Time of Flight Diffraction Technique (TOFD) - An Ultrasonic Testing Method for all Applications?

Time of Flight Diffraction Technique (TOFD) -
An Ultrasonic Testing Method for all Applications?

By A. Hecht *
English by Rolf Diederichs

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Abstract

The article begins with an introduction of TOFD technique principles. The main part reviews some recent literature wherein excellent results for TOFD were obtained, especially regarding speed. The author gives examples of known problems when TOFD is used and recommends not discarding proven test methods in favor of the cost-saving factor of TOFD, however, TOFD can be a valuable add-on for other test methods.

Table of contents

1. Introduction
2. Three recent examples
3. Can TOFD perform all NDT tasks?
4. Conclusion
5. Literature

1. Introduction

The first information on Time of Flight Diffraction Technique (TOFD) for ultrasonic testing on welds was introduced in 1977 [1]. The method was reported extensively in English publications and was also introduced in Germany [2]; nevertheless the method was more or less ignored by German NDT experts. Finally, in 1996, a European pre-standard was announced and thanks to that and some newly published papers [4-6] it seems that TOFD is on the way to replacing radiography and other UT techniques. One paper published in 1995 [7] referred to the wide acceptance of the method with a "TOFD Comes of Age" article.

The following main principles describe TOFD:

Two angle beam probes (usual 45°) are placed as a transmitter-receiver arrangement and are connected together (Fig 1). The distance of the probes is calculated according to the wall thickness. Longitudinal waves are usually applied. The sound beam spread is large to maximize the extent of the scan. The A-scan (Fig 4) [9] shows the so-called lateral wave, the back wall echoes and between both signals other signals can possibly appear, which can occur due to inhomogeneity. The A-Scan is not rectified in the TOFD technique. TOFD technique is always applied with imaging methods (Fig 5) [9].

Fig 5 shows the B-Scan image generated by horizontal probe movement and sound time of flight in a vertical direction. The echo amplitude is displayed as gray scale, usually zero amplitude light gray (negative maximum amplitude black, positive maximum amplitude white). For weld testing it is important to notice that the probes are aligned transversal to the weld, while the image is generated in the direction of the weld. That means the image projection of Fig 5 stands perpendicular to the probe projection shown in Fig 1!
In practice, testing with the TOFD method is only applied by continuously moving the probe pair along the weld seam, while in traditional UT techniques the probe must be also moved perpendicular to the weld seam. Depending on the equipment the scan is performed either manually or by use of an automated manipulator. In any case a computerized data evaluation is necessary. In a very early stage of the TOFD method an instrument called "ZipScan" was applied, while today many instruments which can perform B-Scans can be used - many of those are available worldwide.

Three recent examples

As already mentioned TOFD weld testing was mainly applied outside Germany - here are three recent examples:

A platform in the North Sea was inspected for underwater welds of a repaired construction with a speed of 45 minutes for each. A radiography would need 16 to 29 hours [4]. In West Java 2000 m welds on 8 gas containers, the test was carried out at a very high speed. Every day 60 - 100 m of welds were tested with TOFD /5/. A report of the Netherlands welding institute (NIL) documented a higher probability of detection and lower test costs for the TOFD technique than other NDT methods [6]. So TOFD is twice as reliable than manual UT and by 1,3 more reliable than radiography. The latter is by 1,5 more expensive than TOFD. Besides flaw detection, TOFD can also perform sizing.

Can TOFD perform all NDT tasks?

Does that mean that TOFD is a testing technique which can perform all NDT tasks?
In the author's opinion the three most important drawbacks of TOFD are described herein:


1. Sensitivity level The European pre-standard [3] points out that TOFD only evaluates the time of flight and not the amplitude of the diffracted echoes. If the instrument sensitivity (gain) is set on very low level, the TOFD image would display no diffracted echo. If the instrument sensitivity is set just above electronic noise level, the TOFD image will display a lot of diffracted echoes which are caused by very small inhomogeneities of the weld seam and does not mean that the weld is really bad. Also for the TOFD technique it is necessary to define a gain or an amplitude level because the performed test always demands acceptance criteria. 2. Crack size determination The following case is described: A weld was tested during production according to AD-HP 5/3 with a sensitivity of detection of FBH 3 mm. That means that the weld possibly contains many inhomogeneities of FBH 1mm. In-service by use of the traditional angle beam testing can find a crack. The same crack can only be detected with a much higher gain setting if the TOFD technique is applied, since crack tip echoes respond with a very small amplitude in a range of FBH < 0,7 mm. In practice, diffracted echoes at crack tips are not so clear as they are displayed in Figs 4 and 5. Crack tip echoes are part of a noise area caused by other irrelevant diffracted echoes of inhomogeneity. That can make sizing with the TOFD technique impossible. A TOFD image inspector needs to perform depiction decisions similar to that used in radiography. He or she must distinguish the relevant echoes. 3. Detection of small cracks at backside This is one of the main disadvantages of TOFD. For in-service inspection of welds it is usually not so important to find old defects inside the weld seam. More important is the detection of cracks at the backside of containers or piping. As an inspection example defects of 0.5 mm depth and app. 10 mm length must be tested at a pressure component or container of 30 mm wall thickness The use of diffracted echoes is for that task is not possible. So close to the back wall the crack tip echo amplitude is very small. In that case traditional UT techniques with angle beam probes and use of the mirror effect must be applied . The TOFD technique is not applicable here!

Conclusion

Considering the limitations of the TOFD technique described above, discarding proven test methods in favor of the cost-saving factor of TOFD is not recommended, however, TOFD can be a valuable add-on for other test methods. Let's look at the example of the automated UT of welds for pipelines. By use of mechanized test systems like ROTOSCAN or PIPECAT it was possible to replace the radiography method. The latter uses 8 focused angle beam probes in pulse echo technique in as in conjunction with one probe pair for TOFD. Nobody would take the risk of using only the TOFD technique, however it is a valuable add-on for the complete test.
Of course improvement in the TOFD technique results is possible by post-processing with the SAFT method. The author does not know if that application has been applied.
For those who want to read more on TOFD, we suggest:



• A literature search via the FIZ-W.
• A keyword search (e.g. "tofd") in World Wide Web by use of the search engines like AltaVista, Lycos, Magellan, etc.
  http://www.tabbbsplus.com/other/hottest.htm
• A keyword search in Ultrasonic-Testing-Online Journal also in World Wide Web: http://www.ndt.net
  

Literature

1. M. G. Silk: "Sizing crack-like defects by ultrasonic means", in Research Techniques in Non-destructive Testing, Vol. III, edited by R. S. Sharpe, Academic Press, 1977.
2. H. Heckhäuser, K.-H. Gischler: "Das Zipscan-System bei der Ultraschallprüfung an plattierten Bauteilen und Rohrleitungen", DGZfP-Seminar "Automatisierung in der Ultraschallprufung", Berlin, 7. - 8.11.1988
3. CEN/TC 138/WG 2 N 173, work item 00138051, European Prestandard final draft 1996: "Non-destructive Testing - Ultrasonic Examination - Part 6: Time-of- Flight Diffraction Technique as a Method for Defect Detection and Sizing". Review
4. INSIGHT Vol. 38 No. 8 August 1996, S. 549
5. INSIGHT Vol. 37 No. 8 August 1995, S. 581
6. INSIGHT Vol. 38 No. 6 June 1996, S. 391
7. F. A. Wedgwood, "TOFD Comes of Age"; Inspection, January 1995, S. 35 - 37
8. Personal Information: Mr. R Verhaeghe, Vincotte
9. Shaun Lawson, Ultrasonic testing and image processing for in-progress weld inspection,
   Ultrasonic testing online Journal April 1996.

Author

Dr.-Ing Andreas Hecht
Andreas Hecht is since 1993 in charge of the NDT-unit with a staff of 39 people at BASF´s plant in Ludwigshafen. He is responsible for the NDT at the plant equipment where all typical NDT-methods are applied. Andreas Hecht was born 1953 in Berlin. After his masters degree in material sciences he joined The German Federal Institute for Materials Testing (BAM), where he worked with ultrasonic propagation in structured materials and constructed one of the first computer-aided immersion tank systems with a modular architecture. During his work with BAM he was also involved in the training of level 3 personnel at the German Society for NDT (DGZfP) as well as training of NDT-personnel in Indonesia and Kenya. In 1986 Andreas Hecht finished his thesis on the grain size determination in austenitic sheets by ultrasonic backscattering and moved from BAM to BASF in Ludwigshafen. At the BASF he was first in charge of NDT of composite materials and for the development and application of new scanning inspection-systems before having his present position.
BASF AG
D-67056 Ludwigshafen
Phon: +49-0621-60-56466, Fax: +49-0621-60-54088
Email:

Health risks from fume and gases during welding Health, safety and accident prevention

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Health risks from fume and gases during welding

Health, safety and accident prevention

Job Knowledge 31

Factors affecting composition and quantity of fume and gases

The quantity and composition of fume and gases generated during welding is influenced by the following:

• type of process
• welding consumable
• presence of any material coatings
• nature of MMA flux coating, or core of FCA consumable
• welding parameters
• shielding gas composition

The two most important are the welding process and the choice of consumable.

Welding processes

Gas welding

Gas welding fume contains pollutants formed by combustion of the fuel gas. When an oxidising flame is used, these will be carbon dioxide with oxides of nitrogen but, for a slightly reducing flame, carbon monoxide will also be present.
Safe practice and accident avoidance
Providing there is good ventilation, the levels of fume and gases generated when welding mild steel are normally well below any workplace exposure limits (WELs). No special precautions are required.

Manual metal arc (MMA)

A MMA rod has a suitable composition for the weld metal deposited but the flux covering provides gases for the arc, additional alloying elements and slag for protecting the weld pool. The composition and quantity of the fume generated will depend on the process variables, for example, type of consumable, polarity, voltage and current. The toxic elements in the fume will be similar to those in the consumable, although the proportions are likely to be different.
Safe practice and accident avoidance
For normal welding operations, the exposure levels of fume constituents will generally exceed relevant WELs. Local exhaust ventilation (LEV) should be used to remove fume at source. As stainless steel fume, in particular, causes respiratory tract sensitisation (the welder becomes more susceptible to occupational asthma), special care should be taken to control exposure to as low a level as is reasonably practicable.

Flux cored arc (FCA)

Continuously fed wire in self-shielded FCA welding contains a flux which produces a large amount of gas for the arc and protection/deoxidisation of the weld pool. Self-shielded wires are normally used for welding outdoors. With gas-shielded wires, which are only used in the welding shop, an additional shield (CO₂ or argon-CO₂) is needed to protect the weld pool. As FCA is normally employed at high welding current levels and higher duty cycle, more fume will be produced compared with MMA.
Safe practice and accident avoidance
Providing sensible precautions are taken, self-shielded FCA welding, taking place outside, will not need any fume removal measures. For gas-shielded welding inside a building, similar precautions to MMA welding should be used, such as LEV.

Metal inert gas (MIG/MAG)

MIG/MAG uses a solid wire and a separate gas to form the arc and shield the weld pool. The shielding gas is normally CO₂ or a mixture of argon-O₂ -CO₂ but argon can be partly replaced with helium. As well as the effect of the welding parameters, the mode of metal transfer has a significant effect on fume levels.
Dip transfer mode operates at a low welding current level and has a characteristic short arc length. Fume levels are low. Spray transfer mode which operates at much higher current levels and at a greater arc length can generate higher fume levels, but a spray condition at minimum voltage generates low levels of fume.
Pulsed transfer mode operates at similar low current levels to dip transfer but with a longer arc length and generates intermediate fume levels depending upon the welding parameters.
Special mention must be made of the presence of ozone which is generated by the effect of ultraviolet radiation on the air immediately surrounding the gas shield as shown in the diagram. Welding with an aluminium-silicon wire, in particular, produces high ozone exposures. Over exposure to ozone may also occur during MIG/MAG welding of aluminium, mild and stainless steels.
Safe practice and accident avoidance
Use LEV to remove fume and ozone at source. If ozone is not adequately controlled by this method, the welder must wear Respiratory Protective Equipment (RPE).

Tungsten inert gas (TIG) and plasma

The arc is formed between the tungsten electrode and the workpiece with an inert gas. As the filler metal is added directly to the weld pool, metal does not pass through the arc, so there is hardly any fume generated. In open workshop conditions, exposures will not normally exceed WELs.
Welding of stainless steel may generate unacceptable levels of ozone.

Ozone is generated by the effect of ultraviolet radiation
on the air immediately surrounding the gas shield.
Safe practice and accident avoidance
Good ventilation does not require fume extraction but when welding stainless steel, control of ozone using LEV or LEV plus RPE may be needed.

Submerged arc

Submerged arc welding uses a bed of granulated flux to cover the arc. As the arc is not exposed, there is negligible emission of fume and gaseous pollutants, although exposure to fluorides should be considered when using highly basic fluxes.
Safe practice and accident avoidance
Special precautions are not usually needed but it should be remembered that dust may be produced when filling the hopper with flux.

Exposure to welding fume and gases

The effect of process, and consumable type on exposure assessment is summarised in the Table. The Indication of exposure levels assumes that no control systems are being used.
The generic term Workplace Exposure Limit (WEL) is used and may apply to one or all of the individual constituents of the fume. Assessment indicators are the fume and gas components most likely to exceed their respective WELs during welding.
The Welding Manufacturers Association has produced a standard format for hazard data sheets to enable manufacturers to comply with their legal obligations under the Health and Safety at Work Act 1974 Section 6. The hazard sheet contains the following information:

• chemical analysis of substances present in the fume produced by the consumable
• the appropriate WELs

For safe use of consumables, reference should always be made to the manufacturers' or suppliers' hazard data sheets.
It should be noted that in addition to fume, where appropriate, a risk assessment must also consider exposure to shielding gases, which may be toxic or asphyxiant. Gases are also produced by the action of heat on the welding flux or slag (carbon dioxide and carbon monoxide) and heat or ultraviolet radiation on the atmosphere surrounding the welding arc (nitric oxide, nitrogen dioxide and ozone).

Effect of process, and consumable composition on the assessment of exposure

Process	Material	Indication of exposure levels	Assessment indicators
Gas welding	Mild steel	Below WELSs except in poorly ventilated or confined space	Carbon dioxide, carbon monoxide (reducing flame) nitrogen dioxide
Manual metal arc (MMA)	Mild steel and low alloy steels	Above WELs	Iron oxide, Manganese
	Stainless steel	Above WELs	Hexavalent chromium or nickel
	Aluminium, copper, nickel alloys, cast iron, hardfacing	Generally in excess of WELs	Exposure to aluminium, copper, nickel, chromium barium cobalt, depending on the alloy welded.
TIG and plasma arc	Mild and low alloy steels	Less than WELs	Shielding gas can constitute a hazard in confined spaces by reducing the available oxygen to a level which will not support life
	Stainless steel alloys	Less than WELs for fume, greater than WEL for ozone	Ozone, shielding gases in confined spaces
Metal inert gas/metal active gas (MIG/MAG)	Mild and low alloy steels	Generally in excess of any WELs	Iron oxide, manganese, ozone, shield gases.
	Stainless steel	Greater than WELs	Exposure to chromium, nickel, ozone and shielding gases.
	Aluminium and aluminium alloys	Greater than WELs for ozone and fume	Aluminium oxide fume. Ozone levels can be very high, particularly with aluminium/silicon alloys.
Flux-cored arc	Mild and low alloy steels	Greater than WELs	Iron oxide, manganese. Some self shielded consumables may give rise to soluble barium in fume
	Stainless steel	Greater than WELs	Exposure to Hexavalent chromium, chromium, nickel

Publications and relevant standards

COSHH

Essentials for welding cutting and allied tasks

EH 40 (updated annually)

Workplace Exposure Limits, HSE Books.

Welding Manufacturers Association, Leaflet 236, Hazards from welding fume.
Note: When assessing fume risk, the suppliers' data sheets and hazard information must be fully consulted.
For more information, please contact us.

Job Knowledge > Fabrication Requirements of BS EN 1090 Part 2

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Fabrication Requirements of BS EN 1090 Part 2

Now that CE marking of structural steelwork and aluminium structures is mandatory it may be an appropriate time to look at some of the fabrication requirements within the specification, BS EN 1090 Part 2, that must be complied with in order to be able to apply a CE mark. It is suggested that Job Knowledge No. 120 – Structural Steel, CE Marking and ISO 3834 should be read in conjunction with this article. It should also be noted that no brief article such as this Job Knowledge article can cover every aspect of a specification requirement - the specification itself must be referred to for accuracy.
Part 1 of the BS EN 1090 series specifies the requirements for conformity assessment of what are termed the performance characteristics of the structure – essentially those criteria such as toughness, fire resistance, fatigue performance etc – and requires the implementation of a factory production control (FPC) system – see Job Knowledge No. 120 for further information. BS EN 1090 Part 2 is entitled “Technical requirements for the execution of steel structures” and specifies the requirements for the manufacturing of structural steelwork - this includes both bolting and welding. The steels covered by the specification comprise not only the conventional carbon manganese steels such as EN 10025 S275 but also high strength steels up to grade S960, ferritic, austenitic-ferritic and austenitic steels.
This article will not cover what may be termed the conventional requirements for welding such as procedure qualification, welder qualification, welding plan requirements etc but will attempt to highlight those requirements that may not be normal practice within the fabrication shop. There are several clauses within the specification that may cause some difficulties if the requirements are strictly enforced and no concessions are permitted. For example clause 5.5 states that all welding consumables shallcomply with EN 13479, the general product standard for filler metals and fluxes – the term “shall” makes this mandatory and consumables complying with other specifications only, the AWS standards for example, cannot be used. The same does not apply to parent materials where specifications other than the European material specifications can be used “....if otherwise specified...”
The quality of cut edges is dealt with in clause 6.4. This requires that any ‘free’ cut edge, but not weld preparations, complies with limits on tolerance, hardness and smoothness. A footnote states that hand thermal cutting should only be used if it is not practical to use machine thermal cutting. If specified in the contract documentation thermally cut edge hardness shall not exceed 380HV₁₀ for hot finished or normalised S235 to S460 steels and 450HV₁₀ for cold forming or quenched and tempered S260 to S700 steels. Although this requirement may not be mandatory for all contracts it would be advisable to produce a qualification test record to demonstrate compliance. The clause also requires that the “...capability of thermal cutting processes shall be periodically checked....”. The period between checks is not specified. The checks comprise the production of four samples, one each from the thickest and thinnest product and one each from a sharp corner and a curved cut from products of representative thickness.  Records of such periodic checks should be maintained for audit purposes.
Clause 6.5 is entitled ‘Shaping’ and permits hot and cold forming and flame straightening  “....provided the properties are not reduced below those specified for the worked material...” and calls up the guidance given in TR 10347 ‘Guidance for forming of structural steels in processing’. For many structural steel fabrication companies, hot and cold forming is sub-contracted to specialist benders; these must supply the fabricator with documentary evidence that the properties have not been reduced below those specified for inclusion in the as-built QA documentation pack. Flame straightening, however, may be carried out by the fabricator and, for EXC3 and EXC4, a written procedure is required that details the maximum temperature, method of heating, workers permitted to use the process and the results of mechanical tests carried out for the process approval. A procedure qualification record (PQR), similar to that of a welding PQR, is therefore required.
Clause 7 covers the welding requirements, most of which are relatively conventional but there are some additional requirements that need to be taken into account as follows:

1. Cl 7.4.1.2.b(3) requires procedure qualification tests to be carried out on test pieces with the maximum prefabrication primer thickness if welding is carried out over shop primers. This would need recording on the PQ certificate.
2. Cl 7.4.1.2.c. Transverse stressed fillet welds in steels of a higher grade than S275 and with a fillet weld throat less than half the component thickness need an additional cruciform test piece to be welded and three tensile tests performed. 
3. Table 12 permits EXC 2 welding procedures to be qualified in accordance with ISO 15614-1, ISO 15613, ISO 15612 (standard welding procedure), ISO 15611 (previous welding experience) and ISO 15610 ( tested welding consumables). EXC 3 and EXC 4 must be qualified to either ISO 15614-1 or ISO 15613.
4. Cl 7.4.1.4 requires some additional testing to be performed if a welding process with a relevant PQR has not been used for a period of time –i) between one and three years a production weld test is required to weld steel grades greater than S355 ii) over three years for steels ≤S355 a macro-section is to be taken and for steels>S355 a new procedure qualification test is required.
5. Cl 7.4.2 Welders must be specifically qualified to weld hollow section branches with an angle less than 60O.
6. Table 16 requires electrode quivers to be heated to 100°C minimum rather than the 75°C that is generally used.
7. Cl 7.5.5 requires tack welds and temporary attachments to be preheated in accordance with the relevant welding procedure specification.
8. Cl 7.5.6 covers the requirements for temporary attachments which may be attached in accordance with a written welding procedure. Cutting or chipping of temporary attachments is not permitted for EXC3 and EXC4 components unless otherwise specified. It is not clear if cutting refers to thermal cutting and if this is permitted if the cut surface is clear of the component surface, say by a minimum of 3mm which is normal practice. It is recommended that an attachment removal method statement is written and implemented on the shop floor to remove doubt.
9. Cl 7.5.7 covers the requirements for the qualification of tack welds,  a topic that is being interpreted differently by various companies.  Tack welds on EXC2, EXC3 and EXC4 components are required to be welded in accordance with a qualified WPS. Tack welds are to be a maximum length of 4x the thickness of the thicker part or 50mm “...unless a shorter length can be demonstrated as satisfactory...”. Unfortunately the clause does not define what is “satisfactory” so this is left to individual interpretation. Tack welds that are to be incorporated into the completed weld can be qualified by ensuring that the lengths and positions of the tack welds on the qualification test piece are marked and at least one macro-section is taken through the tack and hardness tested. This would need to be noted in the procedure qualification record. The major issue concerns the qualification of small temporary tack welds that may be used, for example, to tack weld a thin wall small diameter bracing to a large thick beam. How such a tack weld is to be qualified is not clear and has yet to be resolved.
10. Cl 7.5.9 deals with butt welds and Cl 7.5.9.1 specifies that the ends of butt welds shall be terminated in a manner that ensures sound welds. For EXC3 and EXC4 and EXC2, if specified, this shall be achieved by the use of run on/off plates that will be later removed.
11. Cl 7.5.9.2 covers single sided butt welds. Tack welds shall be included in the final weld and permanent or removable backing may be used. For EXC3 and EXC4 this backing shall be continuous for the full length of the joint, if necessary by joining the backing lengths together by full penetration butt welds. Note, however, that many contract specifications are prohibiting the use of backing for EXC3 and EXC4 components and specifying that the welds shall be full penetration unbacked joints.
12.  Cl 12.4.2.2. This clause is entitled scope of inspection and is relatively straightforward – there is just one requirement that needs to be mentioned. This is that the first five joints made to the same new WPS require inspecting to twice the extent specified in Table 24 and to an EN 5817 acceptance level of Quality Level B. The reason for this additional testing is to establish that the new WPS can provide the required quality in production – as distinct from that which can be provided during procedure qualification testing.

Specific industry standards are being developed which, although based on BS EN 1090 Part 2, will contain additional requirements that may mean further qualification testing will be needed. Once such specifications are available then these may form part of a Job Knowledge article covering the key differences and additional requirements.

Plant Inspector/ NDE Engineer Job Location: Singapore and world wide offered by Cutech Process Services Pte Ltd, Singapore

Plant Inspector/ NDE Engineer
Job Location: Singapore and world wide
offered by Cutech Process Services Pte Ltd, Singapore


Job description / Skill Requirement:

Perform Inspection on various types of Plant components (Piping / Storage tank / Pressure vessel / RBI)
Able to perform Visual Inspection, UTG and adhoc site NDT activities as and when required
Shall coordinate with other NDT/ Advanced NDT team. Interpret the results and prepare final Inspection reports.

Liaise with the clients daily

Qualification and Experience Requirements:

Minimum O Level / ITE / Diploma / Degree
Preferably Certified in API ( 510/ 570 / 653 / 580) or having experience in Plant inspection
Preferably a Certified Welding Inspector
Preferably NDT Level II in UTG
At least 2 year experience in Plant Inspection

Best Regards,

P.Pugalendhi (Pugal)
Director – NDT & Projects,
Cutech Process Services Pte Ltd,
Singapore
Email : pugal@cutechgroup.com
Mobile: 65- 91052231

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> Technical Knowledge > Job Knowledge > Distortion - types and causes

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Distortion - Types and causes

Dishing of the steel plate between longitudinal stiffeners can be seen clearly on the bow of this ship (Courtesy MOD)

Job Knowledge 33

This article covers several key issues on distortion in arc welded fabrications, especially basic types of and factors affecting the degree of distortion.

What causes distortion?

Because welding involves highly localised heating of joint edges to fuse the material, non-uniform stresses are set up in the component because of expansion and contraction of the heated material. Initially, compressive stresses are created in the surrounding cold parent metal when the weld pool is formed due to the thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. However, tensile stresses occur on cooling when the contraction of the weld metal and the immediate heat affected zone is resisted by the bulk of the cold parent metal.
The magnitude of thermal stresses induced into the material can be seen by the volume change in the weld area on solidification and subsequent cooling to room temperature. For example, when welding CMn steel, the molten weld metal volume will be reduced by approximately 3% on solidification and the volume of the solidified weld metal/heat affected zone (HAZ) will be reduced by a further 7% as its temperature falls from the melting point of steel to room temperature.
If the stresses generated from thermal expansion/contraction exceed the yield strength of the parent metal, localised plastic deformation of the metal occurs. Plastic deformation causes a permanent reduction in the component dimensions and distorts the structure.

What are the main types of distortion?

Distortion occurs in six main forms:

• Longitudinal shrinkage
• Transverse shrinkage
• Angular distortion
• Bowing and dishing
• Buckling
• Twisting

The principal features of the more common forms of distortion for butt and fillet welds are shown below:

Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage.
Non-uniform contraction (through thickness) produces angular distortion in addition to longitudinal and transverse shrinkage.
For example, in a single V butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Hence, balanced welding in a double side V butt joint can be used to produce uniform contraction and prevent angular distortion.
Similarly, in a single side fillet weld, non-uniform contraction produces angular distortion of the upstanding leg. Double side fillet welds can therefore be used to control distortion in the upstanding fillet but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.
Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding; this produces a dished shape. Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds (see main photograph).
In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling.
Distortion due to elastic buckling is unstable: if you attempt to flatten a buckled plate, it will probably 'snap' through and dish out in the opposite direction.
Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.

How much shall I allow for weld shrinkage?

It is almost impossible to predict accurately the amount of shrinking. Nevertheless, a 'rule of thumb' has been composed based on the size of the weld deposit. When welding steel, the following allowances should be made to cover shrinkage at the assembly stage.

Transverse Shrinkage

Fillet Welds 0.8mm per weld where the leg length does not exceed 3/4 plate thickness
Butt weld 1.5 to 3mm per weld for 60° V joint, depending on number of runs

Longitudinal Shrinkage

Fillet Welds 0.8mm per 3m of weld
Butt Welds 3mm per 3m of weld
Increasing the leg length of fillet welds, in particular, increases shrinkage.

What are the factors affecting distortion?

If a metal is uniformly heated and cooled there would be almost no distortion. However, because the material is locally heated and restrained by the surrounding cold metal, stresses are generated higher than the material yield stress causing permanent distortion. The principal factors affecting the type and degree of distortion, are:

• Parent material properties
• Amount of restraint
• Joint design
• Part fit-up
• Welding procedure

Parent material properties

Parent material properties which influence distortion are coefficient of thermal expansion and specific heat per unit volume. As distortion is determined by expansion and contraction of the material, the coefficient of thermal expansion of the material plays a significant role in determining the stresses generated during welding and, hence, the degree of distortion. For example, as stainless steel has a higher coefficient of expansion than plain carbon steel, it is more likely to suffer from distortion.

Restraint

If a component is welded without any external restraint, it distorts to relieve the welding stresses. So, methods of restraint, such as 'strong-backs' in butt welds, can prevent movement and reduce distortion. As restraint produces higher levels of residual stress in the material, there is a greater risk of cracking in weld metal and HAZ especially in crack-sensitive materials.

Joint design

Both butt and fillet joints are prone to distortion. It can be minimised in butt joints by adopting a joint type which balances the thermal stresses through the plate thickness. For example, a double-sided in preference to a single-sided weld. Double-sided fillet welds should eliminate angular distortion of the upstanding member, especially if the two welds are deposited at the same time.

Part fit-up

Fit-up should be uniform to produce predictable and consistent shrinkage. Excessive joint gap can also increase the degree of distortion by increasing the amount of weld metal needed to fill the joint. The joints should be adequately tacked to prevent relative movement between the parts during welding.

Welding procedure

This influences the degree of distortion mainly through its effect on the heat input. As welding procedure is usually selected for reasons of quality and productivity, the welder has limited scope for reducing distortion. As a general rule, weld volume should be kept to a minimum. Also, the welding sequence and technique should aim to balance the thermally induced stresses around the neutral axis of the component.
The article was prepared by Bill Lucas in collaboration with Geert Verhaeghe and Rick Leggatt.
For more information please contact us.

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Distortion Control - Prevention by fabrication techniques

Distortion caused by welding a plate at the centre of a thin plate before welding into a bridge girder section. Courtesy John Allen

Assembly techniques

In general, the welder has little influence on the choice of welding procedure but assembly techniques can often be crucial in minimising distortion. The principal assembly techniques are:

• tack welding
• back-to-back assembly
• stiffening

Tack welding

Tack welds are ideal for setting and maintaining the joint gap but can also be used to resist transverse shrinkage. To be effective, thought should be given to the number of tack welds, their length and the distance between them. With too few, there is the risk of the joint progressively closing up as welding proceeds. In a long seam, using MMA or MIG, the joint edges may even overlap. It should be noted that when using the submerged arc process, the joint might open up if not adequately tacked.
The tack welding sequence is important to maintain a uniform root gap along the length of the joint. Three alternative tack welding sequences are shown in Fig. 1:
a) tack weld straight through to the end of the joint (Fig 1a). It is necessary to clamp the plates or to use wedges to maintain the joint gap during tacking
b) tack weld one end and then use a back stepping technique for tacking the rest of the joint (Fig 1b)
c) tack weld the centre and complete the tack welding by back stepping (Fig 1c).

Fig. 1. Alternative procedures used for tack welding to prevent transverse shrinkage

a) tack weld straight through to end of joint
b) tack weld one end, then use back-step technique for tacking the rest of the joint
c) tack weld the centre, then complete the tack welding by the back-step technique

Directional tacking is a useful technique for controlling the joint gap, for example closing a joint gap which is (or has become) too wide.
When tack welding, it is important that tacks which are to be fused into the main weld are produced to an approved procedure using appropriately qualified welders. The procedure may require preheat and an approved consumable as specified for the main weld. Removal of the tacks also needs careful control to avoid causing defects in the component surface.

Back-to-back assembly

By tack welding or clamping two identical components back-to-back, welding of both components can be balanced around the neutral axis of the combined assembly (Fig. 2a). It is recommended that the assembly is stress relieved before separating the components. If stress relieving is not done, it may be necessary to insert wedges between the components (Fig. 2b) so when the wedges are removed, the parts will move back to the correct shape or alignment.

Fig. 2. Back-to-back assembly to control distortion when welding two identical components

a) assemblies tacked together before welding
b) use of wedges for components that distort on separation after welding

Stiffening

Fig. 3. Longitudinal stiffeners prevent bowing in butt welded thin plate joints

Longitudinal shrinkage in butt welded seams often results in bowing, especially when fabricating thin plate structures. Longitudinal stiffeners in the form of flats or angles, welded along each side of the seam (Fig. 3) are effective in preventing longitudinal bowing. Stiffener location is important: they must be placed at a sufficient distance from the joint so they do not interfere with welding, unless located on the reverse side of a joint welded from one side.

Welding procedure

A suitable welding procedure is usually determined by productivity and quality requirements rather than the need to control distortion. Nevertheless, the welding process, technique and sequence do influence the distortion level.

Welding process

General rules for selecting a welding process to prevent angular distortion are:

• deposit the weld metal as quickly as possible
• use the least number of runs to fill the joint

Unfortunately, selecting a suitable welding process based on these rules may increase longitudinal shrinkage resulting in bowing and buckling.
In manual welding, MIG, a high deposition rate process, is preferred to MMA. Weld metal should be deposited using the largest diameter electrode (MMA), or the highest current level (MIG), without causing lack-of-fusion imperfections. As heating is much slower and more diffuse, gas welding normally produces more angular distortion than the arc processes.
Mechanised techniques combining high deposition rates and high welding speeds have the greatest potential for preventing distortion. As the distortion is more consistent, simple techniques such as presetting are more effective in controlling angular distortion.

Welding technique

General rules for preventing distortion are:

• keep the weld (fillet) to the minimum specified size
• use balanced welding about the neutral axis
• keep the time between runs to a minimum

Fig. 4. Angular distortion of the joint as determined by the number of runs in the fillet weld

In the absence of restraint, angular distortion in both fillet and butt joints will be a function of the joint geometry, weld size and the number of runs for a given cross section. Angular distortion (measured in degrees) as a function of the number of runs for a 10mm leg length fillet weld is shown in Fig. 4.

If possible, balanced welding around the neutral axis should be done, for example on double sided fillet joints, by two people welding simultaneously. In butt joints, the run order may be crucial in that balanced welding can be used to correct angular distortion as it develops.

Fig. 5. Use of welding direction to control distortion

a) Back-step welding
b) Skip welding

Welding sequence

The sequence, or direction, of welding is important and should be towards the free end of the joint. For long welds, the whole of the weld is not completed in one direction. Short runs, for example using the back-step or skip welding technique, are very effective in distortion control (Fig. 5).

• Back-step welding involves depositing short adjacent weld lengths in the opposite direction to the general progression (Fig. 5a).
• Skip welding is laying short weld lengths in a predetermined, evenly spaced, sequence along the seam (Fig. 5b). Weld lengths and the spaces between them are generally equal to the natural run-out length of one electrode. The direction of deposit for each electrode is the same, but it is not necessary for the welding direction to be opposite to the direction of general progression.

Best practice

The following fabrication techniques are used to control distortion:

• using tack welds to set up and maintain the joint gap
• identical components welded back to back so welding can be balanced about the neutral axis
• attachment of longitudinal stiffeners to prevent longitudinal bowing in butt welds of thin plate structures
• where there is choice of welding procedure, process and technique should aim to deposit the weld metal as quickly as possible; MIG in preference to MMA or gas welding and mechanised rather than manual welding
• in long runs, the whole weld should not be completed in one direction; back-step or skip welding techniques should be used.

Bill Lucas prepared this article in collaboration with Geert Verhaeghe, Rick Leggatt and Gene Mathers.
For more information please contact us.

Resonant Inspection a "new" NDT technique by Godfrey Hands *

Resonant Inspection a "new" NDT technique

by Godfrey Hands *

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ABSTRACT

This new Inspection Technology is not only restricted to conventional NDT applications, but is also able to perform many inspection functions in one test. The technology can detect cracks, voids, hardness variations, dimensional variations, bonding problems, parts with missing manufacturing processes, misshaped parts and changes in material properties. It is primarily suitable for inspecting mass-produced components, although some high value individual components can be condition monitored to detect changes in their structural integrity. The Author's company offers a rapid response testing service using RI to manufacturing industries.

Table of contents

• Introduction
• Operational Theory
• Test Set-Up
• Vibration Modes and Spectra
• What can we detect and with what detection limits ?
  Cracks, Dimensional variations, Hardness variations, Changes in Material Properties, Parts with different shapes, Parts with a missing production process, Lack of Bonding
• What materials can we inspect ?
• Testing Speed
• Applications in mass-production
• Resonant Inspection Services
• Suppliers of Resonant Inspection equipment
• References

Introduction

Resonant Inspection is a "new" NDT technique that was originated by scientists at Los Alamos National Laboratory in the USA, and has been developed for industrial applications during the last four years of commercialisation by an American company Quatrosonics Inc. It is a whole-body resonance inspection that is particularly suited to inspecting smaller mass-produced hard components, and one test will inspect the complete component without radiation, the need for scanning, immersion in liquids, chemicals, abrasives or other consumables.

Operational Theory

Hard components have their own Resonant Frequencies, for example a bell will ring with one specific note. This note is actually a combination of several pure tones, each representing a different resonance mode of the bell or harmonics of them. Wine glasses also have resonant frequencies. The tone of the "ringing" depends upon the size of the glass, a small glass ringing at a higher note than a large glass. This tells us that Resonant Inspection can differentiate between components of different sizes. A bell and a glass of the same size will ring at different frequencies. This tells us that the resonant frequency is dependant upon the material of the tested component. (in practice, it depends upon the material properties or "stiffness" of the object). In addition, a good bell or wine glass will ring true, whilst a cracked bell or wine glass will ring with a "cracked" note or will "clunk" instead of ringing. This tells us that we can detect cracks with Resonant Inspection. So what's new ? People have been "inspecting" things by hitting them with a hammer and listening to them ringing for centuries. Computers and modern electronics technology have enabled us to take the human element out of the inspection process, thus measuring more frequencies and recognising more subtle changes than are detectable with the human ear. This also allows us to automate the process (thereby eliminating "operator error"), and also allows us to move into the ultrasound region to detect smaller differences.
Resonant Inspection operates by exciting a component with a sine wave excitation at one specific frequency (thereby putting all of the energy into that one frequency) then quickly sweeping all of the individual frequencies through the required test range. A hammer striking the component will put all the energy into a broad spectrum (from DC up to hundreds of kilohertz), with only a small amount at the resonant frequencies. This swept sine-wave approach allows a much improved signal to noise compared to the hammer blow technique. A narrow band filtered receiver, typically only several Hertz wide, will follow the swept sine-wave. This .vastly improves the signal to noise ratio and raises the detectability of the inspection by orders of magnitude compared to the old hammer method.

Test Set-Up

For Resonant Inspection, we normally locate the component to be tested on three or four piezo transducers. It is not necessary to scan the component with the transducers, nor to rotate a component past the transducers, as one test will evaluate the whole-body or complete component. One of the transducers normally acts as a transmitter, exciting the component, whilst one or two more of the transducers act as receivers, measuring the amplitude of vibration at the specific frequency of the transmitter or at one of its harmonics. Further transducers can be used to support the component in the test. These transducers have ceramic tips (to prevent wear of the transducers and to provide a good transfer of energy between the component and transducer), which whilst normally being hemispherical, can also be ground to a user specific shape if required.

Vibration Modes and Spectra

Figure of vibration modes Components vibrate typically with Torsional (twisting), Flexural (bending) and Extensional (stretching) modes. The figure here demonstrates some of these modes.
Figure of a typical spectrum . Also shown is a typical spectrum between 100 and 400 kHz from a small cylindrical component. This shows many resonances of different amplitudes.	Figure of a small piece of above spectrum If we concentrate on a small section of this spectrum (between 225 and 250 kHz), we can see three specific resonances.
If a defect is introduced into the component, then two of the resonances will change. A crack will reduce the stiffness of a component, and therfore the resonant frequency will have a lower frequency. If a component is rotationally symmetric (e.g. a cylinder), there will normally be two resonances at the same frequency from the X and Y axes (diameters of the cylinder), plus another resonance from the length (Z axis) of the cylinder. A defect on the outside diameter (only extending for a small amount of the circumference, but precisely in the direction of vibration of one of the axes) will only affect the X or the Y resonance, not both. In this case, the X or the Y component of the two superimposed resonances will shift low, and we have an apparent "splitting" of the resonance into two distinct peaks. This becomes apparent from the spectrum of the component with a defect introduced into it, where one of the resonances is not affected by the defect, one of the resonances splits, so only one axis is affected, and one splits and shifts, showing that the two axes are affected,