 Materials
What does fully killed steel mean?
I see that a specification required by my customer is that the steel shall be fully killed. What does that mean, and why would they need that?
Killed steel is steel that has been treated with a strong
deoxidizing agent during the melting process. The elements
typically used are aluminum and/or silicon. Both have a
strong tendency to form oxides which are floated out during
the melting process, which reduces the overall oxygen content
in the steel. If oxygen remains in the steel in large amounts,
the steel is much more susceptible to tearing during the
forging process. The excess oxygen can also form oxide particles
in conjunction with iron and manganese, which reduce the
notch toughness and impact resistance of the steel. This
can adversely affect the life of the finished part.
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Can you process a Cr-Cu (Approx. 0.90% Cr, Balance
is Cu) alloy using the open die process and what type of
resulting properties can you obtain?
Scot
Forge has processed thousands of pounds of this material
and can definitely achieve the properties for which you
are looking. Achieving these properties may be possible
utilizing cold work to strengthen the material, however,
this processing route sacrifices ductility. This is a precipitation
hardening alloy which allows manipulation of the Cr within
the alloy to achieve a homogenous material with the desired
strength without sacrificing ductility. Scot Forge also
has in-house heat treat facilities that can process this
material without having to utilize an outside source.
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What is the weldability of AISI 8620?
Let me begin my response by reminding you Scot Forge does
not, in general, perform welding on our product and therefore
we should not be considered to have expertise in providing
consultation in this processing area. Please contact a reputable
welding technology consultant to determine the actual processing
parameters for your application and also to determine any
special purchase order requirements such as the appropriate
heat treatment or residual element restrictions. With this
understanding, I can pass along the information below I
located in the ASM Handbook Volume 6: Welding, Brazing,
and Soldering (ASM International, 1993, pp. 668-673).
AISI/SAE 8620 is classified by the ASM Handbook as a "Heat-Treatable
Low-Alloy (HTLA) Steel" that has a greater susceptibility
to weld cracking compared to low-alloy steels used for pressure
vessels and piping (0.10-0.25% C). Although classified as
a HTLA steel, the nominal 0.20% carbon content of 8620 makes
this grade less susceptible to cracking than the higher
carbon HTLA grades such as 4140, 4150, and 4340. Nonetheless,
the handbook recommends that a preheat and interpass temperature
of 250-350 F be maintained for a 1 to 2 inch cross-section
followed by a post-weld stress relieve treatment (typically
50 to 100 F below the final tempering temperature or 1100-1250
F if a subsequent quench and temper heat treatment operation
is to be performed). Because this temperature is below the
M90 temperature of approximately 600 F (see attached figure),
the welded region can be heated directly to the stress relieve
temperature, held for approximately 1 hour per inch of joint
thickness, and still-air cooled. If it is not practical
to start the post weld stress relieve immediately after
completion of welding, the welded region should be allowed
to cool slowly and uniformly to room temperature to await
subsequent post weld stress relieve. Adding a delay between
the welding process and the post weld stress relieve does,
however, put the weld joint at risk for cracking.
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Disclaimer: Although the
information set forth herein is believed to be correct,
Scot Forge makes no recommendation, or offers no warranty
of any kind with respect to the subject matter or
its accuracy. Scot Forge specifically disclaims all
warranties, expressed, implied, or otherwise, including
without limitation, all warranties of merchantability,
fitness, or suitability for a particular purpose or
application.
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847-587-1000 |
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