size ranging from 6 to 90 μm on the fatigue crack growth (FCG) of a tempered 0.41%C lath martensitic steel was investigated in a 90 MPa H2 gas environment at ambient temperature. Under the presence of H, severe FCG accelerations accompanying intergranular (IG) cracking along PAG boundaries as well as quasi-cleavage (QC) fracture along martensite block boundaries and {011} crystallographic planes were systematically found. Smaller PAG mitigated the acceleration at a relatively high-stress intensity via suppressing IG cracking. However, PAG refinement instead escalated the acceleration at a low-stress intensity factor where QC facture prevailed. This inverse grain size effect was discussed in terms of the plasticity criterion in triggering QC and its possible dependence on the microstructural length scale.

Influence of boron segregation on the hot ductility of T23 steel CGHAZ

Boron segregates in a non-equilibrium segregation mode at lath, packet, block boundaries as well as prior austenite grain boundaries (PAGBs) in 2.25Cr1.6WVNb steel coarse grain heat-affected zone (CGHAZ) during the thermal cycle, and tends to segregate only at PAGBs in the same mode during the high-temperature tensile test. Boron segregation along PAGB increases GB plasticity but not inter-granular strength. This is because the boron segregation at PAGBs occupies the vacancies and other defect positions, which inhibits the nucleation and growth of micro-voids, and reduces plasticity consumption. As a result, the hot ductility of CGHAZ is enhanced.

Extraction of reversible hydrogen trapped on prior austenite grain boundaries and promoting intergranular fracture in the elastic region of tempered martensitic steel by utilizing frozen-in hydrogen distribution at -196 °C

Factors that promote hydrogen-related intergranular (IG) fracture in the elastic region of tempered martensitic steel have been identified utilizing frozen-in hydrogen distribution and tensile tests in liquid nitrogen at -196 °C. Tensile test results at room temperature (R.T.) after precharging with 12.1 mass ppm of hydrogen showed hydrogen embrittlement associated with IG fracture. Although tensile test results at -196 °C after precharging with the same amount of hydrogen did not show hydrogen embrittlement with IG fracture, those at -196 °C after preloading with elastic stress just before fracture strength at R.T. showed hydrogen embrittlement with IG fracture. As preloading stresses increased and preloading speeds decreased, subsequent hydrogen embrittlement susceptibility at -196 °C increased. In addition, although tensile testing at -196 °C after hydrogen precharging, preloading at R.T., and unloading at -196 °C resulted in IG fracture, testing conducted after unloading at R.T. did not. It is noted that reversible hydrogen that accumulates on prior austenite grain boundaries (PAGBs) in the presence of load and desorbs in the absence of load in the elastic region at R.T. is directly responsible for IG fracture. These findings revealed that hydrogen-related IG fracture in the elastic region of tempered martensitic steel was not caused by stress-free hydrogen distribution, but was promoted by reversibly accumulated hydrogen mainly due to stress-induced diffusion onto PAGBs during stress loading at R.T.

High temperature deformation behaviour and microstructure evolution in Cu containing High Strength Low Alloy (HSLA) steel

ABSTRACT The hot deformation behaviour of a Cu containing high strength low alloy (HSLA) steel has been evaluated by carrying out hot compression tests over a temperature range of 900 °C to 1100 °C and constant true strain rates of 0.0003 to 1 s−1. Although single peak flow curve is observed predominantly, a tendency to exhibit multiple peaks at low strain rates (0.0003 and/or 0.001 s−1) at temperature greater than 950 °C. The important parameters pertaining to dynamic recrystallisation (DRX), such as critical, peak, steady state stress and strain were ascertained, and the DRX state diagram depicting the variation of critical parameters governing DRX with Zener-Hollomon parameter Z was established. The volume fraction of recrystallised grains, computed from flow stress, as expected, follows a sigmoidal trend indicating higher kinetics at low strain rates for a given temperature. Reconstruction of prior austenite grain boundaries (PAGB) based on electron back scattered diffraction (EBSD) technique for a few select specimens clearly elucidated the effect of temperature and strain rate on the DRX behaviour of the steel. The DRX state diagram established here clearly demarcates the deformation space i.e. temperature-strain rate-stain space into regimes where the material exhibits full DRX, hardening/softening + DRX and only hardening (no DRX) behaviour. This would be useful for designing suitable forging sequence to achieve the desired microstructure and mechanical properties.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Different augmentations of absorbed hydrogen under elastic straining in high-pressure gaseous hydrogen environment by as-quenched and as-tempered martensitic steels: combined experimental and simulation study

Hydrogen embrittlement (HE) behavior of as-quenched and as-tempered martensitic steels under elastic straining in a high-pressure gaseous hydrogen environment was studied for the first time. The difference in the total content of defects acting as H trap sites has the highest contribution to the increase in absorbed hydrogen content under the elastic loading. The hydrogen-induced fracture occurred in the as-quenched specimen during elastic straining in hydrogen environment, while the tempered specimen did not fracture under the elastic loading. Higher accumulation of the dislocations near the main crack initiation sites (prior austenite grain boundaries (PAGBs)) in the as-quenched specimen will be the main source of the hydrogen to the potential flaw to crack to initiate. H accumulated near PAGBs enhances dislocation slip along {011} planes and helps transgranular crack propagation. Due to less possibility to provide the critical local amount of hydrogen at PAGBs for the crack to initiate, the as-tempered specimen remained unfractured.

Austenite grain boundary segregation and precipitation of boron in low-C steels and their role on the heterogeneous nucleation of ferrite

The addition of boron (B) to steels suppresses the austenite to ferrite phase transformation dramatically, thus increasing their hardenability. It achieves this through grain boundary (GB) segregation at the austenitic GBs that delays the ferrite nucleation. Though the effects of B segregation on hardenability have long been known, the mechanisms of B segregation and how exactly B suppresses the ferrite nucleation remain elusive. We designed a B-containing low-C steel to study the B segregation and precipitation behavior. We conducted heat treatments with different austenitization temperatures and cooling rates. Site-specific atom probe tomography, complemented by high-resolution secondary ion mass spectrometry reveals B segregation at prior austenite grain boundaries (PAGBs) along with carbo-boride precipitation. The B segregation mechanisms are discussed in detail based on these observations considering their dependence on austenitization temperature and cooling rate. Furthermore, we analyzed the impact of GB energy reduction through nucleation kinetics calculations. From our analysis, we conclude that the reduction in GB energy affects the grain corner and grain edge nucleation substantially. However, the retarding effects of carbo-boride precipitation on ferrite nucleation cannot be completely excluded. We provide a detailed account of the possibilities of how B-containing precipitates may be suppressing ferrite nucleation.

Crystallographic Analysis on the Upper Bainite Formation at the Austenite Grain Boundary in Fe-0.6C-0.8Mn-1.8Si Steel in the Initial Stage of Transformation

A crystallographic analysis was conducted of the upper bainite nucleated at the austenite grain boundary in Fe-0.6C-0.8Mn-1.8Si (in mass %) steel by the EBSD analysis. The effect of the character of the prior austenite grain boundary (PAGB) on the formation of upper bainite was investigated from several perspectives: PAGB plane, grain boundary energy, and so on. BFs form on both sides of the high-angle PAGBs, while BFs do not form at twin boundaries. It is suggested one of the reasons for the suppression of BF formation at twin boundaries is the lower grain boundary energy. At high-angle grain boundaries, there is no difference in the potency for BFs’ nucleation between the tilt-like PAGBs and twist-like PAGBs, and the formation of BF is not affected by the angle between the rotation axis, the PAGB plane, and grain boundary energy. The variant selection of BFs was investigated. The BFs pair, whose misorientation across the PAGB is small, is formed preferentially. When several variant pairs can form having small misorientation across the PAGB, the variant pair that can reduce the elastic strain energy preferentially forms to accommodate the shape strain.