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By retarding
transformation rates, molybdenum 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
- Improve hardenability by slowing the transformation
(moving the nose of the curve to the right);
- Reduce embrittlement during tempering
- Enhance the creep strength of low alloy steel
grades at higher temperature,
- 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.
Editor's Note: CR4 thanks Milo for sharing this blog entry, which originally appeared here.
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Re: Four Contributions of Molybdenum to Steel
