|If you don't know the acronyms "PQR", "WPS", or "CWI", then you've taken your welding for granted and have been very lucky, or so you think.
Welding is more than just laying down a bead of molten metal. It is a casting operation that, for the most part, is taken for granted in many companies. That's because a lot of bad welding doesn't crack and fail immediately upon completion. It may take months or even years. However, if a crack develops in the same location time and time again, it may be prudent to review your welding procedure.
What is a welding procedure? The welding procedure specification, or WPS, is a specification that spells out all of the various parameters that need to be controlled in order to make a good weld. It is a blueprint that the welder and the inspector can follow. A certified welding inspector, or CWI, is someone who has been trained to inspect the welds per the WPS, and maintain the standards spelled out in that document. You can visit the American Welding Society website for information on various WPS documents for different materials and configurations. You can purchase them online. If a WPS does not exist, then it is necessary to design the weld configuration and other variables and make a practice run of the weld using these new parameters. The practice weld is not on the repair region, but on a dummy mockup of the region to be repaired. After the practice weld is completed, the weld is sent to a lab for tensile and bend testing, radiographic inspection and visual inspection. If it passes all of those tests, then the procedure is documented in a procedure qualification report, or PQR. It is a one-time event. Welding procedure specifications are usually developed from many many PQRs, as they should be. A WPS is a universally accepted procedure that is based upon a solid track record. All of the relevant PQRs are listed in the front page of a WPS. The luxury of having a specific WPS is not always possible. There will be times when there won't be a WPS on which to rely. In that case there are some fundamental parameters that will have to be considered. This is a case study that illustrates some of those parameters.
JOINT CONFIGURATION -The picture to the left shows such a case. It is a crack in a stiffening ring on an 8 1/2 ft. diameter rotary dryer (similar to a kiln, less the interior refractory). The stiffening ring helps keep the shell round. If the stiffening ring cannot maintain its shape then the shell will flex too much. At some point the flexing will cause a crack to initiate and grow in length and depth. If allowed to go unchecked, this condition will eventually lead to a catastrophic failure. It was very important to do this job right the first time around. This crack was only two inches away from a previous repair that was not done correctly. That story is in the article "Material Properties - Case Study No. 112: The Necessity of Requiring a Welding Procedure Specification." The first thing necessary was to chase the cracks (i.e. remove any trace of the crack). It was important to bevel the surface toward the crack to allow enough room for the welder to have good manipulation of the welding rod. The deeper the crack, the greater the need for good rod manipulation. Without good rod manipulation, problems such as lack of fusion and lack of penetration are probable.
NON-DESTRUCTIVE INSPECTION - While starting on the crack shown in the picture to the upper left, a second crack was found about a foot above it (picture lower left). The white dotted lines highlight the metal removed with the crack. The red dotted lines highlight regions that looked ok to the naked eye, but in actuality were cracked. Both regions were ground out. It was important to get to the very bottom of the crack. This didn't happen on the bottom crack the first time around because the region was visually checked without dye-penetrant and looked to be ok. After putting down a couple of weld passes, the crack opened up again, at the root, and on the right side of the weld passes.
This is the point where having an inspector helps. The inspector, especially a certified welding inspector, is highly trained in welding procedures and non-destructive inspection methods, such as liquid dye penetrant. Ultrasonic inspection is also valuable. These particular root cracks were found with liquid dye penetrant.
WELD DESIGN ENGINEERING - The crack went into an inaccessible area which could not be repaired. It was important that the new weld did not tie into the existing crack, otherwise there was a good chance that the new weld could crack if the existing crack flexed in any manner. To isolate the existing crack from the new weld region, the crack was drilled out deep enough so as to go past the existing crack depth. Having done this, the region to the left of the drill hole was gouged out and ground down (picture to the lower right). This time the region was checked with liquid dye penetrant . The bottom of the drill hole showed no signs of a crack, therefore, the region to the left of the drill hole was isolated from the crack. The one region of red that indicated a crack (picture lower right) was in fact the bottom of the stiffening ring. The ring was attached to the shell with fillet welds on either side. It was not a full penetration weld. With a clear indication that the crack had been removed, welding continued.
|WELDING HEAT AND INTERPASS TEMPERATURE - One variable often overlooked when welding is the interpass temperature. This is the temperature of the top weld bead and the vicinity of parent metal around it after several consecutive passes are made without allowing it to cool down. Heat will build up if allowed to go unchecked. If the temperature becomes too hot, there can be induced distortion stresses upon cooling. In an unrestrained joint the result is visible distortion. In a highly restrained joint such as this one, the result is a crack. Another problem from running too high of an interpass temperature is metallurgical. The properties of the material can change, especially if it is an alloy. To minimize these problems, a welding procedure specification for shielded metal arc welding for the specific material was employed. The procedure specified the controlling parameters on many different variables, two of which were the amount of amperage, or "heat," to use for the rod size, and the maximum interpass temperature. To minimize the interpass temperature, the weld passes were staggered (picture lower left). It was quickly observed that running several passes almost on top of each other raised the interpass temperature above the specified limit. This was especially noticeable with the weld repair on the top crack where the groove was narrower and the welds could not be staggered (picture lower right). In this case the welder alternated his weld passes between the top crack and the bottom crack. He used "temp sticks" to measure the temperature. These sticks are designed to melt like wax at a predetermined temperature. If the stick melts, then the interpass temperature has been exceeded. If the stick doesn't melt then the interpass temperature is ok and more weld passes can be laid down over the other passes. The welders also noticed that if they laid thicker passes than that specified in the WPS, or "gobbed" it on as they said, the interpass temperature shot up quickly. Guess what happens when Production personnel scream that they want the equipment back? Welders gob on the weld.|
|INSPECTING BEYOND THE WELD REGION - When checking the region around the gouged out areas with liquid dye penetrant, many hairline cracks were found below the bottom gouged out crack area (picture upper left, white dotted region). They appeared to be insignificant. That was a mistake! When running the outer pass next to the outer diameter of the ring (white dotted region), a crack opened up over 1/4" away. The region was ground out, and this time around, all hairline cracks within three inches of the weld area were ground out completely.
The finished products can be seen in the pictures below. In the picture to the lower left, the red dotted region is where hairline cracks were chased until they disappeared when checked with liquid dye penetrant. These cracks could not be seen with the naked eye. This job took 44 hours from set up to tear down. What started as one crack wound up being seven cracks. Chasing cracks to completion and watching the interpass temperature lengthened the job.
|Bottom Crack Repair||Top Crack Repair|
As a foot note, the same job was done from start to finish in 12 hours time about seven months ago. The welder was a hero to Production. However, five days later the ring broke two inches away (the bottom crack shown here). That failure caused an additional crack to open up on the stiffening ring about 1200 away. Somehow I don't believe the term "interpass temperature" was in that welder's vocabulary.
As the old saying goes ... "Pay me now, or pay me later."
I would like to add that paying me later is ALWAYS MORE EXPENSIVE!!!
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