Quenching and Lubrication – The Hidden Productivity Connection

Quenching metals – or the rapid cooling of a work piece (metal) in a medium to convert the grain structure to a more stable form – is an important part of steel and other metal alloy manufacturing processes. The quenching process is intense, placing equipment components under extreme temperature changes and exposure to contaminants like water.

Maximizing equipment performance and longevity for this type of equipment depends on proper lubrication, the first line of defense when it comes to protecting these components.

Over the years, I’ve received a number of lubrication questions related to quenching (such as, “May I use a heat transfer oil to quench metals?”), and I thought it’d be helpful to take a closer look at this topic to help operators optimize equipment productivity for this important part of the metal-making process.

The quenching process

As mentioned, the simple definition of quenching is the rapid cooling of a work piece (metal) in a medium to convert the grain structure to a more stable form. In this process, a workpiece is heated between 800 - 900°C, followed by a cooling process (quenching). The length of the cooling process depends on the desired grain or crystalline structure.

There are several media used in quenching, including:

  • Water: Applied evenly to the workpiece in order to maintain uniform cooling. Operators must make sure the water does not get too hot.

  • Air and other gases: These media help limit residual stress and brittleness, improves fatigue life.

  • Oil cooling: Operators can use either neat or emulsified oils (water and oil). These oils are used to heat treat alloy steels and is less severe than water cooling as slower cooling rate minimize distortion.

Once quenching is completed, the metal must be tempered to reduce brittleness. The alloy is re-heated to between 200 and 600°C for a period of time, further reducing hardness while increasing ductility. Once the quenching and tempering process is complete, the workpiece is ready to be used in the desired application.

Quenching oils

The cooling rate of hot metals depends on the dimensions of the object, the process temperature, and the quench oil. If an oil or emulsion is used as the coolant, considerations include:

  • Viscosity

  • Specific heat of the oil

  • Latent heat of vaporization

  • Specific heat of the vapor

  • Thermal conductivity

Quenching using oils as the coolant is a three-phase process:

  1. At the start of quenching, the oil is vaporized and a vapor blanket surrounds the workpiece. Heat escapes very slowly as no liquid comes in contact with the hot metal.

  2. As the metal cools, the intermediate or partial oil contact stage is reached. Here, the vapor barrier is penetrated and the oil intermittently contacts the metal surface. This direct oil to surface interface allows the cooling rate to increase.

  3. Finally, as the workpiece continues to cool, the direct lubricant contact stage is reached. This stage offers reduced cooling through convection and conduction.

Neat quench oils are light-bodied mineral oils with a viscosity of 20 cSt at 40°C. Fast quench oils are neat and have a viscosity of 10 cSt at 40°C. Emulsions (water and oil) are used to get accelerated cooling rates but are generally inferior in performance to neat oils. ASTM D6200 the procedure for the evaluation of a quenching oil's characteristics to include cooling rate.

Ideal lubricant characteristics

If it’s not already clear by now, it’s important to choose a lubricant specifically designed to perform in typical quenching operating conditions. Lubricant formulations can vary, and it’s important to choose products based on certain key characteristics.

For example, the Mobil Thermrex Series is a family of three quench oils for the heat treatment of steel and alloy steels. They are comprised of selected solvent-refined base stocks and additives to provide optimum chemical and thermal stability, providing uniform steel hardness, long batch life, and minimum deformation.

The selection of the particular grade of quench oil depends on the cooling rate desired to prevent cracking and distortion. Thermrex D-10, R-21 and V-32 are most effective with agitation at batch temperatures of less than 65°C (150°F). For safety, it is recommended the flash point of Thermrex D-10 through V-32 should be 65°C (150°F) above the batch temperature.

I hope this week’s tip was helpful. If you have any questions or comments, leave a note in the comments section below!