One of the most important and most efficient heat treatment methods is the hardening of steel in order to achieve the desired fine-quality ferromagnetic structure with the strength, stiffness and toughness desired in various sizes and shapes. The steel hardening process involves homogenous heating up to 800-1000 ° C and then quenching it in a suitable fluid to reach the ambient temperature. To produce steel with high hardness properties, without cracks and deformation, the steel should be cooled to a high temperature at Ms point and slowly cool down when it reaches below the Ms point. Point Ms is the transition temperature from the metal austenite state. At this temperature, martensitic areas begin to form. In most steels, the Ms point is in the range of 200-300 ℃. Three parameters of maximum refrigeration speed, temperature at maximum speed and refrigeration speed of 300 ℃, are effective in evaluating the optimal quenching process.
The baths of water and salt are cheap but give a very quick refresh that increases the tendency to crap and warpage in the steel part.
The oil baths have a relatively lower refrigeration speed and have a maximum refrigeration speed of 600 ℃. But these queens are expensive and have a relatively low flash point that brings about the dangers of the fire. And sometimes it creates an unwanted film on the piece.
Polymer quenches include Polyalkylene Glycol (PAG), Polyvinyl Pyrrolidone (PVP), Sodium Polyacrylate (ACR), Polyethyl Oxazoline (PEO). The cooling rate of the polymeric quenches depends on the temperature, concentration, fluid mixing and destruction of the quenching agent.
Advantages of polymer quenches
• Low refrigeration speeds similar to oils
• Flexibility of refrigeration speed by changing the viscosity of the fluid, the quality of mixing the fluid and temperature
• Reduce internal stress, improve fatigue resistance and increase the shelf life of the unit
• Reduced costs due to less viscosity of polymeric quenches that require less energy when changing the bathroom fluid
• Improvement of safety (fire hazard elimination) and work environment health
• Polymer quenches, unlike oils, do not leave damaging products, so there is no need to clean up the waste material before each time the fluid is replaced. It is emphasized that polymer quenches do not cause any sediment and sludge.
Figure 2. Mechanism of Quench Refrigeration Mechanism in Schematic
Polyvinyl Pyrrolidone (PVP) Quenches
Behavior and function like oily quenches.
• The polymer carrier fluid is water, so there is no environmental concern about working with it.
• The bar, rolled sections and forgings are widely used in the steel industry for quenching.
• PVP quenches are available in separate grades for lower alloy steels (replacing fast quench oils) and higher alloy steels (alternatives to medium and slow quench oils).
Figure 3. Coefficient of refractive curve of quenched PVP at various concentrations