Strain ageing of austenitic nitrogen steels

Austenitic steels Cr15Mn17 and Cr18Ni16Mn10 alloyed by nitrogen were cold worked by rolling to a different reduction in thickness and subsequently aged at room temperature (RT) up to 2 years and at 170degreesC for 5 min. Their structure is studied using X-ray diffraction and transmission electron microscopy (TEM). Iron nitrides are observed after prolonged ageing (6 months, and more) at RT. Increase in the content of nitrogen, degree of plastic deformation or temperature of ageing assists precipitation of nitrides. In steel Cr15Mn17, the precipitation of nitrides is accompanied by the formation of the ferromagnetic alpha-phase, which is also confirmed by magnetic measurements. Steel Cr18Ni16Mn10 is rather stable to the gamma-->alpha-transformation although the occurrence of nickel redounds to precipitation of nitrides after smaller degrees of deformation. Mechanisms of ageing are discussed.