Understanding Quenchants and Their Effects
Everyone who has ever seen a movie about medieval life is likely familiar with the image of a sweaty, soot-covered blacksmith pounding away with his hammer on a sword. When finished, he lifts the bright red, heated blade from the anvil and plunges it into a nearby bath. Most audiences probably think the water was meant just to cool the hot blade. But as any metallurgist or heat treater knows, the blacksmith was forging and heat treating the sword and using the water bath to quench it.
Since the beginning of the Bronze Age nearly five thousand years ago, metallurgists have used heat treating and quenching to influence the final properties of metals. Heat treating offers manufacturers control over hardnessa, toughness, and corrosion resistance. Quenching helps to further influence hardness, and reduces the level of distortion and residual stress created during the manufacturing process. The primary and critical role that quenching plays is to control the rate of cooling in a metal. If it is cooled too slowly, the metal will become more brittle, reducing its overall hardness. If it is cooled too rapidly, there will likely be distortion and cracking. The key to controlling the rate of quenching and, in turn, the rate of cooling, is choosing the right quenchant for the application.
No discussion of quenchants would be complete without a brief summary of process variables and cooling mechanisms. The most important variables of the process are equipment design, type of quenchant, quenchant concentration, and bath temperature and agitation rate. Each can have a pronounced effect on the final properties of the workpiece and must be carefully controlled throughout the process. Variables must be carefully fine-tuned with regard to the geometry and specific alloy of the part to be quenched. For example, alloys with low hardenability quenched in water require a high level of bath agitation to provide the desired propertie
Three Stages of Heat RemovalThere are three stages of heat removal associated with quenching in liquids, commonly referred to as A, B and C stages. Stage A, the vapor blanket cooling stage, is the first step in which a vapor blanket forms around the quenched metal. Cooling will continue in stage A until the amount of heat in the metal part is less than the amount of heat needed to evaporate the quenchant and maintain the vapor phase. This leads to stage B, the nucleate boiling stage, in which the vapor pocket collapses and heat is rapidly removed from the surface of the metal as the liquid comes into contact with it and is vaporized. Stage B is when the most rapid cooling takes place. Stage C is the liquid cooling stage, which begins when the temperature of the metal falls below the boiling point of the liquid quenchant. Boiling stops and the metal is further cooled through conduction and convection into the liquid. The rate of C stage cooling is most directly affected by the viscosity of the liquid, with cooling rates decreasing with increased viscosity.
Although there is a wide range of quenchants available, for the majority of today's heat treating applications liquid media are preferred. Water, oils, brines, caustics, and polymers are the five most commonly used liquid quenchants. Each type offers different cooling rates, residual effects, equipment maintenance and product storage considerations, as well as other factors. When selecting the right quenchant for a particular application, heat treaters must consider all these factors, as well as the metal to be quenched, the acceptable level of residual stress, the geometry of the workpiece, and the desired hardness of the final product.
Natural QuenchantsWater is the oldest and still one of the most common quenchants. It is inexpensive, easy to use and has minimal safe handling or disposal considerations. Water, however, does have a number of limitations that makes it a less desirable choice in certain applica-tions, particularly with steels of high hardenability,
Plain water offers almost the maximum cooling rate available in a quenchant liquid and is most commonly used with steels that have low hardenability. When a hot piece of metal enters a water bath, the water heats and begins to boil. This is Stage A cooling in which a vapor pocket (which inhibits cooling) forms around the metal. The vapor pocket is unstable, however, and can lead to non-uniform heat transfer between the quenchant and the metal. Non-uniform heat-transfer creates thermal gradients within the metal that can cause unwanted high residual stress and distortion.
To counteract this effect, quenchant baths need to be constantly agitated, but this action can reduce the water's cooling power and care must be taken when choosing the proper rate of agitation to ensure the water offers the desired quenching effect. The use of water is usually restricted to basic, uniformly shaped parts of plain carbon or low alloy steels that are thin and of low hardenability. Because the minerals found in municipal or well water can lead to inconsistent quenching performance, distilled or demineralized water is preferred.
Brine, which offers faster cooling than plain water, can also be used as a quenchant. Brine solutions are typically made by adding a small concentration (usually five to seven percent) of sodium chloride or calcium chloride to distilled water. Like water, it is the preferred medium for alloys with low hardenability.
When steel is quenched in brine, a layer of salt is precipitated onto the metal. The salt layer disrupts the vapor jacket that forms around the metal, which helps to reduce and eliminate non-uniform heat transfer in the solution. In addition, brine permits a reduced level of agitation compared to water. Temperature is less critical for a brine system, thus reducing its importance as a variable. Brines are most commonly used with high carbon steels or parts requiring high hardness. Brine's primary drawback is its corrosive nature. This an issue for the metal, but it is also a consideration for quenching equipment used, which may be prone to more frequent equipment shutdowns and higher maintenance costs.
Similar to brine are caustic solutions that are water-based with five to ten percent sodium hydroxide. These offer performance characteristics similar to brine and can be used with a range of metals. However, because of its high alkalinity, heat treaters must be aware of the special storage and handling considerations associated with this quenching medium.
OilsOils are excellent quenchants and are valued for their ability to offer rapid cooling over a wide range of temperatures. They can be used on a variety of metal alloys, as well as on parts with complex geometries and of varying thicknesses. Oils are classified in three distinct groups: conventional, fast, and martempering (or hot quenching). They are categorized based on their quenching effect, use temperature, and overall composition. Also, some heat treaters will use emulsions of soluble oils that are more commonly used as cooling liquids during the grinding, cutting and forming processes.
One of the most favorable characteristics of oils is their ability to "thin" when heated. Quenchant liquids in general tend to lose their cooling power as they approach boiling point. The thinning, or loss of viscosity, that oils exhibit during heating helps to counteract this effect.
Oils are highly valued for their ability to operate effectively over wide temperature ranges. Generally, they cool at a slower rate than do water or brine. However, they offer more uniform heat extraction, resulting in potentially less distortion and cracking in the metal. Quenching oils are available with flash points ranging from approximately 130°C to 290°C. It is common practice to operate the quenchant bath from 65°C to 100°C below the flash point to eliminate the possibility of the oil combusting. This "safety zone" can be reduced to 10°C when quenching in a protective atmosphere.
Conventional oils are mineral oils that may contain antioxidants but are free of additional additives that may affect their quenching properties. They are characterized by a long vapor phase with relatively slow cooling, very fast cooling in their boiling range and very slow cooling during the convection stage. The use of conventional oils is generally limited to steels with medium to high hardenability. Fast quenching oils are mineral oils that contain proprietary additives to increase their quenching power. Fast quenching oils may contain additional antioxidants, wetting agents, and other additives to tailor their desired effects. They are characterized by a high initial quenching speed, which can approach that of water, fast cooling during stage B, and a cooling rate similar to that of conventional oils during the convection stage. Usually, fast-quenching oils are used on parts made from steels with low hardenability and/or large cross-sectional areas.
Martempering oils are paraffin mineral oils that offer excellent thermal and oxidation stability. They are primarily used at high (95°C - 230°C) temperatures for the martempering of ferrous metals. They are available with a range of quenching/cooling powers and provide unique benefits because of their various additives and modifiers.
Polymer SolutionsPolymer quenchants are the most versatile, stable, and consistent performers among aqueous quenchants. For these reasons, these products have seen dramatic growth in use during the past sixty years. The first widely used polymer quenchant was polyvinyl alcohol (PVA), but it is no longer commonly used in the U. S. because of regulatory concerns. Currently, there are several proprietary polymers available in the marketplace, but the most commonly used formulations are polyalkylene glycols (PAGs), polyvinyl pyrolidines (PVPs), and polysodium acrylates (PSAs). Today, PAGs are the most commonly used polymer quenchant in the United States.
Polymer quenchants can be formulated to provide quenching power ranging from that similar to fast quenching oils to that greater than water. PAG quenchants have shown significant market penetration, replacing fast and medium speed quenching oils for a range of steels and part geometries. PAGs are useful in nearly all heat treating operations, including batch quenching, integral quenching, continuous quenching, and press quenching. They are especially suited to induction heat treating operations. PAGs are ideal for aluminum quenching, virtually eliminating distortion problems previously en-countered with water quenching. They are so compatible with aluminum that they are specified in quenching aluminum for aerospace applications by AMS 3025B. They offer equal or better cooling rates and can be used in a variety of heat treat-ing operations.
The distinctive formulations of PAGs make them completely soluble in water at room temperature. PAG molecules exhibit a unique attribute of inverse solubility in water at high temperatures. They quench hot metals by surrounding the metal piece with a coating that can control the rate of heat exchange with the entire aqueous system. As the metal part approaches the temperature of the PAG quenchant, the polymer coating dissolves back into a uniform concentration in the quenchant bath.
Compared with water, PAGs offer a more stable vapor blanket cooling mechanism to facilitate uniform heat transfer. In turn, PAGs help to reduce quench cracking and distortion. They degrade slowly, so changes to the quenchant system tend to be gradual and can be monitored and controlled with routine maintenance. In addition, PAGs have a more favorable safety profile than oils, as they do not burn and smoke when heated. They are non-toxic and spills can be cleaned up with only water and without the cost or safe handling considerations of environmentally toxic solvents. They enable heat treaters to lower their overall operating costs because they can employ and maintain less air pollution and fire protection equipment than with oil.
PAG quenchants are stable solutions that offer minimal environmental and health risks when compared to quenching oils. They are easily removed from the workpiece during cleanup and are moderately biodegradable in the environment.