Mechanical & Aerospace Engineering West Virginia University Strengthening by Phase Transformation

Mechanical & Aerospace Engineering West Virginia University Pure Iron

Mechanical & Aerospace Engineering West Virginia University Fe-C Phase Diagram

Mechanical & Aerospace Engineering West Virginia University Solid Phases in Fe-Fe 3 C Phase Diagram

Mechanical & Aerospace Engineering West Virginia University Slow Cooling Structures in Fe-C Alloys

Mechanical & Aerospace Engineering West Virginia University Microstructure of Pearlite

Mechanical & Aerospace Engineering West Virginia University Microstructures during Slow Cooling of Pure Iron

Mechanical & Aerospace Engineering West Virginia University Microstructures during Slow Cooling of Eutectoid Steel

Mechanical & Aerospace Engineering West Virginia University Microstructures during Slow Cooling of Hypo-eutectoid Steel

Mechanical & Aerospace Engineering West Virginia University Microstructure of Hypo-Eutectoid Steel

Mechanical & Aerospace Engineering West Virginia University Microstructures during Slow Cooling of Hyper-Eutectoid Steel

Mechanical & Aerospace Engineering West Virginia University Microstructure of Hyper-Eutectoid Steel

Mechanical & Aerospace Engineering West Virginia University Room Temperature Microstructures of Slow Cooled Steels

Mechanical & Aerospace Engineering West Virginia University Mechanical Properties of Normalised Carbon Steels

Mechanical & Aerospace Engineering West Virginia University T ime -T emperature -T ransformation Diagram C-curves are determined by quench-hold-quench sequences.

Mechanical & Aerospace Engineering West Virginia University T ime -T emperature -T ransformation Diagram Coarse Pearlite  Fine Pearlite  Upper Bainite (  + tidy plate of Fe 3 C)  Lower Bainite (displacive  + tidy rod of Fe 3 C) For continuous cooling, there are Continuous Cooling Diagrams available.

Mechanical & Aerospace Engineering West Virginia University Quenched and tempered Carbon Steels

Mechanical & Aerospace Engineering West Virginia University T ime -T emperature -T ransformation Diagram of Carbon Steel 1.As the Carbon content increases, the “nose” moves toward right 2.As the C content increases, both M S and M F decrease

Mechanical & Aerospace Engineering West Virginia University Effect of Carbon Content on Lattice Structure and Hardness of Martensite

Mechanical & Aerospace Engineering West Virginia University Change of Martensite during Tempering Tempering: Reheating Martensite to 300 – 600C

Mechanical & Aerospace Engineering West Virginia University Change of Martensite during Tempering

Mechanical & Aerospace Engineering West Virginia University Mechanical Properties of Q+A Carbon Steels

Mechanical & Aerospace Engineering West Virginia University Why Alloy Steels Plain Carbon steel 1.Cannot be strengthened beyond about 100,000 psi without significant loss in toughness and ductility. 2.Large sections cannot be made with a martensitic structure throughout, and thus are not deep-hardenable. 3.Rapid quench rates are necessary for full hardening in medium- carbon plain-carbon steels to produce a martensitic structure. This rapid quenching leads to shape distortion and cracking of heat- treated temperatures. 4.Have poor impact resistance at low temperatures. 5.Have corrosion resistance for many engineering environments 6.Oxidized readily at elevated temperatures.

Mechanical & Aerospace Engineering West Virginia University Why Alloy Steels Add Alloying Elements 1.To improve the hardenability of the steel 2.To give solution strengthening and precipitation hardening 3.To give corrosion resistance 4.To stabilize austenite, give a steel that is austenitic (fcc) at room temperature

Mechanical & Aerospace Engineering West Virginia University Hardening of Alloy Steels (1) Solution Hardening  Alloying Elements in low-alloy steels : dissolve in the ferrite  W & Co in tool steels: large amount  No special heat treatments are needed  Do Not upset by overheating

Mechanical & Aerospace Engineering West Virginia University Hardening of Alloy Steels (2) Precipitation Hardening Example: Hot-work tool steel 1C-0.4Si-0.4Mn-4Cr-5Mo-6W-2V-5Co Used as Q+T condition: Fine dispersion of Fe3C + Solid solution of Mo, Mn, Cr & others When it becomes hot: Fe3C dissolve; Formation of Mo2C, W2C + VC Secondary Hardening (Stronger than Q+T)

Mechanical & Aerospace Engineering West Virginia University Classification of Alloy Steels

Mechanical & Aerospace Engineering West Virginia University Classification of Alloy Steels

Mechanical & Aerospace Engineering West Virginia University Classification of Alloy Steels AISI System: First 2 digits indicate the principal alloying element or group of alloying elements Last two digits indicate the approximate nominal carbon content of the alloy

Mechanical & Aerospace Engineering West Virginia University Applications of Alloy Steels

Mechanical & Aerospace Engineering West Virginia University Applications of Alloy Steels

Mechanical & Aerospace Engineering West Virginia University Stainless Steel Adding 13% to 30% Cr to form hard, compact film of Cr2O3 and then, protect the underlying metal

Mechanical & Aerospace Engineering West Virginia University Classification of Stainless Steels Ferritic Stainless Steels: 11 to 30%Cr, <0.12% C. Do not normally undergo the austenite-ferrite transformation and are not considered heat-treatable. Good ductility and relative low strength. Martensitic Stainless Steels: 12 to 17%Cr, 0.1 to 1%C. Can be hardened by heat treatment to form martensite. Very high hardness can be achieved if carbon content is up to 1%.

Mechanical & Aerospace Engineering West Virginia University Classification of Stainless Steels Ferritic Stainless Steels

Mechanical & Aerospace Engineering West Virginia University Classification of Stainless Steels Austenitic Stainless Steel: Essentially ternary alloys containing from6 to 22% Ni. Cannot be hardened by heat treatment. Retain an austenitic structure at room temperature. More ductile, better corrosion resistance than ferritic stainless steel. Typical ASS: Fe-0.1C-1Mn-18Cr-8Ni (18/8)

Mechanical & Aerospace Engineering West Virginia University Classification of Stainless Steels

Mechanical & Aerospace Engineering West Virginia University Classification of Stainless Steels Precipitation-Hardening Stainless Steel: 10 to 30%Cr, varying amounts of Ni and Mo. Precipitation- hardening phases are formed by additions of Cu, Al, Ti & Nb. High mechanical strengths, without significant loss of corrosion resistance for many high temperature applications.