TERMINOLOGY AND WELD PREPARATIONS

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!

Weld preparationWelding process impacts upon weld preparation

Arc welding EB

W

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

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

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

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

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

Weld preparationsAccess impacts upon weld preparation

Weld preparations

Access impacts upon weld preparation

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

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

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

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

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

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

Weld preparations

Reduce weld volume by:• reduce included angle

Thickness of parent material impacts upon weld preparation

• reduce root gap

Weld preparations

Reduce weld volume by:• increase root face

Thickness of parent material impacts upon weld preparation

• use double bevel weld prep

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

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

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

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

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

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

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

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