Uniform Rapid Quenching Enables Austempering In HIP

Austempering is a heat treatment where austenitized parts of iron/steel are quenched to avoid pearlite, but temperature is not decreased down to form martensite but instead kept above M_s for isothermal transformation at 210-400°C, usually in a salt bath.

For steels, bainite is formed. Ductile Irons have typically 2-3% 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 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, resulting in the following advantages:

1.       Faster heating and austenitization kinetics (to 4% less volume) concurrently with elimination of closed casting porosity and residual stresses due to 200 MPa of isostatic argon pressure;

2.       Doubled hardenability due to slower pearlite kinetics reduces alloying cost;

3.       During URQ, argon temperature passes pearlite nose after 5 seconds, while center of Ø22 mm tensile bar head passes after 20 seconds, reaching austempering (<400°C) after 40 seconds;

4.       Freedom to nucleate and grow acicular ferrite at different temperatures, reducing total process time.

ADI by AusFerHIP can reach following properties: R_p0.2=1257±11 MPa; R_m=1604±16 MPa; A₅=8.1±1.4 %.