Cutting&Bending High Strength Steel Hardox Weldox
Cutting&Bending High Strength Steel Hardox Weldox
Shearing and Bending machines are the most common metal working machinery for cutting off one plate, bending some flat plate to certain shape with some regular or customized punch and dies. But for some high tensile strength steel like SSAB Sweden made, including Strenx, Hardox, Weldox and Docol material, it's need to consider more details for the processings. Their special steel mainly used in manufacturers of heavy transport, construction machinery and material handling including mining and recycling industrials.
High Tensile Strength Structural steels:
PREPARATION BEFORE BENDING
• Check the rolling direction of the plate. If possible, orient the rolling direction perpendicular to the bend line. The plate can often be bent tighter this way, than with the bend line parallel to the rolling direction.
• Check the surface quality of the plate. Surface damage can worsen bend ability as it can be the cause of fractures.
For heavy plates, defects on the plate such as scratches and rust can often be removed with careful grinding. Grinding scratches shall preferably be placed perpendicular to the bend line
• Thermal cut and sheared edges should be deburred and rounded with a grinder.
• Check the condition of the tools.
• To avoid excessive tool wear, tooling should be harder
than the work piece.
• Check that the tools and tool setup are in line .
• Pay attention to safety and follow the local safety directions. Only qualified people may be by or in the vicinity of the machine. When high strength steel is being bent, nobody should stand in front of the press brake.
• Check that the punch together with the workpiece do not bottom out in the die.
• Consider springback. Avoid rebending to correct the profile angle. The exposure of a material to previous forming processes reduces its bendability to a great extent.
• Bending force, springback and, in general, minimum recommended punch radius increase with the strength of the steel.
• In many cases for Strenx and Hardox plate products, the indentity of the plate is stamped perpendicular to the rolling direction. Avoid placing the plate so that the stamping occurs in the bend line, due to the risk of cracking
• Excessive blast cleaning can have a negative effect on Bend ability. Recommendations for Strenx and Hardox plate products are based upon tests with blast cleaned and painted surfaces. The recommendations for Strenx strip product and Docol are based on tests without a blasted surface.
• High strain rate may cause local a temperature increase in the bend. This could have an adverse impact on the bendability, especially for thicknesses above 20 mm.
If possible, reduce the punch speed, in order to decrease the temperature difference within the work piece.
Springback increases with increased die width, while punch force is reduced. Make sure that the opening angle of the die allows for over-bending, without bottoming out, to compensate for springback. An increased die opening width can in many cases lower the strain level in the bend. Also, make sure that there is enough room for the chosen punch together with the workpiece, in the die, during bending, without deforming the die. The minimum recommended die opening Width.
The die edge radius should be at least half the plate thickness. Alternatively, the die width should be increased in order to minimize pressure on the die edge radius, and consequently reduce the risk of die marks.
The suitable punch radius, along with the die width, is the most important parameter. When bending high strength steel, the final inner radius often becomes somewhat smaller than the radius of the punch, figure 3. When there is low friction between plate and tools, the phenomenon becomes more obvious.
For steel with yield strength over approximately 500 MPa, a
punch radius of the same size or slightly larger than the desired
bending radius is recommended.
CONDITION OF TOOLS
Due to the increased contact pressure between plate and tools when bending high strength steel, wear on the tools increases somewhat. Check at regular intervals, that the punch radius and die edge radius are both constant. For bends that have cracked in a construction, the crack has in many cases propagated form the compression side of the bend, figure 2. This can often be attributed to poor condition of the punch. The edges of the die should remain clean and undamaged.
Required punch force is often high when bending high strength steel. The static friction coefficient is typically higher than the kinetic. This can cause the plate to lock over the edge
of one die edge radius, and at the same time, slide over the other one. In this way, the workpiece swings down into the die in a discontinuous way during the bending process. This phenomenon, called stick-slip, can result in higher strains over the bend. Use a stable machine and steady tool fastening.
Lubrication of the die edge or use of a rotating die edge radius can be helpful, avoiding stick-slip and also lowering the punch force.
Crowning compensates for the elastic deflection of the bending machine under load, The central part of punch and die deflects the most. By crowning, the deflection (C) can
be compensated for, thus achieving the same bending angle along the entire blank length. If the bend profile becomes curved along the bend line (B) this cannot be compensated
for by crowning. After unloading compressive stresses arise on the stretched side, at the same time as tensile stresses appear on the compressed side.
The stress distribution over the plate thickness causes longitudinal stresses. It is those stresses that tend to curve the profile. The magnitude of curvature depends mainly on the flange height and the profile stiffness. Additional considerations have to be made when setting the crowning at stepwise bending of long profiles.
Springback increases with steel strength and the ratio between die width and plate thickness (W/t). Material yield strength has the biggest influence.
When bending, a varying residual stress distribution is achieved over the bend cross section. The plastic strain level and the distribution of these stresses will control the tendency for springback. All springback is fully elastic.
To compensate for springback, the die should be shaped in such a way to allow overbending without coining the material.
It is very difficult to accurately predict the springback of a material when bending, since this depends to a large extent on each unique tool setup. That is why trials are recommended.
For thinner plate or sheet (t < 10 mm), an estimation of the material’s springback, in degrees, can be achieved by dividing the tensile strength (MPa) by 100.
A precondition is that the die width is approximately 10–12 x the plate thickness.
PARAMETERS THAT AFFECT SPRINGBACK:
• Yield strength of the material–higher yield strength causes greater springback.
• Punch radius–increased punch radius will cause greater springback.
• Die width–larger die width causes greater springback.
• The strain hardening of the material.
Strenx and Hardox is delivered with guaranteed bending performance according to Strenx and Hardox guarrantee.