Understanding Steel Types and Alloying

The question asks about a type of steel where elements are added to modify the mechanical properties of plain carbon steel. This process of adding elements to plain carbon steel to improve specific properties is known as alloying.

Let's consider the definition of different types of steel:

Plain Carbon Steel: This is primarily an alloy of iron and carbon, with carbon content typically ranging from 0.02% to 2.1% by weight. It also contains small amounts of other elements like manganese, silicon, phosphorus, sulfur, and oxygen, which are considered impurities.

Alloy Steel: This type of steel is created by adding various alloying elements (other than carbon) in significant amounts to plain carbon steel. Common alloying elements include chromium, nickel, molybdenum, vanadium, silicon, and manganese (in higher concentrations than in plain carbon steel). The purpose of adding these elements is specifically to enhance or modify the mechanical properties and other characteristics of the steel, such as strength, hardness, toughness, wear resistance, corrosion resistance, and heat resistance.

Let's look at the given options:

• Alloy steel: As explained above, alloy steel is defined by the addition of alloying elements to modify the properties of plain carbon steel. This definition perfectly matches the description given in the question.
• Invar: Invar is a specific alloy, typically consisting of 64% iron and 36% nickel. It is known for its very low coefficient of thermal expansion, not as a general category of steel used for modifying mechanical properties broadly. While it is an alloy of steel, it's a specific application, not the general term for steel modified for mechanical properties.
• Stainless steel: Stainless steel is a type of alloy steel that contains a minimum of 10.5% chromium. The primary purpose of chromium is to provide corrosion resistance. While it is an alloy steel, it's a specific type known for corrosion resistance rather than the general category defined by modifying various mechanical properties.
• High speed steel: High speed steel (HSS) is another specific type of alloy steel, primarily used for cutting tools. It contains elements like tungsten, molybdenum, chromium, and vanadium in significant amounts to provide high hardness, wear resistance, and heat resistance, especially at elevated temperatures encountered during cutting. It is a specific application of alloy steel.

Based on the definitions, the term that generally describes steel whose elements are used for the purpose of modifying the mechanical properties of plain carbon steel is alloy steel.

Therefore, steel whose elements are used for the purpose of modifying the mechanical properties of plain carbon steel is called Alloy steel.