P3 testing of weld joints ii ndt 2

Design and Metallurgy of Weld Joints (MEM-510)

Testing of Weld Joints II

Dr. Chaitanya Sharma 1-1

Non Destructive Testing of Weld Joints Lesson Objectives

In this chapter we shall discuss the following: 1. Development of the grain structure in the fusion zone.

2. The effect of welding parameters on the grain structure.

3. Common solidification mechanisms and different modes of solidification 4. Mechanisms of and techniques for grain refining

Learning Activities 1. Look up Keywords 2. View Slides; 3. Read Notes, 4. Listen to lecture

Keywords:

Weld solidification, epitaxial solidification, actual temperature gradient (G), growth rate (R), axial and columnar grains, macrostructure, inoculation grain refinement, arc pulsation

What is NDT? The use of noninvasive techniques to determine the integrity of a material, component or structure.

Or quantitatively measure some characteristic of an object. i.e. Inspect or measure without doing harm.

Why NDT Is Important?

1. The essential feature of NDT is that the test process itself produces no deleterious effects on the material or structure under test.

2. Detects various subsurface defects in welds, casting and others.

3. Assure safe operation of structure by detecting flaws.

4. Helps in quality control and assessing plant life.

5. It is used for in-service inspection and for condition monitoring.

6. It is also used for the measurement of physical properties such as hardness and internal stress.

7. NDT methods can be adapted to automated production processes as well as to the inspection of localized problem areas.

What are Some Uses of NDE Methods?

1. Flaw detection and evaluation 2. Leak detection 3. Location determination 4. Dimensional measurements 5. Structure and microstructure characterization 6. Estimation of mechanical and physical properties 7. Stress (strain) and dynamic response measurements 8. Material sorting and chemical composition determination

When are NDE Methods Used?

There are NDT application at almost any stage in the production or life cycle of a components.

1. To assist in product development 2. To screen or sort incoming materials 3. To monitor, improve or control manufacturing processes 4. To verify proper processing such as heat treating 5. To verify proper assembly 6. To inspect for in-service damage

Different Methods of NDT

Six Most Common NDT Methods 1.

2.

3.

4.

5.

6.

Visual Liquid Penetrant Magnetic Ultrasonic Eddy Current X-ray

Visual Inspection Most basic and common inspection method. – illuminate the test specimen with light – examine the specimen with the eye Tools include fiberscopes, borescopes, magnifying glasses and mirrors.

Portable video inspection Robotic crawlers permit unit with zoom allows observation in hazardous or inspection of large tanks and tight areas, such as air ducts, vessels, railroad tank cars, reactors, pipelines. sewer lines.

Visual Inspection Equipment Equipments for visual inspection include:

• Magnifying Glass • Magnifying Mirror • Microscope • Borescope – endoscopes or endoprobes • Flexible Fiber Optic Borescope – working lengths are normally 60 to 365 cm with diameters from 3 to 12.5 mm • Video Image scope

Advantages And Disadvantages of Visual Inspection Advantages: 1. Economical 2. Expedient 3. Requires relatively little training 4. Relatively little and less costly equipment 5. Versatile 6. Most widely used of all the nondestructive tests.

7. Simple, easy to apply, quickly carried out and usually low in cost.

Disadvantages: 1. Limited to external defects or surface conditions only 2. Limited to the visual acuity of the inspector

Applications of Visual Inspection • Visual examination are carried out before welding, during welding and after welding.

• Before Welding: For detecting dents, burrs, scales, seams or laps and also dirt grease rust and mill scale on the fusion faces and edge preparation.

• During Welding: For checking the geometry of the edge and joint preparation i.e. root face, bevel and included angles etc.

• After Welding: For cleaning the weld and to remove spatter, scale & slag

Welding Defect To Be Inspected

By Visual Inspection Visual inspection is used for identifying following external defects :

1. Appearance of ripples on the weld 2. Lack of fusion at the root of the butt weld 3. Overlaps 4. Undercuts 5. Unfilled craters 6. External cracks 7. Improper size and shape of beads 8. Misalignment 9. Surface porosity 10.Excessive reinforcement and spatter 11.Arc strikes

Liquid Penetrant Testing • LPT is based on the ability of a penetrating liquid to wet the surface opening of a defect and to be drawn into it. • LPT is used to detect the surface opening likes cracks, surface porosity etc in stainless steel, Al, magnesium, brass and other non porous metals.

LPT is of two types: 1. Dye penetrant testing ( Uses red colour liquid penetrant such as Dy check, Met L check) 2. Fluorescent penetrant testing( Uses Fluorescent liquid such as Zyglo penetrex)

Liquid Penetrant Inspection • A liquid with high surface wetting characteristics is applied to the surface of the part and allowed time to seep into surface breaking defects.

• The excess liquid is removed from the surface of the part.

• A developer (powder) is applied to pull the trapped penetrant out the defect and spread it on the surface where it can be seen.

• Visual inspection is the final step in the process. The penetrant used is often loaded with a fluorescent dye and the inspection is done under UV light to increase test sensitivity.

Steps In Liquid Penetrant Inspection • Liquid penetrant testing involves the following basic steps: 1. Surface preparation 2. Penetrant application 3. Dwell time ( 3-20 min and 30 to 60 min) 4. Removal of excess penetrant 5. Application of developer 6. Surface inspection 7. Post cleaning

Penetrant Application

Penetrant is applied by brushing , flooding, spraying It is good to heat the specimen to widen the surface opening slightly

Application of Developer

Fine chalk like silicate developer is applied after the removal of excess penetrant to soak up the liquid penetrant in order to make the cracks visible by blotting paper action

Surface Inspection After LPT There are two types of indications: True and False indications.

True indications are caused by penetrant bleeding out from actual surface as shown below

Large crack: A line of some width Fatigue crack: A line of dots Porosity : Dots

Advantages & Disadvantages of LPT Advantages: 1. May be used on all non porous materials.

2. Versatile and portable 3. Relatively inexpensive 4. Expedient inspection results 5. Result are easily interpreted 6. Requires no electrical energy except for light source 7. Indications may be further examined visually

Disadvantages: 1. Surface film such as coatings, scale hide reject able defects 2. Bleed out from porous surface can also mask indications 3. Part must be cleaned before and after inspection 4. Defects must be surface breaking 5. Decontamination & precleaning of test surface may be needed 6. Vapour hazard 7. Difficult to find very tight and shallow defects.

8. Depth of flaw not indicated

Applications of LPT Used to detect weld discontinuities open to surface i.e.

Cracks, porosity ,seams 1. Used on nonporous materials 2. Can be applied to welds, tubing, brazing, castings, billets, forgings, aluminium parts, turbine blades and disks, gears

Magnetic Particle Testing • Used to detect surface /sub surface defects in magnetic materials • Utilizes principle that magnetic lines of force, when present in a ferromagnetic material, will be distorted by an interruption in material continuity .

• Thus as long as part to be inspected is free of cracks, magnetic particle will not be attracted.

• When a crack or other discontinuity is present , north and south magnetic are set up at edges of the discontinuity • The magnetic particles will be attracted to the poles which are the edges of the crack and will cluster to form an indication directly over the discontinuity.

Magnetic Particle Inspection •The part is magnetized.

•Finely milled iron particles coated with a dye pigment are then applied to the specimen.

•These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity.

•This indication can be visually detected under proper lighting conditions.

Methods of operation To achieve best results in MPT magnetic flux lines should be oriented in a direction perpendicular to the discontinuity or defect Circular magnetization

It is used to detect longitudinal defects in the conductor

Longitudinal magnetization

It is used to detect circumferential defects in the conductor

The Apparatus Prod method is used to magnetized localized area by passing current through the components by means of hand held contact ort prod

Applications of MPT • MPT is used to detect following defects: 1. Surface and sub surface cracks 2. Incomplete fusion 3. Slag inclusion 4. Porosity 5. Inadequate joint penetration 6. Lamination 7. Seams undercut • In magnetic steels, cast irons, nickel and cobalt alloys

Magnetic Particle Crack Indications

Advantages & Disadvantages of MPT Advantages:

1. Inexpensive and portable apparatus 2. Reliable results 3. Can detect subsurface defects • Disadvantages:

1. limited to only ferromagnetic materials 2. Parts must be cleaned before and after inspection 3. Need to demagnetize parts after inspection

Eddy Current Testing Eddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements. Here a small surface probe is scanned over the part surface in an attempt to detect a crack.

Eddy Current Principle When an energized coil is brought near to the surface of a metal or conducting component, eddy currents are induced into the specimen by electromagnetic induction.

These currents set-up magnetic field that tend to oppose the original magnetic field.

The impedance of coil in close proximity to the specimen is affected by the presence of the induced eddy currents in the specimen.

When the eddy currents in the specimen are distorted by the presence of the flaws or material variations, the impedance in the coil is altered.

This change is measured and displayed in a manner that indicates the type of flaw or material condition.

Eddy Current Testing Coil

Coil's magnetic field

Eddy current's magnetic field Eddy currents Conductive material

Advantages & Disadvantages of ECT Advantages: 1. Relatively expedient 2. Low cost 3. Automation is possible for symmetrical parts 4. No couplant is required 5. Probe does not have to be in intimate contact with the test piece.

Disadvantages: 1. Limited to conductive materials 2. Shallow depth of penetration 3. Some indications may be masked by part geometry due to sensitivity variation 4. Reference standard are required

Applications of ECT Following are the main applications of ECT: 1.

2.

3.

4.

5.

6.

Weld discontinuities open to the surface i.e. cracks, porosity etc Sub surface defects Alloy content Heat treatment variation Wall thickness Range from crack detection, to the rapid sorting of small components for either flaws, size variations, or material variation.

7. Commonly used in aerospace, automotive, marine, and manufacturing industries.

ECT eddy current coils & Inspection probes

Radiography The radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radioactive source.

High Electrical Potential

Electrons +

-

X-ray Generator or Radioactive Source Creates Radiation

Radiation Penetrate the Sample Exposure Recording Device

Principle of radiography • When a test piece or a welded joint is exposed to a penetrating radiation like X rays or γ rays variations in amount of radiation transmitted through it depends upon: – The relative densities of the metal and any inclusion – Thru thickness variation

Non metallic inclusions, pores, aligned cracks and other defects results in less or more radiation reaching the recording film.

The variations in transmitted radiation produce optically contrasting areas on the recording film

X ray tube The source of electrons for Coolidge X ray tube is a heated tungsten wire filament, operating in the range of 6 to 15 V with a current of 3 to 5 amp.

The power requirement for the x ray machine range from 50 to 24000 KV. A 24000KV X ray machine is capable of photographing approximately 500 mm thick steel plate

Film Radiography The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation.

X-ray film

The film darkness (density) will vary with the amount of radiation reaching the film through the test object. = less exposure = more exposure

Top view of developed film

Radiographic Images

Advantages & Disadvantages of Radiography Advantages: 1. Permanent record enables review by parties at later dates 2. Gamma source may be positioned inside of the accessible object 3. Energy efficient

Disadvantages: 1. Radiation is a safety &requires special facilities, monitoring of exposure levels and dosage to personnel 2. Source decay over their half lives and must be periodically replaced 3. Gamma source have constant wavelength and cannot be adjusted 4. It requires highly skilled operating and interpretive personnel 5. High initial cost of x ray equipment

Applications of Radiography It is used to detect most of the sub surface weld defects such as 1. cracks, 2. porosity, 3. lack of fusion, 4. incomplete penetration, 5. Slag and corrosion entrapment 6. wall thickness 7. dimensional evaluations

Radioscopy Radioscopy or real time radiography is the x ray radiography in which the component are inspected for subsurface defects and the image is obtained on the screen and on the recorder while at the it is analyzed with the of computers

Ultrasonic Inspection (Pulse-Echo) High frequency (20000 Hz) sound waves are introduced into a material and they are reflected back from surfaces or flaws.

Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depthf of features that reflect sound. initial pulse

crack echo

back surface echo

crack 0

2

4

6

8

10

Oscilloscope, or flaw detector screen

plate

Types of wave form

Operating system Ultrasonic testing uses following OS: Pulse echo system Through Transmission system Resonance system

Through Transmission system

Ultrasonic Imaging High resolution images can be produced by plotting signal strength or timeof-flight using a computer-controlled scanning system.

Gray scale image produced using the sound reflected from the front surface of the coin

Gray scale image produced using the sound reflected from the back surface of the coin

(inspected from “heads� side)

Common Application of NDT • Inspection of Raw Products • Inspection Following Secondary Processing • In-Services Damage Inspection

Inspection of Raw Products • Forgings, • Castings, • Extrusions, • etc.

Inspection Following Secondary Processing • Machining • Welding • Grinding • Heat treating • Plating • etc.

Inspection For In-Service Damage • Cracking • Corrosion • Erosion/Wear • Heat Damage • etc.

Power Plant Inspection Periodically, power plants are shutdown for inspection.

Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage.

Pipe with damage

Prob Signals eproduced by various amounts of corrosion thinning.

Wire Rope Inspection Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope that is used in chairlifts, cranes and other lifting devices.

Storage Tank Inspection Robotic crawlers use ultrasound to inspect the walls of large above ground tanks for signs of thinning due to corrosion.

Cameras on long articulating arms are used to inspect underground storage tanks for damage.

Aircraft Inspection • Nondestructive testing is used extensively during the manufacturing of aircraft.

• NDT is also used to find cracks and corrosion damage during operation of the aircraft.

• A fatigue crack that started at the site of a lightning strike is shown below.

Jet Engine Inspection • Aircraft engines are overhauled after being in service for a period of time.

• They are completely disassembled, cleaned, inspected and then reassembled.

• Fluorescent penetrant inspection is used to check many of the parts for cracking.

Crash of United Flight 232 Sioux City, Iowa, July 19, 1989

A defect that went undetected in an engine disk was responsible for the crash of United Flight 232.

Pressure Vessel Inspection The failure of a pressure vessel can result in the rapid release of a large amount of energy. To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing.

Rail Inspection Special cars are used to inspect thousands of miles of rail to find cracks that could lead to a derailment.

Bridge Inspection • The US has 578,000 highway bridges.

• Corrosion, cracking and other damage can all affect a bridge’s performance.

• The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives.

• Bridges get a visual inspection about every 2 years.

• Some bridges are fitted with acoustic emission sensors that “listen” for sounds of cracks growing.

Pipeline Inspection NDT is used to inspect pipelines to prevent leaks that could damage the environment. Visual inspection, radiography and electromagnetic testing are some of the NDT methods used.

Remote visual inspection using a robotic crawler.

Magnetic flux leakage inspection. This device, known as a pig, is placed in the pipeline and collects data on the condition of the pipe

Radiography of weld

Special Measurements Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at various points to evaluate its uniformity.

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For More Information on NDT The Collaboration for NDT Education www.ndt-ed.org

The American Society for Nondestructive Testing

www.asnt.org