Lecture 38: Types of Steels, Alloying Effects

Lecture 38 04-11-2024 5 November 2024 17  Module: 6 Metallic Materials Module:6 Metallic Materials 6 hours Steels – Types of Steels, Effect of alloying elements on structure and...

Lecture 38 04-11-2024 5 November 2024 17  Module: 6 Metallic Materials Module:6 Metallic Materials 6 hours Steels – Types of Steels, Effect of alloying elements on structure and properties of steels, Alloy Steel – Tool and Die Steel, Stainless steel, Speciality steel, Cast iron- White, Grey, Malleable and Nodular - Properties and application of cast irons. Non-ferrous Alloys, Aluminium, copper, Nickel, Magnesium and Titanium. 5 November 2024 Lecture 37 18  Steels – Types of Steels 5 November 2024 Lecture 37 19  Effect of alloying elements on structure & properties Alloying elements are added to steels for one, or more, of the following reasons: 1) To improve mechanical properties by controlling hardenability and permitting higher tempering temperature while maintaining high strength and ductility 2) To improve high- or low-temperature mechanical properties 3) To increase resistance to chemical attack or thermal oxidation 4) To influence special properties such as magnetic permeability and neutron absorption 5 November 2024 Lecture 37 20  Steels – Types of Steels 5 November 2024 Lecture 37 21  Effect of alloying elements on structure & properties 5 November 2024 Lecture 37 22  Effect of alloying elements on structure & properties Boron (B):  Small amounts substantially increase hardenability  Boron-treated steels will usually contain 0.0005–0.003% boron  Boron is effective with low-carbon alloy steels  Its effectiveness decreases with increasing carbon content  Boron is not recommended for steels containing >0.6% carbon. Manganese (Mn):  Increases hardenability and is a carbide former (Mn3C) above a threshold value  In constructional steel alloys, it increases the critical cooling rate  It therefore facilitates deep hardening 5 November 2024 Lecture 37 23  Effect of alloying elements on structure & properties Nickel (Ni):  Increases hardenability by decreasing the critical cooling rate necessary to produce hardening as a result of quenching  It affects austenite transformation by depressing Ac and Ar critical temperatures  Nickel does not form carbide structures  When combined with Cr, nickel produces alloy steels with  Greater hardenability  Higher impact strength  Higher fatigue resistance than is possible with carbon steels 5 November 2024 Lecture 37 24  Effect of alloying elements on structure & properties Chromium (Cr):  Chromium is a strong carbide former  In the presence of C and Fe, Cr forms a complex series of carbide structures  Complex Cr carbides dissolve in austenite slowly  Cr greatly increases hardenability and increases oxidation & corrosion resistance Vanadium (V):  Vanadium is a strong carbide-forming element  Increases hardenability & promotes finer grain size  V decreases high-temperature grain growth  Vanadium increases hardness at elevated temperature 5 November 2024 Lecture 37 25  Effect of alloying elements on structure & properties Molybdenum (Mo):  Molybdenum may form complex carbide structures.  Increases hardenability and is more potent than chromium  It is often used in combination with nickel and/or chromium  In solid solution, Mo ↓ transformation rates, and this increases depth of hardening. Silicon (Si):  It increases the critical temperature by amounts that vary with carbon content  Required austenitizing temperatures are increased  Silicon is not a carbide former 5 November 2024 Lecture 37 26  Issues with Stainless Steel materials  Stainless steel shows the sensitivity to  Grain boundary corrosion  Stress corrosion cracking of grain boundary type  Happens in specific corrosive environment  Caused by the deterioration of corrosion resistance near GB  Concentration of Cr is reduced by the precipitation of chromium carbide at GB  Happens when stainless steel is subjected to specific thermal history  This is called sensitization 5 November 2024 Lecture 37 27  Issues with Stainless Steel materials  Sensitization is a dynamic phenomenon: Occurs in shorter time as temperature is higher  Sensitization of austenitic stainless steel occurs when it is heated to 500 to 850C  Sensitization of austenitic stainless steel vanishes by quenching from the solution treatment temperature above 1000C  In the case of ferritic stainless steel, the sensitization occurs when it is quenched from high temperature above 900C  Sensitization vanishes by the annealing around 800C 5 November 2024 Lecture 37 28  Issues with Stainless Steel materials 5 November 2024 Lecture 37 29