INFLUENCE OF CHEMICAL COMPOSITION AND AUSTENITIZING TEMPERATURE ON HARDENABILITY OF PM STEELS
Peter K. Sokolowski and Bruce A. Lindsley
Hoeganaes Corporation Cinnaminson, NJ 08077
ABSTRACT The hardenability of powder metallurgy (PM) steels is an important measure of how well certain alloy systems can be used for sinter hardening. Several options are now available for sinter-hardening applications as new alloys have been developed over the last few years. Alloy composition has been optimized to take advantage of rapid cooling in sinter-hardening furnaces while addressing the cost of alloying elements. One of the most widely used tests for hardenability is the Jominy end-quench, where samples are heated into the austenite range and water quenched on one end of the sample, producing a wide range in cooling rate within one sample. The hardenability of different alloy systems was examined by way of Jominy tests and sintering studies. Austenitizing temperature has an important effect on the measured Jominy hardenability of higher molybdenum containing steels. Selection of the austenitizing temperature for these alloy grades therefore has a profound effect on the predicted hardenability of different alloy systems. INTRODUCTION The benefits of sinter-hardening technology to achieve processing efficiency and promote cost-cutting methods are well understood in the industry.1 This technology has been supported over the years through advancements in alloy design and improved cooling equipment. With the application of convective cooling systems in modern sintering furnaces, accelerated cooling rates have allowed the use of a broader range of PM alloys for sinter-hardening parts. While traditional sinter-hardening alloys are capable of achieving a high level of martensitic transformation under most sintering conditions, because of their high level of alloying, leaner alloy systems have been developed to provide a similar metallurgical response. These leaner alloyed systems however, require rapid cooling conditions to attain similar microstructural transformations and hence comparable mechanical properties. In order to aid PM parts-producers in the selection of suitable alloys for potential sinter-hardening applications with current sintering furnaces, an
in-depth study has been undertaken to evaluate the hardenability in a range of ferrous PM alloys available to the market. Hardenability is generally accepted as a qualitative measure describing the ease and depth to which an alloy is able to transform to martensite upon cooling from an austenitizing temperature. The hardenability of iron alloys has been exhaustively studied over the years, with the majority of the work performed on wrought alloys.2, 3 This body of literature was brought about through the inception of notable test methods, namely the Grossman and Jominy end-quench tests, to determine the degree to which a material will harden. These proven tests can provide a sound baseline indication of what to expect from a given PM alloy in a sinter-hardening route or through standard heat-treat practices. In PM, alloy hardenability depth is reported as the point at which the apparent hardness value drops below 65 HRA, referred to as the J Depth, and is given in 1/16 of an inch.4 It is well known that the measured apparent hardness of PM compacts is influenced by porosity. Work has shown that PM materials will exhibit an enhanced hardenability based on the apparent hardness measurement at increased densities.5, 6 This is also apparent in the provided J Depth values in MPIF Standard 35, where values are stated in relation to density. The caveat to reaching high densities of course lies in the fact that considerable levels of alloying are generally needed to achieve appreciable hardenability and increase mechanical properties in conventional PM materials. This typically leads to a reduction in compressibility and hence a limitation on sintered density. Yet one approach to circumvent this effect over the years has been through alloying with molybdenum, which has negligible influence on compressibility. Molybdenum is an attractive alloying element for many reasons. Its introduction in ferrous alloys, even in small amounts, leads to enhanced mechanical properties and markedly improves hardenability.7, 8 Additionally, its ease of processing has lead to its integration industry-wide as a popular alloying additive to provide high performance PM materials. While density is a contributing factor in measured PM hardenability, the greatest metallurgical variables that influence alloy hardenability include prior austenite grain size, composition, and chemical homogeneity. Hardenability will increase as the austenite grain size increases due to a reduced grain boundary area. Grain boundaries serve as nucleation sites for ferrite and pearlite and as such ultimately reduce the effective volume capable of transforming to martensite.9 Furthermore, the amount and type of elemental alloying can significantly suppress ferrite and pearlite transformations. The critical cooling rate, as determined from a continuous-cooling transformation (CCT) diagram, can be modified as the nose of the CCT curve moves to the right as a result of this alloying behavior. Increased alloying permits a slower cooling rate to provide a martensitic transformation in the material. Certain alloying elements, in particular Mo, Mn, Cr, and Ni, have a greater ability to influence these curves and are thus favorable alloying elements in steels. In alloys that contain both Mo and Ni, a synergistic effect is seen between the two elements increasing their effect on hardenability when Ni is greater than 0.75 wt%. The method of alloying for PM materials greatly influences the hardenability and perhaps more importantly the performance characteristics of the material.1 Whether its through admixing, diffusion alloying, prealloying, or a combination thereof, the chemical homogeneity is modified as a result of the alloying method. Ideally, a completely homogeneous microstructure would better indicate the theoretical or calculated hardenability based on alloy constituent and amount. Generally speaking, a prealloyed material will have a homogeneous microstructure, depending on cooling rate, which fully demonstrates the effect of alloying elements being in solid solution in austenite. Of course, if the austenitizing temperature is below the austenite, , single phase field, some alloying elements will remain in carbide form in high carbon alloys, effectively reducing alloy content in the matrix. In view of this effect, this paper discusses the hardenability of commercially available ferrous PM alloys and the influence austenitizing temperature has on measured J Depth values.
PROCEDURE Several commercially available prealloyed steel powders, which are known to exhibit greater hardenability than admixed copper or nickel steels, were selected for this study, Table I. The powders were produced by water atomization, with a typical particle size distribution, having the alloying elements prealloyed in the melt prior to atomization. Each premix was prepared with 0.75 wt% EBS wax (Acrawax C) as the lubricant and varying amounts of Asbury type 3203H graphite. Admixed copper was used to produce alloys with 1 wt% and 2 wt% Cu. All weight percent will be designated as % hereafter.
Table I: Nominal composition of base prealloys used in current study (in %). Base Iron MPIF Designation Mo Ni Mn Fe
Ancorsteel 2000 FL-4200 0.6 0.5 0.25 Bal. Ancorsteel 4600V FL-4600 0.55 1.8 0.15 Bal. Ancorsteel 721 SH - 0.9 0.5 0.4 Bal. Ancorsteel 737 SH FL-4800 1.25 1.4 0.4 Bal. Ancorsteel 150 HP FL-4900 1.5 - 0.1 Bal.
Large compacts of each mix were pressed to 7.0 g/cm3, courtesy Powder-Tech Associates. Blanks, cut from each compact, were pre-sintered at 870 C (1600 F) to provide enough strength for initial machining into oversized cylindrical test bars. The bars were then sintered in 90/10 (vol%) nitrogen/hydrogen atmosphere at 1120 C (2050 F) for 15 minutes at temperature in a continuous-belt furnace. Finally, the sintered bars were sized to the specified 100 mm (4 inch) length by 25 mm (1 inch) diameter, to account for any difference in dimensional change as a result of the sintering process. Hardenability was evaluated using the Jominy end-quench method following ASTM Standard A 255 and MPIF Standard 65.10,11 Samples were austenitized at 900 C (1650 F) for 30 minutes at temperature in 90/10 (vol%) nitrogen/hydrogen atmosphere prior to water end-quenching. In addition, bars of multiple compositions were evaluated at 845 C (1550 F) and 950 C (1750 F) in order to assess hardenability as a function of austenitizing temperature. Metallographic samples, encompassing the length of a Jominy bar, were prepared by grinding and polishing using standard practices and etched with 2% nital / 4% picral for optical microscopy examination. Phase analysis was performed using a point count method at locations coinciding with that of thermocouple placement to link the microstructure with measured cooling rate. An instrumented Jominy method, as described elsewhere,5 was used to determine cooling rates along the length of a Jominy bar. Type-K thermocouples were inserted to a depth of approximately 3 mm (1/8 inch) from the surface at locations of 10 mm (6/16 inch), 25 mm (15/16 inch), 45 mm (28/16 inch), and 85 mm (54/16 inch) from the quenched end to measure the range in cooling rates. The average cooling rate, as reported, is measured in the sample between 650 C (1200 F) and 315 C (600 F). In order to determine the effect of grain size on hardenability as a result of increased austenitizing temperature, grain size measurements were conducted following ASTM E 112.12 The Abrams Three-Circle intercept method was applied to ten fields of analysis to ensure a statistically