To All:
I read a heat treating article in Cutting Tool Engineering this week that I thought gave a good overview of the processes.
There doesn't seem to be a direct link to the article, but if interested you can go here, click on the PDF and I believe it's the second article in a group of articles.
================================================================= Heat treating By Dr. Laroux K. Gillespie
For any specific material, the results of heat treating depend upon the temperature and the amount of time the material is held at that temperature. The cooling rate for a specific metal, such as steel, determines its microstructure and properties.
Isothermal transformation is the key to understanding what the material properties will be after cooling. For example, if the material is cooled at 1,000° F/sec. until it is below 200° F, a martensite structure is produced, which is magnetic and hard. At a slower cooling rate, such as 60° F/sec., some pearlite, which is relatively soft, will develop, but some of the material will have a martensite structure. If cooled at a rate of 1° to 30° F/sec. or slower, almost all of the material will be pearlite. Steel is in an austenite (?) phase when heated to 1,333° F. Austenite is a soft material. If quenched rapidly, the steel stays in that phase until it reaches about
400° F and then starts turning into martensite. Once the temperature falls to about 200° F, the transformation to martensite is completed. Martensite?s hardness is 53 to 67 HRC, and pearlite is about 15 HRC. Bainite is up to 34 HRC but is less ductile than pearlite. Because tensile strength is almost directly proportional to hardness, a faster cooling rate produces stronger steels?ones that have transformations similar to AISI 1080. By raising the temperature and holding it at just below 400° F for 1 to 2 hours, the steel becomes ?tempered,? attaining a more uniform phase throughout the material. Higher temperatures are also used to temper some of the retained austenite to bainite and cementite, or iron carbide. But for tempering temperatures above 400° F, the steel starts to soften. The higher the temperature, the softer the steel will be. In contrast to making a more uniform and less brittle steel, annealing softens steel. To provide a soft pearlitic structure, 1040 steel, for example, is annealed at 1,450 to 1,600° F for 1 hour and then cooled to 1,200° F. Dropping the material into water results in a quench rate of about 300° F/sec. In contrast, the quench rate in oil is 30° F/sec. and in air is 1° F/sec. Tempering the martensitic steel reduces its hardness and tensile strength. As the figure shows, heating AISI 1040 martensitic steel to 1,200° F and holding it there reduces its tensile strength from 130,000 to less than 100,000 psi, but the material is more ductile with 28 percent elongation rather than 20 percent. =================================================================