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Heat-Treatment of Alloys
Published in Alan Cottrell, An Introduction to Metallurgy, 2019
For many engineering components such as gears, shafts and valves it is an advantage to use a steel with a hard surface and a softer, tough interior. This is usually done by one of several case hardening treatments in which the surface layer of a soft steel (e.g. 0.2 wt per cent C) is enriched with a hardening alloy element, usually carbon or nitrogen, by a diffusion treatment. In carburizing, the steel is heated for some hours at about 900°C in a carbon-rich atmosphere provided by gaseous, liquid or solid carbonaceous substances, to produce a carburized layer by diffusion to a depth of about 10−3m. The surface is then hardened by quenching from about 750°C, a treatment that leaves the interior soft. In nitriding, the metal contains about 1 per cent aluminium and is heated at 500–550°C in ammonia for some hours. Nitrogen atoms diffuse into the surface and form fine stable nitride precipitates with the aluminium, so that the metal is precipitation-hardened in its surface. No subsequent heat-treatment is needed. In cyaniding, a steel containing about 0.2 per cent C is heated at 850–900°C for about 1 hour in molten sodium cyanide. Carbon and nitrogen atoms diffuse into the metal and the surface is then hardened by quenching in oil or water. Carbonitriding is similar, except that the metal is heated in a gas of hydrocarbon and ammonia for some hours at 700–850°C.
Water Based Pack Carburizing Technique: A Heat Treatment Practice for Sustainable Environment
Published in Dzaraini Kamarun, Ramlah Mohd. Tajuddin, Bulan Abdullah, Engineering and Technical Development for a Sustainable Environment, 2017
Muhammad Hafizuddin Jumadin, Bulan Abdullah, Muhammad Hussain Ismail, Siti Khadijah Alias
There are several types of carburizing, but the common used were powder (pack) carburizing, fluid carburizing and gas carburizing [7]. The differences between these carburizing are their carbon compound used (powder, liquid and gases). The cheapest and easiest type is the pack carburizing. It is environmentally friendly compared to fluid carburizing and it is way cheaper and easily operates compared to gas carburizing. The problem with pack carburizing, it requires high temperature and high soaking time. High temperature and high soaking time mean high energy needed for combustion of the furnace for a longer time and this will contribute to destroy of environment and waste of natural resources.
Shaft Design
Published in Wei Tong, Mechanical Design and Manufacturing of Electric Motors, 2022
Spheroidizing is a form of heat treatment for carbon steel, especially for high-carbon steel with a carbon content of more than 0.6%. The spheroidite structure, which contains sphere-like cementitie particles, forms when carbon steel is heated to approximately 700°C for more than 30 h. The purpose of this process is to soften high-carbon steels for improving machinability and increasing ductility of high-carbon steel. An annealing process is often used to relieve residual stresses in the cold-worked steel with a carbon content of 0.3% or above. Fully annealed steel has no residual stresses and becomes ductile. Normalizing helps steel with a fine and more uniform pearlitic structure. Normalized steel still maintains high strength and durability. For low-carbon steel, carburizing can increase the hardness and improve other mechanical properties. During the process, carbon molecules penetrate into the material surface. The affected area and depth of carbon content are dependent on the heating time and temperature. Generally, higher temperatures and longer heating time lead to greater carbon diffusion into steel. The carbon content of quenchable steel must be 0.3% or above. A quenching process can increase yield and ultimate tensile strength, fatigue strength, and hardness but decrease material elongation. Usually, quenched steel is about three or four times harder than the normalized steel. However, residual stresses can be introduced during the quenching process in the bulk of the shaft that may cause cracks on the shaft surface. In all heat treatment processes, the heating and cooling rate and temperature holding time are critical parameters to impact the metallographic microstructure of steel.
Detrimental effects of low-rank coal utilization to the operational condition of large capacity updraft moving-bed gasifier
Published in International Journal of Coal Preparation and Utilization, 2022
Phiciato Phiciato, Miftahul Huda
The main purpose of the metallurgical analysis is to analyze the effect of heat exposure to rotating grate. We recall that steam to carbon ratio dropped significantly at 270 h which promoted very low carbon conversion and thus the large quantity of char in the ashtray. Simultaneously, the rotating grate was exposed to high temperature due to the absence of ash zone. The initial suspect is that the presence of char and high-temperature operation has provoked carburization process of the grate. Carburization is a common process for surface hardening of steel, where carbon diffuses into the surface and creates lattice distortion of the crystal structure. The carburization process of Top sample is analogous to conventional pack carburizing in which the steel part is encased in a bed of charcoal, heated between 850°C and 950°C and held for several hours. As a consequence, the hardness and brittleness of steel surface rise with the increase of carbon content. However, it is not desirable to have maximum carbon content at the surface exceeding 0.8% as the surface tends to fail in a brittle manner (Lucas and Vander Voort 2009). Excessive carburization at the surface can result in premature fatigue failures because of the high gradient of carbon concentration between the surface (carbon rich) and the inner (carbon poor) part. The carbon-rich surface can create high residual tensile stress that results in increased distortion and dramatically reduce fatigue strength (Mackenzie and Scott 2008). In this case, a large quantity of char promotes the embrittlement of grate and fatigue crack as a result of excessive carburizing.
Surface and bulk modification techniques to mitigate silt erosion in hydro turbines: a review of techniques and parameters
Published in Surface Engineering, 2022
Carburizing is a heat treatment process in which the surface of the substrate is diffused with carbon to case harden the substrate. It is generally performed between 850°C to 950°C because of the transformation of the microstructure in austenite [88]. The austenite phase is having higher carbon solubility. Abd-Elrhman et al. performed the carburizing treatment on AISI 5117 steel at different holding times and investigated the hardness of the carburized surface [88]. They found the increase in the case depth with the holding time and subsequently the hardness. Further, they conducted the slurry erosion experiments on the as-received and the carburized steel. The improved hardness was found responsible for the increased slurry erosion resistance of the carburized steel.
NIMS fatigue data sheet on gigacycle fatigue properties of SCr420 (0.20C-1.05Cr) carburizing steel for machine structural use
Published in Science and Technology of Advanced Materials: Methods, 2022
Hisashi Hirukawa, Yoshiyuki Furuya, Hideaki Nishikawa, Nobuo Nagashima, Etsuo Takeuchi
The fracture surface morphologies are also suggestive. Typical fatigue fracture surfaces were not observed. There were only intergranular fracture morphologies or fish-eye patterns. Our interpretation is that the hardened layers of the carburizing steel are too brittle to allow stable fatigue crack growth. Namely, the hardened layers are brittle-fractured immediately after the crack initiation. When the fatigue cracks initiate at interiors, the fatigue cracks can stably grow because of the vacuum environment. Stress gradients due to the rotating-bending tests may also affect the internal stable crack growth, i.e. stress at internal crack initiation sites is lower than that at the surfaces. The fracture surfaces of the carburizing steel are thus distinctive.