Multipass Welding: All You Need To Know

Multipass welding is typically required when thick sections are required to be welded. 

This technique is used in the industry for joining of thick sections, carrying out repairs, build ups, surface hard-facing and so on. As the name suggests, multipass welds consist of multiple passes of welding in a joint. 

One pass of the weld is generally allowed to cool before depositing another pass. Due to more weld metal involved, the strength of the joint is high as compared to a single pass weld. 

Some of the welding nomenclature of a multi-pass weld, the zones affected by the welding heat when a butt weld is made by multi-pass weld, and the terminology used for butt welding are shown in figure 1 below.

Different heat affected zones in a multipass weld

Benefits Of A Multi Pass Weld

A thin section can be welded with a single pass of welding. For sheet metals, a single pass of autogenous weld may be enough, or a single pass with a filler metal can do the job. 

For slightly higher thickness, two pass technique can be deployed, first pass from one side of the joint, followed by a second pass from the opposite side of the joint. 

Single pass and two pass techniques can only be used up to a certain thickness limit (a 1/4’’ plate can be welded in a single pass with a dia 5 mm electrode), but beyond a limit – a single pass or two pass technique would not produce a quality weld. 

The heat involved would be considerably high, which would produce a much larger heat affected zone, and a low strength weld. 

The residual stresses in a thick bead would be high, the bead appearance would be poor too since the shielding of a larger bead is difficult to achieve. Porosity can be expected too.

So, when the thickness of the base materials exceeds the limitation of two-pass techniques, there is no option but to deploy multiple passes in order to achieve requisite strength in the joint. Although, we must also contend with distortion and warpage accompanying the larger volume of weld metal in multipass welding.

In some metals, multipass weld techniques are necessary because the base metal microstructure requires it. Let’s see how. See figure 1 above.

The primary heat affected zone is the zone created by deposition of the first pass of the weld. The secondary heat affected zone is the one created by the deposition of second pass of weld. The secondary zone overlaps the primary zone by a small amount.

The portion of the primary zone that gets overlapped by the secondary zone gets softened or annealed. In this manner, the HAZ of the first pass or layer gets softened by the HAZ of second pass or layer. As the number of layers increase, the HAZ of all the layers gets softened up by the HAZ of the layer above it, except the top layer.

In this manner, refinement of the HAZ’s microstructure takes place. This grain refinement improves the properties of the weld.

The microstructure of multipass welds thus varies significantly from that of single pass welds or two pass welds. 

A similar line of thought is applied behind temper bead techniques, in which heat input of successive layers of welding is stringently monitored and regulated. This is done, as the name suggests, to temper the HAZ created by a bead, by adopting a higher heat input for the next bead so that a bigger HAZ is created which will overlap HAZ of previous bead, and temper it. Temper bead techniques are adopted when it is infeasible to do PWHT of a weld.

A multi pass welding procedure enables welding of a variety of weld joints of different thicknesses, for a variety of base metal grades. 

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. 

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.

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.

Figure 2,3, and 4 below show suggested electrode grades and diameters for multipass welding of various material thicknesses and positions.

suggested electrode grades and diameters for multipass welding of various material thicknesses and positions

Figure 3 and 4 below show commonly adopted welding procedures for various kinds of multipass welds.

suggested electrode type and size for various material thicknesses and welding positions
suggested electrode type and size for various material thicknesses and  positions

Single Pass Versus Multipass Welding Technique

Up to ¼’’ thickness of steel can be certainly welded with a single pass weld. However, beyond this –  multiple welding passes is a better idea. 

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.

Sometimes, choosing a single pass technique is restricted by the Code under which the welding is being performed. 

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. 

So, in essence – 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.

Position of the weld, skill of the welder involved, thickness of the base metal involved etc. are some of the factors. 

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. 

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.

Sometimes people ask, how many passes would it require for a multipass weld?

How Many Weld Passes On A Multi-Pass Weld Joint?

The number of passes in a multipass weld would depend on a number of factors. Some of the factors are as below:

  1. Weld Travel Speed: 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.

How much travel speed you adopt ordinarily depends on the comfort level of the welder. 

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 ½ 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.

This indicates that passes of lower thickness aren’t a problem. Passes of thickness (>13 mm) would attract a restriction as described above.

Weld speed also determines the heat input to the weld (a slower speed means more heat input). 

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.

This indicates that higher weld travel speeds (meaning lower heat input) aren’t 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. 

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.

  1. Weave bead vs Stringer bead: 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.

Both 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.

  1. Type of Groove: 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. 

In Section IX, QW 402.1 (which deals with change of type of groove) appears as a non-essential variable for most processes.

  1. Electrode diameter: 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.

ASME Section IX does not place any restriction on electrode diameter per se, QW 406. – which deals with filler metal size, is a non-essential variable. 

However, a higher size of electrode would entail higher heat input, which is restricted by QW 409.1.

This indicates that using a lower diameter electrode isn’t a problem, but using a higher electrode diameter electrode (over that qualified) would require requalification of the welding procedure. 

The choice of electrode gets thus regulated for us.

What Are The Different Passes In Multipass Welding?

The weld passes deposited at various stages of a multipass weld are called by different terms – such as root pass, cap pass, hot pass, cover pass etc. Each pass serves a specific purpose to achieve a certain end. 

Some of these terms are as below:

  1. Root pass: is the progression of the welding at the root of the joint. This is the first pass to be deposited in a joint. A good root pass is very important because it lays the foundation for the oncoming passes in the joint. A poorly laid root pass can make the joint fail the non-destructive examination. 

A poorly laid root pass can manifest in the form of lack of fusion, insufficient penetration, or excess penetration etc. This is why it takes some skill to properly deposit the root pass.

  1. Hot pass: This is a non-standard term, and is used to refer to the weld pass that immediately follows the root pass. The right amount of current is required in hot pass to avoid burn through over the root pass, and also obtain adequate side-wall fusion.
  2. Fill pass: This is again a non-standard term, and is used to denote the weld pass(es) which fill the joint. That is, the pass(es) that follow the root pass and hot pass, and do the bulk of filling of the joint – may be called as fill pass(es).
  3. Cover pass: Also called cap pass, it is the final pass on the face of the weld. It may be in the form of multiple beads or a single bead. If multiple beads are used, all beads comprising the top layer of the weld may be referred to as the cover pass. The cover pass completes the welding of the joint. Sometimes, this pass is also called as finishing pass or finish pass.

Single pass vs multipass welding wire: There is no difference between a multipass welding wire than an electrode/wire used for single pass welding.

So, this is all that I had to offer on this subject. I hope you could understand what is multipass welding, various aspects involved in multipass welding, and properties of multipass weld metals. Do let me know in the comments about your thoughts.

See Also:

8 thoughts on “Multipass Welding: All You Need To Know”

    • Hi. This blog is mostly about Section IX. Per Section IX, a PQR qualified with multiple passes cannot be used to support a WPS with a single pass. HOWEVER, this is a supplementary essential variable, and therefore applicable only when impact testing is a consideration. Also, only a change from multipass to single pass is restricted, not the reverse.

      Reply
  1. Fillet weld size 4.8 mm. How many number of passes minimum shall be required to complete it?
    Is it practical to do it in a single pass? The first pass shall temper the second pass.

    The minimum number of passes shall be two?

    Reply
  2. According to the standard, what should be the width of the weld?
    In a multi-pass welding process, what percentage should one pass over the other pass?

    Reply
    • Hello Zohrab. I am not sure i understand the question fully. Generally no standard places any requirements on width of the weld. Width depends on electrode angle, and heat input, etc.; but no standard requires width to meet any particular value. Regarding the overlap, for electrodes and wires – a 50% overlap is considered good. For strip welding, 50% overlap would not be feasible, so an overlap in the region of 10-15% is what is generally seen.

      Reply

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