Thermo-Calc and Dictra commercial calculation software programs have been used to simulate the formation of fine microstructure of 9%Ni cryogenic steel processed by quench–lamellarize–temper(QLT)heat treatment.The calculations show many compositional fluctuations in each single-martensite lath after QLT treatment;the peak concentrations of carbon and nickel reach about 0.40%and 18%,respectively.The width of the C or Ni element-enriched zone increases when the critical lamellarizing temperature decreases or the temper temperature increases.Extending the temper time has a similar effect on the element profiles.The simulation results are in accordance with the observed fine microstructures.
The single-pass hot compressions of two 5%Ni steels with and without niobium addition at different temperatures of 800-1 150℃and strain rates of 0.01-1s-1 were performed by using a Gleeble-3500 thermal simulator and the effect of niobium on the dynamic recrystallization ( DRX ) behavior was analyzed.The results showed that the niobium addition of 0.04% can retard DRX in 5%Ni steel significantly by increasing the activation energy for DRX from 394 to 462kJ / mol.The critical strain required for starting DRX in 5%Ni steel was increased by 0.04-0.10 with niobium addition when the steel was deformed at a strain rate of 0.01s-1 and temperatures varied from 950 to 1 000℃.The critical temperature required for starting DRX in 5%Ni steel was also increased from 1 000 to 1 050℃ with niobium addition when the steel was deformed at a strain rate of 0.1s-1 .Such a retarded DRX occurring in Nb-added 5%Ni steel can be attributed to the pinning effect of precipitates containing niobium.
LEI Xiao-rongDING Qing-fengCHEN Yuan-yuanRUI Xiao-longLI Shu-ruiWANG Qing-feng
Mechanical properties of quenching,intercritical quenching and tempering (QLT) treated steel containing Ni of 9% were evaluated from specimens subject to various tempering temperatures. The detailed microstructures of steel containing Ni of 9% at different tempering temperatures were observed by optical microscope (OM) and transmission electron microscope (TEM). The volume fraction of austenite was estimated by XRD. The results show that high strength and cryogenic toughness of steel containing Ni of 9% are obtained when the tempering temperature are between 540 and 580 ℃. The microstructure keeps the dual phase lamellar structure after the intercritical quenching and there is cementite created in the Ni-rich constituents when tempering temperature is 540 ℃. When tempering temperatures are between 560 and 580 ℃,the reversed austenites (γ′) grow up and the dual phase lamellar structure is not clear. The γ′ becomes instable at 600 ℃. When tempered at temperature ranging from 500 to 520 ℃,the increase of dislocation density in the lamellar matrix makes both tensile strength and yield strength decrease. When tempered at 540 ℃ and higher temperature,the yield strength decreases continuously because the C and alloying elements in the matrix are absorbed by the cementite and the γ′,so the yield ratio is decreased by the γ′. There are two toughness mechanisms at different tempering temperatures. One is that the precipitation of cementite absorbs the carbon in the steel which plays a major role in improving cryogenic toughness at lower temperature. Another is that the γ′ and the purified matrix become major role at higher tempering temperature. When the tempering temperature is 600 ℃,the stability of γ′ is decreased quickly,even the transformation takes place at room temperature,which results in a sharp decrease of Charpy-V impact energy at 77 K. The tempering temperature range is enlarged by the special distribution of cementite and the lamellar structure.
