Iron carbide diagram and heat treatment process part 2

Hardness and Hardenability

Hardness is a function of the carbon content of the steel. Hardening of a steel( a carbon and iron mixture) requires a change in crystal structure from the B.C.C. structure found at room temperature to the (F.C.C.) face centered cubic structure present in the Austenitic region. The steel is firstly heated to Austenitic region Then when suddenly quenched, the martensite steel is formed. This steel is a very strong and brittle structure.

Hardenability, which is a calculation of the depth of full hardness reached. Hardness is dependent on amount and type of alloying elements. Alloys having the same amount of Carbon content will get the same amount of maximum hardness value; however, the depth of full hardness will differ with the varying alloy elements.

Hardenability and Jominy End Quench Test

This test is conducted to investigate variance of the hardness of a specimen with the distance from quenched surface. the specimen consists of a cylindrical rod 4 inches long 1 inches in diameter. The specimen material is first heated to a predesigned austenitizing temperature and prolonged at that same condition for a long time to obtain an even austenite structure .The specimen is placed in a jig and stream of water is allowed to strike that end while another end is let air cooled. The cooling conditions within the Jominy bar during quenching varies little across the Dia but very fast at that end and respectively less rapidly at points towards the opposite end.

hardness variation with length aftertest

Time-Temperature-Transformation (TTT) Diagram

It is a curve drawn of temperature versus the logarithm of time for a steel (carbon and iron mixture ) of specific composition. It is used to determine the condition when the phase transformations in steel begin and ends for (constant temperature) an isothermal heat treatment of a previously austenitized alloy.

TTT Curve

A rapid quenching process is disturbed ( the horizontal line represents the interruption) by keeping the specimen in a molten salt bath and keeping at a constant temperature. This is followed by another cooling process that enters into bainite region of TTT diagram. The end product is bainite

If the cooling rate is higher than certain critical rates then product formed is truly martensite. However, on a slow cooling product is pearlite.

Different transformation of austenite while cooling

Quench and Tempering Processes

The various hardening methods are:

(1) Conventional Heat, Quench and Temper process

Austenite is transformed to martensite as a result of highly fast quenching from a furnace to room temperature. One serious disadvantage is the increased chances of distortion and cracking the metal as a result of extreme fast quenching process required to form martensite without converting any of the austenite steel to pearlite form . The outer exposed area is cooled quicker than the inner unexposed center. Thinner parts are cooled quicker with the comparison of the parts with greater areas. As a result of various cooling rates, it can lead to cracking as well.

(2) Stepped Quenching or Martempering

To overcome the disadvantage or defect of conventional quenching and tempering, Martempering process can be used. It allows cooling of steel outer surface and the inner core to convert into martensite at the same temperature. This is done by keeping the material at a constant temperature for a certain time.

(3) Isothermal Quenching or Austempering

It is performed in the same manner principally as martempering but with a longer holding time at hot bath temperature (above the martensite point) to ensure a sufficiently complete austenite decomposition

Advantages of Austempering

1. distortion and cracking in austempering is less than Martempering.
2. final tempering is not needed (more energy efficient and less time consuming).
3. Toughness increases (impact resistance is higher than the conventional quench and tempering).
4. Improved ductility.

Different types of surface hardening process

Carburizing

It is a process in which steel intakes liberated atomic carbon when the metal is heated in the presence of a carbonaceous material, such as charcoal or carbon monoxide, with the intention of making the metal harder. On the basis of the amount of time and temperature, the affected area can vary in carbon content. Higher carburizing temperature and more time lead to higher carbon accumulation into the specimen as well as increased depth of carbon deposition.

There are three different types of carburizing processes:

1. Pack Carburizing

In this process, the specimen to be carburized is kept in a steel container so that it is surrounded by granules of charcoal or coal in all sides. An activating chemical such as Barium Carbonate (BaBO3) that promotes the increased production of Carbon Dioxide (CO 2)

Carbon Monoxide reaction: CO2 + C → 2 CO

Reaction of Cementite to Carbon Monoxide: 2 CO + 3 Fe →Fe3C + CO2

1. Gas Carburizing

Gas carburizing can be done with any carbonaceous gas. Most carburizing gasses are flammable so this process should be done in inert atmosphere at about 1700ºF

1. Liquid Carburizing

Liquid carburizing can be performed by carburizing salt which contains cyanide compounds such as sodium cyanide (NaCN). The disadvantage is that the salt has to be disposed of, which proves to be very costly and it is hazardous to the environment as well.

2Nitriding

Nitrogen is contaminated into the surface of the steel being treated. The reaction of nitrogen with the steel causes the production of very hard iron and alloy nitrogen compounds. The resulting nitride case is harder than carburized steels and that is why they should not be grinded. The advantage of this process is that hardness is achieved without the oil, water or air quench. Further hardening is accomplished in a nitrogen atmosphere that prevents scaling and discoloration.

.3 Carbonitriding

This process involves the diffusion of both carbon and nitrogen into the steel surface. The process is carried out in a gaseous atmosphere furnace using a carburizing gas such as propane or methane mixed with a different suitable percentage (by volume) of ammonia.

4 Cyaniding

It is similar to carbonitriding and involves the diffusion of carbon as well as nitrogen into the surface of the steel. The source of the diffusing element in this method is a molten salt of cyanide such as sodium cyanide or potassium cyanide. It is actually supercritical treatment involving temperatures in the range of 1400ºF to 1600ºF. Depths of diffusion ranges from 0.010 in. to 0.030 in. During cyaniding process , the salts are oxidized with the liberation of atomic nitrogen and carbon, they then contaminate into the steel.

Major defects in a metal or alloy due to faulty heat treatment

1. Overheating:

Prolonged heating of a metal or alloy above the A3 line leads to the formation of very large actual grains. Such structure has reduced ductility and toughness. It is possible to retrieve an overheated metal or alloy by usual annealing.

1. Burning:

Heating a metal or an alloy to still higher temperatures near melting point for a longer time leads to burning. This results in the formation of iron oxide inclusions along the grain boundaries.

1. Oxidation:

Sometimes a metal or alloy is oxidized due to an oxidizing atmosphere in the furnace. It is characterized by a thick layer of scale on the surface of a metal or alloy.

1. Decarburization:

It is the loss of carbon in the surface layers of the metal or alloy. Decarburization causes lowering hardness and decrease in fatigue life. It is caused by the oxidizing furnace atmosphere.

1. Cracks:

The cracks occur during quenching when the internal tensile stresses exceed the resistance of the metal or alloy to separate. The capability of a metal or alloy to crack formation increases with carbon content, hardening temperature and cooling rate.

1. Distortion and warping:

Distortion or deformation consisting of changes in the size and shape of heat-treated work is due to thermal and structural stresses. Asymmetrical distortion of work is often called warping in heat-treating practice.

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