Key Engineering Materials Vol. 647

Abstract: Changing the lamellar morphology of pearlite to a globular morphology significantly enhances the formability of pearlite-ferrite steels. This change is conventionally achieved by soft annealing. Annealed structures possess low yield strength and excellent ductility and this ensures their good cold formability. The problems of these technologies lie not only in long processing times, but also in high energy consumption which makes the final product quite expensive. The time necessary for cementite spheroidization can be shortened by unconventional heat treatment around A_c1 temperature combined with deformation applied at various processing stages. Several processing methods were utilized for spring steel 54SiCr with ferrite-pearlite original microstructure and lamellar pearlite morphology. The hardness of this structure reached 290 HV10. Three main strategies were tested in this work, using either tensile and compression deformation with following hold applied at heating temperature, temperature cycling around A_C1 temperature, or deformation cycles applied at heating temperature. First of all, various heating temperatures in the region of 680-740°C were tested to determine the most suitable heating temperature for this steel. Subsequently, the influence of the character and intensity of applied deformations on cementite spheroidization and ferrite grain refinement were investigated. Carbide morphology and distribution were determined by the means of light and scanning electron microscopy and mechanical properties were determined by hardness measurement. Spheroidized carbides evenly distributed in fine ferrite matrix were obtained after the optimization of processing parameters.

Abstract: Sub-zero processing is an add-on step of heat treatment that includes cooling down, holding at low temperature and re-heating up to room temperature. The aim of this process is to improve some mechanical properties of high-carbon steels. In this research, the Cr-V ledeburitic steel Vanadis 6 produced by powder metallurgy was used. As-received material as well as that after heat treatment contains various types of carbides, which are different in terms of both the nature and the origin. During the austenitizing, a part of the carbides (eutectoid, secondary) undergo dissolution in the austenite while other carbides (eutectic, secondary) remain in the material undissolved. The application of sub-zero treatment leads to increase in the volume fraction of small globular carbides, with a size below 0.3 μm. The volume fraction of secondary carbides was found to be constant, irrespectively to the use of sub-zero processing, but it decreases with increase of the austenitizing temperature. The nature of the carbides was found to be practically unaffected by the sub-zero treatment and it can be described as follows: The eutectic carbides are the vanadium rich MC-phase and the secondary ones are the M₇C₃. Regarding the nature of small globular particles, their nature is not doubtless because the electron diffraction fixed them as alloyed cementite but the EDX analysis revealed very high chromium content in the particles.

Abstract: The high-speed P/M steel Vanadis 30 was austenitized at 1100 °C, quenched and tempered at various combinations of processing parameters. For one set of specimens, also sub-zero period, made at -196 °C/4 hours between quenching and tempering. The microstructure and mechanical properties have been investigated as a function of austenitizing time, parameters of sub-zero processing and tempering. There were used metallographic analysis, hardness measurement and three point bending test for the evaluation effect of heat treatment conditions to observed materials.

Abstract:

A deep cryogenic heat treatment (DCT) was applied to X37CrMoV5 steel, which included soaking at -160°C for 12 and 30 hours followed by tempering at 180°C. Microstructures were compared with those after conventional heat treatment (HT). Microstructures with conspicuous dendritic segregation were observed in all specimens. After HT coarser and finer tempered martensite occurred in depleted and enriched areas of carbon and alloying elements respectively. Coarse molybdenum and vanadium carbides, fine secondary Fe₂MoC carbides and retained austenite were identified after HT. Deep freezing resulted in microstructure refinement, transformation of retained austenite into twinned martensite, spinodal decomposition of martensite plates and precipitation of semicoherent h-carbide. The mechanism of h-carbide precipitation was discussed. Wear rate was measured using pin-on-disc test. The best results were obtained after DCT with cryosoaking for 12 hours.

Abstract: Two types of steels used for production of heavy forgings were selected for the experimental evaluation of the effect of long time dwell at elevated temperatures and cooling during heat treatment on their mechanical properties in order avoid the possibility of grain boundary embrittlement. Samples from evaluated steels 26NiCrMoV14-5 and 22CrNiMoWV8-8 were austenitized for 2 hours at temperature of 1200°C and oil quenched. Subsequently the annealing at temperatures (200 - 700)°C for 1 hour and 100 hours was applied. Selected mechanical properties, especially hardness and impact energy, were monitored. It was found that for steels 26NiCrMoV14‑5 and 22CrMoNiWV8-8 exist the temperature intervals (300 - 400)°C and (500 ‑ 600)°C respectively with the possible potential for toughness decreasing.

Abstract: The aim of this work was to analyze the effect of sample thickness on toughness of the slab surface zone at ambient temperature in IF and microalloy steels. Transitional slabs with different pulling rates at the start and the end of the slab were used as well as slabs with standard pulling rates. Samples of standard size 10x10x55 mm and also non-standard size 5x10x55 mm were used for Charpy impact testing. It was confirmed that higher toughness values were found in thin non-standard size samples (5x10x55 mm), or more precisely for a larger portion of non-standard samples with toughness values higher than 50 J.cm^-2. Using non-standard size samples confirmed the distinct stability of toughness values, as well as their smaller spread between minimum and maximum values. The transitional slabs had very distinct and non-uniform spread of toughness values across the slab width. This suggests greatly deteriorated quality of these slabs. During stable casting of TiNb microalloyed steel at the standard conventional higher slab pulling rate, uniform toughness was confirmed across the whole slab width in both sample types.

Abstract: Metallic biomaterials are currently used in medicine for fabrication of various kinds of implants like joint and bone replacements, dental implants, stents, fixation devices for fractured bones etc. Their advantages over polymeric or ceramic biomaterials are in higher strength, fracture toughness and fatigue life. In addition, metals can be simply processed by established technologies known for centuries. Due to the increasing average age of human population, there are growing requirements for mechanical and functional performance of implants. Therefore, extensive research and development activities are focused on new directions in this area including new surface treatments and alloys with improved biocompatibility and mechanical performance, porous biomaterials, biodegradable metallic materials. Biodegradable materials are explored as alternatives for fabrication of temporary medical implants like stents and fixation devices (screws, plates, nails) for fractured bones. The present paper focuses on new Mg-and Zn-biodegradable alloys. Advantages of these materials are characterized with respect to mechanical performance and corrosion behavior.

Abstract: This paper presents the results of structural phase analysis of MAR-M247 superalloy. Analysis was performed at initial and in as casted state after various type of solution annealing in the range 900 °C to 1240 °C with cooling in water. Presented polycrystalline alloy is heat resisting nickel superalloy, especially usable for highly strained components in the industry producing stationary gas turbines and aircraft engines. Analysis was performed using of a Quanta FEG 450 scanning electron microscope with micro-analytic system Trident APEX-4 which have identified presented minor phase. Attention was also paid to its eventual changes after different type of solution annealing.

Abstract: Germanium is an element which is used in metallurgy in a very small extent. Much more significant is its use as a semiconductor material. Most of magnesium alloys are usually used for applications at ambient temperature. The significant decrease in mechanical properties is observed already at the temperature higher than 150°C. This is the reason for the effort to prepare a new low-priced magnesium based alloys with improved mechanical properties at elevated temperatures, e.g. for components of combustion engines. Therefore, new unconventional alloying elements are studied for increase the thermal stability of magnesium alloys. The effect of germanium on the microstructure and mechanical properties of Mg-Ge alloys at ambient and elevated temperatures was studied in this paper.