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Influence of different post treatments on tungsten carbide–cobalt inserts
Materials Letters 62 (2008) 4403–4406
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Influence of different post treatments on tungsten carbide–cobalt inserts Dinesh Thakur, B. Ramamoorthy ⁎, L. Vijayaraghavan Mechanical Engineering Department, IIT-Madras, Chennai-600 036 TN, India
a r t i c l e
i n f o
Article history: Received 10 November 2007 Accepted 28 July 2008 Available online 30 July 2008 Keywords: WC–Co inserts Cryogenic treatment Microhardness XRD SEM
a b s t r a c t The present work is an attempt to improve some of the mechanical properties of cemented tungsten carbide (WC) cutting tool by subjecting it to different post treatments. The different post treatments that are tried out to the tungsten carbide–cobalt (WC–Co) inserts are a) controlled cryogenic treatment, b) heating and forced air cooling and c) heating and quenching in oil bath. The response of WC–Co inserts to such different post treatments has been evaluated in terms of microhardness, microstructural changes, scanning electron microscope (SEM) micrograph and Co metal phase changes through XRD. The experimental result indicate a remarkable response to all the above mentioned post treatments and the analysis of the same are presented in this paper. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
The microstructure of tungsten carbide is complicated mainly for the following reasons [3]:
Tungsten carbide–cobalt (WC–Co) alloys or a composite consists of tungsten carbide grains surrounded by a cobalt base solid solution. The carbide grains impart hardness and wear resistance whereas metal binder cobalt imparts toughness. The wear resistance and fracture toughness of WC–Co alloys are inversely proportional to each other. The carbide grains are in the order of micrometer size. These WC–Co alloys are produced from carbide and metal powders like iron, nickel or cobalt. This mixture is normally blended together in a rotating mixer or in a ball mill. Sometimes titanium and/or tantalum oxide are added to improve the mechanical properties as well as to withstand the elevated temperature. The properly blended mixture is subjected to compaction at high pressure to get a green mold. The compacted green mold is subjected to presintering and final liquid phase sintering at approximately 1300 °C to 1500 °C. The sintering mechanism of WC–Co involves the WC dissolving into the cobalt binder phase, migrating and re-precipitating on the surface of the original WC. The final product consists of a three dimensional skeleton of WC grains with cobalt as a binder phase matrix. At the eutectic temperature, about 12% of tungsten carbide is dissolved in cobalt. As temperature decreases, the solubility drops substantially and at 800 °C–600 °C range, it amounts to about 1%. The fact that the solubility of tungsten carbide decreases with decreasing temperature provides a basis for changing the properties of WC–Co alloys by heat treatment [1–2].
• Some elements like Fe, Cr, Ni etc from stainless steel and W, C, Co from cemented carbide milling bodies gets added to the powders. Co oxidizes during milling, and the oxide subsequently consumes carbon when reduced during sintering. Also Carbon gets added from the press lubricant. Sintering in vacuum or in an inert gas gives rise to decarburization and denitrification. • Equilibrium cannot be achieved during sintering if more than one hard phase (i.e. carbide, carbo-nitride or nitride phase) remains undissolved at the sintering temperature, because of the slow diffusion of metal atoms in the hard phases. Therefore, non-equilibrium phases are retained. However, the diffusivity of C and N is sometimes sufficient to allow the system approach (para) equilibrium with respect to these elements.
⁎ Corresponding author. Tel.: +91 44 2257 4674; fax: +91 44 2257 5705. E-mail addresses: [email protected] (D. Thakur), [email protected] (B. Ramamoorthy), [email protected] (L. Vijayaraghavan). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.07.043
Fig. 1. Variation of microhardness for various post treatments.
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D. Thakur et al. / Materials Letters 62 (2008) 4403–4406
Cryogenic treatment or deep cryogenic treatment is a process that uses cryogenic temperatures to modify materials properties to enhance their performance. Cryogenic processing involves a controlled reduction in temperature of the material from room temperature to cryogenic temperature in the range of −125 °C to−196 °C. Then the material is held at that temperature for a certain period (generally 24 h) followed by controlled heating back to room temperature. Cryogenic treatment has been successfully applied to die and high speed steel (HSS) ferrous alloys. The cryogenic treatment converts the retained austenite into martensite in ferrous materials which is a distorted BCT structure. In addition to transformation to martensite, the subjected metals also develop a more uniform, refined microstructure with greater density. Hence, the properties of the ferrous materials are enhanced. Since the cryogenic treatment hardens and toughens the HSS, the material is probably more chemically inert at high temperatures [4–6]. Tungsten carbide tool materials generally have a cobalt binder. Cobalt is next to the iron in the periodic table as a part of the VIII B group, has the same valences, and form similar phases in crystalline structures. Consequently, a cryogenic treatment of tungsten carbide tool material may have some effect upon the cobalt binder to enhance the properties. The present study is an attempt to improve the properties of WC–Co insert through different post treatments namely a) controlled cryogenic treatment of WC–Co inserts, b) heating and forced air cooling of WC–Co inserts and c) heating and quenching of WC–Co inserts in oil bath. The details of the experimental study carried out is presented and analyzed in the following sections: 2. Experimental procedure Fig. 2. Optical micrograph of WC–Co untreated insert at 1000×. a) Microstructure showing rectangular, triangular carbide particles b) porosity.
In the present study, a tungsten carbide–cobalt alloy in the form of cutting inserts is used. Tungsten carbide insert compositions are
Fig. 3. SEM of WC–Co insert after different post treatments.
D. Thakur et al. / Materials Letters 62 (2008) 4403–4406
WC 93% and the rest of Co metal binder with other minor elements. The inserts were subjected to different post treatments namely a) controlled cryogenic treatment, b) heating and forced air cooling and c) heating and quenching in oil bath. During cryogenic treatment, the inserts were slowly cooled in the refrigerator for sufficient time before subjecting them directly to the liquid nitrogen (− 196 °C). The inserts were soaked at this liquid nitrogen temperature for around 24 h. After soaking, the inserts were kept in the refrigerator before exposing them to the room temperature. Later in the two post treatments of inserts namely a) heating and forced air cooling and b) heating and quenching in oil bath, the inserts were heated for about 30 min in a controlled argon–hydrogen rich environment at a temperature of 750 °C to avoid oxidation problem. The inserts were soaked at this temperature for some time so as to distribute the temperature uniformly. In first case the inserts were cooled from 750 °C to room temperature by forced air cooling and in another post treatment the inserts were quenched from 750 °C in oil bath at room temperature. The response to all these three post treatments was evaluated through microhardness, microstructural changes, scanning electron microscope (SEM) micrographs and XRD analysis.
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3. Results and discussion 3.1. Microhardness observations The microhardness tests were carried out to see the response of the post treatments on the WC–Co inserts. MATSUZAWA MMT7 microhardness tester was used. Load applied on the samples is 200 g. To plot the microhardness graph (Fig. 1) average value of three tested samples is used. The thermal stresses are generated in WC–Co alloys as a result of the large difference between the coefficients of thermal expansion of the WC and Co-phases. The carbide phase is subjected to compressive stresses and the binder to tensile ones. The magnitude of the stresses in the cementing phase increases with decreasing cobalt content. The increased stresses generated by quenching may be expected to result in decreased ductility of the cobalt phase. The tungsten carbide is subjected to compression from all sides, and an increase of compressive stresses would lead to an increase in the strength of the carbide matrix. The rapid cooling of these alloys causes such an increase of compressive thermal stresses in the tungsten carbide, therefore the strength of low-cobalt alloys as a whole gets slightly increased [1]. It is seen from the Fig.1 that there is a slight increase in the microhardness due to the controlled cryogenic treatment compared to untreated WC–Co sample. But other two post treatments showed a considerable improvement in the hardness. The formation of the complex compounds such as W2C,
Fig. 4. XRD profiles of post treatments of WC–Co inserts.
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D. Thakur et al. / Materials Letters 62 (2008) 4403–4406
Co6W6C or Co3W3C might have increased the hardness in the case of the samples due to forced air cooling and oil quenching. 3.2. Microstructural observations Tungsten carbide inserts were etched with 10 wt.% of aqueous solutions of K3Fe(CN)6 and KaOH (Murakami's solution) at room temperature for 15 to 20 s and were then flushed immediately with water and dried after that the micrographs were taken [7]. Fig. 2a) and b) show untreated WC–Co insert micrograph showing carbide particles in the rectangular, triangular etc shape packed in cobalt rich matrix and porosity in WC–Co insert respectively. 3.3. SEM study for different post treatments The scanning electron micrographs of both untreated tungsten carbide insert and post treated tungsten carbide inserts are shown in Fig. 3a), b), c), and d) respectively. It is observed that cryogenic treatment has less effect on the microstructure compared to conventional heat treatments i.e. forced air cooling and oil quenching. It is seen clearly from the Fig. 3b) that due to cryogenic treatments some physical changes have taken place which could be due to cobalt densification. Due to shrinkage or densification cobalt holds the carbide particles more firmly thereby increasing the wear resistance properties of the insert. Also, cryogenic treatment relieves stresses introduced during the sintering process. In other two post treatments, it is seen [from Fig. 3c) and d)] that the cobalt would have been partially liquified due to which cobalt binder exposing the skeleton of carbide matrix and forming complex carbide surrounded by a relatively softer substrate having more cobalt and thereby enhance the tool performance. The uniform distribution of the carbide particles can be seen in oil quenched compared to the air cooled post treatments. 3.4. XRD study for different post treatments Tungsten carbide inserts were examined to understand whether any structural changes have taken place or not after treating WC inserts using X-ray diffraction method. XRD profiles are shown in Fig. 4a), b), c) and d) for the post treatments given to the inserts. Cryogenic treated inserts showed almost same trend [Fig. 4a) and b)] as that of an untreated insert because due to cryogenic treatment only physical changes takes place. Densification of the cobalt metal binder might have taken place. Whereas for other two post treatments namely heating and cooling in air and oil quenching of WC inserts can lead to recrystalization of the lower melting point cobalt binder exposing the skeleton WC matrix over the surface of the tool material.
Further, cobalt reacts with tungsten carbide resulting in a certain complex compound and carbon-depleted/dissociated tungsten carbide. An η (eta) phase of Co6W6C tertiary carbide forms at the interface between the carbide and the cobalt phase. W2C is a chemical compound. XRD profiles of an irradiated carbide tool shows peaks of WC and W2C; also, peaks corresponding to a complex compound W3CO3C and W6CO6C can be seen. This is predominantly seen in the oil quenched sample. In WC–Co system, any disturbance in stoichiometry of WC can result in the formation of hard and brittle phase like W3CO3C and W6CO6C-phase equilibrium between WC and binder Co. These results in increased surface hardness for air cooled and oil quenched inserts.
4. Conclusions Based on the experiments conducted and the analysis made by subjecting the tool WC–Co inserts to different post treatments, the following conclusions are drawn: • The solubility of tungsten carbide decreases with decreasing temperature (800 °C–600 °C) range provided the basis for understanding the properties of WC–Co alloys by post treatments. Post treatments of WC–Co namely a) controlled cryogenic treatment b) heating and forced cooling c) heating and oil quenching of WC–Co inserts have yielded considerable improvement in micro hardness. • Controlled cryogenic treatment improved the wear resistance. This is due to the physical changes i.e. densification of the cobalt metal binder which holds the carbide particles firmly. In the later two treatments it is seen from microstructure that there is uniform distribution of tungsten carbide particles. • XRD study showed formation of complex phases like W3CO3C and W6CO6C. These complex phases results in the increase in hardness due to exposure of skeleton carbide matrix due to later post treatments. References [1] Tumanov VI, Funke VF, Trukhanova ZS, Novikova TA. Translated from Poroshkovaya Metallurgia 1964;2(20):57. [2] Zhigang ZF, Oladapo OE. Scr Mater 2005;52:785. [3] Hans-Olof A. Materials and Design 2001;22:491. [4] Moore K, Collins DN. Key Eng Mater 1993;86:47. [5] Collins DN. Heat Treat Met 1996;2:40. [6] Yun D, Xiaoping L, Hongshen X. Heat Treat Met 1998;3:55. [7] Seah KHW, Rahman M, Yong KH. Proc Inst Mech Eng Part B 2003;217:29.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Influence of different post treatments on tungsten carbide–cobalt inserts Dinesh Thakur, B. Ramamoorthy ⁎, L. Vijayaraghavan Mechanical Engineering Department, IIT-Madras, Chennai-600 036 TN, India
a r t i c l e
i n f o
Article history: Received 10 November 2007 Accepted 28 July 2008 Available online 30 July 2008 Keywords: WC–Co inserts Cryogenic treatment Microhardness XRD SEM
a b s t r a c t The present work is an attempt to improve some of the mechanical properties of cemented tungsten carbide (WC) cutting tool by subjecting it to different post treatments. The different post treatments that are tried out to the tungsten carbide–cobalt (WC–Co) inserts are a) controlled cryogenic treatment, b) heating and forced air cooling and c) heating and quenching in oil bath. The response of WC–Co inserts to such different post treatments has been evaluated in terms of microhardness, microstructural changes, scanning electron microscope (SEM) micrograph and Co metal phase changes through XRD. The experimental result indicate a remarkable response to all the above mentioned post treatments and the analysis of the same are presented in this paper. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
The microstructure of tungsten carbide is complicated mainly for the following reasons [3]:
Tungsten carbide–cobalt (WC–Co) alloys or a composite consists of tungsten carbide grains surrounded by a cobalt base solid solution. The carbide grains impart hardness and wear resistance whereas metal binder cobalt imparts toughness. The wear resistance and fracture toughness of WC–Co alloys are inversely proportional to each other. The carbide grains are in the order of micrometer size. These WC–Co alloys are produced from carbide and metal powders like iron, nickel or cobalt. This mixture is normally blended together in a rotating mixer or in a ball mill. Sometimes titanium and/or tantalum oxide are added to improve the mechanical properties as well as to withstand the elevated temperature. The properly blended mixture is subjected to compaction at high pressure to get a green mold. The compacted green mold is subjected to presintering and final liquid phase sintering at approximately 1300 °C to 1500 °C. The sintering mechanism of WC–Co involves the WC dissolving into the cobalt binder phase, migrating and re-precipitating on the surface of the original WC. The final product consists of a three dimensional skeleton of WC grains with cobalt as a binder phase matrix. At the eutectic temperature, about 12% of tungsten carbide is dissolved in cobalt. As temperature decreases, the solubility drops substantially and at 800 °C–600 °C range, it amounts to about 1%. The fact that the solubility of tungsten carbide decreases with decreasing temperature provides a basis for changing the properties of WC–Co alloys by heat treatment [1–2].
• Some elements like Fe, Cr, Ni etc from stainless steel and W, C, Co from cemented carbide milling bodies gets added to the powders. Co oxidizes during milling, and the oxide subsequently consumes carbon when reduced during sintering. Also Carbon gets added from the press lubricant. Sintering in vacuum or in an inert gas gives rise to decarburization and denitrification. • Equilibrium cannot be achieved during sintering if more than one hard phase (i.e. carbide, carbo-nitride or nitride phase) remains undissolved at the sintering temperature, because of the slow diffusion of metal atoms in the hard phases. Therefore, non-equilibrium phases are retained. However, the diffusivity of C and N is sometimes sufficient to allow the system approach (para) equilibrium with respect to these elements.
⁎ Corresponding author. Tel.: +91 44 2257 4674; fax: +91 44 2257 5705. E-mail addresses: [email protected] (D. Thakur), [email protected] (B. Ramamoorthy), [email protected] (L. Vijayaraghavan). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.07.043
Fig. 1. Variation of microhardness for various post treatments.
4404
D. Thakur et al. / Materials Letters 62 (2008) 4403–4406
Cryogenic treatment or deep cryogenic treatment is a process that uses cryogenic temperatures to modify materials properties to enhance their performance. Cryogenic processing involves a controlled reduction in temperature of the material from room temperature to cryogenic temperature in the range of −125 °C to−196 °C. Then the material is held at that temperature for a certain period (generally 24 h) followed by controlled heating back to room temperature. Cryogenic treatment has been successfully applied to die and high speed steel (HSS) ferrous alloys. The cryogenic treatment converts the retained austenite into martensite in ferrous materials which is a distorted BCT structure. In addition to transformation to martensite, the subjected metals also develop a more uniform, refined microstructure with greater density. Hence, the properties of the ferrous materials are enhanced. Since the cryogenic treatment hardens and toughens the HSS, the material is probably more chemically inert at high temperatures [4–6]. Tungsten carbide tool materials generally have a cobalt binder. Cobalt is next to the iron in the periodic table as a part of the VIII B group, has the same valences, and form similar phases in crystalline structures. Consequently, a cryogenic treatment of tungsten carbide tool material may have some effect upon the cobalt binder to enhance the properties. The present study is an attempt to improve the properties of WC–Co insert through different post treatments namely a) controlled cryogenic treatment of WC–Co inserts, b) heating and forced air cooling of WC–Co inserts and c) heating and quenching of WC–Co inserts in oil bath. The details of the experimental study carried out is presented and analyzed in the following sections: 2. Experimental procedure Fig. 2. Optical micrograph of WC–Co untreated insert at 1000×. a) Microstructure showing rectangular, triangular carbide particles b) porosity.
In the present study, a tungsten carbide–cobalt alloy in the form of cutting inserts is used. Tungsten carbide insert compositions are
Fig. 3. SEM of WC–Co insert after different post treatments.
D. Thakur et al. / Materials Letters 62 (2008) 4403–4406
WC 93% and the rest of Co metal binder with other minor elements. The inserts were subjected to different post treatments namely a) controlled cryogenic treatment, b) heating and forced air cooling and c) heating and quenching in oil bath. During cryogenic treatment, the inserts were slowly cooled in the refrigerator for sufficient time before subjecting them directly to the liquid nitrogen (− 196 °C). The inserts were soaked at this liquid nitrogen temperature for around 24 h. After soaking, the inserts were kept in the refrigerator before exposing them to the room temperature. Later in the two post treatments of inserts namely a) heating and forced air cooling and b) heating and quenching in oil bath, the inserts were heated for about 30 min in a controlled argon–hydrogen rich environment at a temperature of 750 °C to avoid oxidation problem. The inserts were soaked at this temperature for some time so as to distribute the temperature uniformly. In first case the inserts were cooled from 750 °C to room temperature by forced air cooling and in another post treatment the inserts were quenched from 750 °C in oil bath at room temperature. The response to all these three post treatments was evaluated through microhardness, microstructural changes, scanning electron microscope (SEM) micrographs and XRD analysis.
4405
3. Results and discussion 3.1. Microhardness observations The microhardness tests were carried out to see the response of the post treatments on the WC–Co inserts. MATSUZAWA MMT7 microhardness tester was used. Load applied on the samples is 200 g. To plot the microhardness graph (Fig. 1) average value of three tested samples is used. The thermal stresses are generated in WC–Co alloys as a result of the large difference between the coefficients of thermal expansion of the WC and Co-phases. The carbide phase is subjected to compressive stresses and the binder to tensile ones. The magnitude of the stresses in the cementing phase increases with decreasing cobalt content. The increased stresses generated by quenching may be expected to result in decreased ductility of the cobalt phase. The tungsten carbide is subjected to compression from all sides, and an increase of compressive stresses would lead to an increase in the strength of the carbide matrix. The rapid cooling of these alloys causes such an increase of compressive thermal stresses in the tungsten carbide, therefore the strength of low-cobalt alloys as a whole gets slightly increased [1]. It is seen from the Fig.1 that there is a slight increase in the microhardness due to the controlled cryogenic treatment compared to untreated WC–Co sample. But other two post treatments showed a considerable improvement in the hardness. The formation of the complex compounds such as W2C,
Fig. 4. XRD profiles of post treatments of WC–Co inserts.
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D. Thakur et al. / Materials Letters 62 (2008) 4403–4406
Co6W6C or Co3W3C might have increased the hardness in the case of the samples due to forced air cooling and oil quenching. 3.2. Microstructural observations Tungsten carbide inserts were etched with 10 wt.% of aqueous solutions of K3Fe(CN)6 and KaOH (Murakami's solution) at room temperature for 15 to 20 s and were then flushed immediately with water and dried after that the micrographs were taken [7]. Fig. 2a) and b) show untreated WC–Co insert micrograph showing carbide particles in the rectangular, triangular etc shape packed in cobalt rich matrix and porosity in WC–Co insert respectively. 3.3. SEM study for different post treatments The scanning electron micrographs of both untreated tungsten carbide insert and post treated tungsten carbide inserts are shown in Fig. 3a), b), c), and d) respectively. It is observed that cryogenic treatment has less effect on the microstructure compared to conventional heat treatments i.e. forced air cooling and oil quenching. It is seen clearly from the Fig. 3b) that due to cryogenic treatments some physical changes have taken place which could be due to cobalt densification. Due to shrinkage or densification cobalt holds the carbide particles more firmly thereby increasing the wear resistance properties of the insert. Also, cryogenic treatment relieves stresses introduced during the sintering process. In other two post treatments, it is seen [from Fig. 3c) and d)] that the cobalt would have been partially liquified due to which cobalt binder exposing the skeleton of carbide matrix and forming complex carbide surrounded by a relatively softer substrate having more cobalt and thereby enhance the tool performance. The uniform distribution of the carbide particles can be seen in oil quenched compared to the air cooled post treatments. 3.4. XRD study for different post treatments Tungsten carbide inserts were examined to understand whether any structural changes have taken place or not after treating WC inserts using X-ray diffraction method. XRD profiles are shown in Fig. 4a), b), c) and d) for the post treatments given to the inserts. Cryogenic treated inserts showed almost same trend [Fig. 4a) and b)] as that of an untreated insert because due to cryogenic treatment only physical changes takes place. Densification of the cobalt metal binder might have taken place. Whereas for other two post treatments namely heating and cooling in air and oil quenching of WC inserts can lead to recrystalization of the lower melting point cobalt binder exposing the skeleton WC matrix over the surface of the tool material.
Further, cobalt reacts with tungsten carbide resulting in a certain complex compound and carbon-depleted/dissociated tungsten carbide. An η (eta) phase of Co6W6C tertiary carbide forms at the interface between the carbide and the cobalt phase. W2C is a chemical compound. XRD profiles of an irradiated carbide tool shows peaks of WC and W2C; also, peaks corresponding to a complex compound W3CO3C and W6CO6C can be seen. This is predominantly seen in the oil quenched sample. In WC–Co system, any disturbance in stoichiometry of WC can result in the formation of hard and brittle phase like W3CO3C and W6CO6C-phase equilibrium between WC and binder Co. These results in increased surface hardness for air cooled and oil quenched inserts.
4. Conclusions Based on the experiments conducted and the analysis made by subjecting the tool WC–Co inserts to different post treatments, the following conclusions are drawn: • The solubility of tungsten carbide decreases with decreasing temperature (800 °C–600 °C) range provided the basis for understanding the properties of WC–Co alloys by post treatments. Post treatments of WC–Co namely a) controlled cryogenic treatment b) heating and forced cooling c) heating and oil quenching of WC–Co inserts have yielded considerable improvement in micro hardness. • Controlled cryogenic treatment improved the wear resistance. This is due to the physical changes i.e. densification of the cobalt metal binder which holds the carbide particles firmly. In the later two treatments it is seen from microstructure that there is uniform distribution of tungsten carbide particles. • XRD study showed formation of complex phases like W3CO3C and W6CO6C. These complex phases results in the increase in hardness due to exposure of skeleton carbide matrix due to later post treatments. References [1] Tumanov VI, Funke VF, Trukhanova ZS, Novikova TA. Translated from Poroshkovaya Metallurgia 1964;2(20):57. [2] Zhigang ZF, Oladapo OE. Scr Mater 2005;52:785. [3] Hans-Olof A. Materials and Design 2001;22:491. [4] Moore K, Collins DN. Key Eng Mater 1993;86:47. [5] Collins DN. Heat Treat Met 1996;2:40. [6] Yun D, Xiaoping L, Hongshen X. Heat Treat Met 1998;3:55. [7] Seah KHW, Rahman M, Yong KH. Proc Inst Mech Eng Part B 2003;217:29.