Chapter 9 Testing and Inspection of Welds Objectives

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

Figure 9. 5 Nonmetallic inclusions

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

Figure 9. 6 Inadequate joint penetration

Figure 9. 7 Incomplete root penetration

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

Figure 9. 8 Incomplete 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 removal Remove 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

Figure 9. 10 Arc strikes Source: Courtesy of Larry Jeffus

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

Figure 9. 11 Rollover or overlap

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

Figure 9. 12 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

Figure 9. 13 Crater or star cracks

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

Figure 9. 15 Underfill

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

Figure 9. 16 Lamination and delamination

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

Figure 9. 17 Lamellar tearing

Figure 9. 19 Correct joint design to reduce lamellar tears

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 weld Source: 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 testing The 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 welding Source: Courtesy of Hobart Brothers Company

Figure 9. 25 Longitudinal fillet weld shear specimen

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 nickbreak specimen Source: 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 bend Source: 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 specimen Source: 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

Figure 9. 34 Impact testing (A) Specimen mounted for Izod impact toughness testing and (B) a typical impact tester used for measuring the toughness of metals Source: Courtesy of Tinius Olsen Testing Machine Co. , Inc.

Nondestructive Testing (NDT) • Visual inspection is the most frequently used nondestructive testing method • Penetrant inspection locates minute surface cracks and porosity • Magnetic particle inspection uses finely divided ferromagnetic particles to indicate defects • Radiographic inspection detects flaws inside weldments

Figure 9. 35 Penetrant testing Source: Adapted from Magnaflux Corporation

Figure 9. 36 Magnetic particle inspection Flaws and discontinuities interrupt magnetic fields Source: Adapted from Magnaflux Corporation

Figure 9. 38 Schematic of an X-ray system

Nondestructive Testing (NDT) (continued) • Ultrasonic inspection employs electronically produced high-frequency sound waves • Leak checking can be performed by filling the welded container with either gas or liquid • Eddy current inspection: a magnetic field induces eddy currents within the material • Hardness testing measures the resistance of metal to penetration – An index of the wear resistance and strength

Figure 9. 43 Ultrasonic testing

Figure 9. 44 Rockwell hardness tester Source: Courtesy of Newage Testing Instruments, Inc.

Summary • Quality must be built into a product • A weld must be fit for service • Important for both the welder and inspector to know the appropriate level of weld discontinuities • Welds to an excessively high standard will result in an excessively expensive product • Producing high-quality welds is a matter of skill and knowledge