Grinding–hardening using dry air and liquid nitrogen: Prediction and verification of temperature fields and hardened layer thickness

This paper investigates the grinding–hardening both theoretically and experimentally with a plunge surface-grinding process. Theoretically, the paper presents a temperature-dependent finite element heat transfer model, incorporating a triangular moving heat source and various cooling conditions, to investigate the phase transformation kinetics, thus to predict the thickness of a layer hardened. The temperature variation and thickness of the hardened layer were also investigated experimentally on quenchable steel 1045 using dry air and liquid nitrogen as the cooling media. The predictions were in good agreement with the experimental results. It was found that the phase transformation follows the martensitic kinetics. The application of liquid nitrogen enhances the transformation of retained austenite to martensite and results in a refinement of the martensitic structure.     

双程切入磨削表面硬化层的研究

采用缓进给磨削方式和不同的磨削行程对40Cr钢进行了磨削淬硬，对单程和双程磨削表面硬化层的组织与性能进行了比较。结果表明，尽管单程和双程磨削时的原始组织不同，但两者的表面硬化层组织及其变化趋势完全相同。双程磨削淬硬可使表面硬化层深度及其均匀性、显微硬度及耐磨性得到进一步提高；但其最大残余压应力值及应力影响深度有所降低。双程磨削硬化层组织的形成机制同样适用于多程磨削淬硬或淬硬钢的磨削淬硬等场合。     

Realisation of grinding-hardening in workpieces of curved surfaces—Part 1: Plunge cylindrical grinding

This paper investigates the feasibility to achieve grinding-hardening in a plunge cylindrical grinding process. To understand the mechanisms, a temperature-dependent finite element heat transfer model incorporating a triangular moving heat source was developed to describe the temperature field, thus to predict the thickness of the grinding-hardened layer. The analysis carried out included the variation effect of depth of cut caused by the change in wheel-workpiece engagement. The model was applied on quenchable steel 1045 and the analysis was verified experimentally. It was shown that the heating cycle in plunge cylindrical grinding is the result of consecutive heating and cooling processes, varying from location to location in a workpiece. The ratio of the workpiece speed to the infeed rate plays an important role in the heat treatment cycle.     

The effect of machine stiffness on grinding of silicon nitride

This study investigates the effect of machine stiffness on normal forces, actual depth of cut, and workpiece strength in grinding of silicon nitride. To obtain a grinding system with an adjustable stiffness, a compliant workholder is added to a precision grinder. Single-pass and multi-pass grinding experiments are conducted to evaluate the effect of machine stiffness. Cup-type diamond wheels of two different bond types and three grit sizes are used in the grinding experiments. Static and dynamic simulation is carried out to correlate grinding forces and actual depth of cut with machine stiffness. Since the simulation uses a time-domain model, it can accommodate non-linearities caused by the effect of machine stiffness on grinding forces and actual wheel depth of cut, workpiece regeneration, wheel wear, as well as wheel bond type and grit size effects, etc. Particularly, the model allows simulating grinding instability and the interference phenomenon due to residual material removal in multi-pass grinding. The study concludes that both simulation and experimental results have a good agreement.     

Thermal Analysis of Grinding

Thermal damage is one of the main limitations of the grinding process, so it is important to understand the factors which affect grinding temperatures. This paper presents an overview of analytical methods to calculate grinding temperatures and their effect on thermal damage. The general analytical approach consists of modeling the grinding zone as a heat source which moves along the workpiece surface. A critical factor for calculating grinding temperatures is the energy partition, which is the fraction of the grinding energy transported as heat to the workpiece at the grinding zone. For shallow cut grinding with conventional abrasive wheels, the energy partition is typically 60%-85%. However for creep-feed grinding with slow workspeeds and large depths of cut, the energy partition is only about 5%. Such low energy partitions are attributed to cooling by the fluid at the grinding zone. Heat conduction to the grains can also reduce the energy partition especially with CBN abrasives which have high thermal conductivity. For High Efficiency Deep Grinding (HEDG) using CBN wheels with large depths of cut and fast workspeeds, preheated material ahead of the grinding zone is removed together with the chips, thereby lowering the temperature on the finished surface. Analytical models have been developed which take all of these effects into account. Much more research is needed to better understand and quantify how grinding temperatures affect the surface integrity of the finished workpiece.     

砂轮特性对40Cr钢磨削淬硬层的影响

以平面磨削淬硬试验为基础，研究了不同砂轮特性条件下40Cr钢磨削淬硬层的组织与性能。结果表明，在磨削淬硬加工中的热、机械耦合作用下，砂轮特性对磨削淬硬层的马氏体组织形貌及其高硬度区硬度值无显著影响。随着砂轮粒度或砂轮硬度的提高，磨削淬硬层深度相应增加。与树脂结合剂砂轮相比，采用陶瓷结合剂砂轮可使淬硬层深度增加近40％。     

Micro-Analysis of the Contact Zone of Tribologically Loaded Second-Phase Reinforced Sol-Gel-Abrasives

Abrasives manufactured via the Sol-Gel route have increased the grinding efficiency of conventional abrasive grinding tools to a remarkable degree in the past ten years. Little is known, however, about detailed wear mechanisms of the abrasive on a nanometer scale. Sliding tests on a pin-on-disk tribometer and single grit scratching tests have therefore been performed in order to identify the wear mechanisms of these innovative materials. TEM-analysis has revealed that severe plastic deformation combined with both crack bridging and crack deflecting effects in the second phase characterise the abrasive rim zone. The surface of the abrasive is covered with an oxide debris layer which substantially improves the tribological behaviour.     

Ni-Cr-W-Mo合金曲母线异型件多道次热旋成形数值模拟

针对热旋成形过程中起旋断裂以及边缘开裂等零件质量问题,基于ABAQUS/Explicit平台建立了Ni-Cr-W-Mo高温合金曲母线异型件多道次热旋弹塑性有限元模型,研究了多道次热变形过程中的应力、应变规律以及缺陷形成机制,探讨了工艺参数对第2道次热旋成形过程的影响规律,最后通过实验验证了模型的可靠性。结果表明:在第1道次热旋过程中,由于应力差的存在,很容易使板材内外表面产生不均匀变形,甚至拉裂;在第2道次成形过程起始阶段(2~5 s内),等效应变增大过快,易出现材料堆积及起皱现象;此外,当旋轮圆角半径(R=10 mm)及旋轮进给比(f=1.0 mm·r~(-1))过大时,均会导致旋压件产生褶皱现象。     

中频淬火凸轮轴磨削裂纹产生原因及对策

对球墨铸铁中频淬火凸轮轴磨削裂纹产生的原因进行了分析。认为磨削工艺不当,产生了地高的磨削热,使凸轮浅表层组织回火过度,硬度大幅下降,从而产生极大的拉应力是磨削裂纹产生的主要原因。采取对策后取得了较满意的效果。     

On the mechanics of the grinding process, Part II—thermal analysis of fine grinding

Thermal analysis of fine grinding is conducted taking into consideration the stochastic nature of the distribution of abrasive grains and its role under fine grinding (dry) conditions to determine the grinding temperatures and the heat partition at the contacting interface. The analysis considers the grain–workpiece interactions at the local level and the wheel–workpiece interactions at the global level. The workpiece temperature in the grinding zone is taken as the sum of the background temperature due to distributed action of all the previous active grains operating in the grinding zone (global thermal analysis) and the localized temperature spikes experienced at the current abrasive grain tip–workpiece interfaces (local thermal analysis), similar to the work reported in the literature. Since the Peclet number, N_Pe, in the case of fine grinding is very high (a few hundred), the heat flow between the work and the contacting abrasive grains can be considered to be nearly one-dimensional. In this paper, we consider the interaction between an abrasive grain and the workpiece at the contact interface. Consequently, the heat source relative to the grain is stationary and relative to the workpiece is fast moving. The interface heat source on the grain side as well as on the workpiece side is equivalent to an infinitely large plane heat source (with the same heat liberation intensity as the circular disc heat source). However, it will be shown in the paper that the contacting times are different. For example, the abrasive grain contacts the heat source, as it moves over the interface, for a longer period of time (˜milliseconds) whereas the workpiece contacts the heat source for a shorter period of time (˜a few microseconds). The equivalent thermal model developed in the present investigation is simple and represents the process more realistically, especially the heat partition. The analytical results reported here are found to be in good agreement with both the analytical and experimental results reported in the literature by other researchers.     

超声振动辅助缓进给磨削温度场仿真与试验分析

目的通过对比研究磨削过程中超声振动辅助缓进给磨削工件表面的温度变化,验证超声振动对磨削热的影响,为进一步研究磨削机理提供依据。方法基于磨削温度场解析模型,建立了磨削热源平均强度。运用ANSYS软件热分析模块分别对普通缓进给磨削和超声辅助缓进给磨削进行了工件表面温度场仿真,得到了不同载荷步的温度场分布以及工件表面的温度时间变化曲线,较准确地反映了磨削工件时工件表面的温度变化。结果试验和模拟表明,缓进给磨削工件时,工件表面温度较高,对工件施加超声振动后,能够有效降低磨削力,减少磨削过程中产生的热量,降低工件表面温度20%左右。结论超声振动辅助磨削工件时,由于工件高频振动导致磨粒与工件间断性接触,使磨削过程变为有规律的脉冲状断续磨削,有利于工件散热,降低了磨削温度,为避免缓进给磨削时容易出现的磨削烧伤现象提供了技术支持。     

Surface Form Memory in NiTi: Energy Density of Constrained Recovery During Indent Replication

Spherical indentation of NiTi shape memory alloys (SMA) to depths greater than about 3% of the indenter radius results in two-way shape-memory training in a deformation zone beneath the indent. If deep spherical or cylindrical indents are subsequently machined away just sufficiently to remove traces of the original indent (in the martensitic condition), a thermally induced and cyclically reversible flat-to-protruded surface topography is enabled. We term the phenomenon surface form memory. The amplitude of cyclic protrusions, or ‘exdents’, is related to the existence of a subsurface deformation zone in which indentation has resulted in plastic strains beyond that which can be accomplished by martensite detwinning reactions. Dislocation generation in this zone is thought to underlie the observed two-way shape-memory (TWSME) training effect. In this article, we show that these cyclic exdents can perform appreciable mechanical work when displacing under load against a base-metal substrate (constrained recovery). This “non-Hertzian” indentation, which appears to be able to exert the full energy density of SMA actuation, may have use for assembly of micromachines, bond-release, microforging, microjoining, electrical switching, microconnectors, and variable heat transfer devices, among many other potential applications.     

磨削硬化深度预测

磨削硬化是利用磨削过程中产生的热、机械复合作用直接对工件进行表面淬火的新工艺。通过建立磨削温度三维分析模型和热金属效应分析,实现磨削硬化加工工件硬度的预测。基于瞬时温度分布和运动非稳定三维热传导微分方程,并考虑砂轮与工件及冷却液与工件交互作用时热传导情况和材料本身的热扩散,建立了磨削温度三维分析预测模型,结合对加工过程奥氏体相位比例的计算及珍珠岩、残余奥氏体和马氏体的转变等热冶金效应分析,得出磨削硬化加工后硬化深度,实现随加工参数变化的硬化深度分布预测。将此模型与有限元模型进行对比,并通过实验进行了验证。     

中碳钢缓进干磨表面硬化层特征

在平面磨床上采用缓进干磨方式对两种常用中碳钢进行了磨削淬硬试验，研究了表面硬化层特征。结果表明，中碳钢磨削表面硬化层具有多样化的组织分层以及相应的微观组织特征，这是温度梯度、应变梯度以及时间等因素综合作用的结果。同时，磨削表面硬化层具有相似的硬度分布特征，即高硬度的完全硬化区以及硬度值缓慢下降的过渡区，这是缓进给磨削温度场分布的直接结果。     

基于温度匹配法的平面磨削3D有限元仿真及试验

为了准确预测工件表面及亚表面不同深度的温度场变化，基于反热源原理，以实际测量的磨削温度为基础，采用温度匹配法建立适应真实磨削加工时接触区的热源模型。运用有限元法，仿真计算工件磨削温度场的变化，并与瑞利分布热源模型预测结果对比，对工件表面及亚表面不同深度磨削弧区的磨削温度场进行测量。结果表明:基于温度匹配法建立的热源模型模拟的表面及亚表面不同深度温度场与实测值具有很好的一致性，相对误差在3.0%～7.5%，比瑞利热源模型预测的温度场分布精度提高了近2倍。      

An analysis of grinding power and surface roughness in external cylindrical grinding of hardened SCM440 steel using the response surface method

The aim of this study was to analyze effectively the grinding power spent during the process and the surface roughness of the ground workpiece in the external cylindrical grinding of hardened SCM440 steel using the response surface method. A Hall effect sensor was used for measuring the grinding power of the spindle driving motor. The surface roughness was also measured and evaluated according to the change of the grinding conditions. Response surface models were developed to predict the grinding power and the surface roughness using the experimental results. From adding simply material removal rate to the contour plot of these mathematical models, it was seen that useful grinding conditions for industrial application could be easily determined.     

脆性材料平面磨削温度场的测量及分析

磨削温度直接影响砂轮寿命、加工成本和工件质量,一直是磨削加工领域研究加工过程及其本质的重点.获得磨削弧区温度及工件实际的温度场分布是研究磨削热机理的基础.本文采用NEC TH31-110红外热像仪,测量了平面干磨削脆性材料时的热像图,获得了工件整体温度场分布及沿层深的温度分布数据,确定了工件磨削接触区的最高温度及其准确...     

Temperature distribution in a workpiece during cylindrical plunge grinding

In cylindrical plunge grinding, a large amount of heat flows into the workpiece continuously, accumulates and remains even after the process, which causes dimensional error. Therefore it is necessary to investigate the temperature distribution in the workpiece during grinding and analyze the influence of grinding heat on the dimension. Such an investigation has not been done enough, because the technology to measure the temperature distribution in the rotating workpiece has not matured. Considering such background, an in-process measuring system has been developed, which makes it possible to detect the temperature distribution in a wide range from the outer surface to the inside of the rotating workpiece. The system consists of small temperature sensors which are embedded into the workpiece, a micro computer attached on the workpiece which acquires the data from the sensors and transmits to a personal computer by a wireless communication device. Furthermore the contact type thermocouple which enables to measure the rotating surface temperature is added to the system. Measurement experiments revealed that the grinding heat conducts from the workpiece surface toward the center, accumulates, and remains in the workpiece even after the process. Heat conduction simulation was also performed. Good agreement was achieved between the simulated temperatures and experimental measurements.     

Characterization of laser treated steels using instrumented indentation by cylindrical flat punch

The aim of the present work is to assess the possibility of predicting the thickness of the hardened layers produced by High Power Diode Laser (HPDL) heat treatment. Instrumented indentation by cylindrical flat punch was carried out in order to evaluate the effectiveness of the surface-hardening heat treatments on milled AISI 1040 steel substrates by varying laser operational parameters as well as the morphology of steel substrates. In particular, the trend in load-penetration depth were obtained, supporting the identification of the depth at which the hardened layer starts transferring the deformation to the softer substrate, that is, the Critical Transition Depth (CTD) in addition to identifying the overall degree of hardening reached by the laser treated material. On the other hand, hardness profiles and the thickness of hardened layers were also monitored by using combination of Vickers micro-hardness tests and standard metallographic characterizations.Analytical examinations and simple mathematical simulations of the experimental results coming from indentation tests and conventional testing procedures showed good correlation between the extent of the heat affected zone (HAZ), hardness and CTD. In conclusion, this study demonstrates that indentation tests are an effective, sensitive, fast, and low-cost characterization technique for heat treated steels.     

Inference of plastic deformation in weldment with parallel cracks from infrared thermography

Tensile tests are carried out by using the welded plate with two parallel cracks. The effects of the residual stress, the hardened zone by welding and the interaction between cracks on the plastic deformation near cracks and the crack opening displacement are examined by the experiment as well as the elasto plastic finite element analysis. A new technique for evaluating nondestructively the plastic deformation near cracks by using the infrared thermography is proposed. The heated region and the temperature rise measured by the thermography are compared with the plastic zone and the work of plastic deformation obtained by the analysis. The validity and the limitations of the application of the infrared thermography to the evaluation of the plastic deformation are discussed.