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In induction hardening, power supply, generator issues, and coil problems can all cause damage to parts. Consider one more area where problems may develop: improper care of polymer quenchants. Several key variables play a role in maintaining polymer quenchants, and in protecting the final product. Learn what these variables are in this article by D. Scott MacKenzie, Ph.D., senior research scientist of Metallurgy at Quaker Houghton, Inc.

This Technical Tuesday feature will be published in Heat Treat Today's May 2022 Induction Heating print edition.

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Introduction

Induction hardening is commonly used to heat treat gear teeth, shafting, and other parts that require a high surface hardness for wear or strength. The process uses a power supply, RF generator, induction coil, and quenching mechanism (spray or immersion) to yield a high surface hardness and advantageous residual surface stresses. Heating is very fast, with selective heating of the desired part. An induction hardening line can be integrated readily into cellular manufacturing.

There are many problems that can occur in induction hardening that can have nothing to do with the power supply, RF generator, or coil. These are process-related issues that are often due to improper or inadequate process control. These problems can manifest themselves as improper part hardness or cracking; improper pattern; quenching issues such as foaming or excessive drag-out; corrosion issues; or biological issues such as bacteria and fungus or odors. In this short article we will discuss proper process control of polymer quenchants.

Concentration Control

Concentration control is one of the most important process parameters in induction hardening. Improper control can result in soft parts, cracked parts, or excessive distortion. The concentration of the polymer can change due to quenchant drag-out during operation, or due to evaporation of the water. Another source of inaccurate polymer control is contamination from coolants, or process fluids from prior operations if the parts are not cleaned prior to induction hardening.

The most common method of concentration control is by handheld digital or analog refractometer. A small drop of the quenchant is placed on the sample window of the refractometer, and the refractive index (in °Brix) of the quenchant is determined. The refractometer reading is then multiplied by the factor associated with the quenchant to determine the concentration.

However, contamination from using hard water, or other contamination from coolants, etc., can cause the factor to shift lower, resulting in an error in concentration measurement. The refractometer should be verified using kinematic viscosity at routine intervals, to monitor and correct the proper multiplying factor.

If the concentration is low, the polymer should be added. If the concentration is high, the water should be added.

pH

pH is the measurement of the acidity of the solution and is a measure of the health of the system. It infers the presence of adequate corrosion inhibitor. Steel parts tend to rust when solutions are at a pH of less than 7 and have a passive film at a pH greater than 8.5. Further, biological growth is stunted as the pH is increased. Contamination, especially by chloride containing coolants, or from water containing high levels of chloride can result in the pH dropping, and rust occurring. In water, when evaporation occurs, the chloride will concentrate. Should the pH drop below 8.5, then a pH booster or corrosion inhibitor should be added to increase the pH.

Corrosion Inhibitors

There are two types of corrosion inhibitors commonly used in polymer quenchants — nitrite/nitrate corrosion inhibitors, and amine based corrosion inhibitors. These different types of inhibitors should not be mixed due to incompatibility. Most machining coolants contain amine type of inhibitors, so this type of inhibitor is usually recommended for induction hardening unless the parts are thoroughly cleaned and rinsed prior to induction hardening.

Biological Availability

Biological activity, such as fungus or bacteria, can affect the performance of the quenchant. This generally affects the quench system by clogging filters, and clogging quench spray heads. It is also an odor issue, resulting in a strong mildew or rotten egg smell.

The test for biological activity is usually a simple dip slide. The slide, containing an agar-type growth medium, is washed with the fluid, and allowed to sit for three days. Bacteria growth will be evident on one side, and fungal growth is visible on the other side. The levels of bacteria are usually rated from 1–11, indicating bacteria or fungus in a logarithmic scale. When the bacteria exceed 6 or 106 CFU/ml, the fluid should be treated with a biocide. If the fungus count exceeds 102 CFU/ml, then it should be treated with a spectrum fungicide. The system should also be thoroughly cleaned prior to dumping and recharging to prevent contamination of the new bath.

One thing to note, is that the use of biocides is extremely hazardous. Very small quantities (ounces) are required to kill the biological activity in a 10,000-gallon tank. Proper safety equipment (Tyvek suit, chemical safety goggles, face shield, and chemical resistant gloves) should be used to dose a system to kill biologicals. The use of biostable quenchants, such as Aqua-Quench™ 145 or Aqua-Quench™ 245 can avoid the use of dangerous biocides.

Contamination

Contamination is the most common cause of quenchant failure in induction hardening. This is due to improper or inadequate cleaning of parts prior to induction hardening. The contaminants do not burn off , but act as a source for rusting and other surface defects.

The quench tank is not a cleaning tank. Parts should be free from coolants and other fluids prior to heat treatment. Even a small amount of residue on each part can build up in the system, and thousands of parts are processed. For long life of the quenchant bath, proper cleaning of parts is required.

Conclusions

In this short article, the importance of several key variables was illustrated. Proper control of these variables will lead to properly heat treated parts, and long quench bath life.

About the Author: D. Scott Mackenzie, Ph.D., is senior research scientist in Metallurgy at Quaker Houghton. In 2008, he was awarded the Materials Science and Engineering Departmental Distinguished Alumni award from The Ohio State University. He is the author of several books, and over 100 peer-reviewed papers. Scott received a B.S. in Metallurgical Engineering from The Ohio State University and holds an M.S. and Ph.D in Metallurgical Engineering from the University of Missouri. He has served on the ASM Heat Treating Society Board of Directors, and is past president of the International Federation of Heat Treating and Surface Engineering.

For more information: scott.mackenzie@quakerhoughton.com

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