Discussion on the Free Quenching Heat Treatment Process of Automotive Leaf Springs

Free quenching of automotive leaf springs is a new technology that has gradually started to be applied in the industry in China in recent years.Only a few manufacturers are applying it in the industry.Through more than half a year of on-site practice,the changes in the hot forming of spring plates before free quenching have been explored,and finally a heat treatment process that meets the production requirements of our company has been developed,achieving normal production.

Flexible Furnace Concepts for Vacuum Heat Treatment Combined with High-pressure Gas Quenching

IN the past five years the process combination of vacuum hardening, respectively vacuum carburizing with high-pressure gas quenching was successfully introduced to the market, especially in the manufacture of gears. In the meantime furnace concepts for various applications are available to the industry. In the following report three plant varieties are introduced, which differ in process flexibility and throughput. This report also explains criteria for the selection of a furnace in view of the existing application requirements. Besides this a short introduction is given into the vacuum carburizing process and the high-pressure gas quenching technology.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Quenching and Partitioning Steel

In order to investigate the effect of microstructural characterization on the mechanical properties and retained austenite stability, a different type of quenching and partitioning steel（I-Q＆P） through intercritical annealing before the quenching and partitioning process was designed, which can realize lamellar intercritical microstructure compared to the conventional quenching and partitioning（Q＆P） process. The morphology of ferrite and martensite/retained austenite is lamellar in the I-Q＆P steel while it is equiaxed after being heat-treated by conventional Q＆P process. The I-Q＆P steel is proved to have better formability and mechanical properties than conventional Q＆P steel, which is due to the highervolume fraction of retained austenite in the I-Q＆P steel and confirmed by electron backscattering diffraction patterns and X-ray diffraction. Furthermore, the stability of retained austenite in I-Q＆P steel is also higher than that in conventional Q＆P steel, which is investigated by tensile tests and differential scanning calorimetry.

NUMERICAL SIMULATION OF FORGING AND SUBSEQUENT HEAT TREATMENT OF A ROD BY A FINITE VOLUME METHOD

A method to simulate processes of forging and subsequent heat treatment of an axial symmetric rod is formulated in eulerian description and the feasibility is investigated. This method uses finite volume mushes for troching material deformation and an automatically refined facet surface to accurately trace the free surface of the deforming material.In the method,the deforming work piece flows through fixed finite volume meshes using eulerian formulation to describe the conservation laws,Fixed finite volume meshing is particularly suitable for large three-dimensional deformation such as forging because remeshing techniques are not required, which are commonly considered to be the main bottelencek in the ssimulations of large defromation by using the finite element method,By means of this finite volume method, an approach has been developed in the framework of 'metallo-thermo-mechanics' to simulate metallic structure, temperature and stress/strain coupled in the heat treatment process.In a first step of simulation, the heat treatment solver is limited in small deformation hypothesis,and un- coupled with forging. The material is considered as elastic-plastic and takes into account of strain, strain rate and temperature effects on the yield stress.Heat generation due to deformation,heat con- duction and thermal stress are considered.Temperature - dependent phase transformation,stress-in- duced phase transformation,latent heat,transformation stress and strain are included.These ap- proaches are implemented into the commerical commercial computer program MSC/SuperForge and a verification example with experimental date is given as comparison.

Effect of Heat Treatment on Microstructure and Mechanical Properties of NF6357A Cast Alloy for Wear Resistance Application

The solidification structure of the as-cast consists of the matrix structure that is predominantly austenite and precipitated chromium carbide along the grain boundary. Under these circumstances and where the level of impact is relatively modest, such alloys in as-cast condition will perform. However, at higher levels of impact energy, a point is reached where excessive stress are built up within the component and eventually the materials strength is exceeded and the outcome is complete failure in a characteristic stress fracture mode. If this is to be prevented, it is therefore imperative that the casting be subjected to appropriate heat treatment, to obtain a structure which consist of Cr7C3 carbide and martensite at a hardness range of 650-750HB. The microstructure of NF6357A cast chromium steel containing 2.59% C- 0.7%Si-0.91%Mn-18.54%Cr-0.019%P-0.01%S- balance–Fe after appropriate heat treatment such as quenching and tempering process have been characterised by means of optical microscope, micro hardness tester, optical emission spectrometer and charpy testing machine. The results show that oil quenched samples were found to retained microstructural consistency for casting thicker than 120mm section. For economic argument, air quenched castings of less than 120mm thickness is not only cheaper alternative, but it is also environment friendly. The fracture toughness was found to be fairly consistent between 2.4-2.6%C range. However, at higher carbon level, the fracture process is dominated by the presence of segregated carbide network which act as a weak link in the microstructure. This weak link encourages dislocation pile-up and impaired material toughness.

Heat Treatment of Modified 6005A Alloy for Vehicles

Modified 6005A alloy was reported and its mechanical properties were studied by tensile test, hardness measurement and TEM analysis. Results show that the favorable aging condition for 6005A is 175°Cfor 8h, and the nose temperature of its TTP (Time-temperature-properties) curves is about 370°C. In a moderate temperature zone (270°C <T <390°C), the alloy has much high quench sensitivity. Therefore, in on-line extrusion, water spray quench and moderate temperature should be taken to enhance quench rate, then 6005A alloy can be quench hardened and its quenching distortion can be

Microstructure and Mechanical Properties of 50SiMnNiNb Steel by a Novel Quenching-Partitioning-Austempering Heat Treatment

For the purpose of reducing weight of steel parts, save raw materials and keep or even improve safety standards, the development of advanced high strength steels is increasingly demanded in the automotive industry and engineering applications. We have proposed a novel heat treatment （quenching-partitioning-austempering treatment, Q-P-A） to obtain steel parts with high strength and good ductility. The Q-P-A process is intended to produce microstructure consisted of carbon-depleted martensite, carbon-enriched retained austenite and nanostructured bainite. Quenching（Q） treatment fabricates mixed microstructure of carbon-supersaturated martensite and certain amounts of untransformed austenite. Partitioning（P） thermal treatment accomplishes fully diffusing of carbon from the supersaturated martensite phase to the untransformed austenite phase and enriching the amount of carbon in untransformed austenite. Further low-temperature austempering（A） process induces incredible thin bainite from the carbon-enriched untransformed austenite. A study of the microstructure and mechanical properties of 50SiMnNiNb steel subjected to the novel Q-P-A treatment is presented. Microstructure is assessed by optical microscope（OM）, field emission scanning electron microscope（FESEM） and transmission electron microscope（TEM）, and the corresponding mechanical properties are measured. The experimental results indicate that attractive mechanical properties of steels during the Q-P-A process are attributed to the complex multi-phase structure. Slender plates of bainite with 20-40 nm thick are generated in the medium carbon steel. Meanwhile, with increasing of the volume fraction of nanostructured bainite, yield strength of steel parts is increased with little degradation of ultimate tensile strength. In this paper, a novel quenching-partitioning-austempering heat treatment is proposed, and the attractive mechanical properties of steels are obtained during the Q-P-A process.

Identification of Heat Transfer Coefficients by Use Conditions of Quenching Oil

Heat transfer coefficients of the quench medium are necessary for heat-treatment simulation. Cooling characteristics of quenching oil vary with kinds and usage greatly. Users are selecting oil solutions that come up to their desired hardness and quenching distortion requirements. In particular cooling performance rises by agitation and decompression. Therefore we identified a heat transfer coefficient by usage and kinds of quenching oil. Cooling characteristics are different greatly by a kind of quenching oil. A difference of a cooling characteristic by a kind of oil depends on a temperature range of a boiling stage and the maximum heat transfer coefficient mainly. On the other hand, in a convection stage, there are few changes in a boiling stage. Even if quenching oil temperature is changed, heat transfer coefficients do not change greatly. When quenching oil stirred, heat transfer coefficients of vapor blanket stage and a convection stage rise, but there are a few changes in a boiling stage. When quenching oil is decompressed a temperature range of a high heat transfer coefficient moves to the low temperature side. In addition, a heat transfer coefficient in a vapor blanket stage comes down. For precision improvement of heat-treatment simulation, it is important that the heat transfer coefficient is calculated in conformity to the on-site use reality.

Transformation Plasticity─The Effect on Metallo-Thermo-Mechanical Simulation of Carburized Quenching Process

Theory of metallo-thermo-mechamics and the developed CAE code offer a powerful tool to simulate residual stresses and distortion during heat treatment processes, in which the transformation plasticity is one of factors to be considered being coupled with stress/strain and metallic structure. It is pointed out in this paper that, especially in the case of carburized quenching, transformation plasticity plays very important role on the distortion, which is verified by axisymmetric finite element, employing heat treatment simulation code "HEARTS". Simulated results with the careful consideration on the effect of transformation plasticity reveal to improve remarkably the accuracy of prediction of the displacement and the mode of distortion, compensating the discrepancy between experimental and calculated results. Attention is also paid on the difference in transformation plasticity and conventional plasticity in simulating the volume fraction, stress and strain in ring-shaped specimen during quenching. Moreover some discussions are made on practical use of the effect, and recent experimental results on the coefficient of transformation plasticity are presented.

Flow-Optimization to Enhance Gas Quenching Efficiency for Helical Gears Specimen

Numerical simulations based on a conjugate heat transfer solver have been carried out to analyze various gas quenching configurations involving a helical gear streamed by an air flow at atmospheric pressure in a gas quenching chamber. In order to optimize the heat transfer coefficient distribution at key positions on the specimen, configurations involving layers of gears and flow ducts comprising single to multiple gears have been simulated and compared to standard batch configurations in gas quenching. Measurements have been performed covering the local heat transfer for single gears and batch of gears. The homogeneity of the heat transfer coefficient is improved when setting up a minimal distance between the gears (batch density) and when introducing flow ducts increasing the blocking grade around the gears. An offset between layers of the batch as well as flow channels around the gears plays a significant role in increasing the intensity and the homogeneity of the heat transfer in gas quenching process.

Numerical calculation of temperature and phase change during the process of atomizing spray quenching on drilling pipe

A finite element method (FEM) procedure was developed in order to simulatethe quenching process for drilling pipe (DP). The calculating model was based ontime-temperature-transformation (TTT) diagrams, and incorporated with material properties dependenton temperature. The procedure was used to calculate the temperature-time histories, describe thephase transformations of atomizing spray quenching for DP in the welding zone, and predict thehardness distribution in radius direction after quenching in the zone. The calculated results metwell with that of experiments. It was easy to determine the parameters such as volume and pressureof the cooling water and compressed gas by use of the numerical calculation and experiments, becausethe value of convection coefficient was decided greatly by the mixture of the cooling water andcompressed gas. Moreover, the simulating results were helpful not only to design the quenchingequipment, but also to optimize the quenching process for DP's welding zone.

Preliminary Study to Identify Criterion for Quench Crack Prevention by Computer Simulation

A criterion for preventing quench cracks has not been established although much research has been conducted in this field. A few predictions of cracking have been recently reported using simulated results of heat treatment processes. However, these results were insufficient to predict crack generation clearly because the mechanism relates not only to stress distributions in the parts but also to material characteristics. Systematic studies by the combination of experiment and simulation are therefore required to reliably identify a criterion for crack prevention by computer simulation. As a preliminary work before starting the full-scale research, a review of quench cracking research was conducted and existing test methods were classified. Several test results for cylindrical specimens were simulated for considering a cause of the crack generation. Lastly a tentative plan was made for the future research.

Verification of Inverse Quench-Hardening Phenomena in Bearing Steel Specimens by Experiment and Computer Simulation

The appearance of lower hardness at the surface than at the core after through-hardening of steels is known as "inverse quench-hardening." Pioneering work was performed in the 1970s by establishing a simple test procedure where cylindrical specimens were cooled by air and brine sequentially to produce the effect. This phenomenon was described in relation to the polymer quenching and its effect to the improvement in the fatigue strength of the steel in the 1990s. The hardening mechanism in the same specimens as the pioneering work was recently examined using a finite element simulation method. However, this work has not been completed for lack of experimental quenching cooling curves for the comparisons. In this study, the same experiments using 20mm diameter cylindrical specimens as those of the pioneering work were conducted to obtain cooling curves and hardness distributions for comparing simulated results. Experimental cooling curves showed a temperature recovery as predicted by the simulation. Also the inverse quench-hardening phenomena were found in the measured and simulated hardness distributions of the specimens.

热处理工艺对45钢显微组织及力学性能的影响

为优化45钢的综合力学性能,在标准化退火+淬火回火的基础上,设计了3种不同冷却速率的淬火工艺,分别为油淬火、NaNO3盐浴淬火和水淬火。利用光学显微镜、扫描电子显微镜等设备观察了3种淬火状态下的钢料显微组织,发现冷却速率对45钢的马氏体片层尺寸、密度分布具有显著影响。运用拉伸试验机、砂砾冲击试验机,测试了热处理后钢的抗拉强度、屈服强度、延性及冲击韧性。结果表明:NaNO3盐浴淬火处理后的钢在屈服强度、抗拉强度、延性方面均优于油淬火和水淬火处理后的钢;油淬火处理后的钢的冲击韧性最高。研究证明,适当控制淬火冷却速率,可以有效优化45钢的力学性能。

基于强度梯度的传动轴热处理-硬度梯度匹配研究

以某汽车等速万向传动中间轴承受扭转载荷为例,提出了基于强度梯度的热处理-硬度梯度设计方法,即根据结构危险截面最大应力及其梯度分布得到理想强度梯度分布,再由硬度-强度转换关系得到理想硬度梯度分布,最后根据理想硬度梯度分布结合材料端淬曲线进行实际硬度梯度匹配。分别对采用40Cr、UC2和38B3三种材料的传动轴进行硬度梯度匹配,并对硬度梯度匹配后的传动轴进行了静强度试验。试验结果表明,采用不同材料的传动轴经热处理-硬度梯度匹配后,其静强度均能满足试验要求。

淬回火工艺对Cronidur30钢组织及性能的影响

利用扫描电镜、摩擦磨损试验机和X射线衍射仪等研究了淬火温度、冷处理温度和回火温度对Cronidur30钢的硬度、残余奥氏体含量以及摩擦磨损性能和耐腐蚀性的影响。结果表明:淬火温度为1 025,1 060℃时,回火态试样硬度均不低于59 HRC,1 025℃淬火试样的残余奥氏体含量略低且组织更加均匀细小;淬火温度为1 025℃时,-75和-196℃这2种冷处理温度对试样硬度和残余奥氏体含量影响的差别不大;淬火温度1 025℃,冷处理温度-75℃时,155,480℃回火温度下试样的硬度均高于58 HRC,155℃回火后试样的减摩性优于480℃,耐磨性则略低;随着回火温度升高,试样的耐腐蚀性下降,回火温度550℃时试样的耐腐蚀性最差。

薄规格高强度工程机械用钢板在线淬火工艺开发

通过对薄规格高强度工程机械用钢研发过程中的技术难题进行了分析,并有针对性采取了措施,使得20~29 mm厚度钢板在线淬火工艺得以成功应用。通过对轧制道次的优化提高了轧制板形、通过冷却单元上下水比、边部遮挡优化保证了冷却的均有性,通过钢板头部跟踪位置优化解决了头部过冷问题,为水冷后板形的改善创造了条件。通过头尾遮挡参数及热处理回火工艺优化,保证了钢板性能的均匀性,实现了薄规格钢板的短流程生产,取得了较好的经济效益和社会效益。

冷却速率对钴铬钼合金微观组织及力学性能的影响

钴铬钼合金具有较好的生物相容性及耐磨性,是骨关节植入物用最广泛的金属材料。研究了1 150℃保温2 h后空冷、风冷、水冷、气淬不同冷却速度的热处理工艺对钴铬钼合金材料组织与性能的影响。结果显示:随着冷却速度的提升,钴铬钼合金中的碳化物强化相更为细小、更为弥散地分布于基体中,材料强度及塑性均得到较大幅度提升。在真空条件下1 150℃保温2 h,氩气冷却冷速在150℃/min条件下,材料抗拉强度、屈服强度、伸长率分别达889 MPa、585 MPa、13%。

直连绳索取心钻杆分区热处理技术研究

为了提高绳索取心钻杆的性能,对直连绳索取心钻杆的受力情况、结构特点以及传统处理工艺进行分析,提出了分区热处理的工艺。通过对两端进行热加工处理,使钻杆中间管体区域和两端热处理螺纹区域性能出现差异,提高两端综合强度的同时改善螺纹部分的受力状态,可使钻杆柱既具有高强度又兼具柔性,大幅提高钻杆机械性能以及耐磨性。其中调质硬度HRC33~38能在保证管体延伸率与正火、回火基本一致的情况下,强度提高37.7%,硬度提高40%,抗冲击性提高46.7%。调质硬度HRC38~42时,材料的耐磨性提高9.1%。更适合全液压钻机施工及深孔钻进,为钻探施工提供了钻机、钻杆配套性的优选方案。

基于材料成分波动的22CrMo锥齿轮渗碳淬火性能评估

以22CrMo材料锥齿轮为研究对象,通过有限元方法,研究了材料成分波动对锥齿轮渗碳淬火性能的影响。利用材料数据库软件,基于22CrMo材料元素范围的上、下限建立2组材料性能数据库;利用热处理仿真软件建立模型,并对2组材料成分的被动锥齿轮进行渗碳淬火仿真,对比分析了锥齿轮渗碳淬火后表层碳浓度、微观组织演化及齿轮变形等数据。结果表明,材料成分波动对22CrMo锥齿轮淬火后的相组织分布以及齿轮变形量影响较大,对渗碳层影响较小。