Use of Polymer Quenchants
Application of polymer quenching techniques to aluminum alloy heat treatment has allowed the heat treater to achieve more consistent results when attempting to solve quenching related problems. The two main advantages of using polymer quenchants are (1) the control of distortion (particularly in thin sheet metal products) and (2) the reduction of residual stresses in the quenching of larger parts such as forgings and castings. The polymers allow for a precise control over quenching speeds which is not achievable through the use of hot water.
Polymer quenchants have found wide application for most aluminum products such as sheet material, extrusions, forgings, castings and especially fabricated (machined and formed) parts. The most accepted of the available polymer quenchants are polyalkalene-glycol based materials. These polymers are water soluble and by a simple variation of the water dilution, a range of quenching speeds can be achieved. The unique characteristic of "inverse solubility" causes a thin film of concentrated glycol to coat the hot part as it is quenched resulting in a more uniform rate of heat transfer which aids in controlling distortion.
Controlling Quenching SpeedsThe speed of the glycol quench is controlled by varying the concentration of the glycol. The higher the glycol concentration, the slower the cooling rate. Heating the tank is not necessary as adequate control of quenching speeds is achieved by adjusting the concentration in a room temperature bath once the proper concentration has been properly selected. Slight temperature changes in the tank do not significantly effect the cooling rate. Thus, the problem of obtaining different quench rates due to the temperature variation (as is experienced in a hot water tank) is not a factor. Most heat treating specifications require the glycol tank to be not higher than 90° F at the time of quench, although some allow the tank to be as high as 130° F. The cooling rates do slow down slightly between 75-130° F as shown in Figure 1.
The subject of distortion control, not only during the quenching operation, but through the entire processing sequence of aluminum alloys is an extremely complex subject. The basis for quenching distortion has to do with the (1) expansion and contraction of parts when being heated and cooled and (2) control of the methods of cooling, racking etc. The subjects of distortion and warpage can be found in this in-depth article. However, it is appropriate to discuss the impact of glycol quenchants here to note their significant contribution in controlling quenching distortion. Today, glycol quenchants are the main tool the heat treater has available with which to achieve adequate distortion control in the heat treating plant.
Distortion control of sheet metal parts is achieved through the application of two glycol quenching characteristics:
(1) Inverse Solubility - The coating action of the glycol on the surface of the part allows for more even heat extraction over the entire surface of the part due to the elimination of the vapor pocket which occurs with the water quench. As a result, the part contracts more uniformly as it cools and less distortion occurs.
(2) Slower Cooling - As previously noted cold water quenching of sheet metal aluminum achieves quench rates (2000-10,000° F/second) which are significantly faster than the critical quench rates necessary for achieving full properties in all alloys. Quench rates of from 200-1000° F/second are adequate in most cases and for some alloys, 10-50° F/second will achieve satisfactory properties. Distortion is reduced because as the part is immersed into the quench bath, the slower cooling rate of the glycol quench establishes less severe temperature gradients within the part. As a result, the differential thermal contraction that occurs as the part cools is less severe with the water quench and less distortion occurs.
Glycol quenchants, because of their flexibility in achieving controlled cooling speeds, can be tailored to obtain the precise cooling rates needed to achieve the required mechanical properties while at the same time minimizing the distortion of the part. These results cannot be achieved consistently through the use of water quenching.
Control Of Residual StressesPrior to the introduction of the glycol quenchants, heat treaters attempted to reduce quenching stresses by using hot or boiling water. The less severe quench achieved by increasing the water quenching temperature, did result in lower residual stresses as shown in Figure 3 for various alloys. However, lower properties can result - particularly in the more quench sensitive alloys as is shown in Figure 4. The problem is compounded by the fact that only about 20% residual stress relief occurs by increasing the water temperature from 90° F to approximately 150° F. Above 160° F, lower residual stresses can be accompanied by significant loss in tensile properties. By taking advantage of the unique heat transfer characteristics of the glycol quenchants, a lower level of residual stress can be obtained while at the same time minimizing any loss in tensile properties. Uniform extraction of heat is accomplished at a quench rate adequate for achieving a high level of mechanical properties while imparting a minimal amount of residual stress, especially to complex aluminum alloy components.
Even the control of residual stress is much more consistent when using glycols than with hot water quenching. The effect of reducing the residual stress level of aluminum alloys with Ucon Quenchant A is shown in Figure 6. As with water quenching, the lowest residual stress levels are achieved with the slowest quenching rates. When using glycol quenchants, the optimum levels are usually achieved with concentrations of 34-40% Ucon Quenchant A. The ability to utilize these concentrations depends upon the thickness and quenching sensitivity of the material being quenched.
Summary - Glycol Vs. Hot Water Quenching
The main advantage of glycol
quenching compared to hot water
quenching is that more consistent quenching
is achieved load after load and that
quenching performance is not
significantly affected by normal bath
temperature variations. A ten to twenty
degree change in a room temperature
glycol quench tank temperature will have
little effect on its quenching
performance. By contrast, the same temperature
variation in a hot water tank
significantly affects the bath
performance and can have significant
effects in the resultant tensile
properties of the heat treated parts.
copyright © 2010
by Tom Croucher, a consultant to the metallurgy industry.