| The strengthening effect of Nb was found in155-160ksi grade perforating gun pipe.In the paper, the strengthening and toughening mechanism study of Nb was carried out formedium carbon Cr-Mo-V-Nb steel in heat treatment processes. In compared to25CrMo48V steel, the variation of the precipitation-dissolve behavior of Nb withquenching temperature and the refinement effect of Nb on prior austenite grain andmartensite microstructure were studied in quenching process. The variation ofNb-contained precipitates and the influence of heat treatment process parameters onmicrostructure and mechanical properties were analyzed in tempering process. The mainresults are summarized here.There were three types of niobium-contained precipitates observed in as-rolled20CrMo47NbV steel, including Nb-rich (Nb, Ti, V)(C, N), Ti-rich (Ti, Nb, V)(C, N) and(Nb, V)C. the quantity of particles reduced with the quenching temperature increased from900℃to1200℃. The heating temperature had great effect on precipitation-solutionbehavior of Nb. Nb-contained particles began to precipitate at the heating temperatureabove Ac3. In the specimen quenched at900-1000℃, the quantity of (Nb, V)C particlesreduced due to the precipitated particles decreased and the dissolved particles increased.The solid solution Nb increased. In the specimen quenched at1050-1100℃,(Nb, V)Cparticles dissolved rapidly, which resulted in the increase of Nb solid solution.(Nb, V)Cparticles dissolved entirely and Ti-rich (Ti, Nb, V)(C, N) dissolved partly in the specimenquenched at1150℃. Raising the quenching temperature to1200℃, the majority ofTi-rich (Ti, Nb, V)(C, N) particles had dissolved largely, and the quantity and size ofparticles reduced. The most of Nb was in solid solution.In the quenching temperature range from900℃to1000℃, fine (Nb, V)C particlesprecipitated in the grain boundaries or matrix. These fine (Nb, V)C precipitates andNb-contained precipitates less than80nm in as-rolled steel can refine austenite grain bypining austenite grain boundaries migration. The austenite grain was finer for20CrMo47NbV steel as compared to25CrMo48V steel in quenching temperature from900℃to1200℃. The martenistic packet and block were refined due to austenite grain refinement.NbC particles precipitated from matrix and the tempered alloy carbides nucleated atNb-contained precipitates in20CrMo47NbV steel tempered in temperature range from550℃to700℃, which lead to more diffuse distribution of precipitates. The yield strengthand toughness were increased significantly for20CrMo47NbV steel as compared to25CrMo48V steel due to the refinement and precipitation strengthening effect of Nb inquenching and tempering process, respectively. |
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