Thermomechanical Processing for Improved Mechanical Properties of HT9 Steels

• Published in: Materials Vol. 17; no. 15; p. 3803
• Main Authors: Byun, Thak Sang, Collins, David A, Lach, Timothy G, Choi, Jung Pyung, Maloy, Stuart A
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 01.08.2024; MDPI
• Subjects: Annealing; carbide precipitates; Discount coupons; Ductility; Duplex stainless steels; Equilibrium; Ferritic stainless steels; ferritic-martensitic steels; Fracture toughness; Heat resistant steels; High temperature; Hot rolling; Intermetallic compounds; Low temperature; Low temperature resistance; Mechanical properties; Mechanical tests; Metal industry; Microstructure; Normalizing (heat treatment); Nuclear reactors; Precipitates; Quenching and tempering; Radiation; Radiation damage; Radiation dosage; Rapid quenching (metallurgy); Reactor cores; Steel; Steel alloys; strength and toughness; Temperature; Tempering; thermomechanical processing; Thermomechanical properties; Thermomechanical treatment; Ultrafines; ferritic-martensitic steels; thermomechanical processing; carbide precipitates; strength and toughness
• ISSN: 1996-1944; 1996-1944
• DOI: 10.3390/ma17153803

Thermomechanical processing (TMP) of ferritic-martensitic (FM) steels, such as HT9 (Fe-12Cr-1MoWV) steels, involves normalizing, quenching, and tempering to create a microstructure of fine ferritic/martensitic laths with carbide precipitates. HT9 steels are used in fast reactor core components due to their high-temperature strength and resistance to irradiation damage. However, traditional TMP methods for these steels often result in performance limitations under irradiation, including embrittlement at low temperatures (<~430 °C), insufficient strength and toughness at higher temperatures (>500 °C), and void swelling after high-dose irradiation (>200 dpa). This research aimed to enhance both fracture toughness and strength at high temperatures by creating a quenched and tempered martensitic structure with ultrafine laths and precipitates through rapid quenching and unconventional tempering. Mechanical testing revealed significant variations in strength and fracture toughness depending on the processing route, particularly the tempering conditions. Tailored TMP approaches, combining rapid quenching with limited tempering, elevated strength to levels comparable to nano-oxide strengthened ferritic alloys while preserving fracture toughness. For optimal properties in high-Cr steels for future reactor applications, this study recommends a modified tempering treatment, i.e., post-quench annealing at 500 °C or 600 °C for 1 h, possibly followed by a brief tempering at a slightly higher temperature.