Nitrogen pick-up during spray forming of high-alloyed steels and its influence on microstructure and properties of the final products

Spray forming is now getting established as a production route for high-alloyed steels. Tool steels for hot work, cold work and high-speed cutting applications and heat resistant steels generally contain high amounts of numerous elements including chromium, vanadium and aluminium, elements with high affinity for nitrogen. Within the spray forming process nitrogen is usually used as protective atmosphere as well as for gas atomization. The investigation considers nitrogen interaction with steel alloys during the spray forming in a closed vessel. Atmospheres with different nitrogen partial pressure are applied during the process. The microstructure of the spray formed, hot worked and finally heat treated steels is compared. Impact strength, wear resistance and strength at elevated temperature are analysed for the tool steels with respect to the nitrogen content. During melting intensive interaction of the melt with the atmosphere occurs according to the solubility of nitrogen in the alloy and high amounts of nitrogen are picked up. As a general result for the conventional alloys under investigation in spray formed condition this leads to reduction of toughness. This can be explained by additional precipitation of nitrides or carbonitrides. Nitrogen pick-up during spray forming can be avoided primarily by remelting in an argon atmosphere.