TiCN—high speed steel composites: sinterability and properties

Composites: Part A 33 (2002) 819±827

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TiCNÐhigh speed steel composites: sinterability and properties F. Velasco a,*, R. Isabel a, N. AntoÂn a,b, M.A. MartõÂnez a, J.M. Torralba a a

Departamento de Ciencia de Materiales e Ingenieria Metalurgica, Universidad Carlos III de Madrid, Avda. Universidad 30, 28911 LeganeÂs, Spain b Universidad de Salamanca, Escuela PoliteÂcnica Superior, Av Requejo 30, 49022 Zamora, Spain Received 23 July 2001; revised 14 January 2002; accepted 8 February 2002

Abstract Metal matrix composites, based on M3/2 high speed steel and reinforced with two different percentages of TiCN (2.5 and 5 wt%), were manufactured following a conventional powder metallurgy route: mixing, compacting and sintering. The carbide and base material powders were dry mixed and uniaxially compacted at 700 MPa. After this, vacuum sintering was carried out at different temperatures to study the sinterability of manufactured composites. Effects of sintering temperature on sintering density, dimensional change and hardness with temperature were measured, and this study was completed with a dilatometric analysis. Materials were sintered at optimum sintering temperature (1275 8C), and transverse rupture strength and wear behaviour of sintered materials was examined. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Metal-matrix composites; A. Particle-reinforcement; E. Powder processing; High speed steel

1. Introduction Powder metallurgy (P/M) techniques are becoming increasingly widespread for manufacturing high speed steels (HSS). In addition to the typical advantages of P/M (raw material saving, low energy costs), P/M HSS present better microstructural features than conventional (forged) steels, such as homogeneity of carbide distribution in the matrix and smaller grain and carbide sizes, among others. These advantages produce an improvement of properties. Furthermore, higher alloying contents can be used in P/M HSS. HSS have traditionally been used as cutting materials. Notwithstanding, their good wear resistance allows them a broader ®eld of application [1,2] and they currently compete in many applications with cemented carbides. Both types of materials present advantages and disadvantages, but there is a gap between their properties which is attempted to cover through the development of composite materials with a HSS matrix and ceramic reinforcement. The ceramic particles used to obtain HSS metal matrix composites (MMCs) are alumina, TiN, and carbides such as NbC, TiC, WC or VC [3]. The development of this kind of materials brings great advantages, but also gives rise to many dif®culties, such as * Corresponding author. Tel.: 134-91-624-94-85; fax: 134-91-624-9430. E-mail address: [email protected] (F. Velasco).

poorer mechanical properties or the need for strict manufacturing parameter control. Different works have been carried out to study the sinterability of HSS MMCs, specially with NbC additions [4±7], showing the dif®culties of the sintering process. However, when the processing is carried out properly, the wear behaviour is highly improved [8,9]. The addition of TiCN suggested in this research is justi®ed through the use of TiN as coating for HSS due to its good adherence. TiN has been used as reinforcement in HSS [10] but using Cu3P as liquid phase sintering activator. Moreover, TiN has been used as coating of alumina particles reinforcing HSS [11,12] to improve the bonding between matrix and reinforcement. These properties allow expecting good properties in the composite materials manufactured with this matrix and this reinforcement.

2. Experimental procedure The M3/2 HSS is a water atomised powder (manufactured by Coldstream, Belgium) with the following composition: 1% C, 6.1% Mo, 5.85% W, 0.24% Mn, 4.2% Cr, 0.26% Si, 2.6% V, Fe bal. Sieve analysis of HSS powders indicates as main values: 40% , 53 mm; 90% , 100 mm; 100% , 150 mm: As reinforcement, high purity TiCN powders (from H.C. Starck, Germany) were used. TiCN maximum particle size was 100 mm. Two different carbide percentages were chosen: 2.5 and 5 wt%.

1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1359-835 X(02)00 024-6

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Fig. 1. Sintering density vs. sintering temperature of studied materials.

The base material and TiCN were then dry mixed in a low energy blender (rotating laboratory mill), at 80 rpm for 20 min, using 6 mm diameter stainless steel balls with a material/balls ratio of 1/5. Homogeneity of mixes was checked after mixing. Then, the powders were uniaxially compacted in ¯oating die at a pressure of 700 MPa, using zinc stearate as die wall lubricant. Cylindrical and ¯exural strength samples were compacted. 2.1. Sinterability The materials were vacuum sintered (10 24 bar) at different temperatures between 1100 and 1280 8C for 30 min. In order to study their sinterability, different properties were

measured: sintering density, dimensional change and hardness, in sintered state. Densities of sintered materials were evaluated by a method based on Archimedes principle according to MPIF standard 42. Measured values were compared with theoretical values, to obtain relative densities. Dimensional change was evaluated both in the diameter and the height of cylindrical samples. Rockwell hardness was measured following MPIF standard 43, using the most appropriated scale for each sintering temperature. A complete microstructural study, by scanning electron microscopy (SEM), was carried out. X-ray diffraction of base material was also carried out to clearly identify present carbides. Dilatometry of base material and 5% TiCN composite was also carried out in order to compare the results with those obtained in the sinterability study.

Fig. 2. Dimensional change vs. sintering temperature of studied materials.

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Table 1 Rockwell hardness values of the different compositions Temperature (8C)

M3/2 M3/2 1 2.5% TiCN M3/2 1 5% TiCN

1100

1150

1200

1225

1250

1275

1280

84HRB 86HRB 85HRB

87HRB 86HRB 84HRB

87HRB 88HRB 85HRB

95HRB 96HRB 94HRB

93HRB 97HRB 93HRB

47HRC 50HRC 53HRC

48HRC 49HRC 50HRC

2.2. Optimised materials After sinterability study, all materials were sintered at 1275 8C, which was the optimum sintering temperature for TiCN composites. The measured mechanical property was transverse rupture strength (TRS), following MPIF standard 41. Three-points TRS test was carried out with a loading rate of 1 mm/min. Deformation and energy absorbed during TRS test were also measured to study the ductility of composite materials. Fracture analysis was carried out through SEM. Wear properties were evaluated using a pin-on-disc test, typical for evaluating abrasive wear, according to ASTM G99. In this case the pin was an alumina ball of 6 mm diameter and 2500 HV hardness, and the disc was the material to be tested. The tests were carried out at room temperature, keeping a relative humidity less than 30%, and were repeated at least three times. The load applied was 10 N, the sliding speed 0.1 m/s, and the distance covered during the test (sliding distance) 400 m. The friction coef®cient was measured continuously during the test, and the wear coef®cient was calculated by means of the following expression: k…mm3 =N m† ˆ …Volume loss; mm3 †=‰…Normal load; N† £ …Sliding distance; m†Š Wear tracks were observed through SEM to explain the phenomena that take place in these materials.

3. Results and discussion 3.1. Sinterability Samples of the different compositions mentioned earlier were sintered at temperatures both below and above solidus temperature. Fig. 1, that represents the change of sintering density with temperature, indicates this phenomenon. The evolution of the relative sintering density shows that until solidus temperature is reached, the material retains the green density. A sharp transition occurs on the onset of the liquid phase when particles rearrange and the density rises. In every case, the addition of TiCN slightly reduces the temperature for onset of sintering, although the ultimate

density is similar in all cases. These results (the density achieved in composite materials similar to that of the base material) are in agreement with that reported by ThuÈmmler [13] for NbC±HSS composites, due to a reaction that takes place between the added carbides and the HSS matrix. Dimensional change demonstrates this effect (Fig. 2). Almost no change in this property takes place until high sintering temperature is carried out. In this case, it can be seen that 2.5% TiCN containing materials begin to shrink at temperatures lower than base HSS, although ®nal dimensional change is the same. In addition to the physical properties, hardness was tested to record the evolution of sintering with temperature. The values of hardness of the materials were consistent with those of the relative density: adequate values of hardness were obtained only when the temperature exceeded the solidus temperature. Table 1 gives these values measured in materials sintered at different temperatures, and shows that the hardness of base material is in general lower than MMCs hardness at the same sintering temperature, an indication of the no need for higher temperatures to sinter these composite materials with added TiCN. Results from dilatometric study corroborate this effect (Fig. 3). Table 2 summarises the main points from this study. TiCN addition strongly affects the two ®rst measured temperatures: temperature to start shrinkage and temperature with null length change. This could indicate that the presence of TiCN delays the initiation of sintering. However, these temperatures are very low to proper sintering both base HSS and composite materials. The main value, that corroborates the other results from sinterability study, is the temperature at which length change takes place faster (maximum length change rate). This temperature is very close for both materials, showing that TiCN addition does not really in¯uence the sintering behaviour of HSS. Table 2 Main temperatures from dilatometric study Temperature to start shrinkage (8C) M3/2 1102 M3/2 1 5% TiCN 1154

Temperature with null length change (8C)

Temperature with maximum length change rate (8C)

1222 1243

1273 1278

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Fig. 3. Dilatometry of M3/2 HSS and M3/2±5% TiCN composite. LC: length change; LCR: length change rate.

Fig. 4. Microstructure of M3/2 HSS sintered at (a) 1100 8C and (b) 1225 8C.

Fig. 5. Microstructure of M3/2 HSS±TiCN MMCs. (a) 2.5% TiCN composite sintered at 1150 8C and (b) 5% TiCN composite sintered at 1250 8C.

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Fig. 6. Microstructure of M3/2 HSS sintered at (a) 1275 8C and (b) 1280 8C.

The values obtained match with the optimum sintering temperature obtained form other results and the microstructural study. Moreover, in all cases, it can be seen that 1280 8C is a temperature where oversintering of composite materials begins to take place: hardness (Table 1) starts to decrease and dimensional change (Fig. 2) also varies its direction. Microstructural study con®rms the results obtained in the sinterability study. Low sintering temperatures are not appropriated to sinter both base HSS (Fig. 4) and composites (Fig. 5). M3/2 HSS sintered at 1100 8C (Fig. 4a) shows individual particles, and only small contacts are formed among them. Increasing the temperature up to 1225 8C (Fig. 4b) slightly modi®es this effect, keeping the material almost the green density. The ®rst effect observed in MMCs (Fig. 5) is the good dispersion of reinforcement particles in the matrix, due to a homogeneous mixing. Low energy mixing was used, and it seems to be enough to promote a good distribution of TiCN particles in HSS, but it could be improved using

high-energy mixers. Composite material sintered at 1150 8C (Fig. 5a) also shows individual HSS particles. TiCN particles are also appreciated but highly related to pores. Sintering at 1250 8C (Fig. 5b) improves the link between HSS particles, but pores still appear around TiCN particles. This temperature is not good to expect good mechanical properties, as the addition of carbonitrides, with poor interaction between matrix and reinforcement, introduces failure initiation points in the composite materials, as occurs in other particulate composite materials [6,14]. Higher sintering temperatures promote good sintering both in HSS and composites, as expected from previous results of sintering density, dimensional change and hardness. It is well known that temperatures around 1270 or 1275 8C are the most appropriated to sinter an M3/2 HSS [5,6], and the microstructure obtained at those temperatures (Fig. 6) shows an homogeneous structure, without pores, and with typical MC and M6C carbides. XRD of plain steel (Fig. 7) con®rms the presence of these carbides. At

Fig. 7. X-ray diffraction pattern of M3/2 HSS sintered at 1280 8C, showing present phases.

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Fig. 8. Microstructure of M3/2 HSS±TiCN MMCs. (a) 2.5% TiCN composite sintered at 1275 8C. (b) and (c) 5% TiCN composite sintered at 1280 8C.

1275 8C, a ®ne-grained sintered microstructure was attained, while at 1280 8C continuous grain boundary carbides begin to appear. Composites sintered at 1275 8C (Fig. 8a) show that sintering has been carried out properly. There exist a good bond between added TiCN and the matrix, specially with MC carbides. This effect is also observed in other HSS composites with niobium and tantalum carbides [6,7,15]. At 1280 8C (Fig. 8b and c), the bonding between TiCN and carbides is higher, coarse carbides begin to form, and the material begins to be oversintered, explaining the small reduction in hardness observed at this temperature. Semiquantitative analysis (EDX) indicates that some titanium is present in MC and M6C carbides. The mechanism is not clear, but as occurs with NbC carbides, titanium possibly dissolve in the matrix and then precipitates with HSS carbides.

Tables 3 and 4 show the main results regarding mechanical properties and wear behaviour of materials sintered at the optimum sintering temperature, 1275 8C. TiCN additions have in¯uence on hardness, as previously indicated (Table 1). This behaviour is coincident with that observed by other authors [5] in relation with the macrohardness of

Table 3 Mechanical properties of composite materials sintered at 1275 8C. Con®dence values for each property are included

Table 4 Wear properties of composite materials sintered at 1275 8C

M3/2 M3/2 1 2.5% TiCN M3/2 1 5% TiCN

The quite narrow sintering window shows the dif®culty of attaining well-sintered microstructures in HSS when no type of additive is used [16]. Higher temperatures (1285 8C) promote in NbC±HSS composites an excess of liquid phase, with carbide ®lms and nets surrounding the particles [17], and it is supposed that this should occur in the materials tested in this work. 3.2. Optimised materials

TRS (MPa)

Deformation (%)

Energy (MPa)

Property

Friction coef®cient

k (10 25, mm 3/N m)

1139 ^ 89 1667 ^ 100 1491 ^ 70

1.64 ^ 0.16 1.22 ^ 0.08 1.1 ^ 0.13

11.8 ^ 0.6 10.2 ^ 1.2 8.2 ^ 1.3

M3/2 M3/2 1 2.5% TiCN M3/2 1 5% TiCN

0.85 0.7 0.7

7.2 ^ 0.30 3.8 ^ 0.15 2.5 ^ 0.10

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Fig. 9. Surface fracture of M3/2 HSS±TiCN MMCs sintered at 1275 8C. (a) 2.5% TiCN composite, and (b) 5% TiCN composite.

composite materials with NbC. Flexural strength results show that TiCN particles act as real reinforcement, and this property does not decrease as occur in other HSS particulated composites [18]. As previously indicated, the addition of hard particles introduces failure initiation points that usually reduce mechanical properties in particulate composite materials [14]. Only techniques such as HIP [19] avoid this effect. However, the materials manufactured in this work do not present this effect, and strength increases when using TiCN additions when comparing with plain M3/2. The reason is the good bond between HSS and reinforcement. Moreover, TiCN particles are ®ne enough to avoid the formation of agglomerates (clusters) that reduce the cohesion with the matrix and microstructure shows that there exist less carbide size dispersion than in other composites. However, the addition of hard particles slightly reduces the ductility of these materials, as the results of deformation and absorbed energy shows (Table 3). Surface fracture analysis (Figs. 9 and 10) shows the brittleness of tested materials. These materials break by cleavage mechanisms,

and typical intragranular brittle fracture is appreciated. Fig. 10 shows the same surface fracture by secondary electrons (Fig. 10a) and backscattered electrons (Fig. 10b). Dark grey particles are TiCN additions, specially observed in the second photograph. It can be seen how these additions are also fractured during ¯exural strength test. They do not improve the ductility of material, which is in accord with the results shown in Table 3. The results of wear behaviour (Table 4) are also positive. Friction coef®cient values are very similar for all the materials (between 0.7 and 0.8). Although this variation is not very signi®cant, the tendency is to reduce this property when using TiCN additions. Moreover, the composite materials present lower k (wear) values than the base material, which means that their wear resistance is higher, and the tendency is to decrease k when increasing the carbide addition. Thus, it can be assumed that added carbides help to withstand wear. Observation of wear tracks indicates an abrasive wear mechanism in all the materials. No scratches are appreciated, as occuring in other composite materials [20,21].

Fig. 10. Surface fracture of M3/2 HSS±2.5% TiCN composites sintered at 1275 8C. (a) Secondary electrons (SE) image, and (b) backscattered electrons (BSE) image.

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Fig. 11. Wear tracks of studied materials sintered at 1275 8C. (a) M3/2 HSS. (b) and (c) M3/2 HSS±5% TiCN composites.

Base steel (Fig. 11a) presents areas where material has been detached and some oxidation. Composites also present the elimination of material (Fig. 11b). However, the main difference is that added carbonitrides are well linked to the matrix and cannot be easily detached (Fig. 11c). TiCN remains in the matrix helping to withstand wear, and reducing it by improving the performance of the material. 4. Conclusions 1. TiCN additions do not modify sintering temperatures of M3/2 HSS. 2. At high sintering temperatures, a good link between the matrix and reinforcement exist, and hardness is improved when TiCN particles are added. 3. TiCN addition greatly reduces the wear of the base alloy. The wear mechanism is abrasive in all the materials. 4. TRS is also improved with studied additions.

Acknowledgements This research work has been ®nanced by Spanish

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