Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation


Material Properties

Case Study No. 139: Piping Misalignment and Weld Slag


Sooner or later bad habits have a tendancy to catch up with us. Many times we get away with these poor habits. This is one example of where three bad habits (i.e. weld slag, flange misalignment, and vibration) joined forces to cause a failure that otherwise probably would not have happened.

Stainless Steel Pipe Spool - Failure at Weld

Figure 1

How many times have you seen crater cracks in a weld stop, or poor weld starts? How many times have you seen pump piping break away from the pump when the bolts are removed because of the terrible misalignment? How many times have you seen piping vibrating like a reed because of poor isolation and/or support? We've all seen these poor habits before. Yet most of the time these problems don't cause a catastrophic failure when they happen by themselves.

Figure 1 illustrates a case where all three bad habits occurred in the same place at the same time and together they caused the pipe to rip apart. The pipe ripped along the fillet weld that attached the slip-on flange to the pipe. The crack is circled in Figure 1.

S/S Pipe Spool Crack Along Weld Toe

Figure 2

A closer view of the weld can be seen in Figure 2. Although the spool had been made correctly in the shop, the mating flange had been installed in the field. It had been installed incorrectly. Of course the mechanic making up the flange never thought the flange in the field would be so out of alignment.

Why the mechanic didn't act upon this gross misalignment and fix the misaligned field flange probably had something to do with someone breathing over his neck yelling "Are you done yet? We need this pump back so we can get back on line!" Then again, maybe the mechanic just didn't care. The shoddy welding would support the latter case.


Crack Initiation Located at Beginning of New Weld Pass

Figure 3

An even closer look at the weld revealed a suspicious looking region which is circled in Figure 3. The weld progression was from the right to the left. The welder stopped in the region that is circled. He then started a new pass. Notice the rough looking nature of the weld start. There is an old saying that a good looking weld is a sound weld. It is still true. There is slag or other garbage in the circled region.

For reference purposes, notice the large bead of weld splatter in the upper right. This weld splatter is visible in the cross section of the fracture surface shown in figure 5.

Notice how the crack starts and runs along the weld toe. The weld toe is always a region of high stress even for good welds.

Close Up View of Beginning of Weld Pass - Entrapped Slag

Figure 4
This is another view of the region where the crack initiated. It is a misleading failure in that in earlier pictures the crack looks like it starts at one end and travels to the left until it stops. When the crack was opened up the surface revealed something very different as can be seen in Figure 5. Notice the black stuff in the picture to the left and then notice where it is located in Figure 6.
Cross Section View of Fracture Surface

Figure 5

The crack originated in this region of garbage at the weld stop/start. The chevrons straddle either side of the crack origin. The crack initiated here and then spread in both directions, to the left and to the right.
Close Up View of Crack Initiation Point

Figure 6

Figure 6 is a close up view of the crack origin shown in Figure 5. For orientation purposes the black discoloration to the left of the crack origin is also in Figure 4.
View of Weld Slag and Crack Orientation

Figure 7

Looking at Figure 7, the piece of contaminant on the right is shown in a closer view in Figure 8. Analysis of the contaminant had shown it to be weld slag.
Close Up View of Weld Slag and Crack Path

Figure 8

Notice in Figures 7 and 8 how the grain is oriented toward the weld slag. The crack started at the slag and radiated outwards.

Any kind of inclusion will cause localized stresses to increase dramatically. Most of the time though nothing happens. In this case the flange was severely misaligned and when it was bolted together there were tremendous stresses superimposed upon the inclusion. Add some pipe vibration and it was just too much for the weld to handle and it failed in fatigue.

A red flag of warning should go up in your head any time a welder makes fun of having to make"pretty looking welds." They look pretty for a very good reason.


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