Creep Characteristics and Service Life Evaluation of Modified 12%-Cr-steels III

Project Number 266

Very good comprehension of the metal-physical processes in the creep-stressed 9-12% chromium steels is resulting from the investigation results determined in the already concluded predecessor projects (VGB-No. 200 and 233).

This was particularly due to the close co-operation with the Technical University of Denmark (John Hald) and Elsam/Energi E2 as well as with the European project partners in the COST 522/536.

Actually, it is possible to predict the creep characteristics with good exactness based upon micro-structural measuring results. The size of the sub-grains and of the excretions was determined by means of new investigation methods like EFTEM (Energy-Filtered Transmission Electron Microscopy). Growth of the excretions is determined by means of thermodynamic and kinetic modelling.

In summary, this results in the following actual state of knowledge:

  1. Creep strength is a function of the excretions and the sub-grain size, size, distribution and long-term stability of the strength-increasing excretions being the decisive factors.
  2. The sub-grain size of the different steels varies only very little so that improvement of the creep strength must be realised by optimising the excretion condition. Here, focus is on stabilising this condition throughout the overall planned service life of the components.
  3. Investigations based upon APFIM (Atom Probe Field Ion Microscopy) helped to explain the positive influence of boron on the creep strength. This is mainly due to the reduction of the M23C6-carbide coarsening. This knowledge is being realised in all new steel developments, determination of the optimum boron contents being the subject of actual and future investigations.
  4. Nitrogen forms very resistant vanadium nitrides (VN) when composed with vanadium, making the most important contribution to creep strength. Nevertheless, many of the new steels are subject to a creep strength decrease upon different periods of time (up to 80,000 operating hours). Investigations by John Hald have shown that this is in connection with appearance of a new excretion, the so-called Z-phase. Cobalt alloying in actual material development programmes (i.e. VGB No. 221 “12CrCoMo”) seems to aggravate this problem. Thus, further investigations are required to understand the Z-phase formation mechanism and thus be able to take suitable countermeasures in the development of new steels. This is going to be the most important part of the applied project. Further, alternative additives will be searched for, minimising the austenite-grain growth during heat treatment - as the alloying element niobium does.
  5. Adding molybdenum and tungsten leads to laves-phase formation, developing only at operating temperatures of 600 to 650 °C in contrast to M23C6 and MX. These excretions contribute towards creep strength. In the copper-alloyed steel P 122, fine copper-containing excretions were detected, acting as germ-developing points for laves phase. So far, this mechanism has not yet been made use of in the new COST-project test-melts. Therefore, in this follow-up project some new copper-containing steels are to be characterised by means of EFTEM and their long-term characteristics be modelled.

The results of this project can be made use of for the current research projects qualifying new materials for material optimisation regarding chemical composition and heat treatment as well as determination of the required minimum test period of the creep tests.

The project is being realised as joint venture of two Swedish institutes:

  • Chalmers University of Technology, Göteborg
    Division of Microscopy and Microanalysis
    Department of Experimental Physics
    Head of Project: Prof. Hans-Olof Andrén
  • Royal Institute of Technology, Stockholm
    Division of Physical Metallurgy
    Department of Materials Science and Engineering
    Head of Project: Prof. John Ågren

The investigations, the results of which will be available in December 2007, are supported by the Technical Committee “Materials and Quality Supervision”.