The Institute of Rail Welding 16th Technical Seminar

Rail Welding Metallurgy and Process Developments

The latest IoRW technical seminar was hosted by Tube Lines at Canary Wharf. Delegates were welcomed by Ed Wells, Tube Lines' Head of Assurance. His company is interested in rail welding because of their 30 year contract with TfL/LUL for the operation, maintenance and renewal of the Piccadilly, Northern and Jubilee Lines. Technologies and innovations that will assist the company and make its operations safer and more efficient are of interest, so IoRW membership and seminars are very pertinent.

Ed demonstrated this by recounting some of his company's recent activities related to rails and welding. One project has been addressing a rise in the numbers of rail breaks arising from undetectable defects. The cause turned out to be the large number of old joints in BH track that were FB welded during the 1990s with the bolt-holes left in-situ. Defects in the fusion face, and surface damage left by the back corners of the old fishplates have been found to be the failure initiators. Enhanced ultrasonic testing procedures have allowed identification and removal of defective joints before breaks occur. The Piccadilly Line has now been dealt with, causing a temporary rise in the number of Code 1a defects removed but a corresponding fall in the number of rail breaks. The Northern Line is now being tackled; the Jubilee Line's rails are nearly all too new to suffer from this issue.

Taking the chair for the day, David Philpott, Professional Head of Railway Engineering, Volker Rail welcomed everyone, particularly the delegate who had travelled from Australia! The first speaker introduced was Brian Whitney, Network Rail's Principal Rail Maintenance Engineer, whose talk was Analysis of data on rail breaks.

Brian discussed the latest Network Rail (NWR) statistics, the last full year's data being for 2008/9, when 164 rail breaks occurred. The two most significant causes of breaks were transverse defects in the foot and defects in welds; each contributing 37% of the year's total. Defects at rail ends were next most significant at 13.5%; transverse defects in the head accounted for 8.5%. The percentage of defects attributable to welds was thus higher than in previous years, but this change is due to the large fall in the numbers of rail breaks arising from other causes, not because of more weld defects. Remember, the number of rail breaks was as high as 952 in 1998/9!

Rail break

More worrying is the high percentage due to transverse defects in the foot. An increasing proportion is caused by corrosion pits under the foot. Their location and small size makes these hard to detect, so NWR is working on better ways to find them before a break results.

Brian discussed NWR's classification of breaks, by site location and position in the rail, to assess their potential risk. Greatest risk arises from those caused by RCF damage or by rail end defects; either of which may lead to large pieces of rail being lost, and serious derailment risk. Breaks in S&C may similarly lead to serious derailment risk.

NWR has been reviewing rail breaks to determine why they had not been detected and removed before they failed. On UTU recorded routes in 2008/9, 12 out of 78 breaks were classified as detectable. Four had been detected but not actioned in time, two large tache ovale type defects had been incorrectly sized and consequently were not correctly actioned, and six defects were undetectable, being in badly dipped joints or rail foot corrosion. On non-UTU routes 36 of the 86 breaks were considered detectable ultrasonically. Manual UT detection is clearly less reliable than the UTUs. NWR is considering the adoption of recording equipment for manual operators.

So far in 2009/10 only 40 breaks have been recorded. However, the usual rise in frequency is expected with the cold weather, partly due to the effect of higher longitudinal tensile stresses in CWR during cold weather (as much as 50t at 0°C) and partly due to the higher incidence of wheel flats in winter. NWR installed 50,000 aluminothermic welds in 2008/9; about 1,900 were rejected. This year, 688 have been rejected of about 21,400 installed thus far. Porosity has been the commonest cause of rejection; other causes have been incorrect weld location, incomplete weld collar formation, black holes and excessive flashing.

Flash butt and arc repair welds are more reliable, with rejection rates consistently bettering 1.5% for manual repair welds. Figures for the automatic weld process so far look better still.

Network Rail rail failure reduction initiatives include improved inspection techniques, more frequent inspections, revised standards (more based upon risk, aiming to pick up defects earlier and get them actioned early), improved computer modelling tools for engineers to permit them to identify track defects earlier and prioritise them more appropriately and the use of premium rail steels on curves to allow grinding frequencies to be harmonised with those for straight track.

Dr Robert Gehrmann of Goldschmidt Thermit Group spoke on Aluminothermic welding metallurgy. He examined the different welding techniques applicable to the differing rail types in current use and discussed the microstructure of the resulting welds.

Welds in normal and head hardened pearlitic rails may be use a short pre-heat and large weld portion, or a standard pre-heat and a smaller portion. Different weld portions are used for the head hardened rails to ensure that the hardness of the weld metal matches the parent rail. The welds have pearlitic microstructure and corresponding hardness.

For bainitic rails, bainitic portions have to be used to generate bainitic weld metal to match the parent rails. The alternatives of short pre-heat/large portion or standard pre-heat/smaller portion are again available. However, the rail and weld must be covered to reduce the rate of cooling sufficiently to prevent the formation of brittle martensite.

By examining longitudinal sections through welds Dr Gehrmann showed the effect of pre-heat length. The heat affected zone (HAZ) was shorter with shorter pre-heat times. The length of the HAZ on the running surface is crucial to the weld's performance under traffic because of the hardness variations within it. This fall in hardness of the material in the HAZ each side of the weld metal gets proportionately worse the harder the rail steel and appears to be deeper for welds made with short pre-heat/large portion. It is therefore important to consider carefully the design of weld procedure, balancing the length of HAZ against the magnitude of hardness variation within it to match the application.

Dr Gehrmann described a high performance weld (HPW), where the head is formed from a harder alloy than the web and foot. A standard Z90 portion is used, together with a 'plug' containing vanadium and chromium. The plug and special casting system are so arranged that the alloying elements are directed only into the head area of the mould. The head has a hardness of 375HB, the foot has only 250HB.

Finally, we heard about the use of a post-heat treatment (HC) process. The weld is heated after being cast, then covered to reduce its cooling rate. The HAZ spreads further each side of the weld, making it longer along the running surface, but the intensity of hardness reduction within the HAZ is greatly reduced. The HAZ is thus of much less significance despite being longer.

Dr Satya Kondapalli of Welding Alloys Group spoke next, describing Hardfacing of rail components. S&C components of rail and tram ways and the curved grooved rails of tramways need special attention, as they live particularly hard lives compared with other rails and are especially important to the safe, reliable and economic operation of rail or tram ways. Wear and deformation needs to be corrected appropriately in order to assure performance and minimise costs.

Dr Kondapalli looked at the welding processes available for such repairs, ranging from manual metal arc (MMA) to fully automatic welding. Gas process, open arc or submerged arc techniques are possible. Flux cored wire allows metal to be deposited twice as fast as stick electrodes.

Automated welding avoids the imperfections inherent in the stop/start manual process, reduces heat input and cuts the time taken. Costs are reduced, and the fatigue resistance of the repaired component will be greater.

Welding Alloy Group (WAG) offer welding wires specifically designed for particular railway/tramway repair situations. Dr Kondapalli described these, relating the various products to repairs on crossings, switches, rail heads and grooved tramway rails. The repair process is generally to machine away the damaged material, pre-heat, weld repair, grind to profile and inspect for integrity. For switches it is necessary to minimise heat inputs because of the danger of damaging or distorting the thin section of the blades. For crossing repairs, a hard, tough material is particularly important, and WAG can supply wire which gives a weld metal that work hardens under traffic, resists fatigue cracking and has low friction.

WAG is working on the joining of rails by electroslag or electrogas welding. A technique is being developed using electroslag uphill welding using cored wire between rail ends, with moulds surrounding the rail end gap. This has the potential to join rails with lower heat inputs (and thus reduced HAZ size) while maintaining very close control over weld metal composition. I assume that it would even be possible to change the wire used so as to produce harder weld metal in the head, giving a very similar result to the HPW described by Dr Gehrmann.

Balfour Beatty Rail's Mike Westoby spoke about The metallurgy of switches and crossings. He covered an enormous amount of information in a very short period discussing the factors affecting S&C and the choices of materials to be used.

He spoke of the challenges inherent in the development of new materials; how many trials should be done, where should they be sited, how long should they last, and what happens if the trial is disrupted? Then, after all that, some external factor like a radical change in materials prices, can completely change the market and render it all a waste of effort!

Mike focused on three key S&C components; switches, crossings and baseplates. A range of steels are available for the manufacture of switches and crossings. The principal steels described were pearlitic, bainitic, austenitic and high manganese.

Pearlitic steel consists of soft layers of ferrite and harder layers of cementite. Its relatively low toughness results in a susceptibility to fatigue failures. Many tactics are used to manage this issue and to reduce the rate of wear of the material. The two priorities, reduced wear and increased fatigue resistance, can conflict. Head hardened, through hardened and micro-alloyed pearlitic steels are all in use in the effort to reduce wear rates. Rail grinding is used to remove RCF cracks and to maintain appropriate rail profiles, but care is needed to avoid grinding off the surface hardened layer.

Bainitic steels are low carbon, carbide free materials. Their microstructure is similar to that of pearlite, but the ferrite layers are less distinct. Toughness and RCF resistance are significantly better than for pearlitic steels.

Austenitic steel readily work hardens under compressive load, achieving very high hardness values. Thousands of cast austenitic crossings are in use worldwide. The material has high Charpy shelf energy values, and so crack tips blunt quickly, and failure through conventional fatigue is not a problem. However, the steels have low yield stress, and crossings will fail if, as a result of poor installation or maintenance, they are not properly supported. RCF does occur in austenitic steels, but in a different form; a hard skin forms on the surface and eventually peels off.

High manganese steel rail is effective but costly, and needs good maintenance if problems are to be avoided. A lip may form on the running edge during initial work hardening, and will require grinding off. Explosive deformation hardening before installation is an effective way to prevent this. It is used in many countries, but has not been adopted properly in the UK for some reason.

Mike discussed the use of stainless steel intermediary laminae for the welding of pearlitic rail to cast manganese. This technique is now well established. However, for some time it has been forbidden to carry out weld repairs across these welded leg joints because of problems as a result of the mixing of different types of metal. A new procedure is being developed that should overcome this and permit weld repairs in future.

Mike described the differing properties of the two potential materials for castings, grey iron (conventional cast iron) and spheroidal graphite (SG) cast iron. Grey iron does not contract as it solidifies and is cheap, but due to its microstructure it is brittle, with poor impact resistance and low tensile strength. It is strong in compression though, and is excellent for uses like spacer blocks, where compressive strength is important and tensile strength is not. SG cast iron gets its name from the way the graphite in its microstructure is formed into spheres; this gives it great toughness. It has higher tensile strength and much greater impact resistance than grey iron and is ideal for baseplates etc where the need for these improved properties outweighs its higher cost.

Automation of arc welding was the subject for Geoff Chapman of Network Rail. His talk was an update on the NWR automated repair welding project. Originally the company bought 11 ESAB automatic welding units, and 95 repairs had been done with these. The relatively slow take-up of the process has been due to difficulty in manually grinding the completed repairs; it has been very hard to keep up with the rapid automatic welding process.

Solutions have been identified and NWR has decided to expand significantly its resources for automated welding. 59 more ESAB units have been purchased, a total of 70 units are now available. For even greater efficiency, the company has purchased new generator sets for the welding units. Arcgen's 165A sets were the most suitable generators. To ensure the best reliability and productivity, 140 sets were purchased, two for each of the 70 ESAB units. Connected in parallel, these pairs will also be able to supply power for the grinding units and for site lighting.

Grinding is now to be done using 70 Matweld hydraulic units, resolving the difficulties referred to earlier.

Preheating is carried out by units from Trueflame Rail Equipment. In addition to 800mm long heaters for use on repairs to plain rail, special switch blade preheaters have been procured. 1 metre long, these can be clipped on the back of the blade, between it and the stock rail, and left there throughout the repair welding process. At any time the inter-pass temperature falls below the critical 300°C, the preheater can be used to correct this, minimising interruptions to the welding. The preheater allows repairs to be made in metre lengths, allowing speedy repair of long lengths.

Rail breaks

Correct setting up of the welding units is critical to quality and speed of operation. Skill and care are demanded of the welder and to assist, special support brackets have been designed for the welding drive units. These enable easy setting of the unit at the correct angle and distance from the rail, and permit correction for things like the hogging of the switch during preheating. Four different welding programmes are available on the ESAB units. For switch repairs the P4 stringer bead programme is selected. Preheat temperature is 343°C and the travel speed of the welding head is 35cm/min. Voltage and amperage settings are critical and need to be towards the lower end of the amperage range for the wire. The wire feed unit is the Mobilemaster IV from ESAB. Weld beads are deposited on a 4mm offset, starting at the bottom of the wear scar and finishing right at the top of the switch blade.

In switch welding trials to date, sections of blade 1.5m long have been repaired in two hours, carrying out the repair in 3 x 500mm long sections. Nine blade repair trials have been done in all on test track sites. Work now needs to be done to determine the signalling requirements before trials can be done on running lines. Procedures and work instructions are in production and so is the training and mentoring of welders. It is hoped that implementation and monitoring of repairs on working railway sites will begin shortly.

Meanwhile the repair of plain rail defects with the new units should now become the norm, as the new welding and grinding units begin to be used regularly.

The final talk of the seminar came from Bob Sawdon of Balfour Beatty Rail (BBR). He described a project that he has been leading, the production of the Balfour Beatty welding pod. The objective was to allow the equipment and materials needed by a welding team to be taken to site in advance with no manual handling necessary, and left on site in security. The welders would be able to drive to site in a normal car or van instead of needing specialised transport, and welding time would be maximised by avoiding the need for the welders to manhandle and trolley their equipment and materials. Tools and equipment were to be stored on shadow boards, reducing time wasted and avoiding losses of items. Materials were to be allocated dedicated storage areas, separating incompatible items correctly and keeping them safe and secure. Work benches were to be incorporated.

The Balfour Beatty welding pod

The pod was to fit onto a 2t Harsco trolley which was already VAB approved. It was also to be designed to be safely lifted and carried by RRV, with a dedicated RRV to be provided on each worksite.

Bob and colleagues held stakeholder workshops to confirm the detailed requirements. Items like adjustable legs to cope with uneven ground were added to the design. Hot work, oxy-gas safety and site movements were particular concerns taken into account.

A prototype built has had significant use, at Ashford and Eastleigh, and on the recent Clapham blockade. Potential improvements have been identified for incorporation in production models, but the prototype has been a success in most respects. Some project managers have worried about the cost implications of the dedicated RRV, but Bob is confident that practical experience will demonstrate that this is more than compensated by the resultant benefits of the pod.

RAILSAFE European Seminar

The afternoon was dedicated to presentations about RAILSAFE 2, the second stage of the EU RAILSAFE project. RAILSAFE is a project concerned with the Europe wide education, qualification and certification of railway welders. RAILSAFE 1 has been completed, and dealt with aluminothermic welding. RAILSAFE 2 deals with arc welding, for the joining of rails or for their repair. The first presentation of the session saw Bob Sawdon back, to describe the CEN organisation and the way in which CEN standards are initiated and developed.

The recently published EN 15594:2009 covers the restoration of the rail head by arc welding, and is the technical standard relevant to the repair element of RAILSAFE2. Bob gave a brief resume of its scope and contents; it covers welding consumables, tests, approval of contractors, personnel qualifications, other training requirements and finally, repair applications.

Rail head defect

The second presentation, by Hans van den Brug from RI, gave an overview of the RAILSAFE 2 project. He described the background to its genesis; the lack of uniformity of training, testing and monitoring of welders, the lack of mobility of welders across the EU, the publication of EN 15594:2009 and the EC Leonardo da Vinci Life-long learning programme. He went on to describe the objectives of the project and explained how phase 1 had dealt with aluminothermic welding and phase 2 is covering arc welding.

RAILSAFE 2 began in 2008 and has a two year programme. It is now at the stage of a draft guideline, and copies had been distributed to delegates in advance of this seminar.

When the document has been finally agreed it will be implemented under the direction of the European Welding Federation (EWF). Each country in the EU will have a RAILSAFE Authorised National Body (RANB). These RANBs will oversee implementation in their own countries, setting in place an organisation of Authorised Examiners and Approved Training Bodies, with rules and procedures.

Tim Jessop of TWI Ltd (The Welding Institute) took the stage after Hans. Tim took delegates through the draft guideline and questionnaire that had been issued to them by RAILSAFE 2 beforehand. There were a number of sections of the draft where the project was seeking comments from delegates, and these were discussed. The questionnaire was to be used to provide formal comments on the draft.

Acknowledgements:

The above is an extract of an article by Chris Parker in 'the rail engineer', 'the rail engineer' can be obtained free for engineers who work in rail.
Email tre.subs@railstaff.co.uk
www.therailengineer.co.uk.