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MIG Gas Delivery and the Small Block Chevy
Evolved in a Similar Way and Time!
Both had Advances and
Setbacks Before
They Were Optimized!
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for a PDF Report on the Similarities |
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WELDING “HEAT TREATED” 4130
(QUENCHED
AND TEMPERED) Chrome Moly
This
information relates to welding
heat treated,
high strength
4130 Chrome Moly tubing for race car frames where no post weld heat
treatment is utilized. We are not advocating this be done, in fact in
most cases just the opposite.
The reason for discouraging welding high
strength
4130 tubing without subsequent appropriate heat treatment is that the
area next to the weld will have much lower
strength and
may contain some brittle
areas!
We’re presenting this information because
of recent issues with:
-
AA Fuel Dragsters (Photo Above, Cory McClenathan Crashes at Bristol,
Tenn.) and Funny Cars where this
material was (is) used;
-
Questions received in technical talks at
American Welding Society meetings and those received on our Web Site from
race car and custom motorcycle frame builders;
-
A recent article in a popular car
magazine which reinforced the need to define that increased “strength”
Does Not Mean “increased
stiffness!”
A Definitions Page is
provided to keep this discussion of reasonable length. Just click on the
words in
bold italics
to obtain a non
metallurgist definition.
Note: If your going to heat treat the
finished 4130 part see comments from my original Article published in
The American Welding
Societies Journal. |
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WHY 4130 IS STRONGER WHEN “HEAT TREATED”
AND
WHY STANDARD RECOMMENDED WELDING ROD AND
PROCEDURES FOR WELDING NORMALIZED 4130 CAN NOT BE USED.
In the mid 1970’s while managing a welding
wire/rod R&D facility I recommended to a dragster chassis manufacturer
that AWS ER70S-2 welding rod be used when welding
normalized 4130. For race car frames they did not plan to use preheat or post weld heat
treatment. This rod/wire has become widely
accepted as an excellent solution and has proven successful over many years. Welding
heat treated 4130
has totally different implications. The area next to the weld not just the
weld metal becomes the critical issue.
The
reference to
heat treated steel generally means the material is heated to a
temperature where a structural change occurs in the crystal lattice. In the
case of 4130 about 1600 degs F. At these temperatures the iron atoms have
a Faced Centered
arrangement ( Photo Left) and carbon atoms are interspersed between. If
cooled slowly the atoms will rearrange to their less stressed
Body Centered
state. However if quickly
quenched
from these temperatures the structure is maintained leaving a very
highly stressed material which is very
hard and strong. This
metallurgical structure is called
Martensite. Tensile
strength could be as high as 250,000 psi. Unfortunately it is
also very brittle.
What does brittle
mean? It means this very strong steel can act like glass when subjected
to a shape blow or highly stressed environment! A positive term used to
quantify a steels resistance to brittle failure is a property called
toughness.
One measure of
toughness is the Charpy test (although not useful for
very
thin material its
useful
to understand its use.) The photo left is a broken Charpy specimen which shows how
brittle
steel looks. Very little energy was required to initiate and sustain the
fracture. To reduce this
brittleness the hardened steel can be
tempered.
Tempering
requires reheating the very
quickly cooled steel to usually 500 to 1100 degs F. For gears and items
that need to be
hard, 500 degs F may provide enough
toughness.
However for 4130 used in structures the
tempering
is usually done at about 1000 to 1100 degs. F to provide some resistance
to brittle
failure. At these
tempering temperatures a fully
quenched
and tempered
(Q&T) 4130 will have
strength of about 150,000 psi but will have higher
toughness
and ductility.
There is a
tempering temperature
range between 500 and 700 degrees where the steel may actually not improve
in toughness
and should be avoided (however in the weld
Heat Affected Zone we can't avoid reheating the tubing from
the melting point of the steel, 2500 deg F, to temperatures in and below
that range!)
Tempered
properties are also dependent on
tempering time. This discussion will not provide details about
this other than to say when welding
heat treated,
Q&T 4130 it must be understand the area adjacent to the weld deposit (
Heat Affected Zone ) can have
some unusual and undesirable metallurgical structures.
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WELD HEAT
AFFECTED ZONE PROPERTIES
Weld metal chemistry can be controlled by
the type of welding wire or rod used. The amount of base material melted into the weld
deposit must also be
considered. When very small filet welds are made in 4130 tubing the
large amount of melted high carbon (by welding standards) material in the
weld deposit can significantly increase strength.
The properties if the weld metal can
influenced by the rod/wire selection but there is
little
control of what happens in the area next to the weld called the Heat
Affected Zone (HAZ.) In the photo left, the HAZ is
usually referred to as the etched area between the melted weld deposit (visible
by the coarse grains) and the outer etching boundary which in most steels defines
an area that reached about 1300 degrees F. This line exists because a
metallurgical change occurs in steel at that temperature that can be
revealed by chemical “etching.” In essence the HAZ was
heated to a temperature from about the melting point of the steel (about
2500 degs F) next to the weld all the way to not heated at all. For issues
related to tempering, if the 4130 was
tempered
to 1050 degs F to
achieve the required properties, after welding we have essentially
tempered
the tubing next to the weld from 2500 degs F all the way down to the 1050
degs F and lower! The time
at temperature will depend on weld heat input, wall thickness etc. In
general we will have an area much softer than the “after tempering” 150,000
psi! In fact it will probably be no stronger than if we welded the more
typically used 4130
Normalized.
Normalized 4130 tubing was cooled much more slowly from a high temperature,
never creating the
hard brittle
Martensitic
structure but is also much tougher.
Normalized 4130
is usually about 95,000 psi in
strength.
The mixture of metallurgical structures present in the HAZ of welded
Q&T
4130 may contain some
brittle areas.
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WHAT HAPPENS WHEN WELDED Q&T 4130 IS
STRESSED?
When the structure is stressed, the
~150,000 psi
strength
tubing material will
have a much weaker (perhaps 90,000 psi)
HAZ area between
the joined pieces.
The strength will be reduced in the
HAZ in a band heated from the 2500
degs F area next to the weld all the
way down to a band that reaches the 1050 degs F
tempering
temperature. That weakened band could be wide in
deposits made in thin wall tubing. It addition, if any brittle
metallurgical constituent exists, cracks may form and propagate in the
highly stressed HAZ even at
lower overall stress levels. Because of the potential
brittle areas in the
HAZ, our usual suggestion
which has proven successful for many years of using an very ductile AWS ER70S-2
welding rod and making slightly larger fillets to compensate for the somewhat lower
strength
will not help a weak brittle HAZ!
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STIFFNESS
Why use “high
strength”
4130 in the first place? All else being equal 250,000 psi water quenched
4130 tubing is no “stiffer” then mild steel with 50,000 psi
tensile
strength! Both will bend the same amount when loaded (assuming the
structure does not take a permanent set.) This assumes the same tubing
diameter and thickness - which generally is not the case. Refer to the page
of properties and metallurgical definitions under the section
stiffness, to understand what needs to be done to achieve
a “stiffer” structure. 4130 Chrome Moly is also the same weight
per cubic inch as mild steel. The only way it can be stiffer or
lighter is to use large diameter tubing with a thinner wall thickness.
SUGGESTIONS
What Should be Done to Join Heat Treated
4130?
First be sure it is understood why the
higher strength
is being used. If there is a real structural benefit or are you just
getting into and area where
local buckling
may occur? If the extra
strength is a real benefit, then perhaps welds can be
placed in an area of low stress. Other joining techniques can be
considered. Racing bicycle manufactures use special brackets that capture
the tubing and then braze the joints at much lower temperature. You could
heat treat the final product. Sounds difficult but one Baja race car team
reports they did just that.
My strong recommendation is to have some
testing done by a competent welding research firm. This should include a
measure of resulting weld joint strength, toughness and fatigue properties.
With the large amount of sponsor money spent on most racing series they
surly can be called upon to assist with researching this safety issue.

Email
for recommendations
on
hiring a research firm. |
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If higher strength
steel is felt necessary why not use an alloy that is strong, tough and much
more readily weldable like HY-140.
Click
For Information about this steel.
For More Information on Welding 4130
Chrome Moly: |
| See Welding 4130 Basic Info |
| See Welding 4130 Technical
Details |
| See Welding 4130 Math |
| See Metallurgical
Definitions |
| See High Toughness High Strength Steel Info |
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