In this study, the weldability of lower strength St 37 grade steel plate with a higher strength steel plate (i. e., St 44) using the friction stir welding technique has been studied. St 37 grade steel sheets are widely used in various industries. However, the joining of St 37 steel plates with higher strength grades such as St 44 grade steel is usually required when the strength of St 37 grade is not sufficient. Therefore, microstructural and mechanical properties of friction stir welded St 37-St 44 butt joints were investigated by conducting optical microscopy, microhardness measurements, and tensile testing. A fine-grained recrystallized microstructure was observed in the weld regions of the joints produced. The highest tensile strength, i. e. 385 MPa, was exhibited by the joint produced at 630 rev × min−1. Kurzfassung In der diesem Beitrag zugrunde liegenden Studie wurde die Schweißbarkeit einer Platte eines Stahles St 37 mit einer niedrigeren Festigkeit mit einer Platte aus einem Stahl St 44 mit einer höheren Festigkeit unter Einsatz des Rührreibschweißprozesses (Friction Stir Welding – FSW) untersucht. Bleche aus dem Stahl St 37 werden breitflächig in verschiedenen Industriezweigen verwandt. Allerdings wird ein Verbinden mit Stählen höherer Festigkeit benötigt, wenn die Festigkeit des Stahles St 37 nicht ausreicht. Daher wurden die mikrostrukturellen und mechanischen Eigenschaften von FSW-Stumpfschweißverbindungen des Typs St 37-St 44 untersucht, indem Lichtmikroskopie, Mikrohärtemessungen und Zugversuche angewandt wurden. Es wurde eine feinkristallisierte Mikrostruktur in den Schweißverbindungen beobachtet. Die höchste Zugfestigkeit von 385 MPa ergab sich für die Schweißverbindung, die mit einer Umdrehungsgeschwindigkeit von 630 rev × min−1 hergestellt wurde.

Significant developments in joining technology to emerge in the last 30 years. The technique has originally been developed for joining difficult-to-fusion-weld Al-alloys, particularly for high strength grades and now widely used in various industrial applications, such as transport industries. On the other hand, the application of FSW to high temperature materials such as steels is hindered due to the problems associated with the stirring tools although there is a wide interest for the application of this technique to these materials. Design/methodology/approach: The aim of this review is to address the current state-of-the-art of FSW of steels, focusing particularly on microstructural aspects and the resulting properties of these joints and discuss the future prospects of this technique for steels. For instance, the use of FSW can be advantageous for joining steels in some special applications where conventional fusion welding processes fail to produce sound cost effective joints, and the high tooling costs of FSW can be justified (i.e. underwater joining of steel pipes or hot plate welding in steel mills). In this study, only structural steels (mainly plain C steels), ferritic stainless steels, austenitic stainless steels and duplex stainless steels will be considered and the other types of steels are out of the scope of this work although some examples are included in the discussion. Research limitations/implications: The tools experience high temperatures in FSW of steels, i.e., above 1000°C. The number of tool materials which can withstand such temperatures is very limited. In addition, the welding of many common steels can be readily conducted by various conventional fusion welding methods. These joining methods are very flexible, easy-to-perform and well established in industrial applications, which further prevents the application of FSW to these materials. These limitations are to be overcome for commercial exploitation of this technique for joining steels.