Material Science Lecture 9

54
Heat Treatment of Steel Lecture 9

Transcript of Material Science Lecture 9

Page 1: Material Science Lecture 9

Heat Treatment of SteelLecture 9

Page 2: Material Science Lecture 9

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 chilling, 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

Page 3: Material Science Lecture 9

Types of Heat-Treatment (Steel)

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

Page 4: Material Science Lecture 9

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.

Page 5: Material Science Lecture 9

Time-Temperature-Transformation (TTT)Curve

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

Page 6: Material Science Lecture 9

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

Page 7: Material Science Lecture 9

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

Page 8: Material Science Lecture 9

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

Page 9: Material Science Lecture 9

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

relieving)2. Normalizing3. Isothermal Annealing4. Spheroidizing Annealing (or

Spheroidizing )

Page 10: Material Science Lecture 9

1. Stress-Relief Annealing

It is an annealing process below the transformation temperature Ac1, 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

Page 11: Material Science Lecture 9

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)

Page 12: Material Science Lecture 9

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

Page 13: Material Science Lecture 9

Stress-Relief Annealing Process For plain carbon and low-alloy steels the

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

Page 14: Material Science Lecture 9

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

Page 15: Material Science Lecture 9

Relation between heating temperature and Reduction in Residual Stresses

Higher temperatures and longer times of annealing may reduce residual stresses to lower levels

Page 16: Material Science Lecture 9

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

Page 17: Material Science Lecture 9

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

Page 18: Material Science Lecture 9

Normalizing – Heating and Cooling

Page 19: Material Science Lecture 9

Normalizing – Austenitizing Temperature Range

Page 20: Material Science Lecture 9

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

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

Page 21: Material Science Lecture 9

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

Page 22: Material Science Lecture 9

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 annealing has to be performed

Page 23: Material Science Lecture 9

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

Page 24: Material Science Lecture 9

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).

Page 25: Material Science Lecture 9

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

Page 26: Material Science Lecture 9

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 ferrite2. Uniformly distributed pearlite grains3. Fine lamellar pearlite grains

Page 27: Material Science Lecture 9

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)

Page 28: Material Science Lecture 9

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

Page 29: Material Science Lecture 9

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

Page 30: Material Science Lecture 9

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)a high-speed steel annealed at 820C.

Page 31: Material Science Lecture 9

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

Page 32: Material Science Lecture 9

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

Page 33: Material Science Lecture 9

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

Page 34: Material Science Lecture 9

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,  the  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

Page 35: Material Science Lecture 9

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

Page 36: Material Science Lecture 9

Case-Hardening - Processes Flame/Induction Hardening Carburizing Nitriding Cyaniding Carbonitriding

Page 37: Material Science Lecture 9

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

Page 38: Material Science Lecture 9

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

Page 39: Material Science Lecture 9

Carburizing 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

Page 40: Material Science Lecture 9

Carburizing 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

Page 41: Material Science Lecture 9

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

Page 42: Material Science Lecture 9

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

Page 43: Material Science Lecture 9

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

Page 44: Material Science Lecture 9

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

Page 45: Material Science Lecture 9

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

Page 46: Material Science Lecture 9

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

Page 47: Material Science Lecture 9

Precipitation Mechanism – Aluminum Alloy

Page 48: Material Science Lecture 9

Effect of Aging Time on Precipitates

Page 49: Material Science Lecture 9

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

Page 50: Material Science Lecture 9

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

Page 51: Material Science Lecture 9

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

Page 52: Material Science Lecture 9

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

Page 53: Material Science Lecture 9

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

Page 54: Material Science Lecture 9

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.