The steel can be quenched from the selected austenitizing temperature and isothermally quenched to approximately 1000°F (535°C) and held for equalization. If processing in vacuum and providing that there are requirements of pressure, velocity and volume available, then the method of isothermal cooling for H13 is very practical.
The process can be conducted even with salt bath. This can be accomplished from the austenitize temperature salt bath into a nitrate-mix salt bath. Remember that salt baths will provide a very uniform temperature distribution throughout the molten bath.
The Ms line (martensite start) on the Isothermal Transformation diagram is quite high for H13 at approximately 600°F (315°C), and the “nose of the curve” is positioned to the right-hand side of the diagram at approximately 60 seconds.
It is important to temper as soon as the steel is down to hand warm. This is when you could almost touch the steel surface without burning yourself. All of the tool steels have a sensitivity and risk of cracking, which is the primary reason for tempering immediately.
The steel is also susceptible to incomplete transformation, which means that there is the potential for residual untransformed austenite to be present if the process parameters and the cool down rate are not correct. Retained austenite is the result.
High-temperature tempering will begin to decompose retained austenite. In addition, the steel is also considered to be a secondary-hardening steel when tempered around 930°F (500°C). This simply means that the M?C and the MC types of carbides are beginning to precipitate out of solution from the as-quenched martensite. It should be noted that not all of the carbides are precipitated, nor has all of the austenite been transformed. Hence the need for at least two tempers, preferably three.
More next time in part 4.