RESIDUAL STRESS & DISTORTION

RESIDUAL STRESS & DISTORTION

1. EXPANSION & CONTRACTION

   • Restricted Expansion / Free Contraction
   • Free Expansion / Free Contraction
   • Restricted Expansion / Restricted Contraction

2. COEFFICIENT OF EXPANSION & THERMAL STRESS

   • Thermal stress is created by thermal expansion or contraction
   • Expansion/Contract in a material are proportional to the dimensions of heated area and temp. change
   • Restricted expansion/contract cause deformation and stresses in metal
   • Restricted contraction may set up sufficient internal tensile stress to cause failure if the metal is not ductile enough to accommodate the stretch
   • Even a relative small local temp. change rise can cause distortion

3. RESIDUAL STRESSES

   • INDUCED BY THERMAL PROCESS
     • Spot heated area undergoes upset on cooling. Contraction INDUCES tension around heated area. Stress remains if nothing else is done.
   • INDUCED BY ARC WELDING
     • Expansion is restricted by plates themselves
     • Residual stress in longitudinal may be as high as yield stress of plate
   • STEEL PLATES
     • Slabs with large mass and size cannot expand/contract freely unless through special controlled cooling
   • UNIVERSAL MILL PLATES WITH AS-ROLLED EDGES
     • Univeral; Compressive residual stress at edges and tensile residual in middle
     • Flame Cut; tensile residual at edges
     • As-Roll; compressive residual stress, will cool faster than middle
   • INDUCED BY FLAME CUTTING
     • Oxy Fuel; Temp. on edges approach melting pnt. Uneven heating and subsequent cool will induce residual stress
   • WELDED WIDE FLANGE SHAPES
     • Large tensile residual is induced at the flange tips because localised welding heat and related local Exp/Contr
   • UNIVERSAL MILL ROLLED SHAPES
     • Residual stress created by uneven heating and cooling
     • Flange and web junctions cool last
     • Compressive at the ends and web, tensile at junctions
   • MEASUREMENT OF RESIDUAL STRESSES
     • Stress Relaxation method
       • Mather-Soete Drilling
       • Gunnert Drilling
       • Rosenthal-Norten Sectioning
       • Photoelastic Coating-Drilling
     • X-Ray Diffraction

4. ESTIMATION OF SHRINKAGES

   • Transverse shrinkage of Butt Joint
     • Greater the weld metal area the greater the transverse shrinkage
   • Longitudinal shrinkage of Butt Joints
     • Xsect area of weld is increased, high temp. transverse portion acted upon by shrinkage is larger
   • Transverse shrinkage of Fillets welds of Tee Joint
     • Flange may suffer angular distortion as a result of we to flange welds.
     • Extent varies with flange thickness; larger flange will bend less

5. DISTORTION

   • CAUSED BY OXYFUEL GAS CUTTING
     • Flame cutting induces residual stress and in turn causes distortion
   • CAUSED BY WELDING
     • Shrinkage
       • Longitudinal & Transverse
     • Angular distortion
       • Caused by transverse shrinkage
     • Bending distortion
       • Caused by longitudinal shrinkage
     • Buckling
       • Caused by long (minor Transverse); Welding Large or Thin plates/sheets
     • Twisting
       • Caused by high longitudinal more likely in thin metal
   • TRANSVERSE CONTRACTION (SHRINKAGE) - ANGULAR
     • Non-uniform contraction of weld metal due to greater width of the face of weld compared to root
   • LONGITUDINAL EXPANSION & CONTRACTION (SHRINKAGE)
     • Cool base metal is considerable stress
     • Magnitude of cooling effect depends upon the size of the heated zone in relation to the size of plate
   • OTHER CAUSES
     • Longitudinal Distortion; Shortening in length
     • Bending Distortion; unbalanced residual stresses
     • Angular Distortion; transverse contraction
     • Buckling Distortion; long + Trans
     • Twisiting Distortion; Long contract likely in thin plates or sheet

6. WPS & DISTORTION

   • WELD PASS
     • > # of passes > distortion tendency
     • When dist in Long direct is MORE CRITICAL, the smaller XSect of a bead, the LESS contraction force it can exert against rigidity of plates, and THE MORE it will stretch
   • TRAVEL SPEED OF WELD ARC
     • Slower the rate of travel = The MORE time for heat to spread ahead of weld point
     • Faster the travel, the LESS heat spread occurs ahead of weld point
   • UNIFORMITY OF HEAT INPUT
     • Form of a concentrated zone (weld Point) travels along the joint as weld progresses. At weld point the heated joint edge is expanding and upsetting. And weld metal is deposited in fully expanded condition
   • JOINT DESIGN PREP AND FIT
     • The more symmetrical weld and more balanced transverse contraction = The LESS angular distortion
     • Smaller the weld the better
   • WELD SEQUENCE
     • Groove welds
       • Figure 37; Page 38
       • Transverse fillets
       • Unrestricted Butt joint
       • Butt joint in stiffener
       • Stiffener to Plate
       • Brackets to Vert Plate
       • Bracket to Stiffener
   • SEAM WELD
     • Sequencing applies heat uniformly and simutaneous
     • Spaced intervals
     • Back-step / Skip welding, Staggered / Wandering
   • NON-CONT FILLET
     • Heat input is less = distortion and stress are reduced
   • BUILT-UP - NEUTRAL AXIS
     • Centre plane of plate where neither tension / compression exists
   • COMPLICATED - ACCURATE ASSEMBLY
     • Components to size to obtain close tolerance and increase accuracy

7. CONTROL AND CORRECTION

   • CONTROL
     • Accurate joint prep and fit-up
     • Use of back step o skip
     • Welding progresses outward from central point
     • Balance welds on either side of centre line or N.A.
     • Welding butts before fillets (to allow large contraction 1st)
     • Using intermittent fillet instead of continuos
     • Arrange weld sequence so each joint has Max. freedom of movement
     • Divide weldment into sub-assemblies to reduce cumulative distortions/shrinkage
     • Accurate weldment
       • Apply weld to counteract plate edge curvature
       • Arrange to avoid accumulation of errors due to tolerances on purchased shapes
       • Prepare accurately to reduce stack-up of fit-up tolerances
       • Seq welding so that location necessitate high degree of accuracy are assembled and welded last
       • Allow weld metal shrinkage
       • Latitude in assembly dimensions, weldment can be machined to size
     • Other Means
       • Preheat
         • provide uniform heating and cooling
       • Peening
         • Stretching weld metal
       • Restraint
         • Clamping, rigid tack, maintain min. zero root opening, cooling between passes (reduce restraining req'd)
       • Mech Control
         • Preset to allow recovery of angular distortion. Use of Temp Stiffeners Use of strongbacks/ special jig/fixtures Artificial cooling
   • CORRECTION
     • Mechanical
     • Application of heat

8. FLAME STRAIGHT & FORM

   • PRINCIPILES
     • Heated member expands and grows in unrestrained
     • Restraining expansion produces compressive stresses
     • If the restraining elemnt is not stiff it could buckle under the compressive stress
     • Plastic flow must take place to induce residual stress and change the shape
     • When a heated member cools non-uniformly, portions that cool first go into compression and portions cool last go into tension
   • MAX TEMP - LOW ALLOY
     • 650C metal a dull or cherry red
   • REMOVE BULGE
     • Pattern marked, rapid heat, and move to nest pre-mark
     • Spot Heat 50mm in diameter
     • 100mm w/press or hammer
   • LINE HEAT
     • ow heat line for material < 12mm
   • STRIP HEAT
     • Broader than line heat, but travel speed is slower; weaving action
     • Plastic flow Temp. should be reached throughTotal thickness
     • 2x transverse to heat as parallel to long axis
   • VEE HEAT
     • 2/3 or 3/4 of member width from edge but not directly on the edge
     • Torch moved in ever widening path keeping ahead of dull red @650C
   • BEAM FORM / CAMBER
     • Camber
       • Heat started @ max confinement which is over the web on top flange centreline
         • Vee heat on web
     • Sweep (Lateral camber)
       • Block heat on web
         • Vee on flanges
   • CHANNEL
     • On a channel, Max effect i.e. flange Vee heats done first and cooled completely before block heat
   • ANGLE
     • Vee heats before block heat
   • FLAME CAMBER
     • Strip heating along full length of flange
     • Mounted torches on carriage
     • High compressive in outside flange and top of web; Beam cambers as the tensile and compressive forces reach equilibrium
     • DisAdv
       • Buckling in web and flange edge if too slender to resist high residual compressive.
       • Camber held by internal forces at near yield point
       • Camber loss by vibration/cyclic on transport = not for fatigue loaded structures
   • ALTERNATE
     • (2nd) Block heats on flange and (1st) Vee heats in the web; leaves minimal residual stresses
     • Degree of camber easily calculated in advance and results in good fatigue and static ops.
     • Time consumng
   • HEAT PATTERN
     • One flange must be shorter than the other
     • # of heats can be calculated