Defect and Diffusion Forum Vol. 405

Abstract: Austenitic stainless-steel has been widely used. Although it offers excellent corrosion resistance, processability, and nonmagnetic properties, it is inferior in terms of wear resistance and hardness. Therefore, the purpose of this study is to form iron boride on an austenitic stainless-steel surface using the spark plasma sintering (SPS) method and to evaluate its properties. The SPS method was utilized because the rapid heating involved in the process reduces the processing time. AISI 316L as a sample material was boronized for 3.6 ks at 1073 – 1273 K at applied pressures of 4 MPa and 8 MPa with a powder mixture of B₄C and KBF₄. The Vickers hardness profile results showed that the hardness of the untreated sample was ~ 200 HV whereas that of the boronized sample was ~ 2300 HV. The wear test profile showed that the wear resistance of the boronized sample was significantly improved. Moreover, according to elemental analyses, boron diffused from the sample surface to 200 μm and 60 μm when the applied pressure was 8 MPa and 4 MPa, respectively. This indicated that boron diffused to a greater depth when the applied pressure was increased.

Abstract: Round bars Ø 53 mm were hot-rolled from a 1.4 tonne ingot forged to 165 × 165 mm. The composition of the steel was 0.45 wt. % C and 3.33 wt. % Si plus alloying elements for hardenability. Microstructure after air cooling from 1010 °C on the cooling bed was predominantly ausferritic. Tensile testing of as-rolled bars resulted in yield strength 846 ± 22 MPa, ultimate tensile strength 1169 ± 99 MPa and A₅-elongation of 1.7 ± 0.8 % (without prior necking). When as-rolled steel was baked in air at T = {M_{s initial} -30 K} for six hours, the yield stress raised to 1121 ± 4 MPa, the ultimate tensile stress raised to 1447 ± 5 MPa and the elongation raised to 22.6 ± 1.6 % (with necking > 18 %). For as-rolled bars during continuous cooling, the exposure time within the temperature range 460 – 320 °C was estimated to be about 10 minutes. The microstructure of as-rolled “semi-finished” bars is stable at room temperature. The first baking was done six months after hot-rolling. Optical and scanning electron microscopy showed that remaining areas of austenite, not transformed during continuous cooling but stable at room temperature, transforms to ausferrite when properly baked.

Abstract: In ancient bronze ingots Cu₃As was observed beside other impurities like Sb. Moreover, the Cu-As bronzes were studied concerning the decrease in As during melting respectively remelting. To verify the microstructure and hardness of the eutectic and Cu₃As phase appropriate mixtures were produced by melting pure Cu and As. The eutectic point in the Cu-As system is at 685 °C and 20.8 wt. % As and the Cu₃As phase with 29.56 wt. % As melts at 827 °C. In the sample´s core the microstructure is a homogeneous eutectic, but near the surface it becomes hypoeutectic, i.e. an As loss took place. The lamella thickness of the eutectic was in the range of about 1 µm. The sample with a Cu₃As composition showed a proeutectic microstructure with mainly Cu₃As and a small amount of eutectic. In the large Cu₃As crystals twin lamellae were observed. Additionally, by Vickers indents new twins were introduced. The microhardness of the Cu-As solid solution is 78 HV0.025, of the eutectic 125 HV0.025 and of the Cu₃As phase 158 HV0.025. On some surfaces of the Cu₃As sample a Cu-rich phase was observed. We are not able to explain this phenomenon, but it is definitively no “inverse segregation”.

Abstract: Corrosion behavior of the alloys 1.7386 (P9), 1.4462, 1.4841, 1.4959 (Alloy 800HT) and 2.4816 (Alloy 600) was tested for 24, 72 and 240 h between 480 – 680 °C. The testing gas atmosphere contained 3.8 vol. % HCl, 200 ppm H₂S and CO, CO₂ and N₂. It simulated conditions present in a thermal cracking process for post-consumer plastics. Samples were analyzed by metallography, SEM/EDX and XRD after corrosion experiments. Additionally, their mass loss during the test was evaluated. A multilayered structure of corrosion products grew on the samples during the corrosion experiments. The composition of the corrosion products depended not on the material, but on the testing temperature. At 680 °C chromium sulphide formed the outer layer, followed by a chromium oxide layer. Below these two layers a chlorine containing layer was observed. At 480 °C mainly nickel sulphide was detected, besides chromium oxide and iron- and chromium chloride. Especially at higher testing temperatures FeCl₂ was not observed directly on the samples, but as colorless crystals at the colder parts of the testing equipment. At 680 °C the mass loss of the samples decreased with increasing nickel content. However, this effect changed entirely at lower testing temperatures. At 480 °C 1.7386 and 2.4816 showed nearly the same mass losses.

Abstract: Chemical vapour deposited HfN can be utilised as a component of multilayer systems in protective coatings on cutting tools. In this study, related AlHfN coatings were synthesized through a reaction of metallic hafnium and aluminium with HCl gas forming gaseous HfCl₄ and AlCl₃, which were subsequently transported into a heated coating reactor. Via high temperatures and separately introduced NH₃ and N₂ as reaction gases, AlHfN coatings were deposited on hardmetal inserts. By varying the ratio between AlCl₃ and HfCl₄, compositionally different AlHfN coatings were examined. Additionally, surface morphology, composition as well as crystalline phases of the obtained coatings were analysed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Finally, the microstructure of the cross section of a coating was investigated via transmission electron microscopy. The observations revealed a great impact of the gas composition on the morphology and crystal structures of the coatings. Within the layer, the growth of columnar microstructures was detected. Additionally, the formation of an amorphous HfN intermediate layer between the substrate and the AlHfN with a thickness of approximately 2 nm was found.

Abstract: In order to increase the peripheral speed of grinding wheels of size of Ø 500 × 18 × Ø 200 mm for precise hard metallic surface finishing from conventional 80 m.s^-1 up to 138 m.s^-1 while still ensuring their safety and reliability, the critical locations in the grinding wheel were evaluated using the finite element analysis. The microstructure of grinding wheel was revealed using the materialographic techniques and the scanning electron microscope images were recorded in the back-scattered electrons mode. The image analysis was used on recorded micrographs for separation of individual material components, i.e. an abrasive, a binder, and pores, and to extract their geometries and spatial relationships. Subsequently, the influence of different filling agents (Young's modulus of 5, 10, 20, and 40 GPa) was studied, considering both surface dipping and bulk filling treatments.

Abstract: The aim of this paper was to determine the influence of the mechanical and thermal parameters of the matrix materials on their retentive properties. The term ‘matrix retention’ denotes the capacity of a metallic matrix material to retain diamond particles at the surface of a diamond tool during working. The bonding is obtained during cooling after the hot pressing process. Proper mechanical bonding depends on elastic and plastic properties of the matrix. The model of a diamond particle embedded in a metallic matrix was created using Abaqus software. The analysis has indicated the mechanical parameters that are responsible for the retention of diamond particles in a matrix.

Abstract: Forging of ductile cast iron with pure iron by the Damascus technique, results in a new composite material. The combination of cast iron and pure iron is unusual because of its rather different properties. After forging these two materials a small diffusion zone of about 150 µm was observed. Various heat treatments at 900 °C for 2, 4 or 20 hours and 950 °C for 4 h were performed to increase the diffusion zone up to 2.4 mm. At 900 °C carbon solubility in austenite is about 1.2 wt. % and at 950 °C 1.4 wt. %. During the heat treatment carbon diffuses from cast iron into the pure iron and the diffusion gradient grows with time and temperature. Furthermore, the samples were air cooled or water quenched. In the ductile cast iron, graphite nodules are surrounded by ferrite. During the heat treatment graphite is dissolved and pores are observed. In the diffusion gradient layer, a broad range of microstructures observed in hyper- and hypoeutectoid steels could be found. The microstructures were revealed by different etchants and moreover, hardness measurements were performed.

Abstract: In-situ tensile testing of a 27CrNiMoV alloy which is used for steam turbine rotors was carried out using scanning electron microscope (SEM). Deformation and crack formation and propagation were examined with this test. Small (45 × 10 × 1.5 mm, with gauge length of 20 mm), flat samples based on dog-bone shape were prepared from the steel. The material in its initial state contained high number of defects in form of microcracks. A comparison of behaviour at tensile tests of samples without visible defect and with crack in the gauge section was performed. Apparently, the presence of defect in the initial state showed direct influence on properties like lower tensile strength. The sample, its necking and propagated crack is displayed at different stages of the tensile load. In-situ testing reveals differences in the sample deformation. The defect-free sample is affected in its whole body and regular necking can be observed, whereas the presence of the crack in the defect samples concentrates stress to a smaller area and also changed the shape of the stress-strain curve.