Processing and Metal Strength
Date: August 2005
What makes metal stronger, heat and then cool in air to
room temperature, or heat the metal and then straight to water?
Unfortunately, it is not that simple. Different properties arise from
different cooling techniques. Also, when you say stronger, do you mean
higher ultimate tensile strength (how much a metal can stand before pulled
apart), better hardness (can take a hit better), or better fatigue strength
(can stand to be pushed and pulled better), etc. There are many properties
that designers want for a given application. A car designer usually cares
more about strength and cost when designing a car whereas an airplane
designer cares more about fatigue properties and weight. I digress a
little, but it is important to know that "stronger" can mean different
things to different engineers.
One of the main controlling factors to whether or not a metal is better at
any particular property is its grain structure. When steel, for example, is
made, a given recipe for the steel is used. Here in the US we tend to use
ASTM standards to determine metal designations (i.e. 316 stainless steel,
6061-T6 Aluminum (see
http://www.newton.dep.anl.gov/askasci/mats05/mats05009.htm for another
explanation)). The steel is formed by heating it to a given temperature,
which causes certain types of grains to form. This is why you might hear
the term austenitic or martinsitic steel. These are types of grains that
produce certain properties of the steel. Now, the way you cool it will
determine what types of grains form as it cools down, once again changing
the properties of the steel. By slow cooling the steel (cool in air for
instance), certain grains will change from one form to another. Quenching
(dropping it in water or oil), can cause the grains to be locked in, or even
new types of grains to be formed. So, air cooling one designation of steel
may make a certain property better, but air cooling of another type of steel
may make that same property worse because you are changing the grain
structure. Same goes for water or oil quenching or different metals.
Sorry that the answer isn't so straight forward, but that is what makes
materials engineering so interesting. Everyday material engineers are
working around the world to see what affects certain manufacturing
techniques have on certain materials. That is why today we have lighter
weight materials that have higher tensile strengths for aircraft, plastics
that are strong enough to bounce back after impact, and ceramics that can
withstand going from very hot to very cold.
Thanks for using NEWTON.
Christopher Murphy, P.E.
Air Force Research Laboratory
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Update: June 2012