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Chapter 9. Testing and Inspection of Welds. Objectives. Contrast six differences between mechanical or destructive and nondestructive testing List the 12 most common discontinuities and the nondestructive methods of locating them
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Chapter 9 Testing and Inspection of Welds
Objectives Contrast six differences between mechanical or destructive and nondestructive testing List the 12 most common discontinuities and the nondestructive methods of locating them Describe how three mechanical or destructive and four nondestructive testing methods are performed State five reasons why welds are tested Evaluate a weld for compliance with a given standard or code
Introduction Necessary to ensure quality, reliability, and strength of a weldment Active inspections are needed Extent of testing and inspection depends upon the intended service of the product A weld that passes for one welding application may not meet the needs of another
Quality Control (QC) Two classifications of methods in quality control: Destructive, or mechanical, testing Nondestructive testing Mechanical testing (DT) methods destroy the product Hydrostatic testing is the exception Nondestructive testing (NDT) does not destroy the part being tested
Discontinuities and Defects Discontinuities and flaws are interruptions in the typical structure of a weld A defect is a discontinuity which renders a part unable to meet standards Many acceptable products may have discontinuities The tolerances for welds have been established and are available as codes or standards
Porosity Results from gas that was dissolved in the molten weld pool Bubble trapped as metal cools to become solid Porosity is most often caused by: Improper welding techniques Contamination An improper chemical balance between the filler and base metal
Figure 9.1 Uniformly scattered porosities Figure 9.2 Clustered porosity
Figure 9.3 Linear porosity Figure 9.4 Piping or wormhole porosity
Inclusions Nonmetallic materials, such as slag and oxides, that are trapped: In weld metal Between weld beads Between weld and base metal Sometimes inclusions are jagged Can also form a continuous line Reduces structural integrity
Inadequate Joint Penetration Occurs when the depth that the weld penetrates the joint is less than needed Major causes: Improper welding technique Not enough welding current Improper joint fitup Improper joint design
Incomplete Fusion Lack of coalescence Between the molten filler metal and previously deposited filler metal Between the molten filler metal and the base metal Interpass cold lap Lack of sidewall fusion
Incomplete Fusion (continued) Major causes of lack of fusion: Inadequate agitation Improper welding techniques Wrong welding process Improper edge preparation Improper joint design Improper joint cleaning
Figure 9.9 Gouge removalRemove gouges along the surface of the joint before welding
Arc Strikes Caused by accidentally striking the arc in the wrong place and/or faulty ground connections Even though arc strikes can be ground smooth, they cannot be removed Will always appear if an acid etch test is used Can cause localized hardness zones or the starting point for cracking
Overlap Also called cold lap Occurs in fusion welds when weld deposits are larger than the joint is conditioned to accept Weld metal flows over the surface of the base metal without fusing Generally occurs on the horizontal leg of a horizontal fillet weld To prevent overlap, the fillet weld must be correctly sized Arc must be properly manipulated
Undercut Result of arc force removing metal from joint face Can result from excessive current A common problem with GMA welding when insufficient oxygen is used Incorrect welding technique can cause undercut
Crater Cracks Tiny cracks that develop in the weld craters as the weld pool shrinks and solidifies High shrinkage stresses aggravate crack formation Can be minimized by not interrupting the arc quickly at the end of a weld Some GMAW equipment has a crater filling control
Underfill Deposited metal inadequate to bring the weld's face equal to the original plane For a fillet weld the weld deposited has an insufficient effective throat Usually corrected by: Slowing the travel rate More weld passes
Plate-Generated Problems Some problems result from internal plate defects that the welder cannot control Internal defects are the result of poor steelmaking practices Steel producers try to keep their steels as sound as possible Mistakes that occur in steel production are often blamed on the welding operation
Lamination More extensive than lamellar tearing Involve thicker layers of nonmetallic contaminants Located toward the center of the plate Caused by insufficient cropping of the pipe in ingots Slag and oxidized steel in the pipe is rolled out with the steel Can be caused when the ingot is rolled at too low a temperature or pressure
Delamination The heat and stresses of the weld may cause some laminations to become delaminated Contamination of the weld metal occurs if the lamination contained large amounts of: Slag Mill scale Dirt Other undesirable materials Can cause wormhole porosity or lack-of-fusion defects
Lamellar Tears Appear as cracks parallel to and under the steel surface Not in the heat-affected zone Have a steplike configuration Thin layers of nonmetallic inclusions lie beneath the plate surface These inclusions separate when severely stressed
Destructive Testing (DT) Tensile testing is performed with specimens prepared as round bars or flat strips Two flat specimens are used, commonly for testing thinner sections of metal Weld section Machined to specified dimensions Placed in tensile testing machine
Figure 9.22 Tensile specimen for flat plate weldSource: Courtesy of Hobart Brothers Company
Fatigue Testing Determine weld resistance to repeated fluctuating stresses or cyclic loading Part is subjected to repeated changes in applied stress Specimen may be bent back and forth
Figure 9.23 Fatigue testingThe specimen is placed in the chucks of the machine. As the machine rotates, the specimen is alternately bent twice for each revolution
Shearing Strength of Welds Two forms of shearing strength of welds: Transverse shearing strength Longitudinal shearing strength Transverse shearing strength: Divide the maximum force by twice the width Longitudinal shearing strength: Divide the maximum force by the sum of the length of ruptured welds
Figure 9.24 Transverse fillet weld shearing specimen after weldingSource: Courtesy of Hobart Brothers Company
Welded Butt Joints Three methods of testing welded butt joints: Nick-break test Guided bend-test Free bend-test A jig is commonly used to bend most specimens Not all guided bend testers have the same bending radius Codes specify different bending radii
Figure 9.26 Nick-break specimens(A) Nick-break specimen for butt joints in plate and (B) method of rupturing nick-break specimenSource: Courtesy of Hobart Brothers Company
Figure 9.27 Root and face bend specimens for 3/8-in. (10-mm) plate
Figure 9.32 Free bend test(A) The initial bend can be made in this manner; (B) a vise can be used to make the final bend; and (C) another method used to make the bendSource: Courtesy of Hobart Brothers Company
Alternate Bend Initial bend may be made by placing the specimen in the jaws of a vise Specimen is bent away from the gauge lines Specimen is inserted into the jaws of a vise Pressure is applied by tightening the vise Pressure is continued until a crack or depression appears on the convex face
Fillet Weld Break Test Force is applied to specimen until it ruptures Any convenient means of applying the force may be used Break surface should be examined for soundness Slag inclusions Overlap Porosity Lack of fusion
Figure 9.33 Fillet weld testing(A) Fillet weld break test and (B) method of rupturing fillet weld break specimenSource: Courtesy of Hobart Brothers Company
Testing by Etching Specimens are etched for two purposes: To determine the soundness of a weld To determine the location of a weld Most commonly used etching solutions: Hydrochloric acid Ammonium persulphate Nitric acid
Impact Testing A number of tests can determine impact capability of a weld: Izod test Charpy test Izod test: Specimen is gripped on one end, held vertically Tested at room temperature Charpy specimen: Held horizontally, supported on both ends Tested at a specific temperature
