Dimensional Control in Cu-Ni Containing Ferrous PM Alloys

Transcription

1 Dimensional Control in Cu-Ni Containing Ferrous PM Alloys Bruce Lindsley and Thomas Murphy Hoeganaes Corporation Cinnaminson, NJ 08077, USA ABSTRACT Dimensional precision is a critical parameter in net shape processing of ferrous PM components. Elemental additives, such as copper and nickel, modify the dimensional change in sintered parts. Typically, Cu causes growth and Ni causes shrinkage. Interactions between Cu, Ni and C complicate these simple trends leading to more complex behavior. With the use of prealloyed Mo base materials, these alloy systems can be used for sinter-hardening applications. This paper investigates the dimensional and microstructural changes of Cu-Ni containing PM alloys during the sintering process. INTRODUCTION Dimensional precision is a key attribute in the manufacture of ferrous PM parts, especially in net shape components. As the PM industry competes for larger sized components, the consistency of dimensional change becomes increasingly more important. If absolute tolerances of these larger parts do not change, the dimensional tolerance as a percentage of part diameter must be reduced. Understanding the dimensional change and corresponding sources of variability of PM alloys is critical to control part size and compliance. The dimensional change of sintered compacts is influenced by several factors, including particle size, density, composition, sintering time and temperature, cooling rate and microstructure. Die fill and compaction related issues affect the green density distribution in a part, and may lead to distortion problems. In the sintering furnace, increased sintering temperature and time generally result in shrinkage while the cooling rate will affect the microstructure. Martensite formation causes growth, while retained austenite has the opposite effect. Post sintering thermal treatments will affect both of these structures and also have an effect on dimensions. In addition to these

2 variables, the powder composition plays an important role on microstructure, dimensional change and mechanical properties. The most widely used additives to PM iron are graphite, copper and nickel. Separately, copper and graphite additions cause growth in the sintered compact, while nickel additions cause shrinkage. Dilatometric studies have shown that in Fe-Cu alloys, growth is observed at 1083 C, which is the melting point of copper. The amount of growth is a direct function of Cu content up to levels of 10 wt% [1-3]. Detailed dilatometry work has also shown that generally, upon heating, graphite quickly goes into solution in iron after the alpha to gamma transformation [1, 3-5]. During this solutioning, the sample exhibits considerable growth that is a function of the graphite content. When both copper and carbon are added to a base alloy, there is an interesting interaction between the two alloying elements. The carbon inhibits both Cu bulk diffusion into the iron and copper diffusion along grain boundaries. It has been shown that carbon content changes the wetting angle and dihedral angle of liquid copper with iron; increased carbon increases both angles [1,3]. This has an important ramification on copper diffusion in the compact, as at higher carbon levels, liquid Cu has decreased flow along particle surfaces and especially into grain boundaries, thereby reducing Cu diffusion. The amount of growth observed in a dilatometer at 1083 C is significantly reduced in high carbon alloys. Therefore, growth and dimensional change in a high carbon, high copper mix can be less than if either additive were made alone. It has been shown that for a series of base alloys, the effect of Cu on dimensional change is reduced to almost zero at high (1.0 wt%) levels of graphite [6]. In Figure 1, the effect of graphite and copper content on the dimensional change of iron is presented. The large growth associated with the addition of 2 wt% Cu at 0.6 wt% graphite becomes minimal at 1 wt% graphite. Figure 1. Effect of Cu and graphite on dimensional change of PM iron (from [6]). Nickel additions to PM compacts typically promote shrinkage of the part. The size and distribution of Ni will play a role, but the behavior is well documented for standard Ni grades with particle sizes near 8 µm. Alloys such as FLN and FLN often have negative dimensional changes relative to die size, and these changes become more negative with increasing sintering temperature and time. When copper and nickel are added to the same mix, there is an interaction between the two during sintering [7,8]. The Ni-Cu phase diagram shows

3 that there is complete solid solubility between the two elements at temperatures above 355 C. Given that the two elements can easily alloy together, it is not surprising that an interaction exists in sintered compacts. It has been shown that the initial Ni distribution in the green compact affects the final Cu distribution after sintering [8,9]. It was also found that the addition of both 2 wt% Ni and 2 wt% Cu in combination caused more growth than either separate additions of 4 wt% Ni or 4 wt% Cu, whereas if no interaction was present, one would expect the dimensional change of the 2 wt% Ni plus 2 wt% Cu alloy to lie between that of the 4 wt% Ni alloy and 4 wt% Cu alloy [7]. The purpose of this paper is to investigate the dimensional response of Ni-Cu-Mo steels and determine the mechanisms for the dimensional behaviors of each alloying element and interactions between them. The anomalous dimensional behavior of alloys containing both admixed Ni and Cu will be studied in detail. EXPERIMENTAL PROCEDURE Four commercially available ferrous powders were selected for study, Table I, with the primary focus of the investigation on the 0.85 wt% Mo base alloy. All weight % will be designated as % hereafter. To each of the Fe-Mo bases, 5 levels of Cu (0%, 0.5%, 1%, 1.5%, 2%), 2 levels of graphite (0.6%, 0.9%) and two levels of nickel (0%, 2%) were added for a total of 20 mixes per base material. An atomized Cu with a d 50 of 40µm was primarily used in the study, although additional tests were run with a copper particle size of 11µm. 0.75% EBS wax was used as the lubricant for all mixes. Standard transverse rupture strength bars were pressed at 690 MPa (50 tsi) and sintered in 90% nitrogen / 10% hydrogen at 1120 C (2050 F) in a continuous belt furnace. The cooling rate in the sample measured between 650 C (1200 F) and 315 C (600 F) was 0.7 C/sec (1.3 F/sec). Time at temperature experiments were performed in a pusher furnace, also set at 1120 C with the same atmosphere as the belt furnace. Samples were heated to nominally 705 C (1300 F) for 15 minutes to burn out the lubricant prior to being pushed into the hot zone. Time at temperature started immediately upon the samples entering the hot zone. Dimensional change (DC) was determined by the difference between the length of the die and the sintered length of the bar, divided by the length of the die. Hardness and transverse rupture strength were also measured for all conditions. All testing was performed in accordance with MPIF standards [10]. Table I. Nominal composition of base irons tested (in wt%). Base Iron MPIF Designation Mo Cu Ni Ancorsteel 85 HP FL Ancorsteel 30 HP Distaloy 4600A FD Distaloy 4800A FD Samples for metallographic examination were cross-sectioned, mounted, ground and polished using well established practices. Microstructural evaluations were made using both light optical Ancorsteel is a registered trademark of Hoeganaes Corporation

4 and scanning electron microscopy. In the SEM, backscattered electron imaging was used in combination with electron dispersive spectroscopy (EDS). RESULTS Individual Alloying Element Effects on Dimensional Change The individual contribution to dimensional change of each alloy element was measured in Fe-Ni- Mo-Cu-C alloys. The effect of Mo and C (graphite) with various copper contents and no nickel is shown in Figure 2. There is a clear effect of carbon content, as the slope of the dimensional change vs. copper content curve decreases to roughly zero when graphite is increased from 0.6% to 0.9% (solid lines to dotted lines). At 0.9% graphite, copper additions tend to have little effect on growth. This correlates well with previous work by the authors [6]. There appears to be a small effect of Mo content on dimensional change. The slope of the DC vs. copper content curve decreases slightly with the increase in Mo content (circles vs. squares). It is unknown at this time whether this slight change is a result of the different Mo content or an effect of base alloy particle size distribution. A finer base alloy particle size may cause shrinkage with no Cu present and more growth with the addition of Cu. With the addition of 2% nickel, the behavior of the system changes substantially (Figure 3). The small effect of different Mo base alloys, be it from Mo content or different base alloy particle size, is no longer evident. More importantly, the effect of carbon content on dimensional change has virtually been eliminated. At 0% Cu, the effect of graphite content can be seen, as the FLN type alloys show less growth than the FLN type alloys. This is typical of ferrous PM alloys; an increase in C content increases dimensional change. Above 0.5% Cu, however, there is no longer any effect of graphite content within the graphite content range tested. So in an FLNC-440X type material, carbon content has little effect on dimensional change. This again assumes no accelerated cooling. Increased cooling rates would change the transformation products, which in turn would affect the dimensional change. Any dimensional benefit that could be realized by the independence of dimensional change on carbon content is easily overshadowed by the dependence of dimensional change on copper content. The dimensional change of FLNC-440X type alloys is a very strong function of copper content, as the zero Cu alloys have the least growth and the 2% Cu alloys have the most growth of any compositional conditions studied. This is opposed to the Ni free material, where the dimensional change of FLC-4408 was relatively independent of Cu content (dotted lines in Figure 2). This strong dependence with Cu content at 0.9% graphite is different than for previous alloys studied. With the F-0008, FL-4608 and FL-4808 alloys [6], and FL-4408 and 0.3% Mo plus 0.9% graphite alloys in the current study (Figure 2), Cu has limited effect on dimensional change. These include systems with prealloyed Ni. The obvious difference with FLNC-4408 is the 2% admixed Ni. The addition of both Ni and Cu increase the local hardenability within the alloys considerably. It is possible that the increased hardenability could cause an increase in martensite content, which in turn would increase the dimensional change. Given this possibility, accelerated cooling was used with the Ni free samples to achieve the same hardness level as the conventionally cooled nickel containing samples. This was possible in the sintering furnace in all but the 0% copper specimens. In the Ni-containing and Ni-free samples with 0.9% graphite and

5 0.6 Ni-Free Alloys Mo, 0.6Gr 0.8Mo, 0.6Gr 0.3Mo, 0.9Gr 0.8Mo, 0.9Gr Cu Content (%) Figure 2. The effect of Mo, C and Cu on dimensional change in Ni free PM steels % Ni Alloys 0.3Mo, 0.6Gr 0.8Mo, 0.6Gr 0.3Mo, 0.9Gr 0.8Mo, 0.9Gr Cu Content (%) Figure 3. The effect of Mo, C and Cu on dimensional change in 2% Ni PM steels cooled to achieve similar hardnesses and levels of martensite, the dimensional change remained quite different. The 0% Ni alloys maintained the overall shape of the graph in Figure 2, while the 2% Ni alloys maintained the shape of Figure 3. The transformation products are therefore not the

6 reason of the different behavior. The question is then why does this behavior occur in alloys with admixed Ni and Cu. Nickel-Copper Interaction The reason for the odd behavior is the Ni-Cu interaction that occurs during sintering. This interaction can clearly be seen in Figure 4. In this experiment, different amounts of the Cu were added to a 0.85%Mo-0.9%Gr base with either 0%, 2% or 4% Ni. Again, at 0% Ni, copper has little effect on dimensional change. Increasing Ni contents promote shrinkage, although as copper levels increase in these Ni-containing alloys, the dimensional change increases. This is most apparent with the 2% Ni alloy. Compared with the 0% Ni alloy, the 2% Ni, 1.5% Cu exhibits a small amount of growth while the 2% Ni, 2% Cu alloy shows significantly higher growth Ni Content (%) µ 0.0% Cu 0.5% Cu 1.0% Cu 1.5% Cu 2.0% Cu Figure 4. Dimensional change of FL-4408 with different levels of Cu and Ni. The microstructure of samples sintered for typical times at temperature can not clearly reveal the cause of this interaction. The copper and nickel have diffused with the iron and identification of individual constituents is impossible. Certainly, previous work has been done using EDS in an SEM to analyze the Ni and Cu contents and a tendency was found for the Ni and Cu to be concentrated in the same locations [7]. To understand what happens during sintering, samples were sintered for different times in a pusher furnace. The sample composition was 0.8%Mo, 0.9% graphite, 1.5% copper and either 0 or 2% Ni. The dimensional change of these alloys for different times in the hot zone is shown below in Figure 5. The dimensional change of the samples sintered for zero minutes (lubricant burn out only) was 0.35% for both alloys. The two minute samples continued to grow for both compositions. At times greater than 2 minutes, the Ni-free material shrank with time, with the dimensional change leveling off at times greater than 20 minutes. The 2% Ni sample exhibited growth in the first ten minutes followed by continual shrinkage as sintering continued. The Ni-Cu interaction appears to play an important role in the

7 early minutes of sintering. Therefore, samples within the first ten minutes of sintering were investigated metallographically to determine if the mechanism could be identified. Figure 5. Effect of Ni content and sintering time on dimensional change. Figures 6 and 7 show the effect of time in the sintering furnace on the as-polished microstructures of the Ni-free FLC and the Ni containing FLNC-4408, respectively. The as-polished microstructures revealed that the copper did not melt in the samples sintered for 2 minutes, indicating that the samples had not yet reached 1083 C. The growth up to this time can be associated with carbon diffusion into the iron. Even though the copper has not melted at 2 minutes, the presence of nickel can be seen in Figure 7a. The copper particle in the lower left of the figure had no nickel adjacent to it and maintained the same color as the copper in Figure 6a. The copper particle in the upper right, however, resided next to nickel and alloying of the nickel and copper in the solid state is apparent from the color change. Alloying can occur at relatively low temperatures and short times in this system. In the 5 minute samples, the copper melted and moved throughout the pore network, leaving behind large, characteristic pores. The typical expansion observed in a dilatometer as the copper melts [1,3] was not found in the current study, which is likely an effect of the high carbon content. In the nickel free FLC (Figure 6), the copper is well distributed throughout the sample and is found in the particle boundaries, filling in the long, thin pores. This is consistent with earlier work [11]. The rapid contraction seen in Figure 5 at times 10 minutes coincides with the presence of liquid copper in the boundaries. The location of the liquid copper may enable rapid sintering until the liquid copper is exhausted via copper diffusion into iron at the sintering temperature. Lesser amounts of free copper were observed in the 10 minute sample and very little was found at 20 minutes.

8 The copper also melted within 5 minutes at temperature in the Ni containing FLNC-4408, but the location of the liquid copper is clearly different in this sample (Figure 7). The amount of Cu in the long, thin base iron particle-particle boundaries is greatly reduced and instead, the copper is clustered around the small, admixed nickel particles. The change in color of the copper rich alloy surrounding the Ni particles is now more evident as the unique orange color of copper fades with further Ni alloying. This is clearly different than the Ni free sample, where the amount of free copper decreased with time, but the appearance (color) of copper remained the same. The equilibrium concentration of Fe in liquid Cu is quite low (about 4%) and has little effect on the melting point, and therefore the copper remains as liquid until it diffuses into the solid iron. In the Ni containing material, Cu and Ni have complete solubility, resulting in a Ni-Cu composition gradient. (a) (b) (c) (d) Figure 6. As-polished microstructures of FLC sintered for (a) 2 minutes, (b) and (c) 5 minutes and (d) 10 minutes. Note the higher magnification in (c) and (d).

9 (a) (b) (c) (d) Figure 7. As-polished microstructures of FLNC-4408 sintered for (a) 2 minutes, (b) and (c) 5 minutes and (d) 10 minutes. Note the higher magnification in (c) and (d). In a side experiment, copper and nickel powders were mixed together and placed in the pusher furnace at 1120 C. The compositions varied in Ni content from 0% to 10% in 2.5% increments. The mixtures with 5% Ni melted and formed a button, while the 7.5% and 10% Ni alloys did not fully melt. The compositions that melted at 1120 C match nicely with the Cu-Ni phase diagram, which predicts alloys less than approximately 7% Ni will be fully molten at 1120 C, and those higher than this value will be partially solid until the solidus composition of nominally 15% Ni is reached, at which point the alloy is fully solid. It is also interesting to note that the copper quickly loses its characteristic color as the nickel content is increased. Roughly half of the color fades with a 5% Ni add and with 10% Ni, only a hint of Cu color remains. This whitening effect is evident in the US nickel coin, which is 25% Ni and 75% Cu, and in Nickel Silver alloys, one of which is 12% Ni and 88% Cu (NS-12). No evidence of copper, based on color, is apparent in either the coin or the NS-12 alloy. The addition of 5% iron into the copper-nickel

10 mixes had no measurable effect on the melting temperature, but interestingly, iron negates the whitening effect of nickel on copper. Therefore, more nickel is required to whiten the copper when iron is present. Understanding the color change is useful in interpreting the as polished microstructures of Ni-Cu containing samples sintered for short times. As sintering proceeds and the copper continues to alloy with the nickel in FLNC-4408, it reaches a composition that is solid at 1120 C. Again, according to the phase diagram and the experiment above, this composition is between 7% and 15% Ni, assuming no effects of Fe or C. This is also in the compositional range where copper loses most of its orange color. These two events appear to correspond with the 10 minute sintering sample. As can be seen in Figure 7, the copper-nickeliron alloy is barely visible in the as-polished microstructure. The change in dimensional behavior at ten minutes suggests that the effect of the copper-nickel interaction is minimal after 10 minutes. It is postulated that the liquid Cu - solid Ni interaction is responsible for the growth between 2 and 10 minutes and that once the copper alloy solidifies, the compact begins to behave in a typical manner. Electron backscattered imaging (BEI) was used in the SEM to reveal the compositional differences in the as-polished microstructures of the 5 minute sintered samples. In addition, EDS was used to semi-quantitatively measure the composition of the various regions. Figure 8 shows the difference between the Ni-free and Ni-containing samples. The copper-rich areas are white due to its higher atomic number, with the gray level of nickel between that of copper and iron. In the Ni free sample, the copper was found along particle / grain boundaries. The copper was relatively pure and little copper diffusion into the iron was evident. In the FLNC-4408, the majority of copper was found in the vicinity of the Ni additive and these copper rich regions contain significant amounts of both Ni and Fe after only 5 minutes total time in the hot zone. A range of compositions was found in these bright areas. At the low alloying end, areas contained 6% Ni and 10% Fe, while at the higher end, the copper rich regions contain upwards of 10% Ni and 14% Fe. It is expected that the copper rich regions containing 6% Ni were liquid at 1120 C, while the 10% Ni alloys may have been a mix of liquid and solid. Analysis of the Ni-rich regions revealed that the prior Ni particles contain an average of 10% Cu and 33% Fe. Significant diffusion occurs in these systems during the initial stages of sintering. One possibility for the difference in dimensional behavior between the two alloys is that copper locally infiltrates and fills pores in the Ni-free material, whereas in the Ni-containing alloys, the copper moves preferentially to the Ni-rich regions and causes local swelling via diffusion with Ni and Fe in those areas, resulting in growth of the overall compact. Diffusion of the copper and nickel can also be seen along the rims of the iron particles. In Figure 8b, there is a light gray band on the edges of the iron particles. This gray level contrast is due to the presence of copper and nickel in the base iron. The composition of these regions is nominally 5% Ni and 3% Cu. It is interesting to note that no such alloy rim in apparent in the nickel free alloy. This alloy rim on the iron particles is another difference between the two samples and may contribute to the different dimensional response. Copper does not diffuse quickly into high carbon iron, but the presence of nickel in these areas may change this behavior. Just as the increase in carbon changed the dimensional response in Figure 2 by modifying the copper behavior, the addition of nickel into the high carbon system may cause the copper to behave more like it does in a lower carbon, nickel free alloy.

11 (a) (b) Figure 8. Backscattered electron images of (a) FLC-4408 and (b) FLNC-4408 sintered for 5 minutes. Cu rich regions are white. Effect of Copper Particle Size The role of copper distribution in the green compact can play a large role on the dimensional stability of a sintered part. Assuming a well mixed sample with no segregation, the copper distribution can be changed by modifying the initial copper size. The reduction of particle size can be beneficial with respect to dimensional change [8]. In Ni-free alloys, there is little effect of copper particle size on dimensions. Figure 9 shows the effect on an 0.85% Mo base alloy with 2

12 carbon levels. At 0.6% graphite, little effect is seen, while at 0.9% graphite, increased growth is found at high copper contents with the finer copper. The finer copper is not beneficial in this alloy as liquid copper can freely migrate throughout the pore network; hence initial copper distribution has little effect. The large pores are eliminated with the finer copper, but no benefit to hardness or transverse rupture strength was found. In Ni free materials, the added cost for finer Cu additives is generally not warranted um Cu, 0.6Gr 11um Cu, 0.6Gr 40um Cu, 0.9Gr 11um Cu, 0.9Gr Cu Content (%) Figure 9. Effect of Cu particle size (d 50 ) on dimensional change of FLC2-440X alloys. In the nickel containing alloys, the initial copper particle size and distribution is important for improved dimensional change. Figure 10 shows the effect of Cu size on the dimensional change of FLNC The overall growth is decreased and the slope of the Cu-DC line is reduced, indicating better dimensional control with small variations in Cu content. This reduction in dimensional variation is also demonstrated in Figure 11. The data in Figure 4 was reproduced using a fine Cu additive and both are shown in the figure. The fine Cu shifts the lines toward the 0% Cu, FLNX-4408 curve. In fact, 0.5% 11 µm Cu has nearly identical dimensional change as 0% Cu and 0.1% 11µm Cu is similar to 0% Cu across the range of nickel contents tested. Overall, the spread in dimensional change is greatly reduced. Two diffusion alloy data points, FD-0208 and FD-0408, have been included in the 11 µm Cu graph. Both of these alloys contain 1.5% Cu and interestingly, fall right on the 1.5% 11µm Cu (FLNC-4408) line. Note that the dimensional change of the diffusion alloys do not match up with the 40 µm 1.5% Cu line. The 0.5% molybdenum is diffusion alloyed in the FD alloys, whereas the 0.85% Mo is prealloyed in the FLNC-4408 type materials. Little effect of Mo content was found in Fe-Mo-Ni-Cu alloys (Figures 2 and 3), so the Mo difference between the FL and FD alloys is expected to be small. Given that Mo content has little effect, the use of fine Cu in FLNC-4408 type systems can make these systems behave similarly to diffusion alloys with

13 respect to dimensional change. A good distribution of the fine Cu in the green state is required to take advantage of this effect and a bonding process would be advantageous in this system by fixing the additives to the base iron and preventing segregation um Cu 11um Cu 2% Nickel Alloys Cu Content (%) Figure 10. Effect of copper particle size and copper content on dimensional change of FLNC Figure 11. The effect of copper size on dimensional change of 0.85% Mo + 0.9% graphite alloys with admixed Cu and Ni. Distaloy FD-0208* and FD-0408 were also tested. * Additional Ni was added to FD-0208 to produce a 2% Ni alloy.

14 CONCLUSIONS Dimensional precision is a critical parameter in net shape processing of ferrous PM components. Additive compositions, amounts and size play a critical role on the dimensional change. The sinter-hardening grade FLNC-4408 utilizes both admixed nickel and copper and a strong interaction between these additives has been found that affects the dimensional behavior in this system. The following conclusions have been drawn from this study. The addition of nickel to FLC-440X alloys greatly reduces the effect of carbon on the dimensional change; nevertheless, the dimensional change is strongly dependent on copper content. The addition of both copper and nickel into an FL-4408 alloy can produce growths larger than the addition of either additive alone. The nickel-copper interaction was observed during the measurement of dimensional change with time. A nickel free FLC shrank after the copper melted, whereas the nickel containing FLNC-4408 continued to grow after melting until the copper liquid resolidified. Upon melting, the copper moves to different locations dependent upon Ni content. In Nifree materials, copper moves to pores at particle boundaries and may enhance sintering. In Ni-containing mixes, the majority of copper moves to the nickel-rich regions and alloys with the nickel. This different copper reaction is responsible for the change in dimensional behavior. The use of fine copper was found to improve the dimensional response in FLNC Less growth was found with the finer copper. The dimensional change of FLNC-4408 type alloys made with fine copper was very similar to that of diffusion alloys FD-0208 and FD ACKNOWLEDGEMENTS The authors would like to thank Ron Fitzpatrick for his assistance in collecting the data found within this paper. REFERENCES 1. N. Dautzenberg and H. J. Dorweiler, Dimensional behavior of copper-carbon sintered steels, Powder Metallurgy International, Vol. 17, No. 6, 1985, p Y. Trudel and R. Angers, Properties of iron copper alloys made from elemental or prealloyed powders, Int. Journal of Powder Metallurgy & Powder Technology, Vol. 11, No. 1, 1975, p R. L. Lawcock and T. J. Davies, Effect of carbon on dimensional and microstructural characteristics of Fe-Cu compacts during sintering, Powder Metallurgy, Vol. 33, No. 2, 1990, p. 147.

15 4. F. Chagnon and M. Gagne, Dimensional control of sinter hardened P/M components, Advances in Powder Metallurgy & Particulate Materials, compiled by W. G. Eisen and S. Kassam, Metal Powder Industries Federation, Princeton, NJ, 2001, part 5, p F. J. Semel, Processes determining the dimensional change of P/M steels, Advances in Powder Metallurgy & Particulate Materials, compiled by W. G. Eisen and S. Kassam, Metal Powder Industries Federation, Princeton, NJ, 2001, part 5, p B. Lindsley, G. Fillari and T. Murphy, Effect of composition and cooling rate on physical properties and microstructure of prealloyed P/M steels, Advances in Powder Metallurgy & Particulate Materials, compiled by C. Ruas and T. A. Tomlin, Metal Powder Industries Federation, Princeton, NJ, 2005, part 10, p T. Singh, T. F. Stephenson and S. T. Campbell, Nickel-copper interactions in P/M steels, Advances in Powder Metallurgy & Particulate Materials, compiled by W. B. James and R. A. Chernenkoff, Metal Powder Industries Federation, Princeton, NJ, 2004, part 7, p T. Singh and T. F. Stephenson, Ni-Cu-Mo interactions in sinter-hardening steels, Euro PM2005, EPMA, vol. 1, p L. Azzi, T. Stephenson, S. St-Laurent and S. Pelletier, Effect of nickel type on properties of binder-treated mixes, Advances in Powder Metallurgy & Particulate Materials, compiled by C. Ruas and T. A. Tomlin, Metal Powder Industries Federation, Princeton, NJ, 2005, part 10, p Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries Federation, Princeton, NJ, T. F. Murphy, Quantifying the degree of sinter in ferrous P/M materials, Euro PM2004, Ed. by H. Danninger and R. Ratzi, EPMA, 2004, vol. 3, p. 219.