**Michał Landowski \* , Aleksandra Swierczy ´nska ´ , Grzegorz Rogalski and Dariusz Fydrych**

Faculty of Mechanical Engineering, Gda ´nsk University of Technology, 80-233 Gda ´nsk, Poland; aleksandra.swierczynska@pg.edu.pl (A.S.); grzegorz.rogalski@pg.edu.pl (G.R.); ´ dariusz.fydrych@pg.edu.pl (D.F.)

**\*** Correspondence: michal.landowski@pg.edu.pl; Tel.: +48-583-471-291

Received: 1 June 2020; Accepted: 28 June 2020; Published: 30 June 2020

**Abstract:** This study presents results of experimental tests on quality of dissimilar welded joints between 316L austenitic and 2304 lean duplex stainless steels, welded without ceramic backing. Fiber laser welded butt joints at a thickness of 8 mm were subjected to non-destructive testing (visual and penetrant), destructive testing (static tensile test, bending test, and microhardness measurements) and structure observations (macro- and microscopic examinations, SEM, element distribution characteristics, and ferrite content measurements). Non-destructive tests and metallographic examinations showed that the welded joints meet the acceptance criteria for B level in accordance with EN ISO 13919–1 standard. Also the results of the destructive tests confirmed the high quality of the joints: specimens were fractured in base material with lower strength—316L austenitic stainless steel and a 180◦ bending angle was obtained confirming the high plasticity of the joints. Microscopic examination, SEM and EDS analysis showed the distribution of alloying elements in joints. The microhardness of the autogenous weld metal was higher by about 20 HV0.2 than that of the lean duplex steel. Ferrite content in the root was about 37% higher than in the face of the weld. The Schaeffler phase diagram was used to predict the phase composition of the welded joints and sufficient compliance with the magnetic method was found. The presented procedure can be used for welding of 316L–2304 stainless steels dissimilar welded joints of 8 mm thickness without ceramic backing.

**Keywords:** laser welding; austenitic stainless steel; lean duplex stainless steel; dissimilar welded joints; mechanical properties; microstructure
