2. Warpage
(Shape Distortion)
Warpage is a physical
distortion where there is a
change in geometrical form
or shape of the work piece
and is revealed by changes
of curvature (angular
relations) or curving, bending,
twisting and non-symmetrical
dimensional change without
any volume change
4. WARPAGE DUE TO RESIDUAL
STRESS
Residual stress is the main cause of flatness defects in sheet metal and
stress relaxation after cuts can cause the deformation of cutouts.
Compressive stress is induced on the surface of rolled product if small
diameter rolls are used or if smaller reductions are affected during rolling.
What looks flat off the Cut-to-Length (CTL) line, when cut on the plasma or
laser, is found to be Bow or anything but not flat! The problem is Residual
Stress in the steel.
Flat material can harbour internal stresses, causing defects to re-appear
after secondary processes.
5. Residual stress distribution is
affected by the number of
bending operations the strip
undergoes when passing through
the roller levelling machine.
6. As the material proceeds toward the machine’s exit, the depth of the
up and down bends is gradually reduced. This is comparable to a
decaying Sinusoidal Curve.
7. Sets the Residual Stress distribution with the second load triangle
and ensures flatness via the setting of the last load triangle
Stress relieving operation can be used to relieve the residual stresses of rolled
products
8. MATERIAL BEHAVIOUR TO AVOID
DEFECT
Metal has a Fracture Strain of at least 5 percent and a pronounced yield
strength, it can be levelled.
Material that achieved permanent elongation, even as low as 0.1-0.2%,
consistently maintained flatness in secondary operations indicating
material which has equalized stress.
The net effect is that 90% of the material can be plasticized with the
maximum of only 5.5 yield strains on the material, as opposed to 10 yield
strains necessary using bending alone.
9. Multi-Roll Levelling
In general, the smaller the pitch and the levelling roller diameter, the better
the result.
As a rule, the more levelling rollers, the greater the number of alternating
bends, and the better the levelling result.
At least 13 rollers are needed on a levelling machine to achieve
acceptable tolerances, but 17 to 21 levelling rollers are recommended.
Bending cause elastoplastic deformation, which further causes the
redistribution of residual stress.
When the material fibbers are nearly the same length, the Residual Stress is
gone, and the metal is level.
10. Causes of Warpage
The factors are:
Relief of pre-heat treat residual stresses.
Expansion and contraction of parts during processing of heat treatment.
Inadequate racking procedures in the furnace.
The process of quenching is too fast.
Improper quenching selection.
Uneven quenching due to inadequate agitation or spacing.
Improper quench immersion rates.
Furnace air flow design in relation to part orientation
12. METHODS OF REDUCING RESIDUAL STRESS
Tension leveller and stretch levelling are known as methods of reducing residual stress
Light Press Method for reducing residual stress generated by laser cutting. The method is
to give light press on the sheet metal edge and to reduce tensile residual stress
generated near the laser-cut surface.
Light Indentation Method for reducing residual stress generated by laser cutting. The
method is to give light indentation on the sheet metal edge and to reduce tensile residual
stress generated near the laser cutting surface.
Multi-roll Levelling Method to improve the shape quality of the sheet is to reduce and
eliminate the residual stress.
A Multi-drive System which can apply extra tension in the levelling process, Each roll of
the leveller has an independent gear-motor that can be set at a different speed from the
other drives.
Warpage Control Due To Heat Treatment Process On Conventional Furnace
13. REFERENCE
Light press of sheet metal edge for reducing residual stress generated by
laser cutting considering mechanical properties and intensity of residual
stress by Atsushi Maedaa,∗, Yingjun Jin b, Takashi Kubokia
A strategy for the controlled setting of flatness and residual stress
distribution in sheet metals via roller levelling by Markus Grübera,*, Gerhard Hirta
When Flat Isn’t Flat – Equalizing Internal Stresses Through Roller Levelling by
Jamie Wollenberg Jim Sugars.
Warpage Control Due To Heat Treatment Process On Conventional
Furnace By Muhammad Asif Bin Md Yusof.
Evolution of Residual Stress Based on Curvature Coupling in Multi-Roll
Levelling by Guodong Yi , Ye Liang, Chao Wang and Jinghua Xu.
14. REFERENCE
Defects and Distortion in Heat-Treated Parts by Anil Kumar Sinha, Bohn Piston
Division.
Effects of tension levelling process parameters on cold rolled strip
characteristics using a designed factorial analysis approach by J. W. Morris1, S.
J. Hardy and J. T. Thomas.
Hot rolling and rolling defects by R. Chandramouli.
Method of reducing residual stress generated by laser cutting by light
indentation of sheet metal edge by Atsushi Maedaa, Yingjun Jinb, Takashi Kuboki.
Everything you need to know about flatteners and levelers for coil
processing -- Part I By Eric Theis, Contributing Writer.
15. REFERENCE
Residual Stress in Micro-alloyed Stress Sheet, by J.Miclel, J.Hidveghy, M.Bursak.
Everything you need to know about flatteners and levelers for coil
processing -- Part IV By Eric Theis.
