Superior Mechanical Properties Obtained by HIP Heat Treatment for Machinery Applications

摘要

Austempering is a heat treatment where austenitized parts of iron/steel are quenched to avoid pear-lite, followed by isothermal transformation at 250-400°C (above M_s), usually in a salt bath. For steels, bainite is commonly formed. When Ductile Iron cast with >2 % silicon are austempered to form ADI, carbon released from growing acicular ferrite stabilizes the surrounding austenite instead of forming bainitic carbides, resulting in a fine duplex ferritic-austenitic microstructure named ausferrite. Recent concurrent HIP developments in heat treatment parameters and equipment enable Uniform Rapid Quenching (URQ™) for austempering in the AusFerHIP concept. Numerous advantages include faster heating and austenitization kinetics concurrent with elimination of porosity, and improved hardenability during quenching. In addition, the isothermal transformation to ausferrite can be promoted by rapid nucleation of acicular ferrite at lower temperature followed by rapid growth at higher temperature, this two-step austempering resulting in better properties and reduced process time.