WELDING PROCEDURE SHEETS
AWS defines welding procedure, as the detailed methods and practices including all joint welding procedures involved in the production of a weldment. It is very important that before starting to weld, a welding procedure is drawn up, which will ensure acceptable quality welds at the lowest overall cost. Procedures become more stringent and costly as criticality of the job increases. For example, fabrication of a pressure vessel conforming ASME code requires defect- free welds capable of meeting special mechanical and non-destructive testing requirements demanded by the code. This will mean use of high quality electrodes, skilled and certified welders, moderate currents and travel speeds and welds with little or no porosity or undercut. A commercial quality vessel on the other hand may be fabricated with a more liberal procedure and less skilled welders.
To define and draw up a welding procedure, one may use a standard procedure sheet such as shown below. The sheet can be best prepared by the welding engineer in consultation with welding foreman or shop-floor supervisor. It simplifies welders’ tasks and prevents last minute confusion and faulty work. The preparation of such a sheet provides an opportunity to check on what means and materials are available in the shop, or have to be specially provided to meet the job requirements. The sheet also helps to qualify the welders before they are put on the job. Such sheets serve as references for the future. Important codes demand that such procedure sheets are prepared and the procedures qualified by completing representative welded joints and subjecting them to required destructive and non-destructive tests.
Typical Procedure Sheet for Smaw
(a) Welding procedure number
(b) Related specification and/or drawing number
(c) Material to be welded; specification number or composition
(d) Metallurgical condition of material
(e) Type of weld
f) Preparation of parts:
(i) Angle of bevel
(ii) Root face (iii) Root radius
(g) Cleaning before welding
(h) Set-up of joint (gap, included angle, tolerance on alignment etc.)
(i) Particulars of backing strip or bar (j) Welding position and direction
(k) Make, type and classification of electrode
(l) Electrical supply and electrode polarity (m) Size of electrode for each run (n) Length of run per electrode (o) Current for each run (p) Open circuit voltage (q) Arc voltage (r) Preheating procedure (s) Time between runs (t) Number and arrangement of runs (u) Welding sequence (v) Technique for depositing each run (w) Method of inter-run cleaning (x) Mechanical working of runs (y) Preparation of root before welding reverse side (z) Postweld heat treatment.
8.7.1 Type of Joints
There are six common types of joints, namely, butt, tee, cruciform, lap, corner and edge. These are illustrated in Fig. 8.23, which also illustrates three main types of weld, namely, butt, fillet, and edge. A typical butt weld is shown in the butt joint. A fillet weld is approximately triangular in transverse cross-section, and is used in tee, cruciform, lap and corner joints. An edge weld is a weld in an edge joint, and it covers a part or the whole of the edge widths.
Design of welded joints is based on several considerations, some of which are:
(a) Manner of stress tension, shear, bend, torsion.
(b) Whether loading is static or dynamic; whether fatigue is involved.
(c) Whether subjected to corrosion or erosion.
(d) Joint efficiency, which is defined as the ratio of the strength of the joint to that of the base metal, expressed as a percentage.
(e) Economy; amount of weld metal required to complete the joint and whether high deposition processes and procedures can be used.
(f) Constriction factors: accessibility, control of distortion and shrinkage cracking, production of sound welds.
Fig. 8.23 Major types of joints: (A) Square butt weld (B) Square tee-joint and fillet welds
(C) Cruciform joint with four fillet welds (D) Lap joint with single fillet weld (E) Full open corner joint with fillet welds (F) Edge joint with edge weld.
Various types of joints and welds used in welded strictures are given in Figs. 7.9-7.19 (Chapter 7).
8.7.2 Welding Parameters
To devise a welding procedure, one must choose correct welding parameters, i. e., electrode size, current characteristics and value, welding speed, arc length, angle of electrode, welding position and welding technique. The following notes are meant to help one to arrive at an acceptable procedure.
(a) Electrode size. Each size has a specific current capacity range, which is indicated on the package by the electrode producer. Use of currents above the range will cause the covering
to overheat and breakdown, resulting in increased spatter and low weld quality. Lower currents will give insufficient penetration.
Included angle Angle of bevel
Gap -►] |^-
Electrode size depends on joint thickness, edge preparation and welding position. Largest size that gives quality welds at high production rate should be preferred.
Angle of bevel - ra
Fig. 8.24 Terms pertaining to typical weld preparations
For vertical and overhead welding, smaller diameter electrodes have to be used to restrict the size of the weld puddle, since there is a tendency for the molten metal to flow out of it due to the force of gravity. The largest size which an average welder can manage in these positions is 4 mm diameter in the case of non-iron powder type electrode (say E6013), and 3.15 mm diameter in the case of an iron-powder type (E7018). A skilled welder can weld satisfactorily in vertical and overhead positions with 5 mm diameter electrodes of E6013 as well as E7018 class.
The electrode size is also dictated by the consideration of accessibility to the root of the joint. In a V-grove, for example, electrodes small enough to give correct arc length and to reach the root have to be used for the initial passes, followed by larger size to complete the weld. In a T-joint, on the other hand, a larger diameter electrode (6 mm or 8 mm) can be used for the initial pass, since the access to the root it easy.
Fig. 8.25 Term pertaining to welds
Fig. 8.26 Actual and design throat thicknesses of welds
In some cases, the electrode size has to be restricted to avoid the possibility of burn - through, caused either by bad fit-up (large gap at the root) or thinness of the material. In some metals and alloys, the weldability considerations require that the heat input is restricted by using electrodes of smaller sizes than normally used.
(6) Current-type and amount. The various factors which must be considered in choosing AC or DC, and the polarity in DC, are explained in chapter 4 article 4.2. Current values to be used are indicated under Welding Currents (Table 4.3 p. 77)
Where previous experience is not available, the safest course is to follow the manufacturer’s recommendation regarding the type of current, polarity in the case of DC and the amount of current to be used.
(c) Welding speed. By welding speed is meant the arc travel speed. For a given electrode size and current, the speed is higher with the stringer bead and lower with the weave bead. The wider the weave, lesser is the speed.
In the case of a stringer bead, increase of welding speed under constant arc voltage and current makes the bead narrower and increase penetration until an optimum speed is reached at which penetration is maximum. Increasing the speed further will cause a reduction in the penetration. Too high a speed of travel also results in undercutting, more so when this is coupled with current on the high side. Too low a speed may cause overlapping and overwelding. The travel speed should be somewhere between the maximum without underwelding and the minimum without overwelding. Fillet welding affords a wider latitude with regard to travel speed, but it should be suitably adjusted to obtain the required size of fillet weld.
Electrode melt-off rate is one of the most important factors influencing arc speed. With high-deposition iron powder type electrodes, one can use higher currents to obtain higher melt-off, and considerably increase the speed of travel to obtain a weld bead of a given size. In sheet metal working, the travel speed is kept fairly high to avoid burn through but filling the crater properly as the electrode moves requires additional skill from the welder.
(d) Arc length. Arc length should be kept minimum. Arc length for quality weld deposit also depends upon the electrode coating. Cellulosic electrodes require larger arc than rutile and basic. Low hydrogen types require extremely short arc.
(e) Angle of electrode. Electrode angle determines the uniformity of fusion, weld bead contour, freedom from undercuts and slag inclusions. Welders must learn this skill under experienced welding instructors.
Welding positions have been described in chapter 7.