抄録
It has been known that degradation of toughness in steels accompanying with a decrease in strength occurs in prolonging stress relief (SR) annealing during the fabrication of pressure vessels. This degradation of toughness was named SR embrittlement. The relationship between SR embrittlement and microstructure and fracture appearance was investigated in 1.5Mn, 1.5Mn-0.5Mo, 0.5Mn-1Cr-0.5Mo and 0.5Mn-2.15Cr-1Mo steels heat-treated to martensite structure. SR annealings were carried out at 600 to 725°C.
Carbide size, inter-carbide distance and subgrain size were increased with the progress of SR annealing. Increases in manganese, chromium and/or molybdenum suppressed the growth of carbide and subgrain and caused to retard SR embrittlement.
SR annealing did not change cleavage facet size of Charpy impact fracture surface except for Mn-Mo steel, but increased the distance of tear ridge within cleavage facets in all the steels. The amount of plastic deformation, measured by X-ray line broadening, in cleavage surfaces was decreased by SR annealing.
It is considered that SR embrittlement is mainly caused by a decrease in cleavage fracture strength due to decrease in plastic work done for crack extension. The decrease in plastic work was induced by an increase in distance of a substructure boundaries related to the formation of tear ridge. Since cracking of carbides themselves or cracking between carbides and matrix occur in a steel containing carbides of more than 1 μm in diameter, that also decreased cleavage fracture strength and then caused SR embrittlement.
