This bachelor’s thesis is a study on high-strength steels (HSS) and their welding, concentrating especially on the metallurgy of the heat-affected zone (HAZ). The first part of this thesis introduces general definitions of HSS and their manufacturing methods. This is followed by a literacy review on HSS welding and the special requirements involved. In the latter part of this thesis test welds are used to examine HAZ of HSS.

There is no standardized definition for HSS, but steels are commonly considered to qualify when their yield strength is over 460–550 MPa. There are three main methods of manufacturing HSS, namely QT (quenched and tempered), TMPC (thermomechanical controlled process) and DQ (direct quenching). The manufacturing method of HSS has a significant impact on its weldability.

The grain size in the coarse-grain heat affected zone (CGHAZ) formed in QT steels during welding can grow significantly, especially when higher heat inputs are used. This causes increased hardness in those regions as can be seen in test welds included in this thesis. In TMPC steels this phenomenon is usually not present. The mechanical properties of HSS seem to weaken in CGHAZ when higher heat inputs are used regardless of the manufacturing method.

The quality of HSS welds is under continuous development as research continues on materials with better weldability and welding processes suited to the particular needs of HSS welding.