Fig. 1. Typical cooling curves and cooling-rate curves for new oils

Fig. 2. Typical cooling curves and cooling-rate curves for new and used oils

Cooling curves, cooling-rate curves and oil analyses are provided by suppliers to help heat treaters better control an important process variable – quenching. Unfortunately, in many cases these reports are not well understood or, worse yet, filed to be used merely as ISO documents. It’s time to pull them out of the file cabinet and learn how to interpret them so we know what it is we are being told. Let’s learn more.

The goal of quenching is to produce the desired hardness, strength and toughness in the part while minimizing distortion and residual stress. Uniformity of heat extraction by the quench medium is critical, and mechanical, physical and metallurgical properties developed are directly related to the material and the cooling rates within the part.

Cooling curves and cooling-rate curves (Fig. 1) not only tell us how the oil behaves through the three stages of quenching but also characterize the quenching behavior. In other words, these types of cooling curves measure our “quenching power” and tell us how the oil was performing on the day of the test. When looked at over time and quantified by part test data, they become a powerful predictive tool.

The basic shape of a cooling curve is set by the quenchant formulation. Over time, we see shifts in the cooling curves, and as we build up a history of how the oil is changing, we can tell how different conditions – contamination, oxidation, drag-out, agitation – affect quench performance (Fig. 2 – online only).

Cooling curves are typically generated by immersing a heated thermal probe into an unagitated, heated quenchant and generating time/temperature data as the probe cools. Standard test procedures are based on ASTM D-6200-01/2007 (Standard Test Method for Determination of Cooling Characteristics of Quench Oils by Cooling Curve Analysis) and ISO-9950 (Industrial Quenching Oils – Determination of Cooling Characteristics – Nickel Alloy Probe Test Method). These methods allow comparison of new oil results from supplier to supplier and from plant to plant worldwide.

Three Stages of Quenching

When quenching into any liquid, there are three distinct stages of cooling. Stage 1 is called the vapor-blanket stage, and it occurs immediately upon quenching. It is characterized by the Leidenfrost phenomenon – the formation of an unbroken film or vapor layer that surrounds and insulates the part. It forms because the supply of heat from the surface of the part exceeds the amount of heat that can be carried away by the cooling medium. In this stage, the cooling rate is a function of conduction through the vapor envelope. The cooling rate is slow since the vapor layer prevents the quenchant from contacting the metal surface.

As the temperature of the parts slowly drops, the vapor blanket becomes unstable and collapses, allowing the oil to come into contact with the metal surface. Stage 2 is known as the nucleate-boiling stage and is characterized by violent bubble boiling as heat is rapidly removed from the part surface. The maximum cooling rate (°F/s) is found during this stage along with the highest instantaneous value of the heat-transfer coefficient. Maximum part distortion also occurs during this stage due to differentials in uniform heat extraction from the various part surfaces.

Finally, when the part temperature has dropped below the boiling point of the quenching oil, Stage 3 – the convective heat-transfer stage – begins. This is the slowest of the three stages.

Quenching Speed and Other Useful Data

Quench oils are often classified according to their maximum cooling rate (Table 1). This data is especially useful when reported over time (Table 2). Other data, however, may be of equal or greater benefit to the heat treater such as: the characteristic temperature (Table 3), that is, the transformation point between the first and second stage of cooling – an indication of cooling “efficiency” for a given quench oil; the cooling rate measured at some temperature below the characteristic temperature (Table 4) where boiling ceases – an indication of cracking potential; and H-values (Table 5) – a measure of quench severity.

In the example shown, quarterly checks of cooling-curve data from both new and used quench-oil samples revealed that while the quench oil in an integral-quench furnace had cooling characteristics similar to the previous quarter’s tests, it was significantly slower than a new oil or oil installed in another (open) quench tank. While the new oil and open-tank samples fit the category of a “fast” quenchant, this particular IQ furnace falls in the category of a “slow” oil.

Quenching performance will be different for similar materials, parts and loads quenched in these two tanks. Adding accelerator can move the cooling curve of the oil in the IQ furnace closer to that of the new oil.

How to Benefit the Most from Cooling-Curve Data

Cooling curves are normally determined under controlled conditions in the laboratory as opposed to monitoring quench tanks in real time. Also, the choice of cooling probe further limits our ability to interpret the results because when quenching parts in the heat-treat shop, the quenching power of the oil and the heat-transfer characteristics vary not only over the surface of an individual part but within the workload as well. Thus, for example, the transition between the vapor phase and the nucleate-boiling phase can take place at different times at different points on the various part surfaces throughout the load. Even oxidation of the part surface is known to change the cooling performance. It is, therefore, critical for the heat treater to monitor part hardness, distortion and other properties to understand how his particular quench tank performs as his quench oil ages and changes. IH

Next time: Part Two: What Your Quench Oil Analysis Is Telling You