Title : Mechanical properties and microstructure of AISI D2 tool steel cryogenically treated using controllable cooling
Abstract:
The present work describes an approach to enhance the rate of heat transfer during cryogenic quenching of steel, which is environmentally friendly and low-cost. The approach is based on creating a temporary porous layer made of magnesium sulfate. The role of the layer is to decrease the temperature at the contact between the liquid nitrogen and the hot treated workpiece due to the low conductivity of the magnesium sulfate. The porous layer generates multiple nucleation sites of nitrogen bubbles and increase the rate of bubble formation, which physically breaks the film of steams on the surface. In this way the transition and the film boiling regimes shorten or even been eliminated, thus improve the contact with the liquid. The properties and microstructure, hardness, hardenability and dry abrasive wear of an AISI D2 steel were studied after air and water quenching (conventional treatment), direct liquid nitrogen quenching (deep cryogenic heat treatment) and rapid deep cryogenic heat treatment by creating a temporary porous layer based on magnesium sulfate. The experiments showed an increase in the cooling rate at cryogenic temperatures by using the temporary porous layers. The enhanced cooling rate (in the austenite-martensite transformation range) gave rise to a decrease of the amount of the retained austenite and an increase of the observed hardness of the steel.
Audience Take Away Notes:
- Porous coating layer is increasing the heat flux (cooling rates) at cryogenic temperatures
- The investigated cryogenic treatments (primary or supplementary) using temporary coatings increase the fraction of martensite and improves hardness (in particular surface hardness, 62HRC-68HRC)
- Investigated treatment improves the environmental safety and industrial (economical) process efficiency
- Low cost and environmentally safe material
- Coating creation and removal method is very simple and cheap
- The coating process is adjustable for different steels, shapes and dimensions of the workpiece
- Contribute to an increase in the hardness of existing steels or allow the use of less alloyed steels with the same mechanical properties
