| Material selection
| Fabrication | Surface Finishes
| Cold Forming | Machining
- COLD FORMING
Columbus Stainless Steels supplied in the annealed condition
(No.1, HRA, 2D, 2B or BA) can be cold formed by any of
the conventional processes. These processes include blanking,
bending, piercing, roll forming, coining, embossing, pressing,
spinning, flow forming and deep drawing. In general the
equipment such as presses, press brakes, guillotines,
etc. used for carbon steels can be used for stainless
steels. However, as more power is required to work stainless
steels, the capacity of the equipment is effectively reduced,
eg. Guillotines and press brakes that are rated up to
6mm thickness for carbon steel are restricted to 4mm for
stainless steels. In operations where austenitic grades
are cold worked, eg. Pressing and deep drawing the capacity
of the equipment can be effectively reduced by up to 60%
due to rapid work hardening characteristics of these materials.
addition to the extra power requirements to from stainless
steels, greater demands are made on the form tooling.
Stainless steels not only have greater strength than carbon
steels but work harden more, cause more wear, are susceptible
to pressure welding, exhibit more springback and have
lower heat conductivity. As a result of this, tool materials
must be harder, smoother and better designed than tooling
for carbon steel.
grade tool steels are required and frequently tungsten
carbide or aluminium bronze are utilized. However, in
the case of the latter these materials can be used as
"inserts" in critical areas rather than the
complete tool. In many cases tools designed for carbon
steels are used for stainless steels, in such cases one
must expect to make changes in "setting", use
higher quality lubricants, have more frequent maintenance
and reduced life.
a result of the above, lubrication requirements are critical
and high quality products are essential. Heavy duty pastes
and emulsions, sulphurised or sulphurchlorinated oils,
chlorinated oils or waxes are used depending on the forming
operation. Mineral oils, soap solutions and general purpose
soluble oils are not usually used. Ease of removal after
forming is also a consideration since all traces of lubricant
must be removed prior to subsequent heat treatment or
before putting into service. Surface contamination can
increase finishing coasts and adversely affect corrosion
resistance. In some cases, especially pressing or deep
drawing of ferritic grades, material is PVC or PE coated
prior to forming. This not only acts as a very effective
lubricant, but increases the formability, prevents galling
and/or pressure welding and minimises damage to the surface
following is a summary of the differences between carbon
steel and stainless steel for some of the main cold forming
to its higher shear strength, more power is required along
with greater tool wear when working with stainless steels.
This can be minimised by using angular shear on the punch
and die or by punch staggering during multi hole operations.
regard to the die materials, these should be both hard
and tough, for intricate jobs shock resistant tools must
be used. These materials include D2, D4 and carbide (HRC
60-65) and S1 or S5 (HRC 57-60) for the latter. Clearance
between punch and die has always been a contentious issue
when blanking thin materials. Some manufacturers recommend
clearances below 0.025mm per side, others prefer more
generous clearances. However, for cutting plate the general
rule appears to be 10 to 15% of material thickness per
side. The choice of clearance is always a trade off between
degree of burring, risk of "shear break" and
cut edge, especially a sheared edge is usually rough or
burred, and the material is in the work hardened condition.
The resulting burrs are not only dangerous for the operators,
but can lead to damage and excessive wear to form tools.
In addition the notch effect and the work hardened edge
can reduce the materials ability to be formed in subsequent
operation. Burrs can be removed by manual grinding or
filing or by various automatic deburring equipment.
blanking and piercing are done without lubrication. However,
to prolong tool life or to minimise burr or shear break
lubrication may be used. Sulphurised or chlorinated oils
are usually used. However, care must be taken to ensure
their removal especially if the parts are to be welded
or heat treated.
BREAKING AND ROLL FORMING:
both cases the equipment and tooling used for carbon steels
can be used for stainless steels. However, there are several
differences that need to be taken into account, viz.:
Due to its inherently higher strength and work hardening
tendency, "springback" is much greater for stainless
steels. Therefore, a greater degree of bending is required.
This will depend on the grade of steel, its harness, thickness,
the bend radius and bend angle.
When running both carbon steel and stainless steel on
the same equipment, it is advisable to plan operations
carefully to avoid carbon steel "pick-up" on
the stainless steel. It is recommended that the stainless
steel be run before carbon steels or the equipment be
thoroughly cleaned when changing from carbon steel to
stainless. When changing from stainless to carbon steel,
cleaning is not usually necessary. In cases where the
equipment itself is old and worn and there is a danger
of "pick-up" from this source, it is recommended
that the working surfaces be covered with material such
as PVC or PE.
Lubrication is not normally required, except in cases
where galling and scoring occur. In such cases heavy duty
emulsions can be used.
Prolonged working of stainless on equipment designed for
carbon steel may lead to abnormal tool wear.
These operations are usually done on the softer austenitic
(304, 304L) and ferritic grades (409, 430). Again due
to the higher hardness, harder, stronger dies than those
required for carbon steels must be used. Lubrication is
essential, heavy duty pastes and emulsions are generally
used. Due to the rapid work hardening, interstage annealing
may be required if multiple operations are involved. For
example, in the manufacture of coinage, material is blanked,
coined and annealed prior to embossing.
AND FLOW FORMING:
For the sake of simplicity we will define spinning as
the process whereby a circular blank is centred and held
against a form block in a rigidly built lather. The part
is rapidly rotated and pressure is applied to the metal
surface by means of a spinning tool. This induces material
flow, which is accompanied by limited thinning of the
material and cups, cones, etc. are produced. The process
can be both manual or automatic. Flow forming is a similar
process but in this case extensive thinning of the material
(over 50%) is achieved. Due to the loads required this
process is either fully or semi automatic. Manual spinning
due to its low tooling costs is extensively used to produce
small quantities of any given part. Flow forming on the
other hand due to its higher tooling and equipment costs
is used for a larger number of a smaller range of items.
Figure 4 and 5 details arrangement and tooling for manual
forming generally requires bigger, more robust and sophisticated
purpose built lathes and tooling. These processes can
be used to produce items from as small as
Domestic mixing bowls to dished ends several metres in
diameter. The simple tooling used in spinning can be used
for a range of materials including stainless steels. In
flow forming the chucks are usually manufactured from
cast iron or steel. Other tools are used not only to flow,
but to flange, bevel, trim, etc. and the materials used
will depend on the function of the tool.
most common lubricants used in spinning are adherent bar
soaps and waxes. These must be used to reduce friction,
galling and tool drag. In flow forming, it is important
that the lubricant has a coolant action and that specialised
proprietary brands are used.
austenitic grades can be formed by both processes, but
the lower work hardening types 304, 304L and 304DDQ can
be spun to greater reduction, before intermediate annealing
becomes necessary. The ferritic grades are more readily
flow formed than manually spun.
parts produced by the processes have rough surfaces with
characteristic spiral or helical grooves. Unless being
used for non aesthetic applications, the parts require
extensive finishing operations t make them smooth and
Press forming is an operation where a blank is pressed
between a set of shaped dies to produce a finished or
semi-finished part. Presses can be set to produce a single
part from a single set of dies in one operation, or a
complex part produced in a series of operations in a progression
tool. Due to the capital costs of the equipment and tooling,
presses are usually employed for mass production. Again,
as in most other forming processes, the inherent high
strength of stainless steel increases the power requirements
- up to 60% more force may re required than for carbon
to the above, the tooling used in press forming stainless
steels wear out faster and are more susceptible to fracture
as a result of long runs at high loads. High strength
tool steels such as D2 are used as they offer good resistance
to shock and wear. However, in cases where galling and
pressure welding is prevalent aluminium bronzes are used,
and in cases where high wear is present, carbide is used.
In practice, it is quite common for a tool to be subjected
to the above factors. In such cases compound tools are
made in tool steels with carbide and/or aluminium bronze
inserts in critical areas. Care must be taken to allow
for factors such as "springback" in design of
tools. Also due to their ductility, stainless steels (particularly
the austenitic grades) tend to be susceptible to wrinkling
when subjected to compressive force. If a particular type
of pressing is susceptible to this phenomenon, then a
blank holder or pressure pad may be incorporated into
the tooling. Figure 6 illustrates the above.
regard to lubrication, chlorinated or sulphurised oils,
pigmented pastes, waxes and soaps are used. In high speed
operations, heavy duty emulsions are used due to their
superior coolant effect. In some cases PVC and PE coatings
are used, not only as lubrication, but also to protect
the surface finish.
the Columbus Stainless steels supplied in the annealed
condition can be press formed, although the performance
of the ferritic grades can be enhanced by PVC or PE coating.
Deep drawing is a process by which flat sheet metal is
formed into a cylindrical or box shaped component by means
of a punch and draw ring or die. These actions are usually
performed in a double action press with a blank holder
to prevent wrinkling in the flange as metal is pulled
into the die (see Figure 7).
the workpiece is stretched over the punch nose as it moves
into the die. This stretching continues until the force
is sufficient to overcome the force exerted by the hold
down ring which is controlled to allow the material to
compress and flow into the die. The major mode of deformation
changes from pure stretching to drawing at this stage,
and is accompanied by an increase in material thickness.
This thickening can be as high as 40% for austenitic grades
and 10-15% for ferritic grades.
mechanics of the drawing process are extremely complex
and the drawing characteristics of stainless steels differ
considerably to those of carbon steels. Some of the factors
involved in the process are listed below:
Equipment used for deep drawing stainless steels should
be up to twice as powerful as that used for carbon steel.
Double action hydraulic presses are preferred, single
actions can be used if external "cushion" pressure
can be applied. Mechanical presses can be used in they
can be run at slower speeds than used for carbon steels.
Tool materials are similar to those used in press forming,
viz. D2, aluminium bronzes and carbide. Due to the large
masses of such tools, extensive use is made of cast iron
as a backing material, with the more expensive material
used as inserts in the appropriate areas (especially in
the draw ring). The choice of material for the hold down
ring is not usually critical, and cast irons and steels
are usually used.
Die clearances are similar to carbon steel for ferritic
grades (blank thickness plus 10-15%); for austenitic steels
clearances are much greater blank thickness plus 35-40%).
Draw ring radius should be 4 to 8 times material thickness.
If it is too small it can cause fracture of the material
and if too large increases tendency to wrinkling.
Punch radius should be 4 to 6 times material thickness,
again if too small it leads to tearing and if to large
it leads to wrinkling.
The limiting drawing ratio, i.e. the ratio of blank diameter
to punch diameter is between 2.1 and 2.2 for stainless
steels compared to 2.15 and 2.5 for carbon steels.
Lubricants are selected based on two factors, the most
important being to provide a stable film between the workpiece
and the tooling to prevent welding galling and seizing
as well as reduce friction. Secondly, it must be readily
and completely removable after the drawing operation.
Lubricants include chlorinated or sulphurised oils and
waxes; pigmented pastes; heavy duty emulsions and drawing
soaps. PVC and PE coatings are sometimes used, especially
with ferritic grades, as they improve formability and
protect the surface finish.
Stainless steels can be re-drawn after initial drawing.
This process is used either to reduce the diameter and
increase the height for the full length or for a specified
length or to decrease the radii and/or produce flat bottomed
A spinning operation is sometimes carried out on a drawn
component to reduce the "thickening" effects
and increase the overall wall height.
As a general rule stainless steels cannot be re-drawn
to the same extent as carbon steels without the inclusion
of interstage annealing.
produce two materials, viz. 304 DDQ and 430 DDQ specifically
for deep drawing. When these materials are ordered, it
is advisable to state the exact end use so that the material
most suited to the process can be supplies.
information not listed within this site, please contact:
Tel: (013) 247 3343
Fax: (013) 247 2289