| Postle Newsletter | |
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Volume 1 Issue 2
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| Welcome | |
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Understanding a little
metallurgy will go a long way to understanding wear and how to combat
it. The following is a
little lesson in metallurgy. It
is not intended to make you a metallurgist, but it will help you in
selecting alloys for hardfacing. Lucky for us, pure Iron
and the alloys of Iron, know as Steels, undergo various structural
transformations upon cooling from the molten state to room temperature. Why is this such a fortunate thing? Well, each of the phases exhibits very unique atomic
structures, and with a little manipulation of alloying elements and
cooling rates, one can form a number of alloys with very different
properties and usefulness. For example, by simply varying the cooling
rates of a carbon steel the resultant alloy can be hard, soft, brittle
or ductile. The magnetism of Manganese steels can be altered by simply
changing the cooling rate. Stainless steels can loose their ability to resist corrosion
by simply altering the cooling rate.
The list goes on, but for the sake of this newsletter let us
first consider plain carbon steels, i.e. Iron and Carbon only….no
alloying elements. Upon cooling from the
molten state to room temperature, pure Iron goes through a number of
phases or crystal structures. The
phases are characterized by the way that the Iron atoms stack on top of
one another. For the sake
of simplicity we will only consider three of the most important phases
in terms of hardfacing alloys; Austenite,
Ferrite and Martensite. At
high temperature Iron atoms stack in a face-centered-cube.
This is simply a cube with an Iron atom on each of the eight
corners and one atom in the center of each face. This is known as
Austenite. As it cools it
changes its stacking order into a body-centered-cube.
This arrangement has Iron atoms on each of the eight corners of
the cube and one atom located exactly in the center of the cube.
The latter configuration is stable at room temperature and known
as Ferrite. Unfortunately,
from a structural standpoint Iron in this condition has little value.
It is only when other elements are added does the alloy become
interesting or useful. The introduction of
other elements such as Carbon, Manganese, Silicon, etc. is easily
accomplished in the molten state. Here
all the atoms are not yet in any kind of formation, but as the Iron
cools it enters the first solid phase called the face-centered-mode or
FCC mode or Austenite. This
is where the alloying elements find sites on the cube corners and face
centers and also in the cube center, depending upon their size.
Carbon is a little different however.
It flits around with no real home until the Iron changes form to
the body-centered-cube or BCC mode.
If this transformation happens quickly enough, i.e. by quenching,
the Carbon atom gets trapped in the middle of the cube at room
temperature and distorts the shape of the cube.
The distortion causes the Iron to become very hard and is
commonly referred to as Martensite. If the cooling is slow, the Carbon atoms have time to team up
with Iron atoms in a special compound known as Iron Cabide.
This formation commonly forms layers of Ferrite and Carbide and
is known as Pearlite. Pearlite
is not nearly as hard as Martensite but does serve a useful purpose for
strength and ductility. If
one reheats the hard Martensite up to about 900 degrees Fahrenheit, the
Martensite gives up some of its carbon atoms and upon cooling, the steel
becomes softer and tougher. This
process is known as Tempering. Tempering
softens steel and makes it tougher.
If one heats the steel to even higher temperatures, it becomes
Annealed. This phase is
very soft and workable and is probably the most popular phase for steel
fabrication. After
fabrication, it can be reheated to the FCC or Austenitic phase, quenched
to a desired hardness, and tempered to avoid brittleness. It should be noted here
that the formation of Martensite with just Iron and Carbon is very
difficult. Even at very
high quenching rates, very little Martensite forms at room temperature.
So how do we form Martensite easily?
By introducing alloying elements such as Manganese (Mn), Chromium
(Cr), Molybdenum (Mo), and Nickel (Ni), just to name a few. Various percentages of each of these elements serve to
enhance the formation of Martensite.
To put in another way, they allow the Steel to form Martensite
even a very slow quench rates. Given
the right mix of elements, Steels can form Martensite even if air
cooled. Why are we so interested
in Martensite? Well, it
forms the basis of a group of hardfacing alloys known as Martensitics. They are hard, crack free, fairly tough and resist abrasion
very well. You can think of
a weld nugget or bead as a miniature casting which is rapidly cooled by
the surrounding air and base material.
It goes through the exact stacking and phase changes as described
above. Hardfacing people like Martensite. Sometimes it gives us fits when we want to machine it, but it
goes with the territory. In
addition to helping form Martensite, the various alloying elements also
form their own compounds that range from Carbides to Nitrides to
Borides….. all friends of hardfacing people.
They also impart other characteristics such as corrosion
resistance, toughness, creep resistance, ductility and strength.
The dynamics of alloys is a fascinating subject, and probably way
beyond the scope of a newsletter such as this.
However, suffice it to say that the two most important phases of
Iron base structures from a hardfacing viewpoint are Austenite and
Martensite. These are the
most predominant phases of steels and the ones we design hardfacing
products around. Lets look at a case in
point. If high temperature
Austenite is stabilized with the right combination of alloying elements
it will appear at room temperature instead of transforming to Martensite. This phase is non-magnetic, low hardness (about 25Rc as
welded), work hardens rapidly to 45Rc to 50Rc, and is highly effective
in applications involving impact such as crushers and quarry equipment.
Deposits do not check crack.
Often used as a buildup alloy for more abrasion resistant
products. It is not known
for great abrasion resistance. Popular
Postle alloys in this category are:
If Austenite is not
stabilized and transformation to Martensite occurs, deposits are
characterized with High hardness (>30Rc), low ductility, magnetic,
good abrasion resistance, and often combined with compounds such as
Cabides, Nitrides and Borides. Deposits
do not check crack and are rarely used as a buildup except when Hardness
is in the low 30’s Rc. Popular
Postle alloys in this category are:
Hopefully this brief
introduction will help you understand the nature of just a few of the
Iron base hardfacing alloys. Our
intent is not to make all of our readers metallurgists but a good
understanding of the principles will not only help you in hardfacing
applications but many of the fabrication problems not associated with
hardfacing as well. In the next newsletter,
we will look closer at other forms of Austenitics known as Chrome
Carbides. These are very
effective in combating abrasive wear but have unique properties that you
should be aware of. They
are full of cracks. But we love cracks and we will tell you why.
Believe it or not, hardfacing people love cracks and for a very
good reason. Find out why
in our next newsletter. |
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Postle Industries is
dedicated to providing meaningful education for our customers,
distributors and associates. We
are convinced that the age old axiom “Knowledge is Power”.
This is our most valuable asset, and we are convinced that this
knowledge in your hands is our most powerful ally. In this spirit Postle Industries periodically conducts Online
courses in Welding Metallurgy and Hardfacing.
The classes are free except for a long distance telephone call.
Here is how it works: When a class is scheduled, we will send out invitations via email, along with a meeting name, time, password and a telephone number you can call to attend a group session. On the prescribed time, you may call the number given to you and simultaneously go Online to a meeting place. Here you will join others with the same interest and participate in the meeting. Sessions last from 30 to 60 minutes so it won’t take too much of your day. Right now we are limiting the number of participants so if you want to join a session, be sure to contact us early. In the next few weeks, I will be sending out invitations to the next session. As a subscriber you will be first on the list. |
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Checking
On Customers Before I sign off, I
would like to share this little story with you in A little boy went into a
drug store, reached for a soda carton and pulled it Are
your customers as loyal to you? |
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