THE EXISTENCE OF INTRAGRANULAR FERRITE PLATES AND NUCLEATING INCLUSIONS IN THE HEAT-AFFECTED ZONE OF X-60 PIPE STEEL

In order to improve the heat affected zone (HAZ) toughness of X-60 pip e steel, we have applied intragranular ferrite plate (IFP) technology. The characteristic of IFP is the appearance of fine ferrite plates in side the original austenite grains. By means of suitable Re, Zr and Ti additions at high initial oxygen potentials, and good control of the peak temperature and the cooling rate during welding simulation, one c an obtain IFP contents over 50 vol% with a resultant increase in the t oughness from 55-160 J. It was found that the inclusions that were mos t effective in nucleating the IFP were deformable complex silicates wh ich either entrap Re, Zr and Ti oxides or contain these elements. The greater the number of the evenly distributed and effectively nucleatin g inclusions, the greater the IFP content, and the finer the microstru cture of the HAZ, and the greater the relevant toughness. Generally, t hese silicates behave as fine spheres along a line. The present author s show that these fine spheres result from the remelting of the shuttl e-like silicates due to heating in the process of welding simulation. These silicates contain a high sulfur capacity and thus MnS deposits a re often observed on the periphery of the silicates. The IFP was shown to be directly rooted in the Mn depletion zone which is located besid e the MnS deposits.