Effect of ion bombardment on martensite formation in 1095 steel

Abstract

The effect of argon and helium ion bombardment on the resulting structure and mechanical properties of martensite in 1095 steel was studied as a function of ion dosage. Thin wire (0.005 in) and sheet (0.010 in) samples were resistively heated to the austenitic range in an inert atmosphere for homogenization, were bombarded with the gas ions at this temperature (900°C) and were then quenched by a spray of liquid nitrogen in situ to induce transformation to martensite. Optical microscopy shows a distinct structural difference between the quenched and the ion-implanted-and-quenched martensitic structures attained. Scanning electron microscope analysis on the cross section of the sheet samples indicate that the ion-implanted martensitic structure is uniform throughout the thickness while both field ion microscopy and X-ray diffraction analyses reveal a much finer structure than that of quenched martensite. X-ray diffraction analysis also indicates that 4%–5% retained austenite is present in the final martensitic structures after ion implantation, in a similar amount to that found in the quenched martensite samples. Within the range of ion energies utilized (190 – 240 eV for helium and 200 – 225 eV for argon) the hardness of martensite is increased from 945 kgf mm −2 for the direct-quenched samples to 976–1125 kgf mm −2 for argon-ion implanted martensite and to 960–1095 kgf mm −2 for helium-ion implanted martensite samples. Similarly, tensile strength values are seen to increase, over the quenched value of 85.5 × 10 3 lbf in −2, to (86.8–96.0) × 10 3 lbf in −2 for argon-ion implanted samples and to (85.9 – 94.9) × 10 3 lbf in −2 for martensitic structures produced after helium ion implantation.