Iron carbide diagram and heat treatment process part 1

Iron- Iron Carbide Diagram

This is a phase diagram with the carbon composition (weight percent) is plotted on the horizontal axis and temperature on the vertical axis. The diagram shows the phases present at various temperatures for slowly cooled iron-carbon alloys with carbon content up to 6.7%. The diagram gives us idea about the following points:

1.Solid phases in the phase diagram

2.Invariant reactions in the diagram

3. Critical temperatures

4. Eutectoid, hypo-eutectoid and hypereutectoid steels

iron carbide diagram

(Fe3C) iron carbide or cementite:

carbon percentage 6.67%

It is a typical brittle and hard interstitial solid of low tensile strength (approximately 5,000 psi) but high compressive strength. Crystal structure of Fe3C is orthorhombic structure.

Austenite or (gamma iron):

An interstitial compound of carbon mixed up in iron with the cubic crystal of face-centered (F.C.C) structure. Austenite is usually unstable at room temperature. Under processed conditions we may be able to obtain austenite at room temperature.

high toughness, 150,000psi tensile strength., C40 Rockwell hardness

Ferrite or (alpha iron):

A solid solution of an interstitial compound of a small amount of carbon dissolved in iron with (B.C.C.) crystal structure. Ferrite is actually is the softest structure on the iron-iron carbide diagram.

low toughness, 40,000 psi tensile strength,

Pearlite steel:

It is the eutectoid mixture formed at 1333ºF and containing 0.83 % carbon on very slow cooling. The microscopic study reveals very fine plate-like or a lamellar mixture of ferrite and cementite.

Ledeburite :

It is the eutectic combination of cementite and austenite. It contains 4.3 % Carbon and represents the eutectic form of cast iron. Ledeburite is formed only when the carbon content is greater than 2%. It resembles the separating line on the phase diagram between steel and cast iron.

Delta Iron :

Delta iron exists between 2552º and 2802ºF. It may exist in mixture with the melt at about composition of 0.50 % Carbon, in combination with austenite to about 0.18 % Carbon and in a single phase state out to about 0.10 % carbon. Delta iron has the body-centered cubic (B.C.C) crystal structure and is magnetic.

Other forms of iron and carbon mixture include following

Martensite:

This is formed as a result of martempering i.e. by fast cooling the austenite iron from the austenitic region.

Bainite:

This is formed as a result of austempering i.e by very slow cooling of austenite from austenitic region

Sorbite

Application of this diagram

The iron-iron carbide diagram is useful in finding out what will be the crystal structure and the physical and chemical properties of iron at known carbon percentage and temperature. Helps in the investigation of different types of structures derived from heat treatment of steel.

Limitations

1. Does not say anything about the transformation of austenite at varying cooling rates is missing.
2. The design of heat treatment for obtaining desired properties of a combination of iron and carbon is not possible.
3. How much time is taken to start the phase transformation or how long does the phase transformation take to complete is not explained by it.

Simple Heat Treatments

Annealing

It is a heat process whereby a metal is heated to a specific temp/color and then allowed to cool slowly. Annealing softens the metal, metal now can be cut and shaped more easily. They bend or changes shape easily when pressure is applied.

 Full Annealing

 Process Annealing

 Stress-Relief Annealing

 Spheroidisation

Full Annealing

It is the process of slowly raising the temp about 50ºC (90ºF) above the Austenite temp line A3 line in the case of hypo-eutectoid steels (steels with less than 0.77%). It is held at this temp for sufficient time for all the materials to transform into Austenite or Austenite-Cementite. It is then slowly cooled at the rate of about 20ºC/hr. (36ºF/hr.) in a furnace to about 50ºC(90ºF) into the Ferrite-Cementite.

Process Annealing

It is used to release stress in work-hardened parts made out of low carbon steels (less than 0.25%).The part formed is soft. Iron is heated to a temperature of 727 or less degree C so no phase transformation takes place.

Stress-Relief Annealing

It is used to reduce residual stresses in large castings, welded parts and cold-formed parts. Such parts tend to have stresses developed in them due to thermal cycling or(cold work) work hardening. Parts are heated to a temp of up to 600-650ºC and held for an extended time about 1 hour and more and then slowly cooled in still air.

Spheroidisation

It is an annealing process used for high carbon steels (carbon more than 0.6%) that will be machined or cold formed subsequently.

Applications of Annealing

Annealed metals are comparatively soft and be cut and shaped more easily. Steel or iron bend easily when force or stress is applied. They are heated and allowed to cool slowly. Thus, applications include decorative items, Low-friction applications (locks, gears, and doorknobs), plumbing/electronics, musical instruments

Normalizing

The objective of normalization is to leave the specimen in a normal state, in other words with the absence of internal stresses and uniform distribution of carbon. The high temperatures are kept for this process until the complete conversion of austenite steel with air cooling(leaving in atmospheric air). Normalizing is mostly used as a pre-treatment before quenching and tempering and post-treatment for forging.

Comparison between annealing and normalizing

The main use of annealing treatment is to just remove internal tensions, regenerate overheated steel structures or soften the steel. By completing the normalization of metal (e.g. steel), the strength and toughness of the piece of steel can be improved as compared to the steel(iron-carbon mixture) that has not been not normalized or subjected to another heating process. The ductility property of the steel also gets improvised through normalization; this phenomenon is possible without reducing the value of the hardness or the strength as can happen with other heat treatments.

Quenching and its medium

Quenching is the act of rapidly cooling the hot steel to harden the steel. The various mediums in which quenching can be done are:

 Water:

Quenching is done by placing the heated steel in water. The water touching to the heated steel surfaces vaporizes, and there is no direct contact of the steel with the water. This causes slow down of cooling until the bubbles break and again allow water to contact with the hot steel.

 Salt Water:

The salt solution is actually a rapid quenching medium than water alone because the bubbles of water are broken very fast and allow for rapid cooling of the heated specimen. However, this medium is more corrosion causing than water as a medium , and hence, must be rinsed off quickly.

 Oil:

Oil as quenching medium is used when a slower cooling rate is desired. Since Oil has a high boiling point, as a result the transition from the start of martensite formation to the finish is slow. Oil quenching process causes fumes, spills, and sometimes a fire hazard.

 Polymer:

Polymer quenching medium will produce a cooling rate in between water and oil. The cooling speed can be varied by changing the components in the mixture as these are formed of water and some(organic) glycol polymers.

 Cryogenic quench:

Cryogenics ( extremely low temperature about -172is preferred to make sure there is no remaining Austenite during quenching. This is done in order to guarantee overall conversion to martensite. Since a mixture of martensite and austenite may be formed as a result of another quenching method.

References:
1. D. R. Askeland, “The Science and Engineering of Materials”, PWS- Kent Publishing Co., Boston,
2. Westerman Table ( IS Standard)