Tempered Martensite Embrittlement
Tempered martensite embrittlement is also known as TME. The root causes of TME are the topic of ongoing research. This phenomenon will occur in low-alloy/medium-carbon steels and will occur at tempering temperatures from 500°F up to 700°F.

It is also known as “500°F embrittlement,” and it will occur when cooling the tempered steel. The condition of TME will usually occur within the first hour or so after tempering within the 500-700°F range followed by cooling from that range. The interesting thing about TME is that it occurs fairly quickly after tempering; whereas the TE (Temper Embrittlement) can occur many hours after the cooldown from the tempering temperature.

It would seem that low-alloy steels with a high phosphorous content will be more susceptible to the embrittlement than those with a low phosphorous content. Once again, it is pointing to the simple step on receipt of the steel for heat treatment. Check the test certificate before heat treating the steel.

Temper Embrittlement
Temper embrittlement is a much more difficult condition to observe, simply because it is progressive after the tempering process. There will be a progressive decrease in the impact strength over time. So the metallurgical characteristics will be changing over time.

It is recommended again that the test certificate should be studied for the impurities present in the steel, such as phosphorous, antimony, tin and arsenic.

Very small amounts of these elements/impurities have caused TE at concentrations less than 0.01%. Additionally, silicon in combination with high manganese contents can contribute to TE.

Blue Brittleness
Blue brittleness is generally caused by tempering some of the steels (usually the plain-carbon steels) in the “blue” temperature range, followed by slow cooling. The occurrence of these phenomena will increase as the carbon content increases. The temperature range that blue brittleness is likely to occur at is in the range of 400-800°F. The actual temperature for any particular steel will vary. It will, however, still be within a temperature range of 400-800°F.

Conclusions
It should be understood that the aforementioned embrittlement phases will occur to certain steels and in particular to the plain-carbon/low-alloy steels. The recommendation is to try to not temper within the temperature ranges given, thus avoiding the potential for a problem.

If this is not possible, then one should not let the tempered steel cool down slowly. The tempered steel can be successfully cooled down in a moving air stream or cooled in oil followed by a wash.