INTRODUCTION
Surface modification of type 18/8 stainless steel was carried out using a 3 kW CO2 laser with continuous and pulse wave and with 25%, 50% & 70% overlapping. After laser surface modification, the material was sensitized at 650°C for 9 hours. The degree of sensitization (DOS) was determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the susceptibility to intergranular corrosion (IGC) was determined by practice B, A 262, ASTM. The DL-EPR value of base material was 4.52 whereas on the laser modified surfaces it was in the range of 0.11 to 0.91. After the IGC test, the microstructure was examined over the cross section of the sample which revealed much less attack on laser melted side compared to the base material. X-ray diffraction revealed presence of about 4% ferrite in the laser melted regions. As sensitization is mainly a grain boundary phenomena, the nature of grain boundaries were examined by orientation imaging microscopy (OIM). The results are discussed to explain the reasons for the low DL-EPR values on the laser melted surfaces. The present study demonstrates a clear possibility of remarkable improvement in DOS and resistance to IGC by laser surface modification.
Austenitic stainless steels are used in a variety of industries mainly due to the combination of good corrosion resistance, strength and workability1-3. Though these stainless steels have a high resistance to uniform corrosion, these are prone to localized corrosion like crevice, pitting, intergranular corrosion (IGC) and stress corrosion cracking (SCC). The crevice, pitting and in many cases the susceptibility to stress corrosion cracking is mainly due to the presence of chloride/halide ions in environments in which these stainless steels operate. A particular form of corrosion may dominate in a particular industry. In nuclear fuel reprocessing and waste management industries and in many chemical industries in which nitric acid is the process fluid, the main corrosion problem is intergranular corrosion4,5. The basic cause of IGC is sensitization of stainless steels. Sensitization1-5 refers to the formation of chromium rich carbides at grain boundaries and of chromium depletion regions around these precipitates when the stainless steels are exposed to a temperature range of 500 â?? 800°C. This exposure occurs during welding and also during different fabrication processes/high temperature operation. Once the chromium depletion is below 12 wt%, the depleted regions have a much weaker passive film on the surface that is liable to easy breakage when these stainless steels are exposed to a corrosive environment. In fact, sensitization is also a main reason for intergranular stress corrosion cracking (IGSCC) of stainless steel weldments in certain environments e.g. oxidizing water chemistry in boiling water reactors6.
There have been some attempts earlier at laser surface melting (LSM) on austenitic stainless steels7-12. It has been shown that the microstructure of the LSM layer is different from the base material. LSM of sensitized stainless steel causes dissolution of carbides thereby releasing chromium to erase out the chromium depletion.
