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Evaluating Weld Defects

Best methods to nondestructively check for weld defects.

Weld inspectors can measure the depth of undercut, the size of the groove, the size of the fillet, and how concave or convex is a fillet using gages such as these. Photo: EWI.

Weld defects can be problematic to say the least. A bad weld seam can cause component failure. It can result in costly downtime and rework and hurt a company’s reputation.

A few months ago, Dave Almy, a welding engineer with the Miller Electric Mfg. Co., looked at common weld problems in the story, Troubleshooting Weld Defects, and explained some of these frustrations. Not only did Almy identify these oft occurring problems in the story, which appeared in the March 2015 issue of FAB Shop, he devised simple remedies to prevent these defects from occurring.

Following Almy’s advice may eliminate many of these common weld defects from occurring in the first place, but to be sure that there are no problems you may need to test or evaluate the weld. In some cases, that effort must be undertaken to satisfy a customer requirement. In other cases, testing might be an in-house standard operating procedure.

But what test to choose? FAB Shop approached Ruth Sunderman, project engineer in the nondestructive evaluation (NDE) group at EWI, formerly known as the Edison Welding Institute, WELDING DEFECTS and asked her to consider several different weld defects and determine the best testing method for the particular welding scenario.

EWI intern Kate Namola performs ultrasonic testing on a 1-in. welded plate with a 5MHz, 45-degree shear wave. UT is a detection method that normally uses sound waves in the 1-10MHz frequency range. Photo: EWI.

FAB Shop: Porosity is a common weld defect. It can develop in many types of material—from aluminum to galvanized and carbon steel—and is the result of gas becoming trapped in the weld metal. Porosity can appear at any point on the weld, along its length, and on the inside (subsurface) or outside of the weld. What is the best way to test for porosity issues, especially considering the myriad materials and possible locations of the problem?

Ruth Sunderman: Whenever there is a desire to detect volumetric flaws, like porosity, radiography is the area of nondestructive evaluation (NDE) with the best chance of detection. From an inspection standpoint, radiography includes X-Ray and gamma ray, and all related methods such as real time X-Ray, computed radiography, computed tomography (CT), and even portable X-Ray. The type of radiography used depends on what you intend to inspect. Regardless of the method chosen, the size of the workpiece, geometry, and density are the factors that must be considered when using radiography. Very dense materials such as lead and some high-strength alloys absorb the X-Ray radiation making imaging impossible, but these materials can be useful for limiting radiation exposure to the personnel who may be performing these tests.

With regard to size, very thick parts (the maximum thickness depends on the material) cannot be inspected because they too will absorb theradiation required to perform an inspection. The geometry you wish to inspect is an important factor because complex geometries may make flaws indistinguishable and therefore impossible to detect. Keep in mind that X-Ray condenses all of the features from a 3-dimensional object into a 2-dimensional image. If complex geometry is the only concern, CT is the best option. CT provides a reconstructed model of your part, flaws and all, based on X-Rays collected during a 360-degree sweep of the part

The resulting reconstructed model can be viewed from all orientations inside and out. Internal sections of the part can be viewed without destroying the part using time consuming cutting and polishing processes, and without the fear of missing the targeted flaw.

If no radiographic inspection methods can be used, ultrasonic testing (UT) may be a good alternative. UT may miss an individual pore if it is very small, but a grouping of pores would likely be detected.

This portable, selfcontained instrument is designed to induce a magnetic field in ferromagnetic materials to detect surface defects such as crater cracks in this weld bead. Photo: EWI.

FAB Shop: Although often used interchangeably, cold lap and lack of fusion are two separate welding problems. Lack of fusion results from the weld metal failing to completely fuse to the base metal or to the preceding weld bead in multi-pass applications. Cold lap occurs when the weld overfills and overlaps the toes of the weld. How would you test and examine for them?

Sunderman: For either of these flaws, X-Ray may be used, but ultrasonic inspection would be preferred. Ultrasonic inspection or ultrasonic testing, which is commonly referred to as UT, is a detection method that normally uses sound waves in the 200kHz to approximately 20kHz frequency range. For most industrial applications, 1-10MHz is commonly used. These sound waves are generated by a transducer and move through whatever material you wish to inspect, reflect off of part surfaces, cracks or dissimilar materials and return back to the transducer, providing information about the size and location of these features. Cold lap and lack of fusion [occur] on nice flat surfaces that reflect sound quite well. Weld inspection is commonly performed by introducing sound waves at an angle, such as 45, 60, or 70 deg., so that the sound waves have the opportunity to reflect strongly off of flaws in multiple orientations and in locations that may be difficult to detect using X-Ray. The only types of flaws with which UT may have some problem detecting are small, isolated pores because spherical flaws tend to scatter sound more than reflecting it.

However, some sound will often be reflected and the presence of a group of pores has an even better chance of detection. One special UT technique called time-of-flight-diffraction (TOFD) can be very effective at detecting pores.

Lack of fusion is the result of the weld metal failing to fuse completely to the base material or to the preceding weld bead in multi-pass applications. This close-up side view of a weld (top) shows where the weld and base material didn’t fuse. Photo: Miller Electric

Porosity, as shown here (above), is among the most common weld defects. It can appear at any point on the weld, along its length, and on the inside (subsurface) or outside of the weld. Photo: Miller Electric

Chemicals used for liquid penetrant inspection (LPI). Photo: EWI.

FAB Shop: How about burn through? This occurs when weld metal penetrates through the base material. Materials less than 1/8 in. or around 12 gauge are especially susceptible to burn through.

Sunderman: Burn through can typically be detected using visual inspection. Weld inspection standards will describe how much, if any, is permitted in a weld.

FAB Shop: Excessive spatter can appear when using a variety of different filler metals. This defect results when the weld puddle expels molten metal and scatters it along the weld bead, resulting in molten metal that fuses to the base.

Sunderman: Excessive spatter can typically be detected using visual inspection or even with radiography. Weld inspection standards may define how much spatter is considered to be “excessive” if there is any doubt.

FAB Shop: Welds that are too concave or too convex can compromise the integrity of the finished product and in some cases can be considered a weld defect. How would you tackle these problems?

Sunderman: While concave and convex weld conditions can be detected by radiography, they are usually measured using visual inspection. Gages are used by weld inspectors to measure how concave or convex a weld is and if it is acceptable according to the inspection standards for specific types of weld.

FAB Shop: Crater cracks are small cracks that develop at the end of the weld. They can happen during any welding process and are sometimes called shrink cracks. This weld defect occurs when the weld crater has not been fully filled when completing a pass; the weld pool solidifies and the center of the crater pulls from the center of the weld bead. This is especially common in welding aluminum.

Sunderman: X-ray and UT can detect crater cracks if the crack depths are sufficient for detection. For shallow, surface breaking cracks other nondestructive methods such as visual, liquid penetrant inspection (LPI), magnetic particle inspection (MPI), or eddy current should be considered.

FAB Shop: What other defects do you frequently encounter in terms of welding materials?

Sunderman: Sometimes shallow cracks can form at the surface of a weld. These can be detected using LPI or MPI. These techniques are cheaper and easier to perform, but they are limited to flaws that break the surface.

Also, eddy current inspection can be used to quickly and reliably detect very small surface defects, but this technique is more expensive in terms of initial equipment costs and requires more operator experience than the previous two.

Another common defect is lack of penetration or incomplete penetration. This flaw can generally be detected using visual, ultrasonic, or X-Ray inspection.

In welds with tight fit up, liquid penetrant, magnetic particle, and eddy current could also be effective inspection methods.

EWI

Miller Electric