Intercritical Temperature Range Research Articles

Erosive wear of dual-phase low and medium carbon steels after quenching from the intercritical temperature range

Исследован износ нормализованных (N) и закаленных на двухфазную структуру (мартенсит + феррит) (DP) низкоуглеродистых (0,08 % C) и среднеуглеродистых (0,42 % C) сталей при воздушно-струйной эрозии под действием частиц оксида алюминия под углом удара 15° и 90°. Двухфазная структура DP сталей с разным количеством мартенсита получена путем выдержки при температуре межкритической области 745 °C в течение различного времени (2, 3, 4 мин) с последующей закалкой в воде. Показано, что твердость и предел прочности низко- и среднеуглеродистых DP сталей повышаются с увеличением содержания мартенсита в структуре, а скорость установившейся эрозии уменьшается с увеличением количества мартенсита и угла удара. Установлено, что при эрозии изменение структуры поверхности происходит за счет растрескивания, образования кратеров и срезания при угле удара 90°, тогда как при 15° наблюдаются царапание и выкрашивание.

Dependence of mechanical properties on the phase composition of intercritically annealed medium-Mn steel as the main competitor of high-strength DP steels

Depending on the alloy composition, intercritical annealing may provide different phases in the microstructure. For low-alloyed dual-phase (DP) steels it is usually ferrite and martensite, while for medium-Mn steels retained austenite is also formed. In a present study, a wide intercritical temperature range was applied to a 5% Mn steel to investigate possible microstructure combinations: ranging from fully ferritic, through ferritic-austenitic, multiphase, to fully martensitic, which were next investigated in terms of mechanical properties to clarify the behavior of this type of material. The obtained results together with technological issues and economic indicators were next compared to mechanical properties of typical DP steels in order to assess the possibility of replacing this material in car production. The mechanical properties were evaluated using static tensile and hardness tests. The phase composition was determined qualitatively and quantitatively using dilatometry, X-ray diffraction measurements, and electron backscatter diffraction analysis. The results suggest that both initial austenite and martensite fractions have a decisive influence on the yielding and elongation of steel; however, the tensile strength depends mainly on the sum of martensite initially present in the microstructure and the strain-induced martensite formed from the plastically deformed austenite regardless of the initial retained austenite—martensite ratio. The results indicate superior total elongation of medium-Mn steels reaching 30% compared to DP steels with a similar strength level in the range between 900 and 1400 MPa. However, medium-Mn steels could be a significant competitor to dual phase steels only if some technological problems like discontinuous yielding and serrations are significantly reduced.

Special Features of the α → γ Transformation of Radially Forged Low-Carbon Steel in the Intercritical Temperature Range

Special Features of the α → γ Transformation of Radially Forged Low-Carbon Steel in the Intercritical Temperature Range

Structure formation and properties of sheet structural steels in the modes of ordinary and combined heat treatment with exposure in the intercritical temperature range

The features of the structure, the nature of the distribution of alloying elements and impurities between the structural components of sheet low-alloy steels that have undergone heat treatment with exposure in the intercritical temperature interval (ICTI), the structure formation and mechanical properties of these steels after the combined regimes of heat treatment with exposure in the ICIT were investigated. The analysis of the distribution of elements among the structural components of the studied low-alloy steels allowed us to establish that during heat treatment with direct exposure in the ICTI, such elements as manganese, chromium, aluminum, vanadium, sulfur, and phosphorus are concentrated mainly in the strengthening martensitic-bainite phase, while nickel, silicon and titanium are evenly distributed between the strengthening phase and ferrite. The conducted research on the example of sheet low-alloy steel 14G2 shows that the effects of grinding of ferritic grain, peаrlitic and martensitіс (bainite) components during combined processing with exposure in ICTI can be caused by implementation at low (450-550ºС) temperatures of isothermal decomposition the mechanism of multiple nucleation of ferrite microcrystals. It was established that the combined regimes with exposure in ICTI allow, without the application of hardening and tempering operations, to achieve in sheet low-alloy steel 14G2 mechanical properties at the level of strength class 390 according to DSTU 8541:2015 with yield strength values of 422-425 МPа and with high plasticity and impact viscosity Therefore, the use of these modes in a modified form can be recommended for the creation of end-to-end energy-saving technologies of heat treatment with exposure in the ICTI of low-alloyed steel sheet in the flow of thick-sheet mills equipped with normalization units. Keywords: structure formation, intercritical temperature interval, isothermal decomposition, martensitic-bainite phase, combined regime.

Effect of Heating and Cooling Rates on the Microstructure of a Double Soaked Medium-manganese Al-alloyed Steel

This contribution addresses the effect of heating and cooling rates on the microstructure of a double soaked medium-Mn 0.16C-5 Mn‑1.6Al‑0.2Si steel with an increased Al addition. The obtained results show that the differences in applied heating and cooling rates have a small effect on the microstructure of the investigated steel. Some differences in the dilatometer response during final cooling to room temperature were noted. Moreover, some changes in the fraction of retained austenite at particular heating/cooling variants were identified using the XRD method. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and dilatometry were applied to investigate microstructural changes, which occurred during second annealing step in the intercritical temperature range.

Effect of post-processing heat treatment on hydrogen embrittlement susceptibility of API 5L X70 pipeline steel

Three different heat treatments were applied on X70 pipeline steel: below transformation temperature (X70-720), in the intercritical temperature range (X70-780), and in the intercritical temperature range + tempering (X70-780/720). Then, cathodic hydrogen charging was conducted on these heat-treated samples to study the effect of hydrogen ingress on the degradation of their mechanical properties in relation to their microstructural characteristics. The mechanical test results showed a reduction in elongation and toughness of all hydrogen-charged samples, as well as a strong correlation between hardness and hydrogen embrittlement (HE) susceptibility. However, the treatments, which resulted in the formation of cementite precipitates within the ferritic matrix (X70-780/720), showed improved resistance to HE. Also, the crystallographic texture analysis revealed that a high volume fraction ratio of {111} || ND to {100}<011> contributed to the significant improvement of HE resistance in X70-780/720, while a low volume fraction ratio resulted in the least HE resistance in X70-720 specimens.

Fatigue fracture of steel with ferrite-martensite composite structure

The article presents results of the fatigue tests of natural steel composite material for cyclic bending in a zero loading cycle. Natural ferritemartensite composite (NFMC) has a structure of alternating layers of viscous ferrite and strong martensite, which determines special mechanism of crack deceleration under loading. Zero loading cycle presumes presence of tensile forces directed only in one side, which makes it possible to avoid work hardening of crack edges during its growth. A diagram of fatigue fracture was constructed using data obtained on kinetics of fatigue crack propagation and its growth rate, depending on the number of vibration cycles. Comparison of test results for the samples made of steel of the same chemical composition was carried out. In one case, the secondary sorbite structure ran through the traditional heat treatment. In the other, quenching of the initial row ferrite-pearlite structure in intercritical temperature range, led to obtaining ferrite-martensite composite layered structure. These materials had the same hardness, but the difference in structure organization caused the NFMC structure steel advantage in terms of resistance to fracture under cyclic loading. When crack approaches the martensite-ferrite interface, delamination occurs in ferrite due to tensile stresses parallel to the crack plane. Growth of a crack stops before additional energy is supplied for a new crack generation under conditions close to the uniaxial stress state. Method for determining characteristics of kinetics of crack growth under fatigue loading is presented and recommended for testing steels and alloys under conditions of cyclic load changes.

Hot Ductility Analysis and Flow Stress Prediction of (C-Mn-S-Al-Nb-V-Ti) Micro-alloyed Steel

The objective of this work is to study the hot ductility of (C-Mn-S-Al-Nb-V-Ti) micro-alloy steel of industrial production whose initial structural state is rolling stock. To simulate the thermomechanical treatments imposed we have deformed by pulling our samples after subjecting them to a solution treatment at 1200 °C and a precipitation treatment cycle before deformation. Hot deformations were carried out at temperatures from 700 °C and 1150 °C and deformation rates ranging between 10-2 s-1 and 5.10-4 s-1. The results show a decrease in hot ductility. Minimum values of hot ductility are determined at 800°C, and another decrease in hot ductility was observed at 900°C. Ferrite precipitation is observed at austenitic grain boundaries in the intercritical temperature range, causing intergranular embrittlement. Precipitation makes the hot ductility curve wider and deeper around 900°C. Hot ductility losses can explain by the presence of precipitates in the austenitic region and the presence of the two-phase structure in the intercritical region. Finally, based on experimental stress-deformation data, artificial neuron network (ANN) methods were used to predict flow stress of (C-Mn-S-Al-Nb-V-Ti) micro-alloy steel. This model is more effective in predicting flow stress and results can also be used in the mathematical simulation of hot metal formation processes.

Tailoring the Austenite Fraction of a Cu and Ni Containing Medium-Mn Steel via Warm Rolling

Developing medium-Mn steels (MMnS) demands a better understanding of the microstructure evolution during thermo-mechanical treatments (TMTs). This study demonstrates the relationship among processing, microstructure, and mechanical properties of a warm-rolled medium-Mn steel (MMnS) containing 1.5 wt. % Cu and 1.5 wt. % Ni. After short-time warm rolling (WR) in an intercritical temperature range, a significant quantity (40.6 vol.%) of austenite was reverted and retained after air cooling. The microstructure and tensile properties of the WR specimens were compared with two typical process routes, namely hot rolling+ cold rolling+ annealing+ tempering (CRAT) and warm rolling+ annealing+ tempering (WRAT). The WR specimen exhibited comparable tensile properties with the CRAT specimens (967 MPa yield strength, 1155 MPa tensile strength, 23% total elongation), with a remarkably shortened process route, which was derived from the dislocation accumulation and austenite reversion during rolling. The WR route stands out among the traditional CRAT and the extended WRAT routes for its excellent tensile properties and compact processing route.

Ultrafine Ductile and Austempered Ductile Irons by Solidification in Ultrasonic Field

In this research, ultrasonic melt treatment (UST) was used to produce a new ultrafine grade of spheroidal graphite cast iron (SG iron) and austempered ductile iron (ADI) alloys. Ultrasonic treatment was numerically simulated and evaluated based on acoustic wave streaming. The simulation results revealed that the streaming of the acoustic waves propagated as a stream jet in the molten SG iron along the centerline of the ultrasonic source (sonotrode) with a maximum speed of 0.7 m/s and gradually decreased to zero at the bottom of the mold. The metallographic analysis of the newly developed SG iron alloy showed an extremely ultrafine graphite structure. The graphite nodules’ diameter ranging between 6 and 9 µm with total nodule count ranging between 900 to more than 2000 nodules per mm2, this nodule count has never been mentioned in the literature for castings of the same diameter, i.e., 40 mm. In addition, fully ferritic matrix was observed in all UST SG irons. Further austempering heat treatments were performed to produce different austempered ductile iron (ADI) grades with different ausferrite morphologies. The dilatometry studies for the developed ADI alloys showed that the time required for the completion of the ausferrite formation in UST alloys was four times shorter than that required for statically solidified SG irons. SEM micrographs for the ADI alloys showed an extremely fine and short ausferrite structure together with small austenite blocks in the matrix. A dual-phase intercritically austempered ductile iron (IADI) alloy was also produced by applying partial austenitization heat treatment in the intercritical temperature range, where austenite + ferrite + graphite phases coexist. In dual-phase IADI alloy, it was established that introducing free ferrite in the matrix would provide additional refinement for the ausferrite.

Вплив Q&P-обробки на мікроструктуру та механічні властивості низьковуглецевої TRIP-assisted сталі

. The article describes the effect of heat treatment according the «Quenching and Partitioning» (Q&P) technology on the phase-structural status and mechanical properties of low-alloy 20Si2Mn2Nb steel. It is found that the standard heat treatment which is an isothermal annealing at 300°C (holding for 20 min) after heating in intercritical temperature range (ITR) provides a moderate complex of mechanical properties at low impact toughness. The heat treatment according to the ITR/Q&P scheme (austenitization in intercritical temperature range with a quenching cooling to 200°C and subsequent holding at 350-400°C for 5-20 min) significantly increases the ductile behavior of low carbon TRIP-assisted steels compared to its standard heat treatment. The positive effect of ITR/Q&P heat treatment on steel ductility was caused by the presence of high content of ductile ferrite (54,4 vol. %) in the steel structure. Heat treatment according to the FA/Q&P scheme (FA-full austenitization) with quenching cooling to 235°C and subsequent holding at 350-400°C for 5-20 minutes provided a considerable increase in the strength properties and impact toughness of steel. High strength level was ensured by the formation of a multiphase structure with a high volume fraction of hard martensite phase. The maximum combination of strength and ductility (PSE » 20 GPa·%) was provided by ITR/Q&P treatment conducted according to modes A770Q200P400(5 min) and A770Q200P400(20 min). The improvement in mechanical properties was associated with the formation of multiphase micro-structure which contains an increased amount of retained austenite. It is shown that Q&P treatment with heating in the intercritical temperature range notably improved a steel ability to work-hardening under cold plastic deformation

Hot ductility behavior of a Fe-0.3C-9Mn-2Al medium Mn steel

The hot ductility of a Fe-0.3C-9Mn-2Al medium Mn steel was investigated using a Gleeble3800 thermo-mechanical simulator. Hot tensile tests were conducted at different temperatures (600–1300°C) under a constant strain rate of 4 × 10−3 s−1. The fracture behavior and mechanism of hot ductility evolution were discussed. Results showed that the hot ductility decreased as the temperature was decreased from 1000°C. The reduction of area (RA) decreased rapidly in the specimens tested below 700°C, whereas that in the specimen tested at 650°C was lower than 65%. Mixed brittle-ductile fracture feature is reflected by the coexistence of cleavage step, intergranular facet, and dimple at the surface. The fracture belonged to ductile failure in the specimens tested between 720–1000°C. Large and deep dimples could delay crack propagation. The change in average width of the dimples was in positive proportion with the change in RA. The wide austenite-ferrite inter-critical temperature range was crucial for the hot ductility of medium Mn steel. The formation of ferrite film on austenite grain boundaries led to strain concentration. Yield point elongation occurred at the austenite-ferrite intercritical temperature range during the hot tensile test.

ЗАКОНОМІРНОСТІ ФОРМУВАННЯ ЗАЛИШКОВОГО АУСТЕНІТУ В СКЛАДНОЛЕГОВАНИХ СТАЛЯХ (ОГЛЯД)

The purpose of the analysis is to determine the modes of heat treatment that affect the quality parameters of complex alloy steels. Steels, which contain chromium and significant amounts of other alloying elements (nickel, vanadium, molybdenum, etc.) are used for the production of the different metal products, which are working in hard operating conditions: rolling rolls, power equipment parts, piercing plug, etc. Retained austenite is always present in such steels and affects the properties of the final metal products. Should be include the following factors that increase the amount of retained austenite in complexly alloy steels: thermal stabilization of the austenite; redistribution of the carbon between the α-phase and austenite during cooling in the temperature range of the intermediate or bainitic transformation or during isothermal holding slightly above the Мp point; hardening from the intercritical temperature range; changing the cooling rate; the actual content of carbon and alloying elements; changing in the austenitization temperature, isothermal holding and heating temperature during hardening. The indicated factors should be considered during determining the final parameters of the heat treatment mode to control the amount of retained austenite in complexly alloy steels. Isothermal hardening from the γ-α-area is proposed to obtain a multiphase structure of 38HN3MFA steel. Using of the cryogenic treatment for the strengthening mode of the 38HN3MFA steel promotes the transformation of retained austenite into martensite and significantly increases the microhardness of the structural components of the steel.

Effect of Heating in the Intercritical Temperature Range on Formation of Austenite and Structure of Ultralow-Carbon Steel

Using micrographic, dilatometric, and high-temperature XRD methods, the kinetics of formation of austenite is studied under continuous heating of ultrafine-grained 08G2B steel in two initial states—after quenching and normalization.

Micro-hardness variation of micro-phases during spheroidisation of AISI4340 steel

AISI4340 steel is the medium carbon low-alloy steel has got excellent prominence as a construction material. This steel is spheroidised on heating to the intercritical temperature range by different time intervals with the objective of increasing the number and fineness of the spheroids of cementite particles. A 9 hours, spheroidisation shows better spheroids of cementite particles. To study the effect of initial room temperature structure on spheroidisation primary heat treatments like normalising and hardening are carried out for as-bought steel. Tensile property (yield strength, ultimate tensile strength and percentage elongation), hardness (Rockwell C scale) and micro-structural features and micro-hardness of micro-phases (ferrite and cementite) are studied. A continuous reduction in the tensile strength, hardness and improvement in ductility, is observed with the increase in spheroidisation time is observed in all the spheroidisation paths. Also, increased spheroidisation time shows a lesser aspect ratio for spheroids. The micro-structure shows partial spheroidisation in all the conditions with more number of spheroids in normalised condition and finer size in hardened. Drastic reduction in strength and hardness values are observed in the hardened condition during spheroidisation.

Effect of intercritical processing temperature on mechanical properties, microstructure and microhardness of ferrite - bainite medium carbon dual phase steels

This research focusses on the dual phase (ferrite and bainite) treatment for medium carbon steels containing same carbon content (different alloying elements) and mechanical properties, as well as microhardness and microstructure. Dual phase treatment is carried out by heating normalized specimen to intercritical temperature range (750, 770 and 790 °C), followed by isothermal holding in the bainitic temperature (350 °C) and cooling to room temperature in still air. The effect of chemical composition (alloying elements) of steel on the dual phase morphology intern on the mechanical properties like, tensile strength, elongation, hardness and impact energy is also presented. Mechanical property results are in par with the microstructure and microhardnss of dual phase obtained. Lower intercritical temperature (750 °C) has resulted in lesser amount of bainite phase with poor mechanical properties in dual phase structure, whereas higher temperature (790 °C) increases bainite phase content with considerable improvement in mechanical properties. Microhardness data obtained supports the qualitative analysis of microstructure in all steels under consideration. It is found that intercritical temperature of dual phase treatment and material alloying elements are the major contributing factors on the dual phase microstructure and mechanical properties.

Element partitioning in low-carbon Si2Mn2CrMoVNb TRIP-assisted steel in intercritical temperature range

The present paper is aimed at the study of the kinetics of Mn, Si, Cr partitioning in 0.2wt%C-Si2Mn2CrMoVNb TRIP-assisted steel under the annealing at 770 oC and 830 oC to be within the intercritical temperature range. The work was fulfilled using SEM, EDX, dilatometry, and hardness measurements. It was found that under heating a redistribution of the alloying elements between ferrite and austenite took place. Specifically, silicon partitioned to ferrite while chromium diffused to austenite with distribution coefficient values of 1.12-1.21 (KSi) and 0.75-0.86 (KCr). Manganese was found to partition to a much greater extent resulting in a distribution coefficient of KMn=0.38-0.50 and 2.6 times higher concentration in austenite as compared to ferrite. As annealing temperature raised from 770 oC to 830 oC the elemental partitioning was accelerated, followed by the decrease in manganese content in austenite (by 1.44 time) and ferrite (by 1.34 time) caused by an increase in austenite volume fraction. Silicon featured uneven distribution within ferrite to be accumulated at the “martensite/ferrite” interface and near ferrite grain boundaries, while manganese was concentrated in MC carbides. The recommendation for annealing holding was formulated based on elemental partitioning kinetics.

Mechanical Property Association of Dual Phase and Austempered AISI 4140 Normalized Steel

The present investigation intended to analyse the mechanical properties of medium carbon low alloy steel (AISI 4140) under dual phase heat treatment condition. Formation of dual phase, ferrite and martensite (F+M) from normalized condition followed by heating in the intercritical temperature range to form different volume fractions of ferrite and martensite alters tensile, hardness and impact strength of medium carbon steel. Effect of alloying elements in dual phase treatment will change the mechanical properties. Tensile strength and hardness of austempered steel shows higher value followed by dual phase condition. Ductility of normalized shows better as compared to as bought and dual phase, but austempered shows almost similar to normalized result. Impact strength of the austempered is excellent followed by dual phase condition. Heat treated specimen microstructures reveal the different phase’s present.

Calorimetry of the Phase Transformations in Carbon Steels in the Intercritical Temperature Range

Data on the calorimetric effects during the structural phase transformations in iron–carbon alloys (steels) are presented. No relationships are revealed between temperature points A3 (Am) and the calorimetric effect temperatures during steel thermal cycling. The conclusions in favor of the mechanisms of the phase transformations resulting in the observation of these calorimetric effects are presented. The obtained results can be used to study the conditions of heat treatment with quenching of steels from the intercritical temperature range.

The sensitivity of the microstructure and properties to the peak temperature in an ultrafast heat treated low carbon-steel

In this work, we investigate the sensitivity of the microstructure and mechanical properties of an ultrafast heat treated low carbon-steel to the peak temperature. In all studied cases, the steel was heated within the intercritical temperature range (i.e. between the AC1 and AC3 temperatures). Both the peak temperature and soaking time were varied, and their effect on the size, the fraction of individual microstructural constituents and their tensile mechanical response were investigated. It is shown that the increasing peak temperature and soaking time promote austenite formation and recrystallization processes in the ferritic matrix. The highest nanohardness is shown by martensitic grains, while recovered ferrite demonstrated slightly higher nanohardness compared to recrystallized ferrite. The applied heat treatment parameters have a strong effect on the nanohardness of martensite, whereas the nanohardness of ferrite microconstituents is not sensitive to variations of peak temperature and soaking time. The non-recrystallized ferrite is harder than its recrystallized counterpart due to the higher dislocation density of the former. Increasing peak temperatures promote strengthening in the material at the expense of its ductility mainly due to increased martensite fraction. The steel demonstrates enhanced strain hardening ability independently of the peak temperature. Analysis of the experimental results showed that the industrial processing window of ±10 °C may lead to some heterogeneity of the local microstructure in the ultrafast heat treated sheets. However, the latter should not have any negative effect on the overall mechanical behavior of the ultrafast heat treated steel sheets on the macro-scale.