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Mechanism-based assessment of the influence of powder production and process parameters on the microstructure and the deformation behavior of SLM-compacted C + N steels in air and in corrosive environments

Subject Area Metallurgical, Thermal and Thermomechanical Treatment of Materials
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 372290567
 
Final Report Year 2023

Final Report Abstract

Laser powder bed fusion (PBF-LB/M) combines rapid production of complex material structures from metals with a high degree of individualization and low raw material consumption. One challenge is the limited choice of processable metals, especially in the area of steel alloys. In this research project, fundamental findings were obtained on powder production, PBF-LB/M processing and the chemical and mechanical properties of nitrogen(N)-alloyed, corrosion-resistant steels, the PBF-LB/M processing of which had received little attention prior to the start of the project. In detail, corrosion-resistant austenitic, martensitic and ferritic-austenitic steels were investigated, focusing on the dissolution and precipitation behavior of N during the entire process chain and its influence on the chemical and mechanical properties. In the course of gas atomization, the N contents were adjusted according to the thermodynamic melt solubility limit by melting the different alloys in an N2 atmosphere and using N2 as the atomization medium. By partial substitution of Ni by Mn, austenitic and ferritic-austenitic steel powders with increased N contents compared to established CrNi alloys could be produced. Subsequent gas nitriding in an N2 atmosphere (pN2 = 3 bar) at temperatures below 700 °C also enabled N contents above the maximum N melt solubility to be achieved without significant deterioration of the powder properties. Steel powders with N contents below or within the range of maximum N melt solubility can be produced with low porosity and crack-free. If, on the other hand, the N content is above the N solubility, outgassing of N2 lead to the formation of gas pores. The amount of nitrogen remaining in the component is a function of the laser parameters used and thus of the thermal process conditions as well as the N content of the starting powder. Increased volumetric energy densities lead to increased N2 outgassing due to the enlargement of the melt pool dimensions, local temperature increases, and lower solidification rates. The dissolution and precipitation behavior of N differs depending on the type of primary solidification. While N is predominantly interstitially dissolved in primary austenitic solidifying steels, it precipitates as chromium nitride in primary ferritic solidifying melts, which can be dissolved by downstream solution annealing. The positive influence of N on the mechanical and chemical properties is particularly evident when dense components are produced in the additively manufactured state. If the porosity increases due to partial outgassing by N2, the positive influence is partially compensated. However, with similar porosity between an N-free and an N-containing steel, N leads to an increased damage tolerance under cyclic loading both in the atmosphere and in salt water.

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