Case hardening is achieved with the quenching of the high carbon surface layer that has a good fatigue and wear resistance. This layer is applied on a tough low carbon steel nucleus. Case hardening of the carburized steels is mainly a function of carbon content. When steel carbon contents exceed a certain percentage, hardness is not affected by further carbon addition. Additional carbon at this stage is not dissolved.
|Published (Last):||10 January 2012|
|PDF File Size:||10.31 Mb|
|ePub File Size:||14.84 Mb|
|Price:||Free* [*Free Regsitration Required]|
Case hardening is achieved with the quenching of the high carbon surface layer that has a good fatigue and wear resistance. This layer is applied on a tough low carbon steel nucleus.
Case hardening of the carburized steels is mainly a function of carbon content. When steel carbon contents exceed a certain percentage, hardness is not affected by further carbon addition. Additional carbon at this stage is not dissolved. Steel case hardening depth of the carburized steel depends upon the carburizing time and the surface carbon intensity. When the carburizing time is prolonged to obtain increased case depths, excessive free carbides may be formed.
The micro-structural elements have an unfavorable effect on the distribution of residual stresses. Carburized steels generally have base carbon contents of approximately 0. Most carburized steels are deoxidized by the addition of aluminum. Due to the carbon content difference in the core and the case, their case hardening capability is also different.
Methods of Carburizing While the fundamental carburizing principle has not changed much since its inception, the carbon introduction techniques have been improving. The four carburizing processes are explained below. Pack Carburizing Components are packed in an environment with a high carbon content such as cast iron shavings or carbon powder.
The components are heated with the production of carbon monoxide, which is a reducing agent. The reduction occurs on the steel surface with the release of carbon that is diffused into the surface because of high temperatures. With the carbon absorption inside the component, the materials are hardened.
The surface carbon is in the range of 0. The case depth is approximately 0. Control of pack carburizing is difficult because uniform temperatures may not be maintained. Gas Carburizing Gas carburizing is theoretically similar to pack carburizing aside from the supply of carbon monoxide gas to the heated furnace and the carbon decomposition.
Many of the problems with pack carburizing are eliminated in this process. The carbon monoxide gas needs to be contained safely. The components are enclosed in a carbon bearing environment that is replenished continuously to maintain a high carbon content.
Gas carburizing is one of the most successful and popular carburizing technique used when large quantities of parts are required. Vacuum Carburizing The vacuum carburizing process involves carburizing in a low pressure environment that is oxygen-free.
The atmosphere is significantly simplified although the furnace enclosure is difficult. A single component environment containing uncomplicated gaseous hydrocarbon like methane is used. Since the environment used for heating is oxygen free, the carburizing temperatures can be considerably increased without surface oxidation. The higher temperatures increase the carbon solubility and the diffusion rate.
Thus, the time necessary for case depth is minimized. Liquid Carburizing The steel components are submerged in a liquefied carbon rich environment.
The main component in such baths is cyanide. However, safety issues have led to baths that are non-toxic that accomplish similar results. The components are held in a molten salt that introduces carbon into the metal. Carbon is diffused inwards producing a hardened case by rapid quenching. The case produced by carbon diffusion is similar to that produced by gas carburizing. Cases formed by liquid carburizing have low nitrogen and high carbon content.
Carburizing Mild Steel – How it works
It is especially effective in large cross sections like those of wind-turbine gears. Molybdenum is not oxidised during carburisation, so it does not cause increased surface cracking and spalling. This also means it is not lost by reaction, but remains present in the alloy to provide effective hardening. Wind power — a major driver for carburising steel development Gears used in large wind turbines are subject to extreme loads at the flanks and toes of their teeth, especially when sudden changes in wind speed or hard stops occur. A hard case and tough core result in a more wear-resistant gear capable of handling high impact loads.
Carburizing Process and Techniques – Four Methods of Carburizing
Method[ edit ] Carburization of steel involves a heat treatment of the metallic surface using a source of carbon. The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the remainder of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber. Plasma carburization is increasingly used to improve the surface characteristics such as wear, corrosion resistance, hardness , load-bearing capacity, in addition to quality-based variables of various metals, notably stainless steels.
Main article: Differential heat treatment A flame-hardened sprocket. The discoloration around the teeth represents the area that was rapidly heated and then quenched. Flame or induction hardening are processes in which the surface of the steel is heated very rapidly to high temperatures by direct application of an oxy-gas flame, or by induction heating then cooled rapidly, generally using water; this creates a "case" of martensite on the surface. A carbon content of 0. Typical uses are for the shackle of a lock, where the outer layer is hardened to be file resistant, and mechanical gears, where hard gear mesh surfaces are needed to maintain a long service life while toughness is required to maintain durability and resistance to catastrophic failure. Flame hardening uses direct impingement of an oxy-gas flame onto a defined surface area.