Methodenentwicklung zur Herstellung von stickstofflegierten Stählen mittels Laborschmelzen und deren thermodynamischen Charakterisierung im Hochtemperaturbereich

The control of nitrogen in steel production is of great importance and is becoming increasingly relevant. As an inconspicuous but essential component of steel, nitrogen has a significant influence on material properties. This thesis deals with the development of a method for the production of nitrogen-containing steel samples in the laboratory and the thermal analysis of these steels. For laboratory sample production, a setup for adjusting the N2 partial pressure as a function of the chemical analysis of the melt was developed for a small laboratory induction furnace (high-frequency remelting plant). By means of specific Ar-N2 partial pressure mixtures, samples of the systems Fe-Cr-N, Fe-Mn-N, Fe-Nb-C-N with the desired target nitrogen content were produced as part of this work. Low-alloy steels and Fe-10%Cr alloys in particular could be produced very reliably up to the highest N contents. The second part of the work focussed on the thermal analysis of Fe-Cr-N steels using differential thermal analysis (DTA) and dynamic differential calorimetry (DSC) to determine the phase transformation temperatures in the high temperature range. Preliminary tests on a simultaneous thermal analysis (DTA-TG) with a coupled exhaust gas analysis using a mass spectrometer (QMS) were able to show that the nitrogen diffuses out of the sample from 1200°C onwards when a Fe-10Cr-0.1N alloy is heated under Ar atmospheres, thus changing the chemical analysis of the sample. An experimental concept was developed for the DSC system with a software-controlled Ar-N2 gas mixture to stabilise the N2-partial pressure during the measurement. Unfortunately, undesirable surface oxidation processes dominated due to the residual oxygen content in the nitrogen gas, meaning that reliable DSC-measurement of nitrogen-alloyed steels is not possible with the technical options currently available