Be that as it may, there are also tempering temperature ranges in relation to the steel being treated that will cause: temper embrittlement, tempered-martensite embrittlement (TME) or blue brittleness.
In essence, a simple description of the brittleness that can be experienced during the tempering process can be: “When tempering some martensitic-forming steels, brittleness can occur as a direct result of tempering the steel within a specific tempering-temperature range, followed by slow cooling. The temper brittleness will show itself as an increase in the ductile to brittleness ratio.”
Tempering in the range of 800-1100°F in all of the low- and medium-alloy steels and tool steels should be avoided. Most steels are brittle when tempered in this region and slow cooled.
The reasons for the embrittlement phenomena are not clearly known. However, it is theorized that if retained austenite is present after quenching, it will transform to lower bainite and, secondly (and perhaps the most dramatic), there will be a severe drop in toughness (impact strength).
Temper embrittlement may occur in certain types of martensitic steels that are generally tempered in the range of 800°F up to 1100°F. The occurrence of temper embrittlement can also occur in certain alloy steels, which is a direct result of slow cooling from the tempering temperature. This type of embrittlement is usually found in medium- to high-manganese steels, (1.25% up to 2.00% manganese) and also steels that contain chromium and nickel.
Temper brittleness can be revealed by notched impact testing at room temperature or slightly below room temperature. The simple remedy for this phenomena is not to cool slowly back down to room temperature from the selected tempering temperature. The steel could be air cooled from the selected tempering temperature or cooled in oil.
The high-alloy tool steels are generally not affected by the temper embrittlement phenomena when tempered at high tempering temperatures.
Next time we will discuss TME and blue brittleness.