High Temperature Oxidation Behavior of Creep Resistant Steels in Water Vapour Containing Environments

This study describes the water vapour effect on the oxidation resistance of 9Cr creep resistant steels. Boiler P91 and MarBN steels were oxidized for 3000 h in a simulated humid atmosphere with ~10% water vapour. The oxidation kinetics had a stable course for 1000 h and was evaluated by the weight gain curves for both experimental steels and both oxidation temperatures. The oxidation rate was higher at 650 °C versus 600 °C, as reflected by the oxidation rate coefficient. A significant increase occurred after 1000 h of oxidation, which was related to the local breakdown oxide scale and oxide nodules were formed on steel. This oxidation behavior was influenced by the fact that a compact spinel structure of iron oxides and alloying elements were not formed on the steel. Analysis after 3000 h of exposure showed hematite Fe2O3 formed on the outer layer, magnetite Fe3O4 on the middle layer, and the bottom layer consisted of iron-chromium-spinel (Fe,Cr)2O3.

Modeling and Simulation of Pore Formation in a Bainitic Steel During Creep

In the field of power engineering, where materials are subjected to high pressures at elevated temperatures for many decades, creep-resistant steels are put to work. Their service life is still, however, finite, as the many changes in their microstructure can merely be mitigated and not avoided. Creep cavitation is one of those changes and, in many cases, ultimately causes failure by rupture. In this work, a model is proposed to simulate the nucleation and growth of cavities during creep. This exclusively physics-based model uses modified forms of Classical Nucleation Theory and the Onsager Extremum Principle in a newly developed Kampmann–Wagner framework. The model is validated on P23 steel which underwent creep rupture experiments at 600 °C and stresses of 50, 70, 80, 90 and 100 MPa for creep times up to 46000 hours. The model predicts qualitatively the shape and prevalence of cavities at different sites in the microstructure, and quantitatively the number density, size of cavities and their phase fraction contributing to a reduction in density. Finally, we find good agreement between the simulation and the experimental results especially at low stresses and longer creep times.

Optimization of PWHT of Simulated HAZ Subzones in P91 Steel with Respect to Hardness and Impact Toughness

Appropriate post weld heat treatment (PWHT) is usually obligatory when creep resistant steels are welded for thermal power plant components that operate at elevated temperatures for 30-40 years. The influence of different PWHTs on the microstructure, hardness, and impact toughness of simulated heat affected zone (HAZ) subzones was studied. Thereby, coarse grained HAZ, two different fine grained HAZ areas, and intercritical HAZ were subjected to 20 different PWHTs at temperatures 740–800 °C and durations 0.5–8 h. It was found that the most commonly recommended PWHT, of 3 h or less at 760 °C, is insufficient with respect to the hardness and impact toughness of coarse grained HAZ. To obtain a Vickers hardness ≤ 265 HV and impact toughness at least equal to the impact toughness of the base metal (192 J) in the coarse grained HAZ, it took 8 h at 740 °C, 4 h at 760 °C, more than 1 h at 780 °C, and 0.5 h and 800 °C. Even after 8 h at 800 °C, mechanical properties were still within the target range. The most recommendable post weld heat treatments at 780 °C for 1.2–2 h or at 760 °C for 3–4 h were identified. All specimens subjected to these treatments exhibited appropriate hardness, impact toughness, and microstructure.

Interrelationship between Creep Deformation and Damage for Advanced Creep-Resistant Steels

The components used in power plants generally operate at elevated and/or high temperature and are subjected to internal pressure. Under such conditions creep is of a great concern and there is an urgent demand for methods which can be used to predict the creep life. In this work, using our earlier published creep data for advanced creep-resistant T23 and P92 steels, the interrelationship between creep deformation and damage have been analysed by linking them to the identified acting mechanisms, in terms of empirical formulas for the fracture time assessment. The validity and the applicability of various formulas are examined with the objective to gain insight into the creep deformation and fracture behaviour of the steels under investigation.

Creep Modeling of 9-12%Cr Ferritic Steels Accounting for Subgrain Size Evolution

The enhanced performance of new creep-resistant steels is the result of optimized microstructures. Clearly, the microstructure stability at high temperature is essential for the long-term use of this steels generation. In the recent scientific literature, several research addresses the correlation between the microstructure degradation and the creep performance loss. General aim is to introduce state variables able to describe the metallurgy history of the material affecting its current and future response. The possibility to integrate this metallurgical information in predictive modeling is very attractive. In this work, a new creep model for 9-12%Cr ferritic steels, in the framework of the Continuum Damage Mechanics (CDM), is proposed. The damage variable, usually not related to the underlying physics, may have a metallurgical meaning introducing the kinetic law for subgrain evolution. The microstructure of 9-12%Cr steels is designed to produce the 100% martensite during quenching treatment. Since martensite is not a thermodynamic equilibrium phase, the microstructure evolves exhibiting lath widening and subgrains coarsening. The subgrains growth can be ascribed to the creep strain accumulation and consequently the proposed formulation uses the subgrain size evolution to predict the creep rate beyond the minimum creep rate mainly affected by the recovery processes.

Temperature impact on delayed fracture of creep-resistant steels welds

Purpose: Phenomenon of delayed fracture (or cold cracks formation) of hardenable steels weldments had been widely investigated. But temperature dependence of cracking susceptibility remained discussable, because there was no strict vision of temperature border for the cracking risk appearance, when joints are cooling after welding completion. The proposed paper aimed at assessment of dangerous temperature range at which delayed fracture, mainly for the steels with martensite formation, becomes most probable. Design/methodology/approach: The “Implant” test, conducted under isothermal conditions at the temperatures selected within the range from 160 to 20°C on cooling of the completed test weld joint, was used. Basing on the obtained thermokinetic characteristics of the cracking, the activation energy E of the fracture process was calculated. Comparing of the found E values with the close values of E for the known processes developing in steels, the explanation of the revealed cracking behaviour at different temperatures was proposed. Findings: Delayed cracking of the martensitic weld joints has started to manifest at the temperatures lower than 140°C. Dependence of the cracking period from the temperature is described by C-type curve with the minimum cracking duration within 80-100°C. Using the approach of the activation energy assessment for different temperature ranges (140 to 100°C and 80 to 20°C), the effect of the diffusible hydrogen and a martensite decay on the cracking thermokinetics was considered. Research limitations/implications: Additional investigations of the fine microstructure after different stages of the low-temperature martensite decay could be necessary for deepening understanding of a role of this process in the low-temperature heterogeneity formation and cracking susceptibility. Practical implications: Results widen data on weldability of actual for industry steels and give a ground for consideration of the technological approaches for their welding. Originality/value: Temperature border of the cold cracking risk is specified for the weldments of some commercial steels.