The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel
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INTRODUCTION
NEAR-threshold fatigue crack behavior for long throughcracks has been studied for many different materials, t~-41 This regime, encompassing crack growth rates from 10-8 to 10-~~ m/cycle, is usually where interesting effects of intermittent crack growth, t5'6'71 mixed mode deformation fields around crack tips, tS-Hj crack closure caused by different mechanisms, I9,~2-~6J and microstructurally sensitive crack growth t2'17'~81 occur. While there have been a number of studies in this area, the literature is sparse in the nearthreshold crack growth characteristics of quenched and tempered steels, specifically, on the effects of microstructure on fatigue crack behavior in this regime. In this work, the microstructure of quenched and tempered 4140 steel is varied by tempering at four different temperatures. Near-threshold fatigue crack growth characteristics and threshold values are documented. The effects of crack closure are emphasized along with microstructural interactions.
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common in conventionally processed steel. The resulting microstructure is very "clean" and relatively inclusion-free (Figure 1). Compact tension samples (63.5 • 61 • 6.35 mm) were used for all fatigue crack tests. The samples were machined so that the crack plane was perpendicular to the rolling di-
EXPERIMENTAL
The 4140 steel used in the fatigue tests was a high quality, high purity grade manufactured by the Lukens Steel Company, Coatesville, Pennsylvania. The composition is shown in Table I. The heat was vacuum degassed and calcium treated prior to casting to remove most impurities, especially phosphorus and sulfur. The calcium treatment eliminates the large manganese sulfide stringer inclusions B. LONDON, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA, is Research Scientist, McDonnell Douglas Research Laboratories, St. Louis, MO, 63166. D.V. NELSON is Associate Professor, Department of Mechanical Engineering, Stanford University, Stanford, CA, 94305. J.C. SHYNE, formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA, is with Failure Analysis Associates, Palo Alto, CA. Manuscript submitted August 3, 1987. METALLURGICALTRANSACTIONSA
Fig. 1--Unetched microstructure of the 4140 steel plate used for the fatigue experiments. VOLUME 19A, OCTOBER 1988--2497
Table I.
Element Weight pct
C 0.38
Mn 0.86
4140 Steel Composition
P 0.007
rection and parallel to the "end" face in Figure 1. All machining was done prior to heat treatment. Heat treatment of the compact tension samples involved austenitization at 850 ~ for 1.5 hours, oil quenching to room temperature, tempering at the appropriate temperature for 1.5 hours, and air cooling to room temperature. Tempering temperatures of 200, 400, 550, and 700 ~ were used. The samples were packed in charcoal chips while in the furnace to minimize surface decarburization. The resulting microstructures ranged from a very lightly tempered martensite for the 200 ~ temp
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