C-Cr-Fe-Mo-V (Carbon-Chromium-Iron-Molybdenum-Vanadium)

Abstract

[1972And] prepared more than 150 compositions of three groups of steels, containing Cr-Mo, Cr-V, and Cr-Mo-V, respectively. All steels contained a constant (nominal) amount of 0.12 wt.% C and residual elements of 0.5 wt.% Mn and 0.5 wt.% Si. The samples were annealed at 650 and 700 C up to 1000 h. Phase identification was done by x-ray diffraction of residues obtained by electrolytic extraction. The alloy carbides identified include Fe3C, Cr7C3-based M7C3, Cr23C6-based M23C6, V4C3-based M4C3 (this is denoted as VC-based MC in this review), Fe2MoC ðnÞ; (Fe,Mo)6C-based M6C, and Mo2C-based M2C. The approximate composition of the metallic elements in the extracted carbides was obtained by x-ray fluorescence analysis. The ratios of the partitioning of the alloying elements between matrix and carbide were estimated and listed. For the Cr-Mo-V steels, four isothermal sections were constructed as a function of Cr and Mo at 700 and 650 C and at 0.5 and 1.0 wt.% V. Additional isothermal sections were constructed as a function of Cr and V at 700 C and at constant Mo contents of 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, and 2 wt.% Mo, respectively. [1998Vyr] vacuum-melted four Cr-Mo-V steels containing in weight percent: 0.09-0.12 C, 2.40-2.62 Cr, 0.70-0.95 Mo, and 0.02-0.34 V. Residual Mn and Si contents were about 0.7 and 0.3 wt.%, respectively. The steels were quenched from 1250 C to produce a martensitic structure and then annealed (tempered) between 500 and 720 C up to 1000 h. Extracted carbide particles were analyzed by electron diffraction and energy dispersive x-ray spectroscopy. The alloy carbides found were M3C, M23C6, M2C, M7C3, MC, and M6C. The carbides identified after each annealing treatment were listed along with those predicted from equilibrium calculations for all the four steels. Timetemperature diagrams were constructed to illustrate the carbide precipitation kinetics. In an accompanying paper, [1998Kro] carried out a more detailed thermodynamic analysis of the carbide type and its stability as a function of temperature using the CALPHAD approach. The qualitative conclusions are as follows. With increasing temperature, in steels containing more than 0.1 wt.% V, the equilibrium phases were: a + M23C6 + MC + M6C, a + M23C6 + M7C3 + MC + M6C, a + M7C3 + MC + M6C, and a + M7C3 + MC. The same equilibria were observed without MC in steels containing 0.02 wt.% V. M6C is stable for all investigated compositions at lower temperatures. It dissolves above 620-680 C. Its stability comes mainly from Mo. Full agreement between the computed carbide distribution and the experimental results is found at higher temperatures only, indicating that the carbide distribution is far from equilibrium at lower temperatures. The Mo/V ratio and the absolute Mo and V contents determine primarily the carbide evolution during tempering. [2000For] studied samples of three industrial low-alloy steels exposed to long periods (30,900-141,500 h) in power plants at the temperature interval of 500-510 C. Extracted carbide particles were analyzed by electron diffraction and energy dispersive x-ray spectroscopy. The alloy carbides found were M23C6, M2C (cph), M7C3, MC, and V4C3. The measured compositions of the carbides were listed. The experimental compositions did not agree with the calculated JPEDAV (2007) 28:286–288 DOI: 10.1007/s11669-007-9072-3 1547-7037 ASM International Section II: Phase Diagram Evaluations