Effect of Heat Treatment and Case Carburizing High-Density P/M Steels

Recent advancements in high-density lubricants enable P/M steels to be processed to densities approaching 7.40 g/cm in a single compaction step using heated compaction tools via the AncorMax D process. This broadens the number of suitable applications for P/M steels, including high performance gears. However, in addition to high-density, the microstructural and mechanical property requirements vary depending on the application. This objective of this paper is to quantify and understand the mechanical property differences obtained by subjecting high-density P/M steels to various heat-treatments. Mechanical properties, fatigue properties, and microstructural analysis will be presented in the as sintered, quenched and tempered, and carburized states. INTRODUCTION The introduction of high density compaction technology, known as the AncorMax D process enables P/M part producers to achieve up to 98% pore-free densities in green parts [1]. This premix technology features a proprietary lubricant/binder system requires a high compaction pressure of 550 – 823 MPa (40 – 60 tons/in). Die temperatures must be maintained in the range 60 – 70 °C (140 – 150 °F) and this is achieved using cartridge heaters incorporated within the die. Warm die compaction, coupled with advanced lubricant/binder systems has been shown to increase green densities without the need to heat the powder [2]. Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures generally between 850 and 950 °C (1560 and 1740 °F), at which austenite, with its high solubility for carbon, is the stable crystal structure. Hardening is accomplished when the high-carbon surface layer is quenched to form martensite so that a high-carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core. [3] Case depth of carburized steel is a function of carburizing time and the available carbon potential at the surface. [4] When prolonged carburizing times are used for deep case depths, a high carbon potential produces a high surface-carbon content, which may result in excessive retained austenite or free carbides. These two microstructural elements both have adverse effects on the distribution of residual stress in the case-hardened part. Consequently, a high carbon potential may be suitable for short carburizing times but not for prolonged carburizing. The purpose of this paper will be to investigate the performance levels achievable of FLN2 – 4405 by using conventional sintering, and secondary heat-treatments such as quench and tempered, and case carburizing. ® AncorMax D is a registered trademark of Hoeganaes Corporation EXPERIMENTAL PROCEDURE The press-ready, binder-treated material was tested in a production environment to evaluate its properties through a single press/single sinter, quench and tempered, and the case carburized condition. The base powder Ancorsteel 85HP is water atomized and pre-alloyed with 0.85 w/o molybdenum. The base iron is premixed with 2.0 w/o nickel and 0.40 w/o graphite. The nickel used was INCO 123 and the graphite was Asbury 3203H. The compositions of the test materials that were evaluated in this study are listed in Table I. Table I. Premix Compositions for the Material Studied Base Powder Mo Nickel Graphite lube Bal. w/o w/o w/o organics w/o Ancorsteel 85HP 0.85 2.0 0.40 0.55 TEST SPECIMEN / COMPACTION AND SINTERING Transverse rupture, tensile dog-bone, impact samples were compacted at pressures varying from 415 to 760 MPa, (30 to 60 tsi) (Fatigue samples were compacted to a density of 7.20 g/cm) compaction dies were heated to 63 °C (145 °F). Green density, sintered density, and transverse rupture strength was determined from the average of five compacted transverse rupture (TRS) specimens (ASTM B-528). Tensile strength, yield strength, and maximum elongation were obtained from the average of five dog-bone tensile samples (ASTM E-8). Impact energy was determined from the average of five un-notched Charpy Impact bars (ASTM E-23). Apparent hardness measurements were performed on the surface of the TRS bars using a Rockwell hardness tester. All measurements were conducted using the HRA scale for ease of comparison. All test pieces were sintered under production conditions in an Abbott continuous belt high temperature furnace at the Hoeganaes R&D facility, in Cinnaminson, NJ. The sintering condition used for the test specimen is listed below. Sintering Temperature: 1120 °C (2050 °F) Atmosphere: 90 v/o N2 10 v/o H2 Time in Hot Zone: 20 minutes For the samples that were carburized, the parameters are listed below. Temperature: 925 °C (1700 °F) Time at Temperature 240 minutes Quench: Pressure / Nitrogen Tempering: 205 °C (400 °F) for 1 hour For the samples that were quenched and tempered, the parameters are listed below. Temperature: 900 °C (1650 °F) Atmosphere: 75 v/o H2 – 25 v/o N2 Time at Temperature: 50 minutes Oil Quench: 75 °C (165 °F) Tempering: 205 °C (400 °F) for 1 hour Tensile testing was performed on a 267,000 N (60,000 lb.) Tinius Olsen universal testing machine with a cross-head speed of 0.635 mm/min (0.025 in/min). Elongation values were determined by utilizing an extensometer with a range of 0 20%. The extensometer was attached to the samples up to failure. Rotating bending fatigue samples were pressed to a density of 7.20 g/cm, and machined from blanks. The as-sintered fatigue samples were machined and ground to size after the sintering operation. The heat-treated fatigue samples were rough machined following sintering then heat treated, finished ground, and polished to size. The dimensions of the specimen used for this analysis, along with allowable dimensional tolerances, are shown in Figure 1. Fatigue testing was performed on six randomly selected Fatigue Dynamics RBF-200 machines at a rotational speed of 8000 rpm. These rotating bending machines are of the mechanical and nonresonant type and are an efficient means of inducing fatigue in a specimen of round cross section. [5] A staircase method was used utilizing 30 samples and a run-out limit of 10 cycles. The staircase method of testing was regulated so that there were both failures and run-outs at a minimum of two stress levels. [6] The percentage of failures for each stress level was calculated and plotted on a log-normal graph. From these plots, the fatigue endurance limit (FEL) at 50% and 90% was determined by linear extrapolation. The 50% FEL represents the stress level where 50% of the specimens will break and 50% will run-out. The 90% FEL represents the stress level where 90% of the specimens will "run-out" and 10% will break. Figure 1 Dimensions of Rotating Bending Fatigue Specimen RESULTS AND DISCUSSION Tensile strength for the “as-sintered”, carburized, and quench & tempered conditions are summarized in Tables II through IV. These tables also include information of green and sintered density, apparent hardness, and impact energy. Fatigue properties are listed in Table V. In order to establish a frame of reference for the lubricant/binder processing technology, the compressibility of FLN2 – 4405 was compared to a conventionally pressed premix using Acrawax C as the lubricant pressed at room temperature. A compressibility plot for this material is shown in figure 2. With the AncorMax D process, higher green densities were achieved. Green densities ranged from 6.96 – 7.45 g/cm for the alloy examined. With the advanced lubricant/binder system compared to a standard compaction process a difference of 0.14 g/cm was observed at 415 MPa. (30 tsi). With increasing compaction pressures the difference increased to 0.22 g/cm at a pressure of 825 MPa (60 tsi). This indicates that the AncorMax D process is a more effective process for densification at higher compaction pressures. Table II. As Sintered Properties Compaction Green Sintered Apparent 0.20% Impact Pressure Density Density Hardness TRS UTS Offset Elong Energy (MPa/tsi) (g/cm) (g/cm) HRA (MPa/10 psi) (MPa/10 psi) (MPa/10 psi) % (J/ft*lbf) 415/30 6.96 6.98 46 1028/147 501/72 357/51 1.8 10/7 550/40 7.21 7.22 48 1308/187 613/88 412/59 2.7 17/13 690/50 7.38 7.39 5