In-situ Observation of Nitrogen Release from High Nitrogen Steels

摘要

Nitrogen in high nitrogen steels (HNS) is supersaturated at high temperature and low nitrogen pressure, limiting its use as heat resistant steels and vacuum chamber material. Fe-18Cr-8Mn-N, Fe-15Cr-5Mn-5Ni-N, Fe-12Cr-N, Fe-20Cr-8Mn-N, Fe-19Cr-15Mn-N, containing 0.276mass% to 0.910 mass% nitrogen were prepared by a pressurized induction melting furnace. Specimens were observed with a laser microscope in a high-temperature image heating system under 1 atm pure argon flow. For the sample of Fe-12Cr-N, the solid state precipitation of a compound was observed. The nitrogen gas was found to escape from the surface of the solid Fe-12Cr-0.276N steel sample first and after melting of the steel nitrogen bubbles were observed floating out from steel melt to the surface. The nitrogen release from solid HNS was in good accordance with the previous result obtained with TG/DTA measurements. For the sample of Fe-20Cr-8Mn-0.853N, a phase transformation and a sequential growth of precipitate was observed. No nitrogen release was observed for the sample Fe-19Cr-15Mn-0.576N but the aggregation and growth of nitrogen bubbles on the liquid Fe-19Cr-15Mn-0.910N surface was observed. Also for sample Fe-19Cr-15Mn-0.576N, during the melting and solidification, inclusion movement and crystal growth were observed. From these observations it was found that the behaviors of nitrogen release was related with the nitrogen content in HNS, heating rate and the alloy composition. These observations confirm that the in-situ observation is a powerful method to study the nitrogen behavior in HNS during heating and melting.