Investigation of the Time-dependent Strength and Deformation Characteristics of Superheater Tube Materials after Cold Deformation

Project Number 301

The project continues the investigations begun by the AiF-funded VGB research project 241 "Influence of Cold-Forming on the Creep Rupture Behaviour of Austenitic Steels and Nickel-based Alloys".

In the forerunner project, a series of austenitic steels and a typical nickel-based alloy of type Alloy 617 were examined for the effects of cold deformation on their creep behaviour.

Up to an induced cold deformation of 30 %, the tube web samples, manufactured from straight segments of tube, show the following characteristics:

  • The creep elongations achieved are dependent on the cold deformation.
  • The creep speed decreases as the degree of deformation increases.
  • No worsening of the creep characteristics has yet been established in connection with increasing drawing-out ratios.
  • In the creep test, cold-deformed samples show a distinct reduction in elongation and in local area reduction at fractures.
  • Cold-deformed samples have a tendency to brittle fracture behaviour, which makes evaluation in operation more difficult.

This knowledge is currently limited to experiments lasting up to approx. 10,000 hours, however.

In addition to this, further materials will be examined in the planned project:

  • DMV 304 HCu (DMV),
  • AC 66 mod. (DMV),
  • Sanicro 25 (Sandvik),
  • Alloy 263 (ThyssenKrupp VDM).

In a newly started partial project, the relaxation sensitivity of the materials AC 66 mod., Sanicro 25, Alloy 263 and Alloy 617 will be examined separately for thick and thin-walled tubes using a Gleeble system.

In a further newly started partial project, the effects of cold deformation on creep behaviour of Alloy 740 (Special Metals) will be examined. As for the other cold deformed materials, hot tensile tests, creep tests until 30,000 hours, annealing tests and slow hot tensile tests at 0 and 30 % deformation will be performed. 

The project is being conducted from January 1, 2007, until December 31, 2010, under the supervision of Prof. Christina Berger at the Institute for Materials Science of the Technical University of Darmstadt, with the participation of the Fraunhofer Institute for Material Mechanics (Prof. Peter Gumbsch), Freiburg. It is predominantly being financed by the metalworking industry research association at the Arbeitsgemeinschaft der Eisen und Metall verarbeitenden Industrie e. V. (AVIF, project number 238). From VGB, who are participating in the industrial financing, the Technical Committee Materials and Quality Supervision is accompanying the investigations.

The project falls under the research programme "New Materials for Power Plants”.