Influence of Cold-Forming on the Creep Rupture Behaviour of Austenitic Steels and Nickel Base Alloys

Project Number 241

Cold-forming is a very efficient process to produce high-end boiler-tube applications for superheaters of modern boilers. However, final heat treatment reduces the advantage of the productivity of the cold-forming process.

Creep rupture strength at long term power plant application decreases with an increase of time and temperature due to the cold-forming process. Thereby, the creep rupture strength can fall under the critical value of the "cold-formed" condition. This must be mentioned as a critical aspect of durability of cold-formed components. Also reduction of toughness is critical.

Materials in the cold-formed condition are used for both superheater tubes of modern high-end boilers and for built-in sheets in gas turbines. Here, the long term creep rupture strength is assumed to degenerate. In order to predict degeneration, experimental investigations are necessary. Thus, this project meets the product-oriented aims.

In this project the influence of typical cold-forming rates on creep rupture strength in an applicable range of temperature is investigated. The aim of the project is to determine the creep rupture strength of typical cold-forming grades of austenitic-tube steels for application as superheaters for high-end boilers as well as in selected sheet materials for built-in applications in gas turbines.

Another aspect of this project deals with service-oriented statements of permission of high temperature applications of cold-formed components. The expected benefit is the determination of acceptable cold-forming grades and cold-working parameters. Product and specific forming rates for tubes and built-in sheets are of high industrial exploitation.

Aspired acceptable limits for cold-formed superheater tubes and sheet materials will promote a failure-free operation and cost-effective production of boilers and gas turbines.

The project is carried out from April 2003 to March 2006 by the Institute of Material Technology of Darmstadt Technical University (person in charge: Dr. Alfred Scholz). The budget of the project is sponsored by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF). The project is accompanied by a working group of the Technical Committee "Materials and Quality Supervision".