Visual Inspection Testing (VT) is the most abused form of NDT by any stretch of the imagination. More than likely every fabricator that has a quality department, who don’t drink coffee all day, have steps in their fabrication process for a visual inspection. VT is the best NDT method, but it need to be performed before, during and after welding. But the key to VT is proper training, no matter who performs the inspection, without proper training and technique, you are just doing a dog and pony show. The process seems the easiest and most basic inspection to perform but I say it is actually the most difficult.
Why? Good question and I hope to answer that within this blog.
First, we must know what visual inspection is, ASNT defines VT as “visual observation of the surface of a test object to evaluate the presence of surface discontinuities. VT inspections may be by Direct Viewing, using line-of sight vision, or may be enhanced with the use of optical instruments such as magnifying glasses, mirrors, borescopes, charge-coupled devices (CCDs) and computer-assisted viewing systems (Remote Viewing). Corrosion, misalignment of parts, physical damage and cracks are just some of the discontinuities that may be detected by visual examinations.”
VT is the most basic, cost-effective NDT method. It should take place prior to, during, and after welding. Many standards require it before other methods because there is no point in submitting an obviously bad weld to sophisticated inspection techniques. Welding codes always state that welds subject to nondestructive examination shall have been found acceptable by visual examination. VT requires good eyesight in the technician and sufficient light, a weld size gauge, a magnifying glass, and a 6-in. metal ruler.
Before the first welding arc is struck, materials should be examined to see if they meet specifications for quality, type, size, cleanliness and freedom from defects. Grease, paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and joint preparation should be examined. Finally, process and procedure variables should be verified, including electrode size and type, equipment settings and provisions for preheat or post heat. All of these precautions apply regardless of the inspection method being used.
During fabrication, visual examination of a weld bead and the end crater may reveal problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld detects that can be recognized visually are cracking, surface slag in inclusions, surface porosity and undercut.
On simple welds, inspecting at the beginning of each operation and periodically as work progresses may be adequate. Where more than one layer of filler metal is being deposited, however, it may be desirable to inspect each layer before depositing the next. The root pass of a multipass weld is the most critical to weld soundness. It is especially susceptible to cracking, and because it solidifies quickly, it may trap gas and slag. On subsequent passes, conditions caused by the shape of the weld bead or changes in the joint configuration can cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized if visual inspection detects these flaws before welding progresses.
Visual inspection at an early stage of production can also prevent under welding and over welding. Welds that are smaller than called for in the specifications cannot be tolerated. Beads that are too large increase costs unnecessarily and can cause distortion through added shrinkage stress.
After welding, visual inspection can detect a variety of surface flaws, including cracks, porosity and unfilled craters, regardless of subsequent inspection procedures. Dimensional variances, warpage and appearance flaws, as well as weld size characteristics, can be evaluated.
Before checking for surface flaws, welds must be cleaned of slag. Shot blasting should not be done before examination, because the peening action may seal fine cracks and make them invisible. The AWS D1.1 Structural Welding Code, for example, does not allow peening “on the root or surface layer of the weld or the base metal at the edges of the weld.”
Visual inspection can only locate defects in the weld surface. Specifications or applicable codes may require that the internal portion of the weld and adjoining metal zones also be examined. Nondestructive examinations may be used to determine the presence of a flaw, but they cannot measure its influence on the serviceability of the product unless they are based on a correlation between the flaw and some characteristic that affects service. Otherwise, destructive tests are the only sure way to determine weld serviceability.
The great thing about VT it can be taught to anyone and it should be, with just proper training and technique. Welders, fitters, and even the receiving department should all have some training in the proper use of VT. Catching issue as far upstream will save money and time for all projects, and VT is the first line of defense.