With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots. Water is a good rapid quenching medium, provided good agitation is done. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking. Salt water is a more rapid quench medium than plain water because the bubbles are broken easily and allow for rapid cooling of the part. However, salt water is even more corrosive than plain water, and hence must be rinsed off immediately.
Oil is used when a slower cooling rate is desired.
Since oil has a very high boiling point, the transition from start of Martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard. Polymer quenches that will produce a cooling rate in between water and oil. The cooling rate can be altered by varying the components in the mixture-as these are composed of water and some glycol polymers. Polymer quenches are capable of producing repeatable results with less corrosion than water and less of a fire hazard than oil.