Weldability aspects of offshore steelsFrank Hanus and co-workers, Dillinger Hütte GTS Quality Department - Welding Laboratory 1. Introduction Dillinger Hütte offers a wide range of structural steels for offshore installations. Initially the normalised steel with a nominal yield strength of 355 MPa was dominating, by the way it still is one of our important offshore steels. Some higher strength applications were served with quenched and tempered Grd 450 with the draw back of a more limited plate width. From the late 80ies on we were able to offer thermomechanically rolled plates. “Tiffany” was our first huge platform in TMCP. The next big step came with “Caister Murdoch” when TMCP plates substituted the QT in Grd 450 EM(Z). A lot of experience could be gained in TM rolling of heavy plates in Grd450-EMZ according to BS7191 and S420(NORSOK) before in 2001 the Norwegian platform projects “Grane” and “Valhal” pushed the plate requirements over the 500 MPa yield strength boarder line. Increasing maximum plate thickness or optimisations in the chemical composition of individual grades were intermediate steps in the development and still are ongoing. More recently the exploitation of resources in harsher environments (North of Canada, Sakhalin, Kashagan, Stokman) led to lower service temperatures down to –40°C and expected –60°C and ask for steels that we call “Arctic” or “Super-Arctic”. Some of the weldability test results from such steels will be presented by M. Kulboten in the colloquium. Other projects need even higher strength material in heavy wall thickness. The very extreme being the quenched and tempered plates for gear racks of jack-up-rigs a subject adressed by M. Pickhan and M. Bonn. For all these materials the weldability was an essential aspect in the development of the steels. Finally it is the welded structure that safely has to stand for decades. Sometimes we had to argue with our colleagues who were rather focussed on parent material properties than on HAZ properties. Over the years the welding laboratory has welded many hundreds of welds on offshore steels during the development, weldability qualification and verification tests. A kind of summary of this experience will be given in the paper, the presentation and the workshop. 2. Weldability The term weldability characterises the suitability of a material to produce sound and reliable welded joints. It comprises the tendency of the material to produce hard and brittle areas in the heat affected zone (HAZ) of a fusion weld, it’s susceptibility to form defects like hydrogen induced cold cracks, lamellar tearing, solidification cracks, stress relief cracks or others. Steels of poorer weldability may require either restrictions in the welding process either special measures to avoid defects or inacceptable local material properties. For the fabricator the most significant advantage of the normalised offshore steels compared to conventionel structural steels is their improved weldability. This results from a reduced the carbon content from typically 0,18% to 0,12% and reduced trace and mcroalloying elements due to the limitations in the material standard Table 1. A reduction of the impurity level of oxygen, nitrogen, sulphur has also a beneficial effect on parent material and HAZ properties, especially on toughness. The 1 application of TM rolling allows a further reduction of the carbon content to approximately 0.08% thanks to the grain refinement achieved by the TM process. Compared to normal structural steel plates (EN10025) the plates for offshore are more limited in the carbon content and content of trace elements. Also the microalloys are more restricted. If prequalified steels are ordered, only very limited deviations are allowed against the chemical composition of the material that had been used for the qualification tests. Table 1: Max. product chemical composition according EN 10 025:2004, EN10225 and *) some typical composition plate deliveries by DH Plate thickness 50 mm C S355NL EN10025 S355 G8+N/M S355 G8+N *) S355 G8+M *) S460NL EN10025 S460ML EN10025 S460G2+M/Q S460 G2+M *) typical carbon equivalents S355G8+M S460G2+M Dillimax500ML CE CET Pcm Si 0,55 0,55 0,3 0,3 0,65 0,65 0,55 0,3 Mn 1,60 1,65 1,5 1,5 1,80 1,80 1,65 1,6 CE(IIW) 0,35 0,40 0,44 P 0,030 0,020 <0,015 <0,015 0,030 0,030 0,020 <0,015 S 0,025 0,006 <0,003 <0,003 0,025 0,025 0,007 <0,04 CET 0,25 0,28 0,28 Nb 0,06 0,06 <0,04 <0,03 0,06 0,06 <0,04 <0,03 Pcm 0,18 0,20 0,21 V 0,14 0,06 <0,03 <0,03 0,22 0,14 0,08 <0,04 0,20 0,14 0,12 0,08 0,22 0,18 0,14 0,08 = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15 = C + (Cr + Mo)/10 + (Cr + Cu)/20 + Ni/40 = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B 3 HAZ hardness The hardness measurement in the HAZ gives evidence on very local material property and serves to assess the transformation characteristics of a steel. For a systematic HAZ hardness determination several weld beads were welded on the plate surfaces. By applying different weld heat inputs the cooling rates were varied. The cooling conditions were measured with thermocouples in the weld metal. As it is common practice in the “welding society”, the cooling conditions are characterised by the duration of cooling from 800°C to 500°C, the cooling time t8/5. Cross sections were machined out of the bead on plate welds. They were polished and etched and hardness indents positioned as close as possible to the weld fusion line in the coarse grained HAZ. The HAZ hardness results are shown in figure 1 versus the t8/5 cooling time. For the low heat input welding (FCAW in position welding or laser beam welding) leading to very short cooling times and a microstructure mainly consisting in martensite, the hardness difference is only due to the different carbon content, because carbon governs the martensite hardness. In case of higher heat inputs the influence of the alloy content becomes more pronounced. This is very apparent for the higher alloyed S460N and S690Q (chemical composition suitable for 80 mm plate thickness). Usual welding conditions result in t8/5 times between 8 s and 30 s. In multipass welds of the TM steels the HAZ hardness can be kept below 248HV already in the as welded condition. Due to the different chemical composition the S460G2+M steel is almost 100 HV less hard than the normalized steel in the complete t8/5 range. The delivery condition respectively the initial microstructure is not significant for the hardenability in the coarse grained HAZ but only the chemical composition and the cooling rate. Steels containing Nb and V result in higher HAZ hardness for long cooling times which is caused by precipitation hardening during cooling. During a possible PWHT such reprecipitation becomes more complete and the associated precipitation hardening counteracts HAZ softening by tempering the HAZ. 2 500 usual range of t8/5 HAZ hardness (HV10) 400 S690Q S355G8+N S500M S460(N) S 460 N 300 200 S 460 M S460G2+M 1 5 10 20 cooling time t8/5 s Figure 1 : Bead on plate HAZ hardness for various steels as a function of weld cooling time measured in the as welded condition 4. Cold cracking and the necessary preheat temperature for welding It is an important point for the welding engineer whether preheating and a minimum interpass temperature is necessary to avoid hydrogen assisted cold cracking in the weld. Pre- and postheating, the precautions against this defects, are rather time consuming and cost effective and they are also difficult to be carried out and controlled on the yard. A steel that can safely be welded without preheat, therefore brings a strong advantage for the fabricator. The risk of cold cracking of welded joints is not only depending on the chemical composition of the steel but, this is the steelmaker’s contribution to the problem. For a given chemical composition the risk increases with increasing hydrogen content of the weld metal. It is also increased with the level of stresses in the weld region. Thereby, it is affected by the rigidity of the weld assembly, the plate thickness and the welding conditions. Pre- and post heating may be necessary to avoid cold cracking, by lowering the cooling speed and allowing a more complete effusion of the hydrogen out of the weld prior to cooling to ambient temperature. Detailed explanations about the phenomenon are given by Bailey and co-authors [1]. Formulae for the calculation of the required preheat as a function of the main parameters are given in Stahl-Eisen-Werkstoffblatt 088 [2] and EN1011 [3].CET and Pcm are the carbon equivalents that are commonly used to characterise the susceptibility of the steel for this defect. We see from the formulae that the carbon plays the most detrimental role for this weld defect. The lower carbon content makes the HAZ less hard and less brittle. Both effects reduce the risk for crack formation. According to the CET formulae (see Table 2) each 0.01% C increases the required preheat by 7°C. Table 2 : formulae for calculation of preheat temperature according to DIN EN 1011 Stahl Eisen Werkstoffblatt 088 and DIN EN 1011: CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40 Tv = 700 CET + 160 tanh (t/35) + 62 HD exp 0.35 + (53 CET - 32) Q + 330 t HD Q = plate thickness (mm) = hydrogen (ml/100 g - ISO 3690) = heat input ( kJ/cm) +0,01% CET ~ 7°Chigher preheat 3 If we compare the calculated preheat for a TM steel having 0,08% C and a normalized steel with 0,18%C we conclude that the difference only caused by the different carbon contents raises preheat by 70°C. A comparison of calculated preheat as a function of the plate thickness for combinations of heat input and hydrogen levels is shown in figure 2. The curves represent recommendations according to the CETformulae. Figure 3 allows to compare the preheat recommendations with the results of CTS cold cracking tests. Our tests encompassed normalised offshore steels S355 +N and TM rolled offshore steels from S355 to S500, plate thickness from 25 to120 mm, hydrogen inputs from 2 to13 ml/100g deposit weld metal and heat inputs from 0.7 to 2.0 kJ/mm. The graph shows the percentage of crack length ratio on the polished surfaces prepared from the CTS specimens. Recommended preheat EN1011 150 125 100 Heat input heat input hydrogen hydrogen GSMAW 1.0 kJ/mm 2 ml/100g DM GMAW 0.7 kJ/mm 2 ml/100g SMAW 2.2 kJ/mm 4 ml/100g ml/100g SMAW 2.0 kJ/mm 4 DM SAW 3.3 kJ/mm 7 ml/100g ml/100g SAW 3.3 kJ/mm 7 DM S460 (N) S355J2G3 (N) S460N S355G8+N Offshore 75 50 25 S500M S420G2+M S460M S355G8+M 0 20 40 60 80 100 plate thickness Figure 2 : calculated preheat temperatures as a function of plate thickness 100 90 CTS-specimen 80 70 cracking % polished section from the CTS test weld 60 50 40 30 20 10 0 -150 -100 -50 0 50 100 Tp - Tp(CET) HAZ crack caused by insufficient preheat Figure 3 : CTS-tests Results of CTS tests on S355G8+N (diamonds) and TM-steels (squares) compared with calculated preheat according to the CET concept (EN 1011) 4 The recommendations of EN1011 were conservative against the measurements using the CTS test. As this test configuration produces only a moderate restraint, the welding engineer should consider the actual restraint level before choosing the preheat for a welding job. For highly restraint situations the predictions of the CET concept seem correct. Practical experience has confirmed the essential benefits of the optimised chemical composition of the TM plates. For the huge storm barrier “Maaslant Kering” close to Rotterdam S355 TM plates of 120 mm thickness were welded without preheating. In the Norwegian offshore yards S420M and S460M plates in thickness up to 50 mm are usually fluxcored arc welded without preheat provided that the surfaces are clean and dry. The examples show that especially in heavy constructions the use of TM steel replacing normalised steel allows an essential reduction of preheat temperatures and in many cases preheating ain’t necessary at all.[4]. It is clear that the extension of welding without preheat is related to very low hydrogen input, hence on the appropriate welding consumable, quality assurance and reliable welders. 4. Toughness of the HAZ Thoughness in the HAZ was a main driving force in developping steels suitable for offshore applications. A good cleanness of the steel has a beneficial effect on the HAZ toughness. Good cleanness means low contents of sulphur, the absence of large inclusions and a low level of trace elements. The effect of sulphur and carbon on the HAZ impact energy is focussed in figure 4. The corresponding 355 Y.S. steels had been welded with a heat input of 3.5 kJ/mm. Impact testing at the fusion line revealed that the old S355N steel with the high sulphur content hardly reached the 27 J. The S355N having a reasonably low sulphur content matched 50 J at –20°C and an upper shelf energy of 100J. The normalized offhore steel reaches satisfactory impact results at –40°C. Low sulfur and reduced carbon content led to this improvement. The modern TM-steel with a still lower carbon however reached 200J at –40°C. Impact energy (J) 350 300 250 200 150 100 50 0 -80 S355M (Nb) 0,08% C, 0,001% S S355N Offshore0,12% C, 0.001% S S355N standard0,20% C, 0,003% S S355N old 0,20% C, 0,028% S -60 -40 -20 0 20 40 temperature [°C] Figure 4 : Impact toughness in the HAZ of 3,5 kJ/mm SMAW on 20 mm thick plates of 355 MPa yield strength [5] For plate deliveries for NAM offhore platforms weldability tests were specified. They had to carried out on each heat of steel on the highest plate thickness which kept my welders busy when in a periode of 5 years more than 300 butt welds were produced and tested. The required 36 J average 26 J single value were safely obtained and only very few retests were needed. A statistical evaluation (see figure 5) was prepared after a while, in order to convince NAM to stop requiring the tests. However NAM maintained 5 the requirement with the argument: Since the plates being weldability tested they did not experience any problems during fabrication. Impact energy (J) impact energy Figure 5 : Impact toughness at the fusion line of 3,0 kJ/mm SAW. S355G7+N, 33-63 mm thick plates, V-preparation. Weldability tests for NAM projects. 5. HAZ metallurgy In a systematic way the influence of carbon and other alloys on HAZ toughness can be investigated by means of weld thermal simulation. The principle of weld thermal simulation is shown in figure 6. The mid of the specimen is exposed to a time temperature corresponding to the particular area of interest within the HAZ. The intrinsic properties of such a microstructure can then be determined on the simulated specimens and the steel composition can be optimized e.g. to avoid local brittle areas. In impact testing the notch only samples the aimed microstructure. The results of the simulation test can be far worse than impact testing on real welds because in the simulated specimens there is no energy contribution by adjacent tougher HAZ areas. T 1350°C t thermal cycle coarse grained HAZ microstructure macro etch of HAZ of simulated impact specimen Figure 6 : Thermal cycle for the simulation of coarse grained HAZ microstructure 6 Figure 7 shows the impact transition temperature of steels of different carbon content. The impact transition temperature was determined on specimens having undergone a weld thermal cycle corresponding to the coarse grained HAZ producing martensite or lower bainite. The results show that reducing the carbon content is an effective way to improve the HAZ toughness. Figure 7 : 27J impact transition temperature on specimens with coarse grained HAZ microstructure after Gleeble simulation [6] In the coarse grained HAZ the existence of upper bainite with retained austenite islands (left picture of figure 9) has to be avoided because they can trigger cleavage fracture in impact testing and CTOD testing. The brittle fracture initiation at a retained austenite constituent on the fracture surface is marked with the arrow (right picture of figure 9). The part of coarse grained HAZ heated by a subsequent welding pass to temperatures of partial austenitisation is an other area suspect for low toughness because of carbon enrichment and high carbon martensite developing on the former austenite grain boundaries (middle picture of figure 9). Coarse grained HAZ upper bainite with retained austenite Intercritically reheated coarsed grained HAZ t 8/5 ~ 35 s second peak temperature 775°C Etched in white: high carbon austenite Etched in black high carbon martensite cleavage initiation at a retained austenite island Figure 9 : 7 6. Toughness in welded joints of 500 MPa yield strength steel For the steel qualification for offshore platforms the steel has to show sufficient HAZ toughness on different butt welds in the range from very low heat input of 0.8 kJ/mm to high heat inputs up to 5.0 kJ/mm, sometimes also intermediate heat inputs have to be tested. The welds were impact tested at –40°C at positions shown in figure 10. Figure 10 : Notch positions for impact testing for the weldability qualification (EN10225) testing of bevelled side is optional As can be seen in figure 11 and figure 12 excellent HAZ toughness values were measured. Further results for HAZ impact testing are given in figure 13. These were determined in the course of welding procedure testing for a heavy penstock pipe where also a S500 TM steel is used. Thanks to their low carbon and microalloy content the welds of our TM plates result in good impact toughness and allow even for S500M a wide range of welding heat inputs without deterioration of the toughness. Under many conditions welded constructions built from TM plates do not need to be PWHT because the as welded condition is already sufficiently tough and HAZ hardness moderate. Impact energy at –40°C 300 250 200 150 100 as welded PWHT straight side bevelled side 0 1 2 3 4 5 50 0 heat input [kJ/mm] Figure 11 : Impact results at –40°C of welded joints of a S500M offshore steel in 30mm and 70 mm wall thickness 8 CTOD at -10°C mm 1 0.25 crack positions weld metal fusion line SC-IC-HAZ 0.1 0 1 2 3 4 5 weld heat input kJ/mm Figure 12 : CTOD results on 70 mm thick S500M welded joints, full size BxB SENB specimens, a/W~0.5, through thickness notch, as welded condition, data reference [7] impact energy 300 HAZ-GTAW HAZ-SAW HAZ-GTAW man. 200 - HAZ-SMAW 100 HAZ-FCAW -150 -100 -50 0 50 test temperature Figure 13 : Impact results welded joints of Dillimax 500ML welded with different processes [8] – approval tests for a hydropower penstock 7 Post weld heat treatment A post weld heat treatment (typically 550-580°C) for 2 hours can, however, be performed if this should be stipulated by the code or design requirements. Such a heat treatment hardly alters the parent material properties nor the impact properties of the heat affected zone of our structural TM plates. 9 Steels with higher Vanadium content may suffer a decrease in HAZ toughness during the PWHT [9]. This embrittlement was observed and systematically investigated in the 80ies when S460N became commercial. The effect could mainly be attributed to the precipitation of vanadium carbides. Increasing vanadium shifted the HAZ impact transition temperature in a linear relation (figure 14). Our TM plates for offshore applications do not need vanadium to achieve the required tensile properties. Accordingly the detrimental effect observed on the normalized 460 steels is not a problem with our TM steel. It should, however, be noted that EN 10225 allows up to 0,08% V for the S420 and S460 grades. For a steel with the vanadium content close to the upper limit of the standard, a negative effect of PWHT on the HAZ toughness must be expected. increase of 27J impact transition temperature Figure 14 : Embrittlement in the HAZ of normalized steels during PWHT (560°C 5h) caused by Vanadium [10] 8 Aspects of welding S690 QT steel The high strength QT steels with a yield strength of 690 MPa and higher are much more difficult to weld compared to the beforementioned offshore steels. More precautions are needed due to their much higher carbon and alloy content. Also the weld metal properties are difficult to match with the parent metal properties. In order to produce sufficiently high strength properties in the weld, the heat input must be restricted to approximately 2 kJ/mm. Thereby, the deposition rates and the welding economy are reduced and together with the precautions, the welding time may increase in spite of the reduced weld volume needed for lower wall thickness. To achieve satisfactory weld metal properties the welding parameters must be limited with increasing yield strength. Acceptable properties for an S690 steel are normally obtained with cooling times between 6 s and 20 s. Within this cooling range the coarse grained HAZ a 80 mm thick S690Q steel transforms mainly into martensite with a corresponding hardness above 400 HV (refer to fig.1) 10 Table 2 : Typical carbon equivalents characterising the hardenability (CE) and the HAZ cold cracking susceptibility (CET. Pcm) for some structural steels CE 0.44 0.42 0.44 0.70 CET 0.33 0.26 0.29 0.39 Pcm 0.25 0.18 0.24 0.32 S355J2G3 S500M S690QL S690QL 80mm 50mm 20mm 80mm Fig. 15.1 : Hydrogen induced cracks in the Fig. 15.2 : Positions of the cracks in the weld metal of a submerged arc weld of a weld metal, polished surface etched in 3% S690Q steel nital One example for hydrogen induced cracking of an S690Q weld is shown in figure 15.1. On the polished surface cracks, as well transverse as parallel to the welding direction are present. On the photo the cracks appear bright. After etching it became evident (figure 15.2) that the defects are located in the weld metal in the center of individual welding passes or transverse in the weld metal. The cracks were arrested in the HAZ at the fusion line. How can this behaviour be explained? The welding consumables that produce tensile properties overmatching the ones of S690Q need a relatively high alloying content. A combination of 1-2.5 % nickel, 0.5-1.5% of chromium and about 0.5% molybdenum is typical here. Due to this chemical composition and its as cast microstructure weld metals for steels of 690 MPa yield strength are often more susceptible to hydrogen induced cold cracking than the HAZ of the high strength steels. In order to avoid cracking, preheat and interpass temperature must then be adapted to the weld metal. A steel with reduced carbon equivalent would not allow to drop these precautions in welding. Consequently, high strength parent material with reduced carbon equivalents, as achieved by TM– rolling, need essentially the same precautions as the higher alloyed QT steel. 9. Summary and conclusion Figure 13 compares applicable working ranges for different structural steels. The different plate thicknesses are chosen to produce the same load bearing capacity. The lower limit of the temperature is set for the avoidance of cold cracking, the upper limit for the heat input and interpass temperature are introduced by gaining sufficient tensile and toughness properties. 11 Temperature [°C] 250 max. interpass temp. 200 150 100 50 Figure 16 : S355G8+N 70 mm S690Q 30 mm S355J2G3 70 mm higher heat input min. preheat and interpass temp. S355M - 70 mm S420M - 60 mm S500M - 50 mm 1 2 3 4 5 Heat input [kJ/mm] The Offshore steels permit a wider working range than the traditional structural steels The application of low carbon TM steels still widens the range The excellent weldability as a result of the optimised chemical composition, is a main feature of the thermomechanical rolled plates. Economical advantages for the fabricator can be the omission of preheating, the application of higher weld deposition rates and omission of stress relief or tempering post weld heat treatment. This led to a steadily increasing success of TM plates in the past 2 decades. High strength QT steels belong to another league of materials that need special care in welding as can be estimated from the limited working window drawn in the diagram. Bibliographic reference 1 Baley N. et al.: „Welding steels without hydrogen cracking“, (1973) Abington publishing, ISBN 1855730146 2 STAHL-EISEN-Werkstoffblatt (SEW) 088, Beiblatt 1 'Kaltrißsicherheit beim Schweißen; Ermittlung angemessener Mindestvorwärmtemperaturen' , (1993) Verlag Stahleisen, Düsseldorf 3 EN 1011-2:2001 „Recommendations for welding of metallic materials“ 4 Berghout, C.F., und H. Crucq: 'Kaltrißsicheres Schweißen an einer Brücke ohne Vorwärmung', DVS-Berichte Bd. 131, S. 154/58. Deutscher Verlag für Schweißtechnik DVS-Verlag, Düsseldorf 1990. 5 Dillinger Hütte SVZ Berichte Schweißuntersuchungen zu BS7777, 02/2006, 03/2006 6 K. Richter, F. Hanus, P. Wolf: "Structural steels of 690 MPa yield strength - a state of art", Proc. of 2nd Symposum on High strength steels 2002 , 23.-24 April 2002 Stikklestad Norway 7 Coiffier,J.C., Th. Fröhlich et al.: "Measurement of CTOD properties in welds of 500MPa yield strength steel recently developed for offshore application", USINOR-CRDM Dunkerque, Document no 0002015 (June 2000) 8 EDF: „Etude de la soudabilitée des aciers S690QL et S500ML ...“, Journées Membres Inst. de Soudure, Villepinte,18. Sept. 2007 9 Brenner U., G. Gnirß: ' Schweißtechnische Verarbeitung von Feinkornbaustählen im Druckbehälterbau', Technische Überwachung, Bd.27 (1986), Nr.3, S 131ff. 10 De Boer, H. und U. Schriever: Experience gained in the manufacture of Vanadium-alloyed HSLA-steels, Thyssen Techn. Berichte Heft 2.S.135ff/(1989) 12