Proper Quenching Option Yeilds Better Heat-Treating Results

By Larry Olson, Editor – Modern Applications News

Taken from Modern Applications News, June 2001.

For centuries, quenching with vegetable oils, fats and other non-petrochemical materials was used for hardening swords.

For some time, this process was one of a blacksmith’s most carefully guarded secrets.

The main development in quenching came in the first half of the 20th century with the establishment of transformation diagrams. The contribution of Mr. William Park Woodside is significant in making quenching less of an art and more of a science. A metallurgist by training in blacksmithing, and a founding member of what is now ASM International (American Society for Materials), Woodside was instrumental in the publication of related information in “Metallurgical Transactions.”

In 1911, he founded Park Chemical Company (Detroit, MI), which earned its reputation for developing new processes and production in the field of heat treatment. The company is now known as Heatbath®/Park Metallurgical (Indian Orchard, MA).

Due to its versatile quenching performance, oil is the most widely used quenching medium, next only to water. The worldwide requirement for quench oil today is estimated at between 50 million and 100 million gallons per year. Among the various quenching media, oil continues to be favored because its quenching mechanism and cooling curves are well suited to the TTT (time, temperature, and transformation) and CCT (continuous cooling transformation) diagrams of many types of steel.

Quenching of steel in liquid medium consists of three distinct stages of cooling: the vapor phase; nucleate boiling and convective stage. In the first stage, a vapor blanket is formed immediately upon quenching. This blanket has an insulating effect, and heat transfer in this stage is slow since it is mostly though radiation. As the temperature drops, the vapor blanket becomes unstable and collapses, initiating the nucleate boiling stage. Heat removal is the fastest in this stage, due to the heat of vaporization, and continues until the surface temperature drops below the boiling point of the quenching medium. Further cooling takes place mostly through convection and some conduction.

Heatbath has more than seven types of quenching oils suitable for steels with low to high hardenability. Because of the properties of these oils, it is possible to quench into the martensitic temperature range with minimum distortion, while still obtaining the desired properties in steel parts. Besides hardenability, selection of a quench oil depends on part geometry and thickness, and the degree of distortion that can be tolerated. For example, hot oil is required for smaller parts with high hardenability to achieve the desired mechanical properties with minimum distortion.

Quench oils are available with flash points ranging from 270°F to 560°F. The operating temperature of the oil in an open temperature quench tank should be at least 150°F below its flash point. When the quench tank is operated under a protective atmosphere, oil can be used at as high as 50°F below the flash point. The operating range of Heatbath quench oils is from 50°F to 450°F. A lower operating temperature is helpful in minimizing thermal degradation of the oil.

Although the benefits of using quench oil are great, there are concerns to be addressed: specifically, safety, disposal, and availability. However, these factors are effectively dealt with by technological advancements. By totally enclosing the quenching operations, safety of operators is enhanced to a maximum degree. In addition, by recovering most of the used oil, disposal is nearly eliminated.

These concerns about quench oil have led to the development of water-based polymer quenchants, particularly in the last 30 years. However, due to the inherent differences in the quenching mechanism, oil continues to be favored and still accounts for more quenching than polymer. It is doubtful that polymer will overtake oil in the foreseeable future.

A polymer quenchant is better suited than quench oil for induction or flame hardening. Unlike quench oil, polymer has more variables that require closer attention. The major variables are concentration of polymer in the solution, operating temperature, and agitation, which should be monitored regularly. One advantage that polymer offers that oil cannot is “time or interrupted quenching,” in which the parts are taken out before they are fully cooled. This modification helps to minimize distortion without the fire risk associated with quench oil. For example, in quenching large tubes, only one tube could be quenched safely in oil, where three tubes could be quenched in polymer without fire hazard.

The other quenching media in use are brine, caustic solution, and molten salt. Brine provides faster cooling than water, but is corrosive. Caustic solution provides even faster cooling, but poses a safety hazard to operators. Heatbath has developed a unique product called Speed Quench 1, which is neither corrosive nor caustic, while providing very fast, uniform quenching.

Molten salt has been used for quenching purposes for more than 50 years mainly due to its wide operating temperature range, from 300°F to 1,100°F. It is a unique medium, in which the vapor phase (the first stage) of quenching does not exist. Its high thermal conductivity with good agitation and the addition of water help to achieve fast quenching rates. Some processes, such as austempering, require high operating temperature for which molten salt is the only choice. Salt quenching helps in achieving maximum hardness with minimum distortion. With safety and environmental issues now well addressed, salt quenching is on the rise, particularly for applications where it is desirable to get higher metallurgical performance from lower grades of steel.

In quenching, there are two stresses involved: thermal stresses due to rapid cooling, and transformation stresses due to the increase in volume from austenite to martensite microstructure. These stresses can cause excessive distortion or even cracks. However, quench oil has a unique desirable cooling response in minimizing these effects. Consequently, oil will continue to be used for quenching as long as it is affordable.

APPLICATION DETAILS ARE CRITICAL TO SUCCESS

Proper quenching is an extremely important part of the heat-treating process. Expensive, high value-added parts become scrap if insufficient attention is paid to proper quenching. Selection of a quenchant is primarily governed by the processing specifications, the required physical properties, and the required microstructure. There are cases where either oil or polymer can be used. However, it should be noted that polymer quenching requires greater control over concentration, agitation, and temperature than required for oil.

In order to optimize a quenchant’s performance, it is best to operate in a predictable way. This means knowing what the load will be, how much heat is to be removed, and how much temperature rise of the quenchant this would cause. Such temperature rise should not be more than 40°F in oil and not more than 20°F in polymer. As the quenching of load after load continues, the quenchant temperature may exceed the upper operating limit, requiring use of a heat exchanger to maintain it within the desirable range.

Uniform agitation that provides flow velocities of 50 – 100 feet per minute near the parts is a key to achieving uniform results. Agitation provided by mechanical propeller type systems is commonly used for this purpose. Air agitation by bubbling air should not be used, since this decreases the cooling rate significantly and causes excessive oxidation in quench oil.

Fixturing of the parts is also an important consideration for successful quenching. Even with excellent agitation, parts that are densely ranked will quench out non-uniformly. The racking pattern should be such that it allows enough flow to remove heat without local overheating.

Cleaning of the parts after quenching can be accomplished by hot water combined with mechanical impingement. It becomes easier by using non-foaming and non-emulsifying type cleaners. Heatbath’s Multi-Kleen 840Q has been found to work well in removing quench oil.

Reclaiming quench oil from wash water is becoming increasingly more common. In a wash water bath, oil rises to the top of the bath, where it is captured by skimmers for reclaiming or disposal. The oil-water mixture is allowed to settle for several weeks in a settling tank. The quench oil is removed from the top, heated to 250°F to remove traces of water, and then reused. For especially large quenching operations, systems are able to maximize recovery for about 80% of the oil used. On average, for all operation using recovery, at least one third of the quench oil is reclaimed.

In the case of polymer quenching, some users do not carry out any washing. The thin polymer film left on the parts usually burns off and disappears completely during the tempering step, particularly if it is carried out about 400°F. In the case of salt quenching, washing is always required after quenching to prevent rusting. If tempering also is carried out in a salt bath, washing can be deferred until after tempering.

RESEARCH CONSTANTLY IMPROVING QUENCHANTS

Originally quench, oil was used without any additives. It was slow in cooling and susceptible to oxidation. Research was carried out to overcome these shortcomings by adding certain chemicals to the oil. In addition, the objective was to make oil quenching more reliable and uniform, and to control the vapor phase by starting the nucleate boiling stage sooner. Consequently, the term “fast oil” is applied to quench oil with such additives. Some oils also have additives that extend the nucleate boiling stage to achieve deeper hardening for some steel. Specially formulated oils also are available for vacuum heat-treating operations.

Because of the impact of foreign oil dependence, availability, and cost, it is important to minimize oil use, if possible. Even so, quench oil continues to be used on a large scale since, although alternative quenchants have had some good development, their success has been rather limited. A lot of attention is being given today to vegetable sources now that the industry has the means of removing troublesome components (for example, their susceptibility to oxidation). Although synthetic oils have desirable features, such as low viscosity, high flash point, and properties that would enable a wide application range, their cost remains very high. If costs drop, they would become more attractive.

Heatbath has an ongoing effort aimed at developing new quenchants and improving the performance of existing ones. For example, in induction hardening, the main polymer in use for a long time has been PAG (polyalkaline glycol), which exhibits inverse solubility. This means, when temperature increases above a certain point, the polymer becomes insoluble and comes out of solution. It is usually more desirable to have a polymer that has normal solubility; that is, it becomes more soluble as temperature is increased. Efforts have been successful in developing PVP (polyvinyl pyrrolidone) based quenchants, available under the names Parquench®60 and Parquench®90.

One of Heatbath’s newest quenchants is Polyquench 15XN, which has a polymer with normal solubility. However, in contrast to a PAG-type polymer, this quenchant leaves a film that is non-sticky and easily washable.

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