Improved Performance of Ni-Co Secondary Hardening Steels Through Rapid Processing

Ultra-high strength secondary hardening steels such as HP-9-4-30, AF1410, AerMet100 and Ferrium M54 have good fracture and impact toughness but are limited by their cost, availability, and heat treatment complexity. Reducing the long tempering times (4-10 hours) and increasing toughness in these alloys via processing changes (instead of increased alloying additions) may lead to significant cost savings. Rapid tempering to high temperatures for short (e.g. <1 minute) times has been shown to greatly increase impact toughness in the tempered martensite embrittlement regime in 4340 while maintaining the same strength/hardness relative to the isothermal condition. The proposed mechanism is a combination of a reduction in retained austenite decomposition and finer cementite. The aim of this work was to determine if rapid tempering could similarly increase toughness while maintaining strength in a secondary hardening steel. Both 1‑step and 2-step rapid heating cycles, where samples were heated at 300 °C/s to temperatures higher than peak isothermal tempering temperatures and held for 10-1000 seconds, were studied in a modified HP-9-4-30 steel. Isothermal temper properties were characterized using Vickers microhardness (HV) and Charpy V-notch (CVN) impact testing. Initial rapid heated samples for microhardness testing were processed using a dilatometer while CVN samples were heat-treated using a Gleeble 3800. The results showed that a 1-step rapid temper with hold times up to 1000 seconds could not produce the same hardness as an 8-hour isothermal temper, due to a decreased secondary hardening effect. However, the rapid tempering did lead to significant enhancements in impact toughness. The 2-step heat treatments showed a 44% increase in toughness when tempered to the same hardness as the conventional isothermal temper and a 25 HV increase in hardness when tempered to the same toughness highlighting the possible benefits of rapid thermal processing in these steels.