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  • Effect of Case Carburizing on Mechanical Properties And Fatigue Endurance Limits of P/M Steels

    George Fillari*, Thomas Murphy*, Igor Gabrielov**

    *Hoeganaes CorporationCinnaminson, NJ 08077

    **Borg Warner Automotive Livonia, MI 48150

    ABSTRACT

    Case carburizing has long been a basic technique for the improvement of the wear and fatigue resistance and of PM steel components. The key to the successful improvement in carburizing, however, is understanding, and interpreting the microstructure of the carburized case. The main area for growth in the PM industry is in the high performance gearing applications. The success of penetrating this area depends upon the ability to understand the key components that effect the fatigue endurance limits of PM materials. This paper will examine and illustrate how tensile properties and the fatigue endurance limits of a P/M hybrid alloy are affected by alloying additions and carburizing.

    INTRODUCTION

    Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures generally between 850 C and 950 C (1560 F 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. [1]

    Case depth of carburized steel is a function of carburizing time and the available carbon potential at the surface. [2] When prolonged carburizing times are used for deep case depths, a high carbon potential produces a high surface-carbon content, which may thus result in excessive retained austenite or free carbides. These two microstructural elements both have adverse effects on the distribution of residual stress in the casehardened part. Consequently, a high carbon potential may be suitable for short carburizing times but not for prolonged carburizing.

    In regards to fatigue properties, Low retained austenite content and fine austenitic grain sizes, which create a microstructure of finely dispersed retained austenite and tempered martensite, prevent nucleation of fatigue cracks, or retard fatigue crack initiation until very high stress levels are reached. In contrast, low-stress applications that fracture at low cycles is related to high retained austenite levels and coarse austenite grain sizes. [1]

  • TEST PROGRAM The test program was intended to investigate the performance levels achievable on tensile properties and rotating bend fatigue response of a hybrid PM steel by using a secondary heat-treatments such as case carburizing. This program was divided into three parts. 1. Investigate alloying elements such as graphite and nickel on mechanical properties. 2. To investigate the effect of carburizing times on properties. 3. To estimate the percentage of retained austenite in the carburized case. EXPERIMENTAL PROCEDURE The compositions of the test materials that were evaluated in this study are listed in Table I. The base powder was water atomized and pre-alloyed with 0.85 w/o molybdenum. The 85HP was premixed with nickel and graphite. The nickel used was INCO 123 and the graphite was Asbury 3203H. Each premix contained 0.75 w/o Lonza Acrawax C as the lubricant system.

    Table I. Premix Compositions

    Premix Base Powder Nickel Graphite Acrawax CID Bal. w/o w/o w/oA Ancorsteel 85HP 2.0 0.15 0.75B Ancorsteel 85HP 2.0 0.30 0.75C Ancorsteel 85HP 4.0 0.15 0.75D Ancorsteel 85HP 4.0 0.30 0.75

    TEST SPECIMEN / COMPACTION AND SINTERING Tensile dog-bone, impact, and fatigue samples were compacted to a density of 7.20 g/cm3. 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). Apparent hardness measurements were performed on the surface of the dog-bone tensile samples 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 CYCLE Sintering Temperature: 1150 C (2100 F) (1260 C (2300 F) for premix 3) Atmosphere: 90 v/o N2 - 10 v/o H2Time in Hot Zone: 20 minutes For the samples that were carburized, the parameters are listed below. CARBURIZING CYCLE 1 Temperature: 925 C (1700 F) vacuum furnace Time at Temperature 180 minutes, Quench: Pressure / Nitrogen CARBURIZING CYCLE 2 Temperature: 925 C (1700 F) Time at Temperature 240 minutes, Quench: Pressure / Nitrogen All samples were tempered at 204 C (400 F) in air for 1hr. prior to testing. Tensile testing were 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/cm3, and machined from blanks that were sintered at 1150 C (2100 F) under an atmosphere of 90 v/o N2 -10 v/o H2. 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 non-resonant type and are an efficient means of inducing fatigue in a specimen of round cross section. [3]. A staircase method was used utilizing 30 samples and a run-out limit of 107 cycles. The staircase method of testing was regulated so that there were both failures and run-outs at a minimum of two stress levels. [4] 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 The mechanical properties of the alloys evaluated are summarized in Tables II trough Table VI. Shown in Figure 2 and 3 are the effects of alloy content and carburizing times on ultimate tensile and yield strengths for the alloys tested. In the as sintered condition, the tensile strengths for the alloys at a sintered density of 7.20 g/cm3 are 490 MPa (71*103 psi), 559 MPa (81*103 psi), 621 MPa (90*103 psi) and 738 MPa (107*103 psi) for alloys A, B, C and D. Elongations are in the range of 3.1 5.0 %. For the samples that were carburized in the first cycle, the tensile strengths were increases between 23 70 % to 835 MPa (121*103 psi), 850 MPa (123*103 psi), 850 MPa (123*103 psi) and 910 MPa (131*103 psi), when compared to the as sintered condition. Samples that were subjected to the second carburizing cycle resulted in increases in tensile strengths between 5 20 % to 1000MPa (145*103 psi), 891 MPa (129*103 psi), 987 MPa (143*103 psi) and 959 MPa (139*103 psi) when compared to the first cycle. Increases in yield strengths also follow the same trend. High temperature sintering for alloy 3 resulted in an increase in tensile strength from 3 13 %.

    Table II. As Sintered Properties for The Alloys Tested at 1150 C (2100 F).

    Sintered 0.002Condition Material Density HRA UTS OFFSET Elong

    ID (g/cm) (MPa/103 psi) (MPa/103 psi) %1 7.24 47 490/71 352/51 4.62 7.22 50 559/81 386/56 4.33 7.25 51 621/90 407/59 5.04 7.24 54 738/107 524/76 3.1

    As Sintered

  • Table III. Tensile Properties for Samples Subjected to Both Carburizing Cycles. The Alloys Tested. Sintered at 1150 C (2100 F)

    Sintered Apparent 0.20%Condition Material Density Hardness UTS Offset Elong

    ID (g/cm3) HRA (MPa/103 psi) (MPa/103 psi) %A 7.22 73 835/121 731/106 0.8B 7.22 74 850/123 842/122 0.8C 7.26 70 850/123 607/88 1.1D 7.27 71 910/131 607/88 1.2A 7.22 73 1000/145 918/133 1.0B 7.21 73 890/129 752/109 0.9C 7.27 73 987/143 718/104 1.1D 7.26 72 959/139 676/98 1.1

    Cycle 1 Tempered @ 205 C (400 F) Cycle 2

    Tempered @ 205 C (400 F)

    Table IV. Alloy C Tensile Properties for Samples Subjected to Both Carburizing Cycles. Sintered at 1260 C (2300 F)

    Carburizing Sintered Apparent 0.20%Condition & Density Hardness UTS Offset Elong

    Tempering Temp (g/cm3) HRA (MPa/103 psi) (MPa/103 psi) %Cycle 1 @ 205 C 7.31 73 966/140 621/90 1.3Cycle 2 @ 205 C 7.30 73 1021/148 711/103 1.1

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    Alloy A Alloy B Alloy C Alloy D

    UT

    S (M

    Pa)

    AS SINT CYCLE 1 CYCLE 2

    X

    X

    Figure 2. Ultimate tensile strength as a function of alloy content and carburizing cycle. Samples sintered at 1150 C (2100 F) - X indicates the strength of alloy sintered at 1260 C (2300 F)

  • 200

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    Alloy A Alloy B Alloy C Alloy D

    Yie

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    (MPa

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    AS SINT CYCLE 1 CYCLE 2

    X

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    Figure 3. Yield strength