Subcritical Annealing Research Articles

Subcritical Spheroidization Annealing of Cold‐Rolled Cr–Mo Microalloyed Medium‐Carbon Steel with Different Initial Microstructure

According to the industrial production process of fine‐blanking steel (including hot rolling, cold rolling, and spheroidization annealing), a Cr–Mo‐microalloyed medium‐carbon steel is taken as the research object for laboratory simulation in this paper. The ferrite+pearlite (F + P) and ferrite+bainite (F + B) are obtained by changing coiling temperature during hot rolling conduct cold rolling with 25%, 50%, and 75% total reduction and subcritical annealing at 740 °C for 1–8 h. Microstructure evolution during spheroidization annealing is studied and analyzed by an electron probe microanalyzer, electron back‐scattered diffraction, DICTRA simulation, transmission electron microscopy, and microhardness tester. Compared with F + P, the dislocation density distributed to the ferrite of F + B during cold rolling is higher. The cold‐rolled F + B microstructure shows the rapid spheroidization annealing behavior, which leads to the uniform distribution of finer spherical carbides and the full recrystallization of the ferrite matrix. In addition, properly increasing the reduction of cold rolling can also speed up the spheroidization process of carbide. Herein, a 50% reduction is determined to be the most suitable cold‐rolling process parameter for carbide spheroidization. Moreover, the recrystallization of ferrite is enhanced with the increase of cold‐rolling reduction.

Influence of Long-Term Subcritical Annealing on the Unalloyed Steel Welded Joint Microstructure

The article presents changes in the microstructure of hot-rolled unalloyed structural steel after the arc welding process and in the state after long-term exposure to 600 °C during operation. These studies enable a clear assessment of the effects of long-term exposure to elevated temperature relative to the as-welded condition, which has not been reported. The microstructure examination was carried out on welded joints in eight different zones of the joint. Studies have shown that the welding thermal cycle causes significant changes in the microstructure in the area of the base material heated above the A1 temperature-the heat-affected zone (HAZ)-and in the weld area in the case of multi-pass welding. The long-term exposure of the subcritical temperature of 600 °C on the welded joint leads to the phenomenon of cementite spheroidization in the pearlite in all zones of the joint, while preserving the band structure of the steel after rolling and the structural structure. In the case of the weld, acicular and side-plate ferrite disappearance was observed.

Texture evolution during sub-critical annealing and its effect on yield strength anisotropy of laser directed energy deposited Ti-6Al-2Zr-1Mo-1V alloy

Titanium alloys fabricated by laser directed energy deposition (LDED) are usually subjected to sub-critical annealing to improve mechanical properties. This work mainly investigated the texture evolution of LDEDed Ti–6Al–2Zr–1Mo–1V alloy during sub-critical annealing and its effect on the anisotropy of yield strength. Results indicated that <0001> of α phase in the as-deposited sample exhibited two types of circle-shaped texture, which were perpendicular to the deposition direction (VD) and 45° away from VD, respectively. The intensities of these two types of texture were approximately equal because the α texture in the as-deposited sample were transformed from the strong <001>β//VD fiber texture following Burgers orientation relationship with weak variant selection. During sub-critical annealing, the formation of primary α phase, which tended to be transformed from the {110}β parallel to VD, led to the intensity of circle-shaped texture perpendicular to VD greater than that 45° away from VD. The calculation results suggested that all basal and prismatic slip systems of some α variants transformed from the {110}β parallel to VD were in hardly-activated orientation with Schmid factor (m) equal to zero for loading along the horizontal direction (HD), the slip systems of some α variants transformed from the {110}β 45° to VD were in hardly-activated orientation with m = 0 for loading along 45° direction (OD), while the systems of all α variants were in easily-activated orientation with high Schmid factors for loading along VD. After sub-critical annealing, the content of α phase in hardly-activated orientation increased for loading along HD but decreased for loading along OD, resulting in a smaller difference of yield strength between OD and VD or HD.

Effect of Prior Microstructures on Cementite Dissolution Behavior During Subcritical Annealing of High Carbon Steels

Effect of Prior Microstructures on Cementite Dissolution Behavior During Subcritical Annealing of High Carbon Steels

Enhancement of the AISI 5140 Cold Heading Wire Steel Spheroidization by Adequate Control of the Initial As-Rolled Microstructure

The effect of the initial microstructure and soft annealing temperature on cementite spheroidization and microstructure softening is studied on an AISI 5140 hot-rolled wire. In coarse pearlite microstructure (λ: 0.27 μm), the cementite spheroidization progresses slowly under subcritical treatment, and the microstructure does not achieve the minimum G2/L2 IFI rating defined in the ASTM F2282 to be used in cold forming operations under any of the annealing treatment studies. Fine pearlite (λ: 0.10 μm) and upper bainite microstructures are more prone to spheroidization, and the minimum G2/L2 IFI rating is achieved under subcritical annealing at 720 °C for 6 h. Independent of the initial microstructure, even in the case of martensite, low hardness values within 165–195 HV are attained after imposing a 10 h long treatment at 720 °C. Annealing treatments conducted at 660 °C and 600 °C on pearlitic microstructures give rise to very poor softening. The G2/L2 rating is not achieved in any of the treatments applied at these two temperatures in this study. In pearlitic microstructures, the spheroidization progresses according to a fault migration mechanism, enhanced by the presence of defects such as lamella terminations, holes, and kinks. In the upper bainite, the row-like disposition of the cementite along the ferrite lath interface provides necks where dissolution and consequent lamellae break-up take place quickly under annealing.

Single-Step Heat Treatment for the Restoration of Mechanical Properties of Cold-Strained Mining Support Steel 31Mn4

The steel 31Mn4+QT630 is used frequently for mining support arches. The supports are cold strained, first during service, and again by re-rolling prior to reinstallation, which results in strain hardening and a loss of ductility. Consequently, many of the rerolled arch-segments are not suitable for reinstallation because their mechanical properties are inadequate. The objective of this work was to assess the feasibility of restoration of the required mechanical properties by means of a cost-efficient single step heat treatment. Specimens were cold deformed to different degrees in the range 0 % to 45 % to establish the relation between the degree of cold deformation and the hardness. Differently strain hardened specimens were subjected to subcritical annealing at temperatures 450 °C to 700 °C in the duration 0.5 h to 8 h to determine a suitable time-temperature combinations. Microstructures and mechanical properties were investigated of as-received, cold strained and recrystallized specimens. Tests performed were optical microscopy, scanning electron microscopy, tensile tests, hardness measurements and Charpy impact tests. Elongation at break of the as-received material was below the requirements of the applicable standard, and its microstructure contained significant fractions of pre-eutectoid ferrite and pearlite. Upon cold straining, hardness increased by approximately 2 HV per 1 % of strain. After one-hour recrystallization at 600 °C to 620 °C, the microstructure consisted of a ferritic matrix containing evenly dispersed globular carbide particles. The original ductility was restored, while the elongation, yield strength, and hardness were higher than in the as-received condition. These results confirmed that it is feasible to restore the original mechanical properties of heavily strained profiles with an adequate single-step heat treatment. Furthermore, they indicated that it should be possible to recover the required properties of inhomogeneously strained material with the same set of well optimized heat treatment parameters. Consequently, it should be possible to increase the reuse rate and to decrease the costs for new support arches significantly.

Crack propagation during Charpy impact toughness testing of Ti−Al−V−Mo−Zr alloy tubes containing equiaxed and lamellar microstructures

Numerous reports have elucidated the importance of impact toughness measurements for investigating the damage tolerance of a material. However, the effect of heat treatment parameters on impact toughness and the crack propagation mechanism in titanium alloys has rarely been addressed. Herein, the impact toughness of extruded Ti−Al−V−Mo−Zr alloy that comprises equiaxed and lamellar α microstructures has been investigated by conducting Charpy impact tests. The resulting fracture surface and alloy microstructure were examined by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The obtained results revealed that the colony size decreased after a solid solution and aging treatment, whereas subcritical annealing and slow cooling led to the globularization of lamellar α grains. After globularization, the impact toughness of the alloy increased by 69.2% and 50.0%, respectively; however, the impact toughness of the sample after the solid solution and aging treatment decreased by 61%. During impact testing, the direction of crack propagation was oriented toward the low-angle boundary region. Furthermore, a dissolved secondary phase was formed in the vicinity of the fracture surface, which increased the resistance of the alloy to crack propagation. The present study provides significant guidance for the preparation of titanium alloys with superior impact toughness properties.

Effect of intercritical annealing with high cooling rate associated with cold deformation and subcritical annealing on microstructure and mechanical properties of SAE 52100

Conventional spheroidization heat treatments of bearing steels are long-term, therefore it is necessary to study alternatives to decrease the heat treatment time to save energy consumption. This work aimed to develop a new manufacturing route in order to obtain a completely spheroidized microstructure. The microstructure and mechanical properties of SAE 52100 steel were evaluated after intercritical annealing with higher cooling rate than the conventional one, followed by cold deformation with different area reductions, and subcritical annealings with different soaking times. It has been observed that the route considering the intercritical annealing with cooling rate 8 times higher than the conventional one, followed by a high cold reduction and subcritical annealing for 1 h turns feasible the complete spheroidization of the material.

Correlative analysis of interaction between recrystallization and precipitation during sub-critical annealing of cold-rolled low-carbon V and Ti–V bearing microalloyed steels

In this paper a new insight into fundamentals of static recrystallization, precipitation and their interaction during sub-critical annealing of three cold-rolled low-carbon microalloyed steel grades is presented. The grades under investigation are a base grade containing V as a microalloying element, a Ti + grade containing Ti as microalloying element added into the base grade, and a Ti + Mn + grade containing additional Mn added into the Ti + grade. The cold-rolled steels are sub-critically annealed inside a muffle furnace to simulate industrial continuous annealing parameters in order to investigate the interaction between recrystallization and precipitation across transient stages of the annealing process as a function of temperature and time. The Zener pinning of precipitates and solute drag force of Mn on the recrystallization process are calculated and compared with measured values obtained from experimental studies on the recrystallization kinetics. Results suggest that the recrystallization kinetics is fastest in the base grade. For the Ti + grade, fine (<15 nm) (Ti,V)(C/N) particles retard the recrystallization kinetics. For the Ti+ and Ti + Mn + grades, solute drag effect of Mn solute atoms for dwell time longer than 2 min at annealing temperature of 800 °C is negligible.

New Approach for Multiplying the Strength and the Formability of Cold Rolled BCC Based Structure Steel through Ultra Fine Structure Technique

Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.

Improvement of corrosion resistance of SS316L manufactured by selective laser melting through subcritical annealing

The effect of heat treatment on pitting-corrosion resistance of 316 L stainless steel manufactured via selective laser melting (SLM) was investigated. The result shows that both pitting-corrosion resistance and mechanical properties under Subcritical-temperature heat treatment (950 °C) are better than 1100 °C and as-received. For as-received samples, the most prone to pitting corrosion is melt-pool boundary (MPB). Subcritical-temperature heat treatment not only eliminates MPBs and high-density dislocation but also retains advantages of inclusion nanocrystallisation, cellular substructure. And recrystallisation occurs during 1100 °C heat treatment significantly reduces low-angle grain-boundary density and increases segregation and re-precipitation, which weakens corrosion resistance and mechanical properties.

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.

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.

Processing of Ultrafine-Grained Steels by Warm Rolling and Annealing

Low-carbon microalloyed steel was subjected to warm rolling followed by rapid transformation annealing (RTA) at 800-850 °C and subcritical annealing (SCA) at 600 °C to develop ultrafine ferrite grain structures (UFFG) with grain size less than 3 μm. The present study investigated the influence of light (40%) and heavy (80%) warm rolling deformation (LWR and HWR) applied during the finishing pass of two-pass rolling schedules on the microstructural evolution after rolling and subsequent annealing treatments. RTA treatment of HWR sample at a lower intercritical temperature for an optimum duration (800 °C, 30 s) developed UFFG-martensite dual-phase structure that offered the best combination of strength (YS ~ 900 MPa and UTS ~ 1200 MPa) and ductility (25% elongation). The SCA treatment provided sufficient time to achieve a uniform distribution of carbide particles throughout the ferrite matrix. SCA treatment of HWR at 600 °C for 4 h developed UFFG-carbide structure achieving YS of 800 MPa with 20% ductility. The SCA of LWR resulted in coarser ferrite grain structures (grain size > 5 μm) having higher ductility (more than 30%) but lower strength (UTS of 400-550 MPa) as compared to RTA.

The Inspection of Spheroidized Annealing on SCM435 Cold-Forging Quality Steel Wires with Protective Atmosphere

A cold-forging quality steel rod is usually applied for manufacturing wire which is generally produced by drawing wire coil into wire, followed by spheroidized annealing treatment to achieve the necessary formability for cold forging. The subcritical and intercritical processes are usually used to spheroidize the steel wires. The cold-forging quality SCM435 alloy medium carbon steel wires are widely used to manufacture high tension bolts for mechanical and heavy industry. In this study, the spheroidized annealing experiments on SCM435 alloy steel wires are conducted in a commercial bell furnace with a protective atmosphere of nitrogen or hydrogen. The mechanical properties of annealed steel wires are measured by tensile and Rockwell hardness tests and the process capability is evaluated. It is experimentally revealed that, for SCM435 alloy medium carbon steel wires, the wire quality with intercritical annealing is much better than that with subcritical annealing and is markedly affected by furnace atmospheres. The intercritical annealing quality on SCM435 alloy steel wire in hydrogen atmosphere furnace is better than in nitrogen atmosphere furnace. A comparison between the results obtained using the intercritical annealing with hydrogen atmosphere and the measures using the subcritical annealing shows that the intercritical annealing effectively improves the performance measures of low strength and high ductility over their values at the subcritical annealing. The results presented in this study could be a reference for fasters wire manufacturers.

Contribution to The Knowledge of The Action of Boron in The Dual Structure of a Steel 30MnB5 by Means of X-Ray Diffraction

The new thermal treatment of subcritical annealing and water quenching, applied to boron steels (30MnB5), which we propose in our research, achieve excellent variability in mechanical properties, as a result of obtaining a very versatile dual structure. In addition, there is a significant reduction in energy consumption and process time, which results in the Life Cycle Analysis (LCA) and process costs. The presence of boron favours the success of the thermal treatment. The explanation of the process has been investigated by means of a metallographic study, mechanical characterization and in this work the results obtained by means of the study of the treated and untreated samples by means of X-Ray diffraction are presented. The presence of very small FeB crystals in the structure are the cause of the dual structure and its good characteristics. They have only been detected by X-Ray Diffraction, although we have also tried Transmission Electron Microscopy with unclear results.

Mechanisms of pearlite spheroidization: Insights from 3D phase-field simulations

Morphological evolution of eutectoid phases determine the spheroids' size and distribution post sub-critical annealing of steel. In this work, the spheroidization of the 3-dimensional cementite plates is investigated via phase-field modeling to enhance our understanding of the underlying capillary-mediated mechanisms. Since the interfacial energy plays a key role in the spheroidization process, a phase-field model which efficiently avoids any contribution of the bulk free energy in the interface is employed to recover the sharp interface solutions. It is identified that depending on the cementite aspect ratio, the spheroidization mechanism adopted by the plate, varies. In plates of smaller aspect ratios (<27), the transformation is characterized by the recession of the edges and corners, following which, the entire plate collapses into a single spheroid. However, if the plate aspect ratios are greater than 27, discontinuities set-in due to a curvature-difference between the receding edges and the flat surfaces. Such discontinuities or holes continue to evolve and coalesce during the annealing process. Our phase-field simulation-based analysis of the cementite spheroidization provides the first exposure of the spatiotemporal pathways leading to the spheroidization of cementite. A change in the evolution mechanism from the ‘edge-migration regime ’ to a ‘discontinuities-assisted’ one, is found to be responsible for the fragmentation of cementite plates. The influence of the kinetic pathways on the resulting size and spatial distribution of the spheroids are discussed.

Effect of Ostwald ripening of carbide particles on mechanical properties of SCM435 steel during subcritical annealing

The Ostwald ripening of carbide particles occurs during the process of subcritical annealing in SCM435 steel, and the degree of ripening influences the microstructure and mechanical properties of the steel. The effects of Ostwald ripening were studied by simulating different soaking time at 680 °C using SCM435 steel. The spheroidized specimens were analysed by conducting microstructure and mechanical tests. After increasing the soaking time from 2 to 6 h at 680 °C during subcritical annealing, the number of carbide particles and the spheroidization ratio increased gradually, and the formability was improved. When the soaking time ranged from 6 to 8 h, the spheroidization ratio was similar; however, the number of carbide particles decreased, and the formability gradually worsened. Therefore, by comprehensively comparing the microstructures and mechanical properties, the optimum soaking time was determined to be 6 h at 680 °C during subcritical annealing to obtain preferable cold heading. In addition, the carbide particles gradually coarsened when the soaking time was extended from 2 to 8 h. A formula was presented to quantitatively characterize the progress of Ostwald ripening of the carbide particles during the subcritical annealing of SCM435 steel, and the relative error was less than 8.02%.

Effects of Post-Weld Heat Treatment on the Microstructure and Toughness of Flash Butt Welded High-Strength Low-Alloy Steel

Effect of post-weld heat treatment on the weld microstructure, texture, and its correlation to the toughness of flash butt welded joints were investigated. Upon flash butt welding, the α and γ-fiber in the parent material converted to Goss (110)[001], rotated Goss (110)[1\( \bar{1} \)0], and rotated cube (001)[1\( \bar{1} \)0], (001)[\( \overline{11} \)0] textures along the fracture plane. Formation of these detrimental texture components was a result of shear deformation and recrystallization of austenite at temperatures above Tnr resulting in a drop of toughness at the weld zone. Inter-critical and sub-critical annealing cycles proved to be less effective in reducing the Goss (110)[001], rotated Goss (110)[1\( \bar{1} \)0], and rotated cube (001)[1\( \bar{1} \)0], (001)[\( \overline{11} \)0] texture components, and therefore, toughness values remained unaffected. Post-weld heat treatment in the austenite phase field at 1000 °C for 5 seconds resulted in the formation of new grains with different orientations leading to a reduction in the texture intensities of both Goss and rotated Goss components and therefore improved weld zone toughness. Prolonged annealing time was found to be ineffective in improving the toughness due to grain growth.

Improved Wear Resistance of Boron Steels by Subcritical Annealing and Hardening with Production Cost Savings and Lower Environmental Impact

Boron steels are very interesting as wear resistant materials. In this research we propose a boron steel without alloys, 30MnB5, with a new thermal treatment that exceeds the mechanical characteristics of conventional treatment. The 30MnB5 steel, with the new sub-critical annealing and water quenching heat treatment, also exceeds the RAEX450. The new treatment has important advantages, such as: energy savings and reduction of costs and manufacturing times. It also has a more favourable ACV than 30MnB5 with conventional heat treatment and RAEX450. Its wear resistance is significantly improved compared to the classic heat-treated 30MnB5 and the RAEX450.