Temper<\/a> bead techniques are adopted when it is infeasible to do PWHT of a weld.<\/p>\n\n\n\nA multi pass welding procedure enables welding of a variety of weld joints of different thicknesses, for a variety of base metal grades. <\/p>\n\n\n\n
As far as ASME Code is concerned, Section IX requires that a welding procedure developed with multipass welding must be used in production welding with multiple pass only. <\/p>\n\n\n\n
A change from multipass per side to single pass per side would require requalification of the welding procedure. This appears in QW 410.9, which is a supplementary essential variable, and becomes effective whenever toughness is a consideration.<\/p>\n\n\n\n
In conclusion, benefits of a multipass weld include: higher thicknesses can be welded, strength of the joint is high, tempering of the heat affected zone (HAZ) of the previously deposited weld is achieved (thus achieving a microstructure with lesser residual stress), good evenly sized bead appearance is obtained.<\/p>\n\n\n\n
Figure 2,3, and 4 below show suggested electrode grades and diameters for multipass welding of various material thicknesses and positions. <\/p>\n\n\n\n <\/figure>\n\n\n\nFigure 3 and 4 below show commonly adopted welding procedures for various kinds of multipass welds.<\/p>\n\n\n\n <\/figure>\n\n\n\n <\/figure>\n\n\n\nSingle Pass Versus Multipass Welding Technique<\/strong><\/h2>\n\n\n\nUp to \u00bc\u2019\u2019 thickness of steel can be certainly welded with a single pass weld. However, beyond this \u2013 multiple welding passes is a better idea. <\/p>\n\n\n\n
Single pass welds involve higher heat inputs, so it is difficult to pull them off in overhead or horizontal positions. A multi pass weld in flat position on the other hand is relatively easy.<\/p>\n\n\n\n
Sometimes, choosing a single pass technique is restricted by the Code under which the welding is being performed. <\/p>\n\n\n\n
Also, the skill of a welder plays a role. A skilled welder can produce a quality weld in a single pass using a high size filler. <\/p>\n\n\n\n
So, in essence \u2013 whether you should opt for a single pass or multipass technique for making your weld depends on a number of things. It is important to decide when will a multi-pass weld be needed.<\/p>\n\n\n\n
Position of the weld, skill of the welder involved, thickness of the base metal involved etc. are some of the factors. <\/p>\n\n\n\n
There are a number of variables that determine the quality of a weld, so do a number of trials by varying the different variables, and record the outcome of each trial carefully. Then decide based on the outcome of trials. <\/p>\n\n\n\n
The same goes for multi pass fillet welds too. In a multi pass fillet weld, the root pass ensures the fusion of the root. The number of fillet weld passes depends on the size of fillet weld required. The cover pass in a multiple pass fillet weld helps in merging the toe of the weld to obtain a smooth contour between the fillet and the base metal.<\/p>\n\n\n\n
Sometimes people ask, how many passes would it require for a multipass weld?<\/p>\n\n\n\n
How Many Weld Passes On A Multi-Pass Weld Joint?<\/strong><\/h2>\n\n\n\nThe number of passes in a multipass weld would depend on a number of factors. Some of the factors are as below:<\/p>\n\n\n\n
Weld Travel Speed<\/strong>: A slow travel speed would produce thicker beads, and a groove can be filled that much faster. A faster speed would produce thinner beads, so it would require more passes to fill the same groove. Both travel speeds can produce joints of the same quality.<\/li><\/ol>\n\n\n\nHow much travel speed you adopt ordinarily depends on the comfort level of the welder. <\/p>\n\n\n\n
ASME Section IX generally restricts the maximum pass size. QW 403.9, which appears as an essential variable for most welding processes, says that in a weld in which any passes is greater than \u00bd in. thick, a change in base material thickness beyond 1.1 times that of the qualification test coupon would require requalification of the welding procedure.<\/p>\n\n\n\n
This indicates that passes of lower thickness aren\u2019t a problem. Passes of thickness (>13 mm) would attract a restriction as described above.<\/p>\n\n\n\n
Weld speed also determines the heat input to the weld (a slower speed means more heat input). <\/p>\n\n\n\n
QW 409.1, which appears as a supplementary essential variable for most processes, says that an increase in heat input over that qualified would attract requalification of the welding procedure.<\/p>\n\n\n\n
This indicates that higher weld travel speeds (meaning lower heat input) aren\u2019t a problem. But slower weld travel speed (if it results in heat input exceeding the heat i\/p qualified in PQR) would attract a restriction as given in QW 409.1. <\/p>\n\n\n\n
So, in this manner, Section IX lays down a few boundaries for us, within which we must remain, as far as welding travel speed is concerned.<\/p>\n\n\n\n
Weave bead vs Stringer bead<\/strong>: A weaving bead, in which the welder oscillates his hand to obtain a thicker pass, involves more volume of weld metal per unit length. A stringer bead involves a straight run of hand without any oscillation. Thus a lesser volume of weld metal is deposited in each pass.<\/li><\/ol>\n\n\n\nBoth weaving bead and stringer bead can produce equally acceptable quality welds. And although ASME Section IX does not directly place any restriction on change from stringer bead technique to weave bead technique, or vice versa (QW 410.1 is a non-essential variable), too much weaving would entail more volume of weld metal per unit length, which can run foul of QW 409.1.<\/p>\n\n\n\n
Type of Groove<\/strong>: If all other parameters are same, a U-groove would require more number of passes than a V-groove. Most Codes do not place any restriction of the type of groove, so you are free to adopt any type of groove. <\/li><\/ol>\n\n\n\nIn Section IX, QW 402.1 (which deals with change of type of groove) appears as a non-essential variable for most processes.<\/p>\n\n\n\n
Electrode diameter<\/strong>: A higher dia. Electrode would fill up a groove faster than a lower dia. Electrode. And it is possible that joints made of two different electrode diameters can produce welds of the same quality.<\/li><\/ol>\n\n\n\nASME Section IX does not place any restriction on electrode diameter per se, QW 406. \u2013 which deals with filler metal size, is a non-essential variable. <\/p>\n\n\n\n
However, a higher size of electrode would entail higher heat input, which is restricted by QW 409.1.<\/p>\n\n\n\n
This indicates that using a lower diameter electrode isn\u2019t a problem, but using a higher electrode diameter electrode (over that qualified) would require requalification of the welding procedure. <\/p>\n\n\n\n
The choice of electrode gets thus regulated for us.<\/p>\n\n\n\n