Yoshiaki Toda

Influence of normalizing heat treatment on precipitation behaviour in modified 9Cr–1Mo steel

Abstract Precipitation behaviour during normalizing heat treatment has been investigated on modified 9Cr – 1Mo steel. Heterogeneously distributed spherical MX particles and platelet M3C were observed in the as-normalized condition. The number of the precipitates decreased with increasing normalizing temperature and no precipitates was observed after normalizing at 125°C. The size of MX increased with an increase in normalizing temperature up to 1200°C. For MX, not only size, but also composition of metallic elements was influenced by the normalizing temperature. Since equilibrium composition of MX depends on temperature, an MX particle with non-equilibrium composition dissolves and precipitation of it takes place with its equilibrium composition at the normalizing temperature. A phase field diagram of NbX–VX quasi binary system in modified 9Cr –1Mo steel was experimentally determined. It has been supposed that precipitation of M3C takes place during cooling from normalizing temperature in the surrounding area of MX particles where the concentration of niobium and vanadium in matrix is poor.

Long-term Creep Strength of Creep Strength Enhanced Ferritic Steels

Overestimation of long-term creep strength of creep strength enhanced ferritic steels is caused by inflection of a relation between stress and time to rupture. Creep rupture strength of those steels has been re-evaluated by a region splitting analysis and allowable tensile stress of some steels regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code in Japan has been reduced. A region splitting analysis method evaluates creep rupture strength in the short- and the long-term individually, which is separated by 50% of 0.2% offset yield stress. Inflection of stress vs. time to rupture curve is attributable to longer creep rupture life with a stabilized microstructure of creep strength enhanced ferritic steels, since tensile strength property, which determines short-term creep rupture strength, remains the same level. Accuracy of creep rupture strength evaluation is improved by region splitting analysis. Delta (δ) ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite at the normalizing temperature. It increases driving force of diffusion and promotes recovery of tempered martensite adjacent to δ-ferrite. Concentration gap may be generated also in heat affected zone (HAZ), especially in fine grain HAZ, and it has possibilities to promote recovery and, therefore, to decrease in creep strength.

Region Splitting Analysis on Creep Strength Enhanced Ferritic Steels

Overestimation of long-term creep strength of creep strength enhanced ferritic steels is caused by inflection of a relation between stress and time to rupture. Creep rupture strength of those steels has been re-evaluated by a region splitting analysis and allowable tensile stress of some steels regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code in Japan has been reduced. A region splitting analysis method evaluates creep rupture strength in the short- and the long-term individually, which is separated by 50% of 0.2% offset yield stress. Inflection of stress vs. time to rupture curve is attributable to longer creep rupture life with a stabilized microstructure of creep strength enhanced ferritic steels, since tensile strength property, which determines short-term creep rupture strength, remains the same level. Accuracy of creep rupture strength evaluation is improved by region splitting analysis. Delta ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite at the normalizing temperature. It increases driving force for diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to that in dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength.Copyright

Creep Strength Assessment of High Chromium Ferritic Creep Resistant Steels

Degradation mechanism and life prediction method of high chromium ferritic creep resistant steels have been investigated. In the high stress condition, easy and rapid extension of recovered soft region results in significant decrease in creep strength, however, ductility is high. In the low stress condition, extension of recovered soft region is mainly controlled by diffusion and it is slow, therefore, deformation is concentrated in the recovered soft region along grain boundaries and ductility is extremely low. Delta-ferrite produces concentration gap due to difference in equilibrium composition of austenite and ferrite phases at the normalizing temperature. It increases driving force of diffusion and promotes recovery of tempered martensite adjacent to delta-ferrite. Concentration gap may be produced also in heat affected zone (HAZ), especially in fine grain HAZ similar to dual phase steel, and it has possibilities to promote recovery and, therefore, to decrease creep strength. It has been confirmed the advantage of region splitting analysis of creep rupture strength for high chromium ferritic creep resistant steels, through a residual error analysis. It is important to avoid a generation of concentration gap in order to improve stability of microstructure and to maintain high creep strength.

Development of Ferritic Heat‐Resistant Steels Based on New Materials Design Concept for Advanced High Efficiency Power Plants

To save fossil fuel resources and to reduce CO2 emission, considerable efforts have been directed toward the research and development of heat-resistant materials that can help in improving the energy efficiency of power plants by increasing their steam temperature and pressure. Instead of the conventional 9–12Cr ferritic heat-resistant steels with a tempered martensitic microstructure, the authors have proposed “Precipitation Strengthened 15Cr Ferritic Steel” based on the new material design concept that is a solid-solution treated ferrite matrix strengthened by intermetallic compounds. In this study, the creep strength of the developed 15Cr ferritic steels with various Ni contents was investigated. As a result of the creep tests for the 15Cr steels at 923, 973, 1023 K up to about 65 000 h, it was found that the creep strength of the 15Cr steels was two times higher than that of the conventional 9Cr ferric heat-resistant steels at every temperatures. The creep rupture lives of the steel were 10 to 100 times longer than that of the conventional versions. The creep rupture strength of the 15Cr steels after 100 000 h was estimated to be 100 to 130 MPa at 923 K and 45 to 70 MPa at 973 K from Larson-Miller parameter method. The authors believe the precipitation strengthened 15Cr ferritic steel is a candidate material of the high-temperature structural components in high-efficient thermal power plants.

Effect of Ausageing on the Precipitation Behavior of MX Carbonitride in Modified 9Cr-1Mo Steel

The precipitation behavior of MX carbonitride during a normalizing heat treatment with and without ausageing was investigated in a modified 9Cr-1Mo steel. The normalizing heat treatment was performed at 1150 oC for 1800 s. Ausageing was conducted at 765 and 500 oC for 1800 to 86400 s during the cooling from the heat treatment. The matrix of the steel was austenite single phase during normalizing and ausageing, except for that ausaged at 765 oC for 86400 s. The initial austenite grain size and hardness were not influenced by ausageing, except for the sample ausaged at 765 oC for 86400 s. Although Nb-rich MX (NbX) and cementite were observed, V-rich MX (VX) was not observed under any of the conditions investigated. The amount of NbX in the steel ausaged at 500 oC was at least twice as large as that under the other conditions, and the amount in the steel ausaged at 760 oC was slightly larger than that in the steel that did not undergo ausageing. The precipitation of NbX took place during ausageing in the austenite matrix. On the other hand, it is well known that VX precipitates during tempering. An equilibrium mole fraction of VX in the austenite matrix calculated by Thermo-Calc. was larger than that of NbX at the ausageing temperatures. It is proposed that VX is an equilibrium phase at the ausageing temperature; however, VX nucleation takes much longer in the austenite matrix. It is postulated that the precipitation of VX is more strongly influenced by the interfacial energy rather than supersaturation. It is concluded that the precipitation of MX carbonitride, especially NbX, can be controlled by ausageing during cooling after a normalizing heat treatment.

Improvement in Creep Strength of Heat-Resistant Ferritic Steel Precipitation-Strengthened by Intermetallic Compound

The effects of nickel content and heat treatment conditions on the creep strength of precipitation-strengthened 15Cr ferritic steel were investigated. The creep strength of the 15Cr ferritic steel was drastically improved by solution treatment and water quenching. However, over the long term, the detrimental effect of nickel on the creep strength was pronounced for water-quenched steels. The volume fraction of martensite phase increased with increased nickel content in both the furnace-cooled and water-quenched steels. The volume fraction of martensite phase in the water-quenched steel was smaller than that in the furnace-cooled type, even for the same nickel content. Fine particles, smaller than 500 nm, were precipitated homogeneously within the ferrite phase of the water-quenched steel. On the other hand, coarse block-like particles 1

Effect of cobalt on the microstructure of tempered martensitic 9Cr steel for ultra-supercritical power plants

m in size were precipitated sparsely within the martensite phase. The creep strength of the steels decreased with increased volume fraction of the martensite phase caused by furnace cooling and nickel addition. The lower creep strength and microstructural stability of the martensite phase is attributable to less precipitation strengthening. To enable this steel to be put to practical use, it will be necessary to suppress the formation of the martensite phase caused by addition of nickel by optimizing the chemical composition and heat treatment conditions.