Design of a new generation of 12% chromium steels
From the first generation of martensitic steels, X22CrMoV12 1 developed in 1950’s, to the state of the art, P92 developed in 1980’s, creep strength of this type of steels has doubled at 600°C.
World-wide efforts are trying to push the possible working environment of 9 12% Cr steels to 650°C/325 bar, which is believed to be the upper limit for 9-12% Cr steels. But unfortunately none of the present 9-12% Cr steels fulfil the requirements concerning both creep and corrosion simultaneously. All attempts to achieve the goal turned out to be failures.
A novel steel design concept developed within the European COST Action 536 proposed a new alloy design concept, which makes use of Z-phase as strengthening dispersion for 9-12% Cr martensitic steels, instead of MX. The new steels are supposed to be strengthened by fine Z-phase precipitates, which are formed during heat treatment or at the early stage of application. It is expected that the steels possess both good creep and corrosion resistance, which is attributed to dense distribution of stable and fine Z-phase precipitates and a high Cr content (12 at.%).
A systematic research shall be carried out in the project covering the following parts:
- A number of Z-phase strengthened test alloys with different heat treatments and in the as-tempered state will undergo tensile strength test and impact toughness test. Selected alloys will be characterised by SEM, EFTEM and APT.
- Some experimental steels which have been shown promising comprehensive properties will be selected to carry on long-term creep test at 650°C.
- The optimal heat treatment process has to be elaborated for the new steels and is supported by SEM equipped with EDX and an EBSD system to investigate the microstructure evolution of the steels on a relatively large scale. EFTEM will be used to track the evolution of precipitates of sub-micrometer-size. APT is used to directly investigate the number density of the very small precipitates at the initial stages, and also to track the chemical composition changes of the matrix and the precipitates, thus predicting the possible precipitation reactions.
- The element Ta has compared to other Z-phase forming elements, for example Nb, the special effect of inducing very fine and densely distributed Z-phase precipitates. The underlying mechanisms will be studied in much greater detail by APT and TEM. A combination of the chemical information with phase information can help to understand the mechanisms behind Ta addition and accelerated Z-phase formation.
- The element B is a very beneficial element in terms of creep, which is believed to connect to its ability of hindering the coarsening of M23C6 carbides. How B possibly influences the nucleation and growth process of the precipitates at the initial stages of aging will be studied.
The project is substantially funded by means of Swedish’s public authority and manufacturers.
The project is supported by the VGB-Technical Committee “Materials and Quality Supervision".