Elastic limits and microplastic response in ultrahigh strength carbon steels
- PDF / 420,769 Bytes
- 4 Pages / 590.28 x 785 pts Page_size
- 90 Downloads / 239 Views
Elastic Limits and Microplastic Response in Ultrahigh Strength Carbon Steels M.A. ZACCONE and G. KRAUSS Medium carbon steels quenched to form martensite and tempered at or below 200 ~ have very high hardness and tensile strengths, ranging between HRC50 and 60, and 1660 MPa to 2260 MPa, respectively, m Despite the high strengths of this class of materials, the elastic limits tend to be quite low. The latter behavior was documented by Muir e t a l . ]21 in a series of plain C steels containing from 0.2 to 0.8 pct C. They showed that the elastic limit was very low (138 to 206 MPa) in as-quenched specimens, that the elastic limit increased with increasing tempering temperature to a maximum (690 to 896 MPa) after tempering in the range of 260 to 370 ~ and that for a given tempering treatment the elastic limit stayed constant or decreased with increasing steel carbon content. Recent work on 4130, 4140, and 4150 steels confirms that the elastic limit of hardened specimens tempered at 200 ~ decreases with increasing carbon concentration, tll Muir e t a l . i2j attribute the low elastic limits of hardened steels to residual quenching and transformation stresses. Other explanations for low elastic limits include stressassisted transformation of retained austenite to martensite TM and the high mobility of dislocations in as-quenched martensite. I41 The latter explanations were based on experiments conducted on Fe-30 pct Ni and Fe-Ni (19 to 29 pct)-C (0.02
M.A. ZACCONE, formerly Research Assistant, Colorado School of Mines, is Principal Development Engineer, Rockwell International, Golden, CO 80401. G. KRAUSS is AMAX Foundation Professor and Director, Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted March 16, 1988. 188
VOLUME20A, JANUARY 1989
to 0.57 pct) alloys with subzero Ms temperatures, and thus apply to microstructures quite different from those produced in low alloy medium-carbon steels hardened by quench and tempering. This note presents the results of experiments performed to evaluate the microplastic deformation behavior and the dependence of elastic limits on carbon content of a series of hardened alloy steels. The specimens were tempered at 200 ~ a treatment which effectively reduces residual stresses, ~SJpins the dislocation substructures of the martensite by the precipitation of very fine transition carbides, m retains interlath retained austenite, t61 and substantially increases toughness relative to as-quenched martensitic structures. [7] The compositions and heat treatment of the steels examined are listed in Table I, and the austenitic grain size, hardness, and the retained austenite content for each steel are presented in Table II. Austenitizing temperatures were selected to produce uniform austenite grain sizes, and all specimens were tempered at 200 ~ The 4180 steel is an experimental steel with higher Ni and Mo than a standard 41XX grade, and the 4180 designation is therefore u
Data Loading...
