Taking the temperature - nanofluids for quenching
Dr Paul Stratton, an Independent Consultant for the gas industry, describes the effects nanofluids have on heat transfer and treatment.
Bubbling gases through the cooling water bath has been used for many years to control the cooling rate of aluminium billets after casting. In the late 1990’s this technique was investigated to see if it could be used during quenching of heat treated components to reduce the cooling rate of water closer to that of oil and create an environmentally friendly quenching medium. Unfortunately, the results were inconsistent and there was little correlation between gas flow and quenching rate.
An alternative would be to dissolve a gas in the water, which has been shown to reduce the heat transfer coefficient and, therefore, the cooling rate, especially at lower quenchant temperatures where the solubility of the gas is highest. Carbon dioxide is commonly available, has the highest solubility in water, and the most effect on its quenching characteristics. However, the reduction in cooling rate is still not sufficient to make the water/carbon dioxide mixture a viable alternative to oil for the quenching of alloy steels. Some other means of reducing the cooling rate of water is needed.
Nanofluids are nanotechnology-based heat transfer fluids, engineered by dispersing and stably suspending nanoparticles with typical lengths in the order of one to 50nm. It has been postulated that, although the presence of small quantities of large particles increase the cooling rate, large numbers of nanoparticles might trap evolved steam at the metal surface during quenching and slow the cooling rate. Research by PH Heat Treatments in Johannesburg, South Africa, confirmed by other researchers in 2009, has found that a relatively low proportion of nanoparticles can dramatically affect the properties of the fluid.
Nevertheless, although the reduction in cooling rate is significant, it is still not enough to slow down a water quench sufficiently for more highly alloyed steels, where the risk of cracking is high. It has therefore been considered that a combination of carbon dioxide and nanofluids might be effective in creating an environmentally benign quenchant.
The effect of the alumina nanoparticles on quenching rate is similar to that of carbon dioxide in solution. The vapour blanket stage of quenching is extended and the maximum cooling rate reduced. With both nanoparticle and carbon dioxide, the maximum cooling rate is almost independent of quenchant temperature without the dramatic fall that is observed with water alone. When all three phases are present, the effects of the solid and the gas are additive, each resulting in a reduction of approximately 50% in the maximum cooling rate at 40°C quenchant temperature. Even though the maximum cooling rate of a water/alumina/carbon dioxide mixture can be well below that of a medium quench oil, the image, below, shows that the maximum cooling rate occurs at a much lower temperature than for the oil. The cooling rate is low at high temperatures, whereas, for most applications, it needs to be high to avoid the pearlite nose on the continuous cooling diagram. At low temperatures it is high, whereas it needs to be low to avoid cracking during martensite formation.
Various concentrations of alumina have been investigated for quenching. It has been found that increasing the concentration of alumina nanoparticles decreases the cooling rate at the critical 300°C for a quenchant temperature of 40°C. Some of the alumina was dragged out with the test piece after quenching. The amount of dragged out material visibly increased with higher alumina concentration, but was easily removed by water washing. Higher alumina concentrations above about six per cent w/w did not need stirring to maintain a consistent suspension and therefore might be preferable in practice.
For the very highest solid contents, where the water/alumina mixture has the consistency of liquid mud, the maximum cooling rate is reduced to less than 20°C/s and the peak in cooling rate around 300°C is completely suppressed. This mixture may have applications in the quenching of highly alloyed steels to replace slow quench oils or salt bath quenching.
Further information: Paul Stratton