Determining equivalent concentration for polymer quenchants

Study of one method by which polymer quenchants can be compared to determine equivalent concentrations, even if their cooling curves do not directly compare.

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In this column, I will discuss a method to determine equivalent concentration when selecting alternative quenchants using the Segerberg Hardening Power (HP) for polymer quenchants.

Introduction

When comparing polymer quenchants, there are several things that we look at to make sure that it will quench our parts satisfactorily. First, we look at the cooling curve of the quenchant to make sure that it will properly quench our parts. A typical cooling curve of a polymer quenchant at different concentrations is shown in Figure 1. A table showing the specific values taken from the cooling curve is provided in Table 1.

After verifying that the quenchant will satisfy our required metallurgical properties, we look at other things, such as biostability, available corrosion inhibition, etc.

In Table 1, we see a value for the HP-IVF (polymer). In this article, the meaning of this value will be explained.

Figure 1: Cooling curves of Aqua-Quench® 145, a typical low molecular weight PAG-type polymer. Cooling curves measured using an ASTM D6200 probe, agitated in an ASTM D6259 agitation device (Tensi) at 40°C and 1,000 RPM.
Table 1: Specific values taken from the cooling curves shown in Figure 1.

Hardening Power for Polymer Quenchants

Several decades ago, the late Sorin Segerberg of IVF in Sweden, proposed the concept of Hardening Power for oils and polymer quenchants [1] [2]. During this time, cooling curve testing was not truly established as an international or USA standard. It wasn’t until about 1995 that the ASTM [3] and the ISO method [4] were established as the preferred method for cooling curve measurement.

In this analysis, immersion quenching of 16mm diameter x 48mm long cylinders of SAE 1045 were quenched in many different polymer quenchants, at several different concentrations. A statistical analysis using the cooling rate at the ferrite/pearlite nose (CRP, °C/s), and the cooling rate at the martensite start temperature (Ms, °C):

For alloyed steels, the coefficients in each of the equations for oils and polymer will be different. As can be seen from the above equation, the cooling rate at 300°C will have a greater impact on the hardening power than will the cooling rate at 550°C, for the same range of values.

We can use this equation to compare different polymer quenchants and determine an equivalent concentration.

Comparing Concentrations

Let’s suppose that you are a manufacturer who is making widgets quenching in a polymer quenchant. You are using Polymer Quenchant A with the cooling shown in Figure 2. Issues with delivery, pricing, or other problems are causing you to look at alternative polymer quenchants. You have been presented with cooling curves of two different polymer quenchants of similar chemistry to the one you are currently using (Figure 3 and Figure 4). Commercial items are similar, but the cooling curves look different. Should either of the polymer quenchants be chosen, what would be the equivalent concentration to get similar hardness and distortion results?

Figure 4: Cooling curve of alternative quenchant C.
Figure 3: Cooling Curve of alternative quenchant B.
Figure 2: Cooling curve of existing quenchant A at different concentrations.

Just looking at the cooling curve data, it is difficult to determine an equivalent concentration. However, using the Segerberg Polymer Hardening Power, an estimate of the equivalent concentration can be made by taking the cooling rate at the ferrite/pearlite nose (CRP, at 550°C), and the cooling rate at the martensite start temperature (Ms, 300°C), for each of the concentrations, and computing the Hardening Power, HP. A graph can then be developed, comparing the HP of the different quenchants to the concentration (Figure 5).

Figure 5: Calculated hardening power, HP, for 3 different polymer quenchants.

At a nominal concentration of 12 percent, all the quenchants are equivalent to each other. They will achieve similar hardness and distortion results. However, at concentrations less than 12 percent, Quenchant B would need a lower concentration to achieve the same results. At concentrations above 12 percent, Quenchant C would require less concentration for equivalent properties as the incumbent Quenchant A. Assuming identical pricing for all three quenchants, at concentrations less than 12 percent, Quenchant B is more cost-effective than the incumbent Quenchant A. At concentrations higher than 12 percent, Quenchant C is cheaper than Quenchant A. Specifically, if the manufacturer was using Quenchant A at 5 percent, then 3 percent of Quenchant B would be required, and approximately 7 percent of Quenchant C would be necessary. If pricing were different on each of the quenchants, then the in-tank costs would then be considered by multiplying the expected concentration by the price and the tank size:

Where %C is the concentration, V is the volume of the tank, and P is the price per gallon. All other things being equal, the lowest in-tank cost would be the preferred choice.

Conclusions

In this article, I have shown a method by which polymer quenchants can be compared to determine equivalent concentrations, even if their cooling curves do not directly compare. From the equivalent concentrations, an expected in-tank cost can then be calculated, and the lowest cost option can then be determined.

Should there be any comments on this article, or suggestions for additional columns, please contact the writer, or editor. 

References

  1. S. Segerberg, “Classification of quench oils: A method of comparison,” Heat Treating, vol. 20, no. 12, pp. 30-33, 1988.
  2. S. Segerberg, “The Hardening Power of Polymer Quenchants and its Prediction,” in 12th ASM Heat Treating Conference, 13-15 March, Indianapolis, IN, 1989.
  3. ASTM International, Standard Test Method for Determination of Cooling Characteristics of Quench Oils by Cooling Curve Analysis, ASTM D 6200, Conshocken: ASTM, 2012.
  4. International Organization for Standardization, “Industrial quenching oils — Determination of cooling characteristics — Nickel-alloy probe test method,” Geneva, 1995.