Defect and Diffusion Forum Vol. 395

Abstract: Although heat treatment of high-speed steels has been covered in numerous treatises, their microstructure before treatment still receives insufficient attention. This article therefore explores the relationship between the initial microstructure and resultant properties in heat treated steels for cutting tools and sonotrodes (special tools for ultrasonic welding). As shown in experiments, the size of fragments of primary carbides affects the properties of the matrix after heat treatment and the precipitation of secondary carbides. Hence, the decisive factor is the method by which the initial stock is mechanically worked, as it may produce primary carbides with more uniform distribution and size. In contrast, the article lists some cases where clusters of carbides caused cracking after heat treatment. The article also covers multidirectional forging by which the desired initial microstructure is obtained.

Abstract: The paper presents the results of study of corrosion resistance of Vanadis 6 steel after conventional heat treatment and sub-zero treatment at-140 °C for 17 h. It was found that sub-zero treatment leads generally to decrease in corrosion current, which is in turn reflected in lower corrosion rate of the material subjected to this kind of treatment. The corrosion potential of sub-zero treated specimens was more anodic, suggesting their more noble behavior in 3.5 % NaCl water solution. This behavior was ascribed to the presence of a huge number of small globular carbides in SZT specimens, which seemed to stabilize the protective oxide films on the surfaces. Tempering reduces the corrosion resistance of the steel since this treatment produces fine precipitates of alloyed carbides that deplete the matrix by chromium and thereby destabilizes the protective films on the steel surface.

Abstract: This article concerns distortion of a workpiece after induction-hardening under various conditions. It focuses particularly on the effects of quenching water temperature, PAG polymer concentration and the rotation speed of the workpiece during induction hardening. Electrical as well as non-electrical quantities which affect the process were monitored. They included the current passing through the inductor, the power frequency, quenching water temperature, the flow rate of the quenchant through the spray-quench device, the speed of rotation of the workpiece and some others. The workpiece was a cylinder 70 mm in length which contained a drilled off-axis through hole. Prior to hardening, dimensions of the workpiece and the hole were measured on three planes set in different distances from the bottom face. The measurement was repeated after induction hardening and the findings are reported in this article. Post-process hardness was measured on the cylindrical surface of the workpiece. Hardening depths obtained with different quenchants were measured.

Abstract: The effect of cryogenic processing and tempering on selected mechanical properties and fracture micromechanism of Vanadis 6 high alloy cold work tool steel was analysed. The samples were processed in cold nitrogen gas at -140 °C for 48 h, and tempered at temperatures of 170 – 530 °C. It was found that the hardness of sub-zero treated Vanadis 6 steel decreases with increasing tempering temperature. The highest hardness of the specimen 960 HV10 was achieved by tempering at 170 °C and the lowest hardness 790 HV10 resulted from tempering at the highest tempering temperature, i.e. 530 °C. However, the hardness of conventional heat treated samples was less than 800 HV10 in full range of tempering temperatures. The fracture toughness of sub-zero treated samples does not differ from what was obtained by conventional heat treatment schedule except the case of the high tempering temperature of 530 °C where an increase in fracture toughness by approx.. 3 MPa.m^1/2 has been recorded. The carbides differ clearly in their role in the fracture propagation. While the secondary carbides undergo easily cleavage the eutectic carbides assist more probably decohesive fracture propagation.

Abstract: This paper describes simple metallographic technique for selective etching of individual types of carbides (MC, M₂C and M₆C and M₇C₃) in tool steel. Electrolytic etching in chromic acid was used in order to reveal the MC carbides. Chemical etching in permanganate solution revealed the M₂C and M₆C carbides, while the electrolytic etching in the latter solution enabled to observe M₇C₃, M₂C and M₆C carbides. These techniques were demonstrated on an experimental niobium-containing tool steel prepared by powder metallurgy. The results confirm that the MC carbides are highly thermally stable, while the M₂C carbides decompose during austenitizing at the temperature of 1050 °C and higher. The M₇C₃ carbides dissolve in the austenite significantly. This exact and simple observation of the carbides behaviour enables to describe the role of particular carbides on heat treatment behaviour and also to save the carbide-forming elements, where the important ones (tungsten, vanadium) are listed as critical raw materials and the others (chromium and molybdenum) are also strategic.

Abstract: Rolling bearings are among the most widely widespread components in the engineering industry. A critical issue is damage to functional surfaces of rolling bearings, which has arisen during the operation. An important area is the monitoring of production temperature and its impact on components. We undertook this study to damage of functional surfaces bearings. It was decided that the optimal procedure is comparing three samples, each with another type of damage. This procedure will aid determine the cause of damaged bearings. This work has shown the adverse effect of residual austenite and residual stresses caused by the production process on the dimensional stability of the bearing components.

Abstract: A Cr-V ledeburitic steel Vanadis 6 was powder boronized for different durations, austenitized, quenched and tempered. All the boronized layers are two-phased, i.e. they are formed by MeB and Me₂B compounds. The thickness of boronized compound layers increased moderately with boronizing time. The thickness of MeB individual layer increases more rapidly, and it makes up to 50% of total layer thickness when boronized for 150 min. Boronized layers contain enhanced amount of chromium but they are almost free of other alloying elements. The MeB compound has a microhardness of around 1860 HV 0.1 after short – time boronizing but its microhardness exceeded 2100 HV 0.1 after long – time processing. The microhardness of Me₂B was correspondingly lower, its values were around 1760 – 1850 HV 0.1. Transient region manifested enhanced amount of carbides, resulting from carbon transport from growing boronized layers towards the substrate. This phenomenon was reflected in elevated hardness compared to the substrate.

Abstract: In this work, the microstructure and mechanical properties of four types of high-speed tool steels (Vanadis 30, Vanadis 60, ASP 2052 and S 705) were studied. The steel S 705 was made by conventional ingot metallurgy technology, and other types of steels were manufactured by powder metallurgy technology. All studied steels were examined both in the soft state and further in the hardened condition with subsequent tempering. Microstructure of metallographic samples and fracture areas was studied by electron microscopy. Hardness, tensile properties and notch toughness were determined. Significant differences in the properties of steels in both studied states were documented.

Abstract: A PM made Cr-V ledeburitic tool steel Vanadis 6 has been subjected to conventional austenitizing and quenching, which was followed by sub-zero treatment at different temperatures, and by tempering treatments. The microstructure, hardness and fracture toughness of sub-zero treated steel have been investigated with reference to the same material after conventional room temperature quenching. The main findings are that sub-zero treatments reduce the retained austenite amount, enhance the population density of small carbides, refine the martensite and change the precipitation of carbides during tempering. These alterations are reflected in elevated hardness after low-temperature tempering but slightly lowered hardness after tempering within the normal secondary hardening temperature range, except the specimens treated at-140 °C where the hardness improvement was maintained. The fracture toughness is rather negatively influence by the sub-zero treatments, except the treatment at-140 °C where no impact or rather improvement has been recorded; thus, the treatment at a temperature of-140 °C seems to be a promising way how to improve the hardness and the fracture toughness pf the Vanadis 6 steel simultaneously.