02 Terminology and Weld Preparations
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Transcript of 02 Terminology and Weld Preparations
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TERMINOLOGY AND WELD PREPARATIONS
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Weld preparationWeld preparation depend on:• welding process• preparation method available• access• type of joint• type of parent material• thickness• welding position• in-service type of loading
Weld preparation for arc welding - see BS EN 29692!
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Weld preparationWelding process impacts upon weld preparation
Arc welding EB
W
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Weld preparation
Welding process impacts upon weld preparation
MMA MAG• high heat input process allow a larger root face less weld
metal required, less distortions, higher productivity• on MAG, no root gap possible easier set-up• if gap is too big possible burn-through• if gap is too small lack of penetration problems
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Weld preparation
Preparation method impacts upon weld preparation
• Requires machining slow and expensive
• Tight tolerance easier set-up
• Can be flame/plasma cut fast and cheap
• Large tolerance set-up can be difficult
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Weld preparation
Preparation method impacts upon weld preparation
Variations in cutting line
• Root face too large lack of penetration
• Root face too small burn-through
Variations in cutting angle
• Included angle too large heavy distortions, increase amount of filler metal required
• Included angle too small lack of penetration, lack of side wall fusion
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Weld preparationPreparation method impacts upon weld
preparation - corrective measures
• Backing strip (6 mm or more, same material)
No root face
Root face too large • Recut groove preparation • Increase root gap and use
backing strip
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Weld preparation
Preparation method impacts upon weld preparation - corrective measures
Too big/too small included angle • Recut groove preparation
• Increase root gap and use backing strip
Warning! Backing strips give a built-in crevice
susceptible to corrosion
give a lower fatigue life
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Weld preparationsAccess impacts upon weld preparation
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Weld preparations
Access impacts upon weld preparation
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Weld preparationsAccess impacts upon weld preparation
Pipe weld preparation - one side access only!
for wall thickness up to 3 mmfor wall thickness 3 to 20 mm
for wall thickness over 20 mm
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Weld preparationsAccess impacts upon weld preparation
Pipe weld preparation - one side access only!
•self-adjusting items
•no danger of burn-through
•susceptible to corrosion
•lower fatigue lifeTo ensure minimum root mismatch, the internal bore is sometimes machined with a low angled bevel
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Weld preparations
Type of joint impacts upon weld preparation
•danger of burn-through
•difficult to set-up
•easy set-up
•no burn-through
corner joints require offset
offset
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Weld preparations
Type of joint impacts upon weld preparation
Bevel angle = 30º
Included angle = 60ºIncluded angle = Bevel angle =
50º
lap and square edge butt joints do not require preparation
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Weld preparations
Type of parent material impacts upon weld preparation
Steel Aluminium
• to reduce distortions on stainless steels welds, reduce included angle and increase root face
• to avoid lack of side wall fusion problems aluminium require larger included angles than steel
60º
30º
70-90º
35-45º
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Weld preparationsThickness of parent material impacts upon weld
preparation• a single bevel groove requires a volume of weld
metal proportional to the square of plate thickness
• its lack of symmetry lead to distortions
Reduce shrinkage by:• reduce weld volume• use single pass welding
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Weld preparations
Reduce weld volume by:• reduce included angle
Thickness of parent material impacts upon weld preparation
• reduce root gap
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Weld preparations
Reduce weld volume by:• increase root face
Thickness of parent material impacts upon weld preparation
• use double bevel weld prep
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Weld preparations
Reduce weld volume by:• use U prep instead V prep
Thickness of parent material impacts upon weld preparation
U prep better than V prepV prep better than U prep
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Weld preparations
• Reduce distortions by using an asymmetric V prep instead of a symmetric V prep
Thickness of parent material impacts upon weld preparation
t/3t
• weld first into deeper side• after welding half of depth, root back gouging• weld completely on backside• complete welding on first side
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Weld preparation
PF symmetric preparation
Welding position impacts upon weld preparation
PC asymmetric preparation
• If symmetric preparation is used on PC weld will run out of the groove
60º
30º 60º
15º
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Transitioning is carried out to reduce the wall thickness on a
joint that has two different plate/pipe thickness to match the
thickness of the thinner plate/pipe.
The transition may be applied by a pneumatic beveling machine
or by a disc grinder and it is a normal requirement to have a
minimum length four times the thickness of the misalignment or
four times the thickness of the thinnest plate
The transition may be applied to the inside or out side of the
joint, in the case of a pipeline it is normally applied to the inside
Transition JointsTransition Joints
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Abrupt changes in material thickness, causes stress concentrations and low fatigue strength
A smooth transition is required to reduce the chances of fatigue cracking
A taper of 1 in 4 or 1 in 3 is recommended for maximum fatigue strength
High stress concentrationno taper
Low stress concentration 1 in 4 taper
Transition JointsTransition Joints
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Joints a) and b) are the most common types of transitioning
with c) being used to facilitate non-destructive testing
a)
b)
c)
Transition JointsTransition Joints
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The Purpose of a transition joint on a pipeline is to: Reduce stress concentration, reducing the chances of fatigue
cracking.
Prevent PIG gauges from getting trapped in side the pipe.
Prevent turbulence problems, which may lead to accelerated. erosion
etc.
t. = thinnest plate thicknessor the extent of misalignment
t.
4 x t. min
Transition joints on pipelines are used in situations where a thick walled pipe is to be joined to a thin walled pipe normally where extra safety is required, the transition taper is normally applied to the inside of the pipe
Transition JointsTransition Joints
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Seam offset: Longitudinal seams should be separated at a set distance form
each other The amount of seam offset is dependent upon code
requirements. For pipes typically 250mm or 90o, pressure
vessels 1800
250 mm
or 90oLongitudinal seams are
separated to avoid having welds in close proximity to
each (weld junctions)
On a pipe line the longitudinal seams are separated for the above reason plus if any
crack does initiate it will be isolated to one pipe length minimizing damage
Seam OffsetSeam Offset