The effect of cooling rate on the mechanical and corrosion properties of SAF 2205 (UNS 31803) duplex stainless steel welds

Abstract

Department of Materials Engineering, University of Wales Swansea, Swansea, SA2 8PP, UK(Received July 3, 2000)(Accepted in revised form September 6, 2000)Keywords: Duplex stainless steels; Microstructure; Pitting; Intergranular corrosion; Impact testingIntroductionThe weldability of duplex stainless steels is related to the possibility of forming or maintaining amicrostructure with a ratio of ferrite to austenite phase of about 50:50. Approximately at least 40% ofaustenite is required to produce a nitride free weld microstructure in the heat affected zone and this isobtained by welding a material of balanced composition with a heat input that allows the austenite toform by nitrogen redistribution during cooling. Research work has also been carried out on thecomposition of the filler metals in order to obtain the desired ratio of ferrite to austenite in the weldmetal [1]. During welding and subsequently, several transformations take place in the heat-affectedzone and in the weld metal. The metallurgical aspects of these transformations have been successfullyexplained by many researchers [2], [3], [4]. However, the effect of the cooling rate on the microstructureof the welded joints and as a consequence on the strength and corrosion resistance of DSS weldments,is a matter of continuous interest.In this paper an attempt is made to correlate the impact strength and corrosion resistance of DSSweldments to the cooling rate and to explain their interdependence through microstructural observa-tions.Experimental ProcedureThe DSS used was the SAF 2205 (UNS S31803) grade in the form of 4 mm plate, manufactured byAVESTA, Sweden. The chemical analysis of the material and the welding conditions (GTAW welds)[5] are summarized in Table 1. Eight weldments with a length of 40 cm were prepared. Half of themwere left to cool slowly in air down to the ambient temperature (22°C) and the other half weresubmerged in cold water (set to 10°C) directly after welding.Metallographic specimens were cut transverse to the welding direction. They were examined usingscanning electron microscopy. Quantitative image analysis was performed in order to determine thevolume fraction of austenite and ferrite in the weld zones. Vickers hardness measurements transverseto the weld were performed using a load of 20Kg. In addition, microhardness measurements on the twophases (austenite and ferrite) were recorded, using a load of 200g.