PRELIMINARY PROCESS PROTOTYPING TRAILS FOR PRODUCTION SCALE SINTER (BAINITE) HARDENING OF PM PARTS BY GAS QUENCHING

The microstructure of sintered PM parts are generally ferrite and pearlite where the pearlite content varies with the carbon content. Other than by a separate heat treatment process PM parts can also hardened by sinter hardening in a mesh belt sintering furnace within a module placed immediately after the furnace. The furnace atmosphere is re-circulated across heat exchangers & blasted on parts as they emerge from the sintering furnace. Generally high alloy compositions are required to obtain a predominantly martensitic structure with some bainite being allowed depending on the alloy & application. Fully bainitic microstructure is conventionally obtained in a process called austempering where steel & ductile iron parts are austenised & quenched in a salt bath generally maintained between 280°C to 400°C depending on the required hardness, the carbon & alloy content & section thickness. The parts are held for the desired transformation time required for full bainitic transformation. This ‘austempering’ process is practiced for improving the toughness & fatigue strength of processed parts whether wrought or sintered. Other researchers (1,2) have published their work on methods of obtaining a bainitic microstructure in PM parts by suitable alloying & cooling rate control and resulting properties obtained. However there are very few references on the results of conventional austempering which includes isothermal holding. It was decided to build an (open to public) process prototyping furnace to perform the conventional processes of sintering (Tmax=1150°C in N2+H2) followed by rapid gas cooling (like in sinter hardening but with hot gas) followed by isothermal holding between 250°C to 500°C. The purpose was to study the response of a variety of PM alloys and production parts. As a first trial gears made in Astalloy CRM + 0.5%C & Astaloy Mo + 0.6%C were processed and the results are reported in this paper

کلیدواژه ها:

Bainite Hardening ، Gas quenching ، Sinter hardening ، PM Austempering