1. Heat treatment of steel
Engineering Materials
Engineering alloys
Heat treatment of steel
Tempering
Tempering is the process of reheating the steel at a relatively low temperature leading to precipitation and spheroidization of the carbides present in the microstructure.
 Tempering is done to develop the required combination of hardness, strength and toughness of fully hardened steels.
Steels are never used in the as quenched condition. The combination of quenching and tempering is important to make tough parts.
The tempering temperature and times are generally controlled to produce the final properties required of the steel.

2. Engineering Materials
Engineering alloys
Steel alloys
Alloy agents are added to improve one or more of the following properties
Hardness
Corrosion resistance
Machineability
Ductility
Strength
The following alloying elements
e.g., nickel, manganese, nitrogen, copper, chromium, silicon, aluminum, titanium, vanadium, niobium, molybdenum, tungsten, etc). Could be added to an iron-carbon alloy (steel).

3. Engineering Materials Engineering alloys
Carbon steel
Any steel in the 0.35 to 1.86 % carbon content range can be hardened using a heat-quench-temper cycle. Most commercial steels are classified into one of three groups:
1. Plain carbon steels Low-alloy steels High-alloy steels

4. Engineering Materials Engineering alloys
Carbon steel
Any steel in the 0.35 to 1.86 % carbon content range can be hardened using a heat-quench-temper cycle. Most commercial steels are classified into one of three groups:
1. Plain carbon steels Low-alloy steels High-alloy steels
1. Plain-carbon steel :- is a metal alloy, a combination of two elements, usually iron with less than 1 % carbon, plus small amounts of manganese, phosphorus, sulfur, and silicon.
Steel with low carbon content has the same properties as iron, soft but easily formed. As carbon content rises the metal becomes harder and stronger but less ductile and more difficult to weld. Higher carbon content lowers steel's melting point and its temperature resistance in general.
Plain carbon steels are further subdivided into four groups:
Mild (low carbon) steel (less than 0.3%C) Medium-carbon steels (0.30 to 0.45% C)
3. High carbon steel (0.45 to 0.75 % C) Very high carbon steel (up to 1.50 % C)

5. Engineering Materials Engineering alloys
1. Plain-carbon steel
Plain carbon steels are further subdivided into four groups:
1. Mild (low carbon) steel:- Have less than 0.30 % carbon and are the most commonly used grades. They machine and weld nicely and are more ductile than higher-carbon steels. It is also often used where large amounts of steel need to be formed, for example as structural steel
2. Medium-carbon steels:- Have from 0.30 to 0.45% carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.

6. Engineering Materials
Engineering alloys
1. Plain-carbon steel
Plain carbon steels are further subdivided into four groups:
3. High carbon steel: With 0.45 to 0.75 % carbon, these steels can be challenging to weld. Preheating, postheating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.
4- Very high carbon steel: With up to 1.50 % carbon content, very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.

7. Engineering Materials Engineering alloys
2. Low-alloy steels
Steels with carbon content usually below 0.25 % and often below 0.15 %. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures.
These alloys can, in the right combination,
Improve corrosion resistance
Influence the steel’s response to heat treatment.
Low-alloy steels have good combinations of high strength and toughness and are used extensively in the automotive industry for application such as gears, shafts and axles.

8. Engineering Materials Engineering alloys
3. High-alloy steels (stainless steel)
For the most part, we’re talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:
1. Austenitic Ferritic Martensitic
1. Austenitic stainless steels have 18% Cr and offer excellent weldability, but austenite isn’t stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen
Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium.

9. Engineering Materials Engineering alloys
3. High-alloy steels (stainless steel)
For the most part, we’re talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:
1. Austenitic Ferritic Martensitic
1. Austenitic stainless steels have 18% Cr and offer excellent weldability, but austenite isn’t stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen
2. Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.
3. Martensitic stainless steels have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking.

10. Engineering Materials Engineering alloys
Cast iron :
Iron that contains about 4 % carbon, There are four basic types of cast iron
Gray cast iron,
Relatively soft. Easily machined and welded,
Used for engine cylinder blocks and pipe

11. Engineering Materials Engineering alloys
Cast iron :
Iron that contains about 4 % carbon, There are four basic types of cast iron
2. White cast iron,
Hard, brittle, and not weldable.
When it’s annealed, it becomes malleable cast iron.

12. Engineering Materials Engineering alloys
Cast iron :
Iron that contains about 4 % carbon, There are four basic types of cast iron
3. Malleable cast iron,
which is annealed white cast iron.
It can be welded, machined,
Ductile, and offers good strength and shock resistance.

13. Engineering Materials Engineering alloys
Cast iron :
Iron that contains about 4 % carbon, There are four basic types of cast iron
4. Ductile cast iron,
its carbon is in the shape of small spheres, not flakes. (sometimes called nodular or spheroidal graphite cast iron)
Ductile, malleable and weldable.

14. Engineering Materials
Engineering alloys

15. Engineering Materials Engineering alloys
Ferrous alloys:- Have iron as their principal alloying metals.
Nonferrous alloys:- Have a principal alloying metal other than iron.
Magnesium
Titanium
Aluminium
Nickel
Copper
Zinc
Refractory metals (Mo, W)
Noble metals

16. Engineering Materials Engineering alloys
Magnesium Alloys
Mg Extracted electrolytically from concentrated magnesium chloride in seawater
Crystal structure hexagonal close packed (HCP)
Melting point 650°C
Density 1740 kg/m3 (very low) (lighter then Al)
VERY POOR corrosion resistance
Main alloying additions : Al, Mn, Zn
commercial cast alloy
– “AZ81”: Mg-8Al-0.5Zn-0.3Mn
Automotive applications
Its cost more than Al

17. Engineering Materials Engineering alloys
Magnesium Alloys
Classification of Magnesium alloys:
Usually designated by 2 capital letters followed by 2 or 3 numbers. The letters for the 2 major alloying elements in the alloy, the first is the highest conc., and the second for the 2nd highest conc. The first no. for the wt.% of the first letter, and the 2nd for the second one.
If a letter A, B ..etc, follows the numbers, it indicates that there has been an A, B ..etc modification to the alloy.
A=aluminum K=zirconium M=manganese Q =silver S=silicon T=tin Z=zinc H=thorium.
Example: Explain the meaning of the magnesium alloy designations a) HK31A b)ZH62A.

18. Engineering Materials Engineering alloys
Titanium Alloys
Are expensive but have a combination of strength and lightness not available from any other metal alloy system. They are used extensively for aircraft structure parts.
Crystal structure hexagonal close packed (HCP)
Melting point 1670 °C
Density 4510 kg/m3 (relatively light)
Excellent corrosion resistance
Main alloying additions : Al, V, Sn
commercial alloy
– Ti-5Al-2.5Sn
– Ti-6Al-4V
aircraft applications
Its cost more than Al
ideal for use in components which operate at elevated temperatures, especially where large strength to weight ratios are required.
used widely in the chemical industries

19. Engineering Materials Engineering alloys
Titanium Alloys
Aeroengine Fan Blades
A jet engine produces thrust by giving a large acceleration to a relatively small weight of air. During operations, the fan at the front rotates at about 7500 r.p.m. This sucks air into the engine. As it passes through the engine it is compressed, mixed with fuel and ignited. The resulting expansion causes the gas to exit the jet at a high velocity, giving the required thrust. The diagram to the right shows the temperature, pressure and velocity distribution of the gas inside a typical large aeroengine.
Given the large size and the associated momentum when the engine is spinning, it is imperative that the weight of these blades should be as small as possible. They are therefore made of Ti-6Al-4V.

20. Engineering Materials Engineering alloys
Aluminum Alloys
Crystal structure face centred cubic (FCC)
Melting point 660°C
Density 2700 kg/ m3 (low) (One-third the density of Steel)
Good corrosion resistance
Main alloying additions :
Cu, Mg, Mn, Si, Zn, Sn, Li
→ binary, ternary and higher order alloys
Applications of Al alloys
– Aircraft construction
– Electric conductors
– Building construction
Aluminum alloys can be up to 30 times stronger then pure Aluminum

21. Engineering Materials Engineering alloys
Nickel Alloys
Crystal structure face centred cubic (FCC)
Melting point 1453°C
Density 8900 kg/ m3 (high) (limit its use)
High corrosion and oxidation resistance
Main alloying additions :
Cu, Al, Fe,Ti, Cr, Co
Nickel alloy used in power generation for their ability to resist corrosion, deformation, cracking, and metal fatigue in the presence of high temperatures.
relatively expensive
Nickel when alloyed with chromium and cobalt forms the basis for the nickel-base superalloys needed in gas turbines for jet aircraft and some electric-power generation equipment.

22. Engineering Materials Engineering alloys
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