Ion nitriding behavior of several low alloy steels

Abstract

The ion nitriding behavior of several low alloy steels, U.S. Steel T1 type A, AISI 4140, AISI 6150 and Nitralloy 135M, has been examined under varying process conditions using microhardness-depth correlations, friction and wear measurements, optical microscopy and transmission electron microscopy. The process variables included time (2–48 h) and temperature (400-300 °C). All four steels exhibited a diffusion layer case growth rate which was dependent on the square root of the exposure time. The highest nitrided hardnesses and lowest case depths were observed in the Nitralloy 135M steel while the lowest hardnesses and highest case depths were seen in the T1 steel. In general the behaviors of the T1, AISI 4140 and AISI 6150 steels were similar. The white layer thickness was independent of the material and increased with increasing treatment time. Both the case depth and the white layer thickness were found to increase strongly with temperature while the hardness decreased. Observations made by optical microscopy showed that only slight differences arise in the bulk structure during nitriding. The results of the electron microscope investigation show that nitriding produces platelet or disc precipitates approximately 5–20 nm across on {001} matrix planes. These platelets show a striated morphology which is thought to be the result of strain within the matrix. Electron microscopy has also shown tempering to continue in the core region during nitriding. The wear resistance of the steels was observed to increase by factors ranging from 3.7 to 8.5 after nitriding. The coefficient of friction was seen to increase in the nitrided samples, apparently because of the constraints of the test.