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  • EFFECT OF HEATING RATE AND MICROSTRUCTURAL SCALE ON AUSTENITIZATION

    1Kester D. Clarke, 1Amy J. Clarke, 1Robert E. Hackenberg, 1Chastity J. Vigil, 2Chester J. Van Tyne

    1Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87544 USA 2Colorado School of Mines, 1500 Illinois St., Golden, CO 80401 USA

    Keywords: Induction heating, Austenitization, Steel

    Abstract

    The effect of heating rate and prior microstructure on austenitization kinetics has been evaluated for induction hardenable steels with ferrite-pearlite, quench and tempered martensite, or ferrite-spheroidized carbide initial microstructures. Microstructural scale was varied by initial heat-treatment and dilatometry was used to assess austenitization as a function of heating rate and maximum temperature. As-quenched hardness was determined as a function of heating rate and maximum temperature for each microstructural condition. The initial microstructural scale and alloy composition are shown to cause significant variation in the as-quenched microstructure and material properties, particularly for the highest heating rates. Diffusion simulations support the observed microstructural and material property differences between the various initial microstructures and low-alloy and Cr-alloyed steel compositions.

    Introduction

    After solidification and homogenization, austenitization is a critical thermal cycle influencing microstructure, and therefore performance. Austenitization is particularly important for induction hardening applications. Differences in initial microstructure have been shown to result in significant variations in final microstructure [1-4] after hardening. We have recently examined the effects of microstructure and heating rate in three induction-hardenable steels [5-10], with the goal of understanding the effect of alloy content and microstructure on properties. Here, we summarize the overall effects of heating rate on low-alloy and Cr-alloyed medium carbon steels, and show results from a hypereutectoid alloy with significant Cr content.

    Materials and Experimental Procedures

    Three commercial steel alloys were selected as the experimental materials: 1045, 5150, and 52100. Compositions and equilibrium A3 (or Ac for the hypereutectoid 52100 composition) temperatures, calculated with ThermoCalc, are presented in Table I. The as-received condition for 1045 and 5150 was hot-rolled, and the 52100 was fine spheroidized. For each alloy, half of the as-received material was further heat-treated to produce additional initial conditions: normalized 1045, quench and tempered 5150, and coarse spheroidized 52100. The goal of the subsequent heat-treatments was to produce microstructures with different length-scales than the as-received material and to evaluate the effect of microstructural scale using material with the same composition. Thus, a total of six initial microstructural conditions were examined to characterize the austenitization and austenite homogenization response as a function of heating rate. Starting microstructures for each condition are presented in Fig 1.

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    Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015Edited by: Matthias Militzer, Gianluigi Botton, Long-Qing Chen, James Howe, Chadwick Sinclair, and Hatem Zurob

  • Table I Composition of experimental alloys (wt. pct.) Alloy C Mn Cr P S Si Ni Mo Cu V Al A3/c,C 1045 0.46 0.75 0.06 0.007 0.020 0.25 0.11 0.02 0.25 0.001 0.033 760 5150 0.52 0.85 0.80 0.011 0.020 0.26 0.10 0.03 0.25 0.002 0.028 744 52100 1.04 0.34 1.44 0.016 0.012 0.26 0.11 0.04 0.24 0.008 0.023 902

    (a) (c) (e)

    (b) (d) (f)

    Figure 1. Starting microstructural conditions: (a) 1045 hot-rolled, (b) 1045 normalized, (c) 5150 hot-rolled, (d) 5150 quench and tempered, (e) 52100 fine spheroidized, (f) 52100 coarse spheroidized. Optical micrographs, nital etch. Induction dilatometry was used to simulate continuous induction heat-treatments and to evaluate the material response both on-heating and on-cooling. Dilatometry was performed using an MMC quenching and deformation dilatometer with solid cylindrical samples nominally 10 mm in length and 3 mm in diameter with flat and parallel ends. Heat-treatments consisted of heating at an approximately constant heating rate to a maximum temperature. Experimental heating rates used were nominally 0.3, 3.0, 30, and 300 C/s. For the 0.3 and 3.0 C/s heating rates, in order to reduce the overall testing time, samples were heated at a rate of approximately 30 C/s to 700 C, followed by heating at the experimental heating rates. Upon reaching the maximum temperature, the samples were immediately quenched using helium gas. The helium gas quench achieved an 800 to 500 C cooling time of ~2.4 s, which translates to an average cooling rate of approximately 125 C/s. The microstructures of the as-received and experimentally heat-treated samples were evaluated using standard metallographic sampling and preparation techniques. The experimentally heat-treated dilatometer cylinders were mounted in epoxy with the axis of the cylinder parallel to the polish plane. The samples were then ground to the axial center plane of the cylinders. Metallographic preparation was performed by rotary grinding with successively finer (60 grit to 800 grit) papers. Ground samples were then polished through 6, 1, and 0.25 m diamond polishing paste on metallographic preparation cloths. The initial microstructural scale was evaluated by optical image analysis, with details presented in Table II.

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  • Table II Starting Condition Microstructural Measurements Microstructural Feature 1045 HR 1045 N 5150 HR 5150 QT 52100 F 52100 C Pearlite lamellar spacing, m 0.39 0.36 0.21 Pearlite colony size, m 13.6 8.4 7.2 Ferrite grain size, m 6.7 6.9 2.4 Ferrite volume fraction 0.31 0.41 0.10 Carbide size, m 0.12 0.44 0.70 Carbide volume fraction 0.08 0.25 0.24 Knoop (500 g) microhardness indentations were used to determine the bulk hardness of the experimentally heat-treated samples after metallographic preparation. A minimum of five indentations were placed along the axis of each metallographically prepared dilatometer sample, oriented with the long axis of the indentation perpendicular to the axis of the cylinder. Knoop microhardness indentations were performed on at least two identically heat-treated cylinders for each material and heat-treatment condition. Reported microhardness results are an average of all indentations performed on all samples tested for a given heat-treat condition, with error bars indicating the span of the data. Rockwell C values are reported, which have been converted from Knoop 500 g microhardness values using ASTM standard E140. Thermo-calc and DICTRATM software was used to simulate the microstructure and composition development as a function of maximum temperature and heating rate. Simulation development was based on previous work [11,12] for similar initial microstructures for continuous heating and isothermal heat-treatments. The goal of the diffusion simulations was to evaluate the austenitization and austenite homogenization kinetics, and to provide further understanding of the austenite composition development as a function of heating rate. Two simulation approaches were used, including one for the spheroidized carbide + ferrite microstructure and the quench and tempered microstructure (52100 fine and coarse spheroidized, and 5150 quench and tempered), and another for the ferrite + pearlite microstructures (5150 hot-rolled, and 1045 hot-rolled and 1045 normalized). These approaches are discussed in detail in [5], and results from only the ferrite + pearlite simulations are presented here. The compositions evaluated for all cases are for representative values of iron, carbon, chromium, and manganese.

    Results and Discussion The material response to heat-treatment was measured by as-quenched hardness, as shown in Figure 2. For all continuous heat-treatments, higher heating rates require higher maximum temperatures to reach maximum alloy hardness, which was found to be dependent on alloy carbon level but independent of prior microstructure. With respect to microstructural scale, finer starting microstructures (i.e., 1045 normalized, 5150 quench and tempered, and 52100 fine spheroidized) generally required lower maximum temperatures to reach a given hardness. For example, 1045 hot-rolled material required heating to 875C, even at the slowest heating rate, 0.3C/s, whereas the 1045 normalized material reached maximum hardness with a peak temperature of 825C, and was within 1 HRC of maximum with a peak temperature of 775C. For these types of continuous heat-treatments, this represents a significant difference in thermal cycle required to reach maximum hardness, which increases with increasing heating rates. The 5150 material has higher hardenability, finer starting microstructures, and significantly less ferrite in the starting hot-rolled material than the 1045 material, and thus reached hardnesses above 57 HRC even at the lowest maximum temperatures tested (750C, which is just above the

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  • calculated A3, Table I). Hardness variation between the hot-rolled and quench and tempered starting microstructures were also much less than for the coarser 1045 starting microstructures. The largest hardness differences are found for the highest heating rates. The hypereutectoid 52100 material also shows some difference in hardness as a function of starting microstructure at heating rates of 30/s or more up to approximately 900C. With increasing maximum temperature, the hardness reaches a maximum of roughly 65 HRC, which corresponds to a martensite carbon content of approximately 0.8 wt. pct. [13]. At heat-treatment temperatures above the peak hardness, the hard

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