For many heat treaters, the use of oil gives the best value based on a number of factors including:
  • Overall performance (cooling rate or quench severity)
  • Economics/cost (initial investment, maintenance, upkeep, life)
  • Minimization of distortion (quench-system performance)
  • Variability (controllable cooling rates)
  • Environmental concerns (recycling, waste disposal)
One additional factor – performance over time – should be added to the list since we are always looking for ways to extend the life of our quenchants without sacrificing their performance. Let’s learn more.

Relationship of Physical Properties of Quenching Oils to Performance

Quench oil should be routinely analyzed (quarterly, or monthly if heavily used) to determine its performance characteristics. The testing laboratory or oil supplier’s report (Fig. 1) should be carefully scrutinized as it contains information about the physical property characteristics of the oil. Oil analysis uses standard test methods (Table 1), but in order to gain deeper insights into the meaning of the test results, as opposed to just comparing them with previous results, we need to understand what each category is telling us.

Viscosity
Quenching performance is highly dependent on the viscosity of the oil. In general, viscosity increases as the oil degrades. Degradation can be in the form of oxidation, thermal breakdown or the presence of various contaminants. Oil viscosity changes with time, and the formation of sludge or varnish accelerates the process. Samples should be taken and analyzed for contaminants and a historical record of viscosity variation kept and plotted against a process-control parameter such as part hardness.

Water Content
One of the concerns regarding oil quenching is the presence of water in the quench oil. It is dangerous since, on quenching, water will form steam with a resulting volume expansion of approximately seven times. As the steam bubble rises out of the quench tank, its surface is coated with oil. As it exits from the furnace (usually under extremely high pressure), it is ignited at the burnoff, resulting in a huge evolution of flame.

Water detectors or other in-process monitoring devices with sensitivity in the range of 0.2-0.3% should be provided on all quench tanks. They should be properly maintained and tested daily. Some manufacturers believe that as little as 0.1% may cause dramatic changes in quenching and part surface contamination.

In addition to a fire hazard, a water concentration of 0.05% has been reported to cause soft spots, uneven hardness and staining. When water-contaminated oil is heated, a crackling sound may be heard. This is the basis of a qualitative field test for the presence of water in quench oil. Sources of water include water-to-oil heat exchangers, water-cooled seals, plate coils or water-cooled bearings. The most common laboratory tests for water contamination are either a Karl Fisher analysis (ASTM D1744) or distillation.

Flash Point
The flash point is the lowest temperature where oil vapors will ignite but will not continue to burn when exposed to a spark or flame. The flash point is the maximum safe operating temperature of the oil, and changes in the flash point indicate contamination of the quench bath. There are two types of flash-point values that may be determined – closed-cup or open-cup. In the closed-cup measurement, the liquid and vapor are heated in a closed system. Traces of low-boiling contaminants may concentrate in the vapor phase, resulting in a relatively low value. When conducting the open-cup flash point, the relatively low boiling by-products are lost during heating and have less impact on the final value. The most common open-cup flash-point procedure is the “Cleveland Open Cup” procedure described in ASTM D92. As a general rule, oil should be operated no higher than 150°F (65°C) below the flash point or about 100°F (38°C) below the flash point on quenching a full load.

Oxidation
This variable may also be monitored and is especially important in tanks running marquenching oil or oils being run above their recommended operating range. Oxidation results from the buildup of organic acids and is detected by infrared spectroscopy. It is measured by several methods, including: precipitation number, total acid number, sludge content and viscosity. The cooling curve will change, increasing in speed for cold oil and decreasing in speed for hot/marquenching oils. Nitrogen blanketing of the oil is one way to reduce both oil oxidation and sludge formation.

Precipitation Number
The precipitation number is an indication of the tendency to form sludge. Sludge is one of the biggest problems encountered in quench oils, and high precipitation numbers also indicate a propensity to stain parts. Although other analyses may indicate that the quench oil is performing within specification, the presence of sludge may still be sufficient to cause nonuniform heat transfer, increased thermal gradients, and result in cracking and distortion. Sludge may also plug filters and foul heat-exchanger surfaces (the loss of heat-exchanger efficiency may cause overheating, excessive foaming and possible fires).

Sludge formation is caused by oxidation and polymerization of the quench oil and by localized overheating (“frying”) of the oil. The relative amount of sludge present in quench oil may be quantified and reported as a “precipitation number.” The precipitation number is determined using ASTM D91. The relative propensity of sludge formation of new and used oil may be compared providing an estimate of remaining life.

Neutralization Number or Total Acid Number (TAN)
As oil degrades, it forms acidic by-products. The amount of these by-products may be determined by chemical analysis. The most common method is the neutralization number. The neutralization number is determined by establishing the net acidity against a known standard base such as potassium hydroxide (KOH). This is known as the “total acid number” (TAN) and is reported as milligrams of KOH per gram of sample (mg/g).

The TAN is an indication of the level of oxidation. As the TAN increases, the vapor phase becomes less stable and the maximum cooling rate increases while distortion, cracking and staining tendencies increase. Both precipitation number and total acid number are controlled by filtration.

Quench Speed
Quench speed (see Industrial Heating, August 2007, “Part One: How to Interpret Cooling Curves”) is an important measure of the oil’s ability to achieve its performance properties. It can be determined by two methods, GM Quench-O-Meter (GMQS) and cooling curves. Probe surface condition and the condition of oil are factors that can influence results. Data should always be referenced back to new quench oil.

Accelerator Performance
Accelerants are often added to quench oils to return their performance characteristics close to those of new oils and to extend oil life. In general, it is not a good idea to mix an accelerator package from one supplier with oil from another. Induction coupled plasma (ICP) spectroscopy is one of the most common methods for the analysis of quench-oil additives. When additives (such as metal salts) are used as quench-rate accelerators, their effectiveness can be lost over time by both drag-out and degradation. Their effectiveness can be quantified by performing ICP spectroscopy – a direct analysis for metal ions – and compensating measures can be taken such as the addition of a specific percentage of new accelerator.

Conclusion

No matter what quenchant is used or how confident we are in its performance, routine testing is important. Our ability to interpret test results is equally important so that we can make informed judgments as to how we best control our heat-treating processes. IH