OVERVIEW OF COMMON INSPECTION AND TESTING PRACTICES

The primary aim of any inspection or testing program is to ensure that governing codes, standards, and specifications are followed, and that weld quality meets design standards. In addition, the weld must satisfy the operating conditions to which the weldment will be subjected. To meet these goals, the welder is often required to complete performance qualification tests. These tests are designed to determine the welder’s ability to deposit consistently sound, high-quality welds as required by the codes under which the weldment is being completed. The code-writing organizations that dictate these standards include the following:

American Welding Society (AWS)

Canadian Standard Association (CSA)

American Society of Mechanical Engineers (ASME)

American Society for Testing Materials (ASTM)

American Petroleum Institute (API)

American Bureau of Shipping (ABS)

Inspection and testing practices used by these organizations are divided into two basic categories: destruc­tive testing and nondestructive examination.

DESTRUCTIVE TESTING

Destructive testing involves subjecting weld specimens to a series of mechanical tests that measure various metallurgical properties. These tests ensure that the welding process has been performed properly, that the joint design was correct, and that the proper filler metal was selected. Tests of this nature may make the speci­men unfit for further service. In certain cases where sample weld sections are removed from a unit for testing, the specimen may be repaired or rewelded to restore it to its original condition.

Destructive tests may also be used to certify a welder’s ability to produce sound, high-quality welds. The following list identifies the various destructive tests and the metallurgical properties each measures.

Bend testing—to determine ductility and soundness Tensile testing—to determine tensile strength and ductility Nick break test—to determine structure and soundness Impact test—to determine impact strength Hardness test—to determine hardness

Macro etch test—to determine the depth of weld metal penetration, fusion zone, and heat-affected zone

NONDESTRUCTIVE EXAMINATION

Nondestructive examination practices are used to evaluate weld quality without permanent damage to the weldment. Some of these practices may be used (1) as preparations for the welding operation, (2) as the weld progresses, and (3) after the weld is complete. The following nondestructive practices are described: visual

inspection, ultrasonic inspection, radiographic test, magnetic particle inspection, dye penetrant test, eddy current (electromagnetic) test, leak test, and proof tests.

Visual Examination

Visual examination is one of the most extensively used inspection practices for the following reasons:

■ it is easy to perform,

■ it is used by all workers who are associated with welding practices, and

■ it is relatively inexpensive.

Visual examination reveals the presence of base metal conditions such as mill scale, seams, laminations, and other surface conditions that could contribute to faulty welds. As preparations for welding are being made, visual examination detects incorrect groove angle, root face, and root opening. Visual examination also determines the general uniformity of fitup. As welding progresses, visual examination detects conditions that could result in a faulty weld upon completion. An example of such a condition is inadequate cleaning between passes. This method of inspection is widely used upon completion of the weld and the weldment for the fol­lowing reasons:

■ to determine the dimensional accuracy of the weldment, and

■ to check general conformity to specifications, including weld dimensions and the general acceptability of the welds, including conditions such as rough or irregular deposits; the existence of spatter; arc strike marks; the presence of unfilled craters; overlap; undercut; and weld and base metal cracks.

Ultrasonic Examination

In this method of examination, ultra-high-frequency sound waves are transmitted through the part being tested. These sound waves are then reflected back from the opposite side of the material and are displayed on an oscilloscope screen. Defects in the material or weld cause variations in the echo, and the result is displayed on the oscilloscope, showing the extent and location of the defect. This inspection method may be used on most materials. However, a high degree of operator skill is required to interpret the oscilloscope screen images accurately. This is called straight beam testing, which is not frequently used on welds. Most weld inspection uses angle beam testing, which shows reflections from defects.

Radiographic Examination

The radiographic method makes use of the ability of X rays or gamma rays to penetrate objects. When these rays are directed through a welded joint, a portion of the radiation is absorbed. The amount absorbed depends upon the thickness and density of the weld. If defects exist in the weld, the beam of radiation will have less metal to pass through than if the weld were completely sound. These variations in radiation penetration are recorded on a photographic film in the form of light or dark images that indicate the location, shape, and size of any existing defect (in two dimensions only).

Magnetic Particle Examination

This method requires that a magnetic field be established in the part being inspected. A high current is used to set up this magnetic field. If any discontinuities exist, such as surface or subsurface cracks or laminations, the magnetic field is distorted. The surface of the area being inspected is covered with fine ferromagnetic particles. These particles tend to gather at the area of the imperfection. They indicate not only the location of the fault but also the shape and extent of the fault. Since this method depends on the establishment of magnetic fields, it cannot be used with most nonferrous metals or austenitic steels. However, it has been used with nickel alloys.

Penetration Examination

In this method of examination, a dye penetrant is applied to the surface being examined. The dye is allowed to stand for a period of time, the excess is removed using cleaning agents, and a developer is applied to the surface.

When the developer dries, a white background remains that draws the penetrant trapped in the flaw or fault to the surface to highlight the location of the fault. Another method makes use of a fluorescent penetrant, which is absorbed into the flaw. Ultraviolet light is then applied to the area under examination, thus revealing any flaw.

Eddy Current (Electromagnetic) Examination

In this method, the parts being tested are subjected to the influence of an alternating electromagnetic field, which will induce eddy currents. If the material conducts electricity, magnetic fields will be set up. This method is adaptable to the testing of items on a relatively high-speed production basis. It is useful for the detection of both surface faults and faults just below the surface such as cracks, voids, or laminations. This method is widely used where welded surfaces are smooth, such as in the aircraft industry. It is considered ineffective for the inspection of rough surfaces often present on weldments.

Leak Examination

The most common method of examination is one of the simplest. It is also the most economical method of testing welds on vessels and piping systems. It involves simply filling the vessel with water and examining the outside of it for evidence of leaks. (This is known as hydrostatic testing.) Light oils may also be used in some cases. The oil tends to make small leaks more easily detectable. Another common practice is to increase the internal pressure of the vessel to greater than atmospheric pressure. The exterior of the vessel is then examined for evidence of leaks. When air or nonflammable gas under pressure is used for this purpose, a soapy solution is often applied to the weld on the exterior of the vessel. Any leaks cause bubbles to be formed. For small ves­sels, it is sometimes possible to submerge the vessel completely in water. It can then be examined for evidence of rising bubbles, indicating leaking welds.

Proof Examination

This method of examination is used to determine if the weldment is capable of performing the service for which it was designed. This examination usually involves the application of service loading greater than that to which the weldment would normally be exposed. Closed containers or piping systems are subjected to internal pressure by applying air (pneumatic) or water (hydrostatic) pressure.