Development of a continuous cooling transformation diagram for Eglin steel

Eglin steel is a new ultrahigh strength steel that has been developed at Eglin Air Force Base in the early 2000s. This steel could be subjected to a variety of processing steps during fabrication, each with its own thermal history. This article presents a continuous cooling transformation diagram developed for Eglin steel to be used as a guideline during processing. Dilatometry techniques performed on a Gleeble thermomechanical simulator were combined with microhardness results and microstructural characterisation to develop the diagram. The results show that four distinct microstructures form within Eglin steel depending on the cooling rate. At cooling rates above ∼1°C s −1 , a predominately martensitic microstructure is formed with hardness of ∼520 HV. Intermediate cooling rates of 1 to 0.2°C s −1 produce a mixed martensitic/bainitic microstructure with a hardness that ranges from 520 to 420 HV. Slower cooling rates of 0.1 to 0.03°C s −1 lead to the formation of a bainitic microstructure with a hardness of ∼420 HV. The slowest cooling rate of 0.01°C s −1 formed a bainitic microstructure with an apparent carbide constituent at the prior austenite grain boundaries.