Surface treatment of C80U steel by long CO2 laser pulses

The paper presents the results of laser-melted C80U steel. The processed steel was placed between two permanent magnets and laser beam whose scanning velocity was 10 mm/s. CO2 laser beam was working in pulse mode. Pulses were generated at 100% of the average preset power of 700 W, with 45 ms irradiation, zero interval between pulses and beginning of pulse repetition upon the achievement of the average laser power. During the operation, the laser beam generated plasma. The cyclic process of plasma generation induced by long laser pulses was validated experimentally with the use of a high-speed camera. Calculated speed of the plasma propagation velocity was 5.255 × 102 cm/s. The melted path had characteristic “folds”, and it comprised of a series of overlapping melted areas. The distance between adjacent “folds” correlated with beam displacement velocity which indicates that plasma plumes were generated cyclically at the beginning of the generation of successive laser pulses. Hardness test of the processed steel revealed that surface irregularities were caused by mechanical effects of pulsed plasma. The hardness of the analyzed C80U steel varied significantly between areas of measurement. In selected measurement points, hardness values reached 3200-3100 HV, 1800 HV and 400-300 HV. XRD and SAD investigation revealed in microstructure of processed steel martensite, partially dissolved cementite (FeCrMn)3C and small amount of retained austenite. The microstructure revealed by TEM indicates that in areas with greater hardness there were higher dislocations and twins density. Hardness test with a multiple load cycle with increasing load from 0.5 N to 20 N showed that shock waves formed as a result of plasma plume propagation interacting with the melted steel, thus contributing to the strain hardening of processed steel and providing an occurrence of the biggest residual stresses near the surface and reducing them with the depth of melted steel. Impact stress generated by extending plasma also facilitated the phase transformation of austenite and thus reduced the amount of retained austenite.