Quench oil serves two primary functions. It facilitates hardening of steel by controlling heat transfer during quenching, and it enhances wetting of steel during quenching to minimize the formation of undesirable thermal and transformational gradients which may lead to increased distortion and cracking.
When using quench oil to cool hot metal, a vapor blanket is placed around the hot metal when it is first immersed into the oil. The stability of the vapor layer, and thus the ability of the oil to harden steel, is dependent on the metal’s surface irregularities, oxides present, surface-wetting additives which accelerate the wetting process and destabilize the vapor blanket, and the quench oil’s molecular composition, including the presence of more volatile oil degradation by-products.
Upon further cooling, the vapor blanket collapses resulting in nucleate boiling, which is the fastest heat transfer region. The point at which this transition occurs and the rate of heat transfer in this region depend on the oil’s overall molecular composition.
When the temperature of the hot oil-steel interface is less than the oil’s boiling point, nucleate boiling will cease and convective cooling will begin. Heat transfer in this region is exponentially dependent on the oil’s viscosity, which will vary with the degree of oil decomposition. Increasing oil decomposition will result initially in a reduction of oil viscosity followed by increasing viscosity as the degradation process increases. Heat transfer rates increase with lower viscosities and decrease with increasing viscosity.
Oil degradation is often accompanied by sludge and varnish formation. These by-products typically do not adsorb uniformly on the steel’s surface as it is being quenched, resulting in surface cooling rate variations and increased thermal gradients.
Another source of nonuniform heat transfer is quench oil contamination. For example, water may be introduced into the quench oil through a leak in the heat exchanger. Water, because it is not compatible with oil and possesses different physical properties such as viscosity and boiling point, will cause increases in thermal gradients.
To assure optimal quench process control, it is necessary to monitor quality variations throughout the oil’s lifetime. This is accomplished with quench bath maintenance procedures.
Properties of Quench Oils
Quench oil characterization is necessary to ensure optimal quench process control. Quench oil characterization is readily performed by measuring a series of physical properties including: viscosity, water content, neutralization number, precipitation number and flash point (Table 1). In addition to physical property characterization procedures, cooling curve analysis should also be performed when needed.
There are numerous specific physical property characterization procedures that may be used when evaluating quench oils. The following are examples of testing procedures that may be used and are aimed at providing some insight into the meaning of the results:
As previously discussed, a quench oil’s quenching performance is dependent on its viscosity. Due to degradation, oil viscosity changes with time. For process monitoring and trouble-shooting, the heat treater should develop an historical record, similar to the chart shown in Figure 1, of viscosity variation in his tank. Viscosity should be monitored periodically at 40°C using ASTM D445.
Figure 1. Viscosity of a Martempering Oil as a Function of Time
Water from oil contamination or degradation may cause soft spots, uneven hardness, staining, and worst of all, fires. When water-contaminated oil is heated, a crackling sound may be heard. This is the basis of a qualitative field test for water in quench oil. The most common laboratory tests for water contamination are either Karl Fisher analysis (ASTM D6304) or distillation (ASTM D95). Automated moisture detectors should be used with caution because many of these instruments’ lower sensitivity limits (typically 0.5 percent) are higher than the moisture content allowed for quench oils (typically less than 0.1 percent as described in ASTM D6710).
Flash point is the temperature at which the oil, in equilibrium with its vapor, produces a gas that is ignitable but does not continue to burn when exposed to a spark or flame source.
There are two types of flash point test procedures - closed-cup or open-cup. In the closed-cup measurement (ASTM D93), the liquid and vapor are heated in a closed vapor-confined area. 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. The minimum flash point of an oil, under normal operating conditions, should be 90°C (160°F) above the oil temperature being used.
As oil degrades, it forms acidic by- products. Chemical analysis can identify and measure these by-products. The acid number (AN) is the most common method employed. The AN is determined by a titration procedure using potassium hydroxide (KOH) and is reported as milligrams of KOH per gram of sample (mg KOH/g). Oil oxidation may also be monitored and detected by infrared spectroscopy. Figure 2 illustrates the spectral changes that occur from oil degradation.
Figure 2A. IR Spectra of a New vs. Moderately Degraded Quench Oil
Figure 2B. IR Spectra of a New vs. Severely Degraded Quench Oil
One of the greatest problems with quench oils is sludge formation. Although the various analyses above may indicate that a quench oil is usable, the presence of sludge may cause nonuniform heat transfer, increased thermal gradients and increased cracking and distortion.
Sludge may also plug filters and foul heat-exchanger surfaces. The loss of heat-exchanger efficiency can cause overheating, excessive foaming and fires.
Sludge is caused by quench oil thermal and oxidative degradation. The oxidation reactions lead to polymerized and cross-linked molecules, which are insoluble in the oil. The relative amount of sludge in quench oil may be quantified by the precipitation number. The precipitation number as defined in ASTM D91 is found by adding naphtha to the oil and determining the precipitate volume after centrifuging.
The remaining life of an oil can be estimated by comparing the relative propensity of sludge formation in new vs. used oil. Experimental procedures, such as Conradson carbon number, hot-panel coker test and the rotary pressure vessel oxidation test (RPVOT) may also be used.
When organometallic additives, such as metal salts, are used as quench rate accelerators, their depletion by processes such as chemical degradation and drag-out (mass transfer) can be quantified by performing a direct metal analysis. One of the most common procedures is induction-coupled plasma spectroscopy (ASTM D4951 or D6595). A typical elemental analysis report for metal salts contained in a quench oil is shown in Table 2.
One of the oldest tests to quantify the quench severity of an oil is the GM Quenchometer (“Nickel Ball”) test which is conducted according to ASTM D3520 (Figure 3).
Figure 3. GM Quenchometer Test Apparatus
In this test, a 7/8-inch (22 mm) nickel ball is heated to 885°C (1625°F) and then dropped into a wire basket suspended in a beaker containing 200 milliliters of the quench oil at 21°C to 27°C (70°F to 80°F). A timer is activated as the glowing nickel ball passes a photoelectric sensor. A horseshoe magnet is located outside the beaker as close as possible to the nickel ball. As the ball cools, it passes through the Curie point of nickel (354°C, 670°F), the temperature at which it becomes magnetic. At this point, the ball is attracted to the magnet, activating a sensor that stops the timer. The time required for the nickel ball to cool from 885°C to 354°C (1625°F to 670°F) is recorded. Table 3 provides some illustrative GM Quenchometer times for different quenchants.
Although the GM Quenchometer has been used to classify quench oils for approximately 40 years, it is of limited value today. As illustrated in Figure 4, the GM Quenchometer does not provide any information regarding the cooling pathway, which must be known if the quench oil’s ability to harden steel is to be determined.
Figure 4. Comparison of the GM Quenchometer Test
with Cooling Curve Analysis
In view of this critical deficiency, GM Quenchometer quench oil characterization is increasingly being replaced by the ISO 9950 or ASTM D6200 cooling curve analysis procedures for quench oils. Figure 5 illustrates the use of cooling curve analysis to identify cooling variations of a quench oil oxidation over time.
Figure 5. Effect of Quench Oil Oxidation on Cooling Rates
Figure 6 illustrates the effect of varying amounts of water on cooling a nonaccelerated quench oil.
Figure 6. Effect of Water Contamination on a Normal Speed Quench Oil
It should be noted however, that while cooling curve analysis provides an invaluable tool for monitoring and troubleshooting quench oil performance, physical property characterization is still required to identify the causes of the cooling behaviors.