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For a significant increase in the strength of quenched and tempered, high-strength structural steels with a yield strength > 690 MPa, the addition of micro-alloying elements such as Nb and Ti is essential. The standard specifications for the chemical composition of these steels (e.g. in DIN EN 10025-6) often only specify limit contents for the manufacturer to achieve the specified properties. The effect of the alloying elements or their carbides and/or nitrides is sometimes completely contrary, especially in the case of dissolution and re-precipitation in the HAZ with identical filler metal. This makes it difficult to adequately predict the batch dependency regarding the weldability and the load-bearing behaviour of the welded joint. In addition, modern welding processes, such as GMAW with modified spray arc, can negatively influence the phase transformation in the heat-affected zone due to their high heat input. An undesirable property is the softening of the HAZ, as well as the opposite effect of hardening. Against this background, microalloying routes with varying Ti and Nb contents of the high-strength and quenched and tempered structural steel S690QL are systematically investigated within the framework of a DFG project. For this purpose, GMAW welding with modified spray arc is used, which is characterised by high deposition rate and allows narrower joint opening angles (α = 30°). The effect of metallurgical composition in combination with high heat exposure on the formation of a critical HAZ microstructure zone with softening and/or excessive hardening is investigated on these welds. Special attention is paid to the phase transformations and precipitation behaviour in the microstructure of the heat-affected zone and weld metal. In addition to extensive metallographic investigations of individual HAZ areas, thermodynamic phase calculations were carried out using ThermoCalc while varying the chemical composition. This provides an understanding of phase transformation and precipitation growth and dissolution during welding as a function of temperature and cooling conditions. The aim is to show the influence of the heat effect on the microstructure formation in the HAZ by means of initial investigations. Particular attention is paid here to the effect of the different microalloying concepts (Ti or Nb).
In many industrial steel construction branches, like mobile cranes and the offshore sector, high-strength fine-grained structural steels with a yield strength of over 690 MPa have long been used. To guarantee the necessary load-bearing capacity, the welding processing of these steels requires reliable knowledge of the complex interaction between the welding process, its underlying heat input and corresponding cooling conditions, chemical composition of base and filler materials, and resulting metallurgical phenomena in the weld seam and HAZ. Microalloying elements such as Ti and Nb make an indispensable contribution to increasing the strength by precipitation hardening. Previous investigation has shown that significant influence of the microalloying elements on the phase transformation can be assumed regarding the hardening and/or softening of the heat-affected zone. The standard specs for the chemical composition according to DIN EN 10025-6 often just specify chemical limitations for the manufacturer to achieve the desired mechanical properties. The effect of the alloying elements and the corresponding precipitates (carbides and/or nitrides) is sometimes entirely contrary, especially in case of dissolution and re-precipitation in the heat-affected zone (HAZ) with identical filler metal. This makes an adequate prediction of the batch dependency with regard to weldability and the load-bearing capacity of the welded joint difficult. The joining of these steels is mainly carried out by gas metal arc welding (GMAW). Modern inverter welding sources and micro-electronics control technology enabled the use of controlled arc variants, such as the modified spray arc (Mod. SA). Several characteristics from this arc variant e.g. reduced weld seam opening angles and increased deposition rates. Whereby this high heat exposure can have a negative effect on the phase transformation in the HAZ. An undesirable property is the softening of the HAZ, as well as the opposite effect of hardening.