Optimal Heat Treatment Process Research Articles

Customized heat treatment process enabled excellent mechanical properties in wire arc additively manufactured Mg-RE-Zn-Zr alloys

Abstract Customized heat treatment is essential for enhancing the mechanical properties of additively manufactured metallic materials, especially for the alloys with complex phase constituents and heterogenous microstructure. However, the interrelated evolutions of different microstructure features make it difficult to establish optimal heat treatment process. Herein, we proposed a method for customized heat treatment process exploration and establishment to overcome this challenge for such kind of alloys, and a wire arc additively manufactured (WAAM) Mg-Gd-Y-Zn-Zr alloy with layered heterostructure was used for feasibility verification. Through this method, an optimal microstructure (fine grain, controllable amount of LPSO structure and nano-scale β' precipitates) and the corresponding customized heat treatment process (520 °C/30 min + 200 °C/48 h) were obtained to achieve a combination of a high strength of 364 MPa and a considerable elongation of 6.2 %, which surpassed those of other state-of-the-art WAAM-processed Mg alloys. Furthermore, we evidenced for the first time that the favorable effect of the undeformed LPSO structure on the mechanical properties was emphasized only when the nano-scale β’ precipitates were present. It is believed that the findings promote the development of advanced Mg alloys and help to establish customized heat treatment process for additively manufactured materials.

Effect of quenching and tempering treatments on microstructure and mechanical properties of 300M ultra-high strength steel fabricated by laser powder bed fusion

To achieve high-performance 300 M steel parts through the Laser Powder Bed Fusion (LPBF) additive manufacturing process, post-heat treatments are indispensable. Herein, the effects of quenching and tempering treatments on the microstructure and mechanical properties of LPBF-fabricated 300M ultra-high strength steel were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure of quenched samples primarily consisted of quenched martensite with minor amounts of bainite and some grain boundary allotriomorphic ferrite. The quenched samples presented significantly higher strength than the 300M forgings, primarily attributed to the contributions of solid solution strengthening, grain boundary strengthening and dislocation strengthening. After following tempering treatment, the microstructure of LPBF-fabricated samples mainly consisted of tempered martensite with minor amounts of bainite and residual austenite. Compared to the quenched samples, the tempered samples exhibited a decrease in strength while demonstrating a significant increase in ductility. The samples treated with the optimal heat treatment process (900 °C/1 h/Oil Cooling for quenching + two times of 300 °C/2 h/Air Cooling for tempering) demonstrated a remarkable strength–ductility balance, with a yield strength (YS) of 1834 MPa, an ultimate tensile strength (UTS) of 2099 MPa, and an elongation (EL) of 10.2%, which are all beyond or comparable to those of 300M forgings.

CALPHAD simulation and experimental study on heat treatment process of Al-Cu-Mn-Mg-Sc cast alloys

Because different heat treatment parameters have different effects on the microstructure and mechanical properties, it is necessary to optimize the heat treatment parameters to obtain alloys with better properties. In this study, the heat treatment parameters of Al-Cu-Mn-Mg-Sc cast alloys were optimized by using thermal simulation and experimental methods. The results show that the phase transformation temperature of the low melting point eutectic phase Al2Cu is 542.9 ℃, and the time for element homogenization distribution of the alloy during solid solution at 525 ℃, 535 ℃ and 550 ℃ is 8 h, 7 h and 4.5 h, respectively. With the rise of solid solution temperature, the microhardness increase at first and then decrease, and the microhardness reaches to 96.4 HV at 535 ℃. When solid solution heat treatment is carried out at 535 ℃ for different times, the microhardness increase at first and then decrease with time, and the microhardness reaches to 118.4 HV at 5 h. It is known that the number of metastable phase decreases gradually with the increase of aging heat treatment temperature by using the calculation of thermodynamic software. The optimal aging heat treatment process parameter for the alloy is 170 ℃ for 10 h, and the microhardness at this time reaches to 142.3 HV.

Influence of carbon layer hardness characteristics on RCF performance of carburized bearing steel and optimization of heat treatment process

Being a pivotal element within high-speed train transmission systems, the mechanical attributes of bearings are of great significance to ensure the quality and safety of train operation. In this study, the rolling contact fatigue performance of bearing materials was tested to investigate the effects of different carbon layer characteristics on the rolling contact fatigue behavior. Combined with the thickness of carburizing layer, the elastoplastic coupling model of rolling contact fatigue damage of G20CrNi2MoA carburizing bearing steel was established. The model revealed the relationship between the number of load cycles and the degree of subsequent material damage. The model was used to calculate and predict the loading frequency of G20CrNi2MoA carburized bearing steel with excellent fatigue characteristics, which was 2.02 × 108 times before spalling. In addition, the carburizing heat treatment process for bearings was optimized based on the hardness gradient curves obtained, which provided valuable insights for improving the actual production process.

Heat Treatment Process, Microstructure, and Mechanical Properties of Spring Steel with Ultra-High Strength and Toughness

In this research, a new type of spring steel with ultra-high strength and toughness was designed, and its mechanical properties and microstructure under different heat treatment processes were studied. The results show that the optimal heat treatment process for the steel is oil quenching at 890 °C for 40 min, followed by tempering at 400 °C for 1 h. Its mechanical properties have an optimal combination of 1865MPa tensile strength, a yield strength of 1662 MPa, an elongation of 11.5%, a cross-sectional shrinkage of 51.5%, and a Charpy impact energy of 43.7 J at room temperature. With increasing austenitizing temperature, the austenite grain size increases, the martensite lath becomes thicker, and the strength decreases. With increasing tempering temperature, the lath boundary of martensite becomes blurred, the strength decreases, and the plasticity improves. In addition, it was found that during tempering at higher temperature (450 °C), large particle inclusions and secondary cracks appeared in the fractured surface, and a large number of carbides precipitated, leading to the brittleness of tempered martensite.

A finite element model with multi-flow fields for the quenching process of mill liner component made of bainite and martensite: simulation and experimental validation

Controlling the type and location of microstructure transformation through heat treatment processes is the key to improving the property of workpieces, especially they are large-sized and highly thick irregular. Such as mill liner, the worst wear in mining machinery, of which microstructure inhomogeneity is key to developing cost-efficient mining process. In order for advanced high strength steel (AHSS) to be used in mining machinery, its microstructure uniformity must be optimized, usually by adjusting the process parameters of the heat treatment process to optimize the microstructure distribution uniformity. However, the research on the simulation of large-sized and highly thick irregular heat treatment process is limited. Previous studies usually controlled the heat transfer process by using the heat transfer coefficient as the boundary condition, but only applied to small size and simple structure of the workpiece. As the heat transfer coefficient is a process quantity, its size depends on the thermal property parameters of the fluid on both sides of the wall, the flow rate, the shape of the solid surface, the thermal property number of the material and other factors, which is not suitable for large-sized and highly thick irregular. Therefore, for the large-sized and highly thick irregular heat treatment simulation process, the equivalent heat transfer coefficient of flow field was introduced as the boundary condition, and the temperature field, microstructure field and stress field were coupled to improve the accuracy of the numerical model. The accuracy of the finite element model is verified by comparing the temperature simulation error of less than 10% through temperature measurement experiments of the quenched workpiece. The evolution process of heat transfer in large-sized and highly thick irregular water bath quenching is obtained through the finite element model data. The difference in the flow rate of the wall fluid is the fundamental reason for the difference in cooling effect, and the wetting process of the wall is the secondary reason. At the same time, the corresponding microstructure and surface hardness distribution of different cooling regions are obtained, which can provide more accurate guidance for the optimization of heat treatment process of large size and thick special-shaped parts.

Thermal Stability of Electrodeposited Fe-55wt%Ni Alloy and Effect of Low-Temperature Heat Treatment on Magnetic Properties

Electrodeposited nanocrystalline soft magnetic materials have emerged as one of the hottest research topics in the field of magnetic materials due to they are easy to implement in miniaturization, lightweight, and energy-saving of electronic devices. The thermal stability and grain growth process of electrodeposited Fe-55wt%Ni alloy were investigated. Results indicated that the grain growth was rapid at a temperature of about 678 K, while the exothermic peak appeared in DSC with an exothermic heat of about 12 ± 1 J g−1. The activation energy for grain growth was obtained through the optimized Kissinger equation and isothermal kinetics calculations, and the growth mechanism was evaluated based on the calculation results. Below 678 K, the activation energy required for grain growth was low, which implied the growth mechanism was the rearrangement of atoms at the grain boundary; Above 678 K, the growth mechanism was grain boundary diffusion. After the low-temperature heat treatment, the coercivity (H c) decreased and the saturation magnetization (M s) increased slightly, which was attributed to the reduction of internal stress and the ultra-fine nanocrystalline structure. The optimal heat treatment process was 573 K heat treatment for 5 h, where M s and H c of 160 emu g−1 and below 1 Oe, respectively.

Effect of Heat Treatment Process on Microstructure and Mechanical Properties of High-Carbon H13 Steel

This paper investigated the mechanical properties and microstructures of different samples of H13 steel after they underwent various heat treatment processes. It provided a detailed analysis of the microstructure and mechanical properties under different processes and approached the topic from a theoretical perspective. The phase composition of each sample remained unchanged after undergoing different heat treatment processes. Despite the vacuum gas quenching (H1) sample being guaranteed a hardness of 58.47 HRC, its toughness fell below expectations at a mere 46.75 J. Notably, the microstructure of the sample which underwent the H1 process and the cryogenic (H2) treatment exhibited a finer grain size and higher toughness compared to the sample which only underwent the H1 process without the cryogenic treatment. Its toughness was 70.19 J, but its hardness slightly decreased to 57.47 HRC. Following the application of oil quenching and cryogenic treatment (H3), the hardness of the sample significantly increased, reaching a remarkable 58.38 HRC. Additionally, the sample exhibited good impact resistance, with a measurement of 74.25 J. Before the H2 process, the sample which underwent the spheroidizing annealing process (H4) had a higher hardness compared to the sample without spheroidizing annealing. At the same time, when comparing the above four samples, the sample that underwent the H4 process exhibited the best toughness, with a value of 86.94 J, while still maintaining a hardness of 57.85 HRC; thus, it achieved an ideal balance between strength and toughness. Therefore, the optimal heat treatment process for high-carbon H13 steel was spheroidizing annealing followed by vacuum gas quenching and then cryogenic treatment.

Preparation and Characterization of 3D Printed Hydroxyapatite-Whisker-Strengthened Hydroxyapatite Scaffold Coated with Biphasic Calcium Phosphate

This study investigates the in vitro degradation of calcium-deficient hydroxyapatite powder after heat treatment at different temperatures and analyzes the calculated phase composition, particle size distribution, degradation rate, and bioactivity of the powder after heat treatment. A mixture of hydroxyapatite and β-tricalcium phosphate (BCP) coatings was prepared on the surface of a 3D-printed hydroxyapatite-whisker-strengthened hydroxyapatite scaffold (HAw/HA) by vacuum impregnation and ultraviolet light curing combined with an optimized heat treatment process. The performance of the coatings under different methods was characterized. The composite scaffolds with highly interconnected pores and excellent mechanical properties were prepared, and their biodegradation performance, bioactivity, osteoconductivity, and osteoinductivity of the scaffolds were improved. The results showed that calcium-deficient hydroxyapatite began to transform into BCP between 600 °C and 800 °C, and the powder treated at 800 °C has better bioactivity. The BCP coating prepared by light curing was more uniform, resulting in a higher interfacial bonding strength, and has better osteoconductivity and osteoinductivity than that prepared by vacuum impregnation.

Enhanced strength and ductility of high chromium cast iron/low carbon steel bimetal by heterostructure design

In this study, the heterostructured high chromium cast iron (HCCI)/low carbon steel (LCS) bimetal was designed by optimized heat treatment process. The microstructure, mechanical properties and deformation behavior of bimetal were studied. The bonding zone of HCCI/LCS bimetal is comprised of ferrite and pearlite, while the diffusion zone of HCCI mainly consists of M23C6 and ferrite due to the diffusion of C atoms from the HCCI to the LCS. The shear strength value of the bimetal is 395.2 MPa and no microcracks were observed at the interface. The tensile strength and total elongation of the bimetal are 452.1 MPa and 26.2 %, respectively. The analysis of the deformation behavior shows that the back stresses originate from the cooperative effect between the hard and soft zones, which improves the overall strength and ductility of the bimetal. In the deformed samples, large plastic deformations and stress concentrations were observed in the soft zones due to the dislocation were blocked in the hard zones, which confirmed the typical hetero-deformation induced (HDI) strengthening. The present work sheds light on heterostructure of HCCI/LCS bimetal that can be feasibly employed, and it could guide the optimization of mechanical performance of a wide range of fragile alloys for toughness engineering.

Investigation of heating time of a bundle of steel bars

The paper deals with the problem of determining the heating time of a bundle of flat steel bars. To describe a mathematical model of transient heat transfer the energy balance method was used. The most important element is to determine the effective thermal conductivity coefficient using the electrical analogy. The simulations of heating were performed for increasing temperature of furnace. The results show that using different sizes and bundle arrangement the heating time can be reduced by 5–15%. The analysed problem is important from the practical point of view because it allows for optimization of heat treatment processes of steel bars.

Effect of the CNTs into SiCp-Al interfacial micro-zones on ageing precipitation behavior, microstructure and mechanical properties of SiCp(CNT)/Al–Zn–Mg–Cu composites

For Al–Zn–Mg–Cu matrix composites, the introduction of reinforcements tends to have a significant effect on the aging precipitation behavior of the interfacial micro-zones, which in turn affects the mechanical properties of the composites. In this study, the ageing precipitation behavior, microstructure and mechanical properties of SiCp(CNT)/7075Al composites were systematically investigated, to explore the effect of CNT introduction on SiCp-Al interfacial micro-zones. The results showed that the introduction of SiCp(CNT) hybrid reinforcement could promote the aging precipitation behavior of the composites compared with 7075Al. But the introduction of CNTs leads to a further increase of O content in the composite compared with SiCp due to the adsorption of oxygen by CNTs, leading to more severe segregation of Mg and O, which intensified the degree of adverse interface reactions. This adversely affects the aging precipitation behavior of the interfacial micro-zones and the mechanical properties of the composites. Furthermore, the degree of interface reaction can be effectively regulated by adjusting the heat treatment process of SiCp(CNT)/7075Al composites. Compared with the T6 treatment (solid solution treatment at 475 °C for 1 h, aging treatment at 120 °C for 24 h), the yield strength and ultimate tensile strength of both 15 wt % SiCp/7075Al and SiCp(0.5CNT)/7075Al composites can be increased by more than 100 MPa under the optimal heat treatment process (solid solution treatment at 470 °C for 40 min, aging treatment at 120 °C for 15 h). Moreover, the SiCp(0.5CNT)/7075Al composite showed the best balance between strength and ductility, not only the strength was equivalent to that of SiCp/7075Al, but also the elongation increased by 13.2%. This study can provide guidance and suggestions for regulating the aging precipitation behavior of interfacial micro-zones and mechanical properties of metal matrix composites.

The Effect of Heat Treatment on a 3D-Printed PLA Polymer’s Mechanical Properties

Three-dimensional printing is a useful and common process in additive manufacturing nowadays. The advantage of additive polymer technology is its rapidity and design freedom. Polymer materials' mechanical properties depend on the process parameters and the chemical composition of the polymer used. Mechanical properties are very important in product applicability. The mechanical properties of polymers can be enhanced by heat treatment. Additive-manufactured PLA's mechanical properties and structure can be modified via heat treatment after the 3D printing process. The goal of this research was to test the effect of heat treatment on the mechanical and structural parameters of additive-manufactured PLA. This was achieved via the FDM processing of standard PLA tensile test specimens with longitudinal and vertical printing orientations. After printing, the test specimens were heat-treated at 55 °C, 65 °C and 80 °C for 5 h and after being held at 20 °C for 15 h. The printed and heat-treated specimens were tested using tensile tests and microscopy. Based on the test results, we can conclude that the optimal heat treatment process temperature was 65 °C for 5 h. Under the heat treatment, the test specimens did not show any deformation, the tensile strength increased by 35% and the porosity of the PLA structure decreased.

Study on the Subcritical Quenching Process of High-Chromium Cast Iron Prepared by Squeeze Casting

In this study, the heat treatment process of a high-chromium cast iron (HCCI) alloy prepared via 128 MPa squeeze casting at different subcritical quenching temperatures was investigated. The results showed that subcritical heat treatment can change the martensite content, the carbide type and size of the squeeze casting HCCI microstructure. Furthermore, it was revealed that the subcritical quenching heat treatment can improve the hardness of the liquid-forged HCCIs. When the quenching temperature increased from 500 °C to 530 °C, the hardness of the alloy increased significantly, reaching a maximum value of 57 HRC. Thereafter, if the temperature continued to rise to 630 °C, the hardness decreased rapidly. For impact toughness, when the quenching temperature was 500 °C, the toughness of alloy increased by 0.9 J/cm2 than that of the no heat treatment group. If the quenching temperature continued to increase, the toughness was reduced. Taking hardness and toughness into account, the microstructure evolution diagram of the optimal process-500 °C subcritical quenching process was established, various characterisation techniques were used to gain insights into the optimal heat treatment process. Compared with high temperature heat treatment, subcritical heat treatment can improve the performance of the HCCI alloy and reduce costs.

Effect of Al alloying on microstructures and mechanical properties of Co-free maraging steel

<p indent="0mm">To improve the mechanical properties of maraging steel, it is important to control the precipitation and morphology of high-density nano-scale precipitates. In this work, Co-free maraging steel with a dual-phase structure was prepared by composition design and an optimized heat treatment process. The results showed that semi-coherent η-Ni₃Ti and coherent B2-NiAl ordered phases were precipitated in maraging steel with a content of 1.5 wt% Al. The steel had good mechanical properties with a high ultimate tensile strength of <sc>2065 MPa</sc> and an elongation of 9.5%. With an increase in the Al content, the morphology of the precipitated phase changed from a needle-like structure (η-Ni₃Ti) to a spherical structure (B2-NiAl). The addition of Al was also beneficial for the competitive growth of Ni₃Ti and NiAl, and it accelerated the precipitation process and promoted the size refinement of the nano-sized precipitates. Nano-scale austenite existed in the maraging steel and was distributed at the boundary of lath-like martensite. However, with an increase in the Al content, the sizes of both the nano-scale austenite and the lath martensitic decreased. The good mechanical properties of maraging steel were mainly attributed to the dual-phase structure, high-density precipitation, and fine grain strengthening.

Investigating the Effects of Advanced Heat Treatment Techniques on the Mechanical Properties of Cast Components

This paper presents a comprehensive investigation into the effects of advanced heat treatment techniques on the mechanical properties of cast components. The study employs cutting-edge methodologies, including induction hardening, laser hardening, and cryogenic treatment, to modify the microstructure of various cast alloys. The primary focus is on the impact of these treatments on the hardness, tensile strength, ductility, and fatigue resistance of the materials. The experimental results reveal a significant enhancement in the mechanical properties of the treated components, with notable improvements in wear resistance and structural integrity. The findings also underscore the potential of these advanced heat treatment techniques in extending the service life of cast components, thereby contributing to the sustainability of mechanical systems. This research provides a novel perspective on the optimization of heat treatment processes, offering valuable insights for the design and manufacturing sectors. The outcomes of this study have far-reaching implications for industries that rely heavily on cast components, including automotive, aerospace, and heavy machinery, and pave the way for future research in this critical area of mechanical engineering.

Effect of heat treatment process parameters on the microstructure and properties of GH4720Li superalloy

The solution temperature and aging temperature of GH4720Li superalloy are the critical factors in determining its microstructure stability and mechanical properties. However, the matching of solution temperature, primary aging temperature, and second aging temperature greatly influences the final microstructure and mechanical properties, and the mechanism was rarely studied. In this paper, the effects of solution temperature, aging temperature, and other process parameters on the grain size, precipitated phase, and mechanical properties of GH4720Li superalloy were systematically studied by orthogonal experiment, and a set of optimal heat treatment process parameters were found. The results show that the sample was solution treated in the temperature range of 1060 °C ∼ 1120 °C. When the solution temperature was 1160 °C ∼ 1100 °C, although the primary γ ′ phase gradually dissolved, the secondary γ ′ phase gradually increased, and the primary γ ′ phase was pinned at the grain boundary to hinder the grain growth, and the hardness of the alloy gradually increases. When the solution temperature exceeded 1100 °C, the primary γ ′ phase dissolved in large quantities, the grains grew up rapidly, and the hardness of the alloy decreased. The sample was subjected to two-stage aging treatment in the temperature range of 650 °C ∼ 770 °C and 760 °C ∼ 880 °C. As the aging temperature increased, the primary γ ′ phase of the sample gradually grew, and the ability to pin the grain boundary weakened. The volume fraction of the secondary γ ′ phase of the sample gradually decreased and coarsened, resulting in grain growth and a gradual decrease in the hardness of the sample. When the samples were treated by 1100 °C/OC × 4 h + 650°C /AC × 8 h + 760 °C/AC × 8 h, the grain size of the sample was the smallest, the average grain size was 4.5 μm, the distribution of γ ′ phase was the most uniform, and the mechanical properties are the best, reaching 47 HRC.

Influence of the conditions of internal heat exchange on the process of thermal decomposition of carbonates in iron ore raw materials

This work considers influence of heating rate of iron ore raw materials on chemical and metallurgical processes of thermal dissociation of carbonates, which occur during its high-temperature roasting. The research was conducted for the temperature ranges of the existing roasting and sintering machines, taking into account macro-kinetics of calcination processes. Substantial influence of heating rate of iron ore samples on kinetics of thermal decomposition of carbonates was revealed. The cause of the effect of internal heat exchange conditions in reacting iron ore sample on macro-kinetics of dissociation reaction for carbonates was substantiated scientifically. Influence of nonisothermal properties on precision of thermal analytical investigations, which were conducted using the methods of non-isothermal kinetics with different heating rates of iron ore samples, was studied. The authors suggested the mathematical model, which allows to take into account wide range of variations of the parameters in kinetic equations and volumetric thermal and chemical-metallurgical dissociation processes for carbonates. A series of calculating experiments, aimed on reveal of the relationship between heating rate of iron ore samples and evolution of concentration fields, transformation rates and temperature gradients, was carried out using the suggested model. The obtained results can be used for development of the optimal operating conditions for sintering and roasting machines, which propose optimization of heat treatment processes for iron ore preparation, owing to intensification of heat and mass exchange processes during high-temperature roasting. These conditions are considered as optimal from the point of view of power- and resource efficiency. Forming of the above-mentioned conditions allows to decrease fuel and power expenses for thermal preparation of raw materials at metallurgical production facilities, as well as to lower total economical production charges due to use of thermal decomposition of carbonates. The reported study was funded by RSF according to the&nbsp;research project № 22-11-00335.

Achieving strength-ductility synergy in semi-solid squeeze cast 6TiB2/Al–17Si–4Cu composites by heat treatment

This study aims to achieve synergistic strength-ductility enhancement of semi-solid squeeze cast 6TiB2/Al–17Si–4Cu composites through in-situ particle strengthening and T6 heat treatment. The microstructure and mechanical properties of 6TiB2/Al–17Si–4Cu composites with different heat treatments were investigated. TiB2 particles are semi-coherent with α-Al and primary Si, respectively. The nanoscale precipitate phases were determined by their crystal orientation relationships. The results showed that they are θ and θʹ phases. Meanwhile, the research results pertaining to the mechanical properties imply that the strength and ductility are enhanced by the TiB2 particles and Al2Cu precipitates. The optimum heat treatment process (solution treatment for 9 h at 520 °C and aging for 8 h at 170 °C) of strengthened 6TiB2/Al–17Si–4Cu composites is revealed in this study, when the ultimate tensile strength and elongation of the composites are 263.3 MPa and 0.51%, respectively. The mechanisms governing the enhanced strength and ductility of 6TiB2/Al–17Si–4Cu composites were discussed.

Hardness and Conductivity of Die Forged ZYK530 Magnesium Alloy

The relationship among the microstructure, hardness and electrical conductivity of the as-forged ZYK530 Mg alloy after heat treatment was analyzed and studied using a microscope, X-Ray Diffractometer, eddy current conductivity meter, and Vickers microhardness tester, to explore optimum heat treatment process of ZYK530 Mg alloy. The results show that: with the prolongation of holding time, the electrical conductivity and microhardness show the same change trend, both of which show an oscillatory upward trend, and then decrease in an oscillatory downward trend after reaching the peak value. There is a linear positive correlation between the conductivity and the hardness, and the fitting results of the conductivity and hardness are in good agreement with the measured results; combined with the actual production, when the heat-treatment is 480 ℃ × 8 h + 220 ℃ × 3 h, the highest hardness is 79.2 HV, the electroconductivity is 36.2%IACS, and the comprehensive performance is the best, which is the best heat treatment process.