A Prominent Failure Case in Welding of High Strength Steel: Hydropower Plant Cleuson-Dixence

Transcription

1 A Prominent Failure Case in Welding of High Strength Steel: Hydropower Plant Cleuson-Dixence H. Cerjak, N. Enzinger 1 Institute for Materials Science and Welding, Graz University of Technology, Graz, Austria Norbert.Enzinger@tugraz.at Abstract In the Cleuson-Dixence Hydropower Plant modern high strength steel was used for the pressurized shaft. After the start-up in July 1999 several leakages appeared in the section where high strength steel grade S890 was applied. After shut down of the power plant, full retesting by NDT revealed 99 individual, non acceptable indications in 66 welds. After repair efforts the shaft was taken into service again and collapsed after about four month service in December 12 th After that many questions aroused with respect to properties and behaviour of the used material. An investigation was started to review the design, fabrication and service experience history of the shaft. To make sure that the planed reconstruction does not fail in the same way the owner started a Qualification Program S890 to define quantitative materials parameter as a necessary background for further steps. Comparable material was ordered and welded using original welding procedures leading to test material representing the properties of the shaft. Properties and weldability are reported in this contribution. As a result of this comprehensive investigation it can be stated that beside a high quality production a diligent non destructive testing procedure has to be defined and performed after welding to ensure the detection of cold cracks. Introduction The usage of high strength steels in the pressurized shaft of hydropower plants allows a reduction of the wall thickness and therefore leads to lower costs during production and/or bigger shaft diameters, which increases the efficiency of a plant. This concept was applied in the Cleuson-Dixence Hydropower Plant. In the first period of its service which started in July 1998 several leakages appeared in weldments of Steel S890. After a full retesting by NDT and repairs of 99 found individual non acceptable UT indications in 66 welds the shaft was taken in service again and collapsed after about four month service in December 12 th Thus many questions rose concerning the properties and behavior of Steel Grade S890 which was used in the zone of the catastrophic accident. Beside of the investigations performed in this case by the court experts, the owner started a review on the design, fabrication and service experience history of the shaft. To assure a safe reconstruction concept the owner started a so called Qualification Program S890 to define quantitative materials parameter as a necessary background for further steps. Because no original material of the shaft was available representative material S890 was ordered and welded using original welding procedures representing the properties of the shaft. This material was the basis for the characterization of mechanical properties and weldability reported in this paper; additionally performed fracture mechanic tests and fracture mechanics analysis based calculation of critical flaw sizes reported in [1] and stress corrosion susceptibility investigations reported in [2]. As a short summary of these investigations it can be stated, that steel grade S890 and its weldments shows a satisfactory behavior provided a proper treatment including preheating is performed. Additionally it has to be assured that a diligent non destructive testing (NDT) is applied after a sufficient time span after welding to enable the detection of possibly occurred hydrogen induced, delayed, cold cracking. Case History For the penstock material plates from high strength structural steels were used. Depending on the internal pressure three different grades of steels were selected. In the lower part of the plant the steel grade S 890 QL is used (see Figure 1). The steel plates were ordered by the consortium GSN from Öxeloesund (S), Thyssen Stahl (D), Dillinger Hüttenwerke (D) and Sumitomo Metal Industries Ltd (J). The fabrication of the shaft was divided into four work packages and has been awarded completely to the GSN consortium which included Giovanola at Monthey (leader), SulzerHydro at Kriens and GEC Alsthom, Neyrpic at Grenoble. The manufacturing of pipes for the parts of the shaft with thick walls (41-72 mm) was shared between Sulzer-Hydro (Ravensburg) and ATB Caldereria (Brescia), acting as subcontractors. The other three sections of the shaft were manufactured entirely by Giovanola.

2 in the welds have been reported in the NDT protocols. Amongst them there are also repairs which were performed in the region of the later occurred catastrophic failure. Figure 1: the general lay out of the shaft. The assembly welding was shared between Giovanola, GEC Alsthom Neyrpic and Sulzer Hydro. Giovanola and Sulzer- Hydro chose the coated electrode method for in-shaft assembly welding. GEC-Alsthom Neyrpic opted for the automatic or semi-automatic machine method using the MAG process. Fabrication procedure For the production of the segments of the shaft, plates were cold rolled to rings and longitudinal welded in the shop using the submerged arc welding process (SAW). Two single rings were connected with a circumferential SAW to a section transported to the site location. In front of the tunnels this single or double rings were connected to assemblies using circumferential SAW process. The prefabricated parts were then brought into the shaft and welded in position to the final penstock. Manufacturing and prefabrication of the pipes in the workshop started in The assembly on site was carried out from 1997 to June Finishing the assembly, in July 1998, a pressure test was carried out on the whole shaft. On the basis of the concluding test results, the reception of the shaft was pronounced in July Welding and Quality Assurance (QA) For each of the single fabrication steps procedure specifications and test specifications were developed in the framework of a general QA system executed and the results of the findings recorded in protocols. A certain preheating temperature and a post weld heating procedure were derived from special tests performed by Institut de Soudure. These measures are used to control an expected sensibility of high strength steel to cold cracking. Also in the NDT specification GSN C-3655 this fact is considered by the defined time span between the welding and the NDT. In this specification the possibility of a so called delayed cold cracking is considered by defining a repeating of the NDT after a period of seven days after welding was finished. In addition, repair welds have to be retested seven days after the welding was performed. All welds should be tested by non-destructive methods: 100% by UT-testing and 10% by magnetic particle (MT) testing. The intersections of longitudinal and circumferential welds shall be tested by applying X-ray testing, which can be replaced by UTtesting plus limited MT, when the wall thickness exceeds 34mm. As a result of this NDT procedures applied, a number of repairs Observed leakages and related repair works Observed leakages During the first period of operation of the plant three leakages were detected. In July 1999, by sheer chance, a very small but constant pressure drop has been detected in the penstock by the supervisor of the power station during control work in the emptied shaft. After the systematic control of different installations it was concluded that there might exist a leakage. Using precision leakage measurements the position of the leakage was found downstream of Peroua in the section F III. A systematic visual inspection of the joints led to the detection of a 60 mm long crack in the longitudinal workshop weld of pipe 172-2, at the intersection of a circular workshop weld. GSN applied a standard repair method by digging out the cracked part and repair welding of the affected area. Because of water occurrence this way of repair turned out to be extremely difficult. Subsequent systematic leakage controls after the penstock have been set under pressure again showed that there still is a leakage. Delayed cold cracking and the presence of hydrogen in the weld was suspected as a reason for this crack. A second attempt of repair was made and the plant was taken into operation after these repair attempts in July 1999 again. In the middle of October 1999 a systematic pressure control showed still a leakage. The two failed attempts to apply the so called standard procedure of repair welding described above was abandoned and replaced by a so called heavier solution, by cutting of the failure zone, execution of an excavated niche in the concrete behind the steel penstock, draining of water occurrences, construction of a water tight metal enclosure and welding of a pre-manufactured disc. The repair was finished at February 2 nd, The leakage control of the shaft after the final repair of weld took place on February 3 rd, Contrary to all expectations it reveals that there is still a global leakage present under maximum static pressure. On February 5 th, 2000 the repair weld in pipe was examined. The welding joint of the disc controlled by 100% ultrasonic testing showed no welding defects. The conclusion out of this was that the pipe should be perfectly tight and there must be one or more other points of leakage existing in the penstock. On February 9 th, 2000 a very exact control of the entire pipe section of interest leads to detection of a leakage at a level of pipe 316-1, above Dzerjonna. A 100mm long crack on the external surface of the circular workshop fabrication joint in the area of an intersection with a longitudinal weld was detected. On February 14 th small cracks typical for cold fissuring were discovered in the circular workshop weld of pipe 276-

3 1, downstream of Dzerjonna. These cracks perpendicular to the welding seam showed a length of about 30 mm and a very small opening. The parties involved, the owner as well as GSN became aware that this problem could be in relation to the appearance of cold cracking. Retesting of the Shaft by NDT It has to be mentioned that the detection of cracks in the areas of leakages up to now were made only by visual inspection. Considering this fact it was required to proceed with a systematic control of all welded joints in the zone where the high strength steel S890 QL was used. The application of UT was decided on Feb. 14 th, For the control operation it was decided that the same NDT specification used by GSN for the fabrication and site weldings should be used. The retesting was somewhat more restricted because of the presence of the corrosion protection inside the penstock and, of course, by the limited access only from inside of the penstock. TABLE 1: REPAIRS IN THE SHAFT NECESSARY AFTER RETESTING IN 2000 Joint Lot FI Lot FII Lot FIII Lot F IV Total Circumf Long Total From Table 1 it can be seen, that 66 joints have to be repaired (94 repairs) in accordance with the applied NDT-specification. That means that in average 1.5 repairs per repaired joint has to be performed. All failures to be repaired were found exclusively in workshop welds, performed by one fabricator. Examination of the possible causes of leakages After detection of the third leakage in the penstock in February 2 nd, 2000, the first systematic investigations of the causes of the leakages have started. By order of GSN the Institut de Soudure in Paris was contacted to investigate six different samples cut out from the penstock. The results of this investigation are reported in [3]. In one sample cold cracking perpendicular to the welding direction was found, in all other investigation longitudinal cracks, partly or fully penetrating the wall thickness were detected. In the report [3] in all cases cold cracking in the weld metal is depicted as the main reason for the occurrence of cracks and leakages. In the case of leakages the report describes, that existing cold cracks which more or less penetrate the greater part of the wall thickness propagated step by step until break through to the surface. In all cases the cracks are located in the weld deposit of the multilayer weld. The appearance of the crack surface leads to the conclusion that brittle separation caused the crack during or after the welding process. The crack surface was found to be oxidized, and propagated to a certain extend stepwise through the wall leading to the leakage. The lengths of the cracks were reported to be greater than 100 mm for a penetrating crack (sample 316.1) with a penetration length of 70 mm and in the case of a sample 126 and 129 up to a length of 240 mm, open to the surface. The crack in sample was found in connection with a repair applied during fabrication welding. Repair procedures According to different types of failures found and the local con-ditions four repair procedures were selected. These methods are: Type 1: The Failure does not penetrate to full wallthickness: The failure zone was ground off and welded by using manual welding. Type 2 and type 3: Wall penetrating failure. The problem of handling the water access to the welded zone became virulent. In these cases the failure zone was cut out and replaced by a circular coupon. Depending on the amount of water which has to be drained off two methods, the so called the boss and the disc method, each by welding a circular disc in the wall of the penstock, were applied. In most cases these repairs were applied when the indication was found on the intersection of longitudinal and circular welds. Type 4: In the case of localized cold cracks perpendicular to the welding seam the repair was performed by piercing the steel and sealing it with a screwed stainless steel plug. UT retesting and repair works were performed in a continuous working sequence. The repair job was finished end of July 2000 after 5 ½ months. Figure 2: Appearance of cracks leading to leakages in the shaft [3]

4 The analysis of the chemical composition shows that the involved steel supplier uses different approaches in the production of the steel S890. Significant differences can be observed concerning Ni, Cr and Cu. Concerning the Charpy values at -40 C a minimum of 27J is achieved. Nevertheless there are differences between the different suppliers in the width of the scatter band. Thyssen as well as SSAB show a pretty good performance with respect to isotropy, whereas Sumitomo documents show much higher minimum values. TABLE 2: SUMMARY OF ANALYSIS OF THE CHEMICAL COMPOSITION OF BASE METAL USED IN THE SHAFT. Figure 3: Location of the catastrophic fracture [5] Some examples of the results of the investigations performed by Institute Soudure are given in Figure 2. Investigations of such, but smaller types of cracks are described by Dorsch et al in [2]. The catastrophic failure After finalizing the repair work in July 2000 the shaft was taken into service again. Tightness tests had been carried out on August 15 th, September 15 th and October 15 th On December 12 th 2000, at 20:09:17 after the planed stop of the third turbine the catastrophic accident occurred [5]. The reconstruction of the failure position is given in Figure 3. The shaft suffered a longitudinal crack of about 9m length and opened abound 60cm wide. After the report of the court experts [5] the crack started in the longitudinal weld of the ring crossed the circumferential welds nr. 118 upstream, and nr. 119 downstream, penetrated the base materials of the adjacent rings, each 3 meters wide, and stopped by branching in the circumferential weld seams nr. 117 and nr. 120, respectively. Qualification Program S890 To support further steps regarding to the reconstruction alternatives of the shaft Cleuson-Dixence after the catastrophic collapse in December 2000, the Qualification Program S890 has been started in July The aim of this qualification program was the generation of material data of the steel S890, as base material, heat affected zone and weld material, especially with regard to its toughness and failure tolerance capacity. Analysis of status For the detailed analysis of the material S890 and its weldings applied in the shaft several sources were searched diligently collecting the information in a database. Out of these efforts following results could be collected: chemical analysis, plate geometry, tensile and charpy test data, suppliers material certificates, position in the shaft, fabrication and testing of the welding. C Mn Si Cr Mo Ni Cu Nb Dillinger SSAB < Sumitomo Thyssen Ti V Al P S N B CET <0.015 <0.003 <0.01 < <0.015 < < <0.010 < < <0.015 < A similar analysis was performed concerning the values obtained from tensile tests. Again significant different scatter bands are observed. The fracture elongation shows a relative low scatter for all suppliers. No major influence of the plate thickness on the results could be observed. Materials investigated For a quantification of the material used in the shaft, no original material from the shaft was available. For finding representative materials compared to the one used in the shaft the results of assessed material data from the database have been used. Based on these results, representative materials (lower toughness, alloying concept) have been selected in cooperation between IWS and MPA-Stuttgart. At that time S890 material only was available from the manufacturer Thyssen and Dillinger. Investigation program An extensive specimen plan has been designed to enable the investigation of following welding-material-thickness combinations 3 different welding procedures (SAW, SMAW, MAGW) 2 different material suppliers (Dillingen, Thyssen) 3 different plate-thickness (25mm, 35mm, 55mm) This extensive investigation program had the aim to quantify the toughness for the determination of the critical failure size for collapse of the material S890 used in the shaft. The investigations performed included: Tensile tests, Charpy tests, Wide Plate tests, fracture mechanics tests on small specimens (J-Integral), crack growth measurements, Gleeble simulation for the determination of the weldability, metallographic investigations, hardness tests and residual stress measurements.

5 Generated welded materials In the Qualification Program S890 the welding conditions applied on the shaft should be reproduced as close as possible. The following welding procedures have been used for the fabrication and assembling of this shaft: submerged arc welding procedure (SAW) for all longitudinal joints and circumferential joints in the shop and in front of the tunnel shielded metal arc welding (SMA) for assembling circumferential weldings in the tunnel metal active gas welding (MAG) for assembling circumferential weldings in the tunnel As a consequence of the report of the court expert [5], the qualification program has been reduced to the platethickness 35mm (Thyssen) with the welding-procedure submerged arc welding (SAW) and shielded metal arc welding (SMAW). For the same reason a stress corrosion program was established. The results of the qualification program S890 are partly reported in this paper, the results of fracture mechanic investigations and calculations are reported by Roos et al in [1]. Performed investigations and Results Tensile Test For each microstructure of the 35mm thick joint of the XABO 890 (Thyssen) base material tensile tests were carried out in longitudinal and transversal direction at room temperature. The base material was confirmed to fulfill the specification and was found to be isotropic. The SAW weld metal matched the base metal but showed a slightly decreased rupture elongation in transverse direction. Rupture occurred in the weld metal. The SMAW weld metal showed a significantly lower yield strength compared to the base metal. Hardness The hardness was measured on different distances from the surface in the cross section to find the location of the maximum hardness (Figure 4). In the SAW the hardness maximum was found just underneath the final layer (approximately 4mm) in the heat affected zone in the range of about 450HV10. In more distance to this surface the maximum hardness reached values of about 360HV10. The SMAW showed a similar behaviour. Maximum hardness of about 430HV10 was found in the heat affected zone near the surface. In a more distant region the hardness reached about 360HV10. Figure 4: comparison of the hardness distribution in the final layer and in the middle of the plate of the 35mm SA Welding. Charpy test The toughness of base metal, heat affected zone and weld metal of the generated weldments was measured performing the ISO-V-Impact test at nine temperatures (-60 C up to 100 C) according DIN EN The transition curve was derived by applying the Hofer-Hung method. The base material near the plate surface shows anisotropy in the upper shelf (150J in rolling direction and 125 in transverse direction) whereas in the centre of the plate uniformly 125J are measured. The lower shelf shows values of approximately 50J The upper shelf value of the SAW weld metal is about 125J; the lower shelf value approximately 60J The upper shelf value of the SMAW weld metal is about 125J to 150J; the lower shelf value approximately 50J The Chabelka method was applied at room temperature to quantify the influence of the HAZ of the distinguished weld methods (SAW vs. SMAW). Summarizing the results of the Charpy tests it can be seen, that in every case the specification of 27J at -40 C is fulfilled. Microstructure All investigated microstructures, base metal, heat affected zone, SAW weld metal as well as the SMAW weld metal shows the typical annealed martensitic structure. Weldability Additionally the behavior of the heat affected zone was investigated by loading base metal specimens with representative thermal cycles. To cover reality two different peak temperatures and their combination were applied. The obtained samples then were tested with the following methods: tensile and charpy test, metallography and hardness. Simulated specimens representing the heat affected zone confirmed the high hardness which was observed in the cross section of the weldments.

6 Metallographic investigations showed the expected microstructure due to different, representative thermal cycles. Charpy curves showed an adequate lower shelf and transition temperature. Conclusion These conclusions include also parts of the findings reported in [1-2]. The toughness behaviour of S890 (Thyssen and Dillinger) and its SAWelds were tested by means of Standard tests (Tensile, Charpy V-Curves) (BM, SAW, SMAW) Small scale Fracture Mechanics Tests KIc, Jc, da/dn, COD (BM, SAW) Wide Plate Tests (BM, SAW) Steel type S890 is a high strength steel showing, in comparison to lower strength grades construction steels a limited failure tolerance. This fact can be expressed by a smaller distance from crack initiation point to collapse. This is from importance in case of overloading of a cracked component. The toughness, expressed by crack initiation limit is generally satisfactory. The toughness behavior of SAW-weldings is similar, compared to that of the base material (Thyssen, Dillinger), but show somewhat different behavior. SAW and SMAW welds are highly sensitive for forming of hydrogen induced cold cracking in weld material. Base material weldability is satisfactory (Dillinger, Thyssen). Steel grade S890 can principally be used when the following boundary conditions unrestricted can be fulfilled: Keeping the design stresses as low as possible Assuring crack free welds by strict QA measures and optimized automatic NDT Control of keeping inside the maximum agreed design limits under all conditions of design, assembling and service conditions (incl. emergency) Strict H-control of welding processes and NDT to prevent cold cracking Remark Meanwhile the successful reconstruction of the shaft is finished. Due to several reasons steel with a lower strength (type S690) was applied. Line in line option was chosen for the majority of the shaft only in the area of the accident a bypass was designed. Recommission is planned to be early in 2010 [6] References [1] E. Roos, W. Stadtmüller, H. Cerjak, G. Dimmler,: Results of fracture Mechanics Investigations on Steel S890 and its weldments, Proceedings of High Strength Steels for Hydropower Plants, , Graz [2] Th. Dorsch, K. Saarinen, E. Roos, Th. Böllinghaus,. H. Cerjak, N. Enzinger: Results of Stress Corrosion Investigation on Steel and Weldments on Steels S890 and S690, Proceedings of High Strength Steels for Hydropower Plants, , Graz [3] Institut de Soudure, Paris, Report No , [4] B. Hagin: Project Cleuson-Dixence and the Accident on Dec. 12th 2000, Proceedings of High Strength Steels for Hydropower Plants, , Graz [5] Cachot, R.; Mortensen, A.; Neal, B.: Expertise metallurgieque du blindage du puits blinde de l amenagement hydroelectrique de Cleuson-Dixence, Institut des matèriaux, Lausanne, le 22 juillet 2002 [6] ( )