Small-Scale Alloy Development and Industrial Scale-Up Enabled by Complementary Characterization Methods Based on the Example of a PH-Steel

Precipitation hardening (PH)-steels are corrosion-resistant materials that combine superior strength with sufficient toughness and ductility, which makes them highly suitable for safety-critical structural components in the aerospace industry. Their properties are determined by a complex microstructure consisting of a low-carbon martensitic matrix, from which both the toughness-enhancing austenite and strength-enhancing precipitations evolve during the aging heat treatment. The ratio of strength to toughness can mainly be adjusted by the aging conditions but also by marginal modifications of the chemical composition itself.

Since the use of these materials for aerospace applications demands high purity levels, they often undergo a complex melting and remelting process under vacuum leading to high manufacturing costs, which makes targeted alloy development challenging. Consequently, the evaluation of alloy concepts using material produced on a small scale, followed by upscaling to large-scale manufacturing, is a reasonable approach.

In this study, a PH grade (UNS46500) was investigated by using complementary characterization methods such as light optical microscopy, scanning electron microscopy, electron-backscatter-diffraction, atom-probe-tomography and many more to gain a deeper understanding of how the existing phases determine the mechanical performance as well as to assess its potential for the transfer to small-scale production.

In terms of microstructure and properties, the material which was melted on a small scale was sufficiently comparable with large-scale produced material to investigate individual, composition dependent phenomena. Nevertheless, production process related phenomena such as retained austenite formation were observed in certain areas of the bar, which needs to be considered to make a clean evaluation of the influence of specific alloying elements on the microstructure and properties. Finally, the transferability of promising alloying concepts to large-scale production was evaluated and implemented. Further comprehensive characterization of the manufactured material and mechanical testing concluded the product development process from a research & development perspective.