Strain Rate Effects and Temperature History Effects for Three Different Tempers of 4340 VAR (Vacuum Arc Remelting) Steel

摘要

A series of experiments is described in which specimens of AISI 4340 VAR steel are deformed in shear at temperatures ranging from -190 C to 20 C. The tests were performed in a torsional Kolsky (split-Hopkinson) bar at quasistatic and dynamic strain rates. Before testing, all the specimens were normalized, austenitized and tempered to hardnesses of 55, 44, or 33, on the Rockwell C scale. In addition to constant temperature and constant strain rate tests, a number of experiments were performed to study strain rate and temperature history effects in these three tempers. For this purpose a prestrain was imposed at one temperature and strain rate, followed by continued straining at a new temperature or at a new strain rate. Results provide stress-strain curves in shear for the three tempers of this steel. Temperature effects appear greater between -190 C and -50 C than between -50 C and room temperature, particularly for the 200 C temper, while the strain rate sensitivity is about the same as found in mild steel. History effects are quite small for the 600 C and 425 C tempers, even at large strains. However, for the 200 C temper a prestrain at -50 C followed by a temperature change to -190 C requires a higher flow stress than does deformation imposed entirely at the lower temperature. Anomalous history effects of this nature have been seen before in steels, although this is the first evidence for their association with a particular temper.