Design of 10Cr martensitic steels for improved creep resistance in power plant applications

Abstract

The drive to increase the efficiency of fossil fired power generation to reduce CO2 emissions and to conserve energy resources has led Tata Steel to design new 10Cr martensitic compositions for use at temperatures of 620°C and above with creep properties superior to steel 92 (9Cr, 0·5Mo, and 2W), which is the best currently available steel of this type. In the new alloys, the chromium content was set at 10 to ensure the required oxidation resistance. The long term creep performance of steels with this level of chromium has been limited by the precipitation of Z phase nitrogen rich particles. These form at the expense of vanadium rich MN precipitates that are vital for long term creep strength. Compositions have been designed with the aim of suppressing or delaying Z phase formation. The principles underlying the new steel chemistries are discussed, and a detailed explanation is given of the role of heat treatment in optimising creep rupture strength at temperatures in excess of 600°C. The new steel compositions are based on published work supplemented by thermodynamic calculations to optimise the heat treatment cycle, to produce a fully martensitic microstructure and to maximise the stable nanoprecipitates that are essential to maximise creep rupture life.

The design of a compositionally robust martensitic creep-resistant steel with an optimized combination of precipitation hardening and solid-solution strengthening for high-temperature use

“…Hence a second set of thermodynamic calculations at the service temperature is performed. Three additional go/no-go criteria at this use temperature are enforced: (5) The total amount of non-desirable precipitates, i.e. all precipitates not being MX carbonitrides or Ni3Ti/ NiAl intermetallics should be less than 1 vol.%.…”

“…However, the microstructural stability of existing creep steel grades during long term exposure at elevated temperatures is still insufficient. Much effort has been devoted to the development of new martensitic creep-resistant steel with improved creep strength and a decent stability during long term service [2][3][4][5][6][7][8][9][10][11]. Kimura et al's research [12,13] on some ferritic creepresistant steels has shown that at very long times the creep strength reaches a constant value.…”

Review of Z phase precipitation in 9–12 wt-Cr steels

“…As a general realisation of the importance of Z-phase precipitation set in during the following years, observations of Z-phase precipitation in 9-12wt%Cr steels were made by many separate research groups [13][14][15][16]. While the most continuous research has been presented in papers from Danielsen or Sawada, many groups have been involved in more recent characterising of Z-phase precipitation [17][18][19][20][21][22][23][24] as well as atomistic scale modelling [25][26][27][28].…”

Parsons Turbine Conference

“…Sachadel et al 3 report the development, aided by thermodynamic calculations, of a new 10%Cr martensitic steel for use at temperatures up to 620uC. The composition is designed to suppress Z phase formation, thus giving creep properties superior to those of steel 92.…”