Papers by Keyword: Fractography

Abstract: Austenitic stainless steels are characterised by excellent corrosion resistance and good formability, but their low hardness and fatigue life are limitations in demanding applications. The aim of this study was to analyze the effect of solution annealing and plasma nitriding on the microstructure, hardness and fatigue properties of AISI 304 steel. The experimental material was examined in three states: initial, after solution annealing and after plasma nitriding. Solution annealing resulted in the removal of deformation martensite, giving a homogeneous austenitic structure with a decrease in hardness. On the contrary, plasma nitriding produced a hard nitride layer (1291 HV0.01), while no martensite retransformation took place. The results of the fatigue tests showed that the specimens after plasma nitriding reached the highest fatigue limit (878 MPa), while the specimens in the initial condition had the highest number of cycles to fracture. Fractographic analysis revealed typical fatigue failure characteristics in all conditions. The study highlights the possibility of optimising the fatigue properties of austenitic steels through an appropriate combination of thermal and chemical-heat treatments.

Abstract: The stretch flangeability of ferritic steel grade with tensile strength ≥1 GPa in hole expansion tests can be significantly improved by using the wire electrical discharge machining (W-EDM) process for hole-making instead of conventional punching tools. This improvement is attributed to the notably enhanced cut edge quality of the machined holes. In this study, the average hole expansion ratio (HER) of a novel 0.1C-0.3V-0.25Mo-0.08Ti-0.08Nb steel increased from 24% to 91% when W-EDM was used in hole preparation. A comparison between the fractured surfaces of punched and W-EDM-machined holes after HER testing revealed different failure mechanisms in the steel sheet. At the onset of cracking, fractures in the W-EDM specimens exhibited ductile behavior, whereas quasi-cleavage fracture was observed in the punched specimens. Based on texture measurements and metallographic investigations, it was concluded that reducing the intensity of the adverse shear texture component {112}<111> near the steel sheet surface and eliminating microstructural constituent variations improved the stretch flangeability of the Ti-Mo-V-Nb steel in both hole-making processes.

Abstract: The aim of the work was to assess the nature and causes of defects – cracks in the welds of membrane wall panel tubes (MW) of a thermal power plant boiler made of T24 steel. This steel is intended to produce MW evaporators and superheaters for power plant units operating in the area of supercritical steam conditions. Its advantage, compared to the originally used materials, is higher creep resistance. The main expected advantage of the steel was to obtain optimal properties of welded joints without subsequent post-weld heat treatment (PWHT).

Abstract: Steel hydrogen embrittlement (HE), a complex and multifaceted issue, can lead to sudden and catastrophic failure, without significant plastic deformation, making it a critical concern in the industrial sector. The present investigation focuses on the evaluation of HE effects regarding microstructure, mechanical properties degradation and type of fracture of AISI 1010 low-carbon steel, after accelerated hydrogen cathodic charging. Hydrogen was diffused electrolytically in 0.2 Μ H₂SO₄ solution, containing 3g/L of NH₄SCN, using a cathodic current density of 10 and 20 mA/cm², for 6 and 18 h. Mechanical properties were investigated through slow-rate tensile tests, as well as Charpy V-notch (CVN) impact tests, to determine the value of fracture toughness, both in uncharged and electrochemically pre-charged specimens. Vickers microhardness tests were conducted on the cross-sections of the hydrogen charged samples to evaluate embrittlement susceptibility, due to the presence of dissolved hydrogen. The microstructure modification was carried out through light optical (LOM) and scanning electron microscopy (SEM), in conjunction with an energy-dispersive X-ray detector (EDS). Slow scan X-ray diffraction (SSXRD) was also conducted for crystal structure analysis. The microstructure analysis showed the presence of large amounts of secondary cracks and cavities into the steel matrix, due to hydrogen diffusion and its accumulation at various sites. Hydrogen charging caused a significant gradual elongation decrease of the parent material, from 25% to 6.73%, in case of embrittlement at 20 mA/cm² for 18h. Accordingly, after 18 h of exposure, the impact energy decrement was determined at 31.5%, at a current density of 10 mA/cm², whereas the corresponding reduction at 20 mA/cm² reached 68%.

Abstract: A286 nickel-iron based superalloy used in high temperature applications. Age hardening is done to enhance the creep behavior which is much affected by TiC and eta (ⴄ (Ni₃Ti)) phases. Effect of carbon and titanium (0.02C-2.46Ti, 0.04C-2.54Ti, 0.05C-2.58Ti, and 0.06C-2.62Ti) on tensile behavior of aged A286 superalloy is systematically investigated via TiC and ⴄ (Ni₃Ti) phases. It has been revealed that carbon and titanium contents are in proportional to nucleation of TiC and eta phases in the austenitic matrix of this alloy. Precipitation of these phases enhanced yield strength from 354MPa to 501MPa and ultimate tensile strength (UTS) 543MPa to 651MPa. However, plasticity decreased nearly 4%. Fracture topography showed that the ductile transgranular fracture in low C-Ti alloys are due to TiC particles, whereas in high C-Ti alloys fracture nature is found brittle intergranular due to eta phases.

Abstract: The aim of the current study is to perform a preliminary analysis on specimens extracted from 4 mm sheets of EN AW 1200 alloys that were butt-joined using FSW and two SFSW processes. The purpose of this scientific investigation is to promote the suitability and advantages of new ecological joining processes by implementing SFSW in the automotive, naval, railway and aerospace industry. The papers authors performed destructive testing on specimens extracted from sheets joined with the above mentions processes, namely tensile strength test according to SR EN ISO 6892-1:2020. Researchers also performed topography analysis on samples extracted from the AW 1200 but-joined similar sheets using scanning electron microscopy – SEM, combined with energy dispersive X-ray analysis EDAX. Results from the tensile tests highlighted an improvement in the ultimate tensile strength of the welded butt-joint, if compared to regular FSW results. The highest tensile strength value resulted from the samples extracted from joints performed using the second version of SFSW process. Researchers also noticed that samples broke in the weld or transition area. There was also a change in the chemical composition of the sample performed using the first SFSW method, namely, a high concentration of oxide appeared (≥18%), leading to the conclusion that the module used induces rapid oxidation of joints. Experimental data doesn’t comply with automotive, railway, naval and aerospace-imposed compliances, thus further investigations should be done to improve the application of the technology.

Abstract: Under high cycle and very high cycle fatigue, high strength steels break as a result of internal fracture from inclusions. In order to understand this fracture, “Fisheye” crack has been investigated. In our previous work we found that cracks grew from the boundary between the hard surface and soft core of case-hardened S45C, SUJ2 and SCM415 steel bars under rotating bending fatigue. These cracks were called “Transition area origin (TRO)” cracks. In this study, we closely observed the fracture surface of TRO crack areas in carburized JIS SCM415 specimens (under 734, 776 and 865 MPa). We found three features of the TRO cracks: outside of the TRO cracks had asperities; the shapes of TRO cracks were almost circular, and were different from those in S45C and SUJ2 steels; and the HAZ-TRO area which was located at hardened layer had some ridges, and the Core-TRO area at unheat-treated layer was smooth and flat.

Abstract: Nickel-copper alloys are commonly used in highly corrosive conditions where strength is required. Typical applications are in the marine sector, petrochemical industry, or energy facilities such as chemical tubes, pumps, heat exchangers and superheated steam systems. This paper compares the microstructure and mechanical properties of a cast alloy with a 3D printed alloy processed via a laser powder bed fusion (LPBF) technique. Small cylindrical specimens were used for tensile tests at room temperature (RT) and elevated temperatures up to 750 °C in air. The tensile stress-strain response was determined for both types of materials. At RT, LPBF material has a higher yield strength and ultimate tensile strength than a cast alloy. At elevated temperatures, the strength of both variants is comparable. However, the fracture elongation of the LPBF material is significantly lower over the entire range of investigated temperatures. Fracture surfaces and polished sections parallel to the specimen axis were investigated to compare the microstructure and damage mechanisms of the nickel-coper alloy 400 prepared by conventional casting and via LPBF.

Abstract: A significant number of different metals are present in aluminum alloy scrap and waste. Secondary aluminum cast alloys, made by recycling from scrap and waste, have as the main impurity Fe. Fe reduction is a very economically and technologically expensive process and therefore there is a growing interest in researching such materials. Moreover, the higher content of Fe leads to the formation of brittle Fe-rich phases, leading to faster propagation of fracture in castings. Therefore, this study reflected on secondary aluminum cast alloy with a higher concentration of Fe and research their effect on brittle Fe-rich phase formation (in the needle; plate-like form) and propagation of fracture in the castings. This study confirms the increasing amount of needle Fe-rich phases in the melt with higher content of Fe. The increasing amount of such phases leads to the formation of a large number of cleavage fractures on fracture surfaces. Although the cleavage fracture increased, the experimental results show low changes in the properties of all experimental melts.

Abstract: As part of the project “Thermostat for universal use in (electro) mobility”, an analysis of three currently used thermostats failed during operation was performed. In all three cases, cracks occurred in the brass outer casings. The cracks were open and fractographical observation was performed. The microstructure of the material was evaluated using both the light and scanning electron microscopy. The local composition of the material was determined by EDS microanalysis. Furthermore, hardness profiles were measured. The cracks were predominantly intergranular with a smaller portion of transgranular cleavage. The microstructure was formed by a mixture of α- and β-phase grains and lead particles. In addition to the stress caused by the overpressure of the molten wax, a higher level of residual stresses caused by deformations can be expected. The failure was caused by the mechanism of stress-corrosion cracking. Metal induced embrittlement or/and corrosion fatigue could interact too.