Weld, defects

The subject of weld defects is quite extensive, and complete coverage is well beyond the scope of this chapter. Therefore, this chapter will focus on specific types of weld defects of general concern in cooling water systems. Defects of seam-welded tubes are considered under material defects in Chap. 14. 

A weld defect may be defined as a metallurgical or structural interruption in the weldment that significantly degrades the properties of 

There is no guarantee that crack-free joints will automatically be obtained when fabricating weldable metals. This is a result of the fact that weldability is not a specific material property, but a combination of the properties of the parent metals, filler metal (if used), and various other factors (see Table 11.8). 

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Weld Defects Recognition Using the Digital Hartley Transform. 

We present in this paper an invariant pattern recognition method, applied to radiographic images of welded joints for the extraction of feature vectors of the weld defects and their classification so that they will be recognized automatically by the inspection system. 

An invariant pattern recognition method, based on the Hartley transform, and applied to radiographic images, containing different types of weld defects, is presented. Practical results show that this method is capable to describe weld flaws into a small feature vectors, allowing their recognition automatically by the inspection system we are realizing. 

Table 1 Three Hartley feature vectors of the weld defect of figure 3.b.

S. DARBANE, R. DRAI, Y. CHERFA Y. KABIR, Detection of weld defects by image segmentation, Les Annales de Chimie Sciences des Materiaux, June 97, Vol. 22, N° 3-4. 

In this figure one can clearly see an image of a weld defective area with a superimposed groove defectometer Fe2 4 mm thick. The depth of a minimum groove was 0,5 mm. One can clearly see a groove 0,2 mm deep, which corresponds to sensitivity less than 2% Both incomplete root penetration and metal weld beads can be clearly seen in the image. 

Application of Surface SH Wave s Detection Standard for the Ultrasonic Inspection of Weld Defects in Steel Structures, 3rd Revision, Architectural Institute of Japan, P27, (1996)... 

After many decades of application, industry has completely accepted standard NDT as an inevitable but invaluable part of production and maintenance of components. Its application has been well-regulated, acceptance criteria for weld defects exist, good schemes for personnel qualification are in place and equipment has evolved to a standard of nearperfection. NDT has become a commodity. Pioneering years are over. 

Examples will cover maintenance inspection such as corrosion detection in piping and tanks, but also routine weld inspection. The need for acceptance criteria for weld defects adapted for modern NDT techniques will be highlighted, because these form (in many cases) the key to benefit. 

For many years, the technical capabilities of standard NDT methods did not allow for this approach. If NDT would have produced quantitative data on defect size from the beginning, it is highly probable that current acceptance criteria for weld defects would have used this information. Acceptance criteria would have been completely different from what they are... 

Unfortunately, now that such methods have become available, such as the Time Of Flight Diffraction (TOFD) technique, this revolution does not happen. What we see instead is a much slower process towards quantitative NDT, in combination with adapted acceptance criteria for weld defects. 

For NDT of new construction this implies that, the more one knows about the material properties and operational conditions, the better the acceptance criteria for weld defects can be based on the required weld integrity and fine-tuned to a specific application. In pipeline industry, this is already going to happen. 

F.H. Dijkstra, J.A. de Raad, H hy Develop Acceptance Criteria for Pipeline Girth Weld Defects European-American Workshop Determination of Reliability and Validation Methods of NDE, Berlin 18-20 June 1997... 

O. Farli, R.M. Denys, S. Crutzen, Development of Acceptance Criteria for Pipeline Girth Weld Defects. European Conference on Non-Destructive Testing, Copenhagen, 26 - 29 May 1998... 

Rules for the welded fabrication of pressure vessels cover welding processes, manufacturer s record keeping on welding procedures, welder qualification, cleaning, fit-up alignment tolerances, and repair of weld defects. Procedures for postweld heat treatment are detailed. Checking the procedures and welders and radiographic and ultrasonic examination of welded joints are covered. 

Weld attack. Welds are often more susceptible to corrosion than other areas (see Chap. 15, Welds Defects ). Welds may contain porosity, crevices, high residual stresses, and other imperfections that favor attack. Carbon steel welds are usually ditched by acid attack (Fig. 7.10). 

Although not commonly listed as a weld defect, high-welding stress nevertheless provides an essential ingredient to stress-corrosion cracking and other failures. It differs in an important respect from other types of weld defects in that stresses cannot be visually identified or revealed by conventional nondestructive testing techniques. 

These weld defects not only substantially reduced the mechanical strength of the weld, the pores also formed stress-concentrating notches. Consequently, when the door frame was inadvertently struck during the scrap charging operation, a fracture initiated at the deficient weld and propagated rapidly through the %-in. (1.9-cm) plate, as revealed by the brittle appearance of the plate fracture and the directional chevron markings. 

Galvanic corrosion may occur at stainless steel welds if sensitization has taken place or if welding has produced unfavorable dissimilar phases (see Chap. 15, Weld Defects, particularly Case History 15.1). These forms of microstructural galvanic corrosion do not involve the joining of two different metals in the usual sense. 

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