Tempering process and precipitation behavior of 8%Cr-2%WTa steel

Abstract

In order to clarify the stability of precipitates in 8%Cr-2%W-0.09%Ta steel at high temperatures, tempering and precipitation behavior of the steel has been examined precisely. Tempering was performed mainly at 740°C for 0.5-1 000 h. Hardness peak was found in a tempering curve at 650-740°C for 3.75-8 h. Discontinuous changes in X-ray diffraction intensity of the matrix, apparent lattice strain and apparent crystalline particle size, which were estimated from integral breadth, the amount of extracted residue, and the Cr and Ta content in the residue were observed at 740°C for 2-12 h tempering. These discontinuous changes correspond to the rotation of preferred orientation or the generation of subgrains accompanying the dissolution and the subsequent re-precipitation of M23C6 and TaC. The re-precipitation of fine M23C6 particles is responsible for the hardness peak. Three kinds of TaC with different chemical composition and different size are found. The amount and the size of the coarse and medium TaC particles are affected by the discontinuous changes in microstructure. Fine particles of TaC grow very slowly, which is caused by the continuous dissolution of TaC due to the annihilation of precipitation site, dislocations, and re-precipitation on dislocations. Particle sizes of TaC and M23C6 estimated using X-ray diffraction peak coincide with those observed in transmission electron microscopy. © 2007 ISIJ.