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Atmosphere influence in sintering process of stainless steels matrix composites reinforced with hard particles
Composites Science and Technology 63 (2003) 69–79 www.elsevier.com/locate/compscitech
Atmosphere influence in sintering process of stainless steels matrix composites reinforced with hard particles J. Abenojara,*, F. Velascoa, A. Bautistaa, M. Camposa, J.A. Basb, J.M. Torralbaa a
Departamento de Materiales, Universidad Carlos III de Madrid, Av. de la Universidad, 30, 28911 Legane´s, Madrid, Spain b Aleaciones de Metales Sinterizados, S.A. Ctra. Nacional 340, Km.1242, 08620 Sant Vicenc¸ dels Horts, Spain Received 24 July 2001; received in revised form 20 January 2002; accepted 15 August 2002
Abstract This paper studies the change in properties of powder metallurgy 316L stainless steel, depending on kind of added reinforcement and its amount and the sintering atmosphere. Used reinforcements have been AlCr2, Cr2Ti, VC and SiC; and the amounts were 1.5 and 3% (vol.). All materials have been manufactured mixing the powders with a 0.6% (wt.) of wax, compacting at 700 MPa, and sintering at 1230 C. In order to analyze the sintering atmosphere, three different media were used: 75%H2–25%N2, pure H2 and vacuum. The properties of the 24 different composites materials are evaluated and compared with the base stainless steel sintered in the same conditions. The study carried out includes sintering density, tensile strength, hardness, corrosion and wear and microstructural analysis. Intermetallics seem to be a good solution to improve the studied properties. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Metal-matrix composites; A. Intermetallics; E. Sintering
1. Introduction Powder metallurgy (P/M) process has numerous advantages for fabricating small pieces of complicated shapes, because it allows material and energy savings as well as dimensional accuracy. Sintered stainless steels have a wide range of applications, mainly related to the automotive industry. However, they present lower properties than their wrought counter-parts in terms of strength, corrosion and wear resistance. The main reason for this lower performance is the presence of porosity, and the problems that these steels have when they are sintered in industrial atmospheres [1]. In order to achieve the best performance, P/M stainless steels must be sintered in inert atmospheres at high temperatures. Their properties can also be improved with different additions, with the aim of increasing the final density of the steel, and enhancing the mechanical and corrosion behaviour [2]. When the goal is to improve the wear resistance, ceramic particles [3,4] are used due to their * Corresponding author. E-mail address: [email protected] (J. Abenojar).
hardness. The main problem related to these particulate MMCs is their poor corrosion behaviour compared with the corrosion resistance of the matrix. The low interaction between matrix and reinforcements favours the onset of the corrosive attack. This problem forces to use at the same time other additions with the aim of activating the sintering process. The addition of intermetallic particles can be another option to improve not only the mechanical characteristics of the sintered materials, but also their corrosion resistance. However, the effects of the intermetallic reinforcements on the final properties of the composite material are very dependent on the kind of addition and on the atmosphere where their sintering has been carried out [5–7].
2. Experimental procedure Metal matrix composites were manufactured following a conventional P/M route. Raw materials used in this research were powders of AISI 316L austenitic stainless steel as metallic matrix (from Ho¨gana¨s AB,
0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00179-3
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Sweden), and four different reinforcements: AlCr2 and Cr2Ti intermetallics (particle size < 75 mm), VC (particle size < 45 mm) and SiC (particle size < 75 mm). To obtain the MMCs, two different amounts of reinforcement were considered: 1.5 and 3% (in vol.), and all mixes have a 0.6% (wt.) lubricant to minimise friction during compacting operations. Powders were dry mixed in a rotating laboratory mill at 80 r.p.m. for 20 min and the homogeneity of this mix was checked. Apparent density and flow rate of powder mixes were measured. Afterwards, powders were uniaxially pressed at 700 MPa. After pressing, green density and green strength were evaluated. Then, green specimens were sintered in 25% N2/75% H2, pure H2 and vacuum atmospheres at 1230 C for 30 min. Samples of plain 316L were manufactured following the same process. A complete microstructural study was carried through optical and scanning electron microscopy (SEM), aided by semiquantitative microanalysis through EDX (energy dispersive X-ray analysis). Sintering density, tensile strength, elongation and hardness (HRA) were measured. Fracture analysis was also carried out on tensile tested samples. Salt spray tests (ASTM B118) were carried out to study corrosion behaviour of these materials. The wear behaviour was measured through a pin-ondisk test. Wear tests were performed according to ASTM Standard G99–95 using the following test conditions: Pin: Tool steel (55 HRC), 6 mm diameter; Friction track diameter: 16 mm; Sliding distance: 400 m; Speed: 0.1 m/s; Load applied: 5 N; Relative humidity: less than 30%; Room temperature. Friction coefficient was measured during the test (given value was determined when a steady state in the wear test was reached) and wear was measured through the wear coefficient k:
k m2 =N ¼ volume loss material m3 = ½applied loadðNÞ sliding distanceðmÞ The tracks were also observed through SEM in order to establish the wear mechanisms that take place in the materials.
3. Results and discussion Fig. 1 shows the main characteristics of the powders of studied materials: flow rate and apparent density. The flow rate of the powder mixes does not differ meaningfully from the flow of rate of the plain 316L powders. Although reinforcement particle size could affect, the lubricant reduces the differences between materials. Apparent density is not modified by SiC additions, it increases when AlCr2 is added and decreases with Cr2Ti and VC additions. The increase of the apparent density after mixing 316L powders with AlCr2, is the low hardness of this intermetallic. On the other hand, the small reduction of the apparent density is due to the low specific weight of the added particles. The properties of the different compacted powder mixes before sintering are shown in Fig. 2. The values of the green density are very close for all the materials, and lubricant enhances this property during compaction. The small differences observed are due to lightness of reinforcements studied. The green strength increases for the reinforced materials, except 1.5% VC. The small sized, hard particles used as reinforcements link the ductile stainless steel particles together during compaction, increasing the green strength. However, this increase is not always detectable because the added lubricant acts in the opposite way, reducing the strength of the compacted materials.
Fig. 1. Apparent density and flow rate of powder mix.
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Fig. 2. Green density and green strength of studied materials.
Fig. 3 shows the sintering density of the materials sintered in 75%H2–25%N2, pure H2 and vacuum. As can be seen, most of the composite materials exhibit less density than plain 316L steel, and 3% reinforced materials present lower density than 1.5% reinforced materials. There is no clear influence of the sintering atmosphere on this parameter. The low density of composite materials after sintering is due to two reasons. The former is the lower density of reinforcements that reduces the density of the composite. The later is that added particles hinder the sintering process, owing to the low interaction between intermetallics/carbides and stainless steel. This low interaction has been previously observed for oxide additions [4,8,9] and was expected for carbide additions, specially SiC. Previous results have shown the good interaction between stainless steel and intermetallics, but this did not influence sintering density values [6,10]. Figs. 4 and 5 show the variation of the mechanical properties for the different stainless steels after sintering in the three atmospheres studied. Tensile strength and
hardness results are strongly determined by the atmosphere used to sinter the materials. 75%H2/25%N2 atmosphere promotes highest properties in all materials, although ductility is reduced. All composite materials keep the ductility of stainless steel, as elongation results show. Sintering in 75%H2/25%N2 promotes higher tensile strength (Fig. 4) than other sintering atmospheres, especially for plain 316L stainless steel. The microestructural study of the plain 316L sintered in 75%H2/ 25%N2 atmosphere (Fig. 6a) reveals that there are no chromium nitrides precipitated in the grain boundaries, and only the typical porosity of the sintered materials can be observed. As no precipitated nitrides are observed, the effect of the sintering atmosphere is due to the presence of nitrogen in the metal that promotes solid solution hardening. The solid solution of N2 makes stainless steels sintered in 75%H2/25%N2 less ductile (Fig. 4) and harder (Fig. 5). Reinforcements slightly modify the strength of the steel. Cr2Ti is the only addition that improves the
Fig. 3. Sintering density of materials sintered in 75%H2–25%N2, pure H2 and vacuum.
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Fig. 4. Tensile strength and elongation of materials sintered in 75%H2–25%N2, pure H2 and vacuum.
Fig. 5. Hardness of materials sintered 75%H2–25%N2, pure H2 and vacuum.
strength when sintering in 75%H2/25%N2 because of the good interaction of the intermetallic with the matrix (Fig. 6c). Chromium diffuses from the intermetallic particles to the matrix and layers of different composition appears in the reinforcements, being the inner, darker areas, the ones with higher Ti content. Moreover, Fe and Ni diffuse into the intermetallic. When AlCr2 is added, it reacts with the stainless steel (Fig. 6b). Different areas can be observed in the microstructure: the lightest areas (Fe-rich), the grey areas (rich in nitrogen and chromium) and the dark areas (alumina rich). The formation of an alumina layer in the AlCr2/steel interface in all the sintering atmospheres used (Figs. 6b, 7b and 8a) implies that the reinforcement does not contribute to the strengthening of the steel and the tensile strength of the materials diminishes. The alumina layer appears in
AlCr2 reinforced materials because of the dew point existing in the furnace, that, although is adequate for the steel is too high for the intermetallic, very avid for moisture. Carbide additions decrease tensile strength of H2–N2 atmosphere sintered materials due to different reasons. The use of VC as reinforcement (Fig. 6d) promotes the formation of a vanadium carbonitride (C,N)5V4, whose stoichiometry corresponds to C2N3V4. So, lower nitrogen level is achieved in base steel and tensile strength slightly decreases. It must be emphasised the beneficial effect of 3% VC additions, that increase the hardness of the material, thanks to the great hardness of reinforcements and interaction with the matrix (Fig. 6d). Only SiC additions reduce hardness in this sintering atmosphere. The reason is the low interaction between matrix and reinforcement (Fig. 6e).
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Fig. 6. Microstructures of materials sintered in 75% H2 + 25% N2 at 1230 C: (a) 316L; (b) 316L + 3% Cr2Al; (c) 316L + 3% TiCr2; (d): 316L + 3% VC; (e) 316L + 3% SiC.
If sintering is carried out in vacuum (Fig. 7a) or hydrogen, no special feature appears in the base stainless steel. Mechanical properties are lower than in nitrogen base atmosphere due to the absence of solid solution strengthening. The sintering of 316L reinforced with AlCr2 in vacuum (Fig. 7b) and H2 (Fig. 8a) atmosphere again originates the formation of an alumina layer around the intermetallic particles. So strength values are lower than those of plain stainless steel. Moreover, vacuum sintering
(Fig. 7b) promotes chromium carbide precipitation inside the intermetallic particles and a alumina layer appears surrounding them. The origin of the latter has been previously explained, and the presence of carbides is due to a reaction between lubricant and intermetallic during delubing operation. The same reason explains titanium carbide precipitation observed in Cr2Ti vacuum sintered materials (Fig. 7c). Diffusion of chromium from the particle to the matrix is also observed, thus obtaining a better link and
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Fig. 7. Microstructures of materials sintered in vacuum at 1230 C: (a) 316L; (b) 316L+3% AlCr2; (c) 316L+3% Cr2Ti; (d) 316L+3% VC.
so better strength that AlCr2 containing composites. Diffusion is also appreciated in their hydrogen sintered composites (Fig. 8b). Diffusion of the chromium to the matrix occurs and dark, Ti-rich areas appear inside the particles. Its effect on tensile strength and hardness is then positive. Carbide reinforcements show no reaction with the stainless steel in vacuum and hydrogen. VC agglomerates can be appreciated in both sintering atmospheres (Figs. 7d and 8c). This agglomeration of particles avoids mechanical properties improvement, and 1.5% VC containing materials always present better properties than 3% VC composites. The only material that performs better than plain steel is 1.5% VC MMC sintered in hydrogen, due to its high sintering density (Fig. 3) and less amount of agglomerates. SiC composites present a parallelism between tensile strength and hardness. Vacuum sintering improves both properties, while hydrogen acts in the opposite way. The reason for this behavior is not clear, but it could be due to the presence of a thin layer of SiO2 on SiC particles that hydrogen trends to reduce promoting Si diffusion to the matrix, although this point could not be checked. The fracture analysis is performed on tensile tested samples. The fracture of base material is ductile in all
the sintering atmospheres, as can be explained through the dimples observed on the fracture surface (Figs. 9a, 10a and 11a). The strengthening effect of nitrogen in 316L sintered in dissociated ammonia is not enough to induce brittleness in the steel, as no nitrides are precipitated (Fig. 6a). The partly brittle characteristics of the fracture of the reinforced materials are due to the fact that the steel gives the ductility, while reinforcement gives the brittleness (Figs. 9b, 10b and 11b). Carbides and intermetallics are points where stresses are accumulated and where fracture initiates its propagation. SiC leave holes in the fracture surface due to its bad interaction (Fig. 11b). 316L with 3% addition SiC presents the best wear performance of all studied materials (Fig. 12). In this material, the hardness of the ceramic particles helps to stand wear, although they are not well linked to the steel. AlCr2 and Cr2Ti composites have better wear than the base steel when sintered in H2–N2, but these intermetallic additions increase the wear rate when the sintering is performed in pure H2 and vacuum atmospheres. The reason for this performance can be found in the better link between matrix and intermetallic in 25%H2/75%N2, because intermetallics are not hard enough to withstand wear as SiC does. Finally,
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Fig. 8. Microstructures of materials sintered in H2 at 1230 C: (a) 316L+3% AlCr2; (b) 316L+3% Cr2Ti; (c) 316L+3% VC.
Fig. 9. Fractographic analysis of materials sintered in 75% H2+25% N2 at 1230 C: (a) 316L; (b) 316L + 3% Cr2Ti.
the small particle size of VC and the formation of agglomerates reduce its wear performance. Two different kinds of wear tracks are distinguished after testing the specimens (Fig. 13). Some materials exhibit wide tracks (henceforth ‘‘a-type’’ tracks) and narrow tracks (henceforth ‘‘b-type’’ tracks). Table 1 shows the type of track which appears on each material.
It does not exist a clear relation between type of track (Fig. 13) and the amount of added reinforcement or sintering atmosphere. However, base material always presents ‘‘b-type’’ tracks, as material reinforced with 3% of SiC (Table 1, Fig. 14). ‘‘b-type’’ tracks appear in the materials with lower wear rate and are typically found in composite materials tested in similar conditions
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Fig. 10. Fractographic analysis of materials sintered in vacuum at 1230 C: (a) 316L; (b) 316L + 3% VC.
Fig. 11. Fractographic analysis of materials sintered in H2 at 1230 C: (a) 316L; (b) 316L + 3% SiC.
Fig. 12. Wear rate of materials sintered in 75%H2–25%N2, pure H2 and vacuum.
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Table 1 Type of track Materials
Base +1.5% AlCr2 +3% AlCr2 +1.5% Cr2Ti +3% Cr2Ti +1.5% SiC +3% SiC +1.5% VC +3% VC
Type of track 75% H2+25% N2
H2
Vacuum
b b a a a a b a a
b b a b b a b b b
b b a b b a b b a
that promote abrasion mechanisms in the material [11]. On the other hand, it seems that ‘‘a-type’’ tracks appear mainly in materials sintered in H2–N2. It has been demonstrated by SEM that ‘‘b-type’’ tracks show the characteristics scratches produced by an abrasive wear mechanism. On the other hand, ‘‘a-type’’ tracks exhibit a strong plastic deformation that indicates that the material has plastic behaviour. The reasons that explain this change in wear mechanism are not clear, as they are not related with hardness, sintering density or the characteristics of reinforcements. However, materials sintered in H2–N2 present higher hardness (or at least, very hard phases),
Fig. 13. Macrography of both types of track. Left: wide track. Right: narrow track.
and the materials sintered in hydrogen or vacuum atmospheres presenting ‘‘a-type’’ tracks also have hard phases: carbides in the case of 3% AlCr2 (the lower amount of reinforcement does not promote enough amount of hard carbides), SiC is hard and it can stay on the matrix when added in low amounts, and VC (when added in high amount). The negative effect of nitrogen pick-up on corrosion properties of sintered stainless steel is well known [6,7], and it has been proved again by the obtained salt fog
Fig. 14. Up: 316L + 3% Cr2Al: micrographs corresponding to wide track. Down: 316L: micrographs corresponding to narrow track.
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Fig. 15. Number of hours to 0.25% corrosion in the salt spray test of materials sintered in 75%H2–25%N2, pure H2 and vacuum at 1230 C.
Fig. 16. Corrosion in salt spray chamber. Transverse view of sintered materials in 75%H2+25%N2: (a) 316L; (b) 316L + 3% Cr2Al.
Fig. 17. Corrosion in salt spray chamber. Transverse view of sintered materials in vacuum: (a) 316L; (b) 316L + 3% TiCr2.
results (Fig. 15). Corrosion tests demonstrate that sintering in vacuum or H2 delays the onset of the attack (Fig. 15). Intermetallic reinforced composites sintered in H2–N2 seem to be the most adequate addition from a corrosion point of view. This could be related with the positive effect of Al, Ti, and especially Cr of the inter-
metallic in the stability of the passive layer. AlCr2 reduces the corrosion intensity when the material is sintered in H2, and Cr2Ti has proved its effectiveness when the process is carried out in vacuum. Vacuum sintering of AlCr2 composite materials, without admixed lubricant, promotes the formation of
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Fig. 18. Corrosion in salt spray chamber. Transverse view of sintered materials in H2: (a) 316L + 3% SiC; (b) 316L + 3% VC.
ferritic areas in the steel [11] due to the reaction intermetallic-steel that enhances corrosion resistance of the material. In this case, vacuum sintering shows chromium carbides in the intermetallic (Fig. 7b) and alumina surrounding it. This carbon comes from the wax for lubrication, that during delubing has react with the intermetallic reducing material performance. The corrosion resistance is reduced because the formed carbides hinder a proper matrix-reinforcement link and the formation of high chromium ferritic areas. SEM images on Figs. 16–18 show that the attack is always localised and takes place through pitting. In reinforced materials the corrosion are localised interface between the matrix and the reinforcements (pits are nucleated around the intermetallic or ceramic particles, Figs. 16–18) due to a galvanic effect between reinforcements and steel and to the geometric heterogeneities that sometimes appear between them.
4. Conclusions The main conclusions that can be deduced from the results presented in this work are: Cr2Ti improves tensile strength. The two studied intermetallics and SiC improve hardness. SiC composites present better wear performance, specially when using 3% addition and sintering in vacuum or hydrogen. Corrosion is delayed in all cases, except for 3% SiC and 1.5% VC additions. In general, used intermetallics improve corrosion behaviour without losses in mechanical properties. Nitrogen atmospheres promote good results in mechanical properties (hardness and UTS), while materials sintered in vacuum and hydrogen atmospheres present best corrosion results.
Acknowledgements Authors wish to acknowledge the financial support of Spanish Ministry of Education through project 2FD971390.
References [1] Molins C, Bas JA, Planas J. P/M stainless steels: types and their characteristics and applications. Adv Powder Metall 1992;5:345– 57. [2] Velasco F, Ibars JR, Ruiz Roma´n JM, Torralba JM, Ruiz Prieto JM. Improving P/M austenitic stainless steels corrosion resistance through infiltration. Corr 1996;52(1):47–52. [3] Velasco F, Anto´n N, Torralba JM, Vardavoulias M, Bienvenu Y. Sinterability of Y2O3-Al2O3 particulate stainless steel matrix composites. Appl Comp Mater 1996;3(1):15–27. [4] Lal S, Upadhyaya GS. Effect of ternary additions on the sintered properties of 316L stainless steel and its ytria containing composites. Mod Dev Powder Metall 1988;18:581–94. [5] Moreira-Lima W, Candela N, Anto´n N, da Costa CE, Velasco F, Torralba JM. Corrosion and wear behaviour of AISI 316L reinforced with AlCr2 particulate composite. In: Proceedings of the Powder Metallurgy World Congress, vol. 3, Shrewsbury, UK. UK: EPMA; 1998. p. 413–8. [6] Lima WM, Velasco F, Torralba JM. Stainless steel matrix composites reinforced with AlCr2. Mater Sci Forum 1999;299–300: 431–8. [7] Lima WM. PhD thesis, Universidad Carlos III de Madrid, Spain, 1999. [8] Lal S, Upadhyaya GS. Effect of copper and bronze addition on the sintered properties of 316L austenitic stainless steel and its composite containing 4vol%Y2O3. Powder Metall Int 1988;20: 35–8. [9] Mukherjee SK, Kumar A, Upahyaya GS. Anodic polarization study of sintered 434L ferritic stainless steel and its particulate composites. Powder Metall Int 1985;17(4):172–5. [10] Velasco F, Lima WM, Abeno´jar J, Torralba JM. Sinterability study of stainless steel matrix composites reinforced with intermetallics. In: German RM, Messing GL, Cornwall RG, editors. Sintering Science and Technology. State College (PA): Pennsylvania State University; 2000. p. 21–6. [11] Gordo E, Velasco F, Anto´n N, Torralba JM. Wear mechanisms in high speed steel reinforced with (NbC)p and (TaC)p MMCs. Wear 2000;239:251–9.
Atmosphere influence in sintering process of stainless steels matrix composites reinforced with hard particles J. Abenojara,*, F. Velascoa, A. Bautistaa, M. Camposa, J.A. Basb, J.M. Torralbaa a
Departamento de Materiales, Universidad Carlos III de Madrid, Av. de la Universidad, 30, 28911 Legane´s, Madrid, Spain b Aleaciones de Metales Sinterizados, S.A. Ctra. Nacional 340, Km.1242, 08620 Sant Vicenc¸ dels Horts, Spain Received 24 July 2001; received in revised form 20 January 2002; accepted 15 August 2002
Abstract This paper studies the change in properties of powder metallurgy 316L stainless steel, depending on kind of added reinforcement and its amount and the sintering atmosphere. Used reinforcements have been AlCr2, Cr2Ti, VC and SiC; and the amounts were 1.5 and 3% (vol.). All materials have been manufactured mixing the powders with a 0.6% (wt.) of wax, compacting at 700 MPa, and sintering at 1230 C. In order to analyze the sintering atmosphere, three different media were used: 75%H2–25%N2, pure H2 and vacuum. The properties of the 24 different composites materials are evaluated and compared with the base stainless steel sintered in the same conditions. The study carried out includes sintering density, tensile strength, hardness, corrosion and wear and microstructural analysis. Intermetallics seem to be a good solution to improve the studied properties. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Metal-matrix composites; A. Intermetallics; E. Sintering
1. Introduction Powder metallurgy (P/M) process has numerous advantages for fabricating small pieces of complicated shapes, because it allows material and energy savings as well as dimensional accuracy. Sintered stainless steels have a wide range of applications, mainly related to the automotive industry. However, they present lower properties than their wrought counter-parts in terms of strength, corrosion and wear resistance. The main reason for this lower performance is the presence of porosity, and the problems that these steels have when they are sintered in industrial atmospheres [1]. In order to achieve the best performance, P/M stainless steels must be sintered in inert atmospheres at high temperatures. Their properties can also be improved with different additions, with the aim of increasing the final density of the steel, and enhancing the mechanical and corrosion behaviour [2]. When the goal is to improve the wear resistance, ceramic particles [3,4] are used due to their * Corresponding author. E-mail address: [email protected] (J. Abenojar).
hardness. The main problem related to these particulate MMCs is their poor corrosion behaviour compared with the corrosion resistance of the matrix. The low interaction between matrix and reinforcements favours the onset of the corrosive attack. This problem forces to use at the same time other additions with the aim of activating the sintering process. The addition of intermetallic particles can be another option to improve not only the mechanical characteristics of the sintered materials, but also their corrosion resistance. However, the effects of the intermetallic reinforcements on the final properties of the composite material are very dependent on the kind of addition and on the atmosphere where their sintering has been carried out [5–7].
2. Experimental procedure Metal matrix composites were manufactured following a conventional P/M route. Raw materials used in this research were powders of AISI 316L austenitic stainless steel as metallic matrix (from Ho¨gana¨s AB,
0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00179-3
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Sweden), and four different reinforcements: AlCr2 and Cr2Ti intermetallics (particle size < 75 mm), VC (particle size < 45 mm) and SiC (particle size < 75 mm). To obtain the MMCs, two different amounts of reinforcement were considered: 1.5 and 3% (in vol.), and all mixes have a 0.6% (wt.) lubricant to minimise friction during compacting operations. Powders were dry mixed in a rotating laboratory mill at 80 r.p.m. for 20 min and the homogeneity of this mix was checked. Apparent density and flow rate of powder mixes were measured. Afterwards, powders were uniaxially pressed at 700 MPa. After pressing, green density and green strength were evaluated. Then, green specimens were sintered in 25% N2/75% H2, pure H2 and vacuum atmospheres at 1230 C for 30 min. Samples of plain 316L were manufactured following the same process. A complete microstructural study was carried through optical and scanning electron microscopy (SEM), aided by semiquantitative microanalysis through EDX (energy dispersive X-ray analysis). Sintering density, tensile strength, elongation and hardness (HRA) were measured. Fracture analysis was also carried out on tensile tested samples. Salt spray tests (ASTM B118) were carried out to study corrosion behaviour of these materials. The wear behaviour was measured through a pin-ondisk test. Wear tests were performed according to ASTM Standard G99–95 using the following test conditions: Pin: Tool steel (55 HRC), 6 mm diameter; Friction track diameter: 16 mm; Sliding distance: 400 m; Speed: 0.1 m/s; Load applied: 5 N; Relative humidity: less than 30%; Room temperature. Friction coefficient was measured during the test (given value was determined when a steady state in the wear test was reached) and wear was measured through the wear coefficient k:
k m2 =N ¼ volume loss material m3 = ½applied loadðNÞ sliding distanceðmÞ The tracks were also observed through SEM in order to establish the wear mechanisms that take place in the materials.
3. Results and discussion Fig. 1 shows the main characteristics of the powders of studied materials: flow rate and apparent density. The flow rate of the powder mixes does not differ meaningfully from the flow of rate of the plain 316L powders. Although reinforcement particle size could affect, the lubricant reduces the differences between materials. Apparent density is not modified by SiC additions, it increases when AlCr2 is added and decreases with Cr2Ti and VC additions. The increase of the apparent density after mixing 316L powders with AlCr2, is the low hardness of this intermetallic. On the other hand, the small reduction of the apparent density is due to the low specific weight of the added particles. The properties of the different compacted powder mixes before sintering are shown in Fig. 2. The values of the green density are very close for all the materials, and lubricant enhances this property during compaction. The small differences observed are due to lightness of reinforcements studied. The green strength increases for the reinforced materials, except 1.5% VC. The small sized, hard particles used as reinforcements link the ductile stainless steel particles together during compaction, increasing the green strength. However, this increase is not always detectable because the added lubricant acts in the opposite way, reducing the strength of the compacted materials.
Fig. 1. Apparent density and flow rate of powder mix.
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Fig. 2. Green density and green strength of studied materials.
Fig. 3 shows the sintering density of the materials sintered in 75%H2–25%N2, pure H2 and vacuum. As can be seen, most of the composite materials exhibit less density than plain 316L steel, and 3% reinforced materials present lower density than 1.5% reinforced materials. There is no clear influence of the sintering atmosphere on this parameter. The low density of composite materials after sintering is due to two reasons. The former is the lower density of reinforcements that reduces the density of the composite. The later is that added particles hinder the sintering process, owing to the low interaction between intermetallics/carbides and stainless steel. This low interaction has been previously observed for oxide additions [4,8,9] and was expected for carbide additions, specially SiC. Previous results have shown the good interaction between stainless steel and intermetallics, but this did not influence sintering density values [6,10]. Figs. 4 and 5 show the variation of the mechanical properties for the different stainless steels after sintering in the three atmospheres studied. Tensile strength and
hardness results are strongly determined by the atmosphere used to sinter the materials. 75%H2/25%N2 atmosphere promotes highest properties in all materials, although ductility is reduced. All composite materials keep the ductility of stainless steel, as elongation results show. Sintering in 75%H2/25%N2 promotes higher tensile strength (Fig. 4) than other sintering atmospheres, especially for plain 316L stainless steel. The microestructural study of the plain 316L sintered in 75%H2/ 25%N2 atmosphere (Fig. 6a) reveals that there are no chromium nitrides precipitated in the grain boundaries, and only the typical porosity of the sintered materials can be observed. As no precipitated nitrides are observed, the effect of the sintering atmosphere is due to the presence of nitrogen in the metal that promotes solid solution hardening. The solid solution of N2 makes stainless steels sintered in 75%H2/25%N2 less ductile (Fig. 4) and harder (Fig. 5). Reinforcements slightly modify the strength of the steel. Cr2Ti is the only addition that improves the
Fig. 3. Sintering density of materials sintered in 75%H2–25%N2, pure H2 and vacuum.
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Fig. 4. Tensile strength and elongation of materials sintered in 75%H2–25%N2, pure H2 and vacuum.
Fig. 5. Hardness of materials sintered 75%H2–25%N2, pure H2 and vacuum.
strength when sintering in 75%H2/25%N2 because of the good interaction of the intermetallic with the matrix (Fig. 6c). Chromium diffuses from the intermetallic particles to the matrix and layers of different composition appears in the reinforcements, being the inner, darker areas, the ones with higher Ti content. Moreover, Fe and Ni diffuse into the intermetallic. When AlCr2 is added, it reacts with the stainless steel (Fig. 6b). Different areas can be observed in the microstructure: the lightest areas (Fe-rich), the grey areas (rich in nitrogen and chromium) and the dark areas (alumina rich). The formation of an alumina layer in the AlCr2/steel interface in all the sintering atmospheres used (Figs. 6b, 7b and 8a) implies that the reinforcement does not contribute to the strengthening of the steel and the tensile strength of the materials diminishes. The alumina layer appears in
AlCr2 reinforced materials because of the dew point existing in the furnace, that, although is adequate for the steel is too high for the intermetallic, very avid for moisture. Carbide additions decrease tensile strength of H2–N2 atmosphere sintered materials due to different reasons. The use of VC as reinforcement (Fig. 6d) promotes the formation of a vanadium carbonitride (C,N)5V4, whose stoichiometry corresponds to C2N3V4. So, lower nitrogen level is achieved in base steel and tensile strength slightly decreases. It must be emphasised the beneficial effect of 3% VC additions, that increase the hardness of the material, thanks to the great hardness of reinforcements and interaction with the matrix (Fig. 6d). Only SiC additions reduce hardness in this sintering atmosphere. The reason is the low interaction between matrix and reinforcement (Fig. 6e).
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Fig. 6. Microstructures of materials sintered in 75% H2 + 25% N2 at 1230 C: (a) 316L; (b) 316L + 3% Cr2Al; (c) 316L + 3% TiCr2; (d): 316L + 3% VC; (e) 316L + 3% SiC.
If sintering is carried out in vacuum (Fig. 7a) or hydrogen, no special feature appears in the base stainless steel. Mechanical properties are lower than in nitrogen base atmosphere due to the absence of solid solution strengthening. The sintering of 316L reinforced with AlCr2 in vacuum (Fig. 7b) and H2 (Fig. 8a) atmosphere again originates the formation of an alumina layer around the intermetallic particles. So strength values are lower than those of plain stainless steel. Moreover, vacuum sintering
(Fig. 7b) promotes chromium carbide precipitation inside the intermetallic particles and a alumina layer appears surrounding them. The origin of the latter has been previously explained, and the presence of carbides is due to a reaction between lubricant and intermetallic during delubing operation. The same reason explains titanium carbide precipitation observed in Cr2Ti vacuum sintered materials (Fig. 7c). Diffusion of chromium from the particle to the matrix is also observed, thus obtaining a better link and
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Fig. 7. Microstructures of materials sintered in vacuum at 1230 C: (a) 316L; (b) 316L+3% AlCr2; (c) 316L+3% Cr2Ti; (d) 316L+3% VC.
so better strength that AlCr2 containing composites. Diffusion is also appreciated in their hydrogen sintered composites (Fig. 8b). Diffusion of the chromium to the matrix occurs and dark, Ti-rich areas appear inside the particles. Its effect on tensile strength and hardness is then positive. Carbide reinforcements show no reaction with the stainless steel in vacuum and hydrogen. VC agglomerates can be appreciated in both sintering atmospheres (Figs. 7d and 8c). This agglomeration of particles avoids mechanical properties improvement, and 1.5% VC containing materials always present better properties than 3% VC composites. The only material that performs better than plain steel is 1.5% VC MMC sintered in hydrogen, due to its high sintering density (Fig. 3) and less amount of agglomerates. SiC composites present a parallelism between tensile strength and hardness. Vacuum sintering improves both properties, while hydrogen acts in the opposite way. The reason for this behavior is not clear, but it could be due to the presence of a thin layer of SiO2 on SiC particles that hydrogen trends to reduce promoting Si diffusion to the matrix, although this point could not be checked. The fracture analysis is performed on tensile tested samples. The fracture of base material is ductile in all
the sintering atmospheres, as can be explained through the dimples observed on the fracture surface (Figs. 9a, 10a and 11a). The strengthening effect of nitrogen in 316L sintered in dissociated ammonia is not enough to induce brittleness in the steel, as no nitrides are precipitated (Fig. 6a). The partly brittle characteristics of the fracture of the reinforced materials are due to the fact that the steel gives the ductility, while reinforcement gives the brittleness (Figs. 9b, 10b and 11b). Carbides and intermetallics are points where stresses are accumulated and where fracture initiates its propagation. SiC leave holes in the fracture surface due to its bad interaction (Fig. 11b). 316L with 3% addition SiC presents the best wear performance of all studied materials (Fig. 12). In this material, the hardness of the ceramic particles helps to stand wear, although they are not well linked to the steel. AlCr2 and Cr2Ti composites have better wear than the base steel when sintered in H2–N2, but these intermetallic additions increase the wear rate when the sintering is performed in pure H2 and vacuum atmospheres. The reason for this performance can be found in the better link between matrix and intermetallic in 25%H2/75%N2, because intermetallics are not hard enough to withstand wear as SiC does. Finally,
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Fig. 8. Microstructures of materials sintered in H2 at 1230 C: (a) 316L+3% AlCr2; (b) 316L+3% Cr2Ti; (c) 316L+3% VC.
Fig. 9. Fractographic analysis of materials sintered in 75% H2+25% N2 at 1230 C: (a) 316L; (b) 316L + 3% Cr2Ti.
the small particle size of VC and the formation of agglomerates reduce its wear performance. Two different kinds of wear tracks are distinguished after testing the specimens (Fig. 13). Some materials exhibit wide tracks (henceforth ‘‘a-type’’ tracks) and narrow tracks (henceforth ‘‘b-type’’ tracks). Table 1 shows the type of track which appears on each material.
It does not exist a clear relation between type of track (Fig. 13) and the amount of added reinforcement or sintering atmosphere. However, base material always presents ‘‘b-type’’ tracks, as material reinforced with 3% of SiC (Table 1, Fig. 14). ‘‘b-type’’ tracks appear in the materials with lower wear rate and are typically found in composite materials tested in similar conditions
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Fig. 10. Fractographic analysis of materials sintered in vacuum at 1230 C: (a) 316L; (b) 316L + 3% VC.
Fig. 11. Fractographic analysis of materials sintered in H2 at 1230 C: (a) 316L; (b) 316L + 3% SiC.
Fig. 12. Wear rate of materials sintered in 75%H2–25%N2, pure H2 and vacuum.
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Table 1 Type of track Materials
Base +1.5% AlCr2 +3% AlCr2 +1.5% Cr2Ti +3% Cr2Ti +1.5% SiC +3% SiC +1.5% VC +3% VC
Type of track 75% H2+25% N2
H2
Vacuum
b b a a a a b a a
b b a b b a b b b
b b a b b a b b a
that promote abrasion mechanisms in the material [11]. On the other hand, it seems that ‘‘a-type’’ tracks appear mainly in materials sintered in H2–N2. It has been demonstrated by SEM that ‘‘b-type’’ tracks show the characteristics scratches produced by an abrasive wear mechanism. On the other hand, ‘‘a-type’’ tracks exhibit a strong plastic deformation that indicates that the material has plastic behaviour. The reasons that explain this change in wear mechanism are not clear, as they are not related with hardness, sintering density or the characteristics of reinforcements. However, materials sintered in H2–N2 present higher hardness (or at least, very hard phases),
Fig. 13. Macrography of both types of track. Left: wide track. Right: narrow track.
and the materials sintered in hydrogen or vacuum atmospheres presenting ‘‘a-type’’ tracks also have hard phases: carbides in the case of 3% AlCr2 (the lower amount of reinforcement does not promote enough amount of hard carbides), SiC is hard and it can stay on the matrix when added in low amounts, and VC (when added in high amount). The negative effect of nitrogen pick-up on corrosion properties of sintered stainless steel is well known [6,7], and it has been proved again by the obtained salt fog
Fig. 14. Up: 316L + 3% Cr2Al: micrographs corresponding to wide track. Down: 316L: micrographs corresponding to narrow track.
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Fig. 15. Number of hours to 0.25% corrosion in the salt spray test of materials sintered in 75%H2–25%N2, pure H2 and vacuum at 1230 C.
Fig. 16. Corrosion in salt spray chamber. Transverse view of sintered materials in 75%H2+25%N2: (a) 316L; (b) 316L + 3% Cr2Al.
Fig. 17. Corrosion in salt spray chamber. Transverse view of sintered materials in vacuum: (a) 316L; (b) 316L + 3% TiCr2.
results (Fig. 15). Corrosion tests demonstrate that sintering in vacuum or H2 delays the onset of the attack (Fig. 15). Intermetallic reinforced composites sintered in H2–N2 seem to be the most adequate addition from a corrosion point of view. This could be related with the positive effect of Al, Ti, and especially Cr of the inter-
metallic in the stability of the passive layer. AlCr2 reduces the corrosion intensity when the material is sintered in H2, and Cr2Ti has proved its effectiveness when the process is carried out in vacuum. Vacuum sintering of AlCr2 composite materials, without admixed lubricant, promotes the formation of
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Fig. 18. Corrosion in salt spray chamber. Transverse view of sintered materials in H2: (a) 316L + 3% SiC; (b) 316L + 3% VC.
ferritic areas in the steel [11] due to the reaction intermetallic-steel that enhances corrosion resistance of the material. In this case, vacuum sintering shows chromium carbides in the intermetallic (Fig. 7b) and alumina surrounding it. This carbon comes from the wax for lubrication, that during delubing has react with the intermetallic reducing material performance. The corrosion resistance is reduced because the formed carbides hinder a proper matrix-reinforcement link and the formation of high chromium ferritic areas. SEM images on Figs. 16–18 show that the attack is always localised and takes place through pitting. In reinforced materials the corrosion are localised interface between the matrix and the reinforcements (pits are nucleated around the intermetallic or ceramic particles, Figs. 16–18) due to a galvanic effect between reinforcements and steel and to the geometric heterogeneities that sometimes appear between them.
4. Conclusions The main conclusions that can be deduced from the results presented in this work are: Cr2Ti improves tensile strength. The two studied intermetallics and SiC improve hardness. SiC composites present better wear performance, specially when using 3% addition and sintering in vacuum or hydrogen. Corrosion is delayed in all cases, except for 3% SiC and 1.5% VC additions. In general, used intermetallics improve corrosion behaviour without losses in mechanical properties. Nitrogen atmospheres promote good results in mechanical properties (hardness and UTS), while materials sintered in vacuum and hydrogen atmospheres present best corrosion results.
Acknowledgements Authors wish to acknowledge the financial support of Spanish Ministry of Education through project 2FD971390.
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