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Sintering and properties of MIM M2 high speed steel produced by prealloy and master alloy routes
Metal Powder Report Volume 71, Number 3 May/June 2016
metal-powder.net
SPECIAL FEATURE
Sintering and properties of MIM M2 high speed steel produced by prealloy and master alloy routes Martin A. Kearns1, Keith Murray1, Paul A. Davies1, Viacheslav Ryabinin2 and Erainy Gonzalez2 §
1 2
Sandvik Osprey Ltd., Red Jacket Works, Milland Road, Neath SA11 1NJ, UK TCK S.A., Zona Franca Industrial Las Americas, Santo Domingo, Dominican Republic
The advantages of powder metallurgical processing of tool steels have been recognized for many years with water atomized [1] powder being used in press & sinter valve seat parts for many years and gas atomized tool steel powders consolidated by HIP used in high performance applications [2]. While coarse HIP powder grades dominate high volume tool steel markets, the advent of fine powder gas atomization has enabled the use of MIM for the production of an increasing number of industrial components where hardness and wear resistance is required. Examples include hand- and power-tool components, pump impellers and textile machinery. Tool steels typically have carbon content in the range 0.5–1.5% and heat treatment cycles are carefully controlled to achieve the optimum distribution of matrix carbides to deliver required wear performance. There are six groups of tool steels: water-hardening, cold-work, shock-resisting, high-speed, hot-work, and special purpose types. MIM parts makers have a full range of powder options including popular alloys such as H13, S7, A2, D2, M2 and T15. The choice of alloy for a particular application will depend on cost, working temperature, required surface hardness, strength, shock resistance, and toughness requirements. For more severe service environments, higher levels of refractory carbide formers (e.g. W, Mo) are used to maintain hot hardness and abrasion resistance. The subject of this study, M2, is an example of a popular high speed tool steel which is widely adopted in MIM in spite of the challenges posed by its narrow sintering window [3–7]. The phase diagram in Fig. 1 for M2 [8] illustrates the narrow temperature range within which complex changes in phase stability occur. Earlier studies on sintering of M2 have generally explored the temperature range 1200–1250 8C [3–7] but some studies on coarser water atomized powders have been conducted at higher temperatures [6]. The limited sintering window exhibited by M2 means that
§ This paper was presented at MIM2016 conference in Irvine, California, and is re-printed in Metal Powder Report with permission from MPIF.
E-mail address: [email protected].
achieving reproducible properties and consistent hardness requires close control of carbon levels in particular and it is the case that chemistry specifications for MIM powders can be narrower than those for wrought alloys. Zhang et al. [6] state that decarburization can occur when sintering in H2 and recommends a N2 atmosphere is used with 5–25%H2. Nitrogen raises the solidus temperature and the presence of carbonitrides is reported to inhibit grain growth [6]. The sintering mechanism in tool steels like M2 is Super-solidus Liquid Phase Sintering (SSLPS) and densification occurs rapidly once a liquid phase is formed at grain boundaries. Grain growth also tends to occur rapidly and as the liquid film thickness increases, the likelihood of distortion also increases. In some studies, addition of refractory carbides to the starting mix has been adopted in order to broaden the sintering window and reduce grain growth. This appears successful in some respects [6,7] but additional processing is required and coarse microstructural features appear to limit properties. Table 1 shows a range of published values for properties of M2 MIM parts. In other studies [12–14] relating to AISI 440C, AISI 4xxx low alloy steels and 420 stainless steel, it has been shown that a master alloy (MA) approach, whereby a concentrated alloy is blended with fine 0026-0657/ß 2016 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mprp.2016.04.085
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atmosphere and properties have been determined in the as sintered and heat treated conditions (double tempering to minimize retained austenite). Recognizing the criticality of carbon control on sintering behavior and finished part properties, a focus of the current study has been to evaluate carbon loss during sintering and to relate this to the chemistry of starting powders.
FIGURE 1
Phase diagram showing phase stability in M2 [8]. Construction lines show range of sintering temperatures and carbon content of three alloy powders used in this study.
carbonyl iron powder (CIP), can be advantageous. A reduction in distortion compared with prealloyed (PA) parts has been observed and, compared with blends of elemental ingredients; a more uniform distribution of alloying elements is achieved. Lower distortion is associated with finer average particle size in the green part and this has also been demonstrated in studies on PAs using progressively finer particle size distributions. Among the objectives of this study was therefore to examine the effect of particle size on sintered properties of PA M2 on processing and properties of M2 and to compare this with the behavior of M2 made by the MA route. Parts have been sintered in a nitrogen
A series of PA and MA M2 powders was produced by Sandvik Osprey’s proprietary inert gas atomization process using nitrogen gas. The ‘as-atomized’ powders were air classified to 90%-22 mm and 80%-22 mm size ranges. The chemistry and particle size distribution data of each powder batch used in the study is shown in Tables 2 and 3 respectively. In the case of MAs, additions were made of CIP made by Sintez. A mixture of HC and BC grades with different carbon levels was used to achieve the desired final chemistry. Two distinct MAs were formulated: each with a 3 concentration of main alloying elements but one with 2.9%C to which could be added low-carbon CIP powder and the other with 1.14%C to which could be added a mixture of low- and high-carbon CIP. The carbonyl iron powders are significantly finer than the gas atomized powders and, when added to the MA, mean that the effective size distribution is more akin to a 90%-16 mm powder than the 90% or 80%-22 mm typical of the PA. Feedstocks were prepared by TCK using their proprietary binder formulation to achieve a powder loading level of 61.8% corresponding to a 17.4% shrinkage factor. This shrinkage factor is typical of the target value in previous studies [12–14] with gas
TABLE 1
Published values for M2 tool steel in as sintered (AS) and heat treated (HT) condition. Reference
UTS (MPa)
0.2%PS (MPa)
El %
Density % T.D.
Hardness AS (HRC)
Hardness HT HVN (HRC)
BASF datasheet [9] Zhang, et al. [6] German [10] Indo MIM [11]
1200 1305 1100 –
800 – 1000 –
1 – 1 –
99 99.9 99 99
55–60 43 – 55–60
>820 HVN 62 HRC 60–65 HRC
TABLE 2
Chemical analysis of powders used in this study. Alloy
Fe
Cr
W
Mo
V
Mn
Si
N
C
O
M2 PA lot 031912-1 M2 PA lot 121514-1 M2 MA lot 121514-2 M2 MA low C lot 121514-3 Sintez HC 669 Sintez BC 477
Bal Bal Bal Bal Bal Bal
4.1 4.0 13.3 13.3 – –
6.1 6.0 17.9 18.4 – –
4.8 4.6 15.3 15.0 – –
1.9 1.9 5.8 6.1 – –
0.39 0.28 1.0 1.0 – –
0.31 0.25 1.2 0.9 – –
0.04 N.A. N.A. N.A 0.81 0.005
0.84 0.83 2.90 1.14 0.78 0.02
0.074 N.A. N.A. N.A. 0.25 0.43
TABLE 3
Particle size and tap density data for powders used in this study. Lot
Particle size
Grade
T. D. (g cm 3)
031912-1 121514-1 M2 MA 121514-2 121514-3 Sintez HC 669 Sintez BC 477
90%-22 mm 80%-22 mm 80%-22 mm 80%-22 mm 90%-12 mm 90%-12 mm
M2 PA M2 PA M2 MA M2 MA low C Sintez HC Sintez BC
5.2 5.2 5.2 5.2 4.16 4.23
Particle size data (mm) D10
D50
D90
4.1 4.3 4.8 4.4 2.6 2.6
10.0 12.3 12.3 13.0 5.1 5.6
20.0 27.6 27.5 27.3 10.6 11.2 201
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Experimental procedure
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Metal Powder Report Volume 71, Number 3 May/June 2016
were tested in triplicate in accordance with ASTM E8-08. Vickers hardness testing was carried out on tensile bar tabs using a 10 kg weight and results converted to HRC using standard tables. Polished cross-sections of Charpy bars were prepared for microstructural analysis by etching in dilute aqua regia. The heat treatment applied to all samples involved austenitizing at 1190 8C for 30 min before quenching in oil to room temperature then double tempering at 550 8C. SPECIAL FEATURE
Results
FIGURE 2
Progress of densification of M2 PA & MA with increasing sintering temperature.
atomized powders and is the sizing factor applied to the final part dimensions in order to design the mold. The feedstocks were molded in an Arburg injection molding unit to produce green standard MIMA tensile and Charpy test specimens [15]. Molded green parts were subject to an initial solvent debind followed by thermal debind at 500 8C (932 8F) for 3.5 h and sintered in a nitrogen atmosphere in an Elnik furnace. A series of sintering cycles was run at temperatures in the range 1220– 1250 8C with a standard heating profile of: 2 8C/min ramp to 1000 8C, 60 min hold, 2 8C/min ramp to sintering temperature, 60 min hold and furnace cool under a nitrogen atmosphere. Sintered density measurements were carried out using a Micromeritics Accupyc model 1340 Helium Pycnometer. The carbon level of the final sintered specimens was measured using Leco combustion analysis. For each sintering run, Charpy bars were mounted on refractory supports both cantilever style (15, 20 and 25 mm overhang) and suspended across refractory supports (38 mm separation) to determine the extent of distortion as a function of chemistry, particle size range and sintering temperature. As-sintered tensile samples
FIGURE 3
As sintered microstructures of different PA variants. 202
Three sintering runs were completed with different material types and different temperatures: 1220 8C, 1240 8C and, 1250 8C. Figure 2 shows the progress of densification with increasing sintering temperature for the different variants. The PA specimens achieve >98% density at 1220 8C and 100% density at 1240– 1250 8C while the MA variants achieve only >98% density at 1250 8C. Of the MAs, the high C variant reaches a higher density at each temperature investigated. Figure 3 shows metallographic sections of the as sintered Charpy specimens. This shows that densification of the PA looks almost complete at 1220 8C for the 90%-22 mm feedstock, but significant porosity remains in the 80%-22 mm sample. Low levels of porosity are evident in the PA samples sintered at higher temperatures (1240 8C & 1250 8C) but there is evidence of grain size coarsening with larger carbides at grain boundaries and precipitate-free zones adjacent to the boundaries. As shown in Table 4, this is accompanied by some carbon loss. The carbon losses are low and consistent for the PA samples and are higher, but still consistent, for the MA reflecting the higher levels of oxygen in the starting feedstock. Figure 4 shows microstructures of the sintered MA variants. The images reflect the trends in density shown in Fig. 2 with lower porosity levels evident at higher temperatures. The sample made with low C MA has higher levels of porosity and a smaller grain size than that made with conventional MA with its full (3) concentration of carbon. Both samples made with MA show microstructures with less uniform distributions of carbide than the PA
Metal Powder Report Volume 71, Number 3 May/June 2016
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TABLE 4
Change in carbon after sintering different feedstock variants.
M2 M2 M2 M2
PA, 031912-1 PA, 121514-1 MA, 121514-2 MA low C, 121514-3
%C (M2)
0.84 0.83 2.90 1.14
%O (M2)
0.074 N.A. N.A. N.A.
%C (CIP)
– – 0.02 0.78
%O (CIP)
%C (final part)
– – 0.43 0.25
1220 8C
1240 8C
1250 8C
0.82 0.80 – –
0.81 0.79 0.89 0.85
0.80 0.79 0.90 0.87
0.03 0.04 0.07 0.04
DC
TABLE 5
Mechanical properties of As sintered specimens. Sintering temp, 8C
M2 feedstock
Particle size range
Density %theory
UTS (MPa)
%El
VHN (HRC)
1220
PA (031912-1) PA (121514-1)
90%-22 mm 80%-22 mm
99.1 84.6
978 –
1499 –
– –
673 (57.5) 387 (41)
1240
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
97.4 98.2 96.0
1040 988 776
1418 1377 1329
– 0.5 1.0
673 (57.5) 600 (55) 583 (54)
1250
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
99.9 98.4 99.3
1230 969 1106
1471 1141 1386
1.5 1.5 0.8
579 (54) 622 (56) 599 (55)
samples. Figure 4 shows that the samples made with MA have more coarse carbides present at grain boundaries adjacent to areas which are denuded of carbides. Table 5 shows that the parts made using PA powder exhibit highest levels of hardness and mechanical properties: generally exceeding the values obtained from parts made using MAs. The lower hardness values measured on parts made from MA combinations reflect the lower density levels shown in Fig. 2. The lowcarbon MA material shows highest porosity and lowest hardness. The 0.2% proof strength ranges between 776 and 1230 MPa and UTS ranges between 1141 and 1499 MPa. The PA results average 1080 MPa and 1460 MPa respectively for effectively a fully dense product. The results for MA based products approach these levels at the highest sintering temperature, but results are more variable reflecting differing levels of porosity and less uniform carbide distributions within the microstructure. It may be noted that this is despite the fact that the carbon level in the PA samples is lower than the levels in the MA samples (see Table 4).
0.2%PS (MPa)
On removal from the sintering furnace, it was apparent that all of the suspension and cantilever samples made from PA feedstock were broken, even for the shortest overhang of 15 mm. However, the MA samples survived intact, even up to 25 mm overhang. The deflection of specimens made using the different MA types was determined and these data are plotted in Fig. 5. This shows that deflection increases with increasing overhang and with increasing sintering temperature but there does not seem to be a systematic difference in behavior between the two different master alloys. The drape test specimens suspended between supports separated by a gap of 38 mm show a deflection in line with that of the tip deflection of a 25 mm cantilever test specimen. After austenitizing at 1190 8C and double tempering at 550 8C, samples were subject to tensile and hardness testing. Results are listed in Table 6 and show that in all cases hardness levels have increased significantly reaching 66-67HRC in most cases. Where hardness levels fall below this value it is associated with high
FIGURE 4
Microstructures of different master alloy variants after sintering at 1240 8C & 1250 8C.
FIGURE 5
Condition of suspension test samples after sintering at 1240 & 1250 8C. 203
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Alloy, lot #
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TABLE 6
Mechanical properties of heat treated specimens. HRC in parentheses are calculated from VHN.
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Sintering temp, 8C
M2 feedstock
Particle size dist.
Density %theory
0.2%PS (MPa)
UTS (MPa)
%El
VHN (HRC)
1220
PA (031912-1) PA (121514-1)
90%-22 mm 80%-22 mm
N.D. 84.6
N.D. 876
N.D. 1205
N.D. 1
N.D. 428 (43)
1240
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
97.4 98.2 96.0
1263 – 767
1647 1137 1167
1.5 – –
863 (66) 903 (67) 769 (61)
1250
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
99.9 98.4 99.3
1201 – –
1330 955 1310
– – –
874 (66) 887 (66) 882 (66)
residual porosity levels. In the case of tensile properties these are more erratic and do not always exceed the as sintered values. This is probably related to the reduced ductility after heat treatment leading to premature failures in some cases. An exception is the PA sintered at 1240 8C which achieves the highest property combination in this study (1263 MPa 0.2%PS and 1647 MPa UTS). Figure 7 shows the heat treated microstructures which confirm that the PA samples have an even distribution of fine carbides and carbonitrides. This is particularly so for the materials sintered at 1240 8C; the equivalent material sintered at 1250 8C displays a coarser grain size and coarser precipitates which are reflected in lower tensile properties. The heat treated microstructures of the MA variants are very different from the PA, exhibiting much coarser complex carbonitride islands up to 20 mm in size. These are located at grain boundaries and are evident in all samples sintered at 1240 8C and 1250 8C, though it is apparent that grain size is larger at the higher temperature and the carbontride islands are also larger.
Discussion The density and metallographic results confirm that sintering in a nitrogen atmosphere in the temperature range 1220–1240 8C, is sufficient to achieve near full density, uniform parts in M2 using PA powder and that the finer, 90%-22 mm size fraction achieves full density at a lower temperature than 80%-22 mm powder. This behavior is in line with earlier studies showing that M2 PA has a narrow sintering window in this range. Liu et al. [4] and Varez et al. [5] have determined that in vacuum, densification of M2 occurs at much lower temperatures (<1200 8C) but that an increase of 60 8C is needed in order to achieve similar densification in a nitrogen atmosphere. This is attributed to the stabilizing effect of nitrogen on austenite and the elevation of the liquidus temperature with increasing nitrogen. The hardness measurements reported by Varez et al. [5] as a function of temperature and sintering atmosphere are reproduced here in Fig. 7 and data from the current study are superimposed on this schematic for comparison. There is a reasonable correlation between the 90%-22 mm results from this study and the data from Ref. [5] for sintering in nitrogen. It is also clear that higher sintering temperature is needed to get to peak density and hardness with the 80%-22 mm fraction and the MA material. The reasons for incomplete sintering of the MA at 1240 & 1250 8C were examined in relation to phase stability of the MA/CIP couples using Thermocalc analysis with plots similar to Fig. 1 being calculated for the prealloy and master alloy compositions. Though not 204
shown here, the analysis shows that in the case of the PA the sintering temperature range straddles the liquidus temperature so that a liquid phase sintering mechanism can operate to promote rapid densification. For the MA variant with 2.9%C the sintering temperature range again straddles the liquidus but for the MAlow C alloy with 1.14%C, the sintering temperature range falls some 15 8C below the liquidus temperature which is elevated by the higher concentration of alloying elements in the master alloy. This means that liquid phase sintering is not expected to play as significant a role in early densification of this variant and this is apparently reflected in the lower final density values shown in Fig. 2. As well as exhibiting higher residual porosity, the MA variants show clusters of coarse carbides in grain boundary regions adjacent to areas which are clearly denuded of carbon. It is expected that these clusters are associated with prior MA particles. While carbon will tend to diffuse away from the MA its final distribution will be determined by concentrations of strong carbide formers (W, Mo, V) which are less mobile and will tend to remain at the original sites of MA particles. In the heat treated condition, Fig. 6, the bright carbides shown in backscattered contrast feature the high atomic number elements in M6C precipitates. The black phases that are interspersed with the lighter ones are V-rich carbonitrides which would normally tend to inhibit grain growth. However, it is apparent that while the PA shows an even distribution of precipitates, the MAs show coarse M6C/MC clusters at grain boundaries suggesting that there has been insufficient time for diffusion of heavy elements from the prior MA particles.
Carbon control The sintering window for M2 is narrow and final properties and part distortion is highly sensitive to the carbon level in the alloy. Good control of carbon level in relation to the sintering window is therefore key to achieving best results. The data for carbon loss demonstrate that this is predictable and low for gas atomized PA powders. It is rather higher for MA/CIP combinations but again correlates with the higher oxygen levels present in the CIP components (see Table 4). For the PA, the measured oxygen level of 0.074% in the starting powder would equate to a loss in carbon of 0.03% if converted to and lost as CO2. For the MA, the high level of oxygen present in the CIP contributes to the higher starting O level of 0.29% or 0.17% (in MA and MAlowC variants respectively). A C loss of 0.11% and 0.07% respectively would therefore be anticipated, but these values are higher than the 0.07% and 0.04% losses observed. The C loss from the MA variants is lower
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Metal Powder Report Volume 71, Number 3 May/June 2016
FIGURE 6
Microstructures of heat treated samples after sintering at 1240 8C & 1250 8C.
Hardness and heat treatment
FIGURE 7
Comparison of peak hardness vs sintering temperature for this study with sintering of M2 in different atmospheres (after Varez et al. [5]).
Tables 5 and 6 show the relationship between sintering temperature and hardness levels in the as sintered and heat treated samples. It is apparent that once densification is substantially complete, a high hardness level of 65-67HRC can be attained for M2 made by either PA or MA routes. This range is slightly higher than values reported elsewhere [6,9–11]. Given the very different heat treated microstructures obtained via the different routes, this is perhaps surprising: for example, the MA variants have a much coarser microstructure (grain size and carbide size) than the PA. It is possible that the softening that would be expected to be associated with coarser features is partly off-set by the higher overall C level in these variants compared with the PA (up to 0.10% higher – see Table 4).
Summary and conclusions than may be expected compared with an earlier study on 440C [12]. This may reflect higher stability of carbide formers in M2.
Mechanical properties The microstructure of the sintered parts shows varying amounts of residual porosity particularly in parts sintered at 1220 8C (80%22 mm PA) and MA and this is in part responsible for the variable ductility and therefore UTS seen in these specimens. Mechanical property values bear good comparison with other reported values and the peak of 1263 MPa, 1647 MPa (0.2%PS, UTS) in the double tempered condition is believed to be the highest value reported for MIM M2.
Deflection tests As in other studies, the MA variant shows greater resilience in the cantilever/suspension test than the PAs and this may be advantageous for components with overhang features where full density is also important. Drape tests (Fig. 5) also reveal that at any given temperature, the MA-based materials show less distortion than PA. This is probably related to the presence of weak, low melting point grain boundary phases which will appear at an earlier stage for PA M2 and will be last to form in the MAs.
The present study shows that sintering M2 in nitrogen at 1220– 1250 8C is effective in achieving virtually full part density for 90% and 80%-22 mm PA powders. Higher temperatures are needed to get full density in parts made via a MA route: a 90%-22 mm grade enables full densification at 1220 8C while the 80%-22 mm requires a sintering temperature of 1240 8C to attain full density. The densification behavior can be rationalized with reference to Thermocalc studies which also help explain final microstructures which, in the case of the MA variants contain coarse carbide clusters at the location of original MA particle sites. There has been insufficient time for effective redistribution of heavy elements like Mo, W and therefore inhomogeneous microstructures with coarse carbides are observed. Carbon loss during the sintering process is predictable for gas atomized powders with and without blended CIP and is dependent only on the amount of oxygen present in the starting powder(s) and not apparently on the sintering temperature in the normal sintering range 1220–1250 8C. Hardness and strength levels achieved in the as sintered and heat treated conditions compare very favorably with reference industry values with 67HRC being achieved in the heat treated condition (double tempered at 550 8C) compared with 60-65HRC quoted in the literature. 205
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In common with previous studies on other MA systems, part distortion appears more pronounced in parts made with PA powders compared with MA powders. This may be one reason to consider a MA route albeit a higher sintering temperature will be needed to achieve full density. It is concluded that gas atomized M2 PA can give fully dense components with high strength and hardness in the as sintered and particularly the tempered condition. The MA approach can give similar properties at higher sintering temperatures, but redistribution of carbide forming elements is incomplete and microstructures are inhomogeneous. While MA can give useful resistance to part distortion, it would be prudent to use MA only as a component of an overall feedstock formulation if distortion control is of overriding importance.
Acknowledgements Grateful thanks are due to Ms Linn Larsson of Sandvik Materials Technology for detailed metallographic analyses and to Anders Willson and Peter Harlin, also of Sandvik Materials Technology, for Thermocalc studies. Our appreciation also goes to Mr Chris Phillips of Sandvik Osprey for deflection measurements.
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References [1] J.V. Bee, P. Brewin, P.D. Nurthen, J.V. Wood, Metal Powder Rep. 182 (March) (1988) 177–180. [2] R.A. Mesquita, C.A. Barbosa, Mater. Sci. Forum 416–418 (1) (2003) 235–240. [3] Z.Y. Liu, N.H. Loh, K.A. Khor, S.B. Tor, Mater. Sci. Eng. A293 (2000) 46–55. [4] Z.Y. Liu, N.H. Loh, K.A. Khor, S.B. Tor, Mater. Lett. 45 (2000) 32–38. [5] A. Varez, B. Levenfield, J.M. Torralba, G. Matula, Mater. Sci. Eng. A366 (2004) 318–324. [6] H. Zhang, D.F. Heaney, R.M. German, Effect of Carbide Addition on Sintering of M2 Tool Steel, New Developments in MIM, 4-166 to 4-179 TWC, 2000. [7] G. Herranz, G.P. Rodrı´guez, R. Alonso, G. Matula, PIM Int. 4 (June (2)) (2010) 60–65. [8] G. Hoyle, High Speed Steels, Butterworths & Co. (Publishers), London, 1988. [9] BASF Datasheet – Catamold1 M2, April 2006. www.basf.de/catamold. [10] R.M. German, Powder Injection Molding – Design & Applications, 2003, p. 81. [11] Indo MIM website. www.indo-mim.com. [12] M.A. Kearns, K. Murray, V. Ryabinin, E. Gonzalez, MIM2015 Conference, Tampa, 2015. [13] A.J. Coleman, K. Murray, M. Kearns, T.A. Tingskog, B. Sanford, E. Gonzalez, Properties of MIM AISI 420 via Pre-alloyed and Master Alloy Routes, PowderMet, 2013. [14] A.J. Coleman, K. Murray, M.A. Kearns, T.A. Tingskog, B. Sanford, E. Gonzalez, Effect of Particle Size Distribution on Processing and Properties of Metal Injection Moulded 4140 and 4340, PowderMet, San Francisco, CA, 2011. [15] MPIF Standard 35 Handbook.
metal-powder.net
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Sintering and properties of MIM M2 high speed steel produced by prealloy and master alloy routes Martin A. Kearns1, Keith Murray1, Paul A. Davies1, Viacheslav Ryabinin2 and Erainy Gonzalez2 §
1 2
Sandvik Osprey Ltd., Red Jacket Works, Milland Road, Neath SA11 1NJ, UK TCK S.A., Zona Franca Industrial Las Americas, Santo Domingo, Dominican Republic
The advantages of powder metallurgical processing of tool steels have been recognized for many years with water atomized [1] powder being used in press & sinter valve seat parts for many years and gas atomized tool steel powders consolidated by HIP used in high performance applications [2]. While coarse HIP powder grades dominate high volume tool steel markets, the advent of fine powder gas atomization has enabled the use of MIM for the production of an increasing number of industrial components where hardness and wear resistance is required. Examples include hand- and power-tool components, pump impellers and textile machinery. Tool steels typically have carbon content in the range 0.5–1.5% and heat treatment cycles are carefully controlled to achieve the optimum distribution of matrix carbides to deliver required wear performance. There are six groups of tool steels: water-hardening, cold-work, shock-resisting, high-speed, hot-work, and special purpose types. MIM parts makers have a full range of powder options including popular alloys such as H13, S7, A2, D2, M2 and T15. The choice of alloy for a particular application will depend on cost, working temperature, required surface hardness, strength, shock resistance, and toughness requirements. For more severe service environments, higher levels of refractory carbide formers (e.g. W, Mo) are used to maintain hot hardness and abrasion resistance. The subject of this study, M2, is an example of a popular high speed tool steel which is widely adopted in MIM in spite of the challenges posed by its narrow sintering window [3–7]. The phase diagram in Fig. 1 for M2 [8] illustrates the narrow temperature range within which complex changes in phase stability occur. Earlier studies on sintering of M2 have generally explored the temperature range 1200–1250 8C [3–7] but some studies on coarser water atomized powders have been conducted at higher temperatures [6]. The limited sintering window exhibited by M2 means that
§ This paper was presented at MIM2016 conference in Irvine, California, and is re-printed in Metal Powder Report with permission from MPIF.
E-mail address: [email protected].
achieving reproducible properties and consistent hardness requires close control of carbon levels in particular and it is the case that chemistry specifications for MIM powders can be narrower than those for wrought alloys. Zhang et al. [6] state that decarburization can occur when sintering in H2 and recommends a N2 atmosphere is used with 5–25%H2. Nitrogen raises the solidus temperature and the presence of carbonitrides is reported to inhibit grain growth [6]. The sintering mechanism in tool steels like M2 is Super-solidus Liquid Phase Sintering (SSLPS) and densification occurs rapidly once a liquid phase is formed at grain boundaries. Grain growth also tends to occur rapidly and as the liquid film thickness increases, the likelihood of distortion also increases. In some studies, addition of refractory carbides to the starting mix has been adopted in order to broaden the sintering window and reduce grain growth. This appears successful in some respects [6,7] but additional processing is required and coarse microstructural features appear to limit properties. Table 1 shows a range of published values for properties of M2 MIM parts. In other studies [12–14] relating to AISI 440C, AISI 4xxx low alloy steels and 420 stainless steel, it has been shown that a master alloy (MA) approach, whereby a concentrated alloy is blended with fine 0026-0657/ß 2016 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mprp.2016.04.085
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atmosphere and properties have been determined in the as sintered and heat treated conditions (double tempering to minimize retained austenite). Recognizing the criticality of carbon control on sintering behavior and finished part properties, a focus of the current study has been to evaluate carbon loss during sintering and to relate this to the chemistry of starting powders.
FIGURE 1
Phase diagram showing phase stability in M2 [8]. Construction lines show range of sintering temperatures and carbon content of three alloy powders used in this study.
carbonyl iron powder (CIP), can be advantageous. A reduction in distortion compared with prealloyed (PA) parts has been observed and, compared with blends of elemental ingredients; a more uniform distribution of alloying elements is achieved. Lower distortion is associated with finer average particle size in the green part and this has also been demonstrated in studies on PAs using progressively finer particle size distributions. Among the objectives of this study was therefore to examine the effect of particle size on sintered properties of PA M2 on processing and properties of M2 and to compare this with the behavior of M2 made by the MA route. Parts have been sintered in a nitrogen
A series of PA and MA M2 powders was produced by Sandvik Osprey’s proprietary inert gas atomization process using nitrogen gas. The ‘as-atomized’ powders were air classified to 90%-22 mm and 80%-22 mm size ranges. The chemistry and particle size distribution data of each powder batch used in the study is shown in Tables 2 and 3 respectively. In the case of MAs, additions were made of CIP made by Sintez. A mixture of HC and BC grades with different carbon levels was used to achieve the desired final chemistry. Two distinct MAs were formulated: each with a 3 concentration of main alloying elements but one with 2.9%C to which could be added low-carbon CIP powder and the other with 1.14%C to which could be added a mixture of low- and high-carbon CIP. The carbonyl iron powders are significantly finer than the gas atomized powders and, when added to the MA, mean that the effective size distribution is more akin to a 90%-16 mm powder than the 90% or 80%-22 mm typical of the PA. Feedstocks were prepared by TCK using their proprietary binder formulation to achieve a powder loading level of 61.8% corresponding to a 17.4% shrinkage factor. This shrinkage factor is typical of the target value in previous studies [12–14] with gas
TABLE 1
Published values for M2 tool steel in as sintered (AS) and heat treated (HT) condition. Reference
UTS (MPa)
0.2%PS (MPa)
El %
Density % T.D.
Hardness AS (HRC)
Hardness HT HVN (HRC)
BASF datasheet [9] Zhang, et al. [6] German [10] Indo MIM [11]
1200 1305 1100 –
800 – 1000 –
1 – 1 –
99 99.9 99 99
55–60 43 – 55–60
>820 HVN 62 HRC 60–65 HRC
TABLE 2
Chemical analysis of powders used in this study. Alloy
Fe
Cr
W
Mo
V
Mn
Si
N
C
O
M2 PA lot 031912-1 M2 PA lot 121514-1 M2 MA lot 121514-2 M2 MA low C lot 121514-3 Sintez HC 669 Sintez BC 477
Bal Bal Bal Bal Bal Bal
4.1 4.0 13.3 13.3 – –
6.1 6.0 17.9 18.4 – –
4.8 4.6 15.3 15.0 – –
1.9 1.9 5.8 6.1 – –
0.39 0.28 1.0 1.0 – –
0.31 0.25 1.2 0.9 – –
0.04 N.A. N.A. N.A 0.81 0.005
0.84 0.83 2.90 1.14 0.78 0.02
0.074 N.A. N.A. N.A. 0.25 0.43
TABLE 3
Particle size and tap density data for powders used in this study. Lot
Particle size
Grade
T. D. (g cm 3)
031912-1 121514-1 M2 MA 121514-2 121514-3 Sintez HC 669 Sintez BC 477
90%-22 mm 80%-22 mm 80%-22 mm 80%-22 mm 90%-12 mm 90%-12 mm
M2 PA M2 PA M2 MA M2 MA low C Sintez HC Sintez BC
5.2 5.2 5.2 5.2 4.16 4.23
Particle size data (mm) D10
D50
D90
4.1 4.3 4.8 4.4 2.6 2.6
10.0 12.3 12.3 13.0 5.1 5.6
20.0 27.6 27.5 27.3 10.6 11.2 201
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Experimental procedure
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Metal Powder Report Volume 71, Number 3 May/June 2016
were tested in triplicate in accordance with ASTM E8-08. Vickers hardness testing was carried out on tensile bar tabs using a 10 kg weight and results converted to HRC using standard tables. Polished cross-sections of Charpy bars were prepared for microstructural analysis by etching in dilute aqua regia. The heat treatment applied to all samples involved austenitizing at 1190 8C for 30 min before quenching in oil to room temperature then double tempering at 550 8C. SPECIAL FEATURE
Results
FIGURE 2
Progress of densification of M2 PA & MA with increasing sintering temperature.
atomized powders and is the sizing factor applied to the final part dimensions in order to design the mold. The feedstocks were molded in an Arburg injection molding unit to produce green standard MIMA tensile and Charpy test specimens [15]. Molded green parts were subject to an initial solvent debind followed by thermal debind at 500 8C (932 8F) for 3.5 h and sintered in a nitrogen atmosphere in an Elnik furnace. A series of sintering cycles was run at temperatures in the range 1220– 1250 8C with a standard heating profile of: 2 8C/min ramp to 1000 8C, 60 min hold, 2 8C/min ramp to sintering temperature, 60 min hold and furnace cool under a nitrogen atmosphere. Sintered density measurements were carried out using a Micromeritics Accupyc model 1340 Helium Pycnometer. The carbon level of the final sintered specimens was measured using Leco combustion analysis. For each sintering run, Charpy bars were mounted on refractory supports both cantilever style (15, 20 and 25 mm overhang) and suspended across refractory supports (38 mm separation) to determine the extent of distortion as a function of chemistry, particle size range and sintering temperature. As-sintered tensile samples
FIGURE 3
As sintered microstructures of different PA variants. 202
Three sintering runs were completed with different material types and different temperatures: 1220 8C, 1240 8C and, 1250 8C. Figure 2 shows the progress of densification with increasing sintering temperature for the different variants. The PA specimens achieve >98% density at 1220 8C and 100% density at 1240– 1250 8C while the MA variants achieve only >98% density at 1250 8C. Of the MAs, the high C variant reaches a higher density at each temperature investigated. Figure 3 shows metallographic sections of the as sintered Charpy specimens. This shows that densification of the PA looks almost complete at 1220 8C for the 90%-22 mm feedstock, but significant porosity remains in the 80%-22 mm sample. Low levels of porosity are evident in the PA samples sintered at higher temperatures (1240 8C & 1250 8C) but there is evidence of grain size coarsening with larger carbides at grain boundaries and precipitate-free zones adjacent to the boundaries. As shown in Table 4, this is accompanied by some carbon loss. The carbon losses are low and consistent for the PA samples and are higher, but still consistent, for the MA reflecting the higher levels of oxygen in the starting feedstock. Figure 4 shows microstructures of the sintered MA variants. The images reflect the trends in density shown in Fig. 2 with lower porosity levels evident at higher temperatures. The sample made with low C MA has higher levels of porosity and a smaller grain size than that made with conventional MA with its full (3) concentration of carbon. Both samples made with MA show microstructures with less uniform distributions of carbide than the PA
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TABLE 4
Change in carbon after sintering different feedstock variants.
M2 M2 M2 M2
PA, 031912-1 PA, 121514-1 MA, 121514-2 MA low C, 121514-3
%C (M2)
0.84 0.83 2.90 1.14
%O (M2)
0.074 N.A. N.A. N.A.
%C (CIP)
– – 0.02 0.78
%O (CIP)
%C (final part)
– – 0.43 0.25
1220 8C
1240 8C
1250 8C
0.82 0.80 – –
0.81 0.79 0.89 0.85
0.80 0.79 0.90 0.87
0.03 0.04 0.07 0.04
DC
TABLE 5
Mechanical properties of As sintered specimens. Sintering temp, 8C
M2 feedstock
Particle size range
Density %theory
UTS (MPa)
%El
VHN (HRC)
1220
PA (031912-1) PA (121514-1)
90%-22 mm 80%-22 mm
99.1 84.6
978 –
1499 –
– –
673 (57.5) 387 (41)
1240
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
97.4 98.2 96.0
1040 988 776
1418 1377 1329
– 0.5 1.0
673 (57.5) 600 (55) 583 (54)
1250
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
99.9 98.4 99.3
1230 969 1106
1471 1141 1386
1.5 1.5 0.8
579 (54) 622 (56) 599 (55)
samples. Figure 4 shows that the samples made with MA have more coarse carbides present at grain boundaries adjacent to areas which are denuded of carbides. Table 5 shows that the parts made using PA powder exhibit highest levels of hardness and mechanical properties: generally exceeding the values obtained from parts made using MAs. The lower hardness values measured on parts made from MA combinations reflect the lower density levels shown in Fig. 2. The lowcarbon MA material shows highest porosity and lowest hardness. The 0.2% proof strength ranges between 776 and 1230 MPa and UTS ranges between 1141 and 1499 MPa. The PA results average 1080 MPa and 1460 MPa respectively for effectively a fully dense product. The results for MA based products approach these levels at the highest sintering temperature, but results are more variable reflecting differing levels of porosity and less uniform carbide distributions within the microstructure. It may be noted that this is despite the fact that the carbon level in the PA samples is lower than the levels in the MA samples (see Table 4).
0.2%PS (MPa)
On removal from the sintering furnace, it was apparent that all of the suspension and cantilever samples made from PA feedstock were broken, even for the shortest overhang of 15 mm. However, the MA samples survived intact, even up to 25 mm overhang. The deflection of specimens made using the different MA types was determined and these data are plotted in Fig. 5. This shows that deflection increases with increasing overhang and with increasing sintering temperature but there does not seem to be a systematic difference in behavior between the two different master alloys. The drape test specimens suspended between supports separated by a gap of 38 mm show a deflection in line with that of the tip deflection of a 25 mm cantilever test specimen. After austenitizing at 1190 8C and double tempering at 550 8C, samples were subject to tensile and hardness testing. Results are listed in Table 6 and show that in all cases hardness levels have increased significantly reaching 66-67HRC in most cases. Where hardness levels fall below this value it is associated with high
FIGURE 4
Microstructures of different master alloy variants after sintering at 1240 8C & 1250 8C.
FIGURE 5
Condition of suspension test samples after sintering at 1240 & 1250 8C. 203
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Alloy, lot #
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TABLE 6
Mechanical properties of heat treated specimens. HRC in parentheses are calculated from VHN.
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Sintering temp, 8C
M2 feedstock
Particle size dist.
Density %theory
0.2%PS (MPa)
UTS (MPa)
%El
VHN (HRC)
1220
PA (031912-1) PA (121514-1)
90%-22 mm 80%-22 mm
N.D. 84.6
N.D. 876
N.D. 1205
N.D. 1
N.D. 428 (43)
1240
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
97.4 98.2 96.0
1263 – 767
1647 1137 1167
1.5 – –
863 (66) 903 (67) 769 (61)
1250
PA (121514-1) MA + CIP MA LC + CIP
80%-22 mm 90%-16 mm 90%-16 mm
99.9 98.4 99.3
1201 – –
1330 955 1310
– – –
874 (66) 887 (66) 882 (66)
residual porosity levels. In the case of tensile properties these are more erratic and do not always exceed the as sintered values. This is probably related to the reduced ductility after heat treatment leading to premature failures in some cases. An exception is the PA sintered at 1240 8C which achieves the highest property combination in this study (1263 MPa 0.2%PS and 1647 MPa UTS). Figure 7 shows the heat treated microstructures which confirm that the PA samples have an even distribution of fine carbides and carbonitrides. This is particularly so for the materials sintered at 1240 8C; the equivalent material sintered at 1250 8C displays a coarser grain size and coarser precipitates which are reflected in lower tensile properties. The heat treated microstructures of the MA variants are very different from the PA, exhibiting much coarser complex carbonitride islands up to 20 mm in size. These are located at grain boundaries and are evident in all samples sintered at 1240 8C and 1250 8C, though it is apparent that grain size is larger at the higher temperature and the carbontride islands are also larger.
Discussion The density and metallographic results confirm that sintering in a nitrogen atmosphere in the temperature range 1220–1240 8C, is sufficient to achieve near full density, uniform parts in M2 using PA powder and that the finer, 90%-22 mm size fraction achieves full density at a lower temperature than 80%-22 mm powder. This behavior is in line with earlier studies showing that M2 PA has a narrow sintering window in this range. Liu et al. [4] and Varez et al. [5] have determined that in vacuum, densification of M2 occurs at much lower temperatures (<1200 8C) but that an increase of 60 8C is needed in order to achieve similar densification in a nitrogen atmosphere. This is attributed to the stabilizing effect of nitrogen on austenite and the elevation of the liquidus temperature with increasing nitrogen. The hardness measurements reported by Varez et al. [5] as a function of temperature and sintering atmosphere are reproduced here in Fig. 7 and data from the current study are superimposed on this schematic for comparison. There is a reasonable correlation between the 90%-22 mm results from this study and the data from Ref. [5] for sintering in nitrogen. It is also clear that higher sintering temperature is needed to get to peak density and hardness with the 80%-22 mm fraction and the MA material. The reasons for incomplete sintering of the MA at 1240 & 1250 8C were examined in relation to phase stability of the MA/CIP couples using Thermocalc analysis with plots similar to Fig. 1 being calculated for the prealloy and master alloy compositions. Though not 204
shown here, the analysis shows that in the case of the PA the sintering temperature range straddles the liquidus temperature so that a liquid phase sintering mechanism can operate to promote rapid densification. For the MA variant with 2.9%C the sintering temperature range again straddles the liquidus but for the MAlow C alloy with 1.14%C, the sintering temperature range falls some 15 8C below the liquidus temperature which is elevated by the higher concentration of alloying elements in the master alloy. This means that liquid phase sintering is not expected to play as significant a role in early densification of this variant and this is apparently reflected in the lower final density values shown in Fig. 2. As well as exhibiting higher residual porosity, the MA variants show clusters of coarse carbides in grain boundary regions adjacent to areas which are clearly denuded of carbon. It is expected that these clusters are associated with prior MA particles. While carbon will tend to diffuse away from the MA its final distribution will be determined by concentrations of strong carbide formers (W, Mo, V) which are less mobile and will tend to remain at the original sites of MA particles. In the heat treated condition, Fig. 6, the bright carbides shown in backscattered contrast feature the high atomic number elements in M6C precipitates. The black phases that are interspersed with the lighter ones are V-rich carbonitrides which would normally tend to inhibit grain growth. However, it is apparent that while the PA shows an even distribution of precipitates, the MAs show coarse M6C/MC clusters at grain boundaries suggesting that there has been insufficient time for diffusion of heavy elements from the prior MA particles.
Carbon control The sintering window for M2 is narrow and final properties and part distortion is highly sensitive to the carbon level in the alloy. Good control of carbon level in relation to the sintering window is therefore key to achieving best results. The data for carbon loss demonstrate that this is predictable and low for gas atomized PA powders. It is rather higher for MA/CIP combinations but again correlates with the higher oxygen levels present in the CIP components (see Table 4). For the PA, the measured oxygen level of 0.074% in the starting powder would equate to a loss in carbon of 0.03% if converted to and lost as CO2. For the MA, the high level of oxygen present in the CIP contributes to the higher starting O level of 0.29% or 0.17% (in MA and MAlowC variants respectively). A C loss of 0.11% and 0.07% respectively would therefore be anticipated, but these values are higher than the 0.07% and 0.04% losses observed. The C loss from the MA variants is lower
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Metal Powder Report Volume 71, Number 3 May/June 2016
FIGURE 6
Microstructures of heat treated samples after sintering at 1240 8C & 1250 8C.
Hardness and heat treatment
FIGURE 7
Comparison of peak hardness vs sintering temperature for this study with sintering of M2 in different atmospheres (after Varez et al. [5]).
Tables 5 and 6 show the relationship between sintering temperature and hardness levels in the as sintered and heat treated samples. It is apparent that once densification is substantially complete, a high hardness level of 65-67HRC can be attained for M2 made by either PA or MA routes. This range is slightly higher than values reported elsewhere [6,9–11]. Given the very different heat treated microstructures obtained via the different routes, this is perhaps surprising: for example, the MA variants have a much coarser microstructure (grain size and carbide size) than the PA. It is possible that the softening that would be expected to be associated with coarser features is partly off-set by the higher overall C level in these variants compared with the PA (up to 0.10% higher – see Table 4).
Summary and conclusions than may be expected compared with an earlier study on 440C [12]. This may reflect higher stability of carbide formers in M2.
Mechanical properties The microstructure of the sintered parts shows varying amounts of residual porosity particularly in parts sintered at 1220 8C (80%22 mm PA) and MA and this is in part responsible for the variable ductility and therefore UTS seen in these specimens. Mechanical property values bear good comparison with other reported values and the peak of 1263 MPa, 1647 MPa (0.2%PS, UTS) in the double tempered condition is believed to be the highest value reported for MIM M2.
Deflection tests As in other studies, the MA variant shows greater resilience in the cantilever/suspension test than the PAs and this may be advantageous for components with overhang features where full density is also important. Drape tests (Fig. 5) also reveal that at any given temperature, the MA-based materials show less distortion than PA. This is probably related to the presence of weak, low melting point grain boundary phases which will appear at an earlier stage for PA M2 and will be last to form in the MAs.
The present study shows that sintering M2 in nitrogen at 1220– 1250 8C is effective in achieving virtually full part density for 90% and 80%-22 mm PA powders. Higher temperatures are needed to get full density in parts made via a MA route: a 90%-22 mm grade enables full densification at 1220 8C while the 80%-22 mm requires a sintering temperature of 1240 8C to attain full density. The densification behavior can be rationalized with reference to Thermocalc studies which also help explain final microstructures which, in the case of the MA variants contain coarse carbide clusters at the location of original MA particle sites. There has been insufficient time for effective redistribution of heavy elements like Mo, W and therefore inhomogeneous microstructures with coarse carbides are observed. Carbon loss during the sintering process is predictable for gas atomized powders with and without blended CIP and is dependent only on the amount of oxygen present in the starting powder(s) and not apparently on the sintering temperature in the normal sintering range 1220–1250 8C. Hardness and strength levels achieved in the as sintered and heat treated conditions compare very favorably with reference industry values with 67HRC being achieved in the heat treated condition (double tempered at 550 8C) compared with 60-65HRC quoted in the literature. 205
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In common with previous studies on other MA systems, part distortion appears more pronounced in parts made with PA powders compared with MA powders. This may be one reason to consider a MA route albeit a higher sintering temperature will be needed to achieve full density. It is concluded that gas atomized M2 PA can give fully dense components with high strength and hardness in the as sintered and particularly the tempered condition. The MA approach can give similar properties at higher sintering temperatures, but redistribution of carbide forming elements is incomplete and microstructures are inhomogeneous. While MA can give useful resistance to part distortion, it would be prudent to use MA only as a component of an overall feedstock formulation if distortion control is of overriding importance.
Acknowledgements Grateful thanks are due to Ms Linn Larsson of Sandvik Materials Technology for detailed metallographic analyses and to Anders Willson and Peter Harlin, also of Sandvik Materials Technology, for Thermocalc studies. Our appreciation also goes to Mr Chris Phillips of Sandvik Osprey for deflection measurements.
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