Tempered Martensite Embrittlement Research Articles

Tempering behavior of an ultra-high-strength steel with 1.6 wt% Si at low to medium temperatures

Tempering behavior of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was examined. Water quenching produced lath martensite structure with a 10.5% volume fraction of retained austenite (RA) with film-like shape. Low temperature tempering (LTT) leads to precipitation of transition carbides and carbon partitioning between martensite and RA. The following precipitation sequence was found: martensite →non-stoichiometric η-carbide→stoichiometric η-carbide→Fe3C. Highest density of non-stoichiometric Fe2C η−carbides was found after isochronal tempering at 280 °C that provided attractive combination of high yield stress (YS) (1890 MPa) with a ductility of ∼6% and a Charpy V-notch (CVN) impact energy of ∼10 J/cm2. At 400 °C, the replacement of non-stoichiometric η-carbide by the stoichiometric one decreases strength and increases ductility due to 30% decrease in its volume fraction. No tempered martensite embrittlement was found due to the fact that Si effectively suppresses the precipitation of cementite at T ≤ 400 °C. The replacement of transition η-carbide by boundary cementite after isochronal tempering at 500 °C leads to considerably decrease in YS down to 1360 MPa, while elongation to failure increases up to 9.3%. Effect of decomposition of RA on strength, ductility and fracture toughness is insignificant.

Tempered Martensite Embrittlement of the Metallic Fastener After Austempering Heat Treatment and Coating Process Due to the Existence of Inclusion

Tempered Martensite Embrittlement of the Metallic Fastener After Austempering Heat Treatment and Coating Process Due to the Existence of Inclusion

Quench and Tempered Embrittlement of Ultra-High-Strength Steels with Transition Carbides

The effect of tempering after water quenching on the strength and fracture toughness of two steels with chemical compositions of 0.34%C-1.77%Si-1.35Mn-0.56%Cr-0.2%Mo-0.04%Nb-0.03Ti-0.002B and 0.44%C-1.81%Si-1.33%Mn-0.82%Cr-0.28%Mo was examined. The last steel exhibits quenching embrittlement in an as-quenched condition. At a tempering temperature of 280 °C, the precipitation of transition η–Fe2C carbides in martensitic matrix leads to increasing fracture toughness and eliminates quench embrittlement in the steel with 0.44 wt.%C. Tempered martensite embrittlement at 400 °C appears as decreased values of the Charpy V-notch impact energy, ductility and the product of strength and elongation, σB×δ (MPa×%) and is attributed to increased effective grain size for fracture, mainly. The precipitation of boundary cementite takes place at tempering at 500 °C and provides increased ductility and fracture toughness despite a decohesion along carbide/ferrite interfaces. The low severity of TME in Si-rich low-alloy medium carbon steels is attributed to the suppression of boundary cementite precipitation at tempering temperatures ≤400 °C.

EVOLUTION OF MICROSTRUCTURE AND EMBRITTLEMENT DURING THE TEMPERING PROCESS IN SiCrCu MEDIUM-CARBON STEELS

The effect of Cu and Si contents on the evolution of embrittlement during a heat treatment consisting of quenching into an oil bath and tempering in the range 200–500 °C was investigated in medium-carbon steels with 0.48 w/% and 0.57 w/% of C. The results showed that the higher silicon content shifted the tempered martensite embrittlement to higher tempering temperatures. The steels alloyed with Cu had lower notch-toughness values, which worsened with increasing tempering temperature compared to the Cu-free samples. In the investigated tempering temperature range, the following microstructural changes occurred: formation of transition carbides, decomposition of retained austenite, and precipitation of cementite and Cu-particles.

Tempered martensite embrittlement of an alloy steel forging from an automobile vehicle system

Tempered martensite embrittlement of an alloy steel forging from an automobile vehicle system

Microstructures and Mechanical Properties of an Ultrahigh‐Strength and Ductile Medium‐Carbon High‐Silicon Spring Steel

Herein, the microstructure and mechanical properties of medium‐carbon high‐silicon spring steel are studied, which is alloyed and/or microalloyed with Cr, Mo, Ni, Cu, Nb, and V, and subjected to various austenitizing and tempering processes. The steel has a mixed microstructure consisting of fine lath martensite, nanoscale cementite, ε‐carbide, and an amount of retained austenite (10.4 vol%). At room temperature, the steel exhibits ultrahigh tensile strength (2002 MPa) and yield strength (1775 MPa) with total elongation of 11.1%, while a good toughness is obtained characterized by Charpy impact energy as 38 J. The ultrahigh strength of this steel is attributed to the dislocation and precipitation strengthening, while the excellent total elongation is closely associated with the retained austenite. In addition, tempered martensite embrittlement is found in this steel when tempered at 450 °C. This is due to the retained austenite decomposition and cementite coarsening along the prior austenite grain and martensite laths.

Effect of 1.5 wt% Copper Addition and Various Contents of Silicon on Mechanical Properties of 1.7102 Medium Carbon Steel.

Requirements for mechanical properties of steels are constantly increasing, and the combination of quenching and tempering is the method generally chosen for achieving high strength in medium carbon steels. This study examines the influence of various silicon contents from 1.06 to 2.49 wt% and the addition of copper (1.47 wt%) on the behavior of 1.7102 steel starting with the as-quenched state and ending with the tempered condition at the temperature of 500 °C. The microstructure was characterized by SEM and TEM, the phase composition and dislocation density were studied by XRD analysis, and mechanical properties were assessed by tensile and hardness testing, whereas tempered martensite embrittlement was assessed using Charpy impact test and the activation energy of carbide precipitation was determined by dilatometry. The benefit of copper consists in the improvement of reduction of area by tempering between 150 and 300 °C. The increase in strength due to copper precipitation occurs upon tempering at 500 °C, where strength is generally low due to a drop in dislocation density and changes in microstructure. The increasing content of silicon raises strength and dislocation density in steels, but the plastic properties of steel are limited. It was found that the silicon content of 1.5 wt% is optimum for the materials under study.

The Role of Retained Austenite in Tempered Martensite Embrittlement of 4340 and 300-M Steels Investigated through Rapid Tempering

Tempered martensite embrittlement (TME) is investigated in two medium carbon, high strength steels, 4340 (low silicon) and 300-M (high silicon), via rapid (1, 10, or 100 s) and conventional (3600 s) tempering. Rapid tempering of 4340 diminishes the depth of the TME toughness trough, where improvements in impact toughness correspond to the suppression of retained austenite decomposition. In 300-M, retained austenite decomposition is suppressed to an even greater extent by rapid tempering. While toughness improves overall after rapid tempering, TME severity remains consistent in 300-M across the tempering conditions examined. Through interrupted tensile tests, it was found that the 300-M conditions that exhibit TME are associated with mechanically unstable retained austenite. Unstable retained austenite is shown to mechanically transform early in the deformation process, presumably resulting in fresh martensite adjacent to interlath cementite that ultimately contributes to TME. The present results emphasize the role of both the thermal decomposition and mechanical transformation of retained austenite in the manifestation of TME.

A New General Paradigm for Understanding and Preventing Li Metal Penetration through Solid Electrolytes

Summary The use of lithium (Li) or sodium (Na) metal anodes together with highly ion-conductive solid electrolytes (SEs) could provide batteries with a step improvement in volumetric and gravimetric energy densities. Unfortunately, these SEs face significant technical challenges, in large part because Li and Na dendrites can penetrate through SEs, leading to short circuits. The ability of such a soft material (Li or Na metal) to penetrate through ceramic is surprising from the point of view of models widely used in the Li-battery field. We introduce a concept, new to the battery field, for preventing penetration of lithium dendrites through SEs by putting the SE surfaces into a state of residual compressive stress. For a sufficiently high compressive stress, cracks have difficulty forming, and cracks that do form are forced to close, inhibiting dendrite penetration. This approach is widely used to solve commercially important stress corrosion cracking problems in metals and static fatigue problems in ceramics and glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li-ion transport rate through a SE is substantially reduced when the SE is under compression. Our molecular dynamics calculations for Li-ion transport through a common SE demonstrate that the introduction of even very high residual compressive stresses (∼10 GPa) has only a modest effect on Li-ion transport kinetics, suggesting that this approach is viable and capable of providing a new paradigm for developing high-performance and mechanically stable SEs.

Perspectives on Quenching and Tempering 4340 Steel

Steels are ubiquitous due to their affordability and the landscape of useful properties that can be generated for engineering applications. But to further expand the performance envelope, one must be able to understand and control microstructure development by alloying and processing. Here we use multiscale, advanced characterization to better understand the structural and chemical evolution of AISI 4340 steel after quenching and tempering (Q&T), including the role of quench rate and short-time, isothermal tempering below 573 K (300 °C), with an emphasis on carbide formation. We compare the microstructure and/or property changes produced by conventional tempering to those produced by higher temperature, short-time “rapid” tempering. We underscore that no single characterization technique can fully capture the subtle microstructure changes like carbon redistribution, transition carbide and/or cementite formation, and retained austenite decomposition that occur during Q&T. Only the use of multiple techniques begins to unravel these complexities. After controlled fast or slow quenching, η transition carbides clearly exist in the microstructure, likely associated with autotempering of this high martensite start temperature (Ms) steel. Isothermal tempering below 598 K (325 °C) results in the relief of carbon supersaturation in the martensite, primarily by the formation of η transition carbides that exhibit a range of carbon levels, seemingly without substitutional element partitioning between the carbide and matrix phases. Hagg transition carbide is present between 300 °C and 325 °C. After conventional tempering at or above 598 K (325 °C) for 2 h, cementite is predominant, but small amounts of cementite are also present in other conditions, even after quenching. Previous work has indicated that silicon (Si) and substitutional elements partition between the cementite, which initially forms under paraequilibrium conditions, and the matrix. Phosphorous (P) may also be preferentially located at cementite/matrix interfaces after high temperature tempering. Slower quench rates result in greater amounts of retained austenite compared to those after fast quenching, which we attribute to increased austenite stability resulting from “autopartitioning”. Rapid, high temperature tempering is also found to diminish tempered martensite embrittlement (TME) believed to be associated with the extent of austenite decomposition, resulting in mechanical properties not attainable by conventional tempering, which may have important implications with respect to industrial heat treatment processes like induction tempering. Controlling the amount and stability of retained austenite is not only relevant to the properties of Q&T steels, but also next-generation advanced high strength steels (AHSS) with austenite/martensite mixtures.

Rapid Tempering: Opportunities and Challenges

Rapid tempering involves shorter times and faster heating rates compared to conventional tempering. Utilizing rapid tempering via industrial processes, such as induction heating, not only offers the opportunity for improved mechanical properties, but also for reduced processing times and energy costs. The current study demonstrates an improvement in toughness with rapid tempering by examining impact toughness at a constant tempering parameter for short-time (1 s) and conventional (3600 s) tempering treatments of 4340 steel. Fracture behavior is found to complement the observed toughness behavior, where higher impact energy conditions are associated with a greater percentage of ductile fracture. The role of the Hollomon–Jaffe tempering parameter, as well as the use of hardness as a metric to characterize degree of tempering, is discussed in light of the results comparing rapid and conventional tempering conditions. This study indicates promising mechanical properties associated with short-time tempering, especially within the tempered martensite embrittlement (TME) regime, and exposes challenges associated with predicting performance strictly based on hardness when rapid tempering is utilized.

Effect of high magnetic field on the microstructure evolution and mechanical properties of M50 bearing steel during tempering

The effect of high magnetic field (12 T) on the microstructure evolution and mechanical properties during tempering of M50 bearing steel have been investigated for evaluating the application prospect of magnetic field in steels. Microstructural observation shows that the high magnetic field can obviously accelerate the decomposition of retained austenite (RA) during the moderate tempering (at 300 or 380 °C), which could be ascribed to the increased dislocation in RA by the magnetic field-induced deformation of martensite. Meanwhile, the cementite precipitation is also increased by the applied high magnetic field because of the higher nucleation rate. Moreover, at higher tempering temperature of 530 °C, the Fe content in the precipitated carbides increases after applying high magnetic field, which is related to the different magnetization of carbides caused by various element content. Characterization of mechanical properties indicates that the tempered martensite embrittlement is significantly intensified during tempering at 380 °C, which is resulted from the accelerated RA decomposition and improved cementite precipitation under high magnetic field. When tempered at 530 °C in high magnetic field, the dislocation recovery is retarded and then the Vickers hardness and ultimate tensile strength (UTS) are thus improved by dislocation strengthening. This finding suggests the potential of high magnetic field (12 T) in optimizing the properties of M50 bearing steel during tempering (530 °C).

The Effect of Tempering on the Microstructure and Mechanical Properties of a Novel 0.4C Press-Hardening Steel

In this paper, the effects of different tempering temperatures on a recently developed ultrahigh-strength steel with 0.4 wt.% carbon content were studied. The steel is designed to be used in press-hardening for different wear applications, which require high surface hardness (650 HV/58 HRC). Hot-rolled steel sheet from a hot strip mill was austenitized, water quenched and subjected to 2-h tempering at different temperatures ranging from 150 °C to 400 °C. Mechanical properties, microstructure, dislocation densities, and fracture surfaces of the steels were characterized. Tensile strength greater than 2200 MPa and hardness above 650 HV/58 HRC were measured for the as-quenched variant. Tempering decreased the tensile strength and hardness, but yield strength increased with low-temperature tempering (150 °C and 200 °C). Charpy-V impact toughness improved with low-temperature tempering, but tempered martensite embrittlement at 300 °C and 400 °C decreased the impact toughness at −40 °C. Dislocation densities as estimated using X-ray diffraction showed a linear decrease with increasing tempering temperature. Retained austenite was present in the water quenched and low-temperature tempered samples, but no retained austenite was found in samples subjected to tempering at 300 °C or higher. The substantial changes in the microstructure of the steels caused by the tempering are discussed.

Effects of Short-Time Tempering on Impact Toughness, Strength, and Phase Evolution of 4340 Steel Within the Tempered Martensite Embrittlement Regime

While tempering reactions under furnace conditions have been extensively studied, short-time tempering at higher temperatures, as might apply to induction tempering, has not been thoroughly explored. The mechanical behavior and phase development of short-time (1, 10, and 100 seconds) and conventionally (3600 seconds) tempered 4340 steel are compared at an equivalent degree of tempering defined by the Hollomon–Jaffe tempering parameter. The tempering parameter accurately predicts hardness values across short-time and conventional tempering conditions, but is less able to describe phase evolution associated with short-time tempering. Room temperature Charpy impact toughness and ductile-to-brittle transition temperature systematically improve with shorter tempering times at an equivalent tempering parameter. In particular, rapid tempering significantly increases toughness within the tempered martensite embrittlement regime. Relative to conventional tempering, shorter tempering times exhibit higher retained austenite content for a given tempering parameter, although no systematic or significant difference in cementite or transition carbide content is observed between time conditions. The retained austenite decomposition behavior indicates that the relationship between the classical tempering stages is altered at short tempering times, where increased overlap and/or “re-ordering” of stage II and III tempering are observed.

Effect of Tempering Mode on the Microstructure and Mechanical Properties of a Lean Alloy Martensitic Steel: Conventional Reheating Versus Induction Reheating

The present study elucidates the effect of the reheating rate and tempering temperature on the type, size, and distribution of carbides/cementite formed within martensite (M)/lower bainite (LB) during the tempering of a high-strength low-alloy (HSLA) steel. A comparison is made between conventional reheating and induction reheating. All the experimental samples were quenched from the austenitization temperature of 920 °C for 30 min. An ultra-high yield strength of 1180 MPa and a good toughness of 94 J at 0 °C were obtained after tempering at 400 °C for 15 min by induction reheating (~ 100 °C/s), while in the case of conventional reheating (~ 2 °C/s) and tempering at 400 °C for 15 min, the steel had a high strength of 1159 MPa, but a poor toughness of 47 J at 0 °C. After induction reheating to 500 °C and tempering for 15 min, a higher toughness was obtained (91 J at 0 °C), but the strength was decreased to 967 MPa. Microstructural characterization revealed that fine needle-like carbides (e-Fe2.4C) were obtained by induction reheating to 400 °C and tempering for 15 min. However, samples tempered at 400 °C by conventional reheating and 500 °C by induction reheating both precipitated coarse θ-Fe3C. In the former, θ-Fe3C precipitated at locations that were previously austenite grain boundaries and led to tempered martensite embrittlement (TME). In the latter, Ostwald ripening occurred, and the precipitates could not maintain high strength. The excellent mechanical properties of induction reheated steel (400 °C, 15 min) are attributed to the following aspects: (a) The selection of appropriate tempering temperature did not lead to decarburization and ensured high strength, and (b) a high reheating rate promoted carbide dispersion and avoided TME in steel.

The Relationship between Strength and Toughness in Tempered Steel: Trade‐Off or Invariable?

Two different types of strength‐toughness relations are investigated in tempered steel (tempered at 25–420 °C). It is found that there is a trade‐off relation between strength‐fracture toughness and becomes invariable relation between strength‐impact toughness. The effect of microstructure on the materials toughness is related to the manner of energy consumption, which should be attributed to the notch root radius of the specimen. The coarse carbides and dislocation density inside the martensite lath have distinct effects on the energy for crack initiation (impact toughness) and crack growth (fracture toughness). Additionally, the mechanism of coarse carbides on the tempered martensite embrittlement is elaborated deeply.

Effect of trace ferrite on mechanical properties of a tempered HSLA steel

The influence of trace ferrite on tensile and impact properties of a HSLA steel quenched from 850 °C and tempered at different temperatures has been examined. SEM and TEM analyses show that the quenched microstructure is martensite with ~3 vol% ferrite, and the evolution of martensite at different tempering conditions is same to that of specimens with single martensite. The tensile strength is determined by the strength of ferrite and martensite according to the law of mixtures. The yield strength is a result of precipitation strengthening and tempering softening, and the yielding point appears when the mobile dislocations are insufficient. Both yield and tensile strength are reduced by the trace ferrite when tempering at temperatures 180–350 °C due to the strength difference between ferrite and martensite. Moreover, the ferrite improves the elongation but deteriorates the impact property for its low strength, and the tempered martensite embrittlement is advanced to lower tempering temperature 250 °C due to the carbide precipitation facilitated by the trace ferrite. However, the influence of trace ferrite is reduced after tempering at higher temperatures 450 °C and 550 °C. High temperature tempering is essential for specimens with trace ferrite to achieve optimal comprehensive mechanical properties.

Influence of boron and nitrogen on tempered martensite embrittlement

The propensity of boron-containing and molybdenum-containing steels to tempered martensite embrittlement was carried out. Investigation conducted by serial curves of impact strength and fractographical studying of samples fracture obtained with a scanning electron microscope (SEM). Fracture of samples depending on conditions of their tempering (temperature and cooling rate) is determined. The study has shown that commercial boron-containing steel does not show a tendency to tempered martensite embrittlement. It was found that addition of boron into steel with a low concentration of nitrogen (0.004 mass%) leads to a decrease of impact strength, but does not embrittle steel. An increase of the nitrogen concentration to 0.016 wt. % leads to embrittlement, regardless of the boron concentration introduced up to 0.025% by weight.

Microstructural Characteristics and Mechanical Properties of Low-Alloy, Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV- and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature, regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.

Rapid Thermal Processing to Enhance Steel Toughness

Quenching and Tempering (Q&T) has been utilized for decades to alter steel mechanical properties, particularly strength and toughness. While tempering typically increases toughness, a well-established phenomenon called tempered martensite embrittlement (TME) is known to occur during conventional Q&T. Here we show that short-time, rapid tempering can overcome TME to produce unprecedented property combinations that cannot be attained by conventional Q&T. Toughness is enhanced over 43% at a strength level of 1.7 GPa and strength is improved over 0.5 GPa at an impact toughness of 30 J. We also show that hardness and the tempering parameter (TP), developed by Holloman and Jaffe in 1945 and ubiquitous within the field, is insufficient for characterizing measured strengths, toughnesses, and microstructural conditions after rapid processing. Rapid tempering by energy-saving manufacturing processes like induction heating creates the opportunity for new Q&T steels for energy, defense, and transportation applications.