Heat Treatment

HEAT TREATMENT

Heat treatment is a heating and cooling process of a metal or an alloy in the solid state with the purpose of changing their properties. It can also be said as a process of heating and cooling of ferrous metals especially various kinds of steels in which some special properties like softness, hardness, tensile-strength, toughness etc, are induced in these metals for achieving the special function objective. Mainly comprises of 3 phases,

- Heating of metal - Soaking of metal - Cooling of metal

Heat treatment in furnaces is generally carried in two type of furnaces - Hearth furnaces

- Bath furnaces HEARTH FURNACES

These furnaces are heated by fuel which may be coke, coal, gas (town, blast or natural) and fuel oil. They can also be operated electrically. They are generally of two types.

Stationary type

Car bottom type Movable type

Rotary Type

Direct fuel fired furnace Indirect fuel fired furnace

Multiple furnace Re-circulation furnace

BATH FURNACES

These are further classified as - Liquid bath type - Salt bath type - Lead bath type - Oil bath type

CONSTITUENTS OF IRON AND STEEL

Micro structure of mild steel

White constituent in this figure is very pure iron or having very low free carbon in iron in form of ferrite and dark patches contain carbon in iron is chemically combined form known as carbide (Cementite). Cementite is very hard and brittle.

Micro structure of pearlitic eutectoid steel

These layers arealternatively of ferrite and cementite. This substance is called as pearlite and is made up of 87% ferrite and 13% cementite. But with increase of carbon content in steel portion of pearlite increases up to 0.8% C. The structure of steel at 0.8% C is entirely of pearlite

Micro structure of of high carbon steel

If carbon content in steel is further increased as free constituent up to 1.5% C, such steel will be called as high carbon steel.

ALLOTROPHY OF IRON

Tracing the cooling temperature of iron from 1600 C to ambient temperature is difficult in actual practice but can be traced used TTT curve ( Temperature , Time and Transformation )

If iron cooled from molten condition to solid state following changes will occur 1539-1600°C Molten-Fe (Liquid state of iron)

1400-1539°C Delta-Fe (Body centered)

910-1400°C Gamma-Fe (FCC atomic arrangement and austenite structure) 770- 910°C Beta-Fe (Body centered-nonmagnetic)

Up to 770°C Alpha-Fe (BCC atomic arrangement and ferrite structure)

- First changing occurs at l539°C at which formation of delta iron starts.

- Second changing takes place at 1404°C and where delta iron starts changes into gamma iron or austenite (FCC structure).

- Third changing occurs at 910°C and where gamma iron (FCC structure) starts changes into beta iron (BCC structure) in form of ferrite, leadaburite and austenite.

- Fourth changing takes place at 768°C and where beta iron (BCC structure) starts changes into alpha iron in form of ferrite, pearlite and cementite.

IRON CARBON EQUILIBRIUM DIAGRAM ( FE - C)

Iron carbon diagram helps us to identify the formation of various structures during heating and cooling phases. Main microscopic constituents that forms during the heating and cooling process of iron and steel are

- Austentite - Ferrite - Cementite - Pearlite

AUSTENITE

On heating the steel, after upper critical temperature, the formation of structure completes into austenite which is hard, ductile and non-magnetic. It is able to dissolve large amount of carbon.It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic.

FERRITE

Slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. Ferrite contains very little or no carbon in iron.

CEMENTITE

when the carbon forms definite combinations with iron in form of iron carbides which are extremely hard in nature. The brittleness and hardness of cast iron is mainly controlled by the presence of cementite in it. It is magnetic below 200°C. Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe3C).

PEARLITE

Pearlite is relatively strong, hard and ductile, whilst ferrite is weak, soft and ductile. It is built up of alternate light and dark plates. These layers are alternately ferrite and cementite. When seen with the help of a microscope, the surface has appearance like pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite. Pearlite is a eutectoid alloy of ferrite and cementite. It occurs particularly in medium and low carbon steels in the form of mechanical mixture of ferrite and cementite in the ratio of 87:13.

HEAT TREATMENT PROCESS

Heat treatment Normalizing Annealing Hardening Tempering Case hardening Surface hardening Carburizing

Cyanding Nitriding Induction Flame hardening hardening

NORMALIZING

Softening process in which iron base alloys are heated 40 to 50°C above the upper-critical limit for both hypo and hyper eutectoid steels and held there for a specified period and followed by cooling in still air up to room temperature Objective:

- Soften metals

- Refine grain structure - Improve grain size ANNEALING

Softening process in which iron base alloys are heated above the transformation range held there for proper time and then cool slowly (at the of rate of 30 to 150°C per hour) below the transformation range in the furnace itself.

Heating is carried out 20°C above upper critical temperature point of steel in case of hypo eutectoid steel and the same degree above the lower critical temperature point in case of type eutectoid steel.

Objective : - Soften the steel - Improve grain size - Relieve internal stress SPHEROIDIZATION

Lowest temperature range of annealing process in which iron base alloys are heated 20 to 40°C below the lower critical temperature, held therefore a considerable period of time then allowed to cool very slowly at room temperature in the furnace itself. During this process, the cementite of steel which is in the combined form of carbon becomes globular or spheroidal leaving ferrite in matrix, thus imparting softness to steel.

Objective :

- To reduce tensile strength - To increase ductility HARDENING

Hardening is a hardness inducing kind of heat treatment process in which steel is heated to a temperature above the critical point and held at that temperature for a definite time and then quenched rapidly in water, oil or molten salt bath. Steel is hardened by heating 20-30°C above the upper critical point for hypoeutectoid steel and 20-30°C above the lower critical point for hyper eutectoid steel and held at this temperature for some time and then quenched in water or oil or molten salt bath.

TEMPERING

If high carbon steel is quenched for hardening in a bath, it becomes extra hard, extra brittle and has unequal distribution internal stresses and strain and hence unequal harness and toughness in structure. These extra hardness, brittleness and unwanted induced stress and strain in hardened metal reduce the usability the metal. Therefore, these undesired needs must be reduced for by reheating and cooling at constant bath temperature. In tempering, steel after hardening, is reheated to a temperature below the lower critical temperature and then followed by a desired rate of cooling.

Reheating the of hardened steel is done above critical temperature when the structure is purely of austenite and then quenching it in a molten salt path having temperature in the range of 150-500°C. This is done to avoid transformation to ferrite and pearlite and is held quenching temperature for a time sufficient to give complete formation to an intermediate structure referred to as bainite then cooled to room temperature. It is divided in to 5 categories based on their heating

- Low temperature tempering - Medium temperature tempering - High temperature tempering - Aus tempering

- Mar tempering

LOW TEMPERATURE TEMPERING

Hardened steel parts requiring tempering are heated up to 200°C and then quenched in oil.Tempering is used to retain hard micro-structure of martensite which increases brittleness.

MEDIUM TEMPERATURE TEMPERING

Hardened steel parts requiring tempering are heated in the temperature range of 200-350°C. This process gives troosite structure. Troosite structure is another constituent of steel obtained by quenching tempering martensite. Less hard and brittle than martensite.

HIGH TEMPERATURE TEMPERING

Hardened steel parts requiring tempering are heated in the temperature range of 350-550°C. This process gives sorbite structure. Sorbite structure is produced by the, transformation of tempered martensite.

AUS TEMPERING

It is a special type of tempering process in which and steel is heated above the transformation range then suddenly quenched in a molten salt bath at a

temperature 200 to 450°C. The piece is held at that temperature until the and outside temperature are equalized. The part is then reheated and cooled at moderate rate. Aus-tempering produces fine bainite structure in steel but with minimum distortion and residual stresses.

MAR TEMPERING

Tempering process in which and its base alloys are heated above the transformation range then suddenly quenched in a molten salt bath at a temperature 80 to 300°C. The piece is held at that temperature until the and outside temperature are equalized. The part is then reheated and cooled at moderate rate. Mar-tempering produces martensite in steel but with minimum distortion and residual stresses.

CASE HARDENING

Some times special characteristic are required in metal such as hard outer surface and soft, tough and more strength oriented core or inner structure of metal. This can be obtained by casehardening process. They are as follow

CARBURIZING

Carburizing can be of three types 1. Pack carburizing

2. Liquid carburizing and 3. Gas carburizing

Pack carburizing :

Metals to be carburized such as low carbon steel is placed in cast iron or steel boxes containing a rich material in carbon like charcoal, crushed bones, potassium Ferro-cyanide or charred leather. Such boxes are made of heat resisting steel which are then closed and sealed with clay. The boxes are heated to a temperature 900°C to 950°C according to type of steel for absorbing carbon on the outer surface. carbon enters the on the metal to form a solid solution with iron and converts the outer surface into high carbon steel.

Liquid carburizing :

Liquid carburizing is carried out in a container filled with a molten salt, such as sodium cyanide. This bath is heated by electrical immersion elements or by a gas burner and stirring is done to ensure uniform temperature. This process gives a thin hardened layer up to 0.08 mm thickness. Parts which are to be case-hardened are dipped into liquid bath solution containing calcium cyanide and polymerized hydro-cyanide acid or sodium or potassium cyanide along-with some salt. Bath temperature is kept from 815°C to 900°C.

Gas carburizing :

In gas carburizing method, the parts to be gas carburized are surrounded by a hydrocarbon gas in the furnace. The common carburizing gases are methane, ethane, propane, butane and carbon monoxide are used in this process. Carbon containing gas such as carbon monoxide (CO), methane (CH4), ethane (C2H6) or town gas is introduced in the furnace where low carbon steel is placed. The furnace is either gas fired or electrically heated. Average gas carburizing temperature usually varies from 870° to 950°C.

CYANIDING

Cyanide may also be used to case harden the steel. It is used to give a very thin but hard outer case. Cyaniding is a case hardening process in which both C and N2 in form of cyaniding salt are added to surface of low and medium carbon steel. Sodium cyanide or potassium cyanide may be used as the hardening medium.

NITRIDING

Nitriding is a special case hardening process of saturating the surface of steel with nitrogen by holding it for prolonged period generally in electric furnace at temperature from 480°C to 650°C in atmosphere of Ammonia gas (NH3). The nitrogen from the ammonia gas enters into on the surface of the steel and forms nitrides and that impart extreme hardness to surface of the metal. Nitriding is a case hardening process in which nitrogen instead of carbon is added to the outer skin of the steel.

FLAME HARDENING

It consists of moving an oxyacetylene flame, over the part where hardening is required. Immediately after this, the heated portion is quenched by means of water spray or air passing over it.

INDUCTION HARDENING

Induction hardening is accomplished by placing the part in a high frequency alternating magnetic field. It differs from surface hardening in the way that hardness of surface is not due to the increase in carbon content but due to rapid heating followed by controlled quenching. In this process, a high frequency current is introduced in the metal surface and its temperature is raised up to hardening range. As this temperature is attained, the current supply is cut off instantaneously water is sprayed on the surface.