— Welding procedures are discussed in chapter 2 on welding processes.
— For new jobs, procedure is finalised after welding a few sample joints and subjecting them to the required tests. The aim is to produce a quality job at lowest possible cost.
— Weldable steel should be selected as far as possible.
— A root gap is provided to ensure accessibility to the root of the joint.
— A root face prevents burn through.
— Bevel is usually 30° to 35°.
— J and U preparations save weld metal.
— On butt welds a weld reinforcement of 1.5 mm is adequate.
— Depending upon the application of the joint considerations are given to the following. Impact loading
Fatigue loading Problem of brittle fracture Torrsional loading Vibrational control.
8.3 JOINT PREPARATIONS FOR FUSION WELDING
The objective of edge preparation is to ensure the degree of penetration and ease of welding necessary to obtain sound welds. Type of preparation depends upon:
(a) type and thickness of material
(b) welding process
(c) degree of penetration required for the situation
(d) economy of edge preparation and weld metal
(e) accessibility and welding position
(f) distortion control
(g) type of joint.
These factors are considered in many combinations. Demands of the task must be met at economical cost.
8.4.1 Type of Welds
The major type of welds include “Fillet” and “Butt” welds. Fillet welds do not require edge preparation and are almost triangular in transverse cross-section. In butt welds the weld metal lies substantially within the planes of the surfaces of the parts joined. These terms should not be confused with the joint form. Examples of butt and fillet welds are shown in Fig. 8.8.
8.4.2 Joint Preparations for Different Types of Welds
Joint preparations for different plate thickness are shown in Figs. 8.9 to 8.19.
8.4.3 Fatigue as a Joint Preparation Factor
Factors that affect joint preparation are given in Fig. 8.10. Special consideration has been given to fatigue, its causes and precautions taken to eliminate, reduce or minimise it.
1. SQUARE BUTT PREPARATIONS 1.1. Close Square Butt
- Thickness 1.25 to 3 mm
- Welded from one side only
- Normal electrodes
1.2 Open Square Butt
— Thickness t < 6 mm
— Welded from one side only
— Normal electrodes
— g = 1.5 to 3 mm
1.3 Square Butt with Integral Backing
— Thickness t = 3 to 12.5 mm
— Normal penetration electrodes
— g = 3 to 8 mm
Lack of penetration and lack of fusion are difficult to detect and they cause fatigue failure of material under fluctuating loads
Susceptibility of a joint to this type of loading depends upon the severity of any notch discontinuity or change in section in the joint
Incomplete fusion (superiority is lost)
Unfortunately a weld constitutes a notch. Severity of this notch depends on type of weld and the defect it contains
і Distortion Penetration
SINGLE V PREPARATION
Thickness t < 19 mm Symmetric V a = 60o
s = 1.5 — 3 mm g = 1.5 — 3 mm
V FORMED BY INCLINED PLATES
Root face s = 0 due to increase in solid angle y
V-angle could be reduced by reverse bevelling if excessive weld metal is consumed.
Assymmetric V-preparation helps welding in horizontal-vertical position to reduce gravitational effect on the weld pool a = 55o p. = 10 - 15o
p2 = 40 - 45o sx = 0 — 1.5 mm s0 = 1.5 — 3 mm.
a = 45o g = 6 mm a = 30o g = 6 mm a = 20o g = 9.5 mm.
2.1 Single V with Integral Backing
• To ensure full penetration where the joint is inaccessible from the other side, a backing strip may be employed.
3. SINGLE BEVEL PREPARATION
Thickness t < 19 mm a = 50°
s = 1.6 — 3.2 mm g = 1.6 — 3.2 mm
Also suitable for inside and outside corner provided that there is no possibility of lamellar tear.
Cheapest preparation suitable for horizontal-vertical position butt joints.
If the members are inclined the solid angle y increases and the root-face s may be dispensed with.
3.1 Single Bevel with Integral Backing
All considerations set out in 2.1 apply also to this preparation
Fig. 8.12 Single bevel preparation
4. SINGLE U PREPARATION
Thickness t = 19.5 - 38 mm
a = 20, s = g = 1.6 - 3.2 mm g = 6.3 to 9.5 mm
The objective is to obtain full penetration while welding from one side, lesser volume of weld metal than V prep., distortion is also less. For high efficiency back gouging and welding the other side is necessary. Also needs care during welding due to reduced a.
The shape and dimensions of u-basically remain the same relative position of components may change.
Suitable only for out-side corner
РЛ 25 - 20° Pi $5-10°
Access and economy in deep grooves Increase a1 = 30 - 40° a2 remains 20°
Asymmetric prep. for
5. SINGLE J PREPARATION
This prep. is used for full penet. butt - welds in T and corner joints in plate thicknesses > 19 mm. Lack of fusion may occur, necessitating back gouging for quality joints. As in U prep. a double groove angle = 40° may be used for very thick plates (az = 20°). Here thickness t = 19 — 38 mm, a = 20°
s = g = 1.6 — 3.2 mm, у = 9.5 — 12 mm.