Initial Tempering of Iron-Manganese-Nitrogen Martensite

Abstract

Changes in electrical resistivity, measured at liquid-nitrogen temperatures, have been used to study the tempering of iron- ∼ 0.7 % nitrogen martensites containing up to 4 % manganese over the temperature range 80–160°C. The retarding effect of manganese on nitrogen diffusion permitted a t ^2/3 law, characteristic of the segregation of interstitial atoms to dislocations, to be observed at the start of tempering in alloys with 0.5–2.0% manganese. This process is largely completed during the quenching and prior handling of binary martensites, which undergo α” (Fe₁₆N₂) precipitation on tempering (Stage 1) conforming initially with a t _1/2 time law. The different behaviour of a 4 % manganese alloy is attributed to strong nitrogen interaction with near-neighbour pairs of manganese atoms.