Applied Mechanics and Materials Vol. 315 (2013) pp 241-245 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.315.241
Effect of Tool Wear on Tool Life and Surface Finish when Machining DF-3 Hardened Tool Steel Ali Davoudinejada, M.Y. Noordinb ,Danial Ghodsiyehc, Sina Alizadeh Ashrafid, Mohsen Marani Barzanie Department of Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia a
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Keywords: Hard Turning, Cutting Edge Geometry, Honed, Abrasive wear.
Abstract. Hard turning is a dominant machining operation performed on hardened materials using single-point cutting tools. In recent years, hard turning operation has become more and more capable with respect to various machinability criteria. This work deals with machinability of hardened DF-3 tool steel with 55 ±1 HRC hardness at various cutting conditions in terms of tool life, tool wear mechanism and surface roughness. Continuous dry turning tests were carried out using coated, mixed ceramic insert with honed edge geometry. Two different cutting speeds, 100 and 210 m/min, and feed rate values of 0.05, 0.125 and 0.2 mm/rev were used with a 0.2 mm constant depth of cut for all tests. Additionally scanning electron microscope (SEM) was employed to clarify the different types of wear. As far as tool life was concerned, best result was achieved at lowest cutting condition whereas surface roughness values decreased when operating at higher cutting speed and lower feed rate. Additionally maximum volume of material removed is obtained at low cutting speed and high feed rate. Dominant wear mechanism observed during the experiments were flank and crater wear which is mainly caused by abrasive action of the hard workpiece material with the ceramic cutting tools. Introduction Turning of hardened steels with hardness over 45 HRC has been applied in many industrial productions when producing bearings, gears, cams, shafts, axels and other power transmission and mechanical parts since the early 1980s. Currently, machining of hardened steel components using geometrically defined cutting tools is often an attractive replacement for some grinding operations. Clear attraction to hard turning is as a result of the numerous advantages over grinding process which is costly, not flexible, time-consuming and degrade the environment due to the usage of coolants [1]. Dry machining is a highly interesting topic and ecologically desirable for the environment. The non usage of cutting fluids decrease the cost of machining which is assessed to be more than the labour and overhead costs [2]. There are various factors which are influencing dry machining such as workpiece geometry and material, cutting conditions, cutting tool material and coating, machine tool and operations [3]. Many researchers have researched on these factors and an example is the investigation on tool life and wear mechanism of coated and uncoated mixed ceramic tools when hard turning AISI 52100 steel. TiN coated ceramic was found to be able to increase the wear resistance and tool life. Additionally TiN coating decreases chipping damage to the cutting tool. This can be attributed to the improved toughness of the tool as a result of the TiN coating [4]. Cutting tool edge geometry is critical in hard turning because tools with superior edge strength are required to withstand the large tool stresses produced. In order to achieve desirable tool life, the cutting edge of the inserts must be reinforced and protected with proper edge preparation where various types of edge configurations such as honed and chamfered are applied to the tool insert [5]. An investigation on the effect of corner and edge radius on the tool flank wear when turning AISI 1040 steel bar stock with hardness of 58.5–61.3 HRC revealed the critical impact of a honed radius to a cutting edge where it protects the edge from chipping and resulted in an increase All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 131.175.28.132, Department of Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia, Skudai, Malaysia-05/03/13,09:15:31)
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of tool life. In addition larger honed tool minimizes corner radius wear [6]. The rather limited works on hard turning involving honed edge preparation has prompted the authors to report the findings of the study conducted. Experimental Work The machine used for the turning tests was Maho GR 200E, CNC lathe with a variable spindle speeds from 100 to 6000 rpm and 8.3 KW motor drive. ASSAB DF-3 tool steel with 55 ±1 HRC hardness, compressive strength 2200 MPa, compressive yield strength (Rc 0.2) 1800 MPa and 185 MPa modulus of elasticity was used as workpice material. Table 1 presents the chemical composition of workpiece material. The insert material is Al2O3-TiCN fine-grained ceramic with a PVD TiN coating for added wear resistance and better surface finish which is designated by CNGA120412E05 and KY4400 grade. Inserts were mounted on a tool holder with a standard geometry code of MCLNL1616H12 and following geometry obtained in the experiments: -5° back and side rake angles also 5° end and side relief angles with 1.2 mm nose radius (Fig. 1). The 0.2 mm depth of cut was fixed when the experiments were performed. Three feed rates of 0.05, 0.125 and 0.2 mm/rev were selected while two cutting speed 100 and 210 m/min were chosen. Cutting condition selection was based on the tool manufacturer and previous research on hard turning [7]. Table 2 shows the experimental details and results. The tool life criterion on the width of flank wear is VBmax= 0.2 mm and it is measured after each cut using a tool maker microscope. Both the rake and flank faces were subsequently observed using the Zeiss optical microscope. For revealing the external morphology and information about the worn tools, SEM images have been captured at the end of tool life. Table 1. Chemical composition of ASSAB DF-3
Elements Percentage %
C 0.38
Si 0.9
Mn 0.5
Cr 13.6
V 0.3
Fig. 1, Ceramic Honed insert detail
Experimental Results Tool Life and Wear Mechanism. Based on the experimental results the longest tool life was 36.1 min obtained at 100 m/min cutting speed and 0.05 mm/rev feed rate. On the other hand 6.1 min was shortest cutting time obtained at 210 m/min and 0.2 mm/rev feed rate. Results revealed that tool life is highly dependent on cutting conditions and with increasing feed rate and cutting speed, tool life was inclined to decrease. Fig. 2 shows the effect of cutting speeds and feed rates on tool life. It can be seen that the cutting speed was the most influential factor for reducing tool life and similar results were obtained with previous works carried out when turning hardened 100Cr6 bearing steel,
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with PCBN and ceramics cutting tools [8]. Fig. 3 illustrates the volume of material removed at different cutting speeds and feed rates. It is observed that for a given feed rate, decreasing cutting speed would increase the rate volume of material removed. In addition increasing feed rate from 0.05 to 0.2 mm/rev increased the removal rate.
Fig. 2, Effect of cutting speeds and feed rates on tool life
Fig. 3, Variation of the volume of material removal with cutting speed and feed.
Fig. 4, illustrates the wear curve of ceramic honed inserts when machining DF-3 tool steel at different cutting conditions. It shows that increasing cutting speed and feed rate accelerates flank wear. In addition tool wear growth on the flank and rake faces of the tool is observed at different cutting time when turning at 100 m/min cutting speed and 0.05 mm/rev feed rate. It can be observed that increasing cutting time expand the flank and crater wear area. Generally turning at higher cutting conditions result in tool wear to increase quickly compared to lower cutting conditions.
Fig. 4, Evolution of flank wear land as a function of cutting time for different cutting speeds and feed rates. The typical types of wear modes noticed in the experiments were flank and crater wear, whereas no cutting edge chipping and cracking was observed, mainly due to the honed cutting edge. Additionally in other investigations with honed edge geometry no chipping was observed [5, 6].
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Due to the high chemical stability of the ceramic, no diffusion wear was observed. From the analyses of SEM images it revealed that abrasive wear was the dominant mechanism which is caused by the hard structure of hardened tool steel. Fig. 5(a) and (b) illustrate that at low cutting condition some parts of the workpiece material adhered on the cutting lip of the insert. This confirms that apart from work material, elements also adhered to the insert. EDAX analysis, Fig. 5(c), revealed the percent of relevant element in the tool lip and Fig. 5(d) shows the abrasive area and grooves on the flank face of the tool.
Fig. 5, SEM images of inserts (a) flank face and (b) rake face after 38 min cutting at 100 m/min cutting speed and 0.05 mm/rev feed rate, (c) EDAX analyses on the tool edge and (d) flank face at the end of tool life at 210 m/min cutting speed and 0.2 mm/rev feed rate.
Surface Roughness Variation. Surface quality achieved with the hard turning process is close to grinding and due to the numerous advantages obtainable it has the potential to replace with grinding process [9]. The average surface roughness values, Ra, obtained in the turning experiments were within the range of 0.62–1.7 µm. The variation of the surface roughness observed was dependent on the cutting conditions and cutting time. Fig. 6 shows the average of surface roughness value at different cutting conditions which revealed that feed rate had critical impact on surface finish and by decreasing feed rate better surface roughness is obtained at both cutting speeds. In addition the best surface finish recorded at 210 m/min cutting speed and 0.05 mm/rev feed rate. Fig. 7 illustrates the effect of tool wear on the surface roughness at 100 m/min cutting speed and 0.05 mm/rev feed where it was noticed that at this condition surface roughness value was prone to increase with increasing tool wear.
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Fig. 6, Effect of various cutting speeds and feed rates on surface roughness.
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Fig. 7, Effect of tool wear on surface roughness in different cutting time.
Conclusion This investigation presents the result of tool life, wear mechanism and surface roughness when turning DF-3 hardened tool steel of 55 ±1 HRC using coated ceramic insert with honed edge geometry. Tool life is dependent on the cutting parameters and the longest tool life recorded was at lowest cutting conditions. Tool wear also significantly affected tool life such that as the tool wear accelerates shorter tool life was observed. Flank and crater wear was observed on the flank and rake faces of the tool and abrasive wear is noticed as the main wear mechanism. In terms of volume of material removed, highest amount of material is removed at lower cutting speed and highest feed rate. It also revealed that feed rate had a critical effect on material removal rate. Surface roughness analysis revealed that the surface roughness improved at low feed rate and it was the dominant factor influencing surface quality. It was also remarkable that at low cutting condition by increasing tool wear surface roughness value increased. References [1] W. Grzesik, T. Wanat. Comparative assessment of surface roughness produced by hard machining with mixed ceramic tools including 2D and 3D analysis. Journal of Materials Processing Technology 169 (2005) 364–371. [2] P.S. Sreejith, B.K.A. Ngoi. Dry machining: Machining of the future. Journal of Materials Processing Technology 101 (2000) 287-291. [3] M.Y. Noordin, V.C. Venkatesh, S. Sharif. Dry turning of tempered martensitic stainless tool steel using coated cermet and coated carbide tools. Journal of Materials Processing Technology 185 (2007) 83–90. [4] K. Aslantas, I. Ucun, A. Cicek. Tool life and wear mechanism of coated and uncoated Al2O3/TiCN mixed ceramic tools in turning hardened alloy steel. Wear (2011), 1101. [5] Viktor P. Astakhov. Geometry of Single-point Turning Tools and Drills. Springer-Verlag London Limited 2010. P162. [6] William J. Endres, Raja K. Kountanya. The Effects of Corner Radius and Edge Radius on Tool Flank Wear. Journal of Manufacturing Processes. (2002) Vol. 4/No. 2. [7] Y. Kevin Chou, Hui Song. Tool nose radius effects on finish hard turning. Journal of Materials Processing Technology 148 (2004) 259–268. [8] Gabriel C. Benga, Alexandre M. Abrao. Turning of hardened 100Cr6 bearing steel with ceramic and PCBN cutting tools. Journal of Materials Processing Technology 143–144 (2003) 237–241. [9] Radu Pavel, Ioan Marinescu, Mick Deis, Jim Pillar. Effect of tool wear on surface finish for a case of continuous and interrupted hard turning. Journal of Materials Processing Technology 170 (2005) 341–349.
