Heat Treatment (2)

04/11/23 Heat Treatment 1

Heat Treatment

AENG 587: Automotive Manufacturing Processes


Enhancement of Properties of Materials

There are several methods to enhance the properties of metal, including:

Strain Hardening Heat Treatment

Ferrous Non-ferrous


Strain Hardening

Strain Hardening is also known as “Work hardening”.

Strain Hardening Effects Increase in yield strength as strain

increases below recrystalization temperature.

Reduce Ductility.


Strain Hardening Strain Hardening of metals depends on

the lattice structure of metals. In F.C.C. strain rate is affected by

stacking fault energy. Therefore, metals with F.C.C. strain hardened faster. [ Cu, Ni, Austenite Stainless Steel]

H.C.P. metals subject to twinning and strain hardening at much higher rate as compared to others.


Effect of Cold Work on the Mechanical Properties of Iron and Copper


Recovery and Recrystalization.


Recovery occurs at a certain temperature range below the recrystalization temperature of the metal, the stresses in highly deformed regions are relieved. During recovery there is no appreciable change in mechanical properties such as hardness and strength.

In recrystalization at certain temperature range, new equiaxed and strain free grains are formed replacing the older grains.

The recrystalization temperature approximately ranges between 0.3Tm and 0.6Tm. Tm is melting point temperature.

Recovery and Recrystalization


Heat Treatment for Steel Definition:

Heat treatment is the process of controlling heating and cooling of metals for the purpose of altering their properties

The effect of thermal treatment depends primarily upon

Alloy composition and microstructure Degree of prior cold work Rates of heating and cooling during heat treatment

GoalsPreparing the material for fabrication i.e.

Improving machining characteristics Reducing forming forces and energy consumption Restoring ductility for further deformation


Equilibrium Diagram

Equilibrium diagrams are helpful for understanding and processing a heat treatment job. Figure shows a portion of the iron-iron carbide equilibrium diagram.


Pearlite Microstructure

Figure 4.11 Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite, and the darker regions are carbide. Magnification: 2500X. Source: Courtesy of USX Corporation.


Hypoeutectoid and Hypereutectoid Steel

On iron-iron carbide equilibrium diagram steel having %C less than 0.77 is hypoeutectoid steel. while hypereutectoid steel has %C more than 0.77.


Isothermal Transformation (T-T-T) Diagrams

These diagrams are called as isothermal transformation diagram or time-temperature-transformation diagrams. The diagram shown is for the eutectoid alloy

T T T for 0.77% C


Construction of TTT diagramAustenite to Pearlite Transformation

Figure 5.32 (a) Austenite-to-pearlite transformation of iron-carbon alloy as a function of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675 °C (1247 °F).

Source: Kalpakjian and Schmid


Austenite to Pearlite Transformation (cont.)

Figure 5.32 (c) Microstructures obtained for a eutectoid iron-carbon alloy as a function of cooling rate. Source: ASM International.



Heat Treatment Processes

Hardening or quenching Annealing

Full Annealing Process Annealing Stress Relief Annealing

Normalizing Tempering


Hardening or Quenching

In hardening the steel alloy is heated to Austenite phase then quenched in a quenching media depending on the alloy chemical composition.

Hypoeutectoid steel: the alloy is heated 30 to 50oC above A3 to have a uniform austenite structure, then quenched rapidly to room temperature. This changes the microstructure to martensite


Hardening or Quenching

Hypereutectoid Steel: the alloy is heated to temperature 30 to 60 oC above A1 to have austenite and cementite structure, then quenched to room temperature. The final structure is martensite.


Austenite, Ferrite, and Martensite

Figure 4.9 The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of percentage of carbon (by weight) on the lattice dimensions for martensite is shown in (d). Note the interstitial position of the carbon atoms. Note, also, the increase in dimension c with increasing carbon content; this effect causes the unit cell of martensite to be in the shape of a rectangular prism.



Martensite

When austenite is cooled at a high rate such as quenching in water its FCC structure is transformed to body-centered-tetragonal structure. This microstructure is called as martensite

Martensite is strong, hard and brittle and has feather like microstructure

The strength and hardness of steel in the martensite condition is function of the carbon content of steel as shown in figure.

Source:DeGarmo


Critical cooling rate and Quenching Media

It is the slowest cooling rate that can be given to a steel alloy and to have it transformed to 100% martensite.

Quenching Media Water Brine solution Oil Molten salt bath Air


Annealing Annealing is a general term used to describe the

restoration of a cold-worked or heat treated metal or alloy to its original properties, such as to increase ductility (hence formability) or to modify the microstructure.

Purpose of Annealing Reduce hardness Remove residual stresses Improve toughness Restore ductility Refine grain size Reduce segregation Alter physical properties.


Annealing Process

The annealing process consists of Heating the work piece to a specific range of

temperature. Holding the work piece at that temperature for a period

of time (soaking) Cooling the work piece slowly.

Annealing process may be carried out in an inert or controlled atmosphere or performed at lower temperature to prevent or minimize surface oxidation.

Annealing temperature may be higher than the recrystalization temperature depending upon the degree of cold work.


Full Annealing

Full annealing term is applied to annealing ferrous alloys, generally low-and medium-carbon steel .

The steel is heated to above A1 or A3 and cooling takes place slowly such as 10ºC (20ºF) per hour

The cooling takes place in a furnace after it is turned off.

The structure obtained in full annealing is coarse pearlite, which is soft and ductile and has small uniform grains.


Annealing of Hypoeutectoid Steel

Heating the work piece to a temperature above A3 by 30ºC to 60ºC (50 to 100ºF).

Hold the work piece at this temperature for a period of time (1hr/in of thickness).

Cool the work piece in the furnace at a rate of 10-30ºC/hr to a temperature at least 30ºC below A1, followed by air cooling to room temperature.


Resulting Microstructure.

After full annealing process the microstructure of hypoeutectoid steel will be soft and ductile. The microstructure consists of coarse pearlite and excess ferrite.


Full Annealing of Hypereutectoid Steel

The resulting microstructure of hypereutectoid steel consists of coarse pearlite similar to hypoeutectoid steel and excess cementite in spheroidal form.


Normalizing

To avoid excessive softness in the annealing of steels, the cooling cycle may be done completely in still air. This process is called “normalizing”.

In this process the part is heated to a temperature above A3 or Acm to transform the structure to austenite.

The soaking process is continued until uniform austenite is obtained and then allowed to cool in still air.

Generally normalizing is carried out to refine the grain structure, obtain uniform structure, decrease residual stresses and improve machineability.


Normalizing


Process Annealing

During process annealing the work piece is annealed to restore its ductility, part or all of which may have been exhausted by work hardening during cold working.


Stress-relief Annealing

To reduce or eliminate residual stresses.

The temperature and time required for process depends upon material and magnitude of residual stresses present.

For steels the part is heated below A1 to avoid phase transformation.

Slow cooling rates, such as in still air are generally employed.


Spherodizing Annealing

Spherodizing annealing improves the cold workability and machinability.

When pearlite is heated to just below the eutectoid temperature and held at that temperature for a period of time (usually for a day) at 700ºC the cementite lamellae transforms to spherical shapes.

Spheroidities are less conductive to stress

concentration because of their shapes. This structure has higher toughness and lower

hardness than pearlite structure.


Spherodizing Annealing Methods

Method 1

Prolonged heating at a temperature just below A1 and then slow cooling.

Method 2Prolonged cycling between temperature slightly above and slightly below A1 followed by slow cooling

Method 3 (for tool steel or high alloy steel)Heat upto 750-800ºC soak at this level for several hours and then allow slow cooling.


Spherodizing Annealing Methods


Selection of Heat Treatment Process

Selection of heat treatment process depends upon objectives and material composition.

Type of Steel % Carbon Content Type of Process

Low Carbon Steel Less than 0.3 %Normalising or Process Annealing

Medium Carbon Steel 0.45 to 0.6% Full AnnealingHigh Carbon Steel Greater than 0.6%

Spherodizing Annealing


Hardness and Toughness of Annealed Steels

Figure 4.15 (a) and (b) Hardness and (c) toughness for annealed plain-carbon steels, as a function of carbide shape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. The spheroidite structure has spherelike carbide particles. Note that the percentage of pearlite begins to decrease after 0.77% carbon. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.



Different Annealing Processes

Source:DeGarmo


Design for Heat Treatment Following points should

be considered Avoid non-uniform

sections and thickness.

Avoid sharp corners. This helps in avoiding

Cracking or distortion

Non-uniform properties

Stress concentration Residual stresses.


Techniques for Avoiding Cracking

Austempering The heated work piece is quenched from austenizing

temperature rapidly enough to avoid formation of ferrite or pearlite.

Held at certain temperature until isothermal transformation from austenite to bainite is complete.

Then work piece is cooled to room temperature in still air at a moderate rate to avoid thermal gradient within the part.

The quenching medium most commonly used is molten salt, at temperatures ranging from 160ºC to 750ºC. (320ºF to 1380 ºF).


Martempering (Marquenching)

The steel or cast iron is quenched from austenizing temperature into hot-fluid medium (hot oil, molten salt).

It is held at that temperature until the temperature is uniform through-out the part and then cooled at a moderate rate (in air).

The main purpose is to avoid the temperature gradient within the part.

The part is then tempered to change the untempered martensite.

Martempered steels have less tendency to crack, distort and develop residual stresses during heat treatment.

Used for complicated shape and high precision.


TTT Diagrams for Austempering and Martempering

A1 A1

Source:DeGarmo


Tempering Martensite

Martensite is hard and lacks toughness.

Subsequent heating follows the quenching is called as “tempering.”

Tempering restores some desired degree of toughness at the expense of decrease in strength and hardness.


Tempering of Steel Alloys

Tempering temperature affects the tempering time as shown in the figure. For example to obtain harness of 50 HRc, the work can be tempered at 400 oC for 18 minutes or at 300 oC for 18 hours.



Mechanical Properties of Tempered

Steel

The properties of tempered steel is influenced by the tempering temperature as shown in Figure. As tempering temperature increases the strength decreases and the ductility increases.



Hardenability

Hardenability is defined as the ability of an alloy to be hardened by heat treatment. It is measured by the depth of hardness that can be obtained by heating followed by quenching.

Hardenability of steel alloy is a function of Carbon content Alloying elements

Alloying elements like Cr, Mn, Mo, V and Ni improve the hardenability by shifting the T-T-T diagram to the right so that alloy can be quenched at slower cooling rate.


Hardenability

Hardenability is measured using “Jominy End-Quench” test.

The test piece is quenched from one side and the hardness is measured along the length at various distances from quench end.

Figure shows Hardness decreases as length increases from end. Hardenability increases with increase in carbon content Hardenability of alloy is directly proportional to the

depth of hardness penetration. Hardenability curves are necessary for predicting

hardness of gears, cams.


Jominy End-Quench Hardenability Test

Figure 4.20 (a) End-quench test and cooling rate.Source: Kalpakjian and Schmid


Jominy End-Quench Hardenability Test

Figure 4.20 (b) Hardenability curves for five different steels, as obtained from the end-quench test. Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals, for better control of properties. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.Source: Kalpakjian and Schmid


Surface Hardening of Steel

Objectives To obtain hard, wear-resistant surface

coupled with tough, fracture-resistant core.

Methods Selective Heating of the surface Alter the surface chemistry. Decomposition of an additional layer.


Selective Heating Techniques

For steel alloys more than 0.3 %C, selective heating techniques can be used to alter the surface hardness required. These techniques are

Flame Hardening Induction Heating Laser Beam Hardening Electron Beam Hardening


Flame Hardening

Oxyacetylene flame is used to raise the surface temperature high enough to reform austenite.

Then quenched and tempered to desired properties.

The depth and hardness depends upon carbon content and Hardenability of the alloy.

Characteristics: Flexible Depth of hard surface varies up to ¼ in. Can be used for large objects Equipments vary from simple hand held torch to

fully automated and computerized unit.


Induction Heating

Suitable for surface hardening Rate and depth of hardening can be controlled by current and

frequency of the generator. Method

The part is heat treated rapidly by the electromagnetic field generated by an induction coil carrying alternating current.

The induction coil induces eddy current in the part. Coil is made of copper or copper-base alloy and usually

water cooled. Characteristics

Efficient process Easy to control Good reproducibility Good quality control and possibility of automation.

Generally used for gear teeth hardening


Induction Heating Setup

Figure 4.26 Types of coils used in induction heating of various surfaces of parts.



Induction-Hardened Surface

Figure 4.1 Cross-section of gear teeth showing induction-hardened surfaces. Source: TOCCO Div., Park-Ohio Industries, Inc.



Laser Beam Hardening

Steel surface is treated with absorbing materials like zinc or manganese, phosphate to improve the efficiency of converting light into heat energy.

The surface is then scanned with laser Surface is quenched with water or oil 0.4 %C steel can attain hardness as high as 65Rc. Characteristics

High speed. Induces compressive stresses in the surface. Produce little distortion. Can be controlled with computer.


Electron Beam Hardening

High energy electron beam which is controlled by electromagnetic coils.

Easy to control and can be automated High efficiency up to 90% Process performed in vacuum. This is the

major limitation of the process.


Techniques using Altered Surface Chemistry

For low carbon steel less than 0.3 %C. Carburizing

Pack-carburizing process Gas carburizing Liquid carburizing or cyaniding.

Pack-carburizing process Components are packed in high-carbon solid medium, enclosed

in a gas-tight box. Then heated in a furnace for 6 to 72 hrs at roughly 1650ºF

(900ºC). C produces CO which reacts with austenite that absorbs the

carbon Boxes are then taken from furnace and parts are heat treated to

produce different surface results.


Pack-carburizing Process

Characteristics Slow process Carbon content ranges from 0.7% to 1.2% and

depends upon the process details Depth variation is less than 3/8“

Problems associated with the process Inefficient process Temperature is not uniform Difficult in handling Process is not adaptable for continuous operation.


Gas Carburizing

Overcomes the difficulty in pack carburizing process by replacing solid carbon with gas containing excess of CO

Advantages Faster More easily controlled More accurate and uniform Continuous operation is possible.

But special type of furnace is required for this process.


Nitriding

Surface hardening for steel alloys that contains Aluminum, Chromium, Molybdenum or Vanadium.

Process Parts are heat treated and tempered at 1000-

1250ºF (525-675ºC) Cleaning of surface Heat the part in a dissociated ammonia

atmosphere for 10 to 40 hrs at 950-1150ºF. N2 diffuses in steel forming alloy nitrides and

hardening the metal to a depth of about 0.025”.


Nitriding

Advantages Very hard case Low distortion No subsequent process is required.

Disadvantages Long time at elevated temperature restricts

the application of nitriding for high quantity production.


Surface Hardening Processes



Case Hardening of Gears New methods for case hardening of alloy steel containing

0.4%C. Combination between case hardening and quenching of

0.4%C. Light case hardened by cyanide [0.006-0.010”] on 0.4%C Harden the core to HRC 50. The gear is economically produced because of shallow

case Steel blank is relatively easy to machine. Gear offers resistance to crushing because the core is

stronger, yet the tooth have a desirable surface hardness.


Gear Carburizing

Source: DeGarmo


Heat Treatment of Non-ferrous Metals

Aside from Age hardening, the non-ferrous alloys are heat treated for the following purposes

To obtain uniform microstructure i.e. to remove coring (Al alloys).

Stress relief (heating for several hours at low temperature)

Recrystallization Precipitation or Age hardening

Obtained from non-equilibrium structure Strength obtained from precipitation of

second phase “dispersion hardening”.


Precipitation Hardening

Heat treatment that precipitates fine particles that block the movement of dislocations and thus strengthen and harden the metal

Principal heat treatment for strengthening alloys of aluminum, copper, magnesium, nickel, and other nonferrous metals

Also utilized to strengthen a number of steel alloys that cannot form martensite by the usual heat treatment


Conditions for Precipitation Hardening

The necessary condition for whether an alloy system can be strengthened by precipitation hardening is the presence of sloping solvus line in the phase diagram

A composition in this system that can be precipitation hardened is one that contains two equilibrium phases at room temperature, but which can be heated to a temperature that dissolves the second phase


Figure 27.5 Precipitation hardening: (a) phase diagram of an alloy system consisting of metals A and B that can be precipitation hardened; and (b) heat treatment: (1) solution treatment, (2) quenching, and (3) precipitation treatment.

Precipitation Hardening


Sequence in Precipitation Hardening

1. Solution treatment - alloy is heated to a temperature Ts above the solvus line into the alpha phase region and held for a period sufficient to dissolve the beta phase

2. Quenching - to room temperature to create a supersaturated solid solution

3. Precipitation treatment - alloy is heated to a temperature Tp, below Ts, to cause precipitation of fine particles of the beta phase


Microstructure of Al-Cu Alloy in Age Hardening

Figure 5.33 (a) Phase diagram for the aluminum-copper alloy system. (b) Various microstructures obtained during the age-hardening process. Source: L. H. Van Vlack; Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.



Aging Curves for Al-Cu alloys

Source: DeGramo

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Heat Treatment (2)