"Chopster" wrote in message
oups.com...
I am currently getting into metallurgy and going through the book I
have I came upon a question that I was unsure on the answer.
There is a piece of 1018 cold drawn steel that is not meeting
elongation, what possible fixes are there?
The only thing I could think of is to re-temper the steel and quinch it
in water. Anyone have any suggestions on this?
Is this a real life problem or an essay question in the book? If it's a
real life problem then if we could get a little more information such as the
tensile strength, yield strength and/or perhaps the hardness of the steel
then we could suggest a solution.
Assuming this is a test question in the book and no other information is
provided then you have to go with the facts that are in the test question.
Let's break down the facts and show how you can determine what to do.
Since the questions is specific that this is cold drawn 1018 steel, then you
have two "facts":
1) The (SAE or ANSI) 1018 grade gives you the chemistry, which can be looked
up in a handbook. This would be a plain carbon steel (as opposed to a
medium carbon, high carbon, or alloyed steel). The carbon content is about
0.18% (+/- 0.05%) and the Manganese content would be about 0.30-0.60%.
There's nothing else of appreciable quantities in this steel. It is a grade
of steel that is NOT designed to be heat treated by quenching and tempering.
2) The "cold drawn" tells us the material was mechanically worked. If the
steel is in the form of a sheet the sheet was put through rollers like
cookie dough with a rolling pin only on a big scale. Each time the metal
thickness is reduced when it goes through the rollers, the grain structure
is changed. At an atomic level the atoms move across each other and through
this process the tensile strength and hardness is raised, but also through
that process the elongation is reduced. If the steel is in the form of
barstock the same thing happens, if you start with (for example) a 3 inch
diameter bar and pull it (i.e. "draw" it) through rollers that squeeze it
into a smaller and smaller diameter, as the diameter is reduced the tensile
and hardness go up whilst the elongation goes down.
There are two possible fixes to cause the elongation to be increased. Both
involve heat treat.
First choice would be a SUBCRITICAL anneal. In this case the cold drawn
steel would be heated up to perhaps 1150 deg F and soaked for a time at that
temperature. When the steel is brought out of the furnace and cooled down
(and the rate does not matter) the elongation would definitely improve.
Where steel mills reduce thicknesses of steel in cold drawing operations,
after the steel has undergone a certain percentage of thickness reduction
the sucritical anneal must be done to restore toughness to the steel. If
it's not donw and the steel is rolled thinner it will crack due to low
elongation. The subcritical anneal will cause some grain refining to occur
(i.e. small grain size when examined by microscope).
The second choice would be a FULL anneal. Here the steel is heated up ABOVE
the critical temperature. I don't have my handbooks avaliable, but for 1018
this might be about 1550 to 1600 degrees F. After soaking the parts at that
temperature, so they are through heated, the furnace is turned off and the
parts left in the closed furnace. The furnace and parts are cooled to
perhaps 400 degrees and taken out. The FULL anneal causes a coarsening of
the grain and the steel is returned to it's naturally softest state.
Tensile strength, yield strength and hardness go to a minimum and elongation
goes to it's maximum (theoretically).
The heat treat you described in your email would be called "HARDENING and
QUENCHING" and should be followed by TEMPERING. When you heat a steel up to
about 1550-1600 (approx) and quench in water, you need more carbon and
manganese in the steel to get it to hardnen. 1018 doesn't have enough in
it. After the quench (when you do this to a medium carbon, high carbon or
alloyed steel) a tempering process usually follows. The combination of
Harden, Quench and Temper will ofter give the optimized or best combination
of tensile, yield AND elongation.
Well this is a long answer to your question but if you are getting into
metallurgy perhaps this will help.
Mark
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