ASME Pressure Vessels
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
Nondestructive Methods of Examination
Nondestructive Examination: Nondestructive examination is a general term used to identify the common inspection method for evaluation of welds and related materials without destroying their usefulness.
Discontinuity: It is an interruption of the typical structure of the material, such as a lack of homogeneity in the mechanical, metallurgical or physical characteristics. Discontinuity is a defect when it exceeds the sizes and types of discontinuities defined as rejectable by the applicable specification.
Examination Method Selection Guide
Visual Examination (VT):
- Applications: Weldments that have discontinuities only on the surface
- Advantages: The method is economical and expedient, and requires relative little training and relatively little equipment for many applications.
- Limitations: The method is limited to external or surface conditions only and by the visual acuity of the inspector
Liquid Penetrant (PT):
- Applications: Weldments that have discontinuities only on the surface
- Advantages: The equipment is portable and relatively inexpensive. The inspection results are expedient and easily interpretable. This method requires no electrical energy except for light sources.
- Limitations: Surface films such as coatings, scale, and smeared metal may mask or hide discontinuities. Bleed out from porous surfaces can also mask indications. Parts must be cleaned before and after inspection.
Magnetic Particle (MT):
- Applications: Weldments that have discontinuities on or near the surface.
- Advantages: The method is relatively economical and expedient. Inspection equipment is considered portable. Unlike dye penetrants, magnetic particles can detect some discontinuities slightly below the surface.
- Limitations: The method is applicable only to ferromagnetic materials. Parts must be cleaned before and after inspection. Thick coatings may mask rejectable discontinuities. Some applications require the part to be demagnetized after inspection. MT requires use of electrical energy for most applications.
Radiography - Gamma (RT Gamma):
- Applications: Weldments that have voluminous discontinuities such as porosity, incomplete joint penetration, slag, etc. Lamellar type discontinuities such as crack and incomplete fusion can be detected with a lesser degree of reliability. It may also be used in certain applications to evaluate dimensional requirements such as fit-up, root conditions and wall thickness.
- Advantages: The method is generally not restricted by the type of material or grain structure. The method detects surface and sub-surface discontinuities. Radiographic images aid in characterizing discontinuities. The film provides a permanent record for future review.
- Limitations: Planar discontinuities must be favorably aligned with radiation beam to be reliably detected. Radiation poses a potential hazard to the personnel. Cost of radiographic equipment, facilities, safety programs and related licensing is relatively high. There is a relatively long time between exposure process and availability of results. Accessibility to both sides of weld is required.
Radiography – X Rays (RT X Rays):
- Applications: Same as that for Gamma radiation.
- Advantages: Same as for gamma radiation except x-ray radiography can use adjustable energy levels and it generally produces higher quality radiographs than gamma sources.
- Limitations: Initial cost of x-ray equipment is high. This method is not generally considered portable. Additionally, all the limitations for Gamma Radiation apply.
- Applications: This method can detect most weld discontinuities including cracks, slag, and incomplete fusion. It can also be used to verify base metal thickness.
- Advantages: The method is most sensitive to planar type discontinuities. The test results are known immediately. The method is portable and most UT flaw detectors are battery operated. The method has high penetration capability.
- Limitations: Surface condition must be suitable for coupling of transducer. A liquid couplant is required. Small thin welds may be difficult to inspect. Reference standards and a relatively skilled operator or inspector are required.
Costs of various inspection methods depend on the particular situation. Two factors that should be considered in the selection of a NDE method are that of the equipment and of performing the inspection. Visual examination is usually the least expensive but it is limited to the detection of surface discontinuities. In general, the cost of RT and UT is higher than that of VT, PT or MT. To meet the intended purpose and to minimize the costs, a qualified engineer should be consulted.
Discontinuities may be found in the weld metal, heat affected zones, and base metal of weldments made in the five basic weld joint types: butt, T-, corner, lap and edge joints. A partial list of discontinuities that may be encountered in the fabrication of metals by welding are discussed here. When specific discontinuities are located in the weld metal, heat-affected zone, or base metal, the abbreviations WM, HAZ, and BM, respectively, are used to indicate the location.
The most common types of discontinuities are listed in the Table 1 and depicted in the Figures 1 through 10. Where the list indicates that the discontinuity is generally located in the weld, it may be expected to appear in almost any type of weld. Tungsten inclusions are an exception – they are found only in the welds made by gas tungsten arc or plasma arc welding processes. The location WI refers to the weld interface.
Table 1: Common Types of Discontinuities
|Type of Discontinuity||Location||Remarks|
|1. Porosity||WM||Porosity can also be found in BM and HAZ if base metal is a casting/td>|
|2. Inclusion||WM, WI|
|3. Incomplete fusion||WM, WI||WM between passes|
|4. Incomplete joint penetration||WM, WI||WM between passes|
|5. Undercut||WI||Adjacent to weld toe or weld root in base metal|
|6. Underfill||WM||Weld face or root surface of a groove weld|
|7. Overlap||WI||We’d toe or root surface|
|8. Lamination||BM||Generally near midsection of thickness|
|9. Delamination||BM||Generally near midsection of thickness|
|10. Seam and Lap||BM||Base metal surface generally aligned with rolling direction|
|11. Lamellar tear||BM||Near HAZ|
|12. Cracks||WM, WI, BM, HAZ||Weld metal or base metal adjacent to WI|
|Weld metal (may propagate into HAZ and base metal)|
|Weld metal at point where arc is terminated|
|Parallel to weld axis through the throat of fillet weld|
|Root surface or weld root|
|13. Concavity||BM||Weld face of fillet weld|
|14. Convexity||BM||Weld face of fillet weld|
|15. Convexity||BM||Weld face of groove weld|
Porosity can be of five kinds. Scattered Porosity is uniformly distributed throughout the weld metal. The cause is generally faulty welding technique or materials. The joint preparation techniques or materials may also result in conditions that cause scattered porosity. If a weld solidifies slowly enough to allow most of the gas to pass to the surface before weld solidification, there will be few pores in the weld. Cluster Porosity is a localized array of porosities having a random geometric distribution. It often results from problems in initiation or termination of a weld pass. Piping Porosity, also known as Wormhole Porosity, is a form of porosity having a length greater than its width that lies approximately perpendicular to the weld face. Piping porosity in fillet welds extends from the weld root to the weld surface. Much of the piping porosity found in welds does not extend all the way to the surface. Careful excavation may also reveal subsurface porosity. Aligned Porosity, also known as Linear Porosity, is a localized array of porosity oriented in a line. The pores may be spherical or elongated. It often occurs along a weld interface, the interface of weld beads, or near the weld root, and is caused by Contamination that leads to gas evolutions at these locations. Elongated Porosity is a form of porosity having a length greater than its width that lies approximately parallel to the weld axis.
Inclusions are entrapped foreign solid materials. Slag Inclusions are discontinuities resulting from the entrapment of nonmetallic products within the weld metal. Slag inclusions result from mutual dissolution of flux and nonmetallic impurities in some welding or brazing processes. They can be found in welds made with any arc welding process that employs flux as a shielding medium. In general slag inclusions result from improper welding techniques, the lack of adequate access for welding the joint, or improper cleaning of the weld between passes. Due to its relatively low density and melting point, molten slag will normally flow to the surface of the weld pass. Sharp notches in the weld interface or between passes often cause slag to be entrapped under the molten weld metal. The release of slag from the molten metal will be expedited by any factor that tends to make the metal less viscous or retard its solidification, such as high heat input.
Tungsten Inclusions are tungsten particles trapped in the weld metal. Tungsten inclusions are often associated with the gas tungsten arc welding process and are sometimes associated with the plasma arc welding process. Tungsten inclusions appear as light indications on radiographs because tungsten is denser than steel or aluminum and absorbs more of the radiation.
This is a weld discontinuity in which fusion does not occur between weld metal and fusion faces or adjoining weld beads. It is a result of improper welding techniques, improper preparation of base metal, or improper joint design. Deficiencies causing incomplete fusion include insufficient welding heat or lack of access to all fusion faces, or both. Unless the weld joint is properly cleaned, the tightly adhering oxides can interfere with complete fusion, even when there is proper access for welding and proper welding heats are used.
Incomplete Joint Penetration
This is a joint root condition in which the weld metal does not extend through the joint thickness. The unpenetrated and un-fused area is a discontinuity described as incomplete joint penetration. It may result from insufficient welding heat, improper joint design, or improper lateral control of the welding arc.
Some welding processes have much greater penetrating ability than others. For joints welded from both sides, backgouging may be specified before welding the other side to ensure that there is no incomplete joint penetration. Pipe welds are especially vulnerable to this type of discontinuity since the inside of the pipe is usually inaccessible. Designers may employ a backing ring or consumable inserts to aid welders in such cases. Welds that are required to have complete joint penetration may require examination by visual or some other NDE method.
Undercut is a groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by the weld metal. This groove creates a mechanical notch which is a stress concentrator. When undercut is controlled within the limits of specifications it is not considered a weld defect. Undercut is generally associated with either improper welding techniques or excessive welding currents, or both.
It is a condition in which the weld face or root surface of a groove weld extends below the adjacent surface of base metal. It results from the failure of the welder to completely fill the weld joint.
Lamination is a type of base metal discontinuity with separation or weakness generally aligned parallel to the worked surface of a rolled product. Laminations may be completely internal and are usually detected nondestructively by UT. They may also extend to an edge or end, where they are visible at the surface, and may be detected by visual, PT or MT method. They may be found when cutting or machining exposes internal laminations.
Laminations are formed when gas voids, shrinkage cavities, or nonmetallic inclusions in the original ingot are rolled flat. They generally run parallel to the surface of rolled products and are most commonly found in shapes and plates. Metals containing laminations cannot be relied upon to carry tensile stress in the through thickness direction.
It is a lamination that has separated under stress.
Seams or Laps
These are base metal discontinuities that may be found in rolled, drawn or forged products. They differ from lamination in that they appear on surface of the worked product. When the discontinuity is parallel to the principal stress, it is not generally a critical defect. When seam and laps are perpendicular to the applied or residual stresses, they will often propagate as cracks. While seams and laps are surface discontinuities, their presence may be masked by manufacturing processes that have subsequently modified the surface of the mill product. Welding over seams and laps can cause cracking or porosity.
It is a subsurface terraced or step-like crack in the base metal with a basic orientation parallel to the wrought surface. It is caused by tensile stresses in the through thickness direction of the base metals weakened by the presence of small, dispersed, planar shaped, nonmetallic inclusions which are parallel to the metal surface. Lamellar tearing often occurs in heavy section materials. Lamellar tearing may extend over long distances and generally initiates in regions of base metals that have a high incidence of stringer-like, nonmetallic inclusions in parallel planes and high residual stresses. The fracture usually propagates from one lamellar plane to another by shear lines that are near normal to the rolled surface.
Cracks are defined as fracture type discontinuities characterized by a sharp tip and high ratio of length and width to opening displacement. They can occur in the weld metal zone, heat affected zone, and base metal when localized stresses exceed the ultimate strength of the material. Cracking often initiates at stress concentrations caused by other discontinuities or near mechanical notches associated with the weldment design. Stresses that cause cracking may be either residual or applied. Residual stresses develop as a result of restraint provided by weld joint and thermal contraction of the weld following solidification. Welding related cracks are generally brittle in nature, exhibiting little plastic deformation at the crack boundaries. A crack formed in the first layer of a weld and not completely removed before the next layer is deposited tends to progress into the layer above and then each succeeding layer until finally it may appear at the surface. The final extension to the surface may occur during cooling after welding has been completed.
Concavity is the maximum distance from the face of a concave fillet weld to a line joining the weld toes. It is sometimes called insufficient throat. Concavity is not rejectable unless the weld is undersize. Concave fillet welds must be inspected by using a fillet weld gauge capable of measuring the throat dimension, since that is the limiting dimension in terms of the size of a concave fillet weld. A concave profile fillet weld size cannot be correctly measured by the leg size.
Convexity is the maximum distance from the face of a convex fillet weld to a line joining the weld toes. The convexity results in a mechanical notch at the junction of the weld face and the base metal similar to that produced by overlap. The severity is greater when the convexity is greater.
In groove welds, weld reinforcement is weld metal in excess of the quantity required to fill a joint. Weld reinforcement may be located at both the root and the face of a groove weld. Weld reinforcement is undesirable when it creates high stress concentrations at the weld toes or weld root similar to convexity. It tends to establish notches that create stress concentrations. This condition may result from improper welding technique or insufficient welding current.
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