Carburizing is one of the most widely used surface hardening processes. It has been in use for a long time. The process involves diffusing carbon into a low carbon steel to form a high carbon steel surface. Carburizing process is also referred to as case hardening or case carburizing process.
Full Answer
What is carburization of steel?
Carburization of steel involves a heat treatment of the metallic surface using a source of carbon. Carburization can be used to increase the surface hardness of low carbon steel.
What are the different types of carburizing?
The following are the carburizing processes commonly used by the industry. Pack carburizing – In this process low carbon steel components are packed in an environment with 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.
How has the carburizing process evolved over time?
The carburizing process has evolved with advancements in heat treatment techniques that have improved the hardness and durability of products like carbon steel wire springs and carbon steel forgings.
What gases are used in the carburization process?
In the carburization process, carbon-rich gases, such as methane and CH 4 –CO–H 2 mixtures, are made to flow over low- or medium-carbon steels (0.1–0.4 wt% C) maintained at temperatures of ∼900°C.
What is the result of carburization?
A result of carburization is an increase in hardness of the carburized materials. Low alloy steels are often exposed to carbonaceous environments to produce hard surfaces to increase wear resistance, corrosion resistance, and abrasion resistance.
What is low temperature carburization?
Low temperature carburization is a gaseous carburization process performed at atmospheric pressure, at temperatures where the kinetics of substitutional diffusion are very slow. Low temperature (350–550 °C) carburization hardens the surface of austenitic stainless steels through the diffusion of interstitial carbon, without the formation of carbides. The surface must be activated, by modification and removal of the naturally occurring chromia (chromium oxide) layer, for the diffusion process to work. The process results in a hardened diffusional case on the surface, typically 20–35 µm thick, with carbon content at the surface in excess of 10 at% and near-surface hardness of about HV 1200 (over 70 HRC). The case remains austenitic and retains its ductility.
What is the most widely used diffusional surface treatment?
Carburization of steel is easily the most well known and widely used diffusional surface treatment. The intent is to boost the C concentration at the surface sufficiently so that it can be made harder by a subsequent quenching and tempering heat treatment (described in Section 9-2 ). The result is a case hardened layer at the surface that can be made visible metallographically. In the carburization process, carbon-rich gases, such as methane and CH 4 –CO–H 2 mixtures, are made to flow over low- or medium-carbon steels (0.1–0.4 wt% C) maintained at temperatures of ∼900°C. Pyrolysis at the metal surface releases elemental carbon which diffuses into austenite or γ-Fe, a phase that can dissolve about 1.25 wt% C at this temperature. A surface layer or case typically 1 mm thick is enriched to about 1 wt% C. After heat treatment, a hardened, wear-resistant case consisting of a hard, metastable phase of steel known as martensite will then surround the softer but tougher interior. Many automotive parts, machine components, and tools such as gears, cam-shafts, and chisels require this combination of properties and therefore are carburized. Some carburized parts are schematically reproduced in Fig. 6-5. In designing carburization treatments the value for the diffusion coefficient of carbon in α-iron may be taken as Dα (C) = 0.004 exp (−19.2 kcal/mol/ RT) cm 2 /s. Similarly, in γ-iron the diffusion coefficient of carbon is approximately Dγ ( C) = 0.12 exp (−32.0 kcal/mol/ RT) cm 2 /s. Note that these D values depend on steel composition.
What happens when temperature gradients are applied to heat exchangers?
Temperature gradients along a heat exchanger tube can result in carburization at one location and decarburization in another. Likewise, oxidation potential gradients due to the presence of dissimilar metals in contact with a common environment can preferentially transport ions from one surface to another.
What are the products of corrosion?
The corrosion products are oxides, carbides and graphite in the form of silk, plate and powder. These corrosion products can be blown off at areas with high gas velocities, resulting in severe corrosion. Carburization can also lead to cracking, creep fracture and fatigue of metallic components.
What happens when you decarburize steel?
Decarburization on the steel surface will decrease the surface hardness and fatigue strength.
How is carbon formed?
Free carbon is formed through the decomposition of carbides (such as CO or hydrocarbon) on steel surfaces at high temperatures. These free carbons can destroy oxide scales and penetrate into the steel substrate to form carbides. With the presence of oxygen in the atmosphere, pits will form on the surfaces.
What is carburization used for?
Carburization can be used to increase the surface hardness of low carbon steel. Early carburization used a direct application of charcoal packed around the sample to be treated (initially referred to as case hardening ), but modern techniques use carbon-bearing gases or plasmas (such as carbon dioxide or methane ).
What is gas carburizing?
In general, gas carburizing is used for parts that are large. Liquid carburizing is used for small and medium parts and pack carburizing can be used for large parts and individual processing of small parts in bulk. Vacuum carburizing (low pressure carburizing or LPC) can be applied across a large spectrum of parts when used in conjunction with either oil or high pressure gas quenching (HPGQ), depending on the alloying elements within the base material.
What is a pack carburizing container?
In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickel-chromium alloy and sealed at all openings with fire clay.
What is a carburizing furnace?
Carburising, carburizing (chiefly American English), or carburisation is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder.
What are the advantages of carburizing over carbonitriding?
Also the advantages of carburizing over carbonitriding are greater case depth (case depths of greater than 0.3 inch are possible), less distortion, and better impact strength. This makes it perfect for high strength and wear applications (e.g. scissors or swords).
Why is steel hard?
When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard due to the transformation from austenite to martensite , while the core remains soft and tough as a ferritic and/or pearlite microstructure.
What is the process of absorbing carbon?
Carburizing. Not to be confused with carbonization, carburation, or carbonation. Carburising, carburizing (chiefly American English), or carburisation is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide.
What is carburizing steel?
Carburizing is one of the most widely used surface hardening processes. It has been in use for a long time. The process involves diffusing carbon into a low carbon steel to form a high carbon steel surface. Carburizing process is also referred to as case hardening or case carburizing process. It is a heat treatment process ...
What is the carbon content of carburizing steel?
Carburizing steels for case hardening. Carburizing steels normally have base carbon contents of around 0.2 %, with the carbon content of the carburized layer generally being controlled in the range of 0.8 % to 1 %.
What is the hardenability of case hardening steels?
The hardenability of a case hardening steels must be sufficiently good to result in a martensitic surface case to the required depth. Case hardening steels must therefore contain a certain amount of alloying elements. A further requirement is that steels for carburizing should be fine grain treated.
How is carbon diffused in steel?
The components are held in a molten salt that introduces carbon into the steel. 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.
What temperature does carburizing steel take?
Carburizing is a heat treatment process in which steel absorbs carbon liberated when the steel is heated between 850 deg C to 950 deg C in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of making the steel harder. The heated steel at this temperature has austenitic structure which has got high ...
How does carburetor work?
Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel creating a case while retaining a substantially lesser hardness in the core. This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components.
Why is plasma carburization important?
Plasma carburization is increasingly being used in major industrial establishments to improve the surface characteristics (such as wear and corrosion resistance, hardness and load bearing capacity, in addition to quality based variables) of various steels, especially stainless steels.
What is carburizing steel?
Carburizing is the addition of carbon to the surface of low carbon steels. It is generally accomplished at temperatures between 850-1,000°C. Once quenched, the high-carbon surface layer yields a high hardness martensitic case with excellent wear and fatigue resistance [1]. This carburized case surrounds the tough, low carbon steel core.
What happens after carburizing carbon?
After carburizing, the part is then quenched and tempered. However, one common assumption is “more is better.”. Often, heat treaters will use carbon potentials greater than the saturation point of carbon in austenite in the mistaken perception that the carbon will be driven into the part faster.
What happens after austenite is soaked?
After soaking, the part is then quenched and tempered. This approach has the advantages of a final austenite grain size that is smaller, improving ductility. The low reheat temperature places an upper limit on the amount of carbon dissolved in the austenite, and reduces the amount of retained austenite in the case.
What temperature is carburized?
The part is then quenched and tempered. In some cases, for very deep case depths, the part is carburized at a very high temperature (927°C and above) to shorten process times.
What is surface engineering of steel?
Surface Engineering of Steels: Understanding Carburizing. Carburizing is a widely used, effective technique to increase surface hardness of steel used in gears, and achieve a compressive residual stress. There are several methods, and hotter isn’t always better. Carburizing is the addition of carbon to the surface of low carbon steels.
How many carbon atoms are in a cyclic ring?
This means that each of the carbon atoms is bonded to two hydrogen atoms and two carbon atoms. The carbon atoms form a cyclic ring of six carbon atoms. Cyclohexane is a liquid at room temperature, and readily available. Once heated, the parts act as catalysts and the carbon is released at the part surface.
What happens when carbon content is too high?
Too high carbon content can result in excessive carbide networks or massive carbides. The case depth is a function of time temperature and chemistry, of the process, and the available carbon (carbon potential) at the surface of the steel. This follows Fick’s Second Law of Diffusion for the concentration of a diffusing species as a function of time, ...
What is Carburizing?
Carburizing is a case hardening process in which carbon diffuses into the surface layer of a steel part at a temperature high enough to change the steel grain structure. This change enables the steel to absorb carbon. The result is a wear-resistant layer that makes carburizing an ideal process in the production of strong, safe metals.
Method of Carburization
Carburization of steel involves heat treatment of the metallic surface using a source of carbon. Carburization can be used to increase the surface hardness of low carbon steel. Early carburization used a direct application of charcoal packed around the sample to be treated, but modern techniques use carbon-bearing gases or plasmas.
Types of Carburizing
In the past, depending on the carbon source, there were three types of carburizing methods: solid carburizing, liquid carburizing, and gas carburizing. Charcoal, molten salt, and carbon-containing gases such as natural gas and propane are used accordingly.