What a difference a temperature range can make! Some plain carbon steels, some martensitic (quenched)  carbon and alloy steels, and some high Chromium content steels can become embrittled if the wrong temper temperature is used.

This means premature failure in impact applications.
Blue Brittleness
Upon heating some plain carbon and alloy steels-not necessarily those that have been quenched to martensite or bainite-can exhibit both an increase in strength and a substantial decrease in ductility/ impact strength.
In this low temperature range, we call this effect blue brittleness. 
Blue Brittleness is a strain aging mechanism that occurs in this blue heat  temperature range.

Temper Embrittlement

 Martensitic (Quenched) steels can become embrittled if the wrong temper temperature is used.

Two Ductility troughs, this is in degrees C.

Link to graph.
In the range of 700 to 1070 degrees F ( 375 to 575 degress C) most common low alloy  steels show an increase in their ductile to brittle transition temperatures- regardless of whether they are heated into this range, or slowly cooled through it.
(Think large section parts/weldments).
Lower Manganese (below 0.30 wt %) containing plain carbon steel grades do not seem to be susceptible, although elevated levels of Tin or Phosphorus can make even these grades somewhat suceptible.
For these reasons, we would NOT use any tempering cycle below 1100 degrees F (~595 degrees C) for the common carbon and alloy constructional steels typically designated by AISI and SAE in North America.
This gets us past the ductility troughs seen on the above figure.
500 Degree F Embrittlement
500 degree F Embrittlement also occurs in quenched and tempered High Strength Low Alloy  (HSLA) steels  when they are subjected to a temperature range between 400- 700 degrees F (~ 200-370 degrees C). This differs from Blue Brittleness in that it is a phenomenon of tempered martensite, it is not related to strain aging. I was taught that it is rather a result of  precipitation along prior austenitic grain boundaries.
 Proper selection of steel chemistry is the best defense against this type of embrittlement, with Aluminum additions above 0.1 weight %  usually effective at preventing the problem. (Some steel producers lack the ability to add  Aluminum to their steel melt due to technology constraints on their casters…)
400 to 500 Degree C Embrittlement
If the steel is high Chromium content  (15% or more by weight) it can be subject to embrittlement when held in a 400-500 degree C  (~750 -930 Degrees F) range for a long enough time.
(Think heat affected zone in welding Stainless steels.)
This embrittlement can be eliminated by a proper soak at a higher temperature to redissolve the carbide (and possibly nitride?) precipitates.
These are the primary forms of embrittlement that I have encountered in my career. Other types of embrittlement  can include Liquid Metal Embrittlement, Sigma Phase Embrittlement and Embrittlement driven by neutron irradiation, or environmental factors such as hydrogen absorption, (often in plating) or Stress Corrosion Cracking where outside chemical attack and mechanical stress produce fine cracks in the steel.
Bottom line: Thermal treatments, and post quench temper treatments below 1100 degrees F are not recommended because of their possible embrittling effects on susceptible steel grades in common use in North America.



  With increasing temper temperature, the plasticity of a quenched martensitic structure increases up to around 400 degrees F; decreases to a minimum in the 450-700 degree F  (230-370 degrees C) range, and then continues to increase.

By retarding transformation rates, moly improves the hardenability of its alloy steel grades.

Believe it or not, its name is from the Greek word for lead.

Molybdenum is an essential micronutrient, but large doses can be highly toxic. Fortunately, we don’t eat our alloy steels.
Molybdenum (“moly”) is added to constructional steels to

  1. Improve hardenability by slowing the transformation (moving the nose of the curve to the right);
  2. Reduce embrittlement during tempering
  3. Enhance the creep strength  of low alloy steel grades at higher temperature,
  4. Add resistance to corrosion.

Moly does this in very low quantities, and so it is truly a “synergistic” alloying element. Typical moly additions in constructional steels are around 0.10-0.60% by weight. Moly analysis typically runs 0.20-0.30 in the low hardening 40XX grades; 0.15-0.25 in the 41XX series of alloy steels; and 0.20-0.30 in the deeper hardening 43XX and 48XX steels.
Moly has been reported in Japanese swords as far back as the 14th Century, but its first major military use was for tank armor in World War I. The French firm  Schneider & Company made moly armor plate which at 25 mm was able to stop a direct hit from a shell. The prior manganese armor plate at 75mm thick was not so impervious and  the reduction of steel mass by about 2/3 made the tanks with moly armor much more mobile (speed and manuverable) in combat. Today moly is an indispensable part of many aerospace and high temperature applications including rocket nozzles.
While Moly can be the only alloying element added (40XX steels) it is also used in combination with Chrome (41XX) Nickel (46XX and 48Xx, or in a triple alloy combination  with Chrome and Nickel (43XX or 86XX) as well as other grades (87XX, 88XX,  and grade 9310 come to mind).
But where we see moly in our shops is in our M- series tool steels. That M prefix stands for Molybdenum, which gives these tools steels their characteristic high hot  hardness.  Moly content in M series tool steels ranges from 4.50% up to 9.50% by weight. It is the ability of these steels to resist softening at high temperatures that makes them so useful in our shops at production speeds and feeds.
The moly tool steels also have a tendency to decarburize so careful grinding and attention to details in heat treatment is critical in toolmaking and sharpening.
For more info on Molybdenum, Click on the Mindmap for Molybdenum.
Photo of moly metal.
Trivia: the first commercial heatof Moly High Speed Steel was by Universal Cyclops in 1931. Grade AISI M1. They called it Motung for, you guessed it, MOly TUNGsten.