In order to understand the microstructural evolution during service that 9Cr steels experience it is important to be able to quantify key microstructural parameters that define the characteristics of the secondary phases (e.g. precipitated phases and inclusions) and the steel matrix. The average size of M 23 C 6 , Laves phase and MX particles in these materials have been reported in many studies, however comparability between these studies is compromised by variations in technique and different/incomplete reporting of procedure. This paper provides guidelines on what is required to accurately measure these parameters in a reproducible way, taking into account macro-scale chemical heterogeneities and the statistical number of particles required to make meaningful measurements. Although international standards do exist for inclusion analysis, these standards were not developed to measure the number per unit area of hard particles that can act as creep cavity nucleation sites. In this work a standardized approach for measuring inclusions from this perspective is proposed. In addition the associated need to understand the segregation characteristics of the material are described, which in addition to defining the area that needs to be analysed to measure the average number of inclusions per unit area, also allows the maximum number of inclusions per unit area to be determined, a parameter which is more likely to define the damage tolerance of the material.

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in grade 91 steel. Failed hot tensile specimens have been sectioned at various positions along the specimen axis, and therefore at different levels of hot tensile strain, to obtain material for creep strength evaluation. Because of the limited amount of material available for creep testing obtained in this way, creep testing has been carried out using the specialised small-scale impression creep testing technique. The grade 91 material has been tested in both the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% Ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant with grade 91 pipework systems.

A 9Cr-3W-3Co-VNbBN steel, designated MARBN ( MAR tensitic 9Cr steel strengthened by B oron and N itrides), has been alloy-designed and subjected to long-term creep and oxidation tests for application to thick section boiler components in USC power plant at 650 o C. The stabilization of lath martensitic microstructure in the vicinity of prior austenite grain boundaries (PAGBs) is essential for the improvement of long-term creep strength. This can be achieved by the combined addition of 140ppm boron and 80ppm nitrogen without any formation of boron nitrides during normalizing at high temperature. The addition of small amount of boron reduces the rate of Ostwald ripening of M 23 C 6 carbides in the vicinity of PAGBs during creep, resulting in stabilization of martensitic microstructure. The stabilization of martensitic microstructure retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. The addition of small amount of nitrogen causes the precipitation of fine MX, which further decreases the creep rates in the transient region. The addition of boron also suppresses the Type IV creep-fracture in welded joints by suppressing grain refinement in heat affected zone. The formation of protective Cr 2 O 3 scale is achieved on the surface of 9Cr steel by several methods, such as pre-oxidation treatment in Ar gas, Cr shot-peening and coating of thin layer of Ni-Cr alloy, which significantly improves the oxidation resistance of 9Cr steel in steam at 650 o C. Production of a large diameter and thick section pipe and also fabrication of welds of the pipe have successfully been performed from a 3 ton ingot of MARBN.