41 CHAPTER 3: TYPES OF WELDING PROCESS, WELD DEFECTS AND RADIOGRAPHIC IMAGES 3.0. INTRODUCTION Welding is the process of coalescing more than one material part at their surface of contact by the suitable application of pressure or heat or both at the same time. Welding provides a method for joining metal parts with respect to the cost of fabrication and usage of materials. Good weld renders a permanent joint robust than the parent material. But the weld defect arising during welding due to the lack of skilled labor, technical faults etc., can lead to catastrophic results if left undetected, particularly in pipe line and nuclear industries. 3.1. TYPES OF WELDING PROCESS AND WELD JOINTS The two major groups of welding processes as categorized by the American welding society are Fusion Welding and Solid State Welding. Fusion welding process involves the application of enormous heat energy to melt the base metal facilitated by an electric arc between base metal and electrode or using heat energy derived from the electrical resistance to the current flow or by the usage of hot flame generated by an oxy fuel gas, which becomes the basis for classification that can be categorized into Arc Welding (AW), Resistance Welding (RW) and Oxy Fuel gas welding (OFW) respectively. Fusion welding operations mostly utilize filler
42 metal, but autogenous weld doesn t use any filler metal (or filler material).the common types of AW are Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). Solid State Welding is mainly accomplished through the usage of pressure. Heat energy if used will be less than the melting point of the metal. Diffusion Welding (DFW), Friction Welding (FRW) and Ultrasonic Welding (USW) are the subcategories of welding falling under Solid State Welding. The base metals should be impurity free so that it can fuse completely with the filler material. This is realized by removing the impurities like Slag, Oxides and trapped gases from the base metals using a material agent called flux. The basic types of weld joints are the Butt joint, the Tee joint, the Lap joint, Corner joint and Edge joint. In Butt joint, the joining parts such as sheet metal or plate etc., lies in the same plane and are joined at their edges. However in the Tee joint the parts will be right-angled to each other and the weld joint will possess the shape of letter T. The joining metal parts will be imbricated in case of Lap joint. The edge joint is induced in common edges. The basic standard terms for the elements of the joints are root and groove, where the cross section view of root may be a point or line or an area which illustrates the portion of the joint where the metals flank by each other. The term groove refers to the
43 space or opening provided between the metal parts which are to be welded. 3.2. TYPES OF WELDING DEFECTS The weld defects such as Lack of Penetration, Porosity, Slag Inclusion, Burn Through, Oxide Inclusion, Lack of Fusion, Crack, Cluster Porosity, Internal Concavity, Offset, Tungsten Inclusion etc., deteriorate the mechanical properties of the welded structures thereby increasing the risks of fatigue, failure and disaster. Lack of Penetration is one of the weld defects that occur as a result incomplete penetration of the weld defect through a joint. This is one of the most unpleasant weld defects because this acts as a cause for Natural stress. When this defect is pressurized, it may result in the failure of the entire weld metal. So this should be avoided to the maximum possible extent. It usually appears as a dark straight line of medium width. Internal Concavity defect arises due to the contraction of the weld metal as it cools. This defect resembles Lack of Penetration but has irregular edges and is broader in the middle. Cluster porosity and Porosity weld defect occur due to the presence of moisture, which turns into gas that will be further trapped in the weld when heated. Lack of fusion weld defect occurs when the filler metal doesn t fuse correctly with the base metal. In a radiographic image these defects appear as straight lines between the seam of the metal and they
44 orient themselves in only one direction, along the weld seam in the metal. Oxide Inclusion occurs as irregularities dark in color since they are less intense than the adjoining materials. Burn-Through is a weld defect which arises due to overheating that makes the weld metal to penetrate the weld zone. When the metal is burnt so much, there is an excessive penetration and there are deposits of the weld metal inside the base metal. They appear as large globules of metal inside the base metal. Burn-Through appears as a dark spot, surrounded by whitish grayscale intensities. Root Undercut weld defect take places as a result of erosion of the base metal near the root of the weld. Tungsten Inclusion defect arises when the tungsten get entrapped in the weld. Slag Inclusion defect occurs due to the presence of nonmetallic element in the weld metal. The distinguishing characteristics of Slag inclusion defect are that, they are asymmetric, jagged, rounded edged and may possibly be scattered throughout the image. 3.3. RADIOGRAPHIC WELD IMAGES Non Destructive Testing (NDT) is applied to test a material for internal defects without damaging its material properties. Radiography based weld flaw identification is based on the fact that the defective region would have differential attenuation of the ionizing radiations
45 making it appear different compared to the image of the other sound regions. The radiographic images used in the present study were taken at the Quality Assurance Division of Indira Gandhi Center for Atomic Research (IGCAR), Kalpakkam, India which pertains to the weld joints in components and piping s, radiographs collected from NDT organization, International Institute of Welding (IIW) and simulated images. The welding process carried out at IGCAR is primarily Tungsten inert gas (TIG) welding since it provides a high quality, stronger weld and it gives the operator a much more flexibility of usage and control. This arc welding process makes use of tungsten electrode for welding where the atmospheric contamination is avoided by an inert gas (Argon). X-rays of 145 KV at a Mille Ampere of 11.4 is passed through the TIG welded metal piece for a time period of 5 minutes, which leads to the creation of the latent image in the film. But the latent image created by the interaction of film material with the X-ray radiation is not visible for human eyes. The film is immediately developed in a dark room using developer solution, stop bath and fixer. The radiograph generated is digitized using DICOM 2905 laser film digitizer which is highly reliable and fast in processing. The sampling pitch is 50 nm to 500 nm with an optical resolution of 50 nm and with a maximum optical density of 4.3. The light source used is He-Ne Laser with a pixel depth of 4096. These radiographs are
46 preprocessed for enhancing the clarity and operability of the radiograph for the further processes like segmentation, feature extraction and classification. 3.4. SUMMARY This chapter provides a brief description of the welding processes, materials or components used in welding, radiographic images and the types of weld defect. The various methodologies for the identification of weld defect are described in the forthcoming Chapters.