Hatch Associates Pty. Ltd. ACN 008 630 500

June 2002 Number 15 333

Structural Assets

Group Leader’s Introduction

his is the fifteenth newsletter for Structural Assets and second edition for the year 2002. The Hatch Structural Assets Group focuses on assisting clients in an operational environment to reduce

risk, extend the life of assets and to scope and prioritise sustaining capital and refurbishment work on structures and associated equipment.

The objective of this newsletter is to share knowledge and experience gained during the course of our work. We would be pleased to receive suggestions from readers, via the enclosed fax back form, as to the subjects and types of articles of interest.

Richard Morgan (RMorgan@hatch.com.au)

Welding – Defects and Discontinuities – What Really is Acceptable

have been asked on numerous occasions when carrying out an inspection “how do you tell the difference between a weld designated to be category SP (AS1554) as distinct from a weld designated to be

category GP, or for that matter, how does a welder set about producing a SP versus a GP weld”?. When I reply that in terms of preparation and production there is no difference I often receive the reply “Why do we specify a difference”?. This is usually followed by a lengthy discussion on the definition or level of ‘acceptability’.

This level is either as set down in the relevant standards and codes of practice or as specified by the design engineer due to the specific duty to which the weld is to be subjected.

Quality in a weld can be defined as ”Conforming to a Specification” which sets the limits of discontinuities and flaws that are acceptable for a particular application. It must be understood that there is no such thing as a perfect weld. Any apparently perfect weld will exhibit fine discontinuities if it is examined closely enough, however this does not make that weld defective or rejectable

If a weld has less defects or discontinuities than allowed by the specification it is acceptable and considered to be a “Quality” weld.

Quality specifications are not the same for all types of welds or in all applications. Welds in structural steels may contain small levels of fine porosity, inclusions or other types of discontinuity that are within the limits allowed by welding codes such as AS1554 or ANSI/ AWS D1.1.

The level of quality set down in these codes should be readily achievable by experienced welders using normal arc welding processes with commercially acceptable industry standard levels of fit-up and joint preparation.

Higher levels of preparation would incur a severe cost penalty to produce and would require much higher levels of testing and examination to verify that the increase in standard had been achieved. Unless the application is of a critical nature eg. Aircraft structure, Nuclear power plant, High-pressure piping or vessel, the increase in quality is usually not warranted.

It should also be made clear that welding procedures and workmanship which produce weldments that are borderline for rejection, should not be accepted when with minimal additional care weldments which meet or exceed the minimum requirements can easily be achieved.

Having established the level of ‘acceptable’ discontinuities it then becomes a matter of setting in place the procedures that will achieve the desired quality at a commercially acceptable cost.

In order to clarify the reasoning behind this discussion it is important at this time to define the difference between a ‘defect’ and a ’discontinuity’.

A ‘defect’ is a flaw or flaws that by their nature or accumulated effect render a component or product unable to meet the minimum acceptance standard or specification or render it unfit for its intended purpose. The term designates rejectability.

A ‘discontinuity’ is an interruption of the typical structure of a material such as lack of homogeneity of it’s mechanical, physical or metallurgical properties. A discontinuity is not necessarily a defect.

The control of defects and discontinuities on site or in the open is more difficult than that for a corresponding weld made in the workshop. In order to achieve the best possible result additional effort has to be made to prevent interference with the welding process by external factors.

This may occur from such sources as weather conditions (wind, rain, humidity etc.), adjacent workings (abrasive blasting, painting etc.) or from an adjacent process area that is likely to contaminate the weld preparation or execution. Shielding the welding site with screens and/ or covers is essential in reducing the likelihood of defects particularly porosity.

The following is a brief discussion on the more

common welding defects and discontinuities, their likely cause, possible cure and resultant repair.

INTRODUCTION

SPECIALIST ADVICE

2 Structural Assets MISSALIGNMENT Caused by careless fit-up or poorly aligned plates of different thickness. Can only be repaired by careful blend grinding or cutting the joint apart, re-preparing and re-welding.

UNDERCUT Unfilled groove cut by the welding process at the toe of the weld. Usually caused by current too high, poor electrode angle, arc length too long or rust. Can be cured by more attention to detail in preparation cleaning and by improving process. Can be repaired by welding up groove with smaller electrode, may require gouging first. Allowable limits for undercut are set by codes.

CONCAVITY / CONVEXITY Caused by incorrect current or speed of weld. Can be avoided by adjusting welding procedure. Can be repaired by either filling with further weld material or by blend grinding smoothly to base metal on each side of weld preparation.

REINFORCEMENT Can be too much or insufficient. Usually caused by incorrect combination of travel speed and current. Can be repaired by removing excess weld metal and blend grinding smoothly to base metal or be re-welded. Welds should have a transition angle of at least 135� at toes of weld.

OVERLAP Usually caused by poor welding technique. Can be cured by changing procedure. Must be repaired by grinding off excess weld metal and blend grinding smoothly to base metal.

LACK OF PENETRATION / FUSION Caused when the weld metal does not form a cohesive bond with the base metal or when the weld metal does not extend into the base metal to the required depth. These defects are usually caused by incorrect welding conditions such as current too low, insufficient preheat, welding speed too fast, incorrect edge preparation, short arc length, electrode too small or arc not in centre of seam. This type of defect can only be repaired by grinding / gouging out the defective area and re-welding. This type of defect is sometimes difficult to detect even with high quality non-destructive testing (NDT) methods.

INCLUSIONS As the name implies these are caused by slag/ spent flux becoming entrapped within the weld metal. This defect

is often associated with undercut in multi-pass welds. Usually caused by low current, welding in an area that is too tight or rust or mill scale not cleaned from the base metal. This type of defect can be prevented by increasing the current or pre-heat, grinding out tight areas to give access to the bottom of the joint and by proper preparation of the base metal prior to welding. This defect can only be repaired by grinding / gouging out and re-welding.

MICROPOROSITY and ARC CRATERS An unfilled weld pool at the termination of a weld run. This is usually caused by improper weld termination technique whereby the molten pool shrinks causing a ‘pipe’. If there is no cracking evident this can be repaired by simply welding up. This problem can be cured by using run on run off tabs or improving welding technique. LAMELLAR TEARING This defect is caused by a flaw in the base metal, whereby a non-metallic inclusion (slag / mill scale) was rolled into the hot plate during the manufacturing process. The tearing is caused when the weld metal is deposited on the surface of the plate in a joint where there is high restraint. The most effective way to avoid this defect is where possible to design plate joints that will minimise the likelihood of it occurring i.e.

Other alternatives include the use of castings or forgings where practical.

CRACKING Cracking in weld metal can occur in either the longitudinal or transverse direction and is caused by the procedure used for the placement of the weld.

Longitudinal cracking can be caused by the weld bead being too wide, current or welding speed too high, root gap too large, shrinkage stress in areas of high constraint or weld metal contaminated with carbon, sulphur or phosphors.

This type of defect can be avoided by proper joint preparation i.e. weld width 0.5 to 0.8 times weld depth,

3 Structural Assets choose base metal with �0.6% sulphur and phosphors, pre-heat to even out cooling rates.

Cracks running transversely to direction of the weld are usually caused by a weld metal hardness problem and can be prevented by the correct choice of welding consumable.

Cracking in the base metal at the toe of a weld could indicate a brittleness problem in the heat affected zone (HAZ). This may require an increase in pre-heating or the use of a more ductile filler material.

Under bead cracking (in the unmelted parent metal of the HAZ) may be due to hydrogen embrittlement and indicates that the use of low hydrogen electrodes would be necessary.

Cold cracking (occurs after the weld metal has completely solidified) is usually caused by highly restrained joint preparations and can be avoided by using more ductile filler material or by welding towards the area of least restraint.

Cracking in weldments is unacceptable and must be ground or gouged out and re-welded. Before re-welding the cause of the cracking must be determined and the problem corrected in order to prevent the same thing occurring in the re-welded joint. Generally the repair is required to be made using a smaller electrode.

POROSITY This defect appears in several forms i.e. single pore or pipe, uniformly scattered, cluster, linear and crater pipes. It can be caused by various things such as an unstable arc or incomplete protection at the weld start, poor welding technique, excessive contamination of joint preparation (such as grease, dampness, atmosphere), high sulphur content in consumables or arc gap too short.

Porosity can be minimised by the proper selection of electrodes and /or filler materials, improved welding technique, attention to cleanliness and prevention of contaminants from entering the weld area during weld production and slowing of welding speed to allow gasses time to escape.

HAMMER MARKS AND ARC STRIKES Caused by excessive force in use of chipping hammer and carelessness with handling of welding electrode holder. Can cause localised stress concentration and depending on location, if damage is significant, may have to be ground out and properly filled. This defect is unsightly and unnecessary making repair costs a burden. CONCLUSION

Welds don’t have to be perfect, just within the acceptable tolerances; working to perfection is far too time consuming and commercially unacceptable.

In accordance with our definitions stated earlier it is only necessary to repair defects; discontinuities by definition are acceptable and as such makes their repair unnecessary and not cost effective. A NOTE ON INSPECTION

To successfully carry out inspections on weldments you require several things, (apart from the requisite technical knowledge), not the least of which is good eye sight and a keen eye for detail. As well as that an extremely useful aid is a ‘palmgrin gauge’ which makes

the measurement of the odd shapes found in weldments a little more manageable.

For further information please contact Greg Gabb (Ggabb@hatch.com.au) on 61 7 3834 7725 For further information and detail a valuable reference is ‘The Procedure Handbook of Arc welding’ by the Lincoln Electric Company, Cleveland Ohio.

The Standards We Use n going about our normal daily work we tend to take the use of Standards and Codes of Practice for granted. We become familiar with their contents and feel comfortable that we are undertaking our designs

etc. in accordance with accepted good practice. In an international environment, it is not uncommon to

suddenly find yourself working on a project for an overseas client in a country where the Standards and Codes, and particularly the relevant Government Regulations that you are required to adopt, are difficult to obtain at best and are then most likely written in a different language.

This situation has occurred to the Structural Assets group on several occasions in the recent past and it has proven to be a time consuming and involved part of the project.

In carrying out projects for clients in the Asia Pacific region in particular, where the many of the countries adopt British or European Standards, and where the majority of clients prefer undertake their own project management, it becomes necessary to cross reference our Australian Standards to the relevant British Standards in order to enable the client to carry out the work without difficulty.

It is in the cross referencing that the difficulty becomes apparent. There is not always a direct equivalent of the standard in question and the transposition may have to be made across two or three of the standards you are converting to.

In the recent ‘conversion’ we started by undertaking an investigation into the AS. /BS. Standards by subject heading.

This was undertaken using Australian Standards on Line and the equivalent web based listing for British Standards. This task would be very difficult to carry out if a language transposition was also required.

The following table may be of interest and could possibly save you a time consuming search down the same path we had to travel. The standards in the table all relate to the fabrication of structural steelwork and the application of protective coatings for a close to marine environment.

The assistance of Calliope Lalous in collating the information is acknowledged with many thanks.

For further information please contact Greg Gabb (Ggabb@hatch.com.au) on 61 7 3834 7725

4 Structural Assets

Australian Standard British Standard AS. 1101 Graphical symbols for general engineering BS.1153 Specification for graphical symbols for general

engineering

AS. 1111 ISO Metric hexagon bolts and screws BS. 1083 Specification for precision hexagon bolts, screws and nuts

AS/ NZS.1554-1 to 6 Structural steel welding BS. 1780 Welding – Basic weld joints in steel

AS. 1163 Structural steel hollow sections DD ENV 1090-4 Execution of steel structures – Supplementary rules for hollow steel sections

AS.1171 NDT – Magnetic particle testing of ferromagnetic products and components

Pr EN ISO 9934 – 1 NDT Magnetic particle testing – General principals

AS. 1214 Hot dip galvanized coatings on threaded fasteners

BS. 7371 – 6Coatings on threaded fasteners – Specifically for hot dip galvanized coatings

AS. 1252 High strength steel bolts with associated nuts and washers for structural engineering

BS. 4395 Specification for high strength friction grip bolts and associated nuts and washers for struct. engineering

AS. 1580 Paints and related materials – methods of test BS. 3900 Methods of test for paints

AS. 1627 Metal finishing – Preparation and pretreatment of surfaces

BS. 7079 – D2 Preparation of steel substrates before application of paints and related products

AS. 1654 ISO system of limits and fits BS. 1916 – 2 Limits and fits for engineering

AS. 1710 NDT Ultrasonic testing of carbon and low alloy steel plate

BS. 3923 Methods for ultrasonic examination of welds

AS. 2062 NDT Penetrant testing of products and components

BS EN 571 – 1 NDT Penetrant testing general principals

AS. 2177 NDT Radiography of welded butt joints in metal – methods of testing

BS. 7009 Guide to the application of real time radiography to weld inspection

AS. 2207 NDT Ultrasonic testing of fusion welded joints in carbon and low alloy steels

BS. 3923 Methods of ultrasonic examination of welds

AS./ NZS. 3678 Hot rolled plates, floor plates and slabs BS.- 4 – 1 Structural steel sections – Specification for hot rolled sections

AS./ NZS. 3679.1 Structural Steel - Hot rolled bars and sections

AS Above

AS./ NZS. 3679.2 Welded I Sections BS. EN 10034 Structural steel I & H sections – Tolerances on shape and design

AS./ NZS. 3752 Welding – Method of determination of diffusing hydrogen content of ferrite weld metal produced by arc welding

BS. 6693 – 3 Diffusible hydrogen – Primary method for determination of diffusible hydrogen in manual metal-arc ferritic steel weld metal

AS. 4100 Steel structures Pr EN 1993 – 1 Eurocode 3 Design of steel structures

AS. 4680 Hot-dip galvanized (zinc) coatings on fabricated ferrous articles

BS. 1461 Hot-dip galvanized coatings on fabricated iron and steel articles

ITEMS OF INTEREST ‘World Coal’ magazine - Paper on “Methods Extending the Design Life of Handling Systems” by Richard Morgan and Dr. Frank Gatto - published in the May 2002 edition.

Dr. Sabaratnam Loganathan has been invited to present a keynote address at the Competitive Advantage Engineering Conference, Carlton Crest Hotel, Melbourne on the 6th June 2002. Dr. Loganathan will present his address on the “Applications of Finite Element Analysis for Materials Handling Machines”

Every effort has been made to ensure that the information contained in this newsletter is correct. However, Hatch Associates Pty. Ltd. or its employees take no responsibility for any errors, omissions or inaccuracies.

For any enquires regarding this newsletter including adding your name to the newsletter distribution list please contact Frank Gatto (FGatto@hatch.com.au).