Troubleshooting deficient properties in heat-treated steel — III

This is the third in a series of articles on troubleshooting issues related to heat treating steel.

In the previous column, I discussed the furnace as a source of incorrect hardness during processing. In this column, I will discuss the quench system as a source of incorrect hardness during processing.

Introduction

As I discussed previously, the potential sources of inadequate properties can be traced to a variety of sources, i.e., the material, austenitizing process, furnace, quench system, and the tempering process (Figure 1). Here I am going to be discussing the quench system.

Quench System

The quench system consists of the quench tank, the quenchant, the agitation system (pumps or agitators), and the method of transporting the part from the furnace to the quench tank [1]. Each of these components should be operating properly to achieve good consistent properties.

The quench tank should be adequately sized and properly filled. Assuming that parts previously processed have been heat treated properly, it should be verified that the tank is filled to the proper level. Process instruments should be checked to verify that the quenchant temperature is correct for the part in question.

The quenchant should be checked to verify properties and whether any contamination has occurred [2]. This is generally accomplished by contacting your quenchant supplier and sending a two-liter sample to them. The supplier will typically check for viscosity, flashpoint, and cooling curve tests. Generally, if the cooling curve shows normal characteristics, compared to the reference (new) cooling curve, the oil should be good. Typically, the maximum cooling rate should be within ± 18°C/sec of the reference oil, and the temperature of maximum cooling rate should be within ± 18°C. Large deviations of flashpoint or viscosity from the reference oil are an indication that the oil is contaminated [3]. For instance, contamination with an oil-based hydraulic fluid can result in changes in the cooling curve (Figure 2) and changes in the flashpoint and viscosity (Figure 3). For polymer quenchants, the same principles apply (except for flashpoint), and the supplier will also verify concentration.

Should the oil be determined to be slow, then speed improvers can often be added to the oil to improve the quenching speed. If the oil is badly contaminated, it may be necessary to dump and recharge the system, but this is only a last resort.

The next thing to consider is the agitation system [4]. The agitation system is usually a set of pumps or agitators that direct flow through the quench tank. Verify that the system process controllers are calling for agitation by turning on the agitators at the proper speed. Visually look at the agitator shafts to verify that they are turning, and turning in the proper direction. Comparing the direction to a similar system can help make sure that the agitators are turning in the proper direction.

After verifying that the agitators are turning in the proper direction, take a clamp-on ammeter and measure the amperage of each agitator. It is also useful to compare the amp pull from a similar system. If the amperage for each agitator is the same, then it is likely that the agitation system is operating properly. If the amperage is higher, then it is possible that the resultant duct work for the agitator is clogged with sludge or parts, forcing the agitators to work harder and pull a higher amperage. One additional caveat is to verify that the agitators are installed correctly. It may be that the agitator impeller has been installed upside down, again resulting in a higher amperage draw. This can occur immediately after maintenance. If the amperage from one of the agitators is low, then the agitator could have worn or degraded blades. It could also mean that the impeller has fallen off the shaft and is sitting at the bottom of the tank. It is for this reason that it is always a good idea to measure the amperage from each agitator or pump when the furnace or quench tank is first commissioned, or when the system is brought back on-line after maintenance.

There is the transportation system that moves parts from the furnace to the quench tank, including the quench elevator. The mechanism moving the load from the furnace should be inspected and tested under a full load to see that it operates as when first commissioned or meets the manufacturer’s specifications. The elevator should be inspected for leaks if hydraulically operated, and verified that the pressures used are within the manufacturer’s specifications. If electrically operated, verification of the amperage on the elevator motors will indicate if binding of the mechanism is occurring.

One thing that can cause poor properties in a furnace load is the racking or fixturing of parts. Part contact [5], or improper quenchant flow around a part [6] can result in poor properties, or spotty results.

Conclusion

The reasons for soft or inadequate properties can often be traced to the quenchant or quench system. Proper maintenance of agitation systems, and a systematic examination of the quenchant properties can yield consistent properties.

Should you have any questions or comments regarding this article, or have suggestions for additional articles, please contact the author or editor.

References

1. SAE International, AMS 2759G, Heat Treatment of Steel Parts, General Requirements, Warrendale, PA: SAE International, 2018.
2. B. L. Ferguson and D. S. MacKenzie, “Effect of Oil Contamination on Pinion Gear Distortion,” in Proceedings from the 6th International Quenching and Control of Distortion Conference, 9-13 September, Chicago, IL, 2012.
3. D. S. MacKenzie, G. Graham and J. Jankowski, “Effect of Contamination on the Cooling Rate of Quench Oils,” in Proc. from the 6th International Quenching and Control of Distortion Conf., 9-13 September, Chicago, IL, 2012.
4. D. S. MacKenzie, “Quench System Design,” in ASM Handbook, Volume 4F, Quenchants and Quenching Technology, vol. 4F, G. E. Totten, R. S. Otero, X. Luo and L. C. F. Canale, Eds., Materials Park, OH: ASM International, 2024, p. (to be published 2024).
5. P. Lasne and D. S. MacKenzie, “Design of a Quench Ring for Proper Quenching of Small Cylinders — Initial Investigation,” in European Conference on Heat Treatment and Surface Engineering 2022, Salzburg, Austria, 2022.
6. A. L. Banka, B. L. Ferguson and D. S. MacKenzie, “Evaluation of Flow Fields and Orientation Effects Around Ring Geometries During Quenching,” J. Mat. Eng. Performance, vol. 22, no. 7, pp. 1816-1825, 2013.
7. D. S. MacKenzie, “Quenchant Agitation, Design, and Characterization,” in Steel Heat Treating Technologies, J. L. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM International, 2014, pp. 281-303.