SteelHaseeb Ullah Khan JatoiDepartment of Chemical EngineeringUET Lahore

Recap

• Eutectic phase diagram

Eutectic phase diagram

• Eutectic isotherm

• Invariant point

• Eutectic Reaction

• Compositions of components at invariant point

• Effect of solutes additions on phases

Microstructure development of Sn-Pb system

Iron and Steel

• After iron extraction– Pig Iron– Wrought Iron– Cast Iron

Pig Iron

• Very high carbon content (3.5–4.5%)

• This makes it very brittle with low melting points and not useful directly as a material except for limited applications

• Pig iron is used to make steel where molten pig iron (hot melt) is charged into BOF or Electric Arc Furnace to make steel

Cast Iron

• It contains carbon contents above 2.14%. Most contains between 3 – 4.5%

• It is brittle, wear and tear resistant, cannot be welded, neither malleable nor ductile, does not rust, lack plasticity and has high compressive strength, excellent machineability and M.P is 1150 – 1300 ˚C

• Used for manufacturing of parts of cars, man hole covers, and railings, etc

Types of Cast Iron

Gray Cast Iron

• The carbon and silicon contents of gray cast irons vary between 2.5 and 4.0 wt% and 1.0 and 3.0 wt%, respectively

• Factors that promote the formation of gray cast iron are:– slow rate of cooling– slow rate of solidification

• Mechanically, gray iron is comparatively weak and brittle in tension as a consequence of its microstructure

• the tips of the graphite flakes are sharp and pointed, and may serve as points of stress concentration when an external tensile stress is applied

Ductile (or Nodular) Iron

• Adding a small amount of magnesium and/or cerium to the gray iron before casting produces a distinctly different microstructure and set of mechanical properties

• Graphite still forms, but as nodules or sphere-like particles instead of flakes. The resulting alloy is called nodular or ductile iron

• Typical applications for this material include valves, pump bodies, crankshafts, gears, and other automotive and machine components

a) Gray iron b)Nodular (ductile) ironthe dark graphite flakes the dark graphite nodules

White Cast Iron

• When pig iron converts to cast iron, if cooling is done rapidly then pig iron is changed in to white cast iron

• It is hard and brittle. Its use is limited to applications that necessitate a very hard and wear-resistant surface, without a high degree of ductility—for example, as rollers in rolling mills

Malleable Cast Iron

• Heating white iron at temperatures between 800 to 900˚C for a prolonged time period and in a neutral atmosphere (to prevent oxidation) causes a decomposition of the cementite, forming graphite, which exists in the form of clusters

• Representative applications include connecting rods, transmission gears, and differential cases for the automotive industry, and also flanges, pipe fittings, and valve parts for railroad, marine, and other heavy-duty services

(c) White iron (d) Malleable ironPearlite dark graphite rosettes

Compacted Graphite Cast Iron

• Silicon content ranges between 1.7 and 3.0 wt%, whereas carbon concentration is normally between 3.1 and 4.0 wt%

• Important applications are in diesel engine blocks, exhaust manifolds, gearbox housings, brake discs for high-speed trains, and flywheels

Wrought Iron

• It is tough, can bear shocks and impact, can be welded, M.P is 1500˚C

• Corroded easily

• Purest form of Iron in which impurities does not exceed from 0.5%

• Used for making sheets, for rod making, making of gas pipelines, boiler tubes and frames of windows

Steel

• When carbon in small quantities is added to iron, ‘Steel’ is obtained

• The atomic diameter of carbon is less than the interstices between iron atoms and the carbon goes into solid solution of iron

• As carbon dissolves in the interstices, it distorts the original crystal lattice of iron

Steel

• This mechanical distortion of crystal lattice interferes with the external applied strain to the crystal lattice, by mechanically blocking the dislocation of the crystal lattices (In other words, they provide mechanical strength)

• Adding more and more carbon to iron (upto solubility of iron) results in more and more distortion of the crystal lattices and hence provides increased mechanical strength

Steel

• However, solubility of more carbon influences negatively with another important property of iron called the ductility’

• Carbon contents may vary between 0.2 – 2.1% by wt, depending on the grade. Purest form of iron and the impurities present in it are less than 0.5%

Steel

• Hence we see that when more carbon is added, enhanced mechanical strength is obtained, but ductility is reduced

• Increase in carbon content isnot the only way, and certainly not the desirable way to get increased strength of steels. More amount of carbon causes problems during the welding process

• Phase diagram for different steel types

Next Lecture

• Stainless steel

Steel Making Routes

• Alloy and Special steels made by route 2

Carbon steel. When a steel consists of iron and carbon only, it is known as carbon steel.

Alloy Steel. When steel contains iron, carbon and other alloying elements such as copper, vanadium, nickel, and molybdenum in combined concentrations as high as 10 wt%, and possess higher strengths than the plain low-carbon steels

Mild or Low Carbon Steel

• Mild or Low Carbon Steel. Carbon contents are in the range of 0.15 – 0.25%

• Its structure is fibrous, color is blue, more tough and elastic than cast and wrought iron, corrodes easily, can be welded and magnetized, withstand shocks and impacts.

• Used in manufacturing of various tools, machine parts, rail tracks, industrial buildings, etc.

Medium Carbon Steel

• Carbon contents are in the range of 0.25 – 0.6%. It is not very tough, cannot withstand shocks, and it is easier to weld.

• Used for making number of tools, such as files. Used in roof covering and also in rain water pipes.

High Carbon Steel

• Carbon contents are in the range of 0.6 – 1.4%. High % of carbon in it renders its hardness and toughness. Its structure is granular and it is tougher than mild steel. More difficult to weld and it can withstand shocks and vibrations better.

• Used for making tools e.g. drilling, fine quality of cutlery is made from it. Used to make those parts of machine, which needs hard, tough and durable materials

Stainless Steel

• Stainless steels are iron-based alloys containing a minimum of about 10.5% chromium

• This forms a protective self-healing oxide film, which is the reason why this group of steels has their characteristic "stainlessness" or corrosion resistance

• The ability of the oxide layer to heal itself means that the steel is corrosion resistant, no matter how much of the surface is removed. (This is not the case when carbon or low alloy steels are protected from corrosion by metallic coatings such as zinc or cadmium or by organic coatings such as paint)

• Although all stainless steels depend on the presence of chromium, other alloying elements are often added to enhance their properties

• The categorization of stainless steels is unusual amongst metals in that it is based upon the nature of their metallurgical structure

Role of important alloying elements

• Chromium: Corrosion resistance

• Nickel: Increases ductility, toughness, corrosion resistance

• Molybdenum: Increases mechanical strength, increases localized corrosion resistance

• Copper: Increases corrosion resistance in acid environments

• Manganese: Increases hot ductility, ferrite/austenite balance

• Silicon: Increases resistance to oxidation

• Carbon: Increases mechanical strength, reduces intergranular corrosion

• Titanium: Increases mechanical strength and corrosion resistance

Corrosion Resistance

Stainless Steel

Austenitic stainless steel (300 Family)

• It is a face-centered cubic crystal structure. • Austenite steels make up over 70% of total

stainless steel production.• They contain a maximum of 0.15% carbon,

a minimum of 16% chromium and 6% nickel and/or manganese (Grade 304=18/8)

• Used for high temperature and corrosive environments

Ferritic Stainless Steel (400 Family)

• Generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, because of the lower chromium and nickel content.

• They are also usually less expensive and contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead.

• Most compositions include molybdenum; some, aluminum or titanium

Martensitic

• Martensitic stainless steels are not as corrosion-resistant but are extremely strong and tough, as well as highly machinable, and can be hardened by heat treatment.

• It contains chromium (12–14%), molybdenum (0.2–1%), nickel (less than 2%), and carbon (about 0.1–1%) (giving it more hardness but making the material a bit more brittle).

• It is quenched and magnetic.

• For different grading standards of iron and steel, consult the notes uploaded online

Assignment

• Difference between cast iron and steel

• Composition, properties and applications of nickel steel, vanadium steel, chromium-vanadium steel, manganese steel, silicon steel, molybdenum steel, tungsten steel

– Submission in next class

Mechanical Properties

Next Lecture

• Fabrication and heat treatment methods