Fourth in a series > Polymer quenchants, first introduced in the 1970s, have captured an increasingly large market share of quenchants at the expense of oil quenchants. In this series of articles, the quenchants (covered generally in the July 2024 issue) and individual polymer types will be examined in more detail.
In the previous columns, I discussed polyalkylene glycol (PAG) and polyethyl oxazoline (PEOX) polymer quenchants. In this column, I will discuss the use of polyvinyl pyrrolidone (PVP) as a polymer quenchant.
Introduction
Polyvinyl pyrrolidone (PVP) is derived from the polymerization of N-vinyl-2-pyrrolidone [1] (Figure 1). Polyvinyl pyrrolidone is a water-soluble polymer characterized by its unusual physical and colloidal properties and by its physiological inertness [2] [3].
Polyvinyl pyrrolidone has been available in the United States as a white, free-flowing powder manufactured in multiple grades ranging from low to high molecular weight.
The low molecular weight polymers have narrower distribution curves of molecular entities than the high molecular weight compounds.
PVP polymers are widely used in inks, adhesives, detergents and soaps, as well as being an effective quenchant.
Quenching characteristics of PVP
As with other polymer-type quenchants, concentration, bath temperature, and agitation all play a role in establishing the cooling characteristics. By comparison, the quenching rates tend to be faster during the stable film and nucleate and boiling stages but are slower during the convection stage.
Because PVP does not have inverse solubility in water, only very small amounts of polymer film are retained on quenched parts at quenching temperatures from 30°C (85°F) to near boiling. Thus, a broader working range of temperatures for quenching can be employed.
The effect of concentration is shown in Figure 2. PVP-based polymers are generally used at concentrations of 10 to 25 percent where the quenching characteristics are very similar to quenching oils.
At higher concentrations, the cooling rate at 300°C is substantially reduced. This makes the quenchant extremely suitable for high hardenability forgings or other crack-prone components. The fast maximum cooling rate at lower concentrations makes it effective in hardening low hardenability parts, while minimizing the formation of non-martensitic transformation products (NMTP).
As the temperature of PVP solutions increases, the vapor phase is extended, and the maximum rate of cooling is reduced (Figure 3). At temperatures above 40°C, the cooling rate is decreased at 300°C, making it suitable for high hardenability alloys. Interrupted quenching is also beneficial.
A maximum operating temperature of 60°C is normally recommended to minimize evaporation losses from the system.
As with other polymer quenchants, the overall heat extraction increases as the agitation is increased. The vapor phase of PVP quenchants are less stable than other quenchants. However, it is still important to maintain good agitation.
Typical applications of PVP quenchants
The oil-like quenching characteristics of PVP polymers extend the applications to the heat treatment of high hardenability materials and alloys.
PVP-based products are used widely in the steel industry for the quenching of bars, rolled sections, and forgings, generally at concentrations of 10-25 percent.
The most common application of PVP quenchants is their application in quenching large forgings and castings. Quenching high hardenability rolled rings is a very common application. For high hardenability rolled rings, the concentration is typically about 10-15 percent, depending on the temperature and uniformity of agitation.
The low solids content and low residual film resulting from use of polyvinyl pyrrolidone-based quenchants is very conducive to the use of interrupted quenching or timed quenching. This allows very high carbon, or very crack-sensitive parts to be quenched effectively, while maintaining properties.
Polyvinyl pyrrolidone-based quenchants have also been used successfully in quenching of high hardenability rolled rings at concentrations of 10-12 percent. PVP quenchants are very important to control residual stresses and yet still achieve hardness in large forgings (Figure 5) and in die steels (Figure 6). This quenchant is being used worldwide to quench large steam turbines, as well as high nickel super alloy forgings used in aerospace for jet engines for turbine hubs.
Large 3,000 kg high hardenability castings for the mining industry are commonly quenched in PVP. The concentration is typically 18-22 percent.
Other applications include induction hardening or replacement for oil in continuous-type furnaces, such as mesh-belt furnaces, for processing links and rollers in link chain, or small bearing components.
Conclusion
Polyvinyl pyrrolidone-based quenchants have been proven to be an effective replacement for oil for immersion quenching large forgings, as well as applications in continuous mesh-belt furnaces for quenching fasteners, bearing components, and link chain parts. The slow quenching characteristics compared to other quenchants at the same concentration often make this a suitable replacement for fast to medium speed oils.
Should there be questions regarding this column, or suggestions for additional columns, please contact the editor or me.
References
- F. Haaf, A. Sanner and F. Straub, “Polymers of N-Vinylpyrrolidone: Synthesis, Characterization and Uses,” Polymer Journal, vol. 17, pp. 143-152, 1985.
- B. Liscic, H. M. Tensi, L. C. Canale and G. E. Totten, Eds., Quenching Theory and Technology, Boca Raton, FL: CRC Press, 2010.
- G. Totten, C. Bates and N. Clinton, Eds., Handbook of Quenching and Quenchants, Metals Park, OH: ASM International, 1993.
- P. Cary, Quenching and Control of Distortion, Metals Park, OH: ASM International, 1988.
