Intercritical Heat Research Articles

Effect Of Inter-Critical Heat Treatment On The Microstructure And Properties Of Medium Manganese Steel

Abstract The objective of this work is to study the effect of inter-critical heat-treatment on the microstructure, mechanical and tri-biological property such as wear resistance of medium Mn steel. Initially, the medium Mn steel was forged, rolled, and water quenched. The water quenched sample is then inter-critically heat-treated at 600 and 700°C for 1 hr followed by water quenching. The structural characterization was carried out with the help of XRD and SEM. Inter-critically heat-treated samples are having a higher hardness than the as cast sample. Charpy impact test results reveal the dynamic impact behavior of the inter-critical heat-treated samples is better as compared to the as cast sample. Pin on disc wear test reveals that the wear resistance of the 700°C inter-critically heat-treated sample is higher than the 600°C sample

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.

Corrosion Behavior of Fe-Al-Zn Interphased Dual Phase Steel in Marine Environment

Intercritical heat treatment operation has been in use for the development of Dual Phase Steel (DPS) and has been found to improve the mechanical properties of the steel. In spite of the enhancement a limitation was however observed as to its corrosion susceptibility. In a bid to further enhance the corrosion resistance of the DPS while maintaining its mechanical properties, galvanealing operation was adopted which involving the immersion of the DPS into Al-Zn melt and subsequently subjecting it to annealing operation at 550°C. Weight loss and linear polarization technique were used to measure or evaluate its resistance in 3.5% NaCl (a simulated marine environment). A minimum of 3 samples was used per immersion time. From the result, it was observed that there is a general sharp decrease in the corrosion rate of the GAS as compared to the control sample. The sample immersed and allowed to dwell in the melt for 20 seconds and further annealed with a soaking time of 20 minutes exhibited the highest corrosion resistance. The polarization curve also shows that the substrate was generally passivated, and this is as a result of the Al-Zn/Fe adhesiveness.

Intercritical Heat Treatment Temperature Dependence of Mechanical Properties and Corrosion Resistance of Dual Phase Steel

Abstract This study investigated the effect of intercritical heat treatment temperature on the tensile properties, work hardening and corrosion resistance of dual phase steel. Ferrite-martensite dual phase steel with different martensite volume fractions were obtained after heat treatment at different intercritical temperatures. Microstructure, mechanical properties of steel were measured and the corrosion resistance was evaluated via polarization test. Tensile strength of the specimens increased by increasing the martensite volume fraction up to 48.2%. Further increase in martensite volume fraction led to decrease in tensile strength. Work hardening behavior analyzing showed that in DP steel with less than 50% martensite volume fraction, the work hardening took place in one stage and by increasing the martensite volume fraction two-stage work hardening behavior was observed in the Holloman analysis. The results of polarization test showed that, the corrosion resistance of dual phase steel is higher than that of plain carbon steel with ferrite-pearlite microstructure.

Modeling of Bainite Transformation During Partitioning Process and Atomic‐Scale Characterization of Bainite

This paper presents a detailed investigation of the interaction between bainite transformation and carbon partitioning during isothermal holding of intercritical heating, quenching, and partitioning (I&Q&P) steel. The kinetics of isothermal bainite transformation is modeled based on the assumption of displacive mechanism. A systematic analysis of the autocatalytic parameters extracted from the fitting curves reveals that autocatalysis plays only a minor role in bainite transformation due to quite small size of parent austenite grains transforming into bainite, independent from each other. Atom probe tomography (APT) is used to investigate the localized composition changes at the interface between bainitic ferrite and retained austenite. The results indicate that carbon enrichment of austenite occurs and Mn atoms segregate at phase grain boundary. Furthermore, in terms of the thermodynamic analysis, bainitic ferrite forms under diffusionless growth followed by tempering, and in present work, the growth of bainitic ferrite can be divided into two parts: bainite transformation quickly (BTQ) and bainite transformation slowly (BTS).

Microstructural evolution and its influence on toughness in simulated inter-critical heat affected zone of large thickness bainitic steel

Simulated inter-critical heat affected zones (ICHAZ) of a large thickness bainitic steel were prepared with three different peak temperatures (Tp) of 750 °C, 780 °C and 800 °C. The microstructure and crystallographic feature were investigated. Toughness of simulated ICHAZ specimens was assessed using instrumental Charpy impact test at − 40 °C. It was found that the volume fraction of newly formed bainite increased and the effective grain size decreased with the increase in inter-critical Tp. The toughness decreased slightly with the Tp of 750 °C compared with the base metal (BM), while the toughness improved sharply when the Tp exceeded 780 °C. The network-like martensite was primarily responsible for the low crack initiation energy with the Tp of 750 °C. In contrast, the fine bainite formed with the Tp above 780 °C was effective in increasing the crack nucleation resistance, resulting in the high initiation energy. The matrix softening as well as the grain refinement played important roles in improving the crack propagation energy. Furthermore, it was confirmed that the presence of a multi-phase microstructure can lead to the scattering of toughness.

Thermal Stability of Retained Austenite and Properties of A Multi-Phase Low Alloy Steel

In this work, we elucidate the effects of tempering on the microstructure and properties in a low carbon low alloy steel, with particular emphasis on the thermal stability of retained austenite during high-temperature tempering at 500–700 °C for 1 h. Volume fraction of ~14% of retained austenite was obtained in the studied steel by two-step intercritical heat treatment. Results from transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicated that retained austenite had high thermal stability when tempering at 500 and 600 °C for 1 h. The volume fraction was ~11–12%, the length and width remained ~0.77 and 0.21 μm, and concentration of Mn and Ni in retained austenite remained ~6.2–6.6 and ~1.6 wt %, respectively. However, when tempering at 700 °C for 1 h, the volume fraction of retained austenite was decreased largely to ~8%. The underlying reason could be attributed to the growth of austenite during high-temperature holding, leading to a depletion of alloy contents and a decrease in stability. Moreover, for samples tempered at 700 °C for 1 h, retained austenite rapidly transformed into martensite at a strain of 2–10%, and a dramatic increase in work hardening was observed. This indicated that the mechanical stability of retained austenite decreased.

2D RVE based micro-mechanical modeling with real microstructures of heat-treated 20MnMoNi55 steel

Macro-mechanical response of multi-phase steel depends on its morphological composition with respect to volume fraction of constituent phases, phase distribution, grain size etc. To address these micro-structural inhomogeneity, a RVE (Representative Volume Element) based micro-mechanical model was incorporated in our current analysis. 20MnMoNi55 steel is basically composed of ferrite and bainite as its constituent phases. To introduce variation in constituent phases and its volume fraction, different inter-critical heat cycles were designed based on the inter-critical temperature range obtained by means of dilatometric testing of 20MnMoNi55 steel. Heat treated samples were then characterized to confirm that the variation is achieved with respect to different set of ferrite-martensite volume fraction. The experimental flow curves obtained through tensile testing for different heat treated samples were primarily correlated with respect to volume fraction of martensite in terms of flow stress of the material. 2D RVEs were constructed based on real microstructure obtained through different heat cycles with the help of Python Scripting interface in Abaqus. The flow curve obtained by means of RVE based finite element simulation closely matches with the experimental results. The stress-strain response obtained from the finite element model output database shows inhomogeneity in its distribution between softer ferrite matrix and harder martensite islands. The deformation localizations, evolved in softer ferrite matrix, are hindered by neighboring martensite islands which causes the stress triaxiality buildup in ferrite matrix. A detailed comparison of stress triaxiality distribution between ferrite and martensite matrix has been established with proper justification.

Micromechanical evaluation of DP1000-GI dual-phase high-strength steel resistance spot weld

In situ micro-cantilever bending tests were carried out on resistance spot welded DP1000-GI dual-phase high-strength steel in order to derive the mechanical response of the welds. Notched micro-cantilevers were milled using focused ion beam milling at the base metal, inter-critical, fine-grained and coarse-grained heat affected zones, and fusion zone. It was shown that due to large plastic yielding, linear-elastic fracture mechanics are inapplicable. To evaluate the fracture toughness of different weld zones, cyclic loading was applied to track the crack size and the conditional fracture toughness of weld zones was measured using crack tip opening displacement and J-integral methods. It was found that micro-cantilever bending method provides insight to the fracture toughness and local mechanical response of different weld zones. The results obtained can be used to make an accurate correlation between resistance spot welding process, microstructure and mechanical response of DP1000-GI dual-phase high-strength steel welds.

A novel multi-step intercritical heat treatment induces multi-phase microstructure with ultra-low yield ratio and high ductility in advanced high-strength steel

A significant volume fraction of 26% retained austenite was obtained via a two-step intercritical treatment in a low carbon low alloy steel with ultra-high tensile strength of ~1200 MPa, ultra-low yield ratio of 0.32, excellent uniform elongation of ~16.4% and total elongation of ~23%. The ultra-low yield ratio and excellent uniform elongation implied high work hardening and excellent formability. The high work hardening and ultra-high tensile strength are attributed to deformation-induced twin-martensite.

Comparison Between Vanadium and Niobium Effects on the Mechanical Properties of Intercritically Heat Treated Microalloyed Cast Steels

In this study, a comparison between V- and Nb-containing low-carbon microalloyed cast steels has been made in terms of strength and impact energy after intercritical heat treatment (IHT). After austenitizing at 950 °C and quenching in water, the alloys were intercritically heated at 760, 800 or 840 °C followed by quenching and tempering at 620 °C. In comparison with conventional heat treatment, IHT increased the impact energies at the expense of some reduction in the strength due to the dual ferritic/martensitic microstructure. However, the impact energy of the alloy containing only V, especially after intercritical heating at 760 and 800 °C, was higher than that of the Nb-containing samples. The improvement in the impact energy required an optimum amount of coarse carbonitride precipitates. While, the impact fracture surface of the alloy V was composed of dimples indicating ductile fracture, cleavage facets in the fracture surface of Nb-containing samples indicated brittle fracture.

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.

Processing and characterization of high strength dual-phase steel by two-step intercritical heat treatment process

A simple approach of two-step intercritical heat treatment has been employed to study the effect of heat treatment on the evolution of microstructures and their effect on the mechanical properties of alloy steel (AISI 1012). The selected steel samples were directly placed in the preheated furnace and were progressively heat treated in two steps, intercritically between the Ac1–Ac3 temperature range. Immediate water quenching (preheated at 30 ℃) was carried out after heat treatment cycles. The processed steels were characterized by examining the X-ray diffraction patterns, microstructures, Vickers microhardness, and tensile strength. The normalized X-ray diffraction results of heat-treated steels revealed the substantial growth in the martenistic phases. The microstructures of heat-treated steel revealed the formation of needle-shape-like structures, which corresponds to the martenistic phase. The increased formation of martenistic phase due to the intercritical heat treatment process improved the overall microhardness (from 188 ± 9 HV of the parent steel to 412 ± 32 HV for 800 ℃ heat-treated steel) up to 2.2 times. The presence of soft and ductile (ferritic and pearlite) phases simultaneously with tough and strong (martenistic) phase allowed the improvement in the ultimate tensile strength. In comparison to parent steel with tensile strength of 510 ± 15 MPa, the intercritical heat treatment steel at 800 ℃ revealed 169.6% higher tensile strength of 1375 ± 35 MPa. However, percentage elongation was reduced by 60%, i.e. from 13 ± 1% for parent steel to 5.2 ± 2% of intercritical heat treatment steel (processed at 800 ℃). An overall study revealed that by a proper intercritical heat treatment process, dual-phase steels with better structure–properties correlation can be obtained for industrial applications.

Microstructure and Properties of HAZ in Low-Carbon Bainite E550 Steel during Double-Pass Welding Thermal Cycle

Investigations on the microstructure and properties of the Coarse-Grained Heat-Affected Zone (CGHAZ) and intercritical reheated Coarse-Grained Heat-Affected Zone (ICCGHAZ) of a low-carbon bainite E550 steel were carried out using thermal simulation technology in this paper.Double-pass welding thermal cycle were performed on Gleeble-3800 thermal simulator, tempering heat treatment of the critical coarse crystal zone carried out in a box resistance furnace, low impact energies at -40 °C and Vickers hardness determined, and the microstructure were observed. The experimental results show that the microstructure of CGHAZ (Tp1 is 1320 °C) was dominated by coarse granular bainite and Lath bainite Ferrite, the impact toughness of CGHAZ was poor. The toughness of the CGHAZ was improved after second welding heat cycle except intercritical two-phase heating. When the peak temperature of the second thermal cycle(Tp2) was 650 °C, martensite-austenite (M-A) constituent of original CGHAZ wasdecomposed and refined, impact toughness and hardness were all higher than that of CGHAZ; When Tp2 is 750 °C, there was a ” necklace” distribution of massive M-A constituent in this ICCGHAZ, the impact energy at -40 °C prominently decreased and Hardness went up; When Tp2 was in the temperature range of 850 °C ~1100 °C, the microstructure was mainly finer granular bainite, the toughness of CGHAZ could be effectively improved; When Tp2 was over 1100 °C, M-A constituents become coarse, the toughness declined slightly . The changing of hardness was the opposite of toughness but the hardness fluctuation was comparatively small. After tempering at different temperature (520 °C~640 °C) , the grain boundary "necklace" structure of ICCGHAZ was still obvious, some of the M-A constituent were decomposed, the hardness decreased, the lowest hardness was obtained in 610 °C.

Weldability of TWIP and DP steel dissimilar joint by laser arc hybrid welding with austenitic filler

In this paper, an innovative 18% Mn austenitic TWIP steel, which exhibits a very good combination of strength and ductility, was joined by laser arc hybrid welding to a conventional DP steel, which consists of ferrite and martensite obtained by an inter-critical heat treatment and fast cooling. An austenitic filler wire was used to improve the weld strength by avoiding the formation of Mn segregations and improper microstructures in the fusion zone by Mn dilution. Metallographic examinations, microhardness and tensile tests were carried out to assess the effects of the laser and materials interaction on the weldability of TWIP-DP dissimilar welds. Hybrid welding and austenitic filler can have a positive effect on the weldability of the DP-TWIP dissimilar joint.

Effect of Intercritical Heat Treatment on Mechanical Properties of Plain Carbon Dual Phase Steel

Mechanical properties of DP (Dual Phase) steels are greatly influenced by the microstructural features such as grain size, morphology and martensite volume fraction (V m %). These microstructural features can be altered by changing the soaking time and temperature within intercritical zone. Present study aims to study the effect of intercritical annealing temperature and soaking time on V m % and its effects on mechanical properties of plain low carbon steel grade (AISI 1020) steel having ferrite-martensitemicrostructure. Nine DP steel specimens with various amount of martensite were produced via intercritical heat treatment. Mechanical properties including TS (Tensile Strength), hardness and toughness were characterized and co-related with martensite volume fraction. It was found that increasing the intercritical annealing temperature and soaking time increases the V m %. The optimum TS and hardness were found at 64V m % and then decrease with further increase in V m %. The toughness was found to have linear relationship with V m %.

The Effect of Two-Step Heat Treatment Parameters on Microstructure and Mechanical Properties of 42SiMn Steel

Medium-carbon steel 42SiMn (0.4C-0.6Mn-2Si-0.03Nb) was used for a two-step heat treatment consisting of a soaking hold and an annealing hold at bainite transformation temperature. Various heating temperatures, cooling rates, and bainitic hold temperatures were applied to the steel to obtain microstructures typical for TRIP (Transformation Induced Plasticity) steels. TRIP steels utilize the positive effects of a multiphase microstructure with retained austenite, creating a good combination of strength and total elongation and an excellent deep-drawing ability. Typical microstructures consist of ferrite, bainite, and 10–15% of retained austenite. In this work, tensile strengths in the region of 887–1063 MPa were achieved with total elongation A5mm of 26–47%, and the final microstructures contained 4–16% of retained austenite. The most suitable microstructure and the best combination of high strength and total elongation were achieved for the processing with intercritical heating temperature of 850 °C and cooling at 30 °C/s to the bainitic hold of 400 °C. Very fine pearlite persisted in the microstructures, even after applying a cooling rate of 50 °C/s, however these small areas with extremely fine laths did not prevent the retention of up to 16% of retained austenite, and high total elongation A5mm above 40% was still reached for these microstructures.

Formation mechanism and control methods of acicular ferrite in HSLA steels: A review

High strength low alloy (HSLA) steels have been widely used in pipelines, power plant components, civil structures and so on, due to their outstanding mechanical properties as high strength and toughness, and excellent weldability. Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels. This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels. Effect of austenitizing conditions, continuous cooling rate, and isothermal quenching time and temperature on acicular ferrite transformation was reviewed. Furthermore, the modified process to control the formation of multi-phase microstructures containing acicular ferrite, as intercritical heat treatments, step quenching treatments and thermo-mechanical controlled processing, was summarized. The favorable combination of mechanical properties can be achieved by these modified treatments.

The Role of Retained Austenite on the Mechanical Properties of a Low Carbon 3Mn-1.5Ni Steel

The present studies focus on the correlation between retained austenite characteristics and the cryogenic temperature Charpy impact toughness, strength, and plasticity. The steels with different volume fractions and stabilities of retained austenite were prepared by quenching followed by intercritical heat treatment, and the microstructure was characterized using scanning electron microscope, electron back-scattered diffraction, and X-ray diffraction. The grain size, dislocation density, crack initiation energy, and crack propagation energy were quantified. It has been demonstrated that the volume fraction of retained austenite plays a significant role in the reduction of the measured yield strength and the effect of tempered martensite/ferrite matrix on cryogenic temperature impact toughness can be assumed to be similar due to the similar grain size, dislocation density and element content in solution for different heat treatments. It was found that the stability of retained austenite plays a determining role in the increase of cryogenic temperature impact toughness. Furthermore, the dependence of the crack propagation energy on retained austenite is much greater than that of the crack initiation energy. Generally, an excellent UTS × TEL does not produce good cryogenic temperature impact toughness.

A Correlation Study Between Mechanical Properties and Morphological Variation of 20MnMoNi55 Steel

Mechanical properties of nuclear reactor pressure vessel materials need to be assessed for degradation while operating under an irradiated condition, but conventional destructive tensile testing is not possible in such cases. In the present paper, a methodology is suggested which relates microstructural parameters to macro-mechanical properties of 20MnMoNi55 steel. Morphological variation of 20MnMoNi55 steel has been achieved through inter-critical heat treatment based on the phase fraction of constituent phases. The inter-critical temperature range of as-received material is determined through dilatometric testing. A color etching technique has been adopted for identification of different phases present in as-received and heat treated steel specimens. Vickers hardness values of each constituent phase as obtained from micro-hardness testing is correlated and standardized with the specified color code of each phase through image analysis of colored optical micrographs. Finally macro-hardness, obtained from the standard mixture rule based on micro-hardness and volume fraction of constituent phases, is correlated with tensile properties.