A STUDY OF THE SHIELDING GASES INFLUENCE ON THE LASER BEAM WELDING OF

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1 A STUDY OF THE SHIELDING GASES INFLUENCE ON THE LASER BEAM WELDING OF 22Cr-5Ni-3Mo DUPLEX STAINLES STEEL Tatiana VRTOCHOVÁ a, Ladislav SCHWARZ a, Koloman ULRICH a, Petr KÁBRT b a Faculty of Materials Science and Technology in Trnava, Slovak University of Technology, J. Bottu 23, Trnava, Slovak Republic tatiana.vrtochova@stuba.sk, ladislav.schwarz@stuba.sk, koloman.ulrich@stuba.sk b Sandvik Chomutov, s r.o., Vít zslava Nezvala 5502, Chomutov, Czech Republic, petr.kabrt@sandvik.com Abstract Procedures for welding duplex steel conventional arc welding methods were studied by several authors and nowadays and recently they are well handled. Welding of two-phase materials by laser beam especially required in practice. In the present paper we describe weldability of duplex stainless steel type 22Cr 5Ni 3Mo (SAF 2205) with laser beam welding. The aim of this contribution is to present the results of testing shielding gas by welding duplex stainless steel with laser beam. Major factor in the process of welding duplex steel, is the impact of shielding atmosphere, which affects a large proportion of properties of welded joints in terms of structural components of ferrite - austenite and corrosion resistance. This fact is used in welding the laser beam to achieve an appropriate ratio of structural components of the atmosphere affects trade. Especially as nitrogen element in the process of welding is very important, because more nitrogen promotes austenite. Welding was performed on Gas CO 2 laser machine Ferranti Photonics AF 8 with max. output 8 kw and wave length 10.6 µm. Samples were analyzed in terms of microstructure, we examined the mechanical properties, hardness of welded joints and the proportion of structural components A/F in the weld metal. We supposed that the nitrogen as shielding gas could affect to the temperature at which the austenite begins to form from the ferrite, it has resulting in change of volume proportion ferrite/austenite phase in the weld metal [1,2]. Key words: duplex stainless steel, laser beam welding, nitrogen, ferrite and austenit content, microstructure 1. INTRODUCTION Duplex steels, together with austenite, ferrite, martenzite and precipitation-hardened steels form a group of stainless steels. The whole group of stainless steels is characterized by their monostructure, while duplex steels are the only members of the group which have a biphase structure composed of ferrite and austenite in approximately the same amounts. A balanced ferrite-austenite ratio is ensured by the use of basic alloying elements (chrome and nickel) and other alloying elements (nitrogen, molybdenum, copper, silicon and tungsten). Together with the chemical composition, the heat treatment regime, i.e. the rate of cooling in the first place, plays an important role in obtaining the required structure. Such microstructure contributes to good properties of these steels, thus making them superior to other stainless steels and steels from other groups for certain applications. Duplex steels rank highly primarily due to their good combination of mechanical properties and excellent corrosion resistance. Therefore, they have found a wide field of application in the petrochemical, food, chemical, pulp and paper, petroleum, and transport industries and in tanker building. Standard austenite stainless steels are being replaced by duplex steels in various steel structures [1]. 1

2 1.1 Effect of nitrogen on microstructure of duplex welds Duplex steel solidifies from the melt first fully as ferrite and ferrite is later partially transformed to austenite towards temperature of C (Fig.1). 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. 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 is shown in Fig.1. 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. In the second generation duplex stainless steels, this effect reduces the problem of excess ferrite in the weld and HAZ [3, 4]. 2. EXPERIMENTAL INVESTIGATIONS Fig.1 Vertical section of the ternary Fe-Cr-Ni system at 70 % iron 2.1 Tested material and experimental procedure 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 2205 with dimensions Ø 42 x 2,7 x 200 (mm). The chemical composition and mechanical properties of the used steel are given in Tables 1 and 2. Tab.1 Chemical composition of SAF 2205 steel (%) C max. Si max. Mn max. P max. S max. Cr Ni Mo N 0,030 0,8 1,2 0,035 0, ,3 Tab.2 Mechanical properties of SAF 2205 steel at 20 C Yield point R p0,2 (MPa) min. R p0,1 (MPa) min. Tensile strength R m (MPa) Elongation A (%) min. Hardness max. (HRC) Welding tests were carried out by producing beads on the tubes. The tubes were clamped in flat position and welds were performed in a single pass using a 4 kw transverse flow CO 2 laser. A 20 l.min -1 flow of N 2 was used as shielding gas. In a first phase of the investigation, a factorial plan was carried out in order to optimise welding parameters in terms of metallurgical and geometrical characteristics of the beads. The absence of relevant welding defects such as solidification cracks or porosity and a regular shape with full penetration of the weld bead were considered as reference parameters. Welding speed and focus height (distance of laser beam focus from material surface, positive if over the surface) were selected as factors, while the laser power was kept constant at 4 kw. Outer surface at welding was protected by N 2 (He) gas and the root was shielded with N 2 (He). 2

3 Tab.3 Laser parameters used at welding Sample Laser power [kw] Welding speed [mm.s -1 ] Shielding gas surface/root Gas flow rate [l.min -1 ] Fokus height [mm] Heat input [kj.cm -1 ] He/He , N 2 /N ,6 Transverse to weld samples were cut from the tubes and prepared for metallographic analyses by standard grinding and polishing and by etching. Microexaminations were carried out by optical. Quantitative image analysis was performed on optical micrographs taken from the weld bead in order to calculate the ferrite and austenite volume fractions. Determination of the phase volume fractions was done by the manual point count method on micrographs taken at 500x magnifications, in accordance to ASTM E-562 standard. Microhardness measurements were also performed on the prepared samples, both longitudinally and transversally to the weld axis. A Vickers microhardness tester (HV5) with a load on the indenter of 49 N was used for this purpose. 3. RESULTS Experiments include evaluation of the formation created welds, further analysis of the macrostructure and microstructure and the determination of the proportion of ferrite in the weld metal. 3.1 Characteristic surface of welds Assessed for the formation of the weld is not only an aesthetic character, given that the pores, splash as well as other errors, whether on the outside or the inside (the root) of the weld defects are unacceptable and may cause a reduction in the required properties of welded constructions and weldments and thus may lead to degradation and damage. In the Fig.2 can be seen welds created by the same welding parameters, using two types of shielding gas, helium and nitrogen. Fig.2 Characteristic surface and root with shielding gases a) Helium b) Nitrogen 3

4 3.2 Analysis of macrostructure Macroscopic observation was made on both samples to be accurately observed effects of welding and protective atmosphere for the geometry and shape of the weld. In both macrostructure can observe some of the excess weld metal base material, which means that smelting was a slight increase in volume of the weld. Welded surface and the angle of transition from weld to base material were suitable. Welded joints were also evaluated in terms of integrity. There were no technological defects. In neither case were observed in the weld metal, or heat-affected zone cracks, pores or shrinkage. Fig. 3a documents macrostructure of the cross section at laser beam welding with helium shielding gas. Fig. 3b documents macrostructure of the cross section at laser beam welding with nitrogen shielding gas. Fig.3 Macrostructure of welds with shielding gases a) Helium b) Nitrogen 3.3 Analysis of microstructure Microstructural observations of welded joints were performed by use of light 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.4 shows the microstructure of laser weld joints after welding with helium shielding gas. Fig.4 Microstructure of welds with helium 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.4a). 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 4

5 metal is composed of ferrite and on frontiers grains is excluded austenite (Fig.4b). There are no non-integrity signs like cracks or poruses in the weld that would be visible to the naked eye. Fig. 5 shows the microstructure of laser weld joints after welding with nitrogen shielding gas. The fusion line is distinct, where the fused zone has polyhedral and acicular structure with finer grain than further in weld metal (Fig.5a). Fusion zone between the weld and the base metal is without any integrity defects. These facts point to the good metallurgic joint of the weld and the basic material. The matrix of weld is formed of ferrite and austenite forms a network along the grain boundaries (Fig.5b). Fig.5 Microstructure of welds with nitrogen Austenite is excluded on frontiers grains the ferritic grains in form of massive particles. Structure coarsening was observed in the upper part of weld. Typical columnar ferrite grains can be observed along the boundaries with excluded austenite, which had partially dendritic character and in some zones also acicular morphology was observed. There are no non-integrity signs like cracks or poruses in the weld and his surroundings. 3.4 Quantitative analysis of microstructure Measurement of ferrite proportion was performed by the test according to ASTM E 562 standard, which specifies the ferrite amount in percentual content of ferrite. It employs a classical metallographic procedure for sample preparation and a coordinate network. The test is evaluated on the basis of counting the points of pervading the selected phase with coordinate network. Ferrite content in these samples is shown in Table 4. Tab.4 Ferrite content in welded joints Sample With helium Point of measurement Weld surface Weld centre Weld root Min. ferrite content (%) Max. Ferrite content (%) Aver. Ferrite content (%) In given point Total 76 With nitrogen Weld surface Weld centre Weld root Ferrite content is high in welded joints with shielding gas as helium. The ferrite % in weld metal was in all cases higher than in the base metal. Generally the content of ferrite > 70% is considered as high. Acceptable phase balance is usually limited in range 35-60%.Ferrite content in all weld metals of tested variants is highest in the root zone of penetration runs compared to surface and centre of penetration depth. In welded 5

6 joints with using nitrogen as shielding gas is 44 % what is very favorable and corresponds to requirements for base metal. 4. CONCLUSION The contribution deals with laser welding of duplex stainless steel. The welded joints were fabricated on a CO 2 laser type AF 8 in the First Welding Company Inc. in Bratislava. The quality of welded joints was evaluated metallographic (by optical microscopy), and measurement of ferrite proportion of weld joints. Based on the experiments performed on laser beam welded joints fabricated in duplex stainless steel type SAF 2205, the following can be stated. The structure of reference weld metal consisted of columnar ferrite grains with austenite precipitated on the grain boundaries. In specimens welded with nitrogen as shielding gas, the grain refining and precipitation of austenite also inside ferritic particles occurred. The welded joints were assessed from the viewpoint of soundness and joint shape. In no case the defects like cracks or pores were observed. Weld root overrunning was not observed. The results of ferrite % measurement in weld metal and in base metal performed by to ASTM E 562 allowed determine the ferrite % in individual welded joints. It was proved, that application of nitrogen shielding gas made possible to reduce the ferrite content in weld metal even by 32 %. Based on the results of experimental activities it may be concluded that the CO 2 laser is suitable for welding of the duplex stainless steel type 2205 with using nitrogen shielding gas at welding. Technology of welding with using nitrogen as shielding gas meets preconditions to replace the classical welding technologies for specially application. REFERENCES [1.] REDE, V., GRILEC, K. Microstructural Transformations of a Duplex Steel Weld and their Influence on the Particle and Cavitation Erosion Resistance. In: Strojarstvo, 2007, vol.49, no.3, p [2.] HRIV ÁK, I.: Dual-Phase Stainless Steels material of pipelines for aggressive media. In: Corrosion of Underground Structures Proceedings of the 13th International Conference Corrosion of underground structures 2003, 27 and 28 may Ko ice: technical University in Ko ice, ISBN p [3.] LIPPOLD, J, C., KOTECKI, D,J.: Welding Metallurgy and Weldability of Stainless Steels. New Jersey: WILEY-INTERSCIENCE, p. ISBN [4.] WESTIN, E.M.: Microstructure and properties of welds in the lean duplex stainless steel LDX Stockholm, Sweden, p. ISBN