High Strength Bainitic Steel Research Articles

Influencing TRIP threshold and variant pairing through minor cold and cryo-rolling in bainitic steel

An attempt has been made to compare the effects of small reduction cold and cryo-rolling on the microstructural evolution and stability of retained austenite in a 3rd generation advanced high-strength bainitic steel. The steel was heat treated and isothermally held at 300 °C for 2 h above Ms temperature to obtain bainitic microstructure. The heat-treated steel was subsequently subjected to cold-rolling and cryo-rolling (10 % reduction in thickness). The substructure of bainite consists of packets, blocks, and sub-blocks. Both the crystallite size and micro-strain of bainitic ferrite shows a decreasing trend after rolling compared to the undeformed condition. However, cryo-rolling induces higher micro-strain in retained austenite, which enhances its mechanical stability requiring higher stress to trigger the transformation induced plasticity (TRIP) effect, thus contributing to increased overall strength. Crystallographic variant analysis showed an increase in the boundary density and frequency of specific V1-V3(V5) (high angle block boundaries) and V1-V4 (sub-block boundaries) variant pairs after cryo-rolling following the K-S orientation relationship. While the heat-treated, undeformed state exhibited all the variants without preference for any particular variant pair. The results suggest that the mechanical properties of the steel after minor cryo-rolling are significantly influenced by crystallographic variant pairing and microstrain. Additionally, minor cryo-rolling proved to be superior to cold rolling in bainitic steel as it increased the threshold strain required for the TRIP effect during rolling.

Insights into the microstructural evolution and strengthening mechanisms of friction stir spot welded advanced high strength ultrafine bainitic steel

Unlocking the full potential of advanced high-strength bainitic steels in spot-welding is a pivotal endeavor for harnessing their extraordinary mechanical properties in the automotive industry. This study suggests a shift in paradigm to address the inferior fusion weldability of this type of steel because of the formation of coarse martensite within the welded zone of weldments obtained by liquid-state welding, through the strategy of refining the martensite within the weld zone using the solid-state friction stir spot welding (FSSW) process. This paper delves into an in-depth investigation of the microstructural evolution and load-bearing capacity of a Fe-0.25C-3Cr-1.63Mn-1.5Si (wt%) ultrafine bainitic steel, having a remarkable tensile strength of 1.7 GPa, during the intriguing process of FSSW at rotational speeds of 600 to 2000 rpm. The research uncovers that the stir zone (SZ) undergoes dynamic recrystallization, resulting in a significantly refined microstructure. Regardless of the rotational speed employed, the high hardenability of the steel and rapid cooling during the process inevitably results in martensite formation within this region. However, a comprehensive microstructural characterization conducted by the electron backscatter diffraction (EBSD) analysis proves that the different thermal cycles induced by different rotational speeds lead into various prior austenite grain sizes, martensitic/bainitic packets and blocks attributes, and density of high-angle grain boundaries (HAGBs). Through the evaluation of strengthening mechanisms in different weld zones, the study sheds light on the key factors influencing strength. Block boundaries emerge as the primary contributor to SZ hardening, surpassing the role of geometrically necessary dislocations (GNDs) and solid solution strengthening. The load-bearing capacity observed during the tensile-shear test is governed by a delicate interplay between bonding width and crystallographic features (block size and HAGBs density) of the martensite formed within the SZ. The highest peak load was achieved under optimal welding conditions (rotational speed of 1000 rpm), which enabled the production of a sufficiently large bonding width, providing ample bonding area for load-bearing, along with an abundance of blocks and HAGBs to effectively impeding crack propagation.

Effect of hot rolling mode on crys-tallographic texture and mechanical properties of directly quenched bainitic steel

The paper considers the effect of hot rolling mode on the structure, mechanical properties and crystallographic texture, as determined by EBSD, for a directly quenched thick plate of high strength bainitic steel. According to the obtained results, stronger strain hardening of austenite before the phase transformation leads to appearance of granular bainite and hence diminishes the final steel strength.

Geometry of dimples and its correlation with mechanical properties in high-strength bainitic steel

ABSTRACT The experimental measurements of 2 D micro/nanolayered dimples' dimensions (both geometry and pattern) on the published tensile fractographs of a series of austempered high-strength low-carbon bainitic steel are carried out and interpreted with the corresponding initial microstructural features and tensile mechanical responses as a function of austempering temperature.

Analysis on the Key Parameters to Predict Flow Stress during Ausforming in a High-Carbon Bainitic Steel

Since flow stress is an important parameter in the processing and application of metallic materials, it is necessary to trace the flow stress during austenite deformation. Thermal compression deformation of austenite in a high-strength bainitic steel was conducted using a Gleeble-3500 thermo-mechanical simulator, within the deformation temperature range of 400 °C~900 °C. By analyzing the stress–strain curves and strain-hardening exponent, the effects of strain hardening and dynamic recovery on the dislocation density of the material during the thermal processing were considered in the present work. Based on the general form of the Kocks–Mecking–Estrin (KME) model, the effects of deformation temperature and strain on the key parameters of the model were clarified. Differing from other work which commonly terms m (strain rate sensitivity exponent) and k2 (dimensionless parameters for dynamic recovery) as constants, the current models consider the quantitative relationship between key parameters and deformation temperature and strain. The results show that m is an exponential function related to temperature and strain, which decreases with the increase in strain. Meanwhile, k2 is a temperature-dependent polynomial function that increases as the deformation temperature increases. Finally, a modified constitutive KME model was proposed to predict the austenitic plastic stress with strain. Using established m-ε and k2-T models, the predicted curves are in good agreement with the experimental measurements.

Ultrahigh impact toughness achieved in high strength bainitic ferrite/retained austenite lamellar steels below-M temperature

• Bainite transformation occurs during austempering process below- M f temperature; • Bainite transformation below- M f temperature refines bainitic ferrite laths; • Bainite transformation below- M f temperature reduces the size of blocky M-RA; • The impact toughness of high strength bainitic steel is improved significantly. A novel heat treatment process below- M f for medium carbon steel, with the relationship between the microstructure and mechanical properties was investigated. This study demonstrated that a bainitic ferrite/retained austenite lamellar steel with a tensile strength of 1436 MPa and a Charpy V-notch impact energy of more than 130 J was obtained by an isothermal heat treatment below- M f . This result is considerably higher than that of above- M s (21.3 J at 400 °C) and below- M s (46.0 J at 300 °C). The isothermal heat treatment process below- M f refines the bainitic ferrite laths, reduces the size and amount of blocky martensite/retained austenite, and makes the bainitic ferrite and retained austenite lamellae more uniform. The results demonstrate that the lamellar structure of bainitic ferrite laths and ductile carbon-rich retained austenite significantly improve the impact toughness of bainitic steels.

Achievement of excellent strength and plasticity in the nugget zone of friction stir welded bainitic steel and its deformation behavior

Compared to nonferrous alloy, the microstructure in the nugget zone (NZ) of steel is easy to coarsen during friction stir welding (FSW) because of higher melting point of steel, leading to lower strength and plasticity of the joint. Here, high-strength bainitic steel was subjected to FSW in water + solid CO2 medium, and mechanical properties and deformation behavior of the NZ were studied. The excellent yield strength (617 MPa) being 111% of the basal metal (BM) was obtained in the NZ owing to fine grain strengthening by fine effective grains of 4.3 μm and dislocation strengthening by the high density of dislocations of 6.7 × 1014 m−2. A perfect tensile strength of 664 MPa and uniform elongation of 13.1% were achieved in the NZ depending on the highly sustained work hardening rate, which were up to 112% and 113% of that of the BM. This can be partly explained by the fact that fine effective grains can obviously reduce the mean free path of dislocations. Moreover, dynamic recrystallization of ferrite with low density dislocations provided sufficient space for the multiplication of dislocations during deformation. Furthermore, refined effective grain and dual-phase microstructure (ferrite/bainite) promoted ductile fracture, and the total elongation of sample reached 108% of that of the BM.

Chro-austempering treatment of a medium-carbon high-strength bainitic steel

Chro-austempering treatment of a medium-carbon high-strength bainitic steel

Statement of Retraction

This article refers to:RETRACTED ARTICLE: Effect of heating on the microstructure and mechanical properties of hot rolled high strength bainitic steel rail

Microstructural investigation and mechanical performance of laser cladding repaired bainite steel with AerMet100 steel

In this paper, bainitic railways were repaired designedly by laser cladding repairing method with AerMet100 steel powder as raw material. The microstructural evolution of repaired specimens was studied carefully, and the corresponding mechanical properties and wear resistance were compared to verify the feasibility of this method. Results show that the bainite microstructures formed in the AerMet100 repaired region exhibit a high microhardness of 515 HV due to high-alloyed solution strengthening and microstructure refinement. The thermal cycles during the repairing process lead to the microstructures in HAZ changing with the distance from the fusion line, and the bottom of HAZ consisting of tempered bainite becomes the microhardness valley. Tensile tests indicate that the overall mechanical properties of repaired specimens are comparable to the bainitic substrate. Compared to the bainitic matrix, wear tests present that the wear resistance of the AerMet100 repaired region is better with the mass loss decreasing by about 28%. The current experimental results suggest that laser cladding repairing with AerMet100 steels is suitable for high-performance repairing of high-strength bainitic steels.

Effect of heating on the microstructure and mechanical properties of hot rolled high strength bainitic steel rail

ABSTRACT In this paper, the microstructural evolution and mechanical properties of the hot-rolled U25CrNi high-strength bainitic steel rail under different tempering conditions were investigated. The results indicate that both the hot-rolled and tempered microstructures of the tested steel consist of bainitic, martensite and retained austenite. When the tempering temperature rises to 400°C, relatively large volume fraction of residual austenite as well as a smaller amount of carbide precipitation in the tested steel have been detected. When the tempering time is extended to 360 min at the same temperature, the precipitation of carbide particles in the steel sample increases. When the tempering temperature is 500°C, the bainitic ferrite in the tested steel is obviously coarsened, accompanied by the precipitation of a large amount of carbide. In a word, the optimum tempering process for this hot-rolled U25CrNi high-strength bainitic rail is 400°C × 200 min.

Effect of Hydrogen on Microstructure and Mechanical Behavior of High-Strength Bainitic Steel in Marine Application

Effect of Hydrogen on Microstructure and Mechanical Behavior of High-Strength Bainitic Steel in Marine Application

Viewpoints on Technological Aspects of Advanced High-Strength Bainitic Steels

The development of advanced high-strength bainitic steels has been preceded and linked to different metallurgical advances, both in the field of fundamental materials science and in technological fields closer to the production and final application. The diversity and abundance of documents in literature has favored the co-existence of extensive terminology in the context of advanced high-strength steels and bainitic steels. In this work, the concept of advanced high-strength bainitic steels is briefly revisited from a wide perspective, with the aim of highlighting the main limitations and challenges for further development of these microstructures.

Characterization of the Stretch Flangeabitity of High-Strength Bainitic Steel: The Significance of Variant Pairs.

Variant pairs have an indispensable function on mechanical properties such as low impact toughness. Therefore, it was assumed that they would also affect the HER (Hole Expansion Ratio, an indicator to evaluate stretch flanging performance). To clarify this, a comprehensive analysis of the common influential factors in an 800 MPa grade low carbon micro-alloyed steel, i.e., the retained austenite, the M/A (Martensite/Austenite) island, the titanium precipitations, the grain diameter, the density of high angle grain boundaries and the textures, was first conducted. It was found that they did not match well with the HER, suggesting that they were not the governing factor for HER in this steel. However, the dominating crystallography groups and the variant pairing results indicated that they fitted well with the HER. In the samples with high HERs, the CP (Close Packed) groups dominated the transformation, wherein one individual CP group consisted of two or more Bain groups, whereas it evolved into the domination of joint CP groups and Bain groups for the low HER sample. Further analysis on the variant pairing features indicated that a correlation occurred between the HER and the high angle variant pairs. In the steels with high HERs, high-angle variant pairs of V1/V2, V1/V3 that transformed from the same CP group, particularly of V1/V2 pair, were mostly generated. They turned to V1/V9, V1/V10, V1/V12, V1/V15, V1/V17, and V1/V18 pairs from differential CP groups, especially the V1/V12 and V1/V15 pair for low-HER steel. This result showed that V1/V2, V1/V12, and V1/V15 might have accounted most for the HER in this steel. The underlying reason was that the V1/V2 pair was specialized in supplying a slip passage for dislocation transmission across a grain boundary with little resistance, whereas the dislocation transmission ability for V1/V12 and V1/V15 pair was particularly poor. Thus, to efficiently enhance the HER, one should regulate the variant pairs by augmenting the V1/V2 fraction and suppressing the formation of the V1/V12 and V1/V15 pair.

NEW MATERIALS FOR THE PRODUCTION OF REPLACEABLE PARTS OF TILLAGE AGRICULTURAL MACHINERY

The analysis of results of practical application of cast replaceable details of hinged tillage agricultural machinery at operation in a number of the agricultural enterprises of the Poltava region at processing of more than 60 thousand hectares within several years is presented. Comparative data of the resource of parts made of high-strength bainitic cast iron and steel serial analogues of domestic and foreign production are given. The advantages of the method of manufacturing by casting methods in comparison with the traditional method of manufacturing by stamping. This technology allows you to easily make adjustments to the design of the cultivator legs, thus providing a new quality of tillage and pruning of the root system of weeds, which eliminates the need for herbicides. The basis of this technology is the use of composite complex modifiers made by rolling powder mixtures of plastic and brittle components with the necessary observance of the proportions of their ratio. The use of powder rolled modifiers provides the necessary properties and structure of the cast metal required for further heat treatment. It is substantiated to obtain high wear resistance of cast products due to the appearance of TRIP-effect and constant updating of wear-resistant surface layer during operation.

Suppression of adiabatic shear band formation by martensitic transformation of retained austenite during split Hopkinson pressure bar test for a high-strength bainitic steel

In this study, a high-strength bainitic steel was fabricated and austempered at 350 °C for 15 min, 90 min, and 24 h to control the volume fraction of retained austenite (RA) while keeping an appropriate hardness level. Their dynamic compressive properties were investigated in relation to the ASB formation and cracking by the SHPB. The specimen austempered for 90 min showed the highest volume fraction of RA along with the highest compressive properties. This specimen also possessed the high resistance to form a transformation-ASB (tASB), which was attributed to the active deformation-induced martensitic transformation of RA. Since the applied dynamic energy was consumed by the martensitic transformation as well as the ASB formation, the active transformation effectively delayed the critical strain for initiating formation. Therefore, this critical strain concept, which was well correlated with dynamic compressive properties, would be an important key factor for suppressing the ASB formation and for improving the ballistic performance.

Enhancement of ballistic performance enabled by transformation-induced plasticity in high-strength bainitic steel

High-strength bainitic steels have created a lot of interest in recent times because of their excellent combination of strength, ductility, toughness, and high ballistic mass efficiency. Bainitic steels have great potential in the fabrication of steel armor plates. Although various approaches and methods have been conducted to utilize the retained austenite (RA) in the bainitic matrix to control mechanical properties, very few attempts have been conducted to improve ballistic performance utilizing transformation-induced plasticity (TRIP) mechanism. In this study, high-strength bainitic steels were designed by controlling the time of austempering process to have various volume fractions and stability of RA while maintaining high hardness. The dynamic compressive and ballistic impact tests were conducted, and the relation between the effects of TRIP on ballistic performance and the adiabatic shear band (ASB) formation was analyzed. Our results show for the first time that an active TRIP mechanism achieved from a large quantity of metastable RA can significantly enhance the ballistic performance of high-strength bainitic steels because of the improved resistance to ASB formation. Thus, the ballistic performance can be effectively improved by a very short austempering time, which suggests that the utilization of active TRIP behavior via tuning RA acts as a primary mechanism for significantly enhancing the ballistic performance of high-strength bainitic steels.

On the Significance of Thermal Deformation and Dynamic Recrystallization on Flow Behavior of 25Mn2Si2Cr Bainitic Alloy

Thermal deformation characteristics, dynamic recrystallization behavior, and processing map of 25Mn2Si2Cr high strength bainitic steels are elucidated during wide temperature range (850–1100 °C) and strain rate (0.1–10 s−1) from flow behavior. Thermomechanical analysis by Thermo‐Calc software indicates that microstructure during thermal deformation is austenite, which is conducive to dynamic recrystallization. Phenomenological constitutive equations of thermal deformation, dynamic recrystallization equation, and processing map are established to predict flow behavior. The experimental results indicate that deformation temperature and strain rate have significant effect on thermal deformation process. Flow stress increases with decrease in deformation temperature and increase in strain rate. Increasing deformation temperature and decreasing strain rate promote dynamic recrystallization occurrence. However, the grain size increases with increase in deformation temperature. Thermal simulations predict that optimum thermal deformation parameters are deformation temperature (950–1000 °C), strain rate (0.25–5 s−1) and deformation temperature (850–950 °C), strain rate (1–5 s−1) for processing of Mn–Si–Cr bainitic steels.

Influence of plastic deformation in fatigue crack behavior in bainitic steel

In the present paper, the behavior of fatigue microcracks is studied in a high-strength bainitic steel with the aim of identifying the microstructural features that influence the mode of initiation and propagation of microcracks. For this study, a gradual monitoring of surface damage during low-cycle fatigue test at different plastic strain ranges was carried out by means of light optical and electron microscopy along with EBSD data analysis. Moreover, the dislocation structure near the surface was analyzed and correlated with the crack initiation site. The results showed a change with respect to cracking mechanisms from 0.2% plastic strain.

Comparative Research on Fatigue Performance of High-Strength Bainitic Steel with or without Corrosion

High-strength bainitic steels have acquired a promising place among steels due to their wide applications in engineering structures. It is, therefore, worthwhile to investigate their fatigue performance in different environments. When fatigue is atmospheric, lattice strength of weaker zones including granular bainite, phase boundary and MA constituents determines the nucleation life of fatigue. The deflected microstructural short cracks enhance the early propagation life of fatigue. In lath bainite, fine ferrite laths, high-density dislocations within the grains, small grain size and high proportion of HAGBs inhibit the crack growth process to improve propagation life of fatigue. The fracture reveals transgranular mode for propagation of fatigue. The pre-corrosion strategy is adopted before investigating corrosion fatigue process, so that there is an adequate corrosion effect on samples. The pre-corrosion time is determined to be 3 days through the analysis of electrochemical parameters, viz. corrosion potential and current. For overall fatigue life, corrosion grooves have significant deterioration effect. The transgranular and intergranular features are observed in fracture at initial stage of crack growth. Only transgranular features are found at middle and final stages.