F. Schubert

Precipitation behavior of Ni-Cr-22 Fe-18 Mo (Hastelloy-X) and Ni-Cr-22 Co-12 Mo (Inconel-617) after isothermal aging

The precipitation behavior of the nickel-base alloys Ni-Cr-22 Fe-18 Mo (Hastelloy-X) and Ni-Cr-22 Co-12 Mo (Inconel-617) has been investigated as a function of aging temperature. Hastelloy-X shows ...

Stereological estimation of microstructural parameters of nickel-based superalloy Waspaloy using TEM methods

Abstract The nickel-based superalloy Waspaloy, which is precipitation strengthened by coherent γ′ particles, was investigated using different transmission electron microscopy (TEM) imaging conditions in the as-received condition and after long time exposure at 650°C and 750°C. Scanning–transmission electron microscopy (STEM) bright-field images, conventional TEM dark-field images and convergent-beam electron diffraction (CBED) patterns have been taken for a stereological evaluation of the particle size distributions, particle density and volume fraction of primary and secondary coherent γ′ precipitates.

Structural design criteria for HTR — a summary report —

Abstract The results of the German research and development project ‘HTR Design Criteria’ has been presented during the workshop on ‘Structural Design Criteria for HTR’. The work is organized in four areas: (1) technical safety boundary conditions, (2) metallic structural components, (3) prestressed concrete pressure vessel, and (4) graphitic structural components. In this summary report some essentials are discussed for all four areas. On basis of these results, the formulation of structural design (KTA) rules are in preparation.

Constitutive Equations for the Description of Creep and Creep Rupture Behavior of Metallic Materials at Temperatures Above 800 °C

At high temperatures, the creep deformation of metallic materials is correlated with the accumulation of creep damage. Creep crack growth leads to a decrease of bearing cross section and an increase in stationary creep. Both variables, creep strain rate and creep damage, are described by a system of coupled differential equations. The solutions of these equations are given for the boundary conditions of creep tests under constant load, for creep rupture behavior, for damage accumulation in the creep region, and for creep-regimed low-cycle fatigue. A general correlation between applied stress, cumulative strain, and cumulative damage is given.

Multiaxial Creep Behavior of Nickel-Base Heat-Resistant Alloys Hastelloy XR and Ni-Cr-W Superalloy at Elevated Temperatures

A series of uniaxial and multiaxial creep tests was carried out on Hastelloy XR and Ni-Cr-W superalloy, which were developed as the high temperature structural materials for nuclear application at the Japan Atomic Energy Research Institute, in order to investigate multiaxial creep behavior of these materials and to verify the laws for the description of deformation under multiaxial loadings. Nortons creep law and von Mises flow rule were applied to the prediction of multiaxial creep behavior of a tube under some significant loading conditions. In most cases the multiaxial creep behavior of these materials were successfully described with the constitutive equations based on the material parameters fitting uniaxial creep test results, though a few exceptional cases were observed. The present study has revealed that the method based on Nortons creep law and von Mises flow rule are basically applicable for the description of the multiaxial creep behavior for Hastelloy XR and Ni-Cr-W superalloy as the conventional design method.

Material hardening under multiaxial creep loading

Abstract Real components in operation are exposed to cyclic multiaxial loadings. For the evaluation of the allowable operation time the effect of those cyclic loadings on deformation and creep strain behaviour must be derivated. The theoretical discussion of strain-, time- and energy-hardening theory and their mathematical description by creep laws and constitutive equations (unified models) results in an approach for the strain hardening rule according to the proposals of Nadai and Timoschenko, which allows, in using the von Mises deviatoric hypothesis and the theory of invariants a multiaxial formulation of the stress-strain behaviour. Results of creep experiments at 950°C with tubular specimen of NiCr 23 Co 12 Mo (Alloy 617) loaded over a period of more then hundred hours by a combination of steady axial load and steady internal pressure superposed by cyclic torsional moments are compared with the theoretical results. By defining a scalar typ of hardening function, which depends on the effective strain, the behaviour can satisfactoraly be predicted. Using the ORNL-model, however, the strain behaviour under cyclic loading cannot fairly enough predicted.

Aspects of design codes for metallic high temperature components

Abstract High temperature component design is very important in different industrial fields. This paper discusses work done in Germany on the design of metallic components for the Helium Cooled High Temperature Reactor. Candidate materials for application at the different temperature regions are presented. Criteria are developed to specify the allowable strength values of the materials (S m , S t ) for use in design codes. The following typical relevant phenomena are treated and discussed: • —creep under uniaxial and multiaxial conditions up to temperatures of 1000° C; • —creep fatigue interaction, where holding times during the stress strain cycles have a great influence on the life time; • —creep buckling for pressure loaded shells with initial imperfections; • —creep ratcheting as a phenomenon, where cyclic secondary stresses produce enhanced deformations; • —fracture mechanics at high temperatures with the aspects of creep crack growth and fatigue crack growth.

Experimentally verified creep ratcheting analysis

Abstract In heat exchanging components amongst the primary stresses even thermally induced secondary stresses must be taken into account. Here fluctuating temperatures (e.g. hot streaks) are especially important. At very high temperatures above 800C a superposition of primary stresses and thermally induced fluctuating secondary stresses may lead to enhanced creep (creep ratcheting). At temperatures of 950°C experiments were carried out to verify the creep ratcheting effect. Here INCOLOY 800 H tubes were loaded by an axial force and by cyclically changing temperatures. A creep ratcheting effect could be shown. Calculations were carried out by using a simple two-shell-model code and finite-element programme, respectively. It could be shown that the results obtained with the two-shell model are very conservative, while the results of the finite element analysis were in a good agreement with the experiments. Finally it is discussed, how the two shell model can be improved.

Multiaxial creep of tubes from incoloy 800 H and inconel 617 under static and cyclic loading conditions

Abstract At temperatures above 800°C the material behaviour under mechanical load is determined by creep. The service of heat exchanging components leads to multiaxial loading conditions. For design and inelastic analysis of the component behaviour time dependent design values and suitable constitutive equations are necessary. The present report gives a survey of the approaches to describing creep under multiaxial loading. Nortons law and v. Mises theory are applied. The load combinations of internal pressure, tensile and torsional stress are studied more closely, cyclic stress superposition in the tensile-pulsating range is discussed and cases of partial relaxation are examined. Experimental results are presented for the loading conditions discussed, and satisfactory agreement between theory and experiment has been found up to now for these results. Regarding lifetime determination under multiaxial creep load, a more precise analysis of creep damage is presented suggesting a suitable deviatoric stress for evaluation in the long-time range.

Mechanical design methods for high temperature reactor components

Abstract The work concerned with design codes for high temperature reactor (HTR) components operating at temperatures above 800°C is summarized. Using the experimental results from the German HTR materials development programmes, in particular the time dependent properties, the structural design analysis for an intermediate heat exchanger is discussed, with reference to creep, fatigue, creep buckling and creep ratchetting. The analysis provides the basis for a critical consideration of ASME Code, Case N 47, and the applicability of the code case rules for service temperatures above 800°C.