This section discusses some of the proper methods for evaluating and repairing defective welds. The defects covered here are those created during and after welding, such as those caused by improper manipulation, insufficient protection of the weld metal pool, oxidation and impurities, and willful neglect to procedures such as proper cleaning, edge preparation and spacing of the joint to be welded. Within the industry, some supervisors themselves ignore these procedures for the sake of productivity. Although avoiding defects has its cost, repairs can be even costlier, even more so when the repairs are not handled in a systematic way.
The repairs to be covered here are based on the following defects, which have been discovered by nondestructive testing: 1) porosity, 2) lack of fusion, 3) lack of penetration, 4) Wagon track, and 5) root bead cracking, excessive penetration, and cold lap.
First Step in Repair
The first step in any report is to locate the defect and establish its size. Even those defects that are visible to the eye must be evaluated for size in order to avoid excessive removal of the base and weld metal.
Second Step in Repair
If a welding procedure already exists for the weld being made, then it should be reviewed in terms of the following:
— Material composition
— Electrode classification
— Joint preparation
— Interpass temperature
— Preheating temperature.
This procedural review helps establish those items that will be used as part of the welding repair. It may not be necessary to undertake an elaborate study if other weJds in the piping system are satisfactory,. The failure of the weld may be due to any one of the conditions mentioned above. However, it is important to realize that the weld has gone through a cycle of intense heating and cooling; the internal or shrinkage stress can be quite high. If the pipe material is of medium alloy, or heat resistant steel, then a repair procedure should be prepared to verify the steps needed for the repair. This procedure will include the fol-
— Method of exploring the defect
— Method of removing metal in and around the crack
— Explanation of the applications of preheating and inter pass temperature, both before and during welding
— Postweld heat treatment
— Electrode classification to be used in making the repair
— Whether the weld will be supervised by anyone experi enced in repairs
— Use of qualified welders
— Sketches showing the final shape and detail of the pre pared area to be repaired
On most repair jobs, metai must be removed so that there is no evidence of discontinuity, creating, a sound base upon which to make weld metal deposit. The following factors must be established;
a. the type of alloy
b. the location, size, and depth
c. wall thickness
d. the propensity to harden
e. probing method
The question at this point might well be: Why is there so much fuss over the repair of a simple weld? If the defect is discovered in a medium carbon steel pipe system, the repairs will not need the precaution used with those materials alloyed for services such as high temperature and high pressure services. At the same time, preparation of a defect, regardless of the material involved, is similar in many respects. The repair procedure document has important reference information about similar situations in which repairs were successful.
When repairing a defective root bead, one with cracking, the procedure will be to remove metal. It is important to establish whether the crack was propagated into the second and third passes. If it did, then the defect is more complicated. In this case, it is possible that that crack was caused by shrinkage stress, complemented by part of the root bead being too weak to withstand or accommodate the stress during cooling. However, there are still other reasons which could be associated with such a defect.
For instance, the preheating temperature may have dropped lower than that specified while initially welding the joint. If the alloy material is of the high hardenable type, the weld may crack. If there were
just a few passes, the cracks may not be visible to the naked eye because the area is still at a high temperature and has not experienced full contraction. Furthermore, cracks do not always develop instantly. However, when the temperature falls, the microstructure of the hardened type develops and can lead to cracking.
When developing a welding procedure for weld repairs, the following factors must be considered:
— The type of micro structure
— Heat treatment prior to attempting removal of weid metal for repair
— Exploratory method for removing defective metal, with the hope of not encountering transverse cracking
A crack that is restricted to the root bead can be looked at differently. Here, the shrinkage stress is the cause, perhaps complemented by other factors such as stress risers, inadequate root opening that led to lack of penetration, or poor restart when depositing the root bead.
in other instances, where the material on which the repair is to be made is of a medium-alloy type, the procedure becomes more challenging. The welding procedure in the first instance (making the weld initially) requires preheating and interpass temperature as well as a normalizing or stress relieving postweld heat treatment. After welding, if the postweld heat treatment was not already performed, then it will be required before the repair exercise begins, and to be normalized when the weld (repair) is completed.
In some cases involving a weld on a pipe with a heavy-wall thickness, even if preheating and interpass temperature were part of the initial welding procedure, both would be applied in repairing the weld. In that case, the sequence will be changed because of the variables involved.
Removing the metal by the carbon process (gouging) requires that preheating should be conducted the same way it was during the initial welding, due to terminal shock of the base metal. Gouging on cold metal that has the potential to harden can create microcracks. This effect is not different from that of welding an alloy that will harden when cooling exceeds the critical rate.
If the mass is great and the joint is complicated, then there is a possibility of multiaxial stress. In this case, surface heating is not an accepted part of the procedure. Heating should be slow so that the full thickness of the section as well as a short distance surrounding the area is uniformly brought to the required temperature.
Welding has been ignored in many ways and has often had to overcome a bad reputation. Too often a welder is requested to run a pass over a surface crack which is not properly prepared or badly prepared, yet is still expected to produce a sound weld. If the weld turns out to be sound, it is because of sheer luck and the welder’s ingenuity. In general, either the process or the electrode are to blame for such unsatisfactory workmanship. Often, it is not sufficiently considered that the defective area was not prepared properly, that all the scale and oxide were not properly removed, or that the welder was not instructed to use preheating and interpass temperature.
To produce a good repair weld, one must follow a set of instructions that are stipulated in the procedure, as follows
2. preparation of the joint to be repaired
3. selection of the welding process and electrode
4. execution of proper preheating and interpass temperatures
Carbon arc gouging is actually part of the method used for removing metal when repairing a weld. Therefore, care should be taken to ensure that the temperature of the weld is held at the required level in order to prevent cracking. This type of cracking is known as terminal shock cracking. It is caused when the surface is heated at a rapid rate and then allowed to cool at a rapid rate. In fact, pipes with greater mass (wall thickness) actually quench the surface, which is expanded on heating. The cooling at a rate of quenching can be so fast that the surface actually shrinks beyond the greater mass of the weld. The surface will develop stress.