Prior Martensite Research Articles

The influence of ferrite content, ferrite-austenite morphology, and orientation relationship on bainite transformation in intercritically annealed bainitic steels

This manuscript elucidates the influence of varying ferrite/tempered martensite (TM) content, morphology and orientation relationship (OR) between ferrite/TM with reverted austenite (γ) resulting at different intercritical temperatures on the subsequent bainite transformation behaviour. A comprehensive analysis was conducted using scanning electron microscopy, electron backscattered diffraction, and dilatometry. TM/bainite and ferrite/bainite steels were produced through intercritical annealing at 735 °C, 745 °C, and 765 °C, followed by austempering at 310 °C, using initially quenched and cold-deformed martensite, respectively. In samples without cold rolling, TM and bainite are alternately distributed at all intercritical temperatures. The γ reversion process in these samples led to the formation of acicular γ at prior martensite blocks and laths while globular γ was formed at prior γ boundaries and packet boundaries. Notably, a lower TM content correlated with increased globular γ formation. The acicular γ exhibited a Kurdjumov–Sachs (K-S) OR with prior γ. The globular γ formed at prior γ grain boundaries showed the K-S OR with the prior γ where it grows. However, globular γ formed at packet boundaries maintained the K-S OR with one of its surrounding blocks in prior γ where it lies. In the subsequent austempering process, bainite formed from reversed γ followed the K-S OR with TM. The TM and bainite shared the same variant in the same block. In samples with cold rolling, polygonal ferrite and bainite were distributed at all intercritical temperatures. Decreasing the ferrite content showed a coarsening of the microstructure. All polygonal samples revealed an irrational OR between bainite/martensite and ferrite. In samples with and without cold rolling, the bainitic formation rate decreased with increasing ferrite/TM, likely due to the high C and Mn content at the α/γ interface. Comparatively, the samples without cold rolling showed faster bainitic transformation than samples with cold rolling at similar ferrite/TM percentages, particularly noticeable at higher ferrite/TM content. The alternately arranged elongated TM and reversed γ lamellae increased the α/γ interfacial density for bainitic nucleation. Moreover, K–S oriented α/γ interfaces exhibited a lower activation energy for bainite nucleation. In samples without cold rolling, both high density of α/γ interfaces and K–S oriented α/γ interfaces collectively facilitate the bainitic transformation.

Ultra-Fine Bainite in Medium-Carbon High-Silicon Bainitic Steel.

The effects of austenitizing and austempering temperatures on the bainite transformation kinetics and the microstructural and mechanical properties of a medium-carbon high-silicon ultra-fine bainitic steel were investigated via dilatometric measurements, microstructural characterization and mechanical tests. It is demonstrated that the optimum austenitizing temperature exists for 0.3 wt.%C ultra-fine bainitic steel. Although the finer austenite grain at 950 °C provides more bainite nuclei site and form finer bainitic ferrite plates, the lower dislocation density in plates and the higher volume fraction of the retained austenite reduces the strength and impact toughness of ultra-fine steel. When the austenitizing temperature exceeds 1000 °C, the true thickness of bainitic ferrite plates and the volume fraction of blocky retained austenite in the bainite microstructure increase significantly with the increases in austenitizing temperature, which do harm to the plasticity and impact toughness. The effect of austempering temperature on the transformation behavior and microstructural morphology of ultra-fine bainite is greater than that of austenitizing temperature. The prior martensite, formed when the austempering temperature below Ms, can refine the bainitic ferrite plates and improve the strength and impact toughness. However, the presence of prior martensite divides the untransformed austenite and inhibits the growth of bainite sheaves, thus prolonging the finishing time of bainite transformation. In addition, prior martensite also strengthens the stability of untransformed austenite though carbon partition and enhances the volume fraction of blocky retained austenite, which reduces the plasticity of ultra-fine bainitic steel. According to the experimental results, the optimum austempering process for 0.3 wt. %C ultra-fine bainitic steel is through austenitization at 1000 °C and austempering at 340 °C.

Effect of prior martensite on bainitic transformation of high Si hypereutectoid steel

The paper discusses the effect of prior martensite on the bainitic transformation of a high Si hypereutectoid steel. The research uses a combination of experimental techniques, high-resolution dilatometry and X-ray diffraction analysis, and theoretical calculations to investigate the impact of martensite on the transformation kinetics and microstructure evolution. The study shows that the presence of martensite, regardless of the amount, accelerates the bainitic transformation, particularly in the early and fastest stages, and at higher isothermal transformation temperatures. At lower temperatures, the preconditioning of the austenite prior to the bainitic transformation is such that its mechanical stability increases to the point where the whole transformation is inhibited. It is also suggested that the formation of cementite due to the tempering of martensite at the isothermal stage controls the final fraction of bainitic ferrite. The findings provide valuable insights into the role of prior martensite in bainitic transformation kinetics and microstructure development.

Microstructural refinement by spontaneous recrystallization without prior deformation of a 15-5 PH steel alloy and its mechanism

The recrystallization without previous deformation is reported in literature for a small, selected group of alloys. The present work provides evidence for the first time that the commercial stainless steel 15-5PH also shows this recrystallization phenomenon during austenitization. A set of in-situ and ex-situ high-temperature techniques reveal that, on heating of the martensitic microstructure, recrystallization takes place after phase transformation between 900 and 1000 °C, causing a distinct reduction of the austenite grain size. This work also shows that the recrystallization correlates with the mechanisms involved in the prior martensite to austenite transformation. It is observed that increasing heating rates lead to decreasing grain sizes. This is attributed to increased defect density in the reverted austenite and increased driving pressure for the nucleation of recrystallized grains. It is proposed that, during martensite to austenite reversion, a defect arrangement of highly stable low-angle grain boundaries and, with increased heating rate, an increased density of internal, grown-in dislocations is inherited from martensite laths. This highly defect-loaded microstructure, formed without external plastic deformation, leads to a recrystallization at increased temperatures. The experimental results agree well with thermokinetic calculations based on the proposed defect arrangement, underpinning the mechanism of spontaneous recrystallization in 15-5PH.

Acceleration of bainitic transformation in 0.28C-3.8Mn-1.5Si steel utilizing chemical heterogeneity

This study proposes a novel method for accelerating bainitic transformation by utilizing Mn heterogeneity. The process involves intercritical annealing to create a heterogeneous distribution of Mn prior to full austenitizing. Heterogeneous Mn distribution leads to the rapid nucleation of bainitic ferrite in the Mn-depleted austenite region, and the rapidly formed bainitic ferrite acts as a nucleation site for Mn-enriched austenite region. Consequently, the required austempering time to achieve a similar fraction of bainitic ferrite is nearly halved compared to conventional process. The effectiveness of Mn heterogeneity in accelerating the transformation is comparable to other acceleration methods, such as prior austenite grain refinement and introducing prior martensite. Moreover, the proposed method has an additional advantage in increasing the fraction of retained austenite. The retained austenite is stabilized not only by C partitioning during austempering but also by pre-partitioned Mn during intercritical annealing.

Kinetics of Carbon Enrichment in Austenite during Partitioning Stage Studied via In-Situ Synchrotron XRD

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

Interrupted quenching and bainitising below Ms temperature of EN X37CrMoV5-1 hot-work tool steel: Bainitic transformation kinetics, microstructure and mechanical properties

A series of dilatometric experiments were performed to investigate the influence of martensite content on the kinetics of bainitic transformation during interrupted quenching and bainitising (iQ&B) treatment in EN X37CrMoV5-1 hot-work tool steel. Three iQ&B treatments were designed and conducted based on experiments to evaluate the mechanical properties and microstructure. The proposed heat treatment leads to multiphase structure providing tensile strength up to 1966 MPa, uniform elongation up to 11.33% (3.31% for quenching and tempering) and impact toughness up to 10.3 J/cm2 (6.9 J/cm2 for quenching and tempering). Obtained microstructures were favourable due to nanobainitic regions and a high amount of retained austenite (above 30 vol%) – percolating and susceptible to the TRIP effect. Prior martensite was found not only to improve the strength of the material but also to accelerate bainitic transformation. When bainitising is conducted at temperatures not above 260 °C bainitic transformation is accelerated at the very beginning. The reason for this acceleration effect, called the “left arm uplift effect”, is seen in the occurrence of “fresh” martensite-austenite interfaces with a small level of carbon enrichment, which act as favourable nucleation sites for bainitic ferrite plates.

Isothermal decomposition of austenite in presence of martensite in advanced high strength steels: A review

The development of the quenching and partitioning (Q&P) process has prompted an interest in the process of isothermal transformation in presence of a pre-existing phase such as martensite. The presence of prior martensite is known to accelerate the overall kinetics of bainite formation, both in the 1-step and 2-step Q&P process. The underlying mechanisms behind this phenomenon are not fully understood. Also, the nature of the isothermal product (isothermal martensite and/or bainite) formed in presence of prior martensite seems to differ according to the thermodynamic and kinetic conditions. For certain thermodynamic conditions, depending on alloying, isothermal martensite may also form in the temperature range just above and below M s . In the event that both bainite and isothermal martensite formation are thermodynamically allowed, the competition in kinetics determines the observed transformation product. The effect of martensite on the subsequent isothermal transformation is reviewed with a focus on the nature of the transformation products and kinetics. In that context, the kinetics of the isothermal transformation in presence of other prior phases such as polygonal ferrite and bainite are also compared and discussed, together with the possible mechanisms behind the acceleration of the transformation kinetics. Guidelines for further investigation are also proposed. − Formation of both bainite and isothermal martensite is possible during isothermal heat treatment of austenite in presence of martensite. − Bainite formation kinetics is accelerated in presence of martensite, potentially due to an increase in the nucleation site density. − Opinions are differed whether bainite formed at or near the interface. Therefore the primary mechanism is not established yet. − Complexities in identifying the primary mechanism are discussed and possible research guidelines are proposed.

Effect of prior martensite formation on the bainite transformation kinetics in high-strength 3% Mn multiphase steel

The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10).

Microstructure and Mechanical Properties of Medium-Carbon Si-Rich Steel Processed by Austempering after Intercritical Annealing

In the present paper, the medium-C Si-rich steel with a quenched martensite microstructure was heated to intercritical annealing temperatures at 750 °C, 760 °C and 770 °C after warm rolling deformation to obtain ferrite with varying volume fractions. Subsequently, bainite/ferrite multiphase microstructures were attained via austempering near Ms temperature. The microstructures of the test steel after different heat treatments were characterized by scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction, and corresponding tensile and impact properties were tested. The results showed that, with the increase of intercritical annealing temperature, the austenite content increased, which limited the growth of ferrite grains, and the grain size decreased from ~1.6 μm to ~1.4 μm. In addition, the degree of ferrite recrystallization was almost complete. At the same intercritical annealing temperature, compared with austempering above Ms, prior athermal martensite (PAM) was obtained after austempering below Ms, which effectively refined the size of bainite ferrite lath. Moreover, with the increase of intercritical annealing temperature, the bainite content of the test steel increased after austempering, resulting in the increase of yield strength, tensile strength and impact energy. In contrast, while the decrease in ferrite content led to a significant decrease in uniform elongation. At constant intercritical annealing temperature, the tensile strength decreased slightly, and the impact property improved after austempering above Ms.

Microstructural Characterizations and Kinetics Model of Isothermal Bainite Formation with the Presence of Prior Martensite in High‐Carbon Nanostructured Bainitic Steel

Quenching and subsequent isothermal bainite transformation (QBT) and direct isothermal transformation (DIT) procedures are performed to comparatively evaluate the impact of the introduced prior martensite on bainite transformation behaviors and microstructural characterizations in a high‐carbon nanostructured bainitic steel. A thermodynamic model under a QBT process based on displacive theory is also developed to expound the bainite formation kinetics. The results indicate that the modified kinetics model, by considering austenite/martensite interface nucleation, austenite grain boundary nucleation, and autocatalytic nucleation, evolved here can be used to accurately describe the bainite formation behavior in the presence of prior martensite. A certain fraction of prior austenite/martensite interface can serve as the preferential nucleation site for the bainite transformation and assist in an acceleration of transformation rate. The bainite nucleation event initially takes place at prior martensite plate boundary in the QBT process instead of austenite grain boundary in the DIT process and subsequently continues through autocatalytic nucleation at austenite/bainite interface generated at either austenite/martensite interface or austenite/austenite interface. The introduced prior martensite slightly refines bainite sheaves and lessens the blocky retained austenite size, presenting a feasible approach to shorten the required time for producing a nanosized bainitic microstructure.

Effect of prior martensite on bainite transformation and microstructure of high-carbon nano-bainitic steel

Bainite transformation kinetics, microstructure, and mechanical properties were comparably investigated by X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy observation, and tensile and impact tests for two treatments: with and without quenching before isothermal bainite transformation at 300 °C. Both the incubation time and growth stage of bainite transformation are accelerated by the introduction of prior martensite, especially the former. In addition, the bainite microstructure around the prior martensite is refined and shows a similar orientation with the martensite due to the strain field caused by the existence of prior martensite. The finer bainite microstructure and prior martensite improve the strength and hardness of the high-carbon nano-bainitic steel. Although the presence of prior martensite and low misorientation relationship between the martensite and the adjacent bainite microstructure are prejudicial to impact toughness, the nanoscale bainite microstructure can inhibit the crack propagation and then improve the impact toughness of the high-carbon nano-bainitic steel.

Wear behavior of a multiphase ductile iron produced by quenching and partitioning process

A quenching and partitioning process has been developed for an unalloyed ductile iron. The process includes austenitizing at 890 °C for 0.5 h, quenching to 180 °C below the Ms (the starting temperature of martensite transformation) for 5 s, with a controlled speed, then followed by a partitioning at 190 °C (10 °C lower than the Ms) for 8 h, and finally air cooling to the room temperature. A multiphase microstructure consisting of prior martensite (PM), bainitic ferrite (BF), and fresh martensite formed on final quenching (FM) with retained austenite (RA) is obtained through the microstructural evolution on quenching and partitioning steps. Further observation shows that a unique nano structure including lath BF and film RA with a width less than 100 nm nucleates around the PM. A strong influence of RA content and stability on both the tensile strength and the elongation is observed. In addition, the wear behavior is investigated under various loads without lubrication. A combined wear mechanism of abrasion and oxidation is found in a case of a low load. With increasing the load, the wear behavior correlates to a comprehensive strengthening and toughening effect of the multiphase microstructure in the matrix.

Transformation kinetics of carbide-free bainitic steels during isothermal holding above and below MS

The transformation kinetics of a low carbon bainitic steel was investigated using in situ confocal microscopy and dilatometry for isothermal treatments above and below the martensite start (MS) temperature. Both martensite and bainite are formed during isothermal holding below the MS, but bainite was the dominant transformation product. The prior martensite formed during cooling to the holding temperature strongly influenced the bainite precipitation kinetics. A critical volume fraction of prior martensite, fcPM, was identified. In order to accelerate the bainite transformation kinetics by austempering below the MS, the prior martensite fraction should be less than fcPM. The best combinations of strength and ductility were obtained in steels by austempering below the MS temperature with an optimum martensite fraction (∼5%). Finally, the isothermal bainite transformation kinetics is analyzed by Johnson-Mehl-Avrami-Kolgomorov (JMAK) kinetics model. The analysis suggests that the nucleation of bainite approaches site saturation when the isothermal holding temperature is below the MS.

Effect of prior austenite grain size on bainitic transformation above and below Ms in medium Mn steel

The effect of prior austenite grain (PAG) size on bainitic transformation kinetics below Ms in 0.2C-3.92Mn-1.6Si steel has been investigated using a dilatometer. The samples were austenitized at two different temperature to achieve the average PAG size of ∼10.9 μm and ∼17.3 μm. The results show that above Ms the austenite grain size plays a significant role in bainitic transformation kinetics. However, the influence of austenite grain size on bainitic transformation kinetics below Ms becomes weaker which is ascribed to the acceleration of prior martensite on bainitic transformation below Ms.

Ultrafine bainitic steel produced through ausforming-quenching process

The coupling effects of ausforming and quenching process on isothermal bainite transformation behaviors, microstructures and mechanical properties of ultrafine bainitic steel have been analyzed. The synergism of ausforming and prior martensite transformation not only significantly accelerates subsequent bainite transformation, but also effectively refines bainite laths and greatly reduces the size of harmful blocky retained austenite, providing an improved transformation induced plasticity effect. Ultimate tensile strength of 2000 MPa and total elongation of 24% can be achieved in ultrafine bainitic steel by ausforming-quenching process and austempering at 300 °C for 1 h.

Understanding the effect of prior bainite/martensite on the formation of carbide-free bainite

ABSTRACTThe isothermal bainite transformation in low-carbon low-alloy steel under direct and step-quenching conditions have been investigated in the present work. Varying quantities of prior bainite and prior martensite were formed separately by quenching above and below temperature to understand their effects on the extent and kinetics of subsequent bainite transformation in regime. Formation of prior bainite at higher temperature has been found to stabilise part of remaining austenite against subsequent bainite transformation at lower temperature showing thermal stabilisation of austenite. Hence, the total amount of bainite in two-step treatment is less than that obtained in single-step isothermal bainite transformation at the latter temperature, indicating the inapplicability of ‘additivity rule’ for continuously cooled bainite. A decrease in the bainitic ferrite plate volume due to higher strength of parent austenite and higher nucleation rate, together with limited number of bainite nucleation sites at the latter temperature are believed to be responsible for the thermal stabilisation of austenite. Prior martensite also stabilises part of remaining austenite against further bainite transformation through fragmentation of austenite grains and increased defect density in austenite. The kinetics of bainite transformation in each of the above-mentioned conditions have been analysed quantitatively using an established model and the results are correlated with the microstructures.

Effect of Austenitizing Temperature and Prior Martensite on Ultra-Fine Bainite Transformation Kinetics

An evaluation method for bainite transformation kinetics was established by theoretical derivation, dilatometric curve analysis, and microstructure observation. The isothermal transformation of ultra-fine bainite under different austenitizing temperatures and contents of prior martensite was studied using a DIL805L dilatometer. The kinetic parameters (activation energy Q*, autocatalytic factor λ, temperature rate constant κ, unit volume transformation rate, and the number density of nucleation sites Ni) of ultra-fine bainite transformation under different austenitizing temperatures and contents of prior martensite were calculated based on the displacement growth bainite dynamics model. It was found that the autocatalytic factor λ is linear with the austenite grain size d, and the number density of nucleation sites Ni is closely related to the average volume of the bainite subunit Vb. Moreover, the formation of prior martensite and its increase can increase the number of nucleation sites and the nucleation rate of the ultra-fine bainite; thus, the ultra-fine bainite transformation can be accelerated.

High Performance Unalloyed Ductile Cast Iron with Multiphase Structure

An unalloyed ductile cast iron with a multiphase structure is designed by a novel austempering process. The designed austempering treatment consists of initial rapid quenching to 180°C after austenizing at 890°C for 20min, and finally austempering at 220°C for 240min. A multiphase structure comprising lenticular/needle-like prior martensite, fine needle bainitic ferrite and film retained austenite is obtained. The excellent mechanical properties, with a tensile strength of 1530MPa and an elongation of 3.1% can be achieved by controlling the matrix microstructure of 12% prior martensite, 15% retained austenite with 1.64% carbon content, and 73% bainitic ferrite. This is mainly attributed to prior marteniste which can promote refinement of multiphase colonies.

Microstructure Variation of Ultrasuper Critical 11Cr Ferritic Steel Under Long-Term Thermal Ageing and Creep

The microstructure variation of ultrasuper critical 11Cr ferritic steel was quantitatively investigated under long-term thermal ageing and creep, with the emphasis on the size of precipitates, martensite lath width, and dislocation density. The tests were interrupted at different deformation stages. The precipitates on prior austenite grain and martensite lath boundaries became obscure due to coarsening of secondary particles. The Laves phase grew faster than M23C6 carbides in both creep and thermal ageing. The dislocation density rapidly decreased at the initial stage, while the martensite lath width increased with ageing and creep duration. The microstructure variations under creep were more pronounced than under thermal ageing, indicating that stress exerts an important effect on microstructure degradation.