Martensitic steel in its un-tempered hard form is much brittle for use. One of the methods to alleviate this difficulty is tempering. Tempering of quenched parts is required for majority of its applications. To get toughness tempering includes heating of steel at less than the lower-critical temperature, which is often 400-1105F or 205-595C as per desired results. sometimes higher temperatures are used for imparting further ductility but at the cost of some strength. these higher temperatures are upto 700C or 1,300F. Technique of tempering is also done for normalized steels. There are other tempering methods as well that involve quenching upto some specific temperature, that is more than martensite starting temperature and holding it at that stage until formation of pure bainite of relieving internal stresses. These include martempering or austempering.
Polished or freshly ground steel forms oxide layers upon heating. At specific temperature, iron oxide forms layer with specific thickness, resulting thin film interference. Thos interference causes appearance of colors on the steel`s surface. Thus you can see various colorful patterns that come & go on the surface. By increasing the temperature layer of iron oxide increases in thickness and changes in colors it reflects. These are the temporary colors that are used for gauging metal`s temperature. At almost 176C (350F) steel starts taking-on very light yellow hue. At 204C (400F), steels turns brown whereas at 282C (540F), color of steel becomes dark straw. At 260C (500F), steel turns brown. Purple color of steel appears at 282C (540 F). deep blue color of steel appears at 310C (590F). Color of steel becomes light blue at 337C (640F).