Heat Treatment of Steel

Heat-Treatment Heat treatment is a method used to alter

the physical, and sometimes chemical properties of a material. The most common application is metallurgical

It involves the use of heating or cooling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material

It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally

Types of Heat-Treatment (Steel)

Annealing / Normalizing, Case hardening, Precipitation hardening, Tempering, and Quenching

Figure - Phase diagramam for iron‑carbon system, up to about 6% carbon

Iron has Several Phases, depending on Temperature

The phase at room temperature is alpha (), called ferrite (BCC)

At 912C (1674F), ferrite transforms to gamma (), called austenite (FCC)

This transforms at 1394C (2541F) to delta () (BCC)

Pure iron melts at 1539C (2802F)

Iron as a Commercial Product

Electrolytic iron - the most pure, at about 99.99%, for research and other purposes where the pure metal is required

Ingot iron - contains about 0.1% impurities (including about 0.01% carbon), used in applications where high ductility or corrosion resistance are needed

Wrought iron - contains about 3% slag but very little carbon, and is easily shaped in hot forming operations such as forging

Solubility Limits of Carbon in Iron

Ferrite phase can dissolve only about

0.022% carbon at 723C (1333F)

Austenite can dissolve up to about 2.1%

carbon at 1130C (2066F)

The difference in solubility between alpha

and gamma provides opportunities for

strengthening by heat treatment

Time-Temperature-Transformation (TTT)Curve

TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition.

It is used to determine when transformations begin and end for an isothermal heat treatment of a previously austenitized alloy

TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at 900°C

Time-Temperature-Transformation (TTT)Curve

Decarburization during Heat Treatment

Decrease in content of carbon in metals is called Decarburization

It is based on the oxidation at the surface of carbon that is dissolved in the metal lattice

In heat treatment processes iron and carbon usually oxidize simultaneously

During the oxidation of carbon, gaseous products (CO and CO2) develop

In the case of a scale layer, substantial decarburization is possible only when the gaseous products can escape

Decarburization Effects

The strength of a steel depends on the presence of carbides in its structure

In such a case the wear resistance is obviously decreased

In many circumstances, there can be a serious drop in fatigue resistance

To avoid the real risk of failure of engineering components, it is essential to minimize decarburization at all stages in the processing of steel

Annealing It is a heat treatment wherein a material

is altered, causing changes in its properties such as strength and hardness

It the process of heating solid metal to high temperatures and cooling it slowly so that its particles arrange into a defined lattice

Types of Annealing1. Full Annealing2. Stress-Relief Annealing (or Stress-

relieving)3. Normalizing4. Isothermal Annealing5. Spheroidizing Annealing (or

Spheroidizing )

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Full Annealing

raising the temperature about 50 ºC (90 ºF) above the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC (90 ºF) into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon). cooled at the rate of about 20 ºC/hr (36 ºF/hr) in a furnace to about 50 ºC (90 ºF) into the Ferrite-Cementite range.

The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.

Process Annealing

heating it to about 700 ºC (1292 ºF) should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air.

the only change that occurs is the size, shape and distribution of the grain structure.

Causes of Residual Stresses

1. Thermal factors (e.g., thermal stresses caused by temperature gradients within the workpiece during heating or cooling)2. Mechanical factors (e.g., cold-working)3. Metallurgical factors (e.g., transformation of the microstructure)

Stress-Relief Annealing

Stress-Relief Annealing

It is an annealing process below the

transformation temperature A1 (600 - 650

ºC), with subsequent slow cooling, the aim of

which is to reduce the internal residual

stresses in a workpiece without intentionally

changing its structure and mechanical

properties

How to Remove Residual Stresses? R.S. can be reduced only by a plastic

deformation in the microstructure. This requires that the yield strength of the

material be lowered below the value of the residual stresses.

The more the yield strength is lowered, the greater the plastic deformation and correspondingly the greater the possibility or reducing the residual stresses

The yield strength and the ultimate tensile strength of the steel both decrease with increasing temperature

Stress-Relief Annealing Process

For plain carbon and low-alloy steels the temperature to which the specimen is heated is usually between 450 to 650˚C, whereas for hot-working tool steels and high-speed steels it is between 600 to 750˚C

This treatment will not cause any phase changes, but recrystallization may take place.

Machining allowance sufficient to compensate for any warping resulting from stress relieving should be provided

Stress-Relief Annealing – R.S. In the heat treatment of metals, quenching or

rapid cooling is the cause of the greatest residual stresses

To activate plastic deformations, the local residual stresses must be above the yield strength of the material.

Because of this fact, steels that have a high yield strength at elevated temperatures can withstand higher levels of residual stress than those that have a low yield strength at elevated temperatures

Soaking time also has an influence on the effect of stress-relief annealing

Relation between heating temperature and Reduction in Residual Stresses Higher temperatures and longer times of annealing

may reduce residual stresses to lower levels

Stress Relief Annealing - Cooling

The residual stress level after stress-relief annealing will be maintained only if the cool down from the annealing temperature is controlled and slow enough that no new internal stresses arise.

New stresses that may be induced during cooling depend on the (1) cooling rate, (2) on the cross-sectional size of the workpiece, and (3)on the composition of the steel

2. Normalizing

A heat treatment process consisting of austenitizing at temperatures of 30–80˚C above the AC3 transformation temperature followed by slow cooling (usually in air)

The aim of which is to obtain a fine-grained, uniformly distributed, ferrite–pearlite structure

Normalizing is applied mainly to unalloyed and low-alloy hypoeutectoid steels

For hypereutectoid steels the austenitizing temperature is 30–80˚C above the AC1 or ACm transformation temperature

Normalizing – Heating and Cooling

Normalizing – Austenitizing Temperature Range

Effect of Normalizing on Grain Size

Normalizing refines the grain of a steel that has become coarse-grained as a result of heating to a high temperature, e.g., for forging or welding

Fig. - Carbon steel of 0.5% C. (a) As-rolled or forged; (b) normalized. Magnification 500

Need for Normalizing

Grain refinement or homogenization of the structure by normalizing is usually performed either to improve the mechanical properties of the workpiece or (previous to hardening) to obtain better and more uniform results after hardening

Normalizing is also applied for better machinability of low-carbon steels

Normalizing after Rolling

After hot rolling, the structure of steel is usually oriented in the rolling direction

To remove the oriented structure and obtain the same mechanical properties in all directions, a normalizing has to be performed

Normalizing after Forging

After forging at high temperatures,

especially with workpieces that vary widely

in crosssectional size, because of the

different rates of cooling from the forging

temperature, a heterogeneous structure is

obtained that can be made uniform by

normalizing

Normalizing – Holding Time

Holding time at austenitizing temperature may be calculated using the empirical formula:

t = 60 + D where t is the holding time (min) and D is

the maximum diameter of the workpiece (mm).

Normalizing - Cooling Care should be taken to ensure that the cooling

rate within the workpiece is in a range corresponding to the transformation behavior of the steel-in-question that results in a pure ferrite–pearlite structure

If, for round bars of different diameters cooled in air, the cooling curves in the core have been experimentally measured and recorded, then by using the appropriate CCT diagram for the steel grade in question, it is possible to predict the structure and hardness after normalizing

3. Isothermal Annealing

Hypoeutectoid low-carbon steels as well as

medium-carbon structural steels are often

isothermally annealed, for best machinability

An isothermally annealed structure should

have the following characteristics:

1. High proportion of ferrite

2. Uniformly distributed pearlite grains

3. Fine lamellar pearlite grains

Principle of Isothermal Annealing

Bainite formation can be avoided only by very slow continuous cooling, but with such a slow cooling a textured (elongated ferrite) structure results (hatched area)

Process - Isothermal Annealing Austenitizing followed by a fast cooling to the

temperature range of pearlite formation (usually about 650˚C.)

Holding at this temperature until the complete transformation of pearlite

and cooling to room temperature at an arbitrary cooling rate

4. Spheroidizing Annealing

It is also called as Soft Annealing

Any process of heating and cooling steel that produces a rounded or globular form of carbide

It is an annealing process at temperatures close below or close above the AC1 temperature, with subsequent slow cooling

Spheroidizing - Purpose The aim is to produce a soft structure by changing

all hard constituents like pearlite, bainite, and martensite (especially in steels with carbon contents above 0.5% and in tool steels) into a structure of spheroidized carbides in a ferritic matrix

(a) a medium-carbon low-alloy steel after soft annealing at 720C; (b)h-speed steel annealed at 820C.

Spheroidizing - Uses

Such a soft structure is required for good machinability of steels having more than 0.6%C and for all cold-working processes that include plastic deformation.

Spheroidite steel is the softest and most ductile form of steel

Spheroidizing - Mechanism

The physical mechanism of soft annealing is based on the coagulation of cementite particles within the ferrite matrix, for which the diffusion of carbon is decisive

Globular cementite within the ferritic matrix is the structure having the lowest energy content of all structures in the iron–carbon system

The carbon diffusion depends on temperature and time

Spheroidizing - Mechanism

The solubility of carbon in ferrite, which is very low at room temperature (0.02% C), increases considerably up to the Ac1 temperature

At temperatures close to Ac1, the diffusion of carbon, iron, and alloying atoms is so great that it is possible to change the structure in the direction of minimizing its energy content

Spheroidizing - Process Prolonged  heating  at  a  temperature  just

 below the  lower  critical  temperature,  usually  followed  by  relatively  slow cooling

In  the  case  of  small  objects  of  high  C steels, spheroidizing  result  is  achieved  more  rapidly

by  prolonged heating to temperatures alternately within and slightly below the critical temperature range

Tool steel is generally spheroidized by heating to a temperature of 749°-804°C and higher for many alloy tool steels, holding at heat from 1 to 4 hours, and cooling slowly in the furnace

CASE HARDENING

Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy.

Case hardening is usually done after the part in question has been formed into its final shape

Case-Hardening - Processes

Flame/Induction Hardening Carburizing Nitriding Cyaniding Carbonitriding

Flame and induction hardening

Flame or induction hardening are processes in which the surface of the steel is heated to high temperatures (by direct application of a flame, or by induction heating) then cooled rapidly, generally using water

This creates a case of martensite on the surface.

A carbon content of 0.4–0.6 wt% C is needed for this type of hardening

Application Examples -> Lock shackle and Gears

Carburizing Carburizing is a process used to case harden

steel with a carbon content between 0.1 and 0.3 wt% C.

Steel is introduced to a carbon rich environment and elevated temperatures for a certain amount of time, and then quenched so that the carbon is locked in the structure

Example -> Heat a part with an acetylene torch set with a fuel-rich flame and quench it in a carbon-rich fluid such as oil

Carburizing Contd. Carburization is a diffusion-controlled

process, so the longer the steel is held in the carbon-rich environment the greater the carbon penetration will be and the higher the carbon content.

The carburized section will have a carbon content high enough that it can be hardened again through flame or induction hardening

Carburizing Contd. The carbon can come from a solid, liquid or

gaseous source Solid source -> pack carburizing. Packing low

carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers.

A heating period of a few hours might form a high-carbon layer about one millimeter thick

Liquid Source -> involves placing parts in a bath of a molten carbon-containing material, often a metal cyanide

Gaseous Source -> involves placing the parts in a furnace maintained with a methane-rich interior

Nitriding Nitriding heats the steel part to 482–621°C in

an atmosphere of NH3 gas and broken NH3. The time the part spends in this environment

dictates the depth of the case. The hardness is achieved by the formation of

nitrides. Nitride forming elements must be present in

the workpiece for this method to work. Advantage -> it causes little distortion, so the

part can be case hardened after being quenched, tempered and machined

Cyaniding Cyaniding is mainly used on low carbon steels. The part is heated to 870-950°C in a bath of

sodium cyanide (NaCN)and then is quenched and rinsed, in water or oil, to remove any residual cyanide.

The process produces a thin, hard shell (0.5-0.75mm) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours.

It is typically used on small parts. The major drawback of cyaniding is that

cyanide salts are poisonous

Carbonitriding

Carbonitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons (e.g. CH4)is used instead of sodium cyanide.

If the part is to be quenched then the part is heated to 775–885°C; if not then the part is heated to 649–788°C

PRECIPITATION HARDENING Precipitation hardening (or age

hardening), is a heat treatment technique used to increase the yield strength of malleable materials

Malleable materials are those, which are capable of deforming under compressive stress

It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which blocks the movement of dislocations in a crystal's lattice

Precipitation Hardening Since dislocations are often the

dominant carriers of plasticity, this serves to harden the material

The impurities play the same role as the particle substances in particle-reinforced composite materials.

Alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging

Precipitation Hardening Two different heat treatments involving

precipitates can change the strength of a material:

1. solution heat treating 2. precipitation heat treating Solution treatment involves formation of

a single-phase solid solution via quenching and leaves a material softer

Precipitation treating involves the addition of impurity particles to increase a material's strength

Precipitation Mechanism – Aluminum Alloy

Effect of Aging Time on Precipitates

QUENCHING and TEMPERING In quench hardening, fast cooling

rates, depending on the chemical composition of the steel and its section size, are applied to prevent diffusion-controlled trans formations in the pearlite range and to obtain a structure consisting mainly of martensite and bainite

However, the reduction of undesirable thermal and transformational stresses usually requires slower cooling rates

Quenching To harden by quenching, a

metal must be heated into the austenitic crystal phase and then quickly cooled

Cooling may be done with forced air, oil, polymer dissolved in water, or brine

Upon being rapidly cooled, a portion of austenite (dependent on alloy composition) will transform to martensite

Quenching Cooling speeds, from fastest to slowest, go

from polymer, brine, fresh water, oil, and forced air

However, quenching a certain steel too fast can result in cracking, which is why high-tensile steels such as AISI 4140 should be quenched in oil, tool steels such as H13 should be quenched in forced air, and low alloy such as AISI 1040 should be quenched in brine

Metals such as austenitic stainless steel (304, 316), and copper, produce an opposite effect when these are quenched: they anneal

Tempering Untempered martensite, while very hard, is

too brittle to be useful for most applications.

In tempering, it is required that quenched parts be tempered (heat treated at a low temperature, often 150˚C) to impart some toughness.

Higher tempering temperatures (may be up to 700˚C, depending on alloy and application) are sometimes used to impart further ductility, although some yield strength is lost

Tempering

Tempering is done to toughen the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite

Tempered martensite is much finer-grained than just-quenched martensite

The brittle martensite becomes tough and ductile after it is tempered.

Carbon atoms were trapped in the austenite when it was rapidly cooled, typically by oil or water quenching, forming the martensite

Tempering The martensite becomes tough after

being tempered because when reheated, the microstructure can rearrange and the carbon atoms can diffuse out of the distorted body-centred-tetragonal (BCT) structure.

After the carbon diffuses out, the result is nearly pure ferrite with body-centred structure.