The use of oils, instead of molten salts, as quenchants for the austempering of steels has several potential benefits. Molten salts, currently used as industrial quenchants, can result in sample corrosion, pose environmental hazards, and are challenging to handle. In this study, a mineral-based quenching oil, MarTemp 2565, has been examined for austempering of high-carbon and low-carbon steels at temperatures up to 320°C (i.e., above the 288°C flashpoint of this oil) to enable bainitic transformation without pearlite formation. A unique inert atmosphere (low oxygen partial pressure, pO2) vertical tube furnace system developed for this work allowed for such austempering without oil combustion. In this work, the relationships between austempering conditions, microstructural evolution, and mechanical properties have been examined for two grades of steel: SAE 52100 (high carbon) and AISI 4120 (low carbon).

 

For austempered SAE 52100, microstructural analyses indicated that secondary carbides increased significantly in average length, aspect ratio, and spacing with increasing austempering temperature from 220°C(3 h) to 320°C(1 h), whereas the average size and area fraction of primary carbides remained constant. These secondary carbide microstructural changes coincided with a decrease in average hardness from 59.4 HRC to 51.3 HRC. Charpy impact testing revealed an apparent reduction in the onset of the ductile-to-brittle transition temperature (DBTT) with increasing austempering temperature; that is, SAE 52100 samples austempered at 220°C(3 h) and 270°C(2.25 h) exhibited an apparent DBTT onset of 250°C, whereas samples austempered at 320°C(1 h) exhibited an apparent DBTT onset of 100°C.

 

For austempered AISI 4120, carburized and non-carburized Charpy bars were austempered at 300°C for 1 h. Microhardness profiling of the carburized samples revealed a lower bainitic case (cladding) possessing hardness values of 57+2 HRC and a tempered martensitic core with hardness values of 44+1 HRC. The bainitic case was found to be in a state of compression (residual stress ~ -400 MPa). On the other hand, non-carburized samples exhibited fully-tempered martensitic structures with surfaces in a state of tension (residual stress ~300 MPa).

 

Charpy impact testing indicated an apparent DBTT completion of 150°C for both carburized and non-carburized samples, although the carburized samples exhibited lower impact energies.

 

By utilizing a unique vertical tube furnace system to conduct low-pO2 austempering treatments, this study has demonstrated how a wide range of austempering conditions may be accessed using oil as a quenchant to achieve controlled bainitic microstructures in low-carbon and high-carbon steels for tailorable mechanical properties.