Quenching
The rate of cooling through the critical range determines the form that the steel will retain. In annealing, the heated steel must be furnace cooled to 900 deg.F.; then it may be air cooled to room temperature. Exceptionally slow cooling to 900 deg.F., which is below the critical range, provides sufficient time for complete transition from austenite to pearlite, which is normal, stable condition of steel at atmospheric temperatures. In normalizing, the heated steel is removed from the furnace and allowed to cool in still air. The cooling is more rapid than annealing, and complete transition to pearlite is not attained. Some sorbite remains in normalized steel, which accounts for the improvement in physical properties over annealed material. Air cooling is a very mild form of quenching. In order to harden steels, it is necessary to use a more rapid quenching medium. There are three mediums commonly used, brine, water and oil. Brine is the most severe quenching medium, water is next and oil the least severe. In other words, oil does not cool the heated steel through the critical range as rapidly as water or brine. However, oil does cool rapidly enough to develop sufficient hardness for all practical purposes. In aircraft work high carbon and alloy steels are oil quenched. Medium carbon steel is water quenched and mild carbon steel is quenched in either brine or water. A severe quench is required for steels with relatively low carbon contents in order to develop the required hardness. Small parts when quenched, cool more rapidly than large parts, and harden uniformly throughout. In large parts the inside core is usually softer and weaker than rest of the material. This fact must be given consideration action in design in calculating the cross sectional strength. The quenching oil is normally maintained between 80 deg. and 150 deg.F, but if water is used as the quenching medium it is held at a temperature below 65 deg,F. This control involves a larger reservoir of liquid and some method of providing circulation and cooling. The rate of cooling through the critical range is governed by the temperature maintained in the quenching medium. In as much as cooling, it is obvious that the quenching medium temperature must be held within limits if consistent results are to be obtained. After steel is reheated and soaked for tempering, it is quenched in either air or oil. Chrome nickel steels, however, must be quenched in oil not air, after tempering in order to avoid “temper brittleness” to which this particular group of steels is subject if air quenched.