ONSITE REPAIR WELDING OF RAILS. Sandor T, Ramsey J, Dumovic M and Wiseman R

Transcription

1 ONSITE REPAIR WELDING OF RAILS Sandor T, Ramsey J, Dumovic M and Wiseman R ABSTRACT Railways are of increasing importance to Australia with freight volume increasing by 12% from 2009 to 2010 to some 853 million tonnes [1]. Yet the maintenance back log in many areas is growing as is the pressure to maintain or replace aging track. This paper focuses on weld repairs on railways and proposes a number of solutions to improve the productivity and reliability of rail repairs. Welded rail repairs have been practiced for many years but have relied on a high level of operator skill. Invariably problems with cracking and variable performance have remained. This paper looks at the metallurgical issues of rail welding and considers practical, innovative solutions for on site repair of rails using manual and semi automatic arc welding processes. The main types of carbon-manganese and austenitic manganese materials are considered as well as the key issues of heat input control, weld metal selection and welding mechanisation designed for onsite use. Applications considered are rail head surfacing, crossing rebuilding and rail joining using arc welding. KEYWORDS Welding, Railways, Rail Repair, Onsite Rail Repair AUTHOR DETAILS Tamas Sandor, Global Rail and Repair & Joining Manager, ESAB, Bulgaria Jeff Ramsey, Global Segment Support Manager, Hardfacing Products, ESAB, Sweden Milo Dumovic, Global Segment Manager, Wearplate and Roll Reclamation, ESAB, UK Robert Wiseman, Technical Sales Manager, ESAB, Australia - 1 -

2 INTRODUCTION Railways are of increasing importance to Australia with freight volume increasing by 12% from 2009 to 2010 to some 853 million tonnes. The break down of the tonnage hauled is, 48% iron ore, 37 % coal, 6 % sugar and 2% bauxite [1]. Rail is a vital mine to port link so performance and maintenance of the rail line is of key importance. A typical iron ore train is 2.4 kilometres long carries 30,000 tonnes and travels at 75 k/hr so the track loading is considerable and any disruption due to rail defects or failure have considerable potential safety and economic risk. Welded rail repairs have been standard practice for many years and offer a fast, economical solution compared to rail replacement. However the high carbon content of rail steels means that control of heat input, welding alloy selection and welding procedure are essential if a sound, reliable repair is to be completed. Traditionally rail joining and weld repairs have been successfully carried out using the manual metal arc welding process however highly portable, mechanised welding systems are now in use allowing for rapid repairs and close control over welding parameters. THE METALLURGY OF RAIL STEELS 1. Carbon Manganese Rail Steels The main steels used for straight rails are carbon manganese alloys typically in the range of % carbon and % Mn. The exception are the hypereutectoid grade steels where the carbon is % and % Mn. As indicated in Table 1 in the EN classification system there are also chromium alloyed grades and lower carbon higher manganese variations. Table1 shows the different grouping and specification limits from three different standards. The UIC Standard [6] uses tensile strength as a basis for classification while the EN standard uses hardness. The variation in specifications demonstrates clearly that one of the prerequisites for rail repair is a clear understanding of the rail grade being repaired. Table 1. Rail Steel Specifications. [3,5,6] C Mn Si P S Cr AL V N Mo Sn Ti Ni Nb AS 1085 Rail Steel % % % %max %max %max %max %max %max %max %max %max %max %max 33 kg, 41kg kg,60,kg, 68kg UTS Hardness Mpa HB min 33 kg, 41kg kg,60,kg, 68kg kg,60,kg, 68kg Head Hardened UIC 860 Classification < A B EN Classification R R R R260Mn R320Cr R350HT R350LHT R370CrHT R400HT The iron carbon equilibrium diagram shows that the principle microstructure is pearlite producing a tough high strength steel

3 Figure 1. Iron Carbon equilibrium showing the position of rail steels The key issue from a welding point of view is the high carbon content which on rapid cooling can produce martensite and cracking in the heat affected zone. In repair welding of C-Mn rail steels the heat input and cooling rate are critical in determining the type of microstructure and the cracking tendency in the heat affected zone. Figure 2 is a series of continuous cooling curves for a rail steel and shows the range of hardness depending on the cooling rate. Ms is the martensite start temperature, above this temperature the martensite transformation is suppressed Temperature Time Figure 2. CCT diagram of a rail steel, showing hardness values for a variety of cooling rates. The blue line indicates a desirable cooling rate to avoid martensite formation and the risk of cracking [2] Rail standards therefore specify a preheat level based on the rail grade, designed to avoid hard, crack sensitive heat affected zones. Depending on the grade of rail steel the preheat levels range from o C and are applied 200 mm either side of the area to be welded. Head hardened rails are produced by heat treating the rails after hot rolling and since they have the same chemistry as standard rail they have the same weldability

4 Developments in rail C-Mn steels include the hypereutectoid grades (0, %C ) that give improved wear resistance but are less weldable due to the higher carbon content. In these steels the preheat needs to be higher that the standard C-Mn grades. Other developments are the bainitic rail steels where the carbon is lower and the strength is produced by alloying with chromium. These steels are more weldable and require lower preheats. 2. Austenitic Manganese Steels Usually used in the form of castings, austenitic manganese steels are commonly used on crossings and are typically % C and 10-14% Mn. When water quenched the microstructure is austenitic but under mechanical loading or abrasion the surface of the material work hardens up to 500HV and is resistant to severe impact and abrasion. In the quenched condition the austenitic manganese steel is non magnetic but the work hardened surface will show weak magnetism, a convenient indication of the condition of the surface. Table 2. Austenitic Manganese Steel Specifications C Si Mn P S Ni Mo Cr Cu Al % % % max% max% max% max% % max% max% EN EU < < Standard ASTM A 128/A128M USA <1.00 > Others ASTM A 128/A128M USA < <4.0 <2.1 <2.5 GOSZT Russia JIS G Japan When cooled slowly or reheated above 260 o C manganese carbide precipitation occurs in the grain boundaries causing embrittlement. Severe embrittlement occurs in the o C range. For weld repairs no preheat is necessary and the interpass temperature should be kept below 260 o C. Cooling should be rapid to avoid extended time at the embrittlement temperatures. For weld repair, the work hardened layer must be removed, usually the depth is of the order of 2-4 mm. As a result of the heat input sensitivity of manganese steels, railway standards clearly specify maximum interpass temperatures, in the range of o C RAIL DEFECTS Common rail defects include the following, 1. Rail end deformation Rail end deformation at joins using fish plates where the wheel impacts the end of the rail at the gap between the rails. Figure. 3 Rail end deformation - 4 -

5 2. Wheel Burns Spalling or cracking of the surface of the rail caused by wheel slip generating heat and then rapid cooling from the cold rail head. Figureur 4. Wheel burn spalling 3. Flowed Rail Head (Lipping) Deformation of the rail causes a lip to form on the edge of the rail head. 3. Crossing Wear Figure 5 Rail head deformation and lipping Wear and deformation at the crossing caused by high wheel stress as the train changes from one rail to another. Figure 6 Crossing wear and rail head spalling - 5 -

6 WELDING CONSUMABLES FOR RAIL REPAIRS The key issues for successful rail repair are: Determination of the rail or crossing material specification, be it C-Mn steel, C-Mn plus alloys steel or austenitic manganese steel. Detection and removal of defects, particularly cracks that may run for some distance. A controlled welding procedure that includes preheat and interpass temperature, welding consumable alloy selection, care and conditioning of the welding consumables to minimise weld metal hydrogen and weld bead placement Welding consumables fall into three main types, C-Mn rail joining and rebuilding C-Mn surface hardfacing Austenitic manganese steel rebuilding and hard surfacing Table 3 Welding Consumables for Rail Repair [13] Application Classification Type ESAB C Mn Ni Cr Mo Yield UTS CVN Hardness % % % % % Mpa Mpa CVN -20 o C HRC C-Mn Rail Joining and rebuiliding AWS, 9018-D1 (J) EN, E55 4 Mn Mo B120 20BH SMAW OK C-Mn Rail Surface Hard Facing DIN 8555, E1 UM -300 SMAW OK AS2576, 1430 A3 AS2576, 1430 B7 FCAW OK AS2576, 1435 B7 FCAW OK AS2576, 1430 B2 SAW Wire OK 15.40S Flux OKFlux Austenitic Manganese Steel AWS, E SMAW OK (Crossings ) EN1600, E18 8 Mn B 1 2 Rebuilidng and Surfacing AWS, E SMAW OK EN1600, E18 8 Mn B 4 2 AS2576, 1320 B7 FCAW OK Tubrodur As welded 20 Work Hardened 40 AS2576, 1220 B7 FCAW OK Tubrodur As welded 20 Work Hardened The consumable types in Table 3 are self shield and well suited to onsite repairs. In the austenitic manganese steels there is a choice between stainless steel austenitic welding consumables and austenitic manganese welding consumables. Some users prefer the stainless steel austenitic deposit for repair build up but the experience is that both manganese and stainless austenitic deposits work equally well. Care should be taken with all consumables to keep them dry to minimise weld metal hydrogen. Shielded manual metal arc electrodes should be reconditioned at 350 o C for 2 hours if they are exposed to humid environments, submerges arc flux 300C for 2 hours. WELD REPAIR TECHNIQUES 1. Rail Joining by Mould welding The technique has been used by Swedish National Rail [11] for many years and is also approved by Norwegian and Danish National Railways. The basic set up is shown in Fig 7(a) and 7(b). The rails are set with a 1-2mm preset to compensate for distortion so that on completion the rail is straight. A fused sand backing is placed under the rail ends and the joint preheated to o C depending on the rail grade. The foot of the rail is welded as shown in Fig 7(a) and then copper moulds are fitted as shown in Fig 7(b). While maintaining the inter-pass temperature of o C the area inside the moulds is welding continuously with only short stops for electrode change. The last 6 mm of rail can be completed with a hard facing electrode to produce a cap of similar hardness to the rail. After welding is complete and before the joint cools below 350 o C the joint can be stress relieved at o C for 10 minutes and then allowed to cool slowly covered with mineral wool pads or similar

7 15-18mm Backing Fig 7(a) Run sequence on backing bar at foot of rail. Fig 7 (c ) Welding mould section Fig 7(d) Finished rail weld Fig 7(b) Set up for rail weld with copper moulds in place 2. Rail End and Rail Depression Repairs The approach is to use a mechanised system with self shielded flux cored wire. A pre-programmed welding head is used to produce a smooth consistent layer that can be varied in shape to suit the area to be repaired. The mechanised system allows for accurate control of welding parameters such as travel speed, wire stick out and heat input. The completed layer is smooth and consistent and significantly reduces the amount of grinding to return the rail to original dimensions. The wire used is a self shield flux cored depositing a Ni Cr Mo deposit to give a surface hardness of 34 HRC

8 Fig 8(a) Mechanised system for rail surfacing Fig 8(b) Rail end prepared for resurfacing Fig 8(c) As welded surface rebuild on rail end. 3.Crossing Repair (Austenitic Manganese) In this application a manganese steel crossing was inspected for wear and defects and rebuilt using a self shielded flux cored wire. Being manganese steel, care was taken to keep the temperature of the surrounding material below 260 o C whether gouging or welding. Fig 9(a) Crossing nose after gouging and grinding out defects and dye penetrant testing Fig 9(b) Weld repairs a using mechanised welding head and 1.6 mm self shielded flux cored wire. Essential Welding Parameters used: Current: 170 A, Voltage: 26.7 V Arc Travel Speed: 540 mm/min Electrical Stick Out: 25 mm Heat Input: 0.4 kj/mm [7] - 8 -

9 The detailed welding procedure is in Appendix A and shows how the consumable allowed a very low heat input of 0.4 kj/mm. Run placement was diagonal to the crossing nose to avoid excessive heat build up and weld bead craters at the narrow tip of the nose. The mechanised system allowed consistent weld bead placement and accurate control of heat input. Stringer beads were used throughout to avoid heat build up Table xx shows a summary of the time to repair the crossing and demonstrates that speed of the mechanised welding system. Table 4 Time to Repair the Crossing [7] Minutes Total Grinding and Gouging time 209 Grinding and Gouging NDT time 60 Welding Equipment calibration and set up time 20 Weld repair time 158 NDT Welding time 24 Total Repair Time 471 Figure 10 Repaired crossing after grinding to original dimensions and dye penetrant detection. [7) Conclusion. Rail steels are subject to wear, spalling and cracking and being high carbon - manganese or austenitic alloy steels need special attention to heat input and alloy selection for repair welding. However with careful heat input control, repairs can be successfully completed. Mechanised self shielded flux core wire repairs are a productive technique for rebuilding worn rails and crossings allowing accurate control of the surface contour and heat input

10 Appendix A Welding Procedure to Repair Austenitic Manganese Steel Rail Crossing [7] WELDING REPAIR PROCEDURE RECOMMENDATION PROJECT CLIENT Weld Repair of the nose area of Austenitic Manganese Crossing JOINT DETAILS REV 1 PROJECT Crossing Repair INC ANGLE SIDE N/A SIDE 2 N/A WELDING CODE N/A guidelines AWS D15.2 and AS/NZS 2576, DIN 8555, EN ROOT OPENING N/A CERT. AUTHORITY N/A ROOT FACE N/A MATERIALS SPEC'S Austenitic 14% manganese casting SPECIFIED HEAT TREATMENT N/A MATERIAL THICKNESS 5 to 70 mm PREHEAT No preheat required THICKNESS RANGE QUAL. N/A MAX INTERPASS 260 C (500 F) JOINT TYPE Flat and horizontal. Weld overlay PWHT N/A JOINT POSITION WELDING PROCESS Flat and Horizontal Flux Cored Arc Welding Self Shielded Post Weld Treatment: Penning could be applied for in situ repair DRAWING REF. ARTC No WIRE OK Tubrodur FLUX OPERATORS MIG N/A Layer No. NOTICE Prior to welding remove grease, rust and dirt. NDT inspection for defects and cracks. Remove defects by grinding or air arc gouging-aag (make sure that 260 C or 500 F temperature is not exceeded while Arc Air Gouging ). Establish wear, defect pattern, size and estimate reference points for the repair) nose depth, width, wing through theoretical and actual. Wel d Bea d FLU X GAS ELECTRODE TYPE SIZE mm ± DC AC CV OR VV N/A DATE 10 th May 2011 POWER SOURCE: OrigoMig4001i WIRE FEEDER: Origo Mig 4804 with A23 panel CV or VV: CV Automated control system: Railtrac BV 1000 WIRE FEED SPEED m/min AMPS VOLTS TRAVE L SPEED mm/min 1 1 N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV N/A OK Tdur DC+ CV to N/A OK Tdur DC+ CV Het Input kj/mm JOINT PREPARATION - BEAD PLACEMENT, SIZE, SEQUENCE ELECTRICAL STICKOUT 25 mm CROSSING NOSE AREA REPAIR Longitudinal weld stringer bead technique recommended in order to minimise heat input. WORK ANGLE - DRAG ANGLE 75 to 90, drag angle Overlapping next weld bead to commence in the toe of the previous weld bead

11 ACKNOWLEDGEMENTS Australian Rail Track Corporation for Ltd. Mr Greg Riches, for advice and support on rail repair specifications Imtram Pty Ltd. Mr Robert Cavallo and Sonia Whiteman for advice on rail industry requirements. REFERENCES [1]. Australian Rail Industry Report 2010, Apelbaum Consulting Group Pty Ltd [2]. Seyffarth Dr Peter: Schweiss-ZTU Schaublier VEB Vertag Technik Berlin [3]. European Standard EN :2011, Railway Application Track Rail Part 1, Vignoles Railway Rails > 46 kg/m and above. [4]. Australian Standard AS :2006 Railway Track Materials, Part 20, Welding of Rail Steel. [5]. Australian Standard AS :2002, Railway Track Materials, Part 1, Rail Steels [6]. International Union of Railways (UIC) Leaflet 860, Technical Specification for the Supply of Rails. [7]. Dumovic, Milo. Internal ESAB report ARTC Weld Repair of RMB 402 Austenitic Manganese Steel Rail Crossing. [8]. AWS D15.2:2003 Recommend Practices for the Welding of Rails and Related Rail Components for Use by Rail Vehicles. [9]. Swedish National Standard Banverket BVF , Rail Welding Surfacing by Welding. [10]. Boelen Robert, Fortescue Metals Group The Pilbara Infrastructure, The Requirements for Head Hardened Rail V1.01. [11]. Banverket (Swedish National Rail Administration) BVF Rail Welding, Joining of Rails by Welding, Mould Welding. [12]. Australian Rail Track Corporation, Rail Repair using Wire Feed Welding, Engineering Practices Manual RTS [13]. ESAB Welding Handbook 8 th Ed