The maintenance of quench oil is critical to the hardening operation, be it for carburize or austenitize and quench.
After the process of austenitizing, the quench procedure is perhaps the most critical operation. It is this operation that will determine the final metallurgy of the quenched part. It is worth a mention that the formation of martensite is dependent on three things: carbon, temperature and cooling speed.
If any of the above aspects are lacking – low or slow – full martensite will not form. The purpose of this discussion is once again to focus on cooling speed.
The best cooling speed of quench oil will always be when the oil is new and the quench tank is clean. The characteristics that will govern the effectiveness of the oil quench medium are listed below and are particularly important to the success of the quenching process.
- Particulate matter, such as soot and iron oxides (scale)
- Water contamination from wet parts, condensate inside the furnace quench chamber and from leaking pneumatic cylinder seals
- Changes in viscosity
- Decomposition of the quench medium
- Quench-speed agitation (too little or too much)
- Quench-medium temperature
- The sludge content as a result of oxidation and polymerization
The following are some of ASTM testing methods for the maintenance of quench oil:
- Ash content - ASTM D484
- Flashpoint - D92
- Fire point - D92
- Quenching speed - D3520
- Precipitation number - D91
- Viscosity - D445
- Water content - D958
- Saponification number - D94
- Sludge content - D91
- Carbon content - D189
- Neutralization number - D974
- Specific gravity - D287
While it is recognized that the above recommended preventive-maintenance tests are expensive, it is equally expensive in terms of rework, failure to accomplish hardness, soft spots and, finally, fire risk. When one considers that an oil quench-tank fire can and has burned down large treatment establishments, the cost of maintenance is not very prohibitive. The cost of maintenance can be justified in terms of increasing the longevity of the oil quench medium.
The cost of oil will most likely increase rather than decrease. It behooves the user to maintain the investment of a full tank of quench oil by simply performing the proper preventive maintenance.