AOPA Pilot Magazine - February 2018 - 95

What makes metal
so useful is that it
exhibits all these
properties. It may
not be the best at any
particular thing, but
it sure does a lot of
things well.
up to bright cherry red and then plunging
it into a bucket of water. This quenching
transforms the wire into a very hard condition. In fact, the heat-treated wire is now so
hard that it is quite brittle and would probably snap off in a hard landing.
The final step is to temper the wire back
to somewhat reduced hardness to provide
the required toughness and springiness.
This is done by heating the wire back up
to a medium blue color (about 750 degrees
F) and allowing it to cool slowly. The final
hardness is a function of temperature-the
hotter the metal is heated during the tempering process, the softer, tougher, and less
brittle it will be.
This same process-annealing, bending,
heat-treating, and tempering-is precisely
the way spring-steel landing gear legs on
production aircraft are made.
HOW HEAT-TREATING WORKS

The physics and chemistry behind heattreating of carbon steel are complex, but
the basic principle is this: At room temperature, the atoms of metallic iron are
organized as a crystalline structure known
as body-centered cubic. When iron is
heated above its critical temperature of
about 1,400 degrees F, the structure transforms into face-centered cubic.
Steel is iron with a few percent of
carbon mixed in. When steel is heated
to the critical temperature and the iron
transforms from body-centered cubic to
face-centered cubic, the carbon atoms
migrate into the central position of the
cubes formerly occupied by iron atoms,
creating nonmagnetic austenite.
If this austenite is allowed to cool slowly
(anneal), iron atoms migrate back into the

center of the cubes and force the carbon
atoms back out, resulting in a mixture of pure
iron (Fe) and iron carbide (Fe3C), which is a
soft steel known as pearlite.
On the other hand, if austenite is cooled
quickly by quenching it in water or oil, the
carbon atoms get trapped inside the crystalline structure to form a very hard, very
brittle steel called martensite. This process
is called heat-treating, and the result is too
brittle for most uses.
The martensite may then be tempered
by heating it back up to a temperature
well below critical-typically 200 degrees
F to 800 degrees F, depending on the final
hardness desired. This allows some of the
trapped carbon atoms out of their crystalline jail cells, and relieves some of the
hardness and brittleness. In this way, varying degrees of hardness and toughness may
be achieved. A steel cutting tool needs to
be very hard, while a steel landing gear leg
needs to be tough and springy.
Aluminum alloys can also be hardened through heat-treating and softened
by annealing. The critical temperature is
a lot lower (about 800 degrees F), but the
principle is the same.
STRAIN HARDENING

Another way to harden metal involves
pressure. When soft metal is compressed
beyond its elastic limit, it becomes harder
because the atoms in its crystalline structure are packed together more tightly.
This is known as strain hardening or work
hardening.
Sheet metal may be strain hardened by
passing it between pressure rollers, either
when cold (cold-rolling) or hot. The most
common sheet metal used in aircraft construction is called "2024-T3 Alclad." The
2024 refers to the aluminum alloy (approximately 4 percent copper plus small
amounts of tin and zinc). The T3 means
that the metal is heat-treated and then
cold-rolled. The Alclad means it's then
plated on both sides with a very thin layer
of commercially pure aluminum to provide
corrosion resistance.
Another example of strain hardening
is the humble rivet. The AD rivets most
commonly used in aircraft construction
are made of relatively soft 2017 aluminum

alloy. When the rivet is driven with a rivet
gun and bucking bar, the alloy is strainhardened to form a strong joint.
For large parts like pistons, connecting
rods, crankshafts, and camshafts, strain
hardening often is accomplished by forging, in which a hot metal ingot is pressed
into shape in a hydraulic press. Forged
parts are stronger than cast parts because
the forging process strain hardens the
metal as it is being shaped.
CASE HARDENING

Alloying, heat-treating, and strain hardening are all methods of hardening a metal
part in its entirety-so-called "through
hardening." Sometimes, it's desirable to
harden only the surface (perhaps just one
surface) of a metal part, while leaving the
interior tougher and less brittle. Examples
include cylinder walls, crankshaft journals, cam lobes, and tappets. The process
of hardening only the surface of a part is
known as case hardening, and it can be
accomplished in two ways: mechanical
and chemical.
Mechanical case hardening is commonly accomplished by cold-rolling or
shot peening, both of which harden the
metal surface by strain hardening. The
two most common methods of chemical
case hardening are carburizing and nitriding. Carburizing involves baking the part
in a carbon monoxide atmosphere, causing additional carbon to be absorbed into
the surface and converting a thin outer
layer from mild steel to harder and stronger high-carbon steel. Nitriding is similar,
but the part is baked in an atmosphere of
ammonia gas (NH3), causing the surface to
absorb nitrogen atoms that make their way
into the interstitial regions of the iron lattice much as carbon does. Both methods
result in extremely hard and durable wear
surfaces without embrittling the part's
interior.
It's amazing what metal can be coaxed
into doing. It's fascinating stuff.
AOPA
MIKE BUSCH is an A&P/IA.
EMAIL mike.busch@savvyaviator.com

WEB

www.savvyaviation.com

www.aopa.org/pilot AOPA PILOT | 95
http://www.savvyaviation.com http://www.aopa.org/pilot
AOPA Pilot Magazine - February 2018

Table of Contents for the Digital Edition of AOPA Pilot Magazine - February 2018
