Tuesday, 23 September 2008


Full annealing is accomplished by heating a hypoeutectoid steel to a temperature above the UCT (Upper Critical Temperature). In practice, the steel is heated to about 100 oF above the UCT. It is then cooled in the furnace very slowly to room temperature. The formation of austenite destroys all structures that have existed before heating. Slow cooling yields the original phases of ferrite and pearlite.


Figure 1. Annealing and Spheroidizing Temperatures

Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels. To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied. This process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes the machining easy. Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite. This structure is desirable when minimum hardness, maximum ductility and maximum machinability are required.


Figure 2. Spheroidizing process applied at a temperature below the LCT.


Figure 3. Spheroidizing process applied at a temperature below and above the LCT.

Low carbon steels are seldom spheroidized for machining, because they are excessively soft and gummy in the spheoridized conditions. The cutting tool will tend to push the material rather than cut it, causing excessive heat and wear on the cutting tip.


Figure 4. Spheroidized cementite in a ferrite matrix.

Stress-Relief Annealing is sometimes called subcritical annealing, is useful in removing residual stresses due to heavy machining or other cold-working processes. It is usually carried out at temperatures below the LCT, which is usually selected around 1000oF.

The benefits of annealing are:

Image79Improved ductility

Image79Removal of residual stresses that result from cold-working or machining

Image79Improved machinability

Image79Grain refinement

Full annealing consists of (1) recovery (stress-relief ), (2) recrystallization, (3) grain growth stages. Annealing reduces the hardness, yield strength and tensile strength of the steel.

Cast Iron

Cast irons may often be used in place of steel at considerable cost savings. The design and production advantages of cast iron include:

Image79 Low tooling and production cost

Image79 Good machinability without burring

Image79 Ability to cast into complex shapes

Image79 Excellent wear resistance and high hardness (particularly white cats irons)

Image79 High inherent damping capabilities

The properties of the cast iron are affected by the following factors:

Image79 Chemical composition of the iron

Image79 Rate of cooling of the casting in the mold (which depends on the section thickness in the casting)

Image79 Type of graphite formed (if any)

Types of Cast Iron:

Major types of cast iron are shown in Figure 1.


Figure 1. Types of Cast Iron

Gray Cast Iron:

Gray cast iron is by far the oldest and most common form of cast iron. As a result, it is assumed by many to be the only form of cast iron and the terms "cast iron" and "gray iron" are used interchangeably. Unfortunately the only commonly known property of gray iron- brittleness- is also assigned to "cast iron" and hence to all cast irons. Gray iron, named because its fracture has a gray appearance. It contains carbon in the form of flake graphite in a matrix which consists of ferrite, pearlite or a mixture of the two. The fluidity of liquid gray iron, and its expansion during solidification due to the formation of graphite, have made this metal ideal for the economical production of shrinkage-free, intricate castings such as motor blocks.

The flake-like shape of graphite in Gray iron, see Figures 2 and 3, exerts a dominant influence on its mechanical properties. The graphite flakes act as stress raisers which may prematurely cause localized plastic flow at low stresses, and initiate fracture in the matrix at higher stresses. As a result, Gray iron exhibits no elastic behavior but excellent damping characteristics, and fails in tension without significant plastic deformation. The presence of graphite flakes also gives Gray Iron excellent machinability and self-lubricating properties.


Figure 2. Graphite Flakes in Gray Cast iron


Figure 3. Photomicrograph of Gray Cast iron

Advantages of Gray Cast Iron:

Bullet5 Graphite acts a s a chip breaker and a tool lubricant.

Bullet5 Very high damping capacity.

Bullet5 Good dry bearing qualities due to graphite.

Bullet5 After formation of protective scales, it resists corrosion in many common engineering environments.


Bullet5 Brittle (low impact strength) which severely limits use for critical applications.

Bullet5 Graphite acts as a void and reduces strength. Maximum recommended design stress is 1/4 of the ultimate tensile strength. Maximum fatigue loading limit is 1/3 of fatigue strength.

Bullet5 Changes in section size will cause variations in machining characteristics due to variation in microstructure.

Bullet5 Higher strength gray cast irons are more expensive to produce.

Low Alloy Gray Cast Iron:

Enables gray cast iron to be used in higher duty applications without redesign or need for costly materials.


Bullet5 Reduction in section sensitivity.

Bullet5 Improvement in strength, corrosion resistance, heat and wear resistance or combination of these properties.


Bullet5 Higher cost.

Bullet5 Alloy additions can cause foundry problems with reuse of scrap (runners, risers, etc) and interrupt normal production.

Bullet5 Increase in strength does not bring corresponding increase in fatigue strength.

Bullet5 Cr, Mo and V are carbide stabilizers which improve strength and heat resistance but impair machinability.

White Cast Iron:

White cast iron is unique in that it is the only member of the cast iron family in which carbon is present only as carbide. Due to the absence of graphite, it has a light appearance. The presence of different carbides, depending on the alloy content, makes white cast irons extremely hard and abrasion resistant but very brittle. An improved form of white cast iron is the chilled cast iron.


Figure 4a. Photomicrograph of White Cast Iron

Chilled Cast Iron:

When localized area of a gray cast iron is cooled very rapidly from the melt, cast iron is formed at the place that has been cooled. This type of white cast iron is called chilled iron. A chilled iron casting can be produced by adjusting the carbon composition of the white cast iron so that the normal cooling rate at the surface is just fast enough to produce white cast iron while the slower cooling rate below the surface will produce gray iron. The depth of chill decreases and the hardness of the chilled zone increases with increasing carbon content.

Chromium is used in small amounts to control chill depth. Because of the formation of chromium carbides, chromium is used in amount of 1 to 4 percent in chilled iron to increase hardness and improve abrasion resistance. It also stabilizes carbide and suppresses the formation of graphite in heavy sections. When added in amounts of 12 to 35 percent, chromium will impart resistance to corrosion and oxidation at elevated temperatures.


Figure 4b. Photomicrograph of Chilled Cast Iron

Fast cooling prevents graphite and pearlite formation. If alloys such as nickel, chromium, or molybdenum are added, much of the austenite transforms to martensite instead of pearlite. The hardness of chilled cast iron is generally due to the formation of martensite.

Chilled cast iron is used for railway-car wheels, crushing rolls, stamp shoes and dies, and many heavy-duty machinery parts.

Ductile Cast Iron (Nodular Cast Iron):

This structure is developed from the melt. The carbon forms into spheres when cerium, magnesium, sodium, or other elements are added to a melt of iron with a very low sulfur content that will inhibit carbon from forming. The control of the heat-treating process can yield pearlitic, ferritic, martensitic matrices into which the carbon spheres are embedded.


Figure 5. Nodular (Ductile) Cast Iron and the spherical carbon embedded into the matrix.


Figure 6. Photomicrograph of Nodular Cast iron

The advantages of ductile cast iron which have led to its success are numerous, but they can be summarized easily-versatility and high performance at low cost. Other members of the ferrous casting family may have superior individual properties which might make them the material of choice in some applications, but none have the versatility of ductile cast iron, which often provides the designer with the best combination of overall properties. This is especially evident in the area of mechanical properties where ductile cast iron offers the designer the option of selecting high ductility, with grades guaranteeing more than 18% elongation (as high as 25 %), or high strength, with tensile strengths exceeding 120 Ksi. Austempered ductile iron offers even greater mechanical and wear resistance, providing tensile strengths exceeding 230 Ksi.

In addition to cost advantages offered by all castings, ductile cast iron, when compared to steel and malleable cast iron, also offers further cost savings. Like most commercial cast metal, steel and malleable cast iron decrease in volume during solidification, and as a result, require feeders and risers to offset the shrinkage and prevent the formation of internal or external shrinkage defects. Ductile cast iron offers significantly low shrinkage during casting. In the case of large castings produced in rigid molds, it does not require feeders. In other cases, it requires feeders that are much smaller than those used for malleable cast iron and steel. This reduced requirement for feed metal increases the productivity of ductile cast iron and reduces its material and energy requirements, resulting in substantial cost savings. The use of the most common grades of ductile cast iron "as-cast" eliminates heat treatment costs, offering a further advantage.

Ductile cast iron is used for many structural applications, particularly those requiring strength and toughness combined with good machinability and low cost. The automotive and agricultural industries are the major users of ductile iron castings. Because of economic advantage and high reliability, ductile iron is used for such critical automotive parts as crankshafts, engine connecting rods, idler arms, wheel hubs, truck axles, front wheel spindle supports, disk brake calipers, suspension system parts, power transmission yokes, high temperature applications for turbo housing and manifolds, and high security valves for many applications. The cast iron pipe industry is another major user of ductile iron.

Malleable Cast Iron:

If cast iron is cooled rapidly, the graphite flakes needed for gray cast iron do not get a chance to form. Instead, white cast iron forms. This white cast iron is reheated to about 1700oF for long periods of time in the presence of materials containing oxygen, such as iron oxide. At the elevated temperatures cementite (Fe3C) decomposes into ferrite and free carbon. Upon cooling, the combined carbon further decomposes to small compact particles of graphite (instead of flake -like graphite seen in gray cast iron). If the cooling is very slow, more free carbon is released. This free carbon is referred to as temper carbon, and the process is called malleableizing.


Figure 7. Malleable Cast Iron

Figure 8 shows ferritic malleable cast iron, which has a ferrite matrix and the tempered carbon particles are embedded into the matrix.


Figure 8. Ferritic Malleable Cast iron

Figure 9 shows pearlite malleable cast iron, which has a pearlite matrix. By adding manganese to the structure, carbon is retained in the form of cementite.


Figure 9. Pearlitic Malleable Cast Iron

A wide variety of physical properties can be obtained by heating and cooling through the eutectoid temperature or by adding alloying elements. Slow cooling will cause the cementite to decompose and release more free carbon (temper carbon). Fast cooling will retain some of the cementite. The amount retained, will depend on the rapidity of cooling.

Malleable cast iron is used for connecting rods and universal joint yokes, transmission gears, differential cases and certain gears, compressor crankshafts and hubs, flanges, pipe fittings and valve parts for railroad, marine and other heavy-duty applications.


Bullet5 Excellent machinability

Bullet5 Significant ductility

Bullet5 Good shock resistance properties


The major disadvantage is shrinkage. Malleable cast iron decreases in volume during solidification, and as a result, requires attached reservoirs (feeders and risers) of liquid metal to offset the shrinkage and prevent the formation of internal or external shrinkage defects.

Classification of Steels

The Society of Automotive Engineers (SAE) has established standards for specific analysis of steels. In the 10XX series, the first digit indicates a plain carbon steel. The second digit indicates a modification in the alloys. 10XX means that it is a plain carbon steel where the second digit (zero ) indicates that there is no modification in the alloys. The last two digits denote the carbon content in points. For example SAE 1040 is a carbon steel where 40 points represent 0.40 % Carbon content. Alloy steels are indicated by 2XXX, 3XXX, 4XXX, etc.. The American Iron and Steel Institute (AISI) in cooperation with the Society of Automotive Engineers (SAE) revised the percentages of the alloys to be used in the making of steel, retained the numbering system, and added letter prefixes to indicate the method used in steel making. The letter prefixes are:

A = alloy, basic open hearth

B = carbon, acid Bessemer

C = carbon, basic open hearth

D = carbon, acid open hearth

E = electric furnace

If the prefix is omitted, the steel is assumed to be open hearth. Example: AISI C1050 indicates a plain carbon, basic-open hearth steel that has 0.50 % Carbon content.

Another letter is the hardenability or H-value. Example: 4340H

General representation of steels:

SAE - AISI Number



Carbon steels

Low carbon steels: 0 to 0.25 % C

Medium carbon steels: 0.25 to 0.55 % C

High carbon steels: Above 0.55 % Carbon


Nickel steels

5 % Nickel increases the tensile strength without reducing ductility.

8 to 12 % Nickel increases the resistance to low temperature impact

15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic properties. (Alnicometals)

25 to 35 % Nickel create resistance to corrosion at elevated temperatures.


Nickel-chromium steels

These steels are tough and ductile and exhibit high wear resistance , hardenability and high resistance to corrosion.


Molybdenum steels

Molybdenum is a strong carbide former. It has a strong effect on hardenability and high temperature hardness. Molybdenum also increases the tensile strength of low carbon steels.


Chromium steels

Chromium is a ferrite strengthener in low carbon steels. It increases the core toughness and the wear resistnace of the case in carburized steels.







Triple Alloy steels which include Nickel (Ni), Chromium (Cr), and Molybdenum (Mo).

These steels exhibit high strength and also high strength to weight ratio, good corrosion resistance.

Table 1. Classification of steels




Ferrite hardener

Graphite former



Mild ferrite hardener

Moderate effect on hardenability

Graphite former

Resists corrosion

Resists abrasion


High effect on ferrite as a hardener

High red hardness


Strong effect on hardenability

Strong carbide former

High red hardness

Increases abrasion resistance


Strong ferrite hardener


Ferrite strengthener

Increases toughness of the hypoeutectoid steel

With chromium, retains austenite

Graphite former


Austenite stabilizer

Improves resistance to corrosion


Ferrite hardener

Increases magnetic properties in steel


Ferrite hardener

Improves machinability

Increases hardenability

Table 2. The effect of alloying elements on the properties of steel

Red Hardness: This property , also called hot-hardness, is related to the resistance of the steel to the softening effect of heat. It is reflected to some extent in the resistance of the material to tempering.

Hardenability: This property determines the depth and distribution of hardness induced by quenching.

Hot-shortness: Brittleness at high temperatures is called hot-shortness which is usually caused by sulfur. When sulfur is present, iron and sulfur form iron sulfide (FeS) that is usually concentrated at the grain boundaries and melts at temperatures below the melting point of steel. Due to the melting of iron sulfide, the cohesion between the grains is destroyed, allowing cracks to develop. This occurs when the steel is forged or rolled at elevated temperatures. In the presence of manganese, sulfur tends to form manganese sulfide (MnS) which prevents hot-shortness.

Cold-shortness: Large quantities of phosphorus (in excess of 0.12%P) reduces the ductility, thereby increasing the tendency of the steel to crack when cold worked. This brittle condition at temperatures below the recrystallization temperature is called cold-shortness.

Cold Work

Objective: To study the effect of cold working on the hardness and microstructure of brass.

Introduction: Cold working promotes rapid change in the mechanical properties and in the appearance of copper alloys. The most rapid method of accomplishing it is by cold rolling as in rolling sheet and bar.


Image79 Rockwell Hardness Tester

Image79 Rolling Mill

Image79 Micrometer

Image79 Hacksaw

Image79 Belt Sander

Image79 Polishing and Etching Equipment

Image79 Metallograph

Material: One piece annealed cartridge brass 3/4 " x 4" x 1/8" thick.


1. Using the hacksaw, cut the annealed cartridge brass into 7 pieces of about 1/2 " length and mark them as 0, 10, 20, 30, 40, 50,60.

  1. Measure the RB or RF (Rockwell B or Rockwell F Scale) of the piece marked "0". This means that sample "0" has no cold work. Take three readings and average them.
  2. Using the rolling mill reduce the thickness of sample "10" by 10%. This means that the sample "10" represents 10% cold work. Measure the hardness and record.
  3. Continue rolling and measuring the hardness numbers of samples 20, 30, 40 ,50 and 60 and record.
  4. Examine the microstucture of the cold worked specimens, parallel to the rolling direction.

Experimental Data: Construct the table shown below and include your experimental results. If the % reduction you obtained in the laboratory varies from the nominal value, indicate what the actual % reduction is. For example, if you obtained 11% reduction (Actual value of cold work) instead of 10 % ( nominal value of cold work), use 11 % in the second column. If you measured the hardness in Rockwell B scale (RB), convert each reading to Rockwell F scale (RF) and include them in the last two columns of the table.

% Reduction Nominal

% Reduction Actual

Thickness After Rolling (in.)












Table 1

After the construction of the table plot a graph that shows the effect of cold work on the hardness of the material as shown below:


Figure 1


Bullet7 Prepare a report with the following format:

Bullet5 Objective

Bullet5 Equipment

Bullet5 Experimental Procedure

Bullet5 Laboratory Data

Bullet5 Discussion of Results

Bullet7 Present your experimental data as shown in Table 1.

Bullet7 Plot Hardness vs. % Cold Work as shown in Figure 1.

Bullet7 Label the microstructure photographs of the selected specimens. Discuss the structure differences and their relationship to hardness of the specimen and explain what has happened to the structure after 60 % cold working .

Bullet7 Answer the following questions:

* What is the effect of cold working on the hardness of annealed brass? (Plot the graph before answering this question)


(TTT ) Diagram

T (Time) T(Temperature) T(Transformation) 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 (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. 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.

Cooling rates in the order of increasing severity are achieved by quenching from elevated temperatures as follows: furnace cooling, air cooling, oil quenching, liquid salts, water quenching, and brine. If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to complete the transformation may be found.

In Figure 1 the area on the left of the transformation curve represents the austenite region. Austenite is stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation.)


Figure 1. TTT Diagram

Figure 2 represents the upper half of the TTT diagram. As indicated in Figure 2, when austenite is cooled to temperatures below LCT, it transforms to other crystal structures due to its unstable nature. A specific cooling rate may be chosen so that the transformation of austenite can be 50 %, 100 % etc. If the cooling rate is very slow such as annealing process, the cooling curve passes through the entire transformation area and the end product of this the cooling process becomes 100% Pearlite. In other words, when slow cooling is applied, all the Austenite will transform to Pearlite. If the cooling curve passes through the middle of the transformation area, the end product is 50 % Austenite and 50 % Pearlite, which means that at certain cooling rates we can retain part of the Austenite, without transforming it into Pearlite.


Figure 2. Upper half of TTT Diagram(Austenite-Pearlite Transformation Area)

Figure 3 indicates the types of transformation that can be found at higher cooling rates. If a cooling rate is very high, the cooling curve will remain on the left hand side of the Transformation Start curve. In this case all Austenite will transform to Martensite. If there is no interruption in cooling the end product will be martensite.


Figure 3. Lower half of TTT Diagram (Austenite-Martensite and Bainite Transformation Areas)

In Figure 4 the cooling rates A and B indicate two rapid cooling processes. In this case curve A will cause a higher distortion and a higher internal stresses than the cooling rate B. The end product of both cooling rates will be martensite. Cooling rate B is also known as the Critical Cooling Rate, which is represented by a cooling curve that is tangent to the nose of the TTT diagram. Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite while minimizing the internal stresses and distortions.


Figure 4. Rapid Quench

In Figure 5, a rapid quenching process is interrupted (horizontal line represents the interruption) by immersing the material in a molten salt bath and soaking at a constant temperature followed by another cooling process that passes through Bainite region of TTT diagram. The end product is Bainite, which is not as hard as Martensite. As a result of cooling rate D; more dimensional stability, less distortion and less internal stresses are created.


Figure 5. Interrupted Quench

In Figure 6 cooling curve C represents a slow cooling process, such as furnace cooling. An example for this type of cooling is annealing process where all the Austenite is allowed to transform to Pearlite as a result of slow cooling.


Figure 6. Slow cooling process (Annealing)

Sometimes the cooling curve may pass through the middle of the Austenite-Pearlite transformation zone. In Figure 7, cooling curve E indicates a cooling rate which is not high enough to produce 100% martensite. This can be observed easily by looking at the TTT diagram. Since the cooling curve E is not tangent to the nose of the transformation diagram, austenite is transformed to 50% Pearlite (curve E is tangent to 50% curve). Since curve E leaves the transformation diagram at the Martensite zone, the remaining 50 % of the Austenite will be transformed to Martensite.


Figure 7. Cooling rate that permits both pearlite and martensite formation.


Figure 8. TTT Diagram and microstructures obtained by different types of cooling rates



Figure 9. Austenite

Figure 10. Pearlite



Figure 11. Martensite

Figure 12. Bainite