In this article, you will learn the complete overview of alloy steel such as its definition, types, properties, and uses, as well as the purpose of alloying, effects of alloying, advantages and disadvantages of alloy steel, and many more.
In our previous article, we learned about the different types of metals and their uses.
Metal can be classified into three categories which are: Ferrous Metal, Non-Ferrous Metal, and Alloys.
Alloy steel is a type of ferrous metal that comes under the types of steel.
So without wasting time let's get started.
Definition of Alloy Steel
Alloy steel is a steel in which various elements other than carbon like nickel, manganese, tungsten, aluminum, copper, molybdenum, etc are added to obtain the desired properties.
Generally, these elements are added to increase mechanical, electrical, thermal, magnetic, or resistant properties.
The main cause of adding alloying elements in alloy steel are as follows:
- To increase the hardness and wear resistance.
- To increase corrosion resistance.
- To increase resistance to abrasion and wear.
- To increase ductility and machinability.
- To improve mechanical, electrical, and thermal properties.
 |
| Alloy Steel |
Properties and Uses of Alloying Elements on Alloy Steel
There are the following alloying elements added to alloy steel:
- Carbon (C)
- Sulfur (S)
- Phosphorous (P)
- Silicon (Si)
- Manganese (Mn)
- Nickel (Ni)
- Chromium (Cr)
- Titanium (Ti)
- Tungsten (W)
- Molybdenum (Mo)
- Vanadium (V)
- Cobalt (Co)
- Aluminum (AI)
- Boron (Br)
Carbon (C)
- It increases hardness and strength.
- It increases machinability.
Sulfur (S)
- It forms manganese sulfide which is hard and brittle.
- Manganese sulfide promotes chip formation and hence increases machinability.
Phosphorous (P)
- It dissolves in ferrite.
- It increases tensile strength and hardness.
- It is a solid solution strengthening.
- It forms iron Phosphide (Fe₃P) which is hard and brittle.
- It also increases machinability.
Silicon (Si)
- It is a ferrite solid solution strengthened.
- It improves oxidation resistance.
- It reduces hysteresis losses.
- It increases toughness.
Manganese (Mn)
- It combines with the sulfur present in the steel to form manganese sulfide and improves machinability.
- It dissolves in ferrite and improves strength and toughness.
- It lowers ductility if present in a higher percentage.
Nickel (Ni)
- It is a ferrite solid solution strengthened.
- It dissolves in ferrite.
- It increases tensile strength and hardness.
- It increases corrosion and oxidation resistance.
- It is an austenite stabilizer and lowers critical temperatures.
- It reduces the coefficient of thermal expansion.
Chromium (Cr)
- It forms chromium carbides which increase hardenability.
- It increases wear resistance.
- It increases corrosion and oxidation resistance.
Titanium (Ti)
- It is a strong carbide former.
- It prevents localized precipitation of chromium carbides.
Tungsten (W)
- It increases hardenability.
- It forms carbides and increases wear resistance.
- It reduces de-carburization.
Molybdenum (Mo)
- It increases hardenability.
- It makes grain finer.
- It forms carbides and increases wear resistance.
- It reduces de-carburization.
Vanadium (V)
- It promotes fine-grain structure.
- It increases hardenability.
- Its properties are similar to those of W and Mn.
Cobalt (Co)
- It reduces hardenability.
- It improves strength.
- It promotes fine-grained structure.
- It improves heat resistance.
Aluminum (AI)
- It is used as a de-oxidizer.
- It promotes fine-grained structure.
Boron (Br)
- It increases hardenability.
- It improves wear and tear resistance.
Types of Alloy Steel
Alloy steel can be classified into the following types:
- Low Alloy Steels
- High Strength Low Alloy Steels
- High Alloy Steel
- High-Speed Steel
- Tool Steel
- Free Cutting Steels
- Maraging Steels
- Low Expansion Steels
- Nickel Steel
- Nickel Cobalt Steel
- Nickel Chrome Steel
- Vanadium Steel
- Manganese Steel
- Silicon Steel
- Cobalt Steel
- Molybdenum Steel
- Stainless Steel
Low Alloy Steels
They have similar microstructures and require similar heat treatments to that of plain carbon steels.
When Nickel, chromium, molybdenum, and other alloying elements consist of less than 10.5% by the weight they are defined as low alloy Steel.
For example cr-mo steel, Ni steel, weathering steel, etc.
High Strength Low Alloy Steels
It is also referred to as micro-alloyed steel.
They have the addition of elements such as Al, Nb, and V either singly or in combination.
It contains 0.07 to 0.13% carbon with 0.5% Aluminium.
These steels show good ductility, malleability, formability toughness, and weldability because their strength increases up to 50- 80 Kg/mm² and is widely used in the automotive industry.
High Alloy Steel
When alloying content of more than 10.5% by weight is called high alloy steel.
They are costly and special-purpose alloys of steel.
For example stainless steel, high-speed steel, etc.
High-Speed Steel
High-speed steel is made by adding tungsten to steel, hence it is also known as tungsten steel.
Tungsten steel is very hard and maintains its hardness even at high temperatures.
It is used in making cutting tools, drill bits, hex blades, cutters, reamers, etc.
According to the amount of tungsten, it is of three types:
- Super High-Speed Steel
- Medium-High Speed Steel
- Low High-Speed Steel
Super High-Speed Steel
It contains 22% tungsten, chromium 4%, and vanadium up to 1%.
Medium-High Speed Steel
It contains tungsten 18%, chromium 4%, and vanadium 1%.
Low High-Speed Steel
It contains tungsten 14%, chromium 4%, and vanadium 1%.
Tool Steel
They are tungsten alloyed Steel containing about 0.7 % of carbon, 18% of tungsten, 4% of chromium, and 1% of vanadium.
The alloying elements like tungsten, chromium, and vanadium form carbides which leads to the high hardness of tool steel.
Tool steel is special Steel that is well suited for making mechanical tools like drills, saw blades, etc.
Free Cutting Steels
These steels can be machined or cut with fast speed because of their high machinability and are hence known as free-cutting steels.
Low-carbon steels produce rough surfaces and are responsible for heating the tool, So Mg, S, and P are added to improve machinability.
Mg combines with S and favors chip formation and also increases hardness and strength.
Maraging Steels
Maraging steels are steels, which are known for possessing superior strength and toughness without losing malleability, although they cannot hold a good cutting edge.
These steels contain 0.03% C, 18.25% Ni, 3 to 5% Mo, 3 to 8% Co, 0.2 to 1.6% Ti and a small amount of Al.
This steel shows strength up to 210Kg/mm² and is used for special applications such as rockets, engine components,s and pressure vessels.
Due to the low carbon content maraging steels have good machinability and offer good weldability.
Due to the high alloy content, maraging steels have a high hardenability.
Low Expansion Steels
These are the alloys of nickel and iron containing 36% Ni, 02% C, 0.5% Mn and balance iron.
These steels have a very low coefficient of expansion and it is widely used for application such as gauges, tapes, and micrometers.
corrosion-resistant.
It does not rust.
It cannot be hardened.
It is often used to make household utensils, watch parts, automobile parts, cars, knives, and airplanes.
Nickel Steel
It contains up to 4% nickel metal.
Its hardness, elastic limit, and tensile strength are high and it does not rust easily.
It is used in making rivets, pipes, axles, and parts of aircraft and engines.
Nickel Cobalt Steel
If 30% to 35% nickel and 5% cobalt are added to the steel, it is called nickel-cobalt steel.
It is also called Invar steel.
Its coefficient of expansion is very less that's why it is used as a precision instrument.
Nickel Chrome Steel
The steel in which 0.5% to 1% carbon, 0.3% to 0.8% manganese, 3% to 5% nickel, and 0.5% to 1.8% chromium are mixed is called nickel chrome steel.
This increases the elastic limit, reduces wear, and also increases hardness and tensile strength.
It is used in making cutlery, automobile parts, cutting tools, etc.
Vanadium Steel
The steel in which 1.5% carbon, 12.5% tungsten, 4.5% chromium, 5% cobalt, and 5% vanadium is added is called vanadium steel.
Due to this, its elastic limit and tensile strength increase, and the ability to tolerate strong shocks arise.
It is mostly used for making tools and cutters.
Manganese Steel
The steel in which 1.0% to 1.9% manganese, and 0.4% to 0.8% carbon are mixed is called manganese steel.
It is also called special high alloy steel.
It wears less.
It is used in making rail lines, grinders, etc.
Silicon Steel
Silicon is mixed in this steel in varying amounts from 1% to 14% according to the work.
It is heat resistant.
It does not rust.
Cobalt Steel
The steel which contains 0.5% to 1.5% high carbon and 5% to 35% cobalt is called cobalt steel.
It has more toughness, tensile strength, and magnetic properties, so it is mostly used for making permanent magnets and more sharp tools.
Molybdenum Steel
The amount of molybdenum in it is very heavy.
It contains molybdenum 4.5% to 9%, carbon 0.8 to 1.5%, chromium 4%, and vanadium 1 to 5%.
On mixing these metals in it, a lot of hardness and toughness come.
It is used to make bearings, motor vehicles, airplanes, etc.
Stainless Steel
If nickel 8%, chromium 18%, molybdenum 2%, and carbon 0.2 to 0.6% are mixed in steel, then it is called stainless steel.
Stainless steels generally contain more than 10% chromium as the main alloying element and are valued for high corrosion resistance. It does not stain, corrode or rust as easily as ordinary steel that's why it is called stainless steel.
Stainless steels have a sufficient amount of chromium present so that a passive film of chromium oxide forms which adheres to the metal surface very tightly and prevents further corrosion.
It is very hard, tough, acid-resistant, and corrosion-resistant.
It does not rust.
It cannot be hardened.
It is often used to make household utensils, watch parts, automobile parts, cars, knives, and airplanes.
These stainless steels can be divided into three groups based on their crystalline structure:
- Austenitic Stainless Steel
- Ferritic Stainless Steel
- Martensitic Stainless Steel
Austenitic Stainless Steel
It contains 18-26% chromium (Cr) and 8-22% nickel (Ni) and less than 0.8% carbon.
It is non-magnetic and non-heat-treatable.
Ferritic Stainless Steel
It contains trace amounts of nickel, 12-17% chromium, and less than 0.2% carbon, along with other alloying elements, such as molybdenum, aluminum, or titanium.
These magnetic steels cannot be hardened by heat treatment but can be strengthened by cold working.
Martensitic Stainless Steel
It contains 11-17% chromium, less than 0.4% nickel, and up to 1.2% carbon.
These are magnetic and are similar in composition to the ferritic group but contain higher carbon and lower chromium to permit hardening by heat treatment.
Effect of Alloying Elements on Alloy Steel
There is the following effect of alloying elements on alloy steel:
- Solid Solution Strengthening and Hardening
- Formation of Carbides.
- Formation of Inclusions (Oxides)
- Formation of Intermediate Compounds
- Shifting and Lowering of Critical Cooling Rate
- Changes in Volume during Transformation
Solid Solution Strengthening and Hardening
Alloying elements are soluble in ferrite. They form solid solutions with ferrite.
Solid solutions are harder and stronger than pure metals, thereby increasing the strength and hardness of the steel.
Formation of Carbides
Some alloying elements combine with carbon to form their carbides.
The alloy carbides so formed increase the hardness and wear resistance.
For example Cr, Ti, W, Mn, V, etc.
Formation of Inclusions (Oxides)
Usually, de-oxidants are added to remove dissolved free oxygen from the steel.
These de-oxidations combine with oxygen to form oxide inclusions.
For example Al, Si, Cr, Mn, etc.
Formation of Intermediate Compounds
Some alloying elements form intermediate compounds with iron.
These compounds improve wear resistance and other mechanical properties.
For example Cr, V, W, Ni, Si, etc.
Shifting and Lowering of Critical Cooling Rate
Different alloying elements have different critical temperatures.
Ferrite stabilizers raise the eutectoid temperature while austenite stabilizers lower the eutectoid temperature.
For example Mo, Ti-ferrite stabilizers, Mn, and Ni-austenite stabilizers.
Changes in Volume during Transformation
The changes in microstructure result in changes in the volume of the unit cell.
Alloys are added to reduce the volume impact.
For example Cr, Ni, Si, Mn, Mo, V, Cr, Ni, P, Si, etc.
Other Effects
Besides those mentioned above, alloying elements are added to improve corrosion resistance, creep strength, etc.
Advantages of Alloy Steel
There are the following advantages of alloy steel:
- Greater strength at elevated temperatures.
- High hardenability.
- Improved ductility and cracking.
- Higher elastic ratio and endurance strength.
- Better machinability at high hardness.
- Less internal stresses.
Disadvantages of Alloy Steel
There are the following disadvantages of alloy steel:
- Higher cost than plain carbon steel.
- Care should be taken during handling.
- Certain grades show temper brittleness.
- Tendency to have retained austenite.
So here you have to know all aspects related to alloy steel.
If you have any doubts then you are free to ask me by mail or on the contact us page.
Thank You.
0 Comments