Stanisław Mroziński

Influence of Temperature on the Cyclic Properties of Martensitic Cast Steel

This paper attempts to describe changes of the cyclic properties of martensitic cast steel in the function of the number of loading cycles under temperature of 25 and 600oC was made. The cyclic properties were described by means of three hysteresis loop parameters: stress amplitude, plastic strain amplitude, plastic strain energy. It was stated martensitic cast steel always undergoes clear softening which is independent of the temperature and level of total strain.

Fatigue Life of GX12CrMoVNbN9 -1 Cast Steel in the Energy-Based Approach

The paper presents the research on the controlled strain and temperature on the stress strength of martensitic GX12CrMoVNbN9 – 1 in the energy-based approach. The fatigue tests were carried out for five levels of total strain amplitude ac and two temperatures (the room one and 600oC). Performed research has shown that in terms of energy the investigated cast steel is characterized by changes in cyclic properties. Moreover, it has been proved that the temperature of study significantly influences the per-unit value of strain energy. The influence is slight in the area of large strains and gets stronger as the level of cyclic strains decreases.

Low Cycle Fatigue of GX12CrMoVNbN9 -1 Cast Steel at 600°C Temperature

The paper treats of the low cycle fatigue (LCF) behaviour of GX12CrMoVNbN9-1 (GP91) cast steel after heat treatment. Fatigue tests at the temperature of 600oC within the scope of small amount of cycles to failure were carried out for five levels of controlled amplitude of total strain εac = 0.25, 0.30, 0.35, 0.50 and 0.60 %. The investigated cast steel within the scope of low cycle fatigue life reveals a three-stage course of changes in strength and strain. In the given scope of low cycle fatigue for GP91 cast steel, cyclic weakening was observed without the occurrence of stabilization period of its properties.

About Fatigue Tests End Criterion in Elevated Temperatures

In this paper there has been presented a new quantitative criterion of the end of a low-cycle fatigue test conducted in the conditions of fixed amplitude loads. In this criterion there is used an analytical description of the parameters of the hysteresis loop and the number of cycles till the fracture. The conducted experimental verification of this criterion enabled to obtain satisfactory results. In the paper it has been stated that the presented criterion can have a special application in the case of tests of metal materials in an elevated temperature.

Microstructural Aspects of Low Cycle Fatigue of Aged GX12CrMoVNbN9-1 Cast Steel

The paper presents the results of research on the changes in microstructure of GX12CrMoVNbN9-1 cast steel subject to aging at the temperature of 600°C and holding time of 8000 hours, followed by low-cycle fatigue. The characteristics of the microstructure of the examined cast steel after ageing and low-cycle fatigue was described using transmission electron microscopy (study of the dislocation microstructure) and morphology of precipitations. It has been shown that low cycle fatigue leads to the matrix softening as a result of the processes of recovery, polygonization and repolygonization. Moreover, the processes of precipitation of Laves phase and coagulation of M23C6 carbides were observed in the microstructure. Intensity of these processes depended not only on the temperature of fatigue tests, but also on the level of total strain amplitude εac.

Strain Energy Cumulation at Increased Temperatures

This paper presents results of strain energy ΔWpl cumulation tests on cast steel specimens under constant-amplitude isothermal stress conditions and programed block stress conditions at three different temperatures (20, 400, 600°C). Programed loads (blocks) were composed of five stages of different sequences which included the same strain as during the constant-amplitude isothermal tests. Through the constant-amplitude tests the author demonstrated effect of temperature and the strain level on the fatigue life, strain energy value ΔWpl and accumulated energy ΣΔWpl. While no influence of the load program form on the value of the energy accumulated in the specimen was found.

True and Nominal Stress during Low Cycle Fatigue Tests of Metals

In this paper results of P91 cast steel after static and fatigue tests were presented. During the tests longitudinal and transverse strains of the specimen were measured. Basing on the results a comparative analysis of nominal stresses σn,true stresses σrz, nominal strain energy ΔWpl (n) and true strain energy ΔWpl (n) was carried out. It was stated that differences between determined paramters rise with increasing strain.

Effect of strain level on cyclic properties of S355 steel

Fatigue strength of structural materials is estimated using results of experimental tests performed in the low-cycle fatigue, among others. Computation models used for determination of strength are based on an assumption that there is a stabilization interval in low-cycle testing. Based on a review of literature, a conclusion can be drawn that stabilization of cyclic properties occurs rarely and applies to few metallic materials. Due to variability of loading conditions and the effect of low or high temperature, the occurrence of the phenomena of cyclic weakening or strengthening a material can be hardly anticipated explicitly and its course is affected by various factors related to the testing conditions. This creates important difficulties at the very handling of test results and during identification of models used in the course of fatigue strength computation.

Energy Accumulation in Steel, in the Conditions of Constant-Amplitude

This work presents results of fatigue tests of P91 steel specimens under constant-amplitude and programmed loading in temperature T=600°C. Two strain levels, applied in different order, were used for programmed loading. The loading program level changes were introduced for different levels of damage. The tests provided the basis for analysis of strain energy ΔWpl in the function of the number of load cycles under constant-amplitude and programmed loading. It has been found that that the form of a loading program has an influence on the life and value of energy cumulated in the specimen until crack initiation. Verification of Palmgren Miner hypothesis revealed that its effectiveness depends on the loading program and temperature.

Cast Steel Tests under Thermal Fatigue Conditions

This paper presents results of experimental research and numerical calculations for thermal fatigue of G-X12CrMoVNbN9-1 martensitic cast steel. The tests were performed using hydraulic testing machine equipped with the heating chamber. The experimental research included static tests under mechanical load conditions at four temperature levels (20°C, 400°C, 550°C, 600°C) aimed at determining material characteristics needed for the numerical model. Main cyclic tests were carried out under thermal load conditions. The parameter, which was maintained at unchanged level during tests was the specimen strain ε = const = 0. The specimen temperature was changed. The numerical calculations for the same temperature change program were performed with the use of ABAQUS software. Experimentally and numerically determined stress value versus test time were compared.