Martensite Packet Size Research Articles

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 Hot Deformation Parameters on Heat-Treated Microstructures and Mechanical Properties of 300M Steel.

The high strength of 300M steel originates from the heat treatment process after forging, but how hot deformation affects the heat-treated microstructure and mechanical properties is unclear. In this study, compression tests under different hot deformation parameters and post-deformation heat treatment experiments were carried out, and the martensite transformation process was investigated using in situ observation. The results show that the grain size of the specimen deformed at low temperature and high strain rate is smaller, and annealing twins will be formed. Both austenite grain boundaries and twin boundaries hinder the growth of martensite blocks, reducing the size of martensite units after heat treatment and thus resulting in higher yield strength. Besides, the mathematical models were established to describe the relationship between hot deformation parameters and grain size after deformation, martensite packet size and martensite block width, respectively, after heat treatment. The relationship between yield strength and hot deformation parameters was also analyzed. According to the results and models, the hot deformation parameters would be optimized more reasonably to improve the final mechanical properties of 300M steel forgings.

Ex situ analysis of high-strength quenched and micro-alloyed steel during austenitising bending process: numerical simulation and experimental investigation

This paper compares the microstructure and mechanical evolution in a high-strength quenched and micro-alloyed steel during the austenitising bending process. Simulation results indicated a new finding that the stress neutral layer (SNL) tends to move to the tension zone during straining. The hardness gradient detected from the centre to compression/tension zones was resulted from comprehensive factors: First of all, the location of SNL revealed a prominent impact on strength. Second, the dislocation accumulation would be responsible for the hardness gradient on the surfaces. In addition, the overall strength decrease during straining was mainly ascribed to integrated effects of dynamic recovery (DRV) and dynamic recrystallisation (DRX). Apart from that, overall smaller martensite packet size and coarser prior austenite grains resulted in the increased hardness value at a lower bending degree. Also, the high consistency between experimental and simulation results is instructive for the practical forming process of railway spring fasteners.

Design of a novel austenitising bending process in forming characteristics of high-strength quenched and micro-alloyed steel: Experiment and simulation

A systematic study was carried out on a novel designed austenitising bending process to explore the effects of quenching temperatures on the evolution of microstructure and mechanical properties of high-strength micro-alloyed steel. Results reveal that the qualified parts could be bent into a satisfactory shape without spring-back and cracks in bending temperatures of 850–1000 °C. The simulation was applied to predict the equivalent stress, stress neutral layer (SNL) and plate thickness. Strength presents a decreasing trend with the increase of bending temperature owing to integrated influences of high-temperature dynamic recovery (DRV) and dynamic recrystallisation (DRX). Beyond that, SNL nearby the geometrical centre line throughout the plate thickness and dislocation pile-ups on the tension/compression zones are responsible for the smaller hardness on middle regions while higher hardness on the surface areas. Through austenite reconstruction, a smaller martensite packet size would directly improve the strength of the plates bent at a lower temperature. The prior austenite size may be related to the appearance of martensite variants but not necessary. Furthermore, the high accuracy by comparing the experimental results with simulation would provide a high-value reference for the practical manufacturing process of railway spring clips.

Strengthening mechanisms in an ultrafine grained powder metallurgical hot work tool steel produced by high energy mechanical milling and spark plasma sintering

The strengthening mechanisms in an ultrafine grained (UFG) powder metallurgical (PM) tool steel produced by high energy mechanical milling (MM) and spark plasma sintering (SPS) have been investigated. In order to gain a proper insight to the Hall-Petch relation, bimodal grain size microstructures were produced by mixing different fractions of the nanocrystalline (NC) MM and the coarse grain (CG) gas-atomized powders to achieve different equivalent grain sizes. Dislocation strengthening (~ 435 MPa) and Hall-Petch strengthening (~ 388 MPa) are the major contributing mechanisms in strengthening the as-sintered steel. The presence of 34 vol% retained austenite (RA) had a negative influence on the strength (~ 555 MPa) and highlights the need of heat treatments (i.e., quenching and tempering) to achieve a fully tempered martensitic microstructure with a dispersion of fine secondary carbides. Recovery of dislocations and defects accompanying the precipitation of carbides reduces the effect of grain size (i.e., martensite packet size) on the strength after tempering. However, dislocations acting as preferential sites for secondary carbides nucleation during tempering, provide a finer and more homogenous particles dispersion compared to the CG counterpart. In this condition, the strength is mainly governed by the carbides size and distribution (~ 210 MPa) confirming that the indirect beneficial effects of MM can still be observed after heat treatment.

Optimization of hot workability and microstructure control in a 12Cr-10Ni precipitation hardenable stainless steel: An approach using processing maps

Optimization of hot working parameters and corresponding microstructure control were explored in a 12Cr-10Ni precipitation hardenable martensitic stainless steel. From hot compression tests, 1073–1373 K (800°–1100 °C) and five (0.001–10 s−1) different strain rates, processing maps were developed, and various domains of microstructure evolution (in the austenite phase) were identified. The stable domain of the processing maps appeared to indicate dynamic recrystallization (DRx) in the austenite phase: as evident by the appearance of inflection point in the strain hardening rate versus flow stress plots. The suspected DRx domain was further confirmed by detailed microstructural investigations of the respective water-quenched specimens. The DRx domain showed a refinement of the martensite packet size and of the prior transformation austenite grain size. The later was shown with optical microstructure and reconstructed (modelled) microtexture data of the prior austenite grains. Additionally, the DRx domain showed a significant difference in activation energy as estimated from appropriate constitutive modelling.

Effect of Austenitizing Temperature on Mechanical Properties of the Mixed Bainite - Martensite Microstructure in CrMoV Steel

The effect of austenitizing temperature on mechanical properties of the mixed bainite - martensite microstructure in CrMoV steel was studied in the present work. The result showed that at low austenitizing temperature (910°C - 1000°C), the mixed microstructures containing 12-28% volume fraction of lower bainite showed higher yield and tensile strength than fully martensitic microstructure. The partitioning of the prior austenite grain by lower bainite was found to cause a refinement of the martensite packet size. In addition the strength of the lower bainite in the mixed microstructure is enhanced by plastic constraint induced by the surrounding stronger martensite. By increasing the austenitizing temperature from 1000°C to 1200°C (40min), the YS, UTS, %EL, %RA and CVN impact energy decreased for all samples. This is attributed mainly to the increase in austenite grain size and width of bainite sheaves.

Origin of low temperature toughness in a 12Cr-10Ni martensitic precipitation hardenable stainless steel

Though ductile to brittle transition (DBT) is a typical feature of body centered cubic materials, the present 12Cr-10Ni precipitation hardened martensitic stainless steel exhibited excellent low temperature impact toughness. This was, however, dependant on the aging temperature. 250°C aging led to higher toughness both at room temperature and at −196°C (77K). Specimens aged at 400 and 500°C, on the other hand, displayed significantly lower sub-zero impact properties. Though martensite packet size, and size distribution, were identical between the two ageing treatments; there was clear evidence of second phase coarsening: from very fine precipitates of less than ~ 10nm to relatively coarser second phase of ~ 5–25nm range. It is suggested that precipitate coarsening and associated loss of coherency are the limiting factors to the DBT performance of this important class of material.

Near-Ac3 austenitized ultra-fine-grained quenching and partitioning (Q&P) steel

The influence of an intercritical anneal and a full austenitizing close to the Ac3 temperature on the mechanical properties of a quenching and partitioning (Q&P) processed medium Mn steel is reported. This approach to Q&P processing is shown to result in significantly improved mechanical properties mainly due to a refinement of the prior austenite grain size and, in consequence, the refinement of the lath martensite packet size after Q&P processing. Deformation twinning and strain-induced martensitic transformation were activated during tensile deformation. This resulted in a clear plasticity enhancement in the ultra-fine grained austenite islands after Q&P processing combined with austenitizing near the Ac3 temperature.

Effect of Martensite Fine Structure on Mechanical Properties of an 1100 MPa Grade Ultra-high Strength Steel

An 1100 MPa grade ultra-high strength steel with different martensite fine structures, characterized by prior austenite grain size, martensite packet size, block width and lath width, was studied by various heat treatment processes. The result shows that with decreasing prior austenite grain size, both the packet size and block width decrease, while the lath width has virtually no change. Accordingly, both strength and toughness increase, while total elongation decreases. The yield strength has a Hall-Petch type relationship with the prior austenite grain size, packet size and block width, and the block width may be regarded as a key factor influencing strength. On the other hand, the ductile to brittle transition temperature (DBTT) is found to be more related to the packet size, which may be considered as a dominant factor influencing toughness.

Effect of direct quenching on microstructure and mechanical properties of a wear-resistant steel

A wear-resistant steel was hot rolled at the same finish temperature, and subsequently treated with conventional reheat quenching and tempering process (RQ&T) and direct quenching and tempering process (DQ&T), respectively. The effect of direct quenching on the microstructure and mechanical properties was investigated in detail by using optical microscope, transmission electron microscope and scanning electron microscope equipped with electron backscattered diffraction. The results showed that the microstructures of both the RQ and DQ specimens were complex constituents of lath martensite and lower bainite. Compared with the RQ specimen, the lower bainite content in DQ specimen was much higher. Furthermore, the bainite in the DQ specimen extended into and segmented the prior austenite grains, which can decrease martensite packet size. The proportion of high-angle boundary in the DQ specimen was higher than that in the RQ specimen, which may improve the impact toughness. The carbides in DQ&T specimen were much finer and distributed even dispersively because direct quenching can retain substantive defects which may provide more nucleation sites for carbide precipitation in the tempering process. Besides, the mechanical properties of DQ and DQ&T specimens were superior than those subjected to RQ and RQ&T processes, respectively.

The effect of the martensitic packet size on the machinability of modified AISI P20 prehardened mold steel

The effect of martensitic packet size on the machinability of prehardened mold steel at a hardness of ~40 HRC (typical hardness for prehardened mold steels) was studied in terms of cutting force and tool life. The machinability tests were performed in end milling using coated, cemented carbide tools at three different cutting speeds. The results showed that an increase in the martensite packet size led to higher cutting force and shorter tool life. The increase in cutting force was related to the increase of work hardening. The work material with a coarser martensite packet size showed a higher amount of work hardening that can explain the higher cutting force. The longer tool life in the workpieces with finer structure was correlated to smaller amplitude of the variation in cutting force.

In-Situ Observations of Martensitic Transformation in Blast-Resistant Steel

A hybrid in-situ characterization system, which couples the laser scanning confocal microscopy (LSCM) with the time-resolved X-ray diffraction (TRXRD) measurement with synchrotron radiation, was used to characterize the microstructure evolution during heat-affected zone (HAZ) thermal cycling of high-strength and blast-resistant steel. The combined technique has a time resolution of 0.3 seconds that allows for high-fidelity measurements of transformation kinetics, lattice parameters, and morphological features. The measurements showed a significant reduction in the martensite start transformation temperature with a decrease in the prior austenite grain size. In addition, the LSCM images confirmed the concurrent refinement of martensite packet size with smaller austenite grain sizes. This is consistent with dilatometric observations. The austenite grain size also influenced the rate of transformation (df m /dT); however, the measurements from the hybrid (surface) and dilatometric (volume) measurements were inconsistent. Challenges and future directions of adopting this technique for comprehensive tracking of microstructure evolution in steels are discussed.

The effects of titanium additions on AF1410 ultra-high-strength steel

The mechanical properties and microstructure of two heats of AF1410 steel were compared. The first heat, heat 811, contained a titanium addition of 0.02 wt pct, while the second heat, heat 91, contained no titanium, manganese, or other strong sulfide formers. The sulfur in heat 811 was gettered as titanium carbosulfide, while in heat 91 the sulfides were chromium sulfide. The toughness of heat 811 was found to be much enhanced compared to heat 91, with Charpy impact energies of 176 J and 79 J and KIC fracture toughness values of 235 MPa.m1/2 and 170 MPa.m1/2, respectively. This significant difference in fracture toughness is attributed to the fact that titanium carbosulfide particles are more resistant to void nucleation than the chromium sulfide particles, which appear to nucleate voids at the onset of plastic strain. In addition to altering the sulfide particle type, the titanium addition also results in the presence of undissolved MC carbides in the titanium-modified steel in addition to the M2C carbides found in heat 91. These carbides act as grain growth inhibitors, resulting in a finer prior austenite grain size and martensite packet size in heat 811.

The dependence of mechanical properties on structure in low alloy steel forgings

The mechanical properties of two heat-treated steel forgings, manufactured from medium carbon Ni-Cr-Mo-V steels have been measured and compared. Microstructural characteristics such as martensite packet and lath size, dislocation density and precipitate size and distribution were measured and used in a detailed quantitative analysis of the relation between microstructural characteristics and proof stress. It was found that the microstructural differences identified could well account for the differences in tensile strength between the materials. Electron microcopy indicated that temper embrittlement leading to intergranular fracture may be responsible for the differences in transition temperature.

Ductile-Brittle Transition Temperature of Ultrafine Ferrite/Cementite Microstructure in a Low Carbon Steel Controlled by Effective Grain Size

To analyze the good toughness of ultrafine ferrite/cementite steels, the concept of effective grain size (dEFF) is applied to ductile-to-brittle transition temperature, DBTT, for ultrafine ferrite/cementite (Uf-F/C), ferrite/pearlite (F/P), quenched (Q), and quench-and-tempered (QT) microstructures in a low carbon steel. The dEFF is determined to be 8, 20, 100, and 25 μm for Uf-F/C, F/P, Q, and QT, respectively. In F/P and Q, it is in accordance with the ferrite grain size and the prior austenite grain size, respectively. In QT, the dEFF fits the martensite packet size. In Uf-F/C, the ferrite grain size has a bimodal distribution and the larger grain size corresponds to the dEFF, which is the smallest among the four microstructures. In terms of the relationship between dEFF and DBTT, the Uf-F/C, Q, and QT microstructures can be placed into the same group and the F/P to a different one. Furthermore, the Uf-F/C has the highest estimated fracture stress among the four microstructures. These might be the result of the difference in the surface energy of fracture, namely the former is estimated to have a surface energy of 34.6 J/m2 and latter a surface energy of 7.7 J/m2. Thus, the excellent toughness of the ultrafine ferrite/cementite steel can be attributed to the small dEFF and the high surface energy of fracture.

Effects of heat treatment on microstructure and mechanical properties of Cr–Mo–3·5Ni–V steel

Heat treatments have been carried out on an annealed low alloy Cr–Mo–3·5Ni–V steel (J-steel) to investigate the effects of various normalising and hardening temperatures, before tempering, on the mechanical properties. It was observed that the 0·2% proof stress increased, whereas the impact toughness, as measured by the increase in transition temperature, decreased with increasing hardening temperature for both normalised and non-normalised material. For a constant hardening temperature, the 0·2% proof stress passed through a minimum and the Charpy upper shelf energy passed through a maximum, at a normalising temperature of 890°C. The variation in properties has been correlated with the observed variation in martensite packet size, dislocation density, precipitate size, and volume fraction. It was found that a 0·2% proof stress above 1100 MN m−2, and a Charpy impact energy of greater than 50 J at −40°C located within the upper shelf energy region, was attainable with the following heat treatment: normalising from 870°C and hardening from 850°C, followed by tempering at 620°C and oil quenching to room temperature.MST/1870

Effects of heat treatment on microstructure and mechanical properties of Cr–Mo–3·5Ni–V steel

Heat treatments have been carried out on an annealed low alloy Cr–Mo–3·5Ni–V steel (J-steel) to investigate the effects of various normalising and hardening temperatures, before tempering, on the mechanical properties. It was observed that the 0·2% proof stress increased, whereas the impact toughness, as measured by the increase in transition temperature, decreased with increasing hardening temperature for both normalised and non-normalised material. For a constant hardening temperature, the 0·2% proof stress passed through a minimum and the Charpy upper shelf energy passed through a maximum, at a normalising temperature of 890°C. The variation in properties has been correlated with the observed variation in martensite packet size, dislocation density, precipitate size, and volume fraction. It was found that a 0·2% proof stress above 1100 MN m−2, and a Charpy impact energy of greater than 50 J at −40°C located within the upper shelf energy region, was attainable with the following heat treatment: normalising from 870°C and hardening from 850°C, followed by tempering at 620°C and oil quenching to room temperature.MST/1870

Microstructure–property relationships in tempered low alloy Cr–Mo–3·5Ni–V steel

The effect of varying normalising and hardening temperatures, before tempering at ~620°C, on the strength and toughness of a low alloy Cr–Mo–3·5Ni–V (wt-%) steel has been examined. Microstructural features including martensite packet and lath size, dislocation density, and precipitate size were measured and used in a Hall–Petch analysis of the strengthening components. It was found that a rms summation of the strengthening contributions to the 0·2% proof stress gave values in good agreement with experimental results. The 50% fracture appearance transition temperature could be described by a relationship involving the fracture facet size and the strengthening contributions from dislocations and precipitates.MST/1802

Tensile properties of a 0.34C3NiCrMoV steel with mixed lower-bainitemartensite structures

The primary objective of this work is to study the influence of microstructure on the tensile properties of a 0.34C3NiCrMoV steel with mixed lower-bainitemartensite microstructures. In general, the tensile yield strength and ultimate strength of the steel with mixed structures were found to decrease with an increase in the lower-bainite content, while a few exceptions were observed on mixed structures containing 10–25% lower bainite which showed higher yield strength than fully martensitic structure. The introduction of lower bainite in prior austenite grains was found to cause a refinement of the martensite packet size. However, the improvement of the strength of martensite due to the refinement of its substructure can only account for part of the strengthening in a mixed lower-bainitemartensite structure. The tensile ductility (RA%) of the mixed lower-bainitemartensite structure in general is quite close to that of the fully martensitic structure except for the fine austenite grain condition (8 μm), for which the mixed structures show inferior RA% as compared with the fully martensitic structure. On the other hand, the refinement of prior austenite grain size was found to cause beneficial effects on both tensile strength and ductility for the steel with mixed lower-bainitemartensite structure.