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Laser Beam Welding of Superduplex Stainless Steel With Post-heat Treatment

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   20 LASER BEAM WELDING OF SUPERDUPLEX STAINLESS STEEL WITH POST-HEAT TREATMENT Ladislav SCHWARZ, Tatiana VRTOCHOVÁ, Koloman ULRICH  Authors:  Ladislav Schwarz, MSc. Eng., Tatiana Vrtochová, MSc. Eng.,    Koloman Ulrich, Profesor, PhD.  Workplace:  Department of Welding, Institution of Production Technologies, Faculty  of Materials Science and Technology, Slovak University of Technology   Address: ul.    J. Bottu 25, 917 24 Trnava, Slovak Republic   E-mail: ladislav.schwarz@stuba.sk, tatiana.vrtochova@stuba.sk,  koloman.ulrich@stuba.sk Abstract This paper investigates the structure and ferrite/austenite content of 2507 duplex stainless  steels joints by laser welding. The quality of welded joints was assessed mainly metallographically (including optical microscopy). The results of ferrite content measurement in weld metal and in base metal show, that application of suitable post heat made possible to reduce the ferrite content in weld metal by 12 %. These results suggest that such a procedure leads to positive results. Key words duplex stainless steel, laser beam welding, welding parameters, ferrite and austenite contents Introduction Duplex stainless steels are very attractive constructional materials for service in aggressive environments and in many industry branches like petrochemical, chemical and  paper, energy, gas fuel, power generations, marine transportations, etc. Such steels offer several advantages over the common austenitic stainless steels. The duplex grades are highly resistant to chloride stress corrosion cracking, have excellent pitting and crevice corrosion resistance and are about twice as strong as the common austenitic steels [1, 6]. The duplex stainless steel is two – phase steel with the structure composed of austenite and ferrite. Optimum austenite/ferrite proportion is 50 % [3]. The suitable structure is obtained by heat treatment at approximately 1050 to 1150 °C (  solution annealing  ). The optimum ratio between  both phases can be influenced by the welding processes. The duplex steels are normally weldable using welding procedures generally used for high alloyed steels. The experience with such comparatively new welding method like laser beam welding is still limited [4].   21 Characteristic of welding problem Duplex stainless steels have an optimized content of individual components ( 50 % ferrite - δ  and 50 % austenite - α ) with precondition for desired austenite proportion and also desired mechanical and corrosive properties of weld. The iron-chromium-nickel ternary phase diagram is a roadmap of the metallurgical behavior of the duplex stainless steels (Fig. 2). Duplex steel solidifies from the melt first fully as ferrite and ferrite is later  partially transformed to austenite towards temperature of 1 000 °C. For wrought alloys, the microstructure has morphology of laths of austenite in a ferrite matrix [5]. Essential  parameters for welding duplex steels are therefore heat input, cooling rate. Laser beam processes, which do not use filler metal, are not recommended since they provide welds with a high ferrite proportion, owing to low heat input and too fast cooling weld. Low heat input and too fast cooling rate may result in undesirable proportion of ferrite in weld and in the heat affected zone and in corresponding loss of toughness and corrosion resistance. Maximum average ferrite content should be within 40 to 50 %. Improvement may be achieved only  by bulk annealing after welding at temperatures 1150 to 1050 °C, what however represents an undesired operation increasing the welding costs. The effect of increasing nitrogen content is also shown in Fig. 2. Beneficial effect of nitrogen is that it raises the temperature at which the austenite begins to form from the ferrite. Therefore, even at relatively rapid cooling rates, the equilibrium level of austenite can almost  be reached [2, 4]. Programme of experiments All experiments were performed with material in form of a seamless tube without final forming and surface heat treatment, made of duplex steel type SANDVIK SAF 2507 with dimensions ø 42 x 3.5 x 500 (mm). Sandvik SAF 2507 is a high alloy superduplex ( austenitic-ferritic ) stainless steel for service in highly corrosive conditions [4]. It is characterized by excellent resistance to stress corrosion cracking in chloride-bearing environments, excellent resistance to pitting and crevice corrosion, high resistance to general corrosion, very high mechanical strength,  physical properties that offer design advantages, high resistance to erosion corrosion and corrosion fatigue and good weldability.  Fig. 1  Ternary Fe – Cr – Ni phase diagram at 70 % Fe section   22The chemical composition and mechanical properties of used steel are given in Tables 1 and 2. In order to guarantee a balanced ferrite/austenite ratio of the weld, heat treatment after welding, and solution annealing is recommended, what however represents an undesired operation increasing the welding costs. Low heat input and rapid cooling ( laser beam welding  ) rate may result in undesirable  proportion of ferrite in weld and in the heat affected zone and in corresponding loss of toughness and corrosion resistance. Maximum average ferrite content should be within 40 to 50 %. We tried to solve the mentioned drawbacks of beam welding duplex steels in the  following way:     by reducing the cooling rate applying the post-heat after welding by a defocused beam with several passes ( rotations ) along the weld zone,     by application N 2  gas during welding and post-heat operation. Welding was performed on Gas CO 2  laser machine Ferranti Photonics AF 8 with max. output 8 kW. With wave length 10.6 µm. The mentioned laser is of versatile type suitable for hardening, welding, thermal cutting and cladding. Welding tests were performed in form of  penetration run into solid material with parameters given in Table 3. Fabrication of  penetration runs was immediately followed with post-heat, performed with defocused beam with parameters given in Table 4. PARAMETERS OF WELDING Table 3 PARAMETERS OF POST HEAT Table 4 Power 4,7 kW Power 4,7 kW Welding speed 15 mm.s -1 Speed of post heat 15 mm.s -1 Shielding gas N 2  ( 18 l/min ) Shielding gas N 2  ( 18 l/min ) Thermal input 0,188 kJ.mm -1 Number of passes 3 Defocusing on surface (f = 0) Defocusing f = + 20 mm CHEMICAL COMPOSITION OF SAF 2507 STEEL (%) Table 1 C max. Si max. Mn max. P max. S max. Cr Ni Mo N 0,030 0,8 1,2 0,035 0,015 25 7 4 0,3 MECHANICAL PROPERTIES OF SAF 2507 STEEL AT 20 °C, PIPES WITH WALL THICKNESS MAX. 20 mm Table 2 Yield point R p0,2  (MPa) min. R p0,1     (MPa) min. Tensile strength R m  (MPa) Elongation  A (%) min. HRC hardness max. 550 640 800 – 1000 25 32   23 The results The samples were subjected to metallographic structural ( macrostructure and microstructure ) analysis and ferrite content measurements. Microstructure of base metal, fusion zone and weld metal was examined. Metallographic studies were performed on all specimens of welded joints. Macrostructures of laser welded joints of stainless steel SAF 2507 are given in Fig.2. The macrostuctural observations have shown that all fabricated joints were without any apparent defects like cracks or pores. In case of some weld roots overrunning was observed. Fig.2a shows the macrostructure of laser welded joints after welding. Observation of macrostructure has shown, that the width of the heat affected zone (  HAZ  ) is relatively small, the weld consists of one layers and the weld root is not overrun. Characteristic surface and root of penetration runs of laser weld joint after application post heat by defocusing laser  beam is shown in Fig.2b. Macrostructure show that the width of the heat affected zone is small, the weld consists of two layers and the weld root is slightly overrun. No inhomogeneities were found in the joint.  Fig. 2  Macrostructures of laser welded joints a) after welding, b) after application post heat Microstructural observations of welded joints were performed by use of optical microscopy. Microstructure of base metal (  BM  ) is linear, what corresponds to tubular  products. Bright particles in the photos represent austenite and the dark ones ferrite. Structural character of individual samples actually does not differ. Fig.3 shows the microstructure of laser welded joints after welding without additional heat effect of weld metal ( WM  ). Microstructure of base metal consists of ferrite with austenite islands. The fusion line is distinct, where the fused zone has polyhedral and acicular structure with finer grain than further in weld metal (Fig. 3a). Fusion zone between the weld and the  base metal is contiguous, relatively plain and without any integrity defects. These facts point to the perfect metallurgic joint of the weld and the basic material. Microstructure of weld metal is composed of ferrite and austenite is excluded on frontiers grains. (Fig. 3b). There are no non-integrity signs like cracks or poruses in the weld that would be visible to the naked eye. a Weld root b Basic materialWeld metalHeat affected zone

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