Temperature evolution, phase ratio and corrosion resistance of duplex stainless steels treated by laser surface heat treatment

Abstract

• Laser surface heat treatment (LSHT) was performed on duplex stainless steels 2304, 2205, and 2507. • The effect of laser spot size and cooling time t 12/8 was investigated by numerical simulation and microstructure analysis. • LSHT obtained the peak temperature, the t 12/8 and the effective depth as 1350 °C, 8.9 s and 1.5 mm respectively. • Austenite percentage increased from 2%, 5%, and 38% to 27%, 36% and 55 % respectively for 2304, 2205, and 2507. • LSHT increased the corrosion pitting temperature from 13, 42 and 66 °C to 19, 52 and 76 °C respectively for 2304, 2205, and 2507. Duplex stainless steels (DSS) are promising materials due to excellent corrosion resistance and good mechanical properties. However, the corrosion resistance of DSS welded joints deteriorates unavoidably due to the unbalanced phase ratio, namely a low austenite percentage resulted from the welding thermal cycle. Laser surface heat treatment (LSHT) exhibits tremendous advantages to restore the corrosion resistance especially regarding efficiency and flexibility. By adjusting laser energy input, including laser power and spot size, different LSHT conditions were applied to DSS 2304, 2205 and 2507, whose austenite content were deliberately decreased to a very low percentage. The temperature evolution during the LSHT was calculated out by finite element analysis. Critical pitting temperature (CPT) was measured to evaluate the corrosion resistance. A long length of laser spot and a slow scanning speed were beneficial for increasing the austenite percentage during the LSHT. With the length of laser spot as 100 mm and the scan speed as 2 mm/s, the laser power was adjusted to obtain a target peak temperature of 1350 ℃, which provided a cooling time t 12/8 as 8.9 s and an effective action depth as 1.5 mm. For DSS 2304 and 2205, the austenite percentage increased from 2% and 5% in deteriorated samples to 28 % and 56 % after the LSHT; the CPT increased from 12.5 and 42.3 ℃ to 19.2 and 51.8 ℃ respectively. DSS 2507 remained the highest austenite percentage after the same deterioration and exhibited the best corrosion resistance under all the used conditions. The influence of LSHT processing parameters and material compositions on the phase transformation and the corrosion resistance was discussed accordingly.