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.

As machinists, we seldom encounter microalloy steels. but what do we need to know?

  1. Microalloy steel is manufactured like any other, but the chemical ingredients added at the initial  melt of the steel  to make it a microalloy include elements like Vanadium, Columbium (sorry, Niobium for us IUPAC  purists), Titanium, and higher amounts of Manganese and perhaps Molybdenum or Nickel.
  2. Vanadium, Columbium Niobium, and Titanium are also grain refiners and aggressive Oxygen scavengers, so these steels tend to also have a very fine austenitic grain size.
  3. In forgings, microalloy steels are able to develop higher mechanical properties (yield strengths greater than say 60,000 psi) and  higher toughness as forged by just cooling in air or with a  light mist water spray.
  4. Normal alloy steels  require a full austenitize, quench and temper heat treatment to develop properties greater than as rolled or cold worked.

Since microalloyed steels are able to get higher properties  using forging process heat- rather than an additional heating quenching tempering cycle- they can be less expensive to process to get improved mechanical properties.
 The developed microstructure ultimately makes the difference. The  microstructure developed in the steel depends on the grade and type.

Tempered martensite for normal alloys.

  • Normal alloy steels require a transformation to martensite  that is then tempered in order to achieve higher properties.
Bainite comparable hardness improved toughness.
  • Microalloy steel precipitates out various nitirides or carbides and may result in either a very fine ferrite- pearlite microstructure or may transform to bainite.

For machinists, if the steel is already at  its hardest condition, the microalloyed microstructure of either ferrite pearlite or bainite  is less abrasive than that of a fully quench and tempered alloy steel.
P.S. The non- martensitic structures also have higher toughness.
We don’t tend to machine prehardened steels in the precision machining industry, but if you ever are part of a team developing a process path for machining forgings, or finish cuts after induction hardening, these facts might be good to know.
Georges Basement Bainite 1000X