Study on Surface Engineering of Normalized Steels Subjected To Machine Hardening

IN INDUSTRY different heat treatment methods areused for production of required surface layer properties.A classification of the surface hardening methods isbased upon three main effective mechanisms viz.,mechanical,thermal and thermo chemical alterations ofsurface layers.Based on the specific chip removal and chipformation mechanisms a substantial part of cuttingenergy in machining is transformed into thermal energy.Depending on the machining conditions and type ofmachining process,the he…     

Parameter Optimization Study on Control of Distortion in Bright Bar Used In Rack Application Subjected To Induction Heat Treatment

The engineering properties of metals and alloys are related to their structures. The change in mechanical properties of the metals and alloys can be achieved by the process of heat treatment. Induction hardening is one such a process involves phase transformation by rapid heating and cooling of the outer surface. Induction hardening improves the outer surface hardness and wear resistant properties keeping the original toughness and ductility in the inner core. However past experiences shows that during heat treatment, parts have undergone dimensional changes due to thermal fluctuations and phase transformations. Dimensional changes can lead to excessive distortion in the component which always presented difficulties to the uses of many varieties of steels which can be hardened by induction hardening. The dimensional changes in components which have been induction hardened have for a long time proved costly and troublesome to manufacturers.These difficulties apply particularly in the automotive industry where the amount of distortion in rack and pinion assembly has been related to their noise level in operation and investigators have suggested a link between distortion and the initiation of fatigue failure. [1] Although the complete elimination of distortion would be an ideal aim, manufacturers are reconciled to the fact that a certain amount of distortion is an inherent part of the hardening process. However, if the amount of distortion is uniform and predictable, an allowance could be made in the initial machining operations. It was with these aims that investigations into the control of distortion in bright bar (EN 18D steel) used in rack application (Power steering assembly) have been carded out by parameter optimization study.     

表面纳米化对材料性能影响的研究进展与展望

工程零部件失效常源于表面,微组织结构显著影响甚至决定工程零部件使役性能,表面纳米化技术可诱导材料微组织结构变化产生纳米晶结构表面层,增大表层残余压应力,对材料性能有极其重要的影响。首先综述了表面纳米化诱导微组织结构变化的过程及机理,诱导材料产生晶粒细化、位错运动、残余压应力增大、相变等微观变化,诱因有塑性变形、温度变化、元素渗入等。其次归纳了表面纳米化对材料性能的影响及其机理,上述微观变化对材料疲劳强度、耐腐蚀性、摩擦磨损性能、生物学性能等产生显著影响。总结了各个典型表面纳米化工艺的特点,相比于其他表面纳米化技术,超声振动辅助加工具有不需引入其他元素、不污染环境、原理简单、高速高质量、成本低廉、可依托于各种传统加工工艺等优势,对材料摩擦磨损性能、疲劳性能、生物学性能、表面浸润性和耐腐蚀性等具有积极作用。最后对表面纳米化工艺的未来发展做了展望,其中针对性分析了超声振动辅助加工。针对纳米晶结构表面层的数字化仿真模拟极其匮乏这一现状,将模拟仿真与试验相结合,分析微组织结构与加工参数、微组织结构与材料性能的映射关系并建立模型直观反映尚需更全面系统的研究。材料的某些性能可能不会同时达到最优值,依...     

Mechanical surface treatments of lightweight materials—Effects on fatigue strength and near-surface microstructures

Mechanical surface treatments such as shot peening or deep rolling are well-known processes to improve the fatigue strength of metallic components. This is due to favorable microstructural alterations in relatively thin surface layers as a consequence of near-surface inhomogeneous plastic deformations. Typical examples demonstrate the fatigue-strength increase for mechanically surface-treated specimens. Existing possibilities to improve the fatigue strength of welded joints by mechanical surface treatments are also included. In the case of lightweight materials (e. g. magnesium- or aluminum-base alloys), process parameters must be well adapted in individual cases to achieve optimum near-surface material states, taking into account the wide range of mechanical properties attainable as a result of their specific material microstructure. The effects of process parameters and microstructures on near-surface materials properties resulting from mechanical surface treatments are demonstrated with examples. Depth distributions of macroresidual and microresidual stresses are analyzed together with microstructural observations. An important point for the effectiveness of mechanical surface treatments is the stability of the near-surface material states during loading history. This aspect is treated for the case of fatigue loading.     

冷加工和时效处理顺序对6061铝合金力学性能的影响(英文)

研究不同的析出硬化和冷加工组合对6061铝合金拉伸性能的影响。结果表明,在不同的热处理过程中,在180℃单时效4h能提高合金的强度和伸长率。然而,双时效处理不能改善其力学性能。另外,预时效对随后的析出硬化有负面影响。合金力学性能的变化归因于析出硬化、应变硬化和加工软化的竞争而引起的显微组织演变。     

Surface hardening by strain induced martensitic transformation

The hardness and fatigue strength achieved by strain hardening are normally noticeable lower than those attained by thermal or thermochemical heat treatments. Strain or deformation induced martensitic transformation of residual austenite can increase the strength achieved by mechanical surface hardening processes considerably. In this paper, an approach is presented where workpieces with a high content of metastable austenite are used for hardening the surface layer. The microstructure has to be sufficiently stable, in order to ensure that the material can be machined without being changed by strain induced transformation of the residual austenite. After machining, high Hertzian contact stresses are introduced by deep rolling, so that a strain induced martensitic transformation of the residual austenite takes place. At the same time deep rolling produces the surface finish of the part. By this method, a surface hardening without a heat treatment process within the production line can be realized. A conceivable use of this method could be the production of bearings or guideways.     

腐蚀环境下自冲铆接头竞争失效机制及力学性能分析

为研究腐蚀环境下自冲铆接头失效模式的转变机理和力学性能特征,采用0.6 mol/L的NaCl溶液对多组自冲铆接头进行周浸腐蚀试验,通过力学测试和扫描电子显微镜对接头的力学性能及失效模式进行分析. 结果表明,异质接头的失效模式随腐蚀周期延长由 Ⅰ 向 Ⅲ 转变,转变速率取决于接头材质,同质接头的失效模式不受腐蚀周期的影响. 短期腐蚀下,搭接区板间腐蚀产物使得同质和异质接头失效载荷均有所上升;异质接头上升幅度较大,腐蚀后期其失效载荷较同质接头损失严重. 异质接头稳定性较差,凸模制备的同质组合接头较平模稳定性好. 异质接头电化学腐蚀剧烈,下板铆接点底部材料的脱落由应力集中所产生的撕裂纹所致.     

Management of materials properties in an electro-thermo-metallurgical simulation of quenching following inductive surface heating

A numerical model for simulating quenching following inductive surface heating is proposed. The metallurgical model, which is based on the superposition principle and uses isothermal data, is integrated within a computerized code for heat exchanges (Flux2D) in order to simulate the thermal kinetics at any point on a test piece. The profile of the zones transformed during a rapid cycle can then be computed. This has been made possible by constructing a coupling scheme that considers the interaction effects among heat transfer, magnetic, and metallurgical phenomena. The coupling is based on a new approach that involves separating the physical laws into equation form to describe the physical phenomena and properties of the material. The latter has been investigated in depth in order to manage the properties in a more representative manner by considering the effect of phase transformations on changes in these properties. An illustrative example compares a coupled thermo- metallurgical numerical simulation with experimental results for treated cylindrical test pieces.     

A coupled thermal and metallurgical model for welding simulation of Ti–6Al–4V alloy

The accurate prediction of the mechanical behavior of welded components made of Ti–6Al–4V requires a particular material model considering the significant effects of the material behavior during welding. Especially, phase transformations are assumed to have an influence on the temperature distribution. The flow curves of the material are changed during welding by complex mechanisms which might be describable using time-temperature history dependent flow curves. The following paper shows (as a step forward), how the influence of phase transformations on the transient heat conduction in components made of Ti–6Al–4V during tungsten inert gas (TIG) arc welding is investigated using a coupled thermal and metallurgical model. Kinetics of α+β→β phase transformation during heating and β→α phase transformation during cooling are studied using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. A numerical heat transfer model is used to calculate the transient temperature field during welding. The thermal properties are calculated by a linear mixing rule based on the phase fractions and the thermal properties of each pure phase. Using these obtained thermal properties, the welding process of Ti–6Al–4V alloy is modeled using finite-elements for the spatial discretization and finite-differences to predict the transient temperature field. Additional calculations neglecting the phase changes are carried out to compare the temperatures and visualize the effects of phase transformations on the cooling behavior. The comparison of these models with measurements shows that the model considering the influence of solid phase transformations describes the temperature profile during cooling accurately.     

Generation of hardened steel surfaces with adjustable roughness by means of a pulsed electron beam

In this work we are utilizing a Pulsed Cold Cathode Electron Beam (PCCEB) for steel surface treatment that presents distinct properties as compared to other beam sources for rapid heating of metal surfaces. The possibility of producing different surface transformations like, melting and solid-solid transformations by adjusting the gun control parameters is demonstrated. It is shown that for the same base material, different pulse conditions give rise to distinct structures, a roughened or smooth martensitic surface or a smooth pearlite layer several microns thick. For the liquid-solid transformation, it is shown for the first time that with an adequate impurity content the instability onset threshold (giving rise to a rough topography) is different from the melting threshold. Hence, by controlling the discharge parameters, it is possible to obtain different topographies with similar crystallographic structure and hardness, allowing an adjustment of the final roughness to the particular application requirements.     

金属表层梯度微结构对力学性能和裂纹扩展的影响

表层梯度结构广泛应用于工程材料,经表面强化技术处理后,零件表层结构呈梯度分布,可以改善零件的服役能力.综述了金属表层梯度微结构对力学性能和裂纹扩展的影响,从微观结构和力学性能的角度,结合非均匀材料中裂纹的扩展行为,考虑了微结构和力学性能之间的相互影响规律,分析了梯度结构中影响裂纹扩展的因素.经表面强化后,材料表面晶粒细化,晶粒尺寸沿纵深方向呈现梯度分布,改变裂纹萌生位置,同时对裂纹扩展起到阻碍,使零件疲劳寿命延长.梯度结构中金相组织由表层单一组织到芯部多相组织的转变,引起表层硬度和残余压应力向纵深方向逐渐减小,表面压应力可以减弱外力的作用,从而影响疲劳寿命.然而,梯度结构中微观组织和力学性能互相关联,像晶粒和位错能改变残余应力的分布,也会对裂纹扩展路径产生影响,如何定量表征他们的影响还有待进一步研究.     

Mechanically driven phase transformation in single phase Al62.5Cu25Fe12.5 quasi-crystals: Effect of milling intensity

In this work the effect of mechanical milling on the structure, thermal stability and hardness of single phase Al_62.5Cu₂₅Fe_12.5 icosahedral quasi-crystals has been investigated for different milling intensities. The results indicate that, irrespective of the milling intensity used, the quasi-crystals transform to a body-centered cubic (bcc) phase during milling. This transformation starts when the grain size of the QC phase is about 10 nm, which represents the critical grain size initiating the phase transformation. Upon heating the milled powder displays grain growth of the bcc phase at low temperatures, followed by transformation to the original icosahedral QC phase at higher temperatures. The phase transformations occurring during milling and subsequent annealing have a remarkable effect on the indentation hardness, which can be tuned within a wide range (7–10 GPa) as a function of the volume fractions of the different phases. This suggests that a composite material with optimized mechanical properties can be produced by appropriate thermo-mechanical treatments.     

Some aspects of superplastic flow of eutectic alloys connected with metastability

Some features of the manifestation of superplastic properties in the Pb-62% Sn alloy have been investigated depending on the duration of aging after preliminary reduction. Mechanical tests have been conducted under creep conditions at a constant applied stress. Metallographic studies of the structure transformation in the process of superplastic flow have been performed at mesoscopic and macroscopic levels. X-ray diffraction investigations have been carried out. The experimental results have been analyzed in combination with the earlier data on the internal friction and changes in Young’s modulus depending on the regimes of aging. Based on the results of the analysis, a conclusion has been made which confirms the previously suggested assumptions that the superplasticity effect in the eutectic Pb-62% Sn alloy is determined by the interaction between the processes of plastic deformation and phase transformations under the action of an external tensile load.     

The effect of film composition on the structure and mechanical properties of NiTi shape memory thin films

Shape memory NiTi-based thin films approximately 2 μm thick were deposited onto Si (100) substrates at room temperature by simultaneous DC magnetron sputter deposition from separate elemental Ni and Ti targets. The effect of composition on film structure, surface morphology, transformation temperature and mechanical behavior was studied using variable temperature X-ray diffraction, atomic force microscopy, electrical resistivity, and nanoindentation. The films showed the expected shape memory and superelasticity behavior corresponding to the different film compositions, comparable with bulk properties. The transformation from the low temperature martensitic phase to the high temperature parent phase takes place above room temperature in Ti-rich and near-equiatomic films, and below room temperature in Ni-rich films. Mechanical properties of films investigated at room temperature by a series of nanoindentations at mN loads (indentation depth < 200 nm) with a spherical indenter demonstrate superelasticity in Ni-rich material and martensitic deformation for Ti-rich and near-equiatomic compositions.     

渗碳与激光相变强化复合处理16Cr3NiWMoVNbE 钢的组织演化

目的 增大16Cr3NiWMoVNbE钢经渗碳强化后的强化层深度，细化晶粒尺寸，提高表面力学性能，并减小工件热变形，缩短工艺周期。方法 将渗碳与激光相变强化相结合，利用“短时”渗碳提高表面含碳量，再通过激光快速局部加热，为碳原子扩散提供理想通道，改善强化层深度。通过光学显微镜、扫描电子显微镜、透射电子显微镜，分别评价材料的金相组织、高倍显微组织，并通过显微硬度计、纳米力学探针对激光相变强化处理后的硬化层截面硬度、纳米硬度、弹性模量进行测试，揭示渗碳和激光相变复合强化16Cr3NiWMoVNbE钢的组织演化和强韧化机理。结果 随着激光能量输入量的增加，复合强化层的深度提高了约50%，显微硬度最大值为792HV，显微硬度提高了约30%，弹性模量、显微硬度呈先增加后降低的趋势，强化层显微组织板条逐渐减少，且尺寸不断粗化，残余奥氏体由薄膜状转变为块状，数量逐渐增加，碳化物聚集球化且数量减少。结论 16Cr3NiWMoVNbE钢经渗碳和激光相变复合强化后，得到了塑韧性优异的复合强化层，为航空发动机关键传动部件表面强化提供了新思路和理论支撑。     

Macro- and microscopic simulation of materio-thermo-mechanical fields

Two kinds of computer simulation are made for some engineering processes incorporating phase transformation. One is the macroscopic or continuum mechanics approach based on the theory of metallo-thermo-mechanics developed byt eh authors, and the other is the microscopic, or atomic ledvei simulation by use of molecular dynamics. In each case, emphasis is focussed on the effect of coupling among material structures, temperature and stress/strain during in-solid or liquid-solid phase transformations. Some illustrative examples are presented.     

热处理工艺对KD级抽油杆用钢组织和性能的影响

采用力学、OM、SEM和TEM研究了某KD级抽油杆用试验钢经3种不同的工艺热处理后力学性能及微观组织的变化。结果表明:经室温入炉回火热处理的材料可获得的综合力学性能较好。500 ℃入炉回火的热处理工艺使材料冲击性能最差,其原因是粒状贝氏体中M+A岛分布不均匀、形状各异且以带尖角的大块状居多,造成应力集中;第二相颗粒析出物的存在使得材料具有较高的强度。     

The continuum field approach to modeling microstructural evolution

The majority of advanced engineering materials contain multiphase and/or multidomain structures. Their physical and mechanical properties depend strongly on the number of phases present and their mutual arrangement; the volume fraction of each phase; and the shape, size, and size distribution of domains (or grains). This article describes a continuum diffuse-interface field approach to modeling microstructural evolution and its application to a number of different processes, including precipitation reactions through nucleation and growth, structural transformations involving symmetry changes, and curvature-driven grain growth.     

High-Strength Titanium Alloy VT16 for Manufacturing Fasteners by the Method of Cold Deformation

A titanium alloy hardened by cold working is described. An effective technology for large-scale production of parts from this alloy (bolts, screws, rivets, etc.) is suggested. Its physical and mechanical properties, structure, and phase transformations under heat treatment and cold deformation are studied. It is shown that the alloy and the method for fabricating parts from it are very efficient.     

Mechanical properties of metals for biomedical applications using powder metallurgy process: A review

The uses of biomaterials have been revolutionizing the biomedical field in deployment as implants for humans. During the past five decades, many implant materials made of metals have been put into practical use. Powder metallurgy techniques have been used to produce controlled porous structures, such as porous coatings applied for dental and orthopedic surgical implants, which allow bony tissue ingrowth within the implant surface, thereby improving fixation. This paper examines various important metals using powder metallurgy technology to produce elements of a total hip replacement. These alloys are 316L stainless steel alloy, Co−Cr−Mo alloy, and Ti−6Al−4V alloy. Also, this paper examines current information on the mechanical properties. Mechanical properties are discussed as a function of type of materials and process of fabrication. This article addresses the engineering aspects concerning the advantages and disadvantages of each type of material.