Annealing is an acceptable practice, but it is necessary to take care with the rate of cooling. It is recommended that the cooling rate not be greater than 65°F per hour (20°C per hour) and cool down to approximately 700°F (370°C). H13 has the potential to be at risk of cracking if cooled down any faster, simply because it will air harden and transform to fresh untempered martensite.
The steel will very easily air harden (depending on cross-sectional area), so it is recommended that no normalizing procedure be conducted on this steel after forging.
When austenitizing the steel, it is strongly recommended that the steel be preheated. Do not subject the steel to thermal shock! No steel, especially the tool steels, like to be subjected to thermal shock.
Figure 1 is a suggested method of heat-up to the selected austenitizing temperature.
Because of the alloy content (in particular the carbon and chromium), high austenitizing temperatures are required. Care is necessary when selecting the austenitizing temperature because of grain growth and carbon dissolution.
It is most important that sufficient carbides (interacting with the chromium) go into solution as finely dispersed carbides, uniformly distributed throughout the matrix. The reason for this is to ensure that the maximum amount of carbon (in the steel) is in solution when the austenite phase is reached, which in turn will produce a high-carbon lath to plate martensite phase.
It is not necessary to over-soak the steel at the process temperature because of the potential for excessive grain growth, particularly at the elevated austenitizing temperatures. These steels are also susceptible to grain-boundary carbide networking, which can lead to brittle fracture along the grain boundaries.
More next time in part 3.
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