PROCEDE D'ENRICHISSEMENT THERMOCHIMIQUE SEQUENTIEL EN CARBONE ET EN AZOTE DE L'ACIER

摘要

The method comprises thermochemically enriching carbon in a steel at a temperature equal to a start temperature of an austenitic transformation of steel, thermochemically enriching nitrogen in the steel at a temperature lower than an end temperature of the austenitic transformation of the steel, re-austenitizing the steel, and hardening parts of the steel. The first thermochemical enrichment step is a step of carburizing to enrich carbon in the steel or a step of carbonitriding to enrich nitrogen and carbon in the steel. The method comprises thermochemically enriching carbon in a steel at a temperature equal to a start temperature of an austenitic transformation of steel, thermochemically enriching nitrogen in the steel at a temperature lower than an end temperature of the austenitic transformation of the steel, re-austenitizing the steel, and hardening parts of the steel. The first thermochemical enrichment step is a step of carburizing to enrich carbon in the steel or a step of carbonitriding to enrich nitrogen and carbon in the steel. The first thermochemical enrichment step is followed by a phase of cooling the parts at a temperature lower than the end temperature of the austenitic transformation of steel under a controlled atmosphere to avoid an oxide formation on a surface of the parts. The second thermochemical enrichment step is carried out at a temperature of 300-700[deg] C, and comprises nitrogen enrichment by nitriding or nitrogen and carbon enrichment by nitrocarburizing. The re-austenitizing and hardening steps are carried out by heating until a temperature equalizes the start temperature of austenitic transformation of the steel, and followed by a phase of maintenance at the start temperature for 30 minutes. The re-austenitizing step is carried out under a controlled atmosphere in the presence of nitrogen gas. The steel parts are a low alloy steel comprising 0.1-0.3% of compared to the total weight of the steel. The first thermochemical enrichment step is conducted to increase the surface carbon to 0.9%. The second thermochemical enrichment step is carried out to increase the surface content nitrogen of the parts until a content of nitrogen id 15%. An independent claim is included for a steel mechanical part.