In previous columns, I discussed three different types of polymer quenchants: PAG, PEOX, and PVP. In this column, I will discuss the use of sodium polyacrylate (ACR) polymers as a polymer quenchant.
Introduction
Sodium polyacrylate-based polymer quenchants have oil-like quenching characteristics, which enable the treatment of a wide range of alloy steels and higher hardenability materials.
Sodium polyacrylate (ACR) quenchants are synthesized from a sodium acrylate monomer, with the chemical formula shown in Figure 1. This polymer is a sodium salt of polyacrylic acid and is used extensively in consumer products. By using the salt of an alkali metal, in this instance sodium, the polymer provides solubility in water.
Polyacrylate quenchants represent a class of quenchants completely different from the PAG, PEOX, or PVP types. The latter polymers fall into the type characterized as nonionic, that is, not ionizable or neutral. Polyacrylate quenchants are considered anionic, which is negatively charged. The charged character of the polymer on parts adds another dimension to the quenchant: strong polarity. The strong polarity provides water solubility but also is suspected of causing the polymer to operate by a different mechanism of heat extraction. Unlike other polymer quenchants, polyacrylate solutions do not split on heating and do not form thick fifilms on the surface of the hot work. Their slower rate of cooling is based on the molecular weight of the polyacrylate and the subsequent viscosity of its solutions. By varying the molecular weights of the polymer, and the polymer concentration, a whole family of quenchants can be designed covering a full range of applications, from the fast quenching of water to the slow cooling of oils.
Quenching characteristics of sodium polyacrylate quenchants
The quenching effect of the polyacrylate quenchants is a function of the three basic parameters: polymer concentration (Figure 2), bath temperature (Figure 3), and bath agitation (Figure 4). The cooling curves of the polyacrylate solutions can have an extended vapor phase and reduced heat extraction during the boiling phase. This polymer quenchant also has a very slow quench rate through the martensitic transformation region [2]. This unique property of the polyacrylate quenchants allows their applications for hardening of crack-prone parts made of high-hardenability steels. Applications of this kind usually are unobtainable with any other polymer quenchants or require much higher concentrations of the polymer.
The quenching characteristics of ACR solutions are sensitive to agitation as shown in Figure 4. Under static conditions, ACR solutions exhibit a pronounced vapor phase. As the severity of agitation increases, the vapor phase is shortened and there is a significant increase in maximum cooling rate. As with other polymer quenchants, agitation is important to achieve uniform quenching. Generally, a high degree of agitation is recommended for hardening operations, because of the persistent vapor phase.
The drag-out of sodium polyacrylate is much higher than either PAG or PVP quenchants at the same concentration. It has been reported that the drag-out of ACR quenchants have a drag-out of twice PAG-type quenchants, and four times that of PVP quenchants [3].
Typical applications
The oil-like quenching characteristics of ACR polymers enable the heat treatment of higher hardenability materials. While most applications are for immersion quenching, there have been a few unique applications (Figure 5, Figure 6). These include critical AISI 4140 seamless tubes for the oil industry, AISI 4140 and 4340 forgings and castings, heavy gears, thin-section alloy steel crankshafts, and high-carbon chromium grinding balls. Another application was for thick section gas cylinder applications. It is also used as a replacement for molten lead baths used in wire patenting.
Concentration control of sodium polyacrylates is based on the kinematic viscosities. However, as solutions age, they tend to shear. One of the major control issues is that the sodium may ionize in solution and become sensitive to hard water, forming soluble carboxylic salts. This results in a change in viscosity, which makes concentration control more difficult. The effective quench rate is then based on viscosity, refractive index, and effective cooling rate. Because of the problems with process control of concentration, the application of ACR polymers has a limited market share to specialized operations such as quenching of white iron grinding balls, patenting of wire and quenching of high hardenability forgings.
Conclusions
In this column, another type of polymer quenchant has been introduced. The sodium polyacrylate polymer quenchants are characterized by a very long and stable vapor phase, with a very slow cooling rate through the martensitic transformation rate. This quenchant has been used for quenching of very high hardenability forgings and used for the patenting of steel wire. At the same concentration, sodium polyacrylate quenchants have a much slower overall quench rate than either the PAG or PVP-type quenchants.
Should there be any questions regarding this article, or suggestions for new topics, please contact either the editor or myself.
References
- D. S. MacKenzie, “Quenchants Used for Heat Treating of Ferrous Alloys,” in Steel Heat Treating Technologies, vol. 4B, J. L. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM International, 2014, pp. 255-280.
- F. F. Griffiths, The Quenching Characteristics of Sodium Polyacrylate Solutions – Ph.D. Dissertation, Sheffield, UK: Dept. Metals and Materials Engineering, Sheffield City Polytechnic, 1989.
- Edgar Vaughn & Company, “Quenching Principles and Practice”, Manchester, UK: Edgar Vaughn & Company, 1885.
