Q1: Nickel and __________ are the alloying element added in steel to increase its toughness.

ANS:D - silicon

Silicon is often added to steel as an alloying element to impart certain desirable properties. Here's an explanation of the role of silicon in steel:

1. Deoxidation: One of the primary purposes of adding silicon to steel is for deoxidation during the steelmaking process. Silicon readily reacts with oxygen present in the molten steel to form silicon dioxide (SiO2), which floats to the surface and is easily removed as slag. By removing oxygen from the steel, silicon helps prevent the formation of undesirable oxides that could weaken the steel or introduce defects.
2. Improved Fluidity: Silicon can improve the fluidity of molten steel, facilitating the casting process. This is particularly beneficial in steelmaking operations where complex shapes or thin sections are required. The increased fluidity helps ensure that the molten steel fills the mold cavity properly and produces high-quality castings.
3. Grain Refinement: Silicon can also act as a grain refiner in steel, promoting the formation of smaller and more uniform grain structures. This can lead to improved mechanical properties, such as increased strength and toughness, as well as better surface finish and machinability.
4. Strength and Hardenability: In certain steel alloys, silicon can contribute to increased strength and hardenability. It forms solid solutions with iron and other alloying elements, affecting the microstructure and mechanical properties of the steel. Silicon can also enhance the response of steel to heat treatment processes, such as quenching and tempering, leading to improved hardness and toughness.

Overall, silicon is a versatile alloying element that can provide several benefits to steel, including deoxidation, improved fluidity during casting, grain refinement, and enhanced mechanical properties. Its addition is carefully controlled to achieve the desired balance of properties for specific applications in industries such as automotive, construction, and manufacturing.