Intercritical Annealing Treatment Research Articles

Comparative Life Cycle Analysis (LCA) Study on Two Wear-Resistant Boron Steels: RAEX450 and 30MnB5

The inter-critical annealing and water quenching heat treatment proposed by us is a practical example of environmental sustainability applied to wear-resistant boron steels. In this research we try to compare the LCAs of two wear-resistant steels, a RAEX450 widely used in the industry today and cheaper 30MnB5 boron steel without alloy elements. The 30MnB5 steel has been given an inter-critical annealing and hardening treatment and the RAEX450 a conventional annealing and hardening treatment. The mechanical properties achieved are similar to or better than those of boron steel; but the energy savings with this steel and its environmental impact are notably more positive for the environment. The LCA of the heat treatment applied to 30MnB5 is significantly more positive than that applied to RAEX450, without affecting its mechanical properties as wear-resistant steel.

Microstructural Characteristics and Tensile Behavior of a Hot-Rolled Medium-Mn Steel (0.25C-8.5Mn-0.5Si-2.5Al) Processed by Intercritical Annealing Treatment

Microstructural characteristics and tensile behavior of a novel hot-rolled medium-Mn steel, 0.25C-8.5Mn-0.5Si-2.5Al (wt.%), under different intercritical annealing temperatures (TIA) were evaluated. The results show that an ultrafine duplex lamellar microstructure with a maximum volume fraction of retained austenite (RA) of 65 vol.% was obtained at TIA of 750 °C. It was found that both the stability of the RA and its stability range play critical role in controlling the sustainable TRIP effect. The product of ultimate tensile strength and total elongation as high as ~ 66 GPa% was obtained at TIA of 725 °C, which is much higher than those of previously investigated medium-Mn steels with similar Mn contents, indicating a great potential for the tested steel. The analysis of the fracture surface revealed that the steel exhibited a kind of micro-delamination fracture, which was found to be primarily nucleated at the interfaces of the transformation-induced martensite and ferrite, and the micro-delamination fracture became more significant with increasing TIA.

Strengthening of an Al0.45CoCrFeNi high-entropy alloy via in situ fabricated duplex-structured composites

Phase precipitation and recrystallization process of Al0.45CoCrFeNi high-entropy alloys (HEAs) were investigated by calculation of phase diagram and electron backscattered diffraction. To reveal the influences of microstructures on the response of mechanical behaviors, the tensile properties, hardness, elastic modulus, and fracture characteristics were thoroughly studied here. The as-processed duplex microstructures demonstrate that intercritical annealing treatments in the dual-phase filed could greatly slow down the recrystallization kinetics, attributing to the in situ precipitation of Ni, Al-rich phase formed initiatively along grain boundary. A series of fine-grained dual-phase HEAs, accompanying with yield strength widely ranging from 558 to 1223 MPa, were well fabricated via annealing the cold-rolled HEAs. Surprisingly, present HEAs exhibit excellent resistance to anneal softening, together with a relatively high recrystallization temperature of 900 °C (~ 0.63 Tm). Additionally, the coupled effect of fine-grain strengthening and precipitation strengthening is successful in manipulating the properties of face-centered-cubic HEA system.

Mechanical property development of a 0.15C–6Mn–2Al–1Si third-generation advanced high strength steel using continuous galvanizing heat treatments

The effects of starting microstructure and intercritical annealing (IA) temperature on the mechanical properties of a prototype 0.15C–6Mn-1.5Al–1Si (wt%) third-generation medium manganese advanced high strength steel (3G AHSS) were determined. The general 3G AHSS mechanical property targets UTS × TE of 24,000–40,000 MPa% were met after a continuous galvanizing line (CGL)-compatible IA treatments at 710 °C and 740 °C for 120 s from an 80% martensite – 20% ferrite starting microstructure due to the as-annealed microstructure having large volume fractions of stable retained austenite. The deformation-induced retained austenite to martensite transformation – or TRansformation-induced plasticity (TRIP) – kinetics were a strong function of IA temperature and resulting retained austenite stability. In particular, the 665 °C intercritical anneal produced retained austenite which only partially transformed during deformation and the 740 °C intercritical anneal produced retained austenite which exhausted the TRIP effect at significantly lower strains than those observed for the 710 °C treatment. It was also shown that significant deformation-induced mechanical twinning occurred in the retained austenite, aiding in sustaining the work hardening rate. Overall, it was determined that medium-Mn 3G AHSS can be produced using CGL-compatible thermal cycles, which holds promise for cost-effective production of these alloys.

Microstructure tailoring for property improvements of DP steel via cyclic intercritical annealing

Multi-step intercritical annealing treatment was applied on the quenched and cold rolled martesnitic microstructures to unravel the effects of thermal cycling on dual phase (DP) steel. During intercritical annealing of the quenched microstructure, hardness decreased with increasing holding time and reached a plateau. However, during intercritical annealing of the cold rolled microstructure, there was a rapid fall of hardness due to recrystallization. Afterwards, hardness increased as a result of austenitization, and then reached a plateau. A rapid grain growth was observed during repetition of intercritical annealing cycle for the DP steel originated from the quenched microstructure with adverse effects on the tensile properties. Conversely, decreased size of martensite islands and formation of chain like morphology of martensite around ferrite grains was observed during thermal cycling of the DP steel originated from the cold rolled microstructure with the consequent enhanced work hardening behavior. Accordingly, the combination of cold rolling and cyclic intercritical annealing was recommended in this work to enhance the work hardening behavior and mechanical properties of DP steels.

Ferrite recrystallisation and intercritical annealing of cold-rolled low alloy medium carbon steel

Static recrystallisation of cold-rolled AISI 4130 medium carbon steel in the ferritic regime and its response to intercritical annealing treatment are studied. A fine and recrystallised microstructure with improved mechanical properties is obtained via subcritical annealing of cold-rolled sheet, where the subsequent intercritical annealing results in the enhancement of tensile strength via the formation of dual-phase microstructure. Intercritical annealing of the cold-rolled sheet is characterised by an initial drop in hardness due to recrystallisation and subsequent rise in hardness as a result of austenitisation. It is revealed that continuous martensite phase can result in a higher deformation resistance. Finally, the effects of intercritical annealing at temperatures below pearlite dissolution finish temperature (Ac1f) are discussed.

The Influence of Microstructural Characteristics on Austenite Formation Kinetics in a Plain Carbon Steel

Since the condition of austenite phase formed during intercritical annealing treatment has a crucial impact on the final microstructure and mechanical properties of dual-phase (DP) steels, detailed investigations with regard to austenite formation in this heat treatment process need to be done. In this study, the effects of different microstructural features, such as ferrite grain size, cementite particle size, and pearlite morphology, on austenite formation in a plain carbon steel (0.165 wt pct C, 1.15 wt pct Mn) during isothermal intercritical annealing treatment have been evaluated. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model was used for modeling the kinetics of austenite formation in this steel during isothermal annealing treatment. The volume fraction of austenite (martensite at room temperature) at different intercritical annealing holding times was calculated by this model using the corresponding results obtained from the experiments. It was found that the starting steel microstructure from which austenite phase is formed has a significant effect on both austenite nucleation and growth. The effect of microstructural parameters on the kinetics of austenite formation in ferrite-cementite steel microstructures was more significant than that in ferrite-pearlite (F-P) steels. An increase in the average cementite particle size or ferrite grain size in ferrite-cementite steels caused a significant decrease in the rate of austenite formation. In F-P steels, on the other hand, pearlite morphology exhibited a small effect on the kinetics of austenite formation while ferrite grain size had a pronounced effect on the rate of austenite formation at the later stage of intercritical annealing, i.e., ferrite to austenite transformation stage.

Effect of intercritical annealing of normalised Nb-Ti-V microalloyed plate steel on microstructural evolution

A homogeneous microstructure is required for consistent mechanical properties in normalised Nb-Ti-V microalloyed plate steels. Frequently, as-hot rolled microalloyed plate steels have a banded microstructure that is persistent even after normalising heat treatment (NHT), and this leads to inconsistencies and some scatter in mechanical properties. Therefore, this work focused on the influence of single-cycle normalising heat treatment (SNHT), double-cycle normalising heat treatment (DNHT) followed by intercritical annealing normalising heat treatment (INHT) on the homogenisation and mitigation of a banded microstructure. The study was conducted on a 0.13C-Nb-Ti-V plate steel grade. The as-hot rolled microstructure was banded and had a 1.13 Anisotropy Index (AI) value. Results from the three thermal cycles revealed that the DNHT and INHT mitigated the pearlite microstructural banding and gave a more homogenized pearlite phase distribution throughout the microstructure, unlike the SNHT that retained the banding. The DNHT also exhibited the finest ferrite grain size, while the INHT exhibited the coarsest. From Vickers hardness measurements (153±5.8 HV, 157±3.6 HV and 166±4.5 HV), the UTS was approximately deduced as, 480, 490 and 530 MPa for the SNHT, DNHT and INHT respectively.

Effect of prior austenite grain size on the evolution of microstructure and mechanical properties of an intercritically annealed medium manganese steel

This paper presents the influence of prior austenite grain (PAG) size on the evolution of microstructure and mechanical properties in an intercritically annealed medium manganese steel (Fe- 8.1 Mn - 0.3 C- 1.1 Si- 1.5 Al). The hot forged alloy was austenitized at two different temperature-time combination to achieve the PAG size of ∼20 μm (sample 1) and ∼40 μm (sample 2). Both the samples retained a small amount of austenite after the austenitization treatment. The sample 1 with smaller PAG size stabilized a significantly higher amount of austenite (∼54.4 ± 0.6 vol%) after the intercritical annealing treatment than the sample 2 (∼39.3 ± 0.7 vol%). Composition as well as dimension of the austenite films were found to be different in the two samples. Sample 1 displayed a better combination of strength and ductility (1096 ± 30 MPa and 32 ± 4.2%, respectively) as compared to the sample 2 (1026 ± 30 MPa and 22 ± 1.9%, respectively). The superior mechanical properties of sample 1 were ascribed to the cumulative effect of the higher volume fraction and lower stability of retained austenite in sample 1 as compared to the sample 2. Fracture surface investigation revealed that the sample 1 experienced microvoid mediated ductile fracture, whereas sample 2 displayed brittle fracture facilitated by crack propagation along certain cleavage facets.

Improved Mechanical Properties of Structural Steel via Developing Bimodal Grain Size Distribution and Intercritical Heat Treatment

Intercritical annealing and tempering heat treatment combined with cold rolling were applied for improvement of tensile properties and strain-hardening rate of structural steel. Coarse dual-phase (DP) microstructure was processed via intercritical heat treatment of the ferritic–pearlitic sheet, which resulted in the increase in tensile strength and disappearance of the yield point phenomenon. However, the tensile toughness was not effectively enhanced, which is indicative of unaltered strength–ductility trade-off. A bimodal-sized ferrite structure was obtained by cold rolling of the DP microstructure followed by subcritical annealing (tempering), where its characteristic features are high yield stress and relatively poor work-hardening response. Afterward, the intercritical annealing of the bimodal steel resulted in the development of fine DP steel with excellent work-hardening capacity and pronounced enhancement of tensile toughness. Conclusively, this work demonstrates the possibility of controlling the tensile properties and work-hardening behavior of conventional steels by simple heat treatment routes with the aid of cold rolling.

Bi-lamellar microstructure in Ti–6Al–4V: Microstructure evolution and mechanical properties

Some characteristic microstructures in Ti–6Al–4V alloy, including lamellar microstructure, martensite microstructure, bimodal microstructure and equiaxed microstructure, have been intensively investigated in the last 30 years. Nevertheless, the bi-lamellar microstructure, which was first defined by G. Lutjering, has not attracted much attention, despite of its significant benefits on various mechanical properties. In the present study, the bi-lamellar microstructure having fine secondary α lamellae (~300 nm) in-between coarse primary α lamellae was obtained after intercritical annealing treatments in the α+β region of a lamellar microstructure. The effects of intercritical annealing temperature (800–960 °C) and cooling rate (water quench~100 °C/min) on the bi-lamellar microstructure as well as their yield strength at room temperature were systematically studied. It was found that the thickness (and volume fraction) of transformed β (βtrans) regions including fine secondary α lamellae with various orientations increased with the increase of the annealing temperature. The thickness of the secondary α lamellae increased with the decrease of the cooling rate. Yield strength of the bi-lamellar microstructure was significantly improved compared with that of the lamellar microstructure composed of coarse primary α plates. Appearance of the peak yield strength of the bi-lamellar microstructure at intermediate annealing temperature was found for the first time. By means of TEM observation, slip line analysis as well as elemental analysis of Al, this peak yield strength was attributed to the combined effects of both increased volume and thickness of βtrans as well as decreased solid solution strengthening of primary α lamellae by Al with the increase of the annealing temperature. The yield strength decreased monotonically with the decrease of the cooling rate. It was believed that the reduced volume and thickness of βtrans and similarity in orientation between primary and secondary α lamellae led to the gradual drop of strength with decreasing the cooling rate.

The Role of Intercritical Annealing in Enhancing Low-temperature Toughness of Fe-C-Mn-Ni-Cu Structural Steel

In this article, an intercritical annealing (IA) process was introduced to the conventional quenching and tempering (QT) heat treatment for a Fe-C-Mn-Ni-Cu structural steel. The corresponding microstructures and mechanical properties of this steel were characterized by scanning electron microscope (SEM) equipped with electron back scattering diffraction (EBSD) and mechanical properties test. The results showed that IA process could lead to a considerable increase in low-temperature toughness for this steel. A mixed microstructure was obtained after IA process had been adopted containing intercritical ferrite and tempered martensite together with a small amount of retained austenite. This steel with mixed microstructure exhibited tensile strength of 961 MPa, relatively lower yield strength of 830 MPa, and a lower yield-to-tensile ratio (Y/T ratio) of 0.86, while a higher total elongation of 22.2 pct was achieved. The reason for this could be attributed to the multiple effect of multi-phase microstructure and deformation-induced transformation of the retained austenite during tensile deformation. The excellent low-temperature toughness was characterized by the Charpy impact energy as 183 J at 153 K (− 120 °C), which was associated with highly stable retained austenite and finer microstructure through reversed transformation during intercritical annealing treatment. These can be considered to increase the resistance to crack initiation and propagation and decrease the ductile-brittle transformation temperature (DBTT).

Retained Austenite Attributes and Mechanical Performance of Different Compositions of TRIP Steel Alloys

The main objective of this work is to correlate the different attributes of retained austenite phase in TRIP steel structure with its mechanical properties. Three different chemical compositions were used to produce TRIP steel alloys. The first alloy contains only silicon as a carbide precipitation inhibitor; in the second alloy, silicon is partially replaced with aluminum, while in the third alloy phosphorus is added to the silicon–aluminum alloy. These compositions were proposed in order to avoid the negative effect of silicon during galvanizing. These three steel alloys were cast, homogenized, hot-forged and then subjected to intercritical annealing treatment followed by cooling and isothermal treatment in salt bath with subsequent air-cooling. Microstructural characterization, using optical microscope, SEM and XRD, along with tensile testing was carried out. Retained austenite attributes include volume fraction, carbon content, total carbon content, retained austenite transformable index and its stability coefficient against strain-induced martensite (SIM) transformation. The effect of these attributes on the mechanical properties of the studied TRIP steel alloys was evaluated using tensile test date, i.e., ultimate tensile strength (UTS), ductility, yield strength to UTS ratio, performance index and strain-hardening exponent. The variation of strain-hardening rate and strain-hardening exponent with strain was also considered. The failure behavior of the studied steel alloys was investigated using SEM on tensile-tested specimens. The results show that Si/Al-alloy has the lowest retained austenite transformable index, lowest stability coefficient against SIM transformation, highest performance index and more stable and uniform variation in strain-hardening rate and strain-hardening exponent with strain. These properties satisfy the industrial constrains of the sheets used in automotive industry relating to better mechanical properties and galvanizability.

Relative effect of C and Mn on strength-toughness of medium Mn steels

ABSTRACTIn order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.

Mechanical Characterization and Microstructural Analysis of AISI 4340 Ferrite-Martensite Dual Phase Steel

—Present investigation deals with the mechanical properties and microstructure of dual phase (DP) steel. Normalizing and intercritical annealing heat treatments were carried out at 900, 750, 770, and 790 ˚C and DP ferrite–martensite steel was obtained. Interaction behavior of both phases has been analyzed through tensile, hardness and impact strength. Comparison of the DP steel with normalized steel has revealed that hardness and tensile strength increases as intercritical temperature increases. Similarly ductility and toughness decreases as the intercritical temperature increases and normalized steel shows better elongation and toughness. The microstructure analysis shows, as the intercritical temperature increases, martensite quantity increases with decrease in ferrite content, which results in improvement in the tensile strength and hardness. The results have shown a decrease in the ductility and toughness as the volume of martensite increases.

Enhancing the Mechanical Properties of Al‐Containing Medium Mn Steel Through Warm Rolling and Intercritical Annealing

This investigation attempts to further enhance the mechanical properties of an Al‐containing medium Mn steel, Fe‐0.25C‐8.5Mn‐0.5Si‐2.5Al (wt%), through warm rolling (WR) and subsequent intercritical annealing (IA) treatments. Two schedules of WR processes were applied with a total thickness reduction of ≈60%. The results show that an ultrafine‐grained (UFG) lamellar structure was well developed after WR, while subsequent IA led to a coarsening of the microstructure as well as the formation of bimodal‐morphological ferrite and retained austenite (RA) grains, i.e., lamellar and equiaxed. Compared with warm rolled samples with clear discontinuous yielding and large yield point elongation, the IA treatment after WR could eliminate this phenomenon mainly due to microstructure coarsening. A significantly enhanced combination of strength and ductility (the product of the ultimate tensile strength and total elongation) as high as ≈75 GPa% was obtained for the WR + IA sample. This enhancement is notably superior to that of previously investigated medium Mn steels with similar Mn contents and is similar or even better than that of twinning‐induced plasticity (TWIP) steels with high‐Mn content. It is thus suggested that WR plus subsequent IA is a promising and feasible approach to further enhance the mechanical properties of medium Mn steel.

Effects of Heat Treatment on Morphology, Texture, and Mechanical Properties of a MnSiAl Multiphase Steel with TRIP Behavior

The effect that the microstructure exerts on the Transformation-Induced Plasticity (TRIP) phenomenon and on the mechanical properties in a multiphase steel was studied. Samples of an initially cold-rolled ferrite–pearlite steel underwent different intercritical annealing treatments at 750 °C until equal fractions of austenite/ferrite were reached; the intercritical treatment was followed by isothermal bainitic treatments before cooling the samples to room temperature. Samples in the first treatment were heated directly to the intercritical temperature, whereas other samples were heated to either 900 °C or 1100 °C to obtain a fully homogenized, single-phase austenitic microstructure before performing the intercritical treatment. The high-temperature homogenization of austenite resulted in a decrease in its stability, so a considerable austenite fraction transformed into martensite by cooling to room temperature after the bainitic heat treatment. Most of the retained austenite transformed during the tensile tests, and, consequently, the previously homogenized steels showed the highest Ultimate Tensile Strength (UTS). In turn, the steel with a ferritic–pearlitic initial microstructure exhibited higher ductility than the other steels and texture components that favor forming processes.

Effect of Different Intercritical Annealing Treatments without and with Overaging on the Mechanical Material Behavior

In the last decade, press‐hardening of manganese‐boron steels for the manufacturing of safety‐relevant components has become a well‐established industrial process. Thereby, very high strengths are achieved with pure martensitic microstructures. However, in certain areas of the car body structure more ductile components are necessary to absorb crash energy. For tailoring the material properties, two‐ or multiphase microstructures can be induced by rolling and heat treatment processes. The press‐hardening steel 22MnB5 which is employed in the present study, is used to manufacture three different ferritic‐martensitic microstructures. These different materials are characterized by classical mechanical models, based on experimental results. For the mechanical description of the initial yield stress depending on the stress state of different multiphase steels, the Hill model is calibrated. The elastic limit and the kinematic hardening of three various heat treatment conditions are determined using conventional tensile tests and the Galimel test, respectively. Therefore, the applied mathematical relations are derived and adapted for the performed investigations. The relationship between the load conditions and the proper application of the equations are explained. The resulting microstructures are characterized and the ramifications with respect to their energy absorption capability are discussed.

Impact of Intercritical Annealing on Retained Austenite and Toughness of a 460 MPa Grade Multiphase Heavy Gauge Plate Steel

The objective of the study described here is to elucidate structure‐property relationship in a 460 MPa multiphase steel with particular emphasis on the influence of intercritical annealing process and retained austenite on mechanical properties. The study suggests that there is significant effect on comprehensive mechanical properties of heavy gauge plate (60 mm thickness) on intercritical annealing, especially the low temperature toughness is increased from 135 to 254 J at the test temperature of −40 °C. Fine retained austenite can be obtained by optimization of interrupted on‐line cooling and partitioning during intercritical annealing heat treatment. The retained austenite stabilized by enrichment with alloying elements during heat treatment replacing coarse martensite and austenite (M/A) constituent is the underlying reason for superior toughness. The study underscores that a practical approach to enhance the toughness of heavy plate is via multiphase control during thermo‐mechanical control processing (TMCP) and off line intercritical annealing.

Microstructural characteristics and tensile behavior of medium manganese steels with different manganese additions

Manganese is normally considered as the most important alloying element in medium Mn steels. However, the influence of Mn additions on the microstructural characteristics and the ensuring mechanical properties has not been much studied. This was addressed in the present study by investigating two variants of medium Mn steels with compositions of 0.2C-7/10Mn-3Al (in wt%), subjected to intercritical annealing (IA) and tensile testing. Results showed that a higher Mn addition can effectively increase the fraction of retained austenite (RA) at ambient temperature. However, the mechanical stability of RA was reduced, due to the lower C concentration partitioned into austenite during IA treatments. The higher fraction and lower mechanical stability of RA in the 10Mn steel gave rise to an enhanced transformation-induced plasticity (TRIP) effect, which was the main contributor to its higher strain hardening rate and improved mechanical properties, confirmed by a dislocation density-based strain hardening model. The yielding behavior and the Portevin-Le Chatelier (PLC) effect of the investigated steels were also found to be altered with different austenite mechanical stability as a consequence of different Mn additions and IA temperatures. Higher deformation-induced martensite transformation (DIMT) kinetics, i.e. lower mechanical stability of austenite, resulted in a lower yield point elongation, and eventually changed the yielding to a continuous manner due to the formation of stress-induced martensite. The PLC effect only occurred in the intermediate austenite stability range, and the critical strain for the onset of jerky flow showed a first decrease and then an increasing trend with higher DIMT kinetics. The observed beneficial effect of Mn additions on mechanical property improvement in medium Mn steels offers a significant aspect for further alloy design of such steels.