Which is better, water quenching or oil quenching

Water, salt water is a strong cooling capacity of the medium. Oil is a medium with weak cooling capacity.
It is mainly the cooling curve of the steel, the steel choice of oil quenching or water quenching, the general carbon steel is moved up the left, for the oil can not meet the requirements of v velocity, will produce other types of phase change. So it's cooled with water. However, it can not be generalized, such as the workpiece is small and appropriate to increase the heating temperature and strengthen the flow of oil. It can also be hardened to a higher hardness. In a general sense, carbon steel due to poor hardenability, in order to obtain higher mechanical properties, generally need to choose a strong cooling capacity of the medium. So water or salt water is usually used. Alloy steel due to the addition of alloying elements (except cobalt) to improve the stability of austenite, can choose a weak cooling capacity of the medium (oil)