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Residual stress - Coggle Diagram
Residual stress
Magnitude of strain and stress
thermal strain
E_TH= ∆L/L= α∆T
Change in length
α = Co-efficient of expansion
ΔT = Temperature rise
L = Original length
ΔL = Increase in length
ΔL=ΔTLα
Strain
ε= ∆L/L
thermal Stress
σ= -Eα∆T
Stress > yield stress = distortion
Youngs modulus
E= Stress α / Strain ε
Factors affecting residual stresses and distortion
Procedural
Number of passes
Every pass adds to the total contraction
each weld pass partially stress relieves previous passes.
Preheat
may increase the level of stresses.
Thickness
Restraint and stress usually increases with thickness
Misalignment
may reduce the stresses in some cases
Travel speed during welding
welding speed increases = reduced heat input and stress
Heat input
increased total heat input = increases shrinkage
Root gap
increases root gap = increases shrinkage
Weld sequence:
Balance weld:
neutral axis of weld
Back step and skip back welding.
Stitch welding
Stress relief.
Peening
Mechanical
Pre-setting
Back-to-back assembly
Assembly method
Tack welding, .
strongbacks,
jigging
Design
Material properties
Yield strength
Higher yield - higher residual stress.
Modulus of elasticity
Measure of stiffness, greater stiffness resists distortion.
Coefficient of expansion
High value - greater local expansion and yielding – higher level of stress on cooling
Transformation temperature
The lower the transformation temperature, the lower the residual stresses.
Thermal conductivity
Low value – higher heat retained in welded zone – higher level of stress on cooling..
Shrinkage
transverse
S = O.1Aw / t
very accurate for multipass welds
assumes shrinkage has a linear relationship with thickness.
t = plate thickness
Aw = cross-sectional area of weld, mm2
Longitudinal
s = 3IL / 100,000t
I = Welding current
L = Weld length, mm
t = Plate thickness
Flange/angular distortion
W = Flange width, mm
I = Fillet leg length, mm
t = Plate thickness
d= 〖0.2WI〗^1.3/t^2
Longitudinal bending distortion:
db = Bending deflection, mm.
A = Cross-sectional area of weld group, mm2.
L = Weld length, mm.
D = Distance of the C of G of the welds from the neutral axis, mm.
I = Moment of inertia of the assembly, mm.
d_b= (0.005 A L^2 d)/I
Correcting distortion
Stress relieving weldments
Vibratory stress relief
Mechanically inducing vibrations
Explosive stress relief
Causing shock waves from an explosive charge to counteract the residual tensile stresses.
Mechanical peening
Mechanical deformation of individual runs by mechanical methods
Thermal heating process
Furnace
local
BS EN 13445
Flame straightening
Furnace Procure requirements
Method - furnace or local?
Method of heating - gas, electrical?
Number of thermocouples - sufficient plus spares.
Position of thermocouples - must be as specified.
Method of attachment of thermocouples - spark discharge or brick.
Calibration of thermocouples - valid and within date?
Calibration of recording equipment - valid?
Speed of chart - correct for paperwork?
Component supports - must be adequate to avoid collapse.
Identification on chart - job no, date, time, signature.
Heating rate - maximum.
Soaking temperature - range allowed.
Soaking time - minimum.
Cooling rate - maximum allowed.
Withdrawal temperature - into cold air.
local procedural requirements
Heated band width.
Insulated band width.
Proximity of fittings such as nozzles and attachments.
Same as furnace +