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Tool life and surface integrity when hard turning stainless steel using wiper coated carbide tool

Master's Thesis 2008 102 Pages

Engineering - Mechanical Engineering

Excerpt

TABLE OF CONTENTS

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS

LIST OF APPENDICES

1 INTRODUCTION
1.1 Background
1.2 Problem Statement
1.3 Objectives of the Research
1.4 Scope of the Study
1.5 Significance of the Study
1.6 Overview of the Methodology
1.7 Structure of the Thesis

2 A REVIEW ON HARD TURNING
2.1 Machining: Hard Turning
2.2 Wiper Coated Carbide Tool
2.3 Martensitic Stainless Steel
2.4 Tool Life and Wear Mechanisms
2.5 Surface Integrity
2.5.1 Surface Roughness
2.5.2 Surface and Subsurface Microstructure
2.5.3 Hardness Variations
2.5.4 Residual Stresses
2.6 Chip Morphology

3 TOOL LIFE AND WEAR MECHANISMS
3.1 Introduction
3.2 Experimental Details
3.3 Tool Life and Total Material Removed
3.4 Modeling of the Tool Life
3.5 Wear mechanisms
3.6 Concluding Remarks

4 SURFACE INTEGRITY
4.1 Introduction
4.2 Experimental Details
4.3 Results and Discussion
4.3.1 Surface Roughness and Its Empirical Model
4.3.2 Surface Microstructure
4.3.3 Residual Stresses
4.4 Concluding Remarks

5 CHIP MORPHOLOGY
5.1 Introduction
5.2 Experimental Details
5.3 Results and Discussion
5.4 Concluding Remarks

6 CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
6.2 Recommendations

REFERENCES

APPENDICES

APPENDIX A

APPENDIX B

LIST OF TABLES

2.1 Comparison between hard turning and grinding as a finish machining operation as compiled by Grzesik and Wanat (2005)

3.1 The tool life and total material removed data obtained
3.2 Design layout of the experiment
3.3 Sequential model sum of squares for the tool life data
3.4 Analysis of variance (partial sum of squares) for linear model of the tool life as the response

4.1 The obtained Ra resulting from the hard turning
4.2 Sequential model sum of squares for the Ra data
4.3 Analysis of variance (partial sum of squares) for two factor interaction model of the Ra data
4.4 Residual stress values on the machined surface caused by hard turning at various conditions

5.1 Chip cross sectional area generated at feed of 0.125 mm/rev

LIST OF FIGURES

2.1 Schematical representation of tool positioning on its holder resulting -5 0 back rake angle, -5 0 side rake angle, 5 0 end relief angle, 5 0 side relief angle, 5 0 end cutting edge angle, and -5 0 side cutting edge angle for an 80 0 diamond-shaped cutting tool with 0.8 mm nose radius
2.2 Comparison between (a) conventional and (b) wiper cutting tool, where f is feed, r is nose radius, rw is wiper radius, and Ra is surface roughness
2.3 Vertical section of Fe-Cr-C diagram for 0.1wt% C (Sourmail and Bhadeshia, 2005)
2.4 Schematical representation of flank wear of a cutting tool according to ANSI/ASME B94.55M standard
2.5 Topography of flank face (a) when new, (b) after 3 minutes, and (c) after 20 minutes of cutting at 103.9 m/min and 0.2 mm/rev, showing original dimple-like TiC surface features being worn by discrete plastic deformation resulting in a ridge-and-furrow appearance (Lim et al., 1999)
2.6 Appearance white layer in 27MnCr5 steel surface microstructure as the effect of hard turning (after Rech and Moisan, 2003)
2.7 Temperature of machined surface and chip against cutting speed (after Bosheh and Mativenga, 2006)

3.1 Images of the cross-sectioned wiper coated carbide tool showing (a) the coating thickness of 3.0 - 3.5 μm and (b) the tool’s chip breaker profile featuring positive rake angle
3.2 Tool life of wiper coated carbide tool at various cutting speed and feed
3.3 Total volume of material removed of the wiper coated carbide tool
3.4 Diagnostic plot of predicted to actual values of the tool life data
3.5 Diagnostic plots of the tool life model (a) normal plot of residuals and (b) residuals vs. predicted
3.6 Response surface graphs of (a) contours and (b) 3D surface for tool life
3.7 Perturbation plot for tool life where x1 and x2 are cutting speed and feed in coded factors, respectively
3.8 SEM images of the wiper coated carbide tool when its tool life criterion is reached (VBmax = 0.14 mm) (a) the rake face and (b) the flank face and (c) the element identification results of the particular regions (v = 100 m/min, f = 0.125 mm/rev, a = 0.4 mm)
3.9 Distribution of (a) normal and (b) shear stresses on tool tip (Zhou et al., 2003)
3.10 Microstructure of the martensitic stainless steel: (a) workpiece and (b) chip cross-section (Vilella’s reagent etched)

4.1 Illustration showing peak-to-valley roughness obtained by cutting tool with nose radius of r where the depth of cut, a, is less than the nose radius
4.2 Center line average surface finish generated by wiper coated carbide tool
4.3 Diagnostic plots of the surface roughness model (a) normal plot of residuals and (b) residuals vs. predicted
4.4 Response surface graphs of (a) contours and (b) 3D surface for surface roughness
4.5 Relation between Ra and cutting speed with the presence of interaction between cutting speed and feed
4.6 Perturbation plot for tool life where x1 and x2are cutting speed and feed in coded factors, respectively
4.7 Estimation of r and wiper radius measured by CMM
4.8 Appearance of the machined workpiece at v = 100 m/min and f = 0.125 mm/rev
4.9 Surface microstructure of stainless steel workpieces machined by sharp wiper tool at (a) v = 100 m/min, f = 0.125 mm/rev and (b) v = 170 m/min, f = 0.25 mm/rev
4.10 Surface microstructure of the stainless steel workpiece machined by severely worn tool (VBmax 0.14 mm) at cutting speed of 100 m/min and feed of 0.125 mm/rev

5.1 Area of process parameter combinations that would result the predetermined response criteria of minimum 10 minutes of tool life and of maximum 0.4 μm of surface roughness
5.2 Desirability plot of the input factors to obtain the maximum tool life and minimum surface roughness
5.3 Material removal rate for the process parameter combinations
5.4 Images of the chip’s cross section generated by the wiper coated carbide tool at various cutting speed and feed
5.5 Microstructure images of the chip’s longitudinal section generated at cutting speed of 100 m/min and feed of 0.125 mm/rev using (a) new tool and (b) worn tool
5.6 Microstructure of the chip’s longitudinal section generated at v = 170 m/min and f = 0.25 mm/rev
5.7 The uncut chip cross-sectional area (Ac) theoretically in contact with a cutting tool with nose radius of r where the depth of cut, a, is less than the nose radius
5.8 Image of the rake face of the wiper coated carbide tool at v = 100 m/min and f = 0.125 mm/rev in (a) sharp and (b) terminally worn condition
5.9 Chip contact area on the rake face of the terminally worn wiper coated carbide tool at v = 100 m/min and f = 0.125 mm/rev
5.10 Images of the chip’s (a) free surface and (b) underside generated by sharp wiper coated carbide tool at v = 100 m/min and f = 0.125 mm/rev (inset: the corresponding EDS spectra)
5.11 Images of the chip’s (a) free surface and (b) underside generated by worn wiper coated carbide tool at v = 100 m/min and f = 0.125 mm/rev (inset: the corresponding EDS spectra)
5.12 Image of chip generating action by the wiper coated carbide tool at v = 100 m/min and f = 0.125 mm/rev

LIST OF SYMBOLS

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LIST OF APPENDICES

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ACKNOWLEDGEMENTS

Intentionally and sincerely, I would like to first deliver my appreciation to my supervisors. To Associate Professor Dr. Noordin Mohd. Yusof, for accepting me in the very beginning, for involving me in the research project, for the support, guidance, lesson, and facility given, and for everything. To Professor Dr. Safian Sharif, for the lesson, support, opportunity, and facility given and for everything.

The acknowledgement is also addressed to Ministry of Science, Technology, and Innovation, Malaysia for the financial support through the IRPA funding vote no. 74268.

I would also like to thank fellow graduate students and the Heads and technicians of Production Laboratory, Materials Science Laboratory, and Metrology Laboratory for the teaching and help, and for everything.

The appreciation also goes to the reviewers of our publications in journals and conferences and to the examiners of this thesis for the valuable corrections and input.

Again and always, to those addressed above and to everybody involved in conducting the research and making this manuscript, thank you, for everything.

ABSTRACT

The feasibility of implementing hard turning partially depends on the performance of the cutting tool in generating the required machined parts. Since process efficiency is a continuous pursue, there is always a need of having inexpensive cutting tools able to deliver the expected quality of machined surfaces and having reasonable life times. For this purpose, the selection of cutting conditions and its performance are important. In this study, a carbide tool with TiAlN coating is proposed as the low cost alternative for performing moderate range of hard turning. Specifically, a coated carbide tool with wiper geometry was used to machine hardened martensitic stainless steel (47 -48 HRC). The tool’s performance is evaluated based on its tool life and the resulting surface finish when hard turning at various cutting speeds (100, 130, and 170 m/min) and feeds (0.125, 0.16, 0.2, and 0.25 mm/rev) and at constant depth of cut of 0.4 mm under dry condition. Further observation was made on the worn tool, the machined surface, and the generated chip. The wiper coated carbide tool lasts mostly beyond 2 minutes and even reaches a maximum of almost 18 minutes of service life time. Combination of abrasion and diffusion are suggested to be the main wear mechanisms of the cutting tool. The resulting surface finish is very fine, being entirely finer than 0.8 μm in Ra which is one level better than the theoretically expected value. Empirical models are developed to quantify the effect of cutting speed and feed to the tool life and Ra. Further observation at the optimum process parameter combination, for example the lowest cutting speed-lowest feed, reveals minimum machining-induced surface microstructure alteration and compressive residual stress on the machined surface and continuous type of the generated chip are resulted.

ABSTRAK

Kemungkinan untuk penerapan larik keras bergantung sebahagiannya pada prestasi mata alat dalam menghasilkan komponen termesin yang dikehendaki. Memandangkan kecekapan proses merupakan satu usaha yang berterusan, maka terdapat keperluan untuk memiliki mata alat yang murah yang dapat menghasilkan kualiti permukaan termesin seperti yang diharapkan dan mempunyai hayat mata alat yang munasabah. Untuk tujuan ini, pemilihan keadaan pemotongan yang sesuai dan prestasinya adalah amat penting. Dalam kajian ini, mata alat karbida bersalut TiAlN dicadangkan sebagai satu alternatif kos rendah untuk melakukan larik keras sederhana. Lebih spesifik, mata alat karbida bersalut dengan geometri penyeka digunakan untuk memotong keluli tahan karat martensitik yang dikeraskan hingga 47 - 48 HRC. Prestasi mata alat ini diuji berdasarkan jangka hayat dan kehalusan permukaan yang dihasilkan ketika melarik keras pada beberapa kelajuan pemotongan (100, 130, dan 170 m/min) dan suapan (0.125, 0.16, 0.2, dan 0.25 mm/rev) dan pada satu kedalaman pemotongan iaitu 0.4 mm dalam keadaan kering. Kajian lebih lanjut dilakukan pada mata alat yang telah haus, permukaan termesin, dan tatal terhasil. Mata alat bersalut bergeometri penyeka ini didapati mampu bertahan melebihi 2 minit malah ia boleh mencecah jangka hayat sehingga 18 minit. Kehausan mata alat didapati disebabkan oleh gabungan mekanisma lelasan dan bauran. Permukaan yang dihasilkan sangat halus, dianggarkan kekasaran permukaan aritmetiknya lebih halus daripada 0.8 m, iaitu satu tahap lebih baik daripada anggaran teori. Model-model empirik dibangunkan untuk mentakrifkan pengaruh kelajuan pemotongan dan suapan pada jangka hayat mata alat dan kehalusan permukaan yang dihasilkan. Pemerhatian lebih lanjut pada gabungan parameter proses yang optimum, iaitu pada kelajuan dan suapan yang paling rendah, menunjukkan bahawa ianya menghasilkan perubahan mikrostruktur yang minimum dan tegasan baki berjenis mampatan pada permukaan termesin serta tatal berterusan terhasil.

INTRODUCTION

1.1 Background

Driven by the need for high performance, more components for various applications are made of hardened steels. Hardened steel’s properties of high wear resistance and compressive strength meet the demands faced by components used in, for example, the automotive and the tool and die industries. Yet, these properties tend to sacrifice the steel’s machinability. Steels at their hardened state cause premature failure when cutting tools suitable to machine unhardened materials are being used. Machine shops, being integral portion of the manufacturing system, modify the machining process flow in order to be able to machine parts made of these hard-to-cut materials. The modification involves three sequential steps, namely rough machining of unhardened steel blank, heat treating the steel to the required hardness, and finish machining to achieve the final dimension required. This technique was and is still using grinding as the only option for finish machining operation.

The introduction of advanced cutting tools with high hot hardness, namely ceramic and polycrystalline cubic boron nitride (PCBN), has enabled the machining of hard materials. Since then, it is possible to machine steels to their final dimensions in their hardened state. This technique became a profitable alternative for finish machining compared to the incumbent grinding operation. High material removal rate and relatively low tool cost are some of the economic benefits. Additionally, stricter health and environment regulations and also post-production cost consideration have led to the minimized use of coolant whenever feasible, and hard turning has been successfully performed in dry condition (Mamalis et al., 2002).

Despite these potential advantages, the penetration of hard turning in the manufacturing industry is still low. The lack of data on surface quality and tool wear for the many combinations of workpiece and cutting tool is the main concern (Pavel et al., 2005). Moreover, the common tools used for hard turning, PCBN and ceramic, are relatively high in price. Some applications in the mold and die industry have been identified to require parts made of hardened steels within moderate range of hard turning (45 - 50 HRC). Using advanced and, consequently, expensive cutting tools for this hardness range may hinder the economic benefit of hard turning. The need for lower cost tool materials to perform hard turning is still on demand. For this purpose, commercially available coated carbide tools are proposed to be the alternatives.

Successful turning of hardened stainless steel (43 - 45 HRC) using coated carbide tool has been reported by Noordin et al. (2007). It is likely that coated carbide has the potential to machine stainless steels with even higher hardness within moderate range of hard turning. This is partially contributed by the continuous development of carbide tool taking place in the form of fine grained substrate, better binder that optimizes strength and toughness, and improved coating using plasma vapor deposition (PVD) technique. Therefore, the potential use of inexpensive and widely used coated carbide cutting tools for hard turning applications is worth exploring.

Finish machining is intended to achieve high level of surface finish and is characterized with low feed and depth of cut (Shaw, 2005). In order to improve productivity, tools with wiper geometry have been developed by tool manufacturers. This tool geometry has wiper radii adjacent to the designated nose radius and has little or no clearance angle to improve the surface finish by burnishing action by the flank face of the insert (Shaw, 2005). This geometry is expected to enable doubling the current feed while still achieving surface finish comparable to single nose-radius tool or, alternatively, if surface finish is the most important consideration, to achieve better surface roughness by maintaining the current feed (Castner, 2000). However, the study on wiper geometry is still limited. Evidences on the effectiveness of using cutting tools with wiper geometry when performing hard turning are still required. Particularly, for coated carbide tool where using low feed is the only option.

1.2 Problem Statement

The problems of interest that appear from the previously defined background are as follow:

1. The feasibility of hard turning using coated carbide tools, which offer low cost per tool, needs to be evaluated when performing moderate range of hard turning.
2. The beneficial effects of using a wiper geometry tool for a particular hard turning application require experimental evidences.
3. It is necessary to observe the main aspects related to post-hard turning in order to make a comprehensive evaluation and to further understand the said process. The aspects are the worn tool, the resulted machined surface, and the generated chip.

1.3 Objectives of the Research

The objectives of this investigation are as follows:

1. To evaluate the suitability of using wiper coated carbide tool for moderate range of hard turning in terms of tool life and surface finish. Empirical models for these machining responses are developed for this purpose.
2. To provide experimental evidences on the use of wiper coated carbide tool for the particular hard turning application by determining its wear mechanisms, examining the quality of the machined surface, and analyzing the generated chip.

1.4 Scope of the Study

Using wiper coated carbide tool to perform moderate hard turning, this study is set within the following scopes:

1. The cutting tool is a commercially available coated carbide insert which features a wiper geometry.
2. The workpiece is heat-treated martensitic stainless steel with through hardness of 47 - 48 HRC.
3. The hard turning is performed dry at various cutting speed and feed and keeping the depth of cut constant. These parameters are set within the range of finish turning.

1.5 Significance of the Study

This study contributes towards determining the suitability of a wiper coated carbide tool for performing hard turning of stainless steel by considering post- process observation of the worn tool, the machined surface, and the generated chip. For practical uses, it also demonstrates the usability of wiper coated carbide tool for hard turning and thereby providing an alternative technique for finish machining of hardened stainless steel. The observed evidences are also expected to provide the validation data for the purpose of further fundamental research on predictive theory of metal cutting.

1.6 Overview of the Methodology

The methodology to achieve the set forth objectives can be outlined as follows:

1. Selecting the cutting speed and feed as independent variables and specifying their appropriate levels.
2. Experimental design and planning.
3. Conducting the experiment. This includes the wear progression and surface finish measurements.
4. Mathematical model development from the tool life and surface roughness data.
5. Setting up post-hard turning observation. The worn tool, the machined workpiece, and the chips were treated separately for specimen preparation purposes. This is followed by measurement and image capturing.

a. Specimen preparation.

i) The worn tool was cleaned from the adhering workpiece.

ii) Small sections from the machined workpiece were mounted, ground, polished, and etched. Another set of face turning experiments was also conducted to obtain planar machined surface for residual stress measurement.

iii) One batch of the chip was cleaned for cross sectional area measurement while another batch was mounted, ground, polished, and etched.

b. Measurement and image capturing of the specimen.

i) The cutting tool specimen was the object of optical and scanning electron microscope. Images and element identification were obtained. ii) The workpiece specimen was sent to scanning electron microscope for image capturing. The face turned specimen was sent to the x-ray diffraction system for measuring its residual stress.

iii) The cleansed chip specimen was put under optical microscope to measure its length and was scaled to measure its weight. The length and weight data obtained were used to calculate its cross-section area. This specimen was also sent to scanning electron microscope for image capturing and element identification. The metallurgically prepared chip specimen was put under optical microscope in order to capture its microstructure images.

6. Data analysis and validation. Another set of hard turning trials was then conducted for validation of the mathematical model proposed.

1.7 Structure of the Thesis

The study reported in this thesis is presented in six chapters. After the introduction in Chapter 1, a literature review on hard turning is compiled in Chapter 2. Chapters 3, 4, and 5 cover the aspects of tool life and wear mechanisms, surface integrity, and chip morphology, respectively. These three chapters are presented in paper-like format with sections of introduction, description of the experimental techniques, results and discussion, and concluding remarks. Finally, Chapter 6 notes the conclusion of the study and recommendations for future work in this area.

CHAPTER 2 A REVIEW ON HARD TURNING

2.1 Machining: Hard Turning

To manufacture a mechanical part, machining is performed in order to change the geometry of a blank by removing its unnecessary material. Turning is a machining process to produce a surface of revolution using single-point cutting tool (Shaw, 2005).

With time, more blank materials need to be machined for broader range of applications. Some applications require the use of parts made of hardened steels.

Traditionally, the machining process to manufacture parts made of hardened steel involved three sequential steps, namely machining a soft or unhardened blank, hardening to the targeted hardness by heat treatment, and grinding to meet the required surface finish. Grinding was the only available finishing operation since no single point turning tool was feasible to finish machine the hard materials. Currently, advanced cutting tools with high hot hardness are widely available and they have made it possible to turn hardened materials.

Turning these materials with high hardness is termed hard turning. In many applications, hard turning has the potential to simplify the aforementioned steps by directly machining the workpiece to its final dimension in the hardened state (Poulachon et al., 2003). Hard turning has become a profitable alternative to grinding as a finishing operation due to the advantages in economical aspects, flexibility, and ecological aspects (Grzesik and Wanat, 2005). However, despite its significant advantages, as seen in Table 2.1, the overall quality of hard turning is still questioned. The lack of data concerning surface quality and tool wear for the many combinations of workpiece and cutting tool impedes the acceptance of hard turning by the manufacturing industry (Pavel et al., 2005).

Table 2.1 Comparison between hard turning and grinding as a finish machining operation as compiled by Grzesik and Wanat (2005).

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From the machining point of view, hard materials are characterized by the properties of high indentation hardness which relates to the requirement of a much harder cutting tool and the strong impact and stress on the small area of the tool- workpiece contact, high abrasiveness which relates to the requirement of cutting tool having high resistance to abrasive wear, low ductility which may cause the formation of segmented or even discontinuous chips, as well as high value of the hardness to modulus of elasticity ratio which induces an appreciable amount of local elastic recovery after the cutting tool passes over (Nakayama et al., 1988).

Due to the hardness of the workpiece materials, hard turning has some special features (Shaw, 2005). Some of them are related to the cutting tool. The cutting tool used is usually set at negative rake angle in order to reduce chipping of its cutting edge. Figure 2.1 shows a tool mounted in a tool holder that gives negative rake angle. Large nose radius of the cutting tool is usually selected as theoretically, the larger the nose radius the better the surface finish. Other features include the segmented type of chip generated, relatively low specific energy consumption compared to that of grinding, low cutting forces since the high temperature generated facilitates the thermal softening in the cutting zone and, hence, reducing the cutting forces, and also low feed and depth of cut which are related to the fact that hard turning is established as a finishing process to meet certain required tolerance. In addition, hard turning is commonly performed dry (Mamalis et al., 2002). Although it is neither possible nor desirable to completely eliminate the use of coolants from machining operations, stricter health and environment regulations as well as post-production cost consideration lead to the minimized use of coolant whenever feasible.

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Figure 2.1 Schematical representation of tool positioning on its holder resulting -50 back rake angle, -50 side rake angle, 50 end relief angle, 50 side relief angle, 50 end cutting edge angle, and -50 side cutting edge angle for an 800 diamond-shaped cutting tool with 0.8 mm nose radius.

It is essential to conduct investigation in the area of hard turning in order to improve the cutting technique, to produce parts with greater precision and longer service life, to increase the productivity, and to enable the machining of a wider range of parts using the available cutting tool.

2.2 Wiper Coated Carbide Tool

Tungsten carbide tool cemented by cobalt binder has been developed since 1920s. Currently, the improvements include finer grained WC substrate, versatile binder that combines the toughness and hardness, and coating over the tool’s substrate that enhances the overall properties of the carbide cutting tool (Jindal et al., 1999). Coatings primarily increase wear resistance and were also reported to induce reduction of the friction force on the rake face, the tool-chip contact length, the heat flux transmitted to the tool, and the tool-chip-workpiece interface temperatures (Jindal et al., 1999; Rech, 2006). These characteristics endow the coated tool with higher resistance to abrasive wear and crater wear, thereby providing longer tool life and higher speed capability on a broad range of workpiece materials (Jindal et al., 1999).

Theoretically, it has been suggested that better surface finish will be achieved by applying low feed besides using tool with large nose radius. Moreover, finish turning is characterized with low cutting parameters, including feed (Shaw, 2005). On the other hand, low feed corresponds with low material removal rate which is one measure of a tool’s productivity. As a result, tool manufacturers developed cutting tool geometry that is capable of achieving the targeted Ra at high feed. This geometry is termed wiper where the tool is designed to have additional radii adjacent to the main tool nose radius. This geometrical modification can double the current feed and still achieve surface finish comparable to that of a conventional tool, as shown in Figure 2.2. Alternatively, the surface finish can be improved if the current feed is maintained (Castner, 2000).

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Figure 2.2 Comparison between (a) conventional and (b) wiper cutting tool, where f is feed, r is nose radius, rw is wiper radius, and Ra is surface roughness.

2.3 Martensitic Stainless Steel

Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10% or more chromium. Its history dated back to 1821 when Berthier found that iron when alloyed with chromium was resistant to some acids. In 1904, Leon Guillet published research on alloys with composition that today would be known as 410, 420, 442, 446, and 440-C. Later on, the first true stainless steel was melted on the 13th August 1913 by Harry Brearley. He, who was working on 430, initiated the industrial era of stainless steel with cutlery as the first product. All stainless steels have a high resistance to corrosion. This resistance is due to the naturally occurring chromium-rich oxide film formed on its surface. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. The film is rapidly self- repairing in the presence of oxygen. Damages caused by abrasion or machining is quickly self-repaired. Stainless steel may also contain nickel, molybdenum, niobium, and chromium to enhance its corrosion resistance. It has low thermal conductivity compared to carbon steel, and hence, the cutting edge temperatures are higher in stainless steel than in carbon steel. This naturally imposes the requirements on the high-temperature hardness of the cutting tool and on its ability to withstand high temperatures (www.ssina.com; www.stainless-steel-world.net; www.outokumpu.com).

Martensitic stainless steel was the first type commercially developed (as cutlery). This type of stainless steel has relatively high carbon content (0.1 - 1.2%) compared to other stainless steel types. It contains between 12 and 18% chromium. In the hardened state, they possess fully martensitic microstructure, a distorted body- centered cubic (bcc) crystal structure, at room temperature. From the Fe-Cr-C diagram in Figure 2.3 for 0.1% C, chances are the microstructure of heat-treated martensitic stainless steel contains M23C6 carbide phase. Some basic properties of martensitic stainless steel are moderate corrosion resistance, poor weldability, magnetic, and can be hardened by heat treatment and therefore high strength and hardness levels can be achieved (Sourmail and Bhadeshia, 2005).

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Figure 2.3 Vertical section of Fe-Cr-C diagram for 0.1% C (Sourmail and Bhadeshia, 2005).

Martensitic stainless steel at its hardened state combines the properties of high corrosion resistance and wear resistance. These properties are beneficial in producing consistent products over very long running periods with less maintenance and operable in harsh environment. One of its applications is as plastic mold. Various plastic and thermoset plastic products can be produced using hardened stainless steel molds with hardness of 45 - 50 HRC.

2.4 Tool Life and Wear Mechanisms

While machining, a cutting tool undergoes changes from its original shape until a certain criterion is met, where it is no longer machining effectively. It is the gradual loss of its materials that is measured to determine its life time. A guide to determine the criteria to determine a tool’s end of service life is provided by ANSI/ASME B94.55M standard. Tool wear measurement considers either flank wear land (Figure 2.4) or depth of crater. When a predetermined criterion is reached, the tool is considered terminally worn.

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Figure 2.4 Schematical representation of flank wear of a cutting tool according to ANSI/ASME B94.55M standard.

Assessing how a tool wears as its reacts to the loading during machining will complement the recognition of the overall cutting process. Common method for this purpose is by evaluating the tool at the end of its life. Some of the basic wear mechanisms exist on the worn carbide tool can be classified as follows (Trent and Wright, 2000):

1. Plastic deformation. Carbide cutting tools are prone to plastic deformation by compression under the influence of high temperature and compressive stress. Since carbide can withstand only limited deformation, crack may form as the tool’s surface is being stressed as the edge is depressed.
2. Diffusion wear. Crater formed on a tungsten carbide tool’s rake face is contributed by diffusion of the tool’s atoms to the chip. Diffusion wear is more affected by chemical load during the cutting process (Ljungberg and Castner, 2001). The chemical properties of the tool material and the affinity of the tool material to the workpiece determine the development of the diffusion wear mechanism. While hardness of the tool material does not affect the process very much.
3. Abrasion wear. Abrasion wear is caused mainly, but not entirely, by the hard particles of the workpiece material. The evidence of such mechanism is the appearance of grooves on the tool flank. Abrasion is less likely to be a significant wear mechanism for carbide tool since the hardness of tungsten carbide is higher than the hardest carbide contained in steels. However, Kim and Kwon (2002) suggested the contrary. A typical cutting tool’s hardness is much higher than that of steel’s carbides under isothermal condition. While during cutting, the stationary cutting tool experiences a constant heat flux at steady state while the carbide phase experiences only thermal transients. This accounts for the ability of the carbide phase to abrade the ‘‘harder’’ surface of a cutting tool. The cutting tool's ability to resist abrasive wear is connected to its hardness (Ljungberg and Castner, 2001). A tool material which is densely packed with hard particles will stand up well to abrasive wear.
4. Fatigue wear. Fatigue wear is caused by fluctuations in temperature and loading forces. Cracks due to fatigue are formed normal to the cutting edge. Intermittent cutting action leads to temperature cycles that create shocks as well as alternating expansion and contraction of the tool’s surface layer at the point where the cutting edge engages the workpiece.
5. Adhesion wear (Su et al., 2006). Generally, during dry turning, a large amount of workpiece material will adhere to the flank face and start to accumulate. As cutting continues, the adhered steel workpiece material reaches a critical size and subsequently detaches and then the underlying coating, as well as small pieces of WC substrate, follows. These mechanisms are generally present in combination. For the case of coated carbide, the coating was also reported to be worn.
6. Discrete plastic deformation (Lim et al., 1999). As reported on the use of TiC coated carbide tools during machining, high compressive stresses and intimate contact between the atomically clean surfaces of the newly-machined workpiece and the flank face results in seizure over much of the tool-work contact area. However, cutting is able to continue as the work material moves by shear in the layers of the work adjacent to this interface. Such conditions generate high shear stresses on the tool surface that plastically deform and ‘smear’ the original dimple-like features of the coating in the direction of workpiece rotation (Figure 2.5). With time, these deformed dimples flow and merge into the ridge and furrow topography. It was proposed that this process culminates in the ductile fracture of tiny fragments of the coating, which are then swept away by the passing work. Discrete plastic deformation gradually reduces the thickness of the TiC coating during machining. As wear progresses, localized areas of the underlying carbide substrate become exposed, and eventually merge into a continuous band of exposed substrate.

illustration not visible in this excerpt

Figure 2.5 Topography of flank face (a) when new, (b) after 3 minutes, and (c) after

20 minutes of cutting at 103.9 m/min and 0.2 mm/rev, showing original dimple-like TiC surface features being worn by discrete plastic deformation resulting in a ridgeand-furrow appearance (Lim et al., 1999).

The wear mechanisms are related to the high temperature generated during cutting. Therefore, the life time of a cutting tool is affected by the cutting temperature. Some reports provided the relation between process conditions and cutting temperature. One of the process conditions that are inversely proportional to cutting temperature in hard turning is cutting speed (Trent and Wright, 2000; Kishawy, 2002; Abukhshim, 2005; Al Huda et al., 2002). Yet, the contrary was reported by Bosheh and Mativenga (2006) that applied a wide range of cutting speeds when high speed turning (100 - 700 m/min) hot work tool steel and by Skvarenina and Shin (2006) when machining compacted graphite iron. Feed is also reported to be proportional to cutting temperature (Kishawy, 2002; Dhar et al., 2006), although Skvarenina and Shin (2006) reported the contrary when machining compacted graphite iron. Depth of cut also influenced the cutting temperature by being proportionally related (Outeiro et al., 2004; Al Huda et al., 2002).

The conditions of the cutting tool also influenced the cutting temperature. Using tool with higher nose radius reduced the cutting temperature (Kishawy, 2002). It was also reported that cutting temperature reduced when the tool was set at more negative effective rake angle (Kishawy, 2002). Using a sharp edged tool could lower the cutting temperature when compared to using a honed edged one (Kishawy, 2002). Tool’s coating was also reported to reduce cutting temperature (Lee et al., 2005). Also, it was concluded that the cutting temperature generated increases as the tool wear land develops (Kishawy, 2002).

The cutting temperature is also influenced by the conditions of the workpiece. When the workpiece diameter is large, the generated cutting temperature is lower than that generated when the workpiece diameter is small (Skvarenina and Shin, 2006). It was also reported that initial temperature of the workpiece was proportional to the cutting temperature (Skvarenina and Shin, 2006). This was reported when machining compacted graphite iron using laser pre-heater, where the higher the temperature induced by the laser beam the higher the process cutting temperature. However, this may also apply for hard turning when the workpiece’s temperature has not been relieved from the previous revolution, causing higher initial workpiece temperature. Additionally for this case, the initial workpiece temperature steadily increases with cutting time.

Another process condition that was reported to influence the cutting temperature is coolant. Coolant, in the form of flooding fluid, sprayed mist, or cryogenic substance has a positive impact on the tool life of carbide cutting tools (Che Haron et al., 2001; Hong, 2001; Varadarajan et al., 2002), especially at low cutting speeds. Although a report on turning free-machining stainless steel using CVD coated carbide tool with wiper geometry (nose radius of 0.8 mm) stated no benefit of applying mist coolant with regards to flank wear width (Bruni et al., 2006). Other than cutting fluid, the use of cutting tool that contained solid lubricant also helps in reducing the process cutting temperature (Jianxin et al., 2005).

2.5 Surface Integrity

Surface integrity is a generic term for describing quality of the surface and subsurface of a machined component. Surface roughness, metallurgical transformation, microstructural hardness variation, and residual stresses are some measures of surface integrity. In order to encourage machine shops to fully adopt hard turning, assessments should be made to clarify the aspects of the tool life and the surface quality. The machining cost per part is a function of tool life and, thus, machine shops demand long tool life. On the other hand, finish machining should produce fine surface finish as requested by the customer of the machined part to meet the specific requirements of certain application (Gillibrand et al., 1996).

Important part properties influenced by hard machining are hardness, wear resistance, fatigue strength, and corrosion resistance. These properties are influenced by the microstructure of the surface layer and also by the residual stresses. These microstructural alterations are deeper when the machined material is harder (Barbacki et al., 2003).

2.5.1 Surface Roughness

In engineering design and production, the degree of surface roughness as a measure of finish quality is commonly specified (Shaw, 2005). A set of surface roughness values stands in the progression of 0.025, 0.5, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 10 μm instead of being linearly spaced. Some measures of surface roughness are the peak-to-valley roughness (Rt) and the center line average (Ra) which is also called arithmetic average roughness. The center line average roughness (Ra) is obtained by measuring the mean deviation of the peaks from the center line of a trace. The center line itself is a line that divides areas above and below which equally. The peak-to-valley roughness for a ground surface is usually about five to ten times its Ra value. In finishing operations, as in hard turning, the end of tool life is usually based on a predetermined surface roughness level of the machined surface.

In a longitudinal turning operation, the finish resulted on the bar is produced by the secondary cutting edge (Shaw, 2005). A characteristic of this turning operation is the feed marks in the form of ridges corresponding to the geometry of the tool at its nose and having a pitch equal to the feed. Roughness due to wear on the tool’s end cutting edge, built-up edge, and tool vibration may also appear.

Özel et al. (2007) reported that surface finish of machined parts is improved as cutting speed was elevated and is deteriorated with the increased in feed. It has also been analyzed that better surface finish could be produced using a tool with certain degree of tool wear, due to the increase in the tool nose radius, but excessive wear will result in a rough surface (Chen, 2000). A geometrical phenomenon that is observed in hard turning is the side flow (a displacement of workpiece material in a direction opposite to the feed direction such that burrs form on the feed mark ridges) on the machined surface. Kishawy and Elbestawi (1999) observed that the material side flow increased with increasing tool wear and nose radius but decreased with lower feed. Chen (2000) also reported that this lateral plastic flow of the workpiece material in front of a cutting edge increases the roughness of the machined surface and suggested that the harder the workpiece the better the surface finish.

2.5.2 Surface and Subsurface Microstructures

The appearance of a machined surface layer is one of the important aspects of hard turning as it influences the functional behavior and dimensional stability of finished components.

Ramesh et al. (2005) reported that by varying cutting speed (100 - 300 m/min), the same workpiece material showed different surface microstructure. High speed resulted in coarser grain size and white layer is formed by martensitic phase transformation, while low speed generated fine-grained microstructure similar to the composition of the bulk metal. For the case of soft martensitic stainless steel, surface alteration was reported by Ezugwu and Olajire (2002) when turning using coated carbide tools. Microstructural alterations of the machined surfaces are predominantly plastic deformation of grain boundaries in addition to untempered and overtempered martensites.

One phenomena of interest in hard turning is the appearance of white layer, as figuratively shown in Figure 2.6. White layers have been associated with grinding for many years. Machined surface is more sensitive to damage from grinding because the relative heat flow into the workpiece surface is significantly higher in grinding than in normal cutting. This is due to the poor heat transfer properties of the grinding wheels such as the aluminum oxide wheels and the ploughing effect produced by wheels of negative rake angle. Therefore, the potential for thermal damage in grinding is higher than in metal cutting with defined cutting edges. Under abusive grinding conditions, the grinding burn produces white layers and untempered martensite (Bosheh and Mativenga, 2006).

illustration not visible in this excerpt

Figure 2.6 Appearance white layer in 27MnCr5 steel surface microstructure as the effect of hard turning (after Rech and Moisan, 2003).

The composition of white layers can be distinguished to three types (Barbacki et al., 2003):

a. In the case of medium and high carbon hardened steels, the white layer consists mainly of untempered martensite (so-called friction martensite). This martensite originates from austenite formed in the subsurface zone during machining.
b. In the case of eutectoid or hypereutectoid hardened steels, the white layer consists mainly of austenite.
c. The white layer produced by hard turning in high carbon steel consists mainly of extremely small (of the order of several nanometers) ferritic grains.

2.5.3 Hardness Variations

As a result of microstructural changes, hardness of the machined surface also alters. Similar causes that altered the microstructure of machined surface simultaneously affected the hardness. Warren et al. (2005) observed that white layer increases the nanohardness and dark layer (subsequent layer beneath the white layer) decreases the nanohardness in subsurface, while strain hardening only slightly increases the subsurface hardness. The effect of cutting parameter to the hardness profile was reported by Ramesh et al. (2005) stating that nanoindentation hardness data reveals a general trend of increased hardness of white layers with an increase in cutting speed.

2.5.4 Residual Stresses

Residual stress is a tension or compression that exists in the bulk of a material without the application of any external load (applied force, displacement of thermal gradient). Residual stress is produced by heterogeneous plastic deformations, thermal contractions and phase transformations induced by the manufacturing process (www.physiqueindustrie.com/residual_stress.htm . X-ray diffraction together with the other diffraction techniques of residual stress measurement uses the distance between crystallographic planes as a strain gage (Noyan and Cohen, 1987). The deformations cause changes in the spacing of the lattice planes from their stress free value to a new value that corresponds to the magnitude of the residual stress.

Details

Pages
102
Year
2008
ISBN (Book)
9783656193234
File size
3.5 MB
Language
English
Catalog Number
v193998
Institution / College
University College of Technology and Innovation, Malaysia – Universiti Teknologi Malaysia
Grade
Satisfactory
Tags
machining carbide stainless steel

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Title: Tool life and surface integrity when hard turning stainless steel using wiper coated carbide tool