Uniform Rapid Quenching enables Austempering Heat Treatment in HIP

摘要

Austempering is a heat treatment where austenitized parts of iron/steel are quenched to avoid pearlite, followed by isothermal transformation at 250-400℃ (above M_s), usually in a salt bath. For steels, bainite is commonly formed. Ductile Irons have typically 2-3 wt% Si to stabilize carbon as spheroidal graphite instead of carbides (as in white irons). During austempering of Ductile Iron castings to form ADI, the silicon level opens a process window where 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.Hot Isostatic Pressing (HIP) is a process widely used for densifying castings and powder-based materials as well as diffusion bonding of metals and ceramics. Recent concurrent developments in heat treatment parameters and equipment make it possible to utilize Uniform Rapid Quenching (URQ™) for austempering in the AusFerHIP concept. The high isostatic argon gas pressure results in numerous advantages such as faster heating and austenitizing kinetics concurrently with elimination of closed casting porosity and residual stresses, combined with a contribution to hardenability due to slower pearlite kinetics during quenching, allowing reduction of alloying cost and casting segregation. In addition, the isothermal transformation to ausferrite can be promoted by rapid nucleation of acicular ferrite at a lower temperature followed by rapid growth at a higher temperature, this two-step austempering resulting in a better combination of mechanical properties and a reduction of total process time.