Cold Drawn Seamless ProMoly™ (4130) High Performance Tubing
Terminology and Manufacturing of MIL/AMS-T-6736B Frame Tubing There has been a great deal of confusion in all the race industry frame communities regarding the terminology and manufacturing methods specified in
MIL/AMS-T-6736B. It is very important to clarify what
the five types specified in the standard intend, options/requirements of how to develop the mechanical properties for each and their effect on the manufacturing processes involved.
Material specification type:
►Type I-Round
►Type II- Rectangular or square
►Type III-Streamline
►Type IV- Oval
Physical Conditions:
►(A) -Annealed
►(N) -Normalized or stress
►(HT-125) - Heat treated to a minimum tensile strength of 125,000 p.s.i
►(HT-150) -Heat treated to a
minimum tensile strength of 150,000 p.s.i.
►(HT-180) - Heat treated to a
minimum tensile strength of 180,000 p.s.i.
Along with the mechanical
properties, the specification covers raw material
requirements, chemical composition, grain size,
decarburization limit, sampling frequency and
conformance inspection requirements.
►
Production Methods The two methods for producing the
starting hollow for the tube product differ greatly.
The seamless method starts with some form of solid
bar stock and either extruding or piercing to
produce the hollow. The welding method starts with
rolled flat stock getting formed through a series of
rolls until the edges meet.
Steel is composed of thousands of crystals that are bonded together. The crystals shape is referred to as
equiaxed which means that if a grain was measured in any direction, the dimensions would be nearly the same. In other words, that means that they are tending towards looking like a rough skinned spheroid. The cold drawing process elongates and flattens the grains producing a pancake shape. This forming process strains the crystals structure and increased the mechanical properties.
►Cold Drawing Process The cold drawing process involves first swaging one end of the tube shell to facilitate inserting it through a carbide die. The swaging or pointing operation, allows enough of the tube to be exposed on the back side of the die to be able to grip the material and produce the drawing forces.
After pointing, the tube is loaded on a mandrel, attached to a long rod. This rod is fastened into the draw bench to allow for precise location in the die.
Up to a point, the higher the reduction percentage,
the higher the mechanical properties that are
achieved. Beyond forty percent reduction, little
increase in mechanical properties occurs. By making
adjustments in the reduction amount, either
condition (N) or HT-125 level properties can be
developed. To produce condition (A), a soft anneal
cycle is applied at this point. The final step
following the cold reduction in the processing is
either a stress relief anneal for conditions N and
HT-125 or the soft anneal for condition A.
►The ProMoly Advantage Due to variations in the different
tube manufacturer’s methods, issues have arisen in
both the forming and performance of performance
racing frames. Consequently, Plymouth Tube has
introduced their trademarked product, ProMoly™ which
is designed to produce consistent quality, strength and formability.
