Phase stability of reduced activation ferritic steel: 8%Cr-2%W-0.2%V-0.04%Ta-Fe

Issues relating to numerical modelling of creep crack growth

A neural network model has been developed on the basis of published experimental data. This allows the creep rupture strength of bainitic and martensitic electric power plant steels with compositions based on Fe–2·25Cr–1Mo and Fe–(9–12)Cr to be estimated as a function of chemical composition, heat treatment and time at temperature. This model, together with a variety of thermodynamic and kinetic calculations, has been used to propose two new alloys related to the 10CrMoW steel which in theory have outstanding creep rupture properties.

https://www.tandfonline.com/doi/pdf/10.1179/026708399101506085

Theoretical design of ferritic creep resistant steels using neural network, kinetic, and thermodynamic models

Effect of V and Nb on precipitation behavior and mechanical properties of high Cr steel

A study was done to investigate the effect of tungsten (W) addition on the microstructure, tensile properties and low cycle fatigue (LCF) properties of 9Cr-lMo steels at 298K and 873K. Four steels containing different amounts of W (0W, 1.2W, 1.8W and 2.7W) were normalized at 1323K for 1 hour and tempered at 1023K for 1 hour. Microstructural analysis revealed that no significant differences wete observed in their tempered martensitic microstructure of 0W, 1.2W and 1.8W alloys, but d-ferrite was observed to form at the prior austenite grain boundaries of the 2.7W alloy. With the increase in W content, yield and tensile strength increased at both temperatures. Low cycle fatigue life also increased with the W content up to 1.8%, but decreased in the 2.7W alloy, which was primarily due to the presence of soft d-ferrite acting as the crack initiation site. The fatigue life at 873K was reduced compared to that at 298K, due not only to the decrease in strength at high temperature but also to the formation of oxide layers along the slip bands, which increases slip irre-versibility during cyclic deformation.

Effect of W addition on the low cycle fatigue behavior of high Cr ferritic steels

The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05 pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the precipitation of Fe2Mo.

T. Takamatsu

Papers

Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb