Defect and Diffusion Forum Vol. 405

Abstract: Three CT specimens from stainless steel AISI 304L were subjected to constant amplitude cyclic loadings with various asymmetries. Crack growth was recorded in detail. Fracture surfaces were documented by 3D maps in about 110 locations in the crack growth direction. 3D maps and their local gradients were represented by 2D wavelet decompositions in 10 levels resulting in 60 textural features. Statistical models expressing crack growth rate as a function of textural features were optimized. Training and testing approach, a high ratio of overfitting, and testing of significance of components ensured model's robustness. Quality of results is documented by graphs confronting model outputs with real data known from experiment. Results are acceptable in all cases.

Abstract: Characteristic features of fatigue damage of 316L austenitic stainless steel cyclically strained axially in tension-compression, reversed torsion and combined axial-torsional mode were studied. All loading modes resulted in the formation of persistent slip markings (PSMs). Predominantly one slip system was activated in the case of axial and torsional loading while at biaxial loading, activation of several slip systems was involved. PSMs acted as sites of multiple fatigue crack initiation. The path of subsequent crack growth at a macroscopic scale differed considerably in dependence on loading mode and applied amplitude. The hardening-softening curves and fatigue life curves were evaluated and results were compared and discussed in terms of the type of applied loading.

Abstract: The effect of surface roughness of virgin specimens (prior roughness) made of low-alloyed high strength steel on their fatigue life after the case-hardening treatment was studied by rotating bending tests of virgin and nitrided samples. In a whole range of S-N curves, the fatigue strength of virgin samples after lathe-turning machining (high roughness) was naturally much higher than that of samples after grinding (low roughness). As expected, the fatigue strength of nitrided specimens was higher than that of virgin samples when averaged through the whole fatigue life range. When distinguishing the rough and smooth nitrided samples, the low-cycle fatigue strength of rough samples revealed to be lower than that of the smooth ones and vice a versa in the high-cycle region. This could be explained by the fact that, in the low cycle region, the cracks in the rough samples nucleated from deep surface defects while, in the smooth specimens, they nucleated from shallow defects (or as fish eyes) which prolonged their initiation stage. Almost all cracks in the high cycle region initiated as fish-eyes but the extent of nitrided layers in the rough specimens was slightly higher than that in the smooth specimens. Therefore, the fish-eye centers were shifted further to the interior of the rough specimens which increased their fatigue life.

Abstract: In this work, the fatigue of P265GH steel acc. to EN 10216-2 has been evaluated by measuring a residual magnetic field (RMF). During experiment the notched-specimen has been loaded with a servo-hydraulic uniaxial MTS test machine and a special magnetic sensor was applied. The measurement distribution of the residual magnetic field was performed in two axes. In the first stage of experiment the specimen was gradually loaded with quasi-static force in range of 0 to 16 kN (~ 0.7 Rm). The increase of strength of residual magnetic field was observed only from 0 to 8 kN, whereas in range 8 to 16 kN this effect was not noticed. In the second stage the controlled tensile fatigue test was performed (R = 0, F_max = 16 kN). Up to 21k load cycles no changes to residual magnetic field were noticed. At 31k cycles the significant increase of amplitude of strength of residual magnetic field change was observed but visual inspection does not show any visible crack, while at 35k cycles crack was visible. It means that applied methodology allow to find the initiation of crack. The performed observation on SEM showed ductile fatigue of fracture.

Abstract: Fatigue cracks can start from non-metallic inclusions in the subsurface zone at low stress amplitude level and high number of cycles. This phenomenon was observed mainly at high strength steels and parts with hardened surface zone. Fatigue strength of steel can be then estimated on the basis of hardness and maximum size of defect/inclusions measurements. Many inclusions rating methods exist, some of which have been adopted as the standards. However, “standard” methods for characterization of inclusion content are not convenient for assessment of their influence on fatigue strength of materials. This contribution deals with assessment of maximum sizes of inclusions in three types of martensitic steels used for production of rotating blades of steam turbines (Böhler T552, Böhler T671, and MLX 17) using extreme value statistics methods. Murakami´s concept of √AREA_max parameter and procedure according to ASTM E2283-08 were applied. Results were compared one another and also with the result of standard methods, too. Founded parameters were used for calculation of expected limit of the fatigue strength and compared with results of fatigue test of these steels.

Abstract: Specimens from Ni-Cr-Fe wrought superalloy INCONEL 718 were used for fatigue push-pull test at elevated temperature 700 °C. Fatigue loading was with the coefficient of cycle asymmetry R = -1. Temperature of fatigue test was chosen from two reasons; one is that limit operating temperature for this type of alloy is 650 – 700 °C due to precipitation of stable but incoherent orthorhombic Ni₃Nb delta phase; the second reason for this temperature is fact that metastable body-centred tetragonal Ni₃Nb gamma double prime phase starts to transform to delta phase and from that reason there is an expectation for mechanical properties decreasing due to increased volume of delta phase. For evaluation of fatigued specimens were Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observation used. Also methods of quantitative metallography – coherent testing grids were used for evaluation of delta phase volume. Some references describe that higher volume of delta phase, mostly situated at grain boundaries, act as initiation sites for cracks and therefore decreasing mechanical properties. Employed SEM and TEM analysis confirm the increased volume of delta phase in specimens after fatigue loading but a negative influence on dynamic properties, such fatigue lifetime, for this alloy was not confirmed.

Abstract: Conventional long-term creep test (CCT) to the rupture and so called accelerated creep test (ACT) of the dissimilar weld joint made of FB2 and F martensitic steels and of the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa. Assessment of microstructure development and changes of hardness was correlated with the creep strength. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centers for cavities. As the fine grained heat affected zone of F steel was the softest part of the weld joint, many cavities originated and cause failure of samples. The aim of this paper is to compare results and possibilities of the “standard” methods and advanced scanning electron microscopy performed by instrument equipped with a concentric backscatter electron detector (CBS). Filtering of the signal enables improving and/or diminishing of selected type of contrast caused by various types of particles of secondary phases. The images were used as an input data for image analysis and developments of microstructures during CCT and ACT were compared. Results have shown that specimens after ACT contains significantly lower content of the Laves phase.

Abstract: Prolongation of the service life of key components of fossil-fuelled power plants beyond their original design limit must be accompanied by thorough and extensive monitoring of the actual material state and particularly creep damage. The extent of cavitation is nowadays routinely tested in-situ by using replica method and its quantification is based on practices stated in VGB-TW 507, NORDTEST NT TR 302 derived from the original Neubauer's classification of cavitation damage. Evaluation of cavitation damage based on the measurement of the number of cavities and/or creep micro-cracks has become a routine activity, but in cases when steel contains large amount of non-metallic inclusions, they can be wrongly identified as cavities and to invalidate the result of calculation of cavitation damage. Comparative analysis of cavitation damage was performed in a pipe bend and its weldment made of a low-alloy 0.5Cr-0.5Mo-0.3V steel creep exposed at 540 °C for more than 225,000 hours by using OM (SEM) and replica method and revealed the comparative extent of cavitation damage decreasing from the outer to inner surface of the pipe wall.

Abstract: Challenging structural applications such as customized jet engine parts are increasingly fabricated by Selective Laser Melting (SLM) of Inconel 718 powder. The as-built surface quality of SLM parts is however inferior of the machined version and the fatigue behavior is negatively affected. The as-built fatigue response of SLM Inconel 718 was quantified here using three sets of directional specimens. Since the surface quality is influenced by powder characteristics, process parameters and layer-wise fabrication, fatigue results showed a directional contribution that was interpreted using metallography and fractography.

Abstract: This paper deals with applications of biomaterial in the human body. Each biomaterial is characterized by biofunctionality and biocompatibility [1]. The choice of biomaterial for medical applications is established on mechanical properties. Therefor the Ti6Al4V alloys, which properties are relatively low modulus, good fatigue strength, formability, machinability, corrosion resistance [2], are most widely used in biomedical replacements, implants, and prosthesis. Despite the excellent properties of the titanium alloy, endoprosthesis often fails and the hip replacement is necessary. Common causes are overloading and cracking, static or dynamic. Other causes of failure include injury, implantation failure, manufacturing inaccuracies, and non-compliance with the manufacturing process.