Two design strategies for enhancing the thermal stability of bainitic structures

Abstract

In response to the growing interest of the industry in advanced high-strength steels subjected to extreme operating conditions, two novel grades BainTS and BainNiAlCu bainitic steels have been developed. Since the in-use properties of bainitic steel are directly related to the thermal stability of the bainitic structure, two different alloy design concepts to improve tempering resistance considering various strengthening mechanisms were proposed. BainTS steel follows the standard assumptions of nanocrystalline bainitic steel design, where the retained austenite is stabilized with silicon. A potential secondary hardening was also considered using Cr, Mo, and V alloying additives. On the other hand, BainNiAlCu steel was designed taking into account the precipitation strengthening with intermetallic compounds (nickel aluminide and copper particles). It was revealed that both steels after heat treatment consist of bainitic ferrite laths and a significant content of retained austenite (above 25%) with film and blocky morphologies. After the tempering process (350 °C–650 °C), there was significant structural evolution following the gradual decomposition of the metastable bainitic structure. It was proved that BainTS steel is characterized by the higher thermal stability of both bainitic ferrite and retained austenite compared to BainNiAlCu. Despite this, the hardening effect after tempering BainNiAlCu steel was higher (by about 140 HV compared to heat treatment, approx. 30% increase). The presented results suggest that bainitic steels strengthened with intermetallic phases are extremely promising in the context of future industrialization in applications where operating conditions are exposed to elevated temperatures.

Keywords

• Retained austenite stability
• Nanostructured bainite
• Tempered bainite
• Decomposition of retained austenite
• Secondary hardening
• Intermetallic strengthening