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Difference Between Alloy Steel and Stainless Steel

Difference Between Alloy Steel and Stainless Steel
Among the types of steel available nowadays, stainless steel and alloy steel are the most popular varieties due to their versatility of applications. In this article, we will discuss more about both these types of steel through a comparison.
Anuj Mudaliar
Did You Know?
Stainless steel can be used to remove odors from the skin. An old chef's trick is to rub the hands on a stainless steel surface, such as a counter top or knife, to get rid of the smell of garlic or fish. In fact, nowadays, stainless steel blocks in the shape of soap bars are manufactured specifically for this purpose.

Steels are alloys which are made mainly of iron and carbon, which have high tensile strength and durability. This material has an exceedingly large range of applications in various industries, such as construction, medicine, transport, etc.
Steels can be categorized into carbon steel, stainless steel, and alloy steel, and although carbon steel covers around 90% of all steel production in the world, it has a few disadvantages, such as low impact resistance, limitations in hardening, low resistance to oxidation and corrosion, etc. To combat these problems, stainless and alloy steel were developed. Let us look at the properties of both these steel types.
Properties of Stainless Steel
The main difference between carbon steel and stainless steel is the amount of chromium that is added to the alloy. While the amount of chromium in carbon steel is usually negligible, stainless steel has 10-20% chromium, and may have 7-9% of nickel. This forms a layer of chromium oxide, which gives the metal a smooth and lustrous look, makes it impervious to water and air, protecting it from corrosion. The nickel adds hardness without sacrificing the material's ductility and durability, and also reduces expansion of the metal under heat. Stainless steel can be categorized into 3 grades, depending on the inclusion of carbon and nickel.
Ferritic Stainless Steel: This grade of steel contains 10-20% of chromium, with less than 0.1% carbon. This material has stable iron content up to melting point. It can be worked mechanically to get greater hardness. However, it is ductile only over a certain temperature, and overheating the material for a long time makes it brittle. It is used in applications where high strength and low weight is required.
Martensitic Stainless Steel: This steel too contains 10-20% chromium. However, the carbon content is higher, ranging between 0.1 and 1.5%. This increase in carbon means that the metal can be heated to higher temperatures, helping to increase its hardness, and lower its brittleness. It is used in making cutlery and multipurpose tools.
Austenitic Stainless Steel: This steel is similar to martensitic steel in carbon content, but it also contains 7-9% nickel, along with chromium. This makes the material non-magnetic, more ductile, and more reactive to temperature. It is used mostly for creating cooking utensils and for industrial equipment.
Properties of Alloy Steel
Alloy steel can be classified into two main types, i.e., high strength low alloy steel, which has less than 8% of alloying elements in the composition, and high alloy steel, which has more than 8% of alloying elements in the mix. The properties of low alloy steel are having increased hardness and resistance against wear and tear, as compared to carbon steel. However, in the bargain, it might lose out on tensile strength.
High alloy/tool and die steels have a higher inclusion of alloying elements, which do not lose out on strength, while gaining hardness, and resist wear and tear even under high temperatures.
Advantages and Disadvantages of Alloy Steel
Pros:
  • Alloy steels provide high strength-to-weight ratios.
  • These are available in numerous varieties, which makes them suitable for a wide range of applications.
  • Many alloy steels have a low density, which means that for a low weight, they offer more bulk, enabling the material to handle more stress and pressure.
  • After a simple heat treatment, they can be worked easily into various forms, such as sheets, bars, plates, tubes, etc.

Cons:
  • Many alloy steels are prone to corrosion, and hence, require protective finishes after manufacture.
  • Alloys tend to have low strength overall, especially at high temperatures.

Advantages and Disadvantages of Stainless Steel
Pros:
  • Due to its anti-corrosive nature, stainless steel items are easy to clean.
  • This type of steel is highly resistant to any kind of corrosion or rust.
  • Stainless steel is very strong in structure. Hence, one can use less material for construction purposes, saving money on costs.
  • Stainless steel usually has a good finish, which makes furnishings made of this material attractive, without the need for any external finishing.
  • It is resistant to fire and heat, and retains its strength even under high temperatures.
  • It is difficult to contaminate due to its non-porous nature.
  • It is highly durable, which means that, for long-term use, it is a very inexpensive option.

Cons:
  • Stainless steel is expensive to purchase as compared to other materials such as carbon steel.
  • Processes such as welding and polishing stainless steel is not easy, and requires people with high-level skills, and can be costly too.
  • It tends to get dents and scratches.
  • This material needs regular cleaning, as grease and oily substances get stuck to it easily.

Alloy Steel vs. Stainless Steel
Chemical Composition: Alloy steel contains the presence of elements other than iron and carbon, such as manganese, silicon, nickel, boron, chromium, vanadium, etc. These elements are added under heat treatment. As such, technically, stainless steel is also a high alloy steel, with the addition of chromium and nickel, but due to its specific properties and applications, it is considered as a separate category in the steel manufacturing industry.
Thermal Conductivity: Thermal conductivity can be defined as the property of a material to conduct heat. While most alloy steels, such as titanium and nickel steels, are materials with low thermal conductivity of around 26-48.6 W/m-K, the thermal conductivity of stainless steel is even lower, and ranges around 11.2-36.7 W/m-K.
Tensile Strength: In general, alloy steels have a higher tensile strength than stainless steel. While the tensile strength of stainless steel ranges from 515-827 MPa, that of alloy steels range from 758-1882 MPa.
Applications: Out of the types of alloy steels, manganese, silicon, and nickel steel are the most widely produced varieties. Manganese steel contains 10-18% manganese, and is used to create objects such as rail tracks, security safes, and armor plating. Another popular alloyed steel is silicon steel, which contains 1-5% of silicon, and is used to create magnets. Nickel steel contains 2-4% of nickel, and is largely used in construction materials, such as metal gears, steel shafts, and steel cables. It is also used in creating vehicle bodies, ship hulls, construction and industrial equipment, etc.
On the other hand, the various grades of stainless steel differ due to the amount of chromium (between 10-20%) and nickel (7-9 %) in the material composition. This material is widely used in the manufacture of many items, such as home furnishings, automobile accessories, cutlery, utensils, etc.
Both, alloy steel and stainless steel have various advantages over carbon steel. Aside from this, their varied characteristics makes them suitable for different applications. Being well-informed of each of their properties can help in choosing one type of steel over the other.