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Selecting Filler Metal
Selecting the proper welding filler metal and techniques
requires an understanding of
the metallurgy of the base material and the weld metal that develops from an
admixture of the 4130 and the filler metal.
The graph on the left was
taken from a US Steel "Atlas of Isothermal Transformation Diagrams."
These graphs define the metallurgical structure that occurs in various
steels when they are cooled. This one defines that we can expect to
have high hardness, brittle Martensite form when 4130 is cooled at a
particular rate.
The graph at the bottom of the page shows the structure that formed when
a bar of 4130 was heated to 1550 deg F then one end spayed with water.
Note that from about 3/16 inch from the quenched end the microstructure is
almost all Martensite. The hardness in this area is 50 RC. The
equivalent tensile strength for that hardness is about 250,000 psi.
Very strong but also brittle meaning a small crack will easily propagate.
If one made a chisel from that material it would break on the first blow!
If we wanted a chisel we would heat it after quenching to "Temper" the
Martensite to a lower strength and make it far less brittle.
The reason for understanding the 4130 IT diagram, particularly the End
Quench Hardenability Test (Called a Jominy Bar, after the inventor,
Walter E. Jominy) will become apparent as we present additional information
below.
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 Many years
ago, for a Masters Thesis, I made a Jominy bar from a weld
deposited in a steel that had about 105,000 psi yield and about 120,000 psi
ultimate strength. I also plotted the cooling curves at numerous
points along the Jominy Bar. (No Excel at that time, had to use
graph paper! Note that Walter Jominy only sited cooling rates
at 1100 deg F, not sufficient for my Professor! Had to use a fast
response Light Beam Visicorder, thermal coupes at 1/8 inch intervals along
the bar etc!) Calculations were also made defining the
cooling rate in welds made in 1 inch thick plate with the submerged arc
welding process. The accompanying graph was developed from that data.
The cooling rates for TIG welds made in thin material are also noted on the
graph.
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 Estimated cooling
rates for several TIG welds in tubing are shown in the accompanying graph in
red and blue lines. The cooling rates are fast since welding volts and amps are relatively the energy efficiency of TIG is low , ~50
to 60%.
From the IT
diagram we can estimate a continuous cooling diagram using a technique
suggested by Grange and Kiefer. It is shown in green superimposed on the
IT diagram. Also shown is the cooling rate of the 1/4 inch Jominy
point which has a cooling rate of 50 deg F/sec at about 1100 deg F.
This is approximately the cooling rate for TIG welds in 0.040 inch
material. For 1/16 inch thick material the TIG cooling rate
would be about 62 deg F/sec. This information is directly
related to the weld heat affected zone (HAZ). These cooling rates can
produce a significant amount of Martensite in the HAZ. The structure
in the weld metal will depend on the weld rod or wire selected as well as
the amount of admixture of 4130. For example, very small fillet welds
may have mostly melted 4130 in the deposit which can create potential
cracking problems.
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 The table
on the left shows the estimated weld metal structure of: 1) a weld made in
4130 without filler metal (called an autogenous deposit), 2) with 30%
ER80S-2 diluted into the 4130 and 3) ER70S-2 rod diluted the same 30% into the 4130. The resulting chemistry
is a simple ratio of the materials assuming TIG welding with Argon shielding gas and
minimum carbon or other
element loss.
The data showing "Critical Diameter" was developed from a
book on Steel Hardenability by Crafts and Lamont. It defines the
diameter of a bar, that when heated to 1500 deg F and quenched in water
will have 50% Martensite in the center. Notice the deposit with 30%
ER80S-D2 filler rod is much more hardenable than even the 4130.
An austenitized and quenched bar 3.9 inches in diameter of this material would have 50% Martensite
in the center
versus only a 2.8 inch bar for 4130 . The deposit has significantly
higher Manganese and Moly than the 4130 or the deposit made with ER70S-2.
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A deposit made with ER70S-2 will most likely have a
slightly lower tensile strength than Normalized 4130. If diluted as
noted, probably 85,000 to 90,000 psi versus the 100,000 psi in 4130
depending on how the material was processed. This reduced strength may
be acceptable if gussets are used or for intersecting tube joints by making
the fillet size slightly larger.
CHECK WELD QUALITY
It is very important to check
weld quality and understand the types of defects that could be encountered when
welding 4130.
Check your weld procedures and keep them consistent. You should make some
sample welds and bend them to destruction to assure failure occurs only after
considerable bending has taken place. Look for porosity or cracks that may
have been present in the weld. It would be a wise investment to hire the
services of an American Welding Society
(AWS)
Certified Welding Inspector (CWI). There are some 20,000 registered.
In fact many of them are members of the 50,000 member AWS. They can
advise on procedures and what to check for such as small undercuts at the weld
toe of fillet welds that can lead to premature failure.
Consistently following the
proper weld procedures and knowing how to check for possible weld problems is of
major importance. Be sure to employ the skills of a qualified
welder who has experience welding this material. Also inspection of the
final welds by an Certified Welding Inspector (Certified by The American Welding
Society) is highly recommended.
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 This
is a photo from an interesting article in the March 2007 issue of "Hot Rod
Magazine." The whole ridged elaborate cage was made from 4130 tubing.
Note they used ER70S-2 welding rod
and made all joints with TIG welding. Have to believe this was a
mocked up photo since the description says the tube ends were ground to
almost a "press fit." This is far from that and unacceptable for
making any type of weld!
Cleaver idea to use the drilled
hole to relieve hot gases in closed tubing joints. |
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If a Search Engine Found This Page 1st- - - We'd Suggest
a Visit to The Basic Welding 4130 Page; Then Return. Click
Here
This page presents the technical information that supports
the reasons for the suggestions found on the "Basic, Welding 4130" page.
Like Math?
Want to
see the calculations for weld cooling rates and references?
CLICK
HERE.
.
Check Out Welding
Math Site
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This Page Presents Technical Details of Welding 4130
for Other Information:
Click for BASIC 4130 WELDING
DETAILS
Click for
EQUATIONS
defining weld cooling rate in tubing
Click for
WELDING HEAT TREATED 4130 CHROME MOLY
Click for
METALLURGICAL DEFINITIONS
Click for WELDING A BETTER
STEEL; HY 130 |
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