Effect of tempering on quasi-static and impact fracture toughness and mechanical properties for 5140 H steel
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I.
INTRODUCTION
IT is
well known that alloy steel 5140 H is used after thermal treatment for highly to moderately stressed machine parts. Consequently, it is of great interest for practical applications to verify relations between such mechanical parameters as yield stress, tensile strength, reduction in area, hardness, etc., from one side, and fracture toughness from the other. All those parameters should be referred to a specific thermal treatment like quenching and subsequent tempering. The primary purpose of tempering is to impart plasticity or toughness to the steel, and the loss in strength is only incidental to this very important increase of toughness. Special interest is focused on comparison of quasi-static and impact fracture toughness of 5140 H steel after quenching and different tempering. The effect of thermal treatment on the mechanical properties together with quasi-static and impact fracture toughness of this steel is rarely examined. The second purpose of this paper is to correlate parameters obtainable from ordinary tensile tests using round bar specimen with fracture toughness, defined as the critical stress intensity factor in the opening mode, i.e., K~c. The SEM fractography of broken specimen after quasistatic and impact tests is usually very helpful in the interpretation of fracture behavior. Thus, a complete picture can be obtained concerning the effect of tempering temperature on fracture and strength characteristics of 5140 H steel.
II.
MATERIAL, SPECIMEN PREPARATION, AND EXPERIMENTAL TECHNIQUES
All specimens were cut from the plate in a direction perpendicular to the rolling direction. Chemical comJ.R. KLEPACZKO is Visiting Professor at the University of Nantes, School of Mechanics, 44072 Nantes Cedex, France, on leave of absence from Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland. A. SOLECKI is Assistant Professor at Lodz Polytechnics, Bielsko-Biala, Poland. Manuscript submitted December 16, 1982. METALLURGICALTRANSACTIONS A
position of the specimen material is given in Table I. This composition exactly fits the U.S. standard for the chromium steel 5140 H, ASTM A322-64, or AISI-SAE. The suffix H is used to distinguish specific hardenability requirements (SAE Standard J 407 d). In Table I are given limits for the alloying elements complying with the U.S. Standards for 5140 H steel. Three types of specimen were prepared, including ordinary tensile specimen with diameter D = 15.6 mm and gage length l0 = 80 mm. Notched round tensile specimen (NRT) in the quasi-static version with diameter D = 15.6 mm, length of cylindrical portion 10 = 80 mm, notch diameter d = 11.7 mm, the V-notch with ~ = 60 deg, and the radius of the notch tip p = 0.1 mm. The third version, used during impact fracture toughness testing, was a reduced version of the notched round tensile specimen. Its dimensions were as follows: D = 12 mm, d = 9.6 mm, = 45 deg, p = 0.1 mm. This change in size was crucial in a substantial reduction of vibrating masses and consequently "ri
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