Influence of material microstructure changes on surface integrity in hard machining of AISI 52100 steel

Residual stresses are a consequence of thermo-mechanical and microstructural phenomena generated during the machining operation. Therefore, for improving product performance in machined hardened steels, material microstructure changes (commonly referred to as white and dark layers) must be taken into account. This paper presents a finite element model for white and dark layers formation in orthogonal machining of hardened AISI 52100 steel. In particular, a hardness-based flow stress and empirical models for describing the white and dark layers formation were developed and implemented in the finite element code. A series of experiments was carried out in order to validate the proposed simulation strategy and to investigate the influence of material microstructure changes on residual stresses. As main results, it was firstly demonstrated by surface topography analysis as both the white and dark layer are the result of microstructural alterations mainly due to rapid heating and quenching. Furthermore, it was found as both the presence of white and dark layers influence the residual stress profile. Particularly, the former significant impacts on the magnitude of maximum residual stress and on the location of the peak compressive residual stress; the latter reduces the compressive area.     

H13淬硬模具钢精车过程的数值模拟

采用热力学耦合有限元方法研究了淬硬钢精车过程中切屑形成规律。运用H13 淬硬模具钢流动应力模型进行数值模拟,考查了H13淬硬模具钢精车过程中工艺参数对工件性能和刀具的影响。结果表明:切削速度愈高,进给量愈小,刀具刀尖半径愈大,则工件加工层上的静水拉应力愈小,表面质量愈好; 淬硬钢精车时径向力起主要作用,大于切削力;切削速度愈大,切削力和径向力则愈小,愈有助于改善工件加工层上的表面质量;切削速度、进给量和刀具刀尖圆角半径愈大,工件和刀具温度愈高,愈易导致刀具前刀面扩散磨损和刀具后刀面磨损。研究结论有助于优化H13淬硬模具钢精车过程中工艺参数选择和改进刀具镶片设计。     

Analysis of the white layers formed during machining of hardened AISI 52100 steel under dry and cryogenic cooling conditions

The present work aims at understanding the effects of cryogenic coolant application and machined surface alterations during orthogonal machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial hardness and tool shape. Several experimental techniques were used in order to analyze the machined surface. In particular, optical and scanning electron microscopes were used for characterizing the surface topography, whereas the microstructural phase composition analysis and chemical characterization have been performed by means of X-ray diffraction and energy-dispersive spectroscopy techniques. The experimental results prove that the white layer is partially reduced or can be totally eliminated under certain process parameters and cryogenic cooling condition.     

Numerical modeling of the influence of process parameters and workpiece hardness on white layer formation in AISI 52100 steel

White layer formation is considered to be one of the most important aspects to take into account in hard machining. Therefore, a large number of experimental investigations have been carried out in recent times on the formation mechanisms and properties of the white layer. However, up to now, only very few studies have been reported on modeling of the white layer formation. This paper presents a finite element model which predicts the white layer formation during machining of hardened AISI 52100 steel. This numerical model was properly calibrated by means of an iterative procedure based on the comparison between experimental and numerical data. The empirical model was also validated for a range of cutting speeds, uncut chip thickness, and material hardness values. This study provides excellent results concerning cutting force, temperature, chip morphology, and white layer. From this study, it was also possible to properly analyze the influence of process variables on the white layer formation.     

An experimental investigation of affected layers formed in grinding of AISI 52100 steel

A systemic experimental investigation involving scanning electron microscopy, Vickers microhardness tester, X-ray diffraction, three-axis piezoelectric dynamometer, and thermocouple was carried out to analyze the affected layers formed in grinding of AISI 52100 steel. The formation mechanisms and properties of affected layer at different grinding conditions were investigated. It is found that the phase transformation, as well as retained austenite and white layer, can be formed at the grinding temperature below the nominal phase transformation temperature of the workpiece material. Mechanical effect associated with plastic deformation also can influence the white layer formation, and play an important role for the phase transformation of the ground workpiece. Furthermore, higher hardness and residual tensile stress are observed on the ground surface of hardened steel, but it shows residual compressive stress on the ground surface of annealed steel.     

高速硬切削加工表面白层形成机理研究

白层是高速硬切削的特有现象,对加工表面完整性和零件的服役性能有着重要影响。针对高速硬切削加工表面白层问题,进行了对GCrl5淬硬轴承钢高速硬切削试验和表面白层测试,研究了不同切削条件下的白层形成机理,分析了切削速度和刀具磨损状态对白层特征的影响规律。分析结果表明,白层厚度随切削速度和后刀面磨损的增大而增大,而其分布的均匀性和连续性也将变差;切削速度和后刀面磨损的增加引起切削温度升高,导致加工表面快速淬火效应,使得白层厚度增大,其中切削速度的影响较为显著;在切削速度较低(100 m/min左右)时白层的形成机理主要为塑性变形,切削速度超过300 m/min则主要是马氏体相变所致,而在中间切削速度(200 m/min左右)时为2种机理的混合作用结果。     

Implications of the reduction of cutting fluid in drilling AISI P20 steel with carbide tools

The machining of hardened steel is becoming increasingly important in manufacturing processes. Machined parts made with hardened steel are often subjected to high service demands, which require great resistance and quality. The machining of this material submits the tools to high mechanical and thermal loads, which increases the tool wear and affects the surface integrity of the part. In that context, this work presents a study of drilling of AISI P20 steel with carbide tools, analyzing the effects on the process caused by the reduction of cutting fluid supply and its relation with the tool wear and the surface integrity of the piece. The major problem observed in the tests was a difficulty for chips to flow through the drill flute, compromising their expulsion from the hole. After a careful analysis, a different machining strategy was adopted to solve the problem.     

Some Observations of the Chip Formation Process and the White Layer Formation in High Speed Milling of Hardened Steel

With the advent of recent advances in machine tools design (main spindle, feed drives, etc.), high-speed milling has become a cost-effective manufacturing process to produce products with high surface quality, low variations in the machined surface characteristics, and excellent dimensional accuracy. In taking into account the most obvious advantages of high-speed machining over conventional machining, a key issue is to identify the effective range of cutting speed that corresponds to high-speed machining producing improved machining performance. The simple reason for this is the fact that machining performance improves when entering the high-speed region but, large increase in cutting speed is not cost-effective due to rapidly increasing tool-wear rates and high power consumption. In order to address this issue requiring a trade-off, an attempt has been made in this paper by formulating an approximate procedure which is based on the analysis of chip-formation mechanisms and a chip-shape analysis, together with the use of metallographic methods. This procedure includes fundamental understanding of the well-known phenomena of white layer formation during the high-speed machining of hardened steels. Essentially, the white layer generated on a machined surface represents a surface defect. Therefore, it is necessary to determine the factors influencing its generation and its prevalent characteristics. There is lack of knowledge in this area, which tends to present the influence of the white layer on the surface integrity and performance of the machined part as a function of machining conditions. This article provides a basis for the determination of the optimal range of cutting speeds and feed rates in high-speed milling of hardened steels ensuring minimized influence of the white layer on the workpiece quality and machined surface integrity.     

Some Observations of the Chip Formation Process and the White Layer Formation in High Speed Milling of Hardened Steel

Abstract

With the advent of recent advances in machine tools design (main spindle, feed drives, etc.), high-speed milling has become a cost-effective manufacturing process to produce products with high surface quality, low variations in the machined surface characteristics, and excellent dimensional accuracy. In taking into account the most obvious advantages of high-speed machining over conventional machining, a key issue is to identify the effective range of cutting speed that corresponds to high-speed machining producing improved machining performance. The simple reason for this is the fact that machining performance improves when entering the high-speed region but, large increase in cutting speed is not cost-effective due to rapidly increasing tool-wear rates and high power consumption. In order to address this issue requiring a trade-off, an attempt has been made in this paper by formulating an approximate procedure which is based on the analysis of chip-formation mechanisms and a chip-shape analysis, together with the use of metallographic methods. This procedure includes fundamental understanding of the well-known phenomena of white layer formation during the high-speed machining of hardened steels. Essentially, the white layer generated on a machined surface represents a surface defect. Therefore, it is necessary to determine the factors influencing its generation and its prevalent characteristics. There is lack of knowledge in this area, which tends to present the influence of the white layer on the surface integrity and performance of the machined part as a function of machining conditions. This article provides a basis for the determination of the optimal range of cutting speeds and feed rates in high-speed milling of hardened steels ensuring minimized influence of the white layer on the workpiece quality and machined surface integrity.     

HARD TURNING PLUS GRINDING–A COMBINATION TO OBTAIN GOOD SURFACE INTEGRITY IN AISI O1 TOOL STEEL MACHINED PARTS

The establishment of adequate machining guidelines requires the study of several factors (residual stresses, roughness, hardness, microstructural changes, etc.) that define the surface integrity generated in the part by a machining operation. This work studies the surface integrity generated in AISI O1 tool steel by four hard turning (conventional, laser assisted, MQL and conventional with worn tool) and two grinding (production and finishing) processes, as well as by a combined machining process (conventional hard turning + finishing grinding). Hard turning generates tensile stresses and strong structural changes in the machined surface while grinding causes compressive stresses and negligible structural changes. Below the surface, grinding generates slightly tensile or nearly null stresses whereas turning generates strong compressive stresses. The results obtained show that an optimum machining process would imply the combination of hard turning plus a slight final grinding.     

HARD TURNING PLUS GRINDING-A COMBINATION TO OBTAIN GOOD SURFACE INTEGRITY IN AISI O1 TOOL STEEL MACHINED PARTS

The establishment of adequate machining guidelines requires the study of several factors (residual stresses, roughness, hardness, microstructural changes, etc.) that define the surface integrity generated in the part by a machining operation. This work studies the surface integrity generated in AISI O1 tool steel by four hard turning (conventional, laser assisted, MQL and conventional with worn tool) and two grinding (production and finishing) processes, as well as by a combined machining process (conventional hard turning + finishing grinding). Hard turning generates tensile stresses and strong structural changes in the machined surface while grinding causes compressive stresses and negligible structural changes. Below the surface, grinding generates slightly tensile or nearly null stresses whereas turning generates strong compressive stresses. The results obtained show that an optimum machining process would imply the combination of hard turning plus a slight final grinding.     

INFLUENCE OF MACHINING PARAMETERS ON THE WHITE LAYER FORMATION PROCESS AND ITS CHARACTERISTICS IN TURNING OF HARDENED STEEL

This article contains results of experimental research activities of white layer formation (WLF) and its characteristics during a process of turning hardened steels (THS), which have been carried out in laboratories of DD Cimos TMD Ai Grada?ac. WLF. Characteristics during the THS process were analyzed from the aspects of influence caused by machining parameters as well as tool flank wear width. Experimental tests of tool wear have been performed. The tool used in experimental tool wear was ceramic cutting insert CNGA 120408T, catalogue mark IN22 Al2O3-TiCN. In accordance with achieved results, value of tool flank wear 220 µm has been set as the criterion of wear. In accordance with defined wear criterion, determination of level and type of influence that machining parameters have on WLF and its characteristics were carried out in accordance with planned experimental methodology. Experiments have shown that cutting speed and tool flank wear width (for all other machining conditions unchanged) can be used for control of WLF and its characteristics. Structural changes in surface layer of the working piece, during the cutting process of hardened material, except for the WLF are also presented through a transition zone, e.g., dark layer, which has lower hardness than the initial material. Hard WL can take over a protection role for a machined surface from abrasive actions, while softened zone (dark layer) can take over a function of WL solder with the initial material. Analysis of achieved results points to a possibility of controlling the WLF and its characteristics, and therefore a possibility of using WL in a positive context. The basis for the above mentioned is the effect of additional plastic deformation of WL (APDWL), which occurs only under certain machining conditions. The effect seen, if follows WLF, results in decrease of machined surface roughness compared to its expected value. Accordingly there is a possibility for identification of WL on a machined surface by measuring the roughness parameters without previous metallographic preparation of samples.     

Study on laser cladding remanufacturing process with FeCrNiCu alloy powder for thin-wall impeller blade

The main objective in machining processes is to produce a high-quality surface finish which, however, can be measured only at the end of the machining cycle. A more preferable method would be to monitor the quality during the cycle, what result a real-time, low-cost, and accurate monitoring method that can dynamically adjust the machining parameters and keep the target surface finish. Motivated by this premise, results of investigation on the relationship between emitted sound signal and surface finish during turning process are reported in this paper. Through experiments with AISI 52100 hardened steel, this work shows that such a correlation does exist presenting strong evidences that Mel-Frequency Cepstral Coefficients, extracted from sound energy, can detect different surface roughness levels, what makes it a promising feature for real-time process quality monitoring methods.     

Experimental investigation and economic analysis of surfactant (Span-20) in powder mixed electrical discharge machining (PMEDM) of AISI D2 hardened steel

Abstract

Powder mixed EDM (PMEDM) is recognized as an advanced and innovative technique with enhanced performance and limited drawbacks in comparison to conventional EDM method. This study investigates the effect of powder particle size, various powder concentrations (C_p), and surfactant concentrations (C_s) on the performance of EDM. Since the machining characteristics are highly dependent on the dielectric performances, significant attention has been directed to introduce Cr powder and Span-20 surfactant into the dielectric fluid to achieve higher productivity and enhanced surface integrity. The EDM machining was carried out on AISI D2 hardened steel through ´Plug & Plaý dielectric circulating system attached to the main machine in order to evaluate the machining performances (i.e. MRR, EWR, and R_a). Interestingly, machining performance was improved with combination of Cr powder mixed and span-20 surfactant. By comparing the performance of span-20 surfactant and micro-nano chromium, the result within selected parameters shows that the span-20 surfactant and nano-chromium is the better choice for the EDM of AISI D2 hardened steel. In the machinability studies, the EDM machining of AISI D2 hardened steel by using span-20 surfactant and nano-chromium has exhibited the excellent machining performances, which led to 45.08% MRR enhancement and 68.89% R_a enhancement comparing to micro-chromium powder and span-20 surfactant led to 35.28% MRR and 28.96% R_a. Furthermore, cost analysis revealed that the nano-Cr powder size was approximately 4 times more economical than micro-Cr powder in machining of AISI D2 hardened steel, although the price for 1?kg is quite expensive.     

Case-based reasoning (CBR) model for hard machining process

In this research paper, hard machining of two materials viz. AISI 52100 (bearing steel) and AISI D2 (tool steel) at a hardness of 55?HRC is addressed. Taguchi’s technique is used for the design of experiments. Eight different parameters are considered for the experimentation in order to perform comprehensive investigations on hard machining process. Case-based reasoning (CBR) model is developed for predicting the machining performance and its capability is evaluated by conducting validation experiments. The root mean squared error, mean absolute percentage error, and the correlation coefficient between the actual and the model-predicted values of surface roughness and tool life are evaluated to confirm the validity of the CBR model.     

Effects of cutting conditions on microhardness and microstructure in high-speed milling of H13 tool steel

In the present study, high-speed side milling experiments of H13 tool steel with coated carbide inserts were conducted under different cutting parameters. The microhardness and microstructure changes of the machined surface and subsurface were investigated. A finite element model, taking into account the actual milling process, was established based on the commercial FE package ABAQUS/Explicit. Instantaneous temperature distributions beneath the machined surface were analyzed under different cutting speeds and feed per tooth based on the model. It was found that the microhardness on the machined surface is much higher than that in the subsurface, which indicates that the surface materials experienced severe strain hardening induced by plastic deformation during the milling process. Furthermore, the hardness of machined surface decreases with the increase of cutting speed and feed per tooth due to thermal softening effects. In addition, optical and scanning electron microscope (SEM) was used to characterize the microstructures of cross sections. Elongated grains due to material plastic deformation can be observed in the subsurface, and white and dark layers are not obvious under present milling conditions. The thickness of plastic deformation layer beneath the machined surface increases from 3 to 10 μm with the increase of cutting speed and feed per tooth. The corresponding results were found to be consistent and in good agreement with the depth of heat-affected zone in finite element analysis (FEA).     

TURNING STUDIES OF DEEP CRYOGENIC TREATED P-40 TUNGSTEN CARBIDE CUTTING TOOL INSERTS – TECHNICAL COMMUNICATION

In the present work, coated tungsten carbide tool inserts of ISO P-40 grade were subjected to deep cryogenic treatment at ?176°C. Turning studies were conducted on AISI 1040 workpieces using both untreated and deep cryogenic treated tungsten carbide cutting tool inserts. The turning performance was evaluated in terms of flank wear of the cutting tool inserts, main cutting force and surface finish of the machined workpieces. The flank wear of deep cryogenic treated carbide tools was observed to be lower than that of untreated carbide tools in machining of AISI 1040 steel. The cutting force during machining of AISI 1040 steel was lower with the deep cryogenic treated carbide tools when compared with the untreated carbide tools. The surface finish produced on machined AISI 1040 steel workpieces was superior with the deep cryogenic treated carbide tools as compared to the untreated carbide tools.     

MICROSTRUCTURAL ALTERATION AND MICROHARDNESS AT NEAR-SURFACE OF AISI H13 STEEL BY HARD MILLING

In the present research, the microstructural alteration and microhardness at near-surface of AISI H13 steel by hard milling under different cutting parameters with coated cutting tools have been investigated. Very thin white layer forms or even no obvious microstructural alteration layer appears at the near-surface. It is reasonable that the formation of the very thin white layer is primarily due to mechanical effect (severe plastic deformation) rather than thermal effect (rapid heating and quenching). The ‘hook’ shape of the microhardness profile indicates that the highest microhardness appears on the very top surface; while the smallest microhardness occurs at the depth of 25 µm below the machined surface. Moreover, the microhardness profiles below the machined surfaces well correlate with the microstructural change of the machined near-surface. It is expected that the experimental results will provide a useful guide to control or minimize the white layer formation and, more significantly, to promote the application of hard milling technique in die and mold industry.     

Microlubrication machining of 1018 steel: the effect of a biodegradable lubricant on the microstructural integrity

Microlubrication is a green machining technique that reduces the amount of cutting fluid during the machining process. Its effect on subsurface microstructural integrity is a very important aspect for the functionality of a machined component, but it is often neglected and requires advanced characterisation techniques. The focus of this study is to investigate the influence of microlubrication using a biodegradable vegetable oil‐based cutting fluid to characterise the subsurface microstructural integrity during end milling of AISI 1018 steel. Vickers microhardness was measured along the cross section of the machined component from the surface edge to inside the bulk, while transmission electron microscopy was conducted inside the subsurface deformation zone to quantify the dislocation densities. It was determined that increased dislocation densities near the workpiece edge resulted in increased microhardness with reduced tool wear. Thus, microlubrication machining is a sustainable green machining process that does not significantly compromise the subsurface integrity of the workpiece material. Copyright © 2017 John Wiley & Sons, Ltd.     

硬态切削与磨削工艺的表面完整性

硬态切削具有良好的加工柔性、经济性和环保性,在对工件性能起关键作用的精加工中,已成为磨削加工的有力挑战者。表面完整性是评价加工质量和工件机械性能的重要指标。本文介绍了硬态切削和磨削工艺的加工表面完整性(包括白层、表面形貌、表面粗糙度、表面残余应力、表面硬化层等),并对二者进行了比较。此外,还分析了硬态切削与磨削工艺相互结合的应用前景。