The Effect of Induction Hardening on the Mechanical Properties of Steel With Controlled Prior Microstructures

摘要

The effects of induction processing on the microstructures and mechanical properties of SAE 1541 and 4140 steel with controlled prior microstructures were evaluated. Cylindrical test samples were laboratory-processed to produce starting materials with microstructures that are representative of a wide variety of common materials available for use in induction hardened components. The microstructural matrix included ferrite plus pearlite produced from materials with different prior austenite grain sizes, bainite produced at different transformation temperatures, spheroidized carbides, and martensite, both in the as-quenched and tempered conditions. All samples were induction processed with the same induction processing parameters (250 kHz frequency, 12.5 kW power input, and a single-turn inductor coil with an integrated quench ring) followed by a conventional low-temperature temper. The response to induction hardening had a sensitive dependence on prior microstructure for both alloys. More uniform case microstructures, increased case depths, and higher surface hardnesses were obtained with prior microstructures containing constituents that resulted in more uniform carbon distributions upon heating. That is, full induction austenitizing was promoted by finer microstructures, reduced alloying (i.e. chromium) levels, and cold-drawing deformation. The bulk mechanical properties evaluated by three-point bend testing were shown primarily to reflect the resulting hardness, i.e. strength, profiles in the bend beam bars. The results suggest that control of prior microstructure is required in order to optimize new bar steel grades and/or induction processing parameters.