CQI-9 compliance demands adherence to the standards for the purpose of excellence in automotive heat treating. Poorly maintained quench oil can cost heat treaters in many areas.
In this Heat Treat Today Technical Tuesday feature, Greg Steiger, senior key account manager at Idemitsu Lubricants America, shares how costly quench oil issues can be addressed through proper adherence to the CQI-9 quench oil testing protocols. Let us know if you’d like to see more Original Content features by emailing editor@heattreattoday.com.
Sr. Key Account Manager
Idemitsu Lubricants America
Introduction
A poorly maintained quench oil can cost a heat treater in more ways than simply the cost of having to replace the oil. The costs can quickly expand to include those associated with poor quality. For example, costs associated with part rejects, or rework and downstream costs for shot blasting, or third-party inspection are often the cause of poor quench oil maintenance. Dirty or poorly maintained oils can affect part cleanliness, surface hardness, and surface finish. For instance, it is well known that a heavily oxidized oil may create surface stains that must be shot blasted to remove. High molecular weight sludge or excessive water can create surface hardness issues. Many of these issues can be addressed through proper adherence to the quench oil testing protocols established by CQI-9.
How can CQI-9 help?
CQI-9 is designed as a tool to help heat treaters produce consistent parts. Using a CQI-9 compliant quench oil analysis can also be a very powerful tool in a heat treaters tool kit. Just as the level of carburization is influenced by the carbon potential of a carburizing atmosphere, the cooling speed of the oil influences microstructure formation and microstructure composition along with mechanical properties such as hardness as well as tensile and yield strength. Furthermore, the cooling speed is dependent upon the viscosity of the oil, the amount of sludge, moisture level, and oxidation of the oil. All of these are tested on a regular basis under the requirements of CQI-9, ISO TS 16949, and most quality systems adopted by modern heat treaters. All of the tested parameters required under CQI-9 will be addressed individually later in this paper.
What is CQI-9?
The member companies of the Automotive Industry Action Group (AIAG) encompassing automotive manufacturers and their Tier I suppliers have enacted an industry heat treating standard called CQI-91. This standard was originally a standalone standard designed and adhered to primarily by North American OEMs and Tier I suppliers as a quality tool to create consistent documented processes within the heat treating industry with the goal of producing consistent reproducible results. Since that first implementation of CQI-9, the standard has now been incorporated into the ISO TS 16949 standard and is now adhered to by most automotive OEMs and their Tier I suppliers. The full range of management responsibilities, material handling, and equipment operations of the CQI-9 standard is beyond the scope of this paper. Instead we will be discussing the used quench oil analysis requirements of CQI-9, why the tests are required, and how heat treaters need a CQI-9 compliant quench oil analysis to properly care for their quench oils.
Utilizing a compliant CQI-9 analysis and the supplier provided operating parameters for the CQI-9 required tests is the first step in the proper care of a quench oil.
CQI-9 Compliant Analysis
Most quench oil suppliers provide a quench oil analysis. Although the quench oil supplier may provide a quench oil analysis, for the analysis to be CQI-9 compliant the analysis must contain the following tests or their equivalent:
- Water content; ASTM D6304
- Suspended solids; ASTM D4055
- Viscosity; ILASD509
- Total acid value; ASTM D664
- Flash point; ASTM D92
- Cooling curve; JIS K2242
The frequency of the above testing must be a minimum of semiannually. A more frequent sampling interval does not violate CQI-9. In fact, the more often a quench oil is analyzed, the easier it is to use the quench oil analysis as a tool in the proper care of a quench oil. It is important to note that the CQI-9 standard does not prescribe specific test methods be used in the above testing; however, they must be performed to a traceable standard. The CQI-9 standard only states that the above values, along with a cooling curve, must be reported. The following sections will describe each test in a CQI-9 compliant analysis.
Water Content
Everyone knows water in a quench oil can be have catastrophic safety and performance consequences. However how much water is too much? That is a question that is difficult to answer. The answer depends on a variety of factors such as the quench oil used and all of the variables associated with a furnace atmosphere. A general rule of thumb when it comes to water levels is to keep the water level below 200PPM. At levels above 200PPM of water, uneven cooling begins to occur.2 It is important to remember a quench oil is not a pure homogenous fluid. Samples taken at various places throughout the quench tank will be similar but will also have differences. These differences will include water and solids levels. Therefore, in areas where the water content exceeds the 200PPM level, uneven cooling will begin. Parts coming into contact with this “localized” quench oil with high water can potentially begin to crack, have a high surface hardness, or have staining problems. Yet parts in other areas of the load continue to behave normally. For this reason, and also because water is much heavier than oil, it is imperative the oil be under agitation. In addition to the potential uneven cooling issues high water may create, a high level of water can also influence the rate of oxidation in an oil.
Suspended Solids
Because solids are typically denser and more viscous than liquids they do not have the same heat transfer properties as a liquid. Due to the inequality of heat transfer capacities between liquids and solids, it is very important to keep the solids level, especially high molecular weight sludge, at a minimum. Sludge reacts in an opposite manner of water. Where water can increase quench speed, high molecular weight sludge will decrease quench speed through uneven cooling.2 The result of the uneven cooling from sludge is typically seen in soft surface microstructures or soft surface hardness. Also, like water, sludge is heavier than oil and the lack of homogeneity in the oil means having proper agitation is paramount when sampling.
Viscosity
Changes in viscosity can lead to both faster quench rates and slower quench rates. As the quench oil is used in the quench process, it undergoes thermal degradation.3 This degradation process can be seen when the oil becomes thinner or less viscous. During this process, a small portion of the base oil and a small amount of the quench oil additives undergo a process called thermal cracking. In this process, heavier molecules are broken into smaller molecules through the use of heat. This thermal cracking creates lighter less viscous oil from heavier oils. The newer lighter viscosity of the quench oil can potentially lead to changes in the quench speed of the oil. These changes can have an impact on the microstructure, case depth, core hardness, and surface hardness on the quenched parts.
As an oil is subjected to the high temperatures of a quenching operation, oxidation is a natural occurrence in the oil. As the oil oxidizes it will begin to increase in viscosity until it reaches the point of forming an insoluble sludge. Therefore, an increase in viscosity typically means the oil is oxidizing. Just as an oil that becomes thinner and less viscous may have a change in cooling properties, an oil that becomes thicker and more viscous may see a change in cooling performance. A thicker oxidized quench oil may affect surface hardness, microstructure, case depth, and core hardness. In severe cases of oxidation staining may result. Such stains typically require post quench and temper processing such as shot blasting.
Total Acid Value
The Total Acid Value, or TAV, is a measure of the level of oxidation in a quench oil. The amount of oxygen in a quench oil cannot be measured without a sophisticated laboratory analysis. However, the formation of organic acids within a quench oil can be easily determined via a titration method. It is well understood that these organic acids are the precursors in a chain of chemical reactions that will eventually form sludge. As the TAV increases so will the levels of oxidation, and in turn, the amount of sludge will also increase. Consequently, as the TAV increases, the amount of staining due to oxidation may increase. The cooling properties of the oil may decrease due to the increased sludge formation as well. Figure #1 shows an example of how the acid value increases the viscosity of a quench oil due to the formation of polymeric sludge in the quench oil.2
Flash point
The flash point of a quench oil is another check to ensure the safety of the quench oil user. As oil thermally cracks, the heavier base oils become not only lighter in viscosity, but their flash points also decrease. If left unchecked, the decrease in flash point could result in a higher risk of fire. In addition to serving as a watchdog against the results of excessive thermal cracking, a flash point is also a safeguard against human error and adding the wrong quench oil to a quench tank. High temperature oils typically have a higher flash point than conventional oils. An increase in flash point, along with no change in TAV, and an increase in viscosity could indicate a contamination issue between oils has occurred.
Cooling curve
There are many different methods of running a cooling curve. Many Asian suppliers of quench oil will use the Japanese Industrial Standard (JIS) K 2242. European suppliers will use the ISO 9950 and North American suppliers rely on the ASTM D 6200 method. All of these standards measure the same basic property, the ability of an oil to reach martensite formation. However, they differ in one basic item. The JIS K-2242 and methods used in China and France use a 99.99% silver probe that is smaller than the size of the Inconel probe used in the ASTM and ISO methods of Europe and North America. Because of this difference, it is important to note that cooling curves and cooling rates between the methods should not be compared. Figure # 2 shows the comparison between the two probes and their dimensions.
ISO/ASTM Inconel probe 12.5mm x 60mm.
JIS K 2242 Silver probe 10mm x 30 mm
In addition to comparing the cooling curve against the standard for the quench oil used, the Grossman H value should also be calculated and used as an indicator of cooling performance. Unlike the old GM nickel ball test that tracked the time to cool a 12mm nickel ball to 352°C, the Grossman H value measures the severity of the quench6.
In using the Grossman H value, the lower the value, the slower and less severe the quench. For use as a rough guide in comparing the quench speed in seconds to the Grossman H value measured in cm-1 the table below can be used.
For example, air has an approximate H value of 0.01 cm-1 and water has an approximate H value of 0.4 cm-1 compared to oil with an approximate H value of ___ cm-1
The calculation used to determine the Grossman H factor has historically been:
H=h/2k
Where h is the heat transfer coefficient of the part when measured at the surface of the part and k is the thermal conductivity of the steel. Typically the heat transfer coefficient is measured at 705°C. A steel’s thermal conductivity does not typically change according to alloy composition or temperature. Therefore, the Grossman H value is proportional to the heat transfer coefficient of the part.
Interpreting a CQI-9 quench oil analysis
Discussion
In examining the test parameters for CQI-9, it becomes apparent that many of the test results should be compared with other test results. For example an increase in the amount of sludge or solids should also increase the viscosity of the quench oil. As the sludge increases, the level of oxidation increases, and therefore, the level of organic acids formed in the quench oil should be increasing the TAV. Finally, as the sludge increases, the cooling property of the quench oil should decline as indicated in the lower H value.
Likewise, as the flash point decreases the amount of thermal cracking is increasing, which should reduce the viscosity and thereby increase the H value and the overall cooling speed of the quench oil. Conversely, if the test parameters are not working in concert with each other, there may be other issues going on within the quench oil. For instance, an increase in the water content can be detected before the increased water levels begin the oxidation process thereby increasing the TAV. Or a viscosity change without a change in other parameters could be an addition of the wrong quench oil to the quench tank. The graph below for Idemitsu Daphne Hi Temp A helps illustrate this point.
In the graph above, it can be seen when the water H value increases and the viscosity remains stable, the likely explanation is an increase in water. When both the H value and viscosity decrease, additive consumption is the most likely reason. Likewise, when the viscosity increases and the H value decreases, the formation of sludge from oxidation is the culprit.
Having test parameters that work in conjunction with each other is only beneficial if sample frequencies are often enough. While CQI-9 only stipulates a semi-annual sampling frequency, the conditions of a quench tank can change in very short order. There are the obvious changes when water is added to the tank. However, many of the changes are more subtle, and left unchecked over time can create potential costly solutions such as a partial dump and recharge of the quench tank, poor part quality, or an increase in downstream processing such as shot blasting. For this reason, many quench oil suppliers request a minimum of quarterly sampling. In addition, if a sample is missed on a quarterly sample frequency, there is still time to sample the quench tank and remain in compliance with CQI-9.
Conclusion
Over time the condition of a quench oil will change and corrective measures will be needed to bring the quench oil back into the suggested supplier’s operating parameters. The chart below helps understand what some of the methods need to be.
With proper care and maintenance, a quench oil can last a very long time. A conventional oil should last 10 to 15 years or longer while a marquench oil should last seven to 10 years. The proper care of a quench is simple and straight forward. A quality quench oil should not need the use of additives to improve oxidation resistance or quench speed. Simply adding enough fresh virgin oil to replace the oil that is being dragged out through normal operations should replenish the oxidation protection and quench speed to within the normal operating parameters. The table below offers recommendations for treating out of normal operating parameters for the required CQI-9 tests.
Most heat treaters make weekly quench oil additions to their quench tanks. The most popular type of filtration system is a kidney loop style where the quench oil is constantly filtered. There are two basic types of these systems. They differ in the number of filters used. For a single filter system, a 25 micron filter is sufficient for quench oil filtration. In a two-stage filtration system, a 50 micron filter is typically used in the first stage and a 25 micron filter is used in the second stage. In a two-stage filter, the cheaper 50 micron filter will be replaced more often than the 25 micron filter in the second stage.
Utilizing a compliant CQI-9 analysis and the supplier provided operating parameters for the CQI-9 required tests is the first step in the proper care of a quench oil. The next basic steps are ensuring there is enough fresh quench oil available for regular additions to replace the oil that is lost through drag out and proper filtration of the quench oil in a constant kidney loop type of a system. With these steps in place, a quench oil will offer consistent performance for years and will be one less concern heat treaters face in the operation of their furnaces.
References:
- Automotive Industry Action Group, “CQI9 “Special Process: Heat Treatment System Assessment;” AIAG version 3, 10/2011.
- Rikki Homma, K. Ichitani, M. Matsumoto, and G. Steiger, “Evaluation and Control Technique of Cooling Unevenness by Quenching Oil,” 2017 ASM Heat Treat Expo, https://asm.confex.com/asm/ht2017/webprogram/Paper43594.html.
- G. Steiger, “Preventing the Degradation of Quench Oils in the Heat Treatment Process,” Metal Treating Institute, https://www.heattreat.net/blogs/greg-steiger/2018/10/03/preventing-degradation-of-quench-oils-in-the-heat.
- M.A. Grossman and M. Asimov. Hardenability and Quenching. 1940 Iron Age Vol. 107 No.17 Pp 25-29.
About the Author:
Greg Steiger is the senior key account manager of Idemitsu Lubricants America for quench products. Previous to this position, Steiger served in a variety of technical service, research and development, and sales marketing roles for Chemtool, Inc., Witco Chemical Company, Inc., D.A. Stuart Company, and Safety-Kleen, Inc. He obtained a BSc in Chemistry from the University of Illinois at Chicago and is currently pursuing a Master’s Degree in Materials Engineering at Auburn University. He is also a member of ASM International.
