Jozef Zrník

Influence of hold period on creep-fatigue deformation behaviour of nickel base superalloy

Abstract The deformation behaviour and damage mechanisms of the wrought nickel base superalloy EI 698 VD has been investigated in condition of creep–fatigue loading. The alloy was subjected to load controlled isothermal low cycle fatigue at 650°C with superimposed hold periods of constant tensile load simulating creep stress component. The introduced hold periods were in range of 3 min to 10 h. The pure fatigue and pure creep tests were conducted as well. The test were of tension–tension type defined by stress ratio R =0.027. During hold periods a constant stress of σ =740 MPa was maintained leading to pure creep. The tests were conducted until fracture. The influence of hold time was evaluated through the deformation behaviour, fatigue life, and fracture mechanisms participating in failure process. The time to fracture decreased continuously with increasing hold period. Considering the fatigue life achieved after shortening the hold time to 1 min, the life time measured was more than 10 times higher than in case of pure creep. The strain rate dependence achieved within this testing showed inverse dependence with respect to the hold time. The microstructure examination of the fractured specimens, using transmission electron microscopy (TEM), enabled to reveal the participating dislocation mechanisms for combined loading. The fracture analysis of broken up specimens showed that fatigue mechanisms became active at crack nucleation sites and crack propagation in time of testing when hold time was 30 min and less.

Effect of thermomechanical processing on the microstructure and mechanical properties of Nb–Ti microalloyed steel

Abstract Based on analysis of austenite deformation behaviour during thermomechanical processing of Nb–Ti microalloyed steel, the rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, and (iii) ferrite-pearlite. The effects of austenite conditioning on the final ferrite-pearlite microstructure and mechanical properties of steel were investigated. To rationalise the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the ferrite grain growth in addition, to the density of ferrite nucleation sites. Mechanical properties were the means to evaluate the variation in austenite solutioning and deformation conditions introduced into individual applied rolling schedules. The benefit of tensile tests, especially yield strength and ductility values, in determining the optimum deformation schedule and coiling condition for given steel is demonstrated.

Influence of cycling frequency on cyclic creep characteristics of nickel base single-crystal superalloy

The static and the cyclic creep load-controlled tests of the nickel base single-crystal superalloy, CMSX 3 have been studied. In this study, two different testing procedures of cyclic creep were carried out under specific loading schedules defined by the stress amplitude, the stress range, stress ratio, and cycling frequency. The different cycling frequencies for two cycling hold periods were investigated to influence the creep/fatigue behaviour of the alloy. The influence of these variables was evaluated through the deformation behaviour and the fracture life data. It is concluded that the additional fatigue stress component superimposed onto creep stress component is of great significance influencing the deformation process and fracture life and accelerating the creep strain rate for chosen types of testing.

Creep Deformation and Microstructural Examination of a Prior Thermally Exposed Nickel Base Superalloy

When exposed for long time at elevated temperatures of 430°C and 650°C the nickelbase superalloy EI 698 VD can experience a significant decrease in the creep. The cause of the creep degradation of nickel-base superalloy has never been unequivocally explained, but it has generally attributed to the microstructural instability at prolonged high temperature exposure. The creep lifetime decrease was more expressive as time of thermal exposure was extended up to 52 000 hours. In this article, the creep life data, generated on prior long thermally exposed wrought nickelbase superalloy were related to the microstructural changes observed using SEM and TEM analysing techniques. Qualitative and quantitative structure analyses provided supporting evidence concerning changes associated with grain boundary carbide precipitation and with volume fraction changes of gamma prime the exposure time prolonged. In order to clarify the role of gamma prime distribution modification in thermally exposed superalloy on creep degradation the SANS (Small Angle Neutron Scattering) diffraction experiment was employed in the characterization of the gamma prime morphology and the size distribution with respect to the period of thermal exposure.

In Situ Neutron Diffraction Analysis of Phase Transformation Kinetics in TRIP Steel

The precise characterization of the multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. In-situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for retained austenite stability evaluation during subsequent mechanical loading. The progress in austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of transformation is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles over the time of transformation are then assumed as a measure of the volume fractions of both phases in dependence on transformation temperature. Useful information was also obtained on retained austenite stability in TRIP steel during mechanical testing. The in-situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of neutron diffraction technique in monitoring the transformation of retained austenite during room temperature tensile test. In both experiments the neutron investigation was focused on the volume fraction quantification of retained austenite as well as on internal stresses rising in structure phases due to retained austenite transformation.

Evaluation procedure for anisotropic SANS

A brief description of an improvement of a special evaluation procedure for treatment of data from anisotropic Small-Angle Neutron Scattering (SANS) is presented. A spatial orientation distribution of particles, the improved calculation of a size distribution, the option to fit a data from a sample containing two types of particles as well as the raw-data treatment are the main extensions. The application of the procedure is demonstrated on data taken from investigation of precipitation in single-crystal nickel-base superalloys.

Controlled Rolling of Hastelloy-N

This paper provides information of the uniform grain structure after application of hot and cold rolling conditions and recrystallization processes followed by various annealing times in NiMoCr alloy. The different structure would result in high temperature mechanical properties such as yield strength, tensile strength, ductility, fatigue, and creep strength. The microstructural changes as results from all tested programs were investigated by optical microscope. The obtained results showed that the uniformity of recrystallized structure relates strongly to amount of deformation during hot and cold rolling and period of annealing time. It was also found that the uniformity of recrystallized structure increased with higher deformation and longer annealing times. The formability was tested and the base deformation resistances (BDR) were also calculated.

Small-Angle Neutron Scattering Investigation of Precipitation in Single-Crystal Nickel-Based Superalloy ZS26

The influence of different heat treatments on the microstructure of ordered cuboidal γ′ precipitates in the ZS26 superalloy (wt%: Ni 64, W 11, Co 9, Al 6, Cr 5, Nb 1.5, Ti 1, Mo 1, V 1) was studied by small-angle neutron scattering. The average shape and size of the precipitates were determined by modelling the scattering curves and by fitting them to the measured 2D data. The volume fraction of the precipitates was deduced from the fitted parameters under the assumption of a homogeneous distribution of particles inside the sample.

Effect of Preliminary Treatment on Grain Refinement of Medium Carbon Steel Using ECAP at Increased Temperature

By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 μm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (εef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

Influence of Plastic Deformation on Creep Behaviour of NiMoCr Alloy

Creep behaviour of nickel-base solid solution strengthened NiMoCr alloy after different applied hot and cold rolling conditions was investigated. The results of creep tests showed that the creep characteristics including strain rate, fracture strain, and lifetime were greatly dependent on the plastic deformation conditions carried out prior to the creep. Failure process including crack nucleation and crack propagation was strongly dependent on grain size. Fine recrystallized structures have provided much less resistance to creep deformation compared to coarse grain size. A comprehensive study of creep deformation as a function of plastic deformation conditions of NiMoCr alloy is also presented.