Welding of duplex stainless steel SAF 2507 by laser beam.

Pikna, Lukas ; Ulrich, Koloman ; Kolenic, Frantisek 等

1. INTRODUCTION

Modern duplex (dual-phase) stainless steels have an optimized content of particular elements (50 % ferrite [alpha], 50 % austenite [gamma]) with assumption for desired ratio of austenite in HAZ and thereby also required mechanical and corrosion properties of weld joint. Finding the role of nitrogen, which is strongly austenite forming element and optimizes ratio of [alpha]/[gamma], is weldability of duplex steels comparable with weldability of austenitic steels.

Laser beam is a technology with a high concentration of energy. Laser welding may be used for almost all weldable materials The welding process is protected by inert gas.

We are focused on research of weld joint produced by laser beam with regard to anticorrosion properties, that are critical for pre exploitation of welded construction, mainly when the laser beam welding are not suitable methods of welding for these steels (1,2).

2. USED MATERIAL

Welding tests were performed on tube with diameter 42 mm, wall thickness 3,5 mm from duplex steel Sandvik SAF 2507.Chemical composition is in table 1. Steel have following properties : high resistance to uniform, pitting, intergranular corrosion, resistance to corrosion cracking, eroson and cavitation, good resistance to high temperatures and physical properties (1,2).

Application: material is suitable for in tube production, refinery, in solution contamined with chlorine. This steel is also partially sutiable for heat excharger, where chloride or brackich water are used as coolant. Vessels for manipulation with organic acids and sulphuric acid and acid mixture. High strength makes this material an attractive alterante of austenitic steels (3,4,5).

Duplex Sandvik SAF 2507 have austenite--ferrite microstructure, which does not contain carbides in boundaries and intermetallic phases. Ferrite content is 40-60% .(fig. 1)

[FIGURE 1 OMITTED]

3. WELDING

Welding was performed on:

--Gas C[O.sub.2] laser machine Ferranti Photonics AF 8 with max. output 8 kW. With wave length 10,6 [micro]m.

On the base of effect of weld parameters to weld forming, its showed that the best weld joint are produced by parameters listed in the next table.

[FIGURE 2 OMITTED]

4. EVALUATION

The corrosion test ASTM A 923-01 and metallographic research were performed on specimens (laser-beam welding 15mm/s).

Metallographic analysis

Determination of ferrite content was performed in specimens in high, head part of joint. Both structural phases were specified by ASTM E 562, with these results:

Minimal ferrite content 39 %.

Corrosion test

For examination the corrosion resistance of joint of duplex steels we used ASTM G48--in 6% Fe[Cl.sub.3], which serves to estimating the parameters to sensitivity to piting and crevice corrosion. In ASTM is specified the test temperature for duplex steel in testing solution to 40[degrees]C for products with joint (for products without joint is specified to 50[degrees]C).

Test result was evaluated by visual examinating under microscop with 20x zoom.

Surface of these specimens are showed on next figures:

[FIGURE 3 OMITTED]

Pitting attack only in place, when the joint was finished, on internal and external surface of tube. Base material is not attacked. Validation, that the pitting attack is not only the function of surface status of joint, is the weld etching on the side of treated specimen. This detection may be the reason for the discussion about the efficiency of chemical surfacing of joint surface.

5. RESULTS AND CONCLUSION

Specimen complied the corrosion test G48 after welding and STS at basic temperature 40[degrees]C, but at the temperature 45[degrees]C is unsufficient. Continuation in development of this type of welding may be suitable. We must focus on elimination of surface uncompactibilities of joint, finishing of joint without piping and elimination of spatter on internal side of tube.

Heat treatment, simultaneous thermal treatment on laser machine increase the austenite content. It's application is practical and decreases costs on production of final product when the classical thermal treatment i sused, solution annealing.

It is possible predict, that corrosion resistance on external surface may be increased,

--post heat treatment 10500 C/15 min. rapid cooling (best in water), which increase austenite content--unpractical for weldment--applicable in welding of tubes,

--suitable chemical treatment of external surface (elimination degenerated layer),

--preheating or heating of joint by defocusing beam,

--to achieving balanced content of austenite and ferite weld metal is required heat input 0,2-1,5 kJ/mm for SAF 2507

--improvement of austenitization of weld metal can be achieved using additional material with increased nickel content and using shielded gas with 2-4% Nitrogen content (both elements are autenitoforming).

Acknowledgements

The contribution was prepared under the support of AP VV-0057-07, VMSP-P-0008-07 projects.

6. REFERENCES

Krecek, Ivan. Duplex stainless steels, metalurgy and their welding. In ZVARAC 1/2006.

Pikna, Lukas. Welding of stainless duplex steel by laser beam. In ZVARAC 1/2008.

Pikna, Lukas--Kolenic, Frantisek--Izdinska, Zita Fodrek, P.--Ulrich, Koloman--Emmer, Stefan--Kovac, L'.: Influence of EB Welding Parameters on Properties of Welded Joints in Duplex steel type SAF 2205. APVT-20-P01305. In: 60th Annual Assembly and International Conference: Proceedings of the IIW International Conference Welding and Materials. Technical, Economic and Ecological Aspects. 01-08 July 2007 Dubrovnik & Cavtat. Croatia.--Zagreb : Croatian welding society, 2007.

Pikna, Lukas--Izdinska, Zita--Kolenic, Frantisek--Izdinsky, Karol: Welding parameters influence to the weld properties of duplex stainless steel created by electron beam. In: Technologia 2007: Zbornlk abstraktov. 10. medzinarodna konferencia. Bratislava, 19-20.9.2007.--Bratislava : STU v Bratislave, 2007

Pikna, Lukas--Izdinska, Zita--Kolenic, Frantisek--Izdinsky, Karol: Welding of stainless duplex steel SAF 2205 by electron beam. In: Technologia 2007 : Zbornlk abstraktov. 10. medzinarodna konferencia. Bratislava, 19-20.9.2007. --Bratislava : STU v Bratislave, 2007

Tab. 1. Chemical composition of duplex
steel Sandvik

C max. Si Mn P S max.
 max. max. max.

0,030 0,8 1,2 0,035 0,015

C max. Cr Ni Mo N

0,030 25 7 4 0,3

Tab. 2. Parameters of welding

Specimen 29

Power 4,5 kW

Welding speed 15 mm.[s.sup.-1]

Shielded gas He

Welding Focus on surface

Simultaneous Thermal Treatment + 20 mm (above
(STT) Focus the surface)