BASICS OF CRYSTALLOGRAPHY• Iron is a metal, Steel is an alloy made by combining iron and other elements.• Iron and steel are crystalline in nature. This means that there is a repeat pattern at microscopic level.• Anything that is not crystalline is amorphous.(glass/wood etc.)
ALLOTROPES• Diamond and coal are both pure carbon. The chemical composition of both are identical.• They exist in different physical forms just because the arrangement of atoms is different.
IMPORTANT ATOMIC ARRANGEMENTSSimple cubic Body centeredFace centered
ALLOTROPES OF IRONIn pure IRON • Below 910 °C (ie. at room temperature also) ferrite form is stable. This form is body centered. • From 910 to 1390 °C the face centered form of iron, called austenite (gamma-iron) is stable. • Above 1,390 °C up to the melting point the body-centered cubic crystal structure is again the more stable form called delta iron.
TYPES OF STAINLESS STEELS• Ferritic• Austenitic• Martensitic• Duplex
FERRITIC STAINLESS STEEL• Ferritic stainless steels generally have better engineering properties than austenitic grades.• Contains between 10.5% and 27% chromium• Corrosion resistance is not good. (low chromium and nickel content).• They are usually less expensive.• Ferritic steel is magnetic.• Common ferritic grades include • 18Cr-2Mo • 26Cr-1Mo • 29Cr-4Mo • 29Cr-4Mo-2Ni
AUSTENITIC STAINLESS STEEL• Also called series 300 steel• Make up over 70% of total stainless steel production.• Contains atleast 16% chromium.• Nickel is added to stablize austenite at room temperature• Superaustenitic stainless steels, such as alloy AL-6XN and 254SMO exhibit great resistance to chloride pitting and crevice corrosion but are very costly.• Austenitic steel is non-magnetic.• Type 304 is a surgical steel. Also used for chemical processing equipment, for food, dairy, and beverage industries, for heat exchangers, and for the mild chemicals.• Type 316 is used in construction, for dispensing and in the most corrosive environments.• Type 316L used in jewelry and watches.• Type 347 is used in aircraft industry.
MARTENSITIC STAINLESS STEEL• Also called series 400 steel.• The martensite is formed by quenching (heating and rapid cooling ) of austenite.• These are not very corrosion resistant but are strong ,tough and machinable.• chromium (12–14%)• molybdenum (0.2–1%)• nickel (less than 2%)• carbon (about 0.1–1%) (extra carbon increases the hardness but make the steel more brittle)• It is magnetic.
GRADES OF MARTENSITIC STEEL• Type 410 is used widely where corrosion is not severe• Type 410S used against high temperature corrosion.• Type 414 used in springs and cutlery.• Type 416 screw machine parts.• Type 420 surgical instruments.• Type 431 high strength parts such as valves and pumps.• Type 440 fabrication of instruments.
Duplex steel• Duplex steel is a better form of stainless steel . It has Chromium content more than 22% whereas stainless steel has 18% or less.• Duplex steel is named so because it has about 50- 50 ratio of austenite and ferrite forms.
PROPERTIES: • Strength: Duplex stainless steels are much stronger than regular stainless steels. • Toughness and ductility: Duplex stainless steels have better toughness and ductility than ferritic grades but not as good as austenitic stainless steel. • Corrosion resistance: corrosion resistance is nearly similar to normal stainless steel (austenitic/ferritic).. • Stress corrosion resistance:( corrosion at increased rate under stress) Duplex stainless steels show very good stress corrosion cracking (SCC) resistance like ferritic steel • Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts (of similar corrosion resistance)..this reduces the alloying costs.It may often be possible to reduce the section thickness ofduplex stainless steel fabrications due to its increased yieldstrength compared to austenitic stainless steel. This resultsin significant cost and weight reduction.
APPLICATIONS• Duplex stainless steels are used in the chemical industry in refineries, gas-processing plants, pulp and paper plants, and sea water piping installations.(good resistance to salt water)• Grade 2205 is most widely used grade of duplex stainless steel due to its excellent corrosion resistance and high strength.