Effect of Microstructure Evolution on the Mechanical Properties of P92 Steel during Long-Term Thermal Aging

Tensile properties and Charpy impact toughness of P92 steel in the normalized and tempered (NT) and thermal aged (TA) at 923 K for different durations (10,000, 20,000, 30,000, and 54,300 h) were investigated. The yield stress and ultimate tensile strength of the thermal ex-posed up to 30,000-h steels were (30-24 %) lower than the NT steel over the investigated tests temperature range (298-923 K). The tensile strengths of thermal exposed for different durations up to 30,000-h steels were comparable. The drastic reduction in tensile strengths about 15-20 % was observed in the 54,300-h exposed steel as compared to the 30,000-h aged steel. The Charpy impact toughness of the P92 steel has decreased significantly in the TA con-dition (37 J) than in the NT condition (142 J). Formation and coarsening of Laves phase by loss of tungsten in the matrix, lower dislocation density, and coarser lath width in the TA steel have been attributed to the deterioration of the tensile strengths of TA steel. Martensitic lath struc-ture was recovered predominantly in the 54,300-h aged steel. Coarsening rate of Laves phase (3.5963 x 10-28 m3/s) is higher than the M23C6 (6.0877 x 10-30 m3/s) precipitates. The pres-ence of coarser Laves phase and reduction in boundary density in the TA steel has resulted in a drastic reduction in the impact toughness of the steel.