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

摘要

In order to improve the heat affected zone (HAZ) toughness of X-60 pipe steel, we have applied intragranular ferrite plate (IFP) technology. The characteristic of IFP is the appearance of fine ferrite plates inside 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 can obtain IFP contents over 50 vol% with a resultant increase in the toughness from 55-160 J. It was found that the inclusions that were most effective in nucleating the IFP were deformable complex silicates which either entrap Re, Zr and Ti oxides or contain these elements. The greater the number of the evenly distributed and effectively nucleating inclusions, the greater the IFP content, and the finer the microstructure of the HAZ, and the greater the relevant toughness. Generally, these silicates behave as fine spheres along a line. The present authors show that these fine spheres result from the remelting of the shuttle-like silicates due to heating in the process of welding simulation. These silicates contain a high sulfur capacity and thus MnS deposits are often observed on the periphery of the silicates. The IFP was shown to be directly rooted in the Mn depletion zone which is located beside the MnS deposits. [References: 5]