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Stainless SteelsThese are stain less as these have a minimum of 11 to 12% Cr which ishaving more affinity for oxygen than iron, forms a very thin, continuousprotective and stable oxide (Cr2O3) film on the surface. Thus Cr impartscorrosion and oxidation resistance and pleasing appearance. Ni. Mo and Mnenhance strength and improve corrosion resistance. Based on amount andtype of alloying elements these steels are classified into 5 types.

Ferritic: (% Cr – 17 × %C) > 12.7 i.e usually 17 to 26 % Cr. These alloysare ferritic in structure up to the melting point. These can’t be heat treated.Cold working is the only strengthening mechanism. These may containsmall additions of Mn, Si, Ni, Al, Mo and Ti. Carbon is kept very low toincrease toughness to avoid sensitization. Mo is added to improve resistanceto pitting corrosion; Nb and Ta are added to stabilize against intergranularcorrosion. These are not susceptible to stress corrosion cracking, thus usedin chemical plants. These are used as heatresisting elements in the makingof furnace components. Less expensive, good machinability, higher thermalconductivity, low thermal expansion than austenitic. These steels showductile to brittle transition. Ridging or roping, temper embrittlement, sigmaphase and grain growth in HAZ are few problems associated with thesesteels.

Martensitic: (% Cr – 17 × %C) ≤ 12.7. Heat treatable steels (as these areaustenitic at temps 950 1000°C). As Cr content is more, air cooling isenough to form martensite which is not brittle due to low carbon content.(1) Low carbon high strength martensitic types are developed to ensuregood weldability, formability and impact toughness. Tempering temperatureof 440 – 550°C should be avoided (temper embrittlement). These are usedin petrochemical, chemical plants, compressors, discs, aircraft structural andengine parts, propeller shafts. (2) High carbon high hardness martensitictypes (poor eldability, formability and ductility) are used in cutlery, surgicalinstruments, springs, high quality ball bearings, razor blades, cool hammers.

Austenitic: (17 – 18% Cr, 8 – 10% Ni, C < 0.03%) these contain sufficientamount of austenitic stabilizers Ni, Mn or N so that steels are austeniticeven at RT. To avoid intergranular corrosion and weld decay ‘C’ should beless than 0.03%. Mo is added to improve pitting resistance and sulphuricacid corrosion. Nb and Ti are added to take care of weld decay. Excellentformability, high work hardening (low stacking fault energy), large uniformelongation, nonmagnetic, no ductile to brittle transition (tough even at verylow temperatures; cryogenic applications). These are not susceptible totemper embrittlement, but shows reduced ductility in temperature range 750– 950°C due to brittle intermetallic formation (sigma phase). These aresusceptible to stress corrosion cracking. These can be strengthened by coldworking or solid solution strengthening. These are used in chemicalindustry, house hold, sanitary, biomedical, architectural, food industries,nuclear and marine applications.

Duplex: these are developed to utilize combination of properties of ferriteand austenite (Toughness, weldability with strength and localized corrosionresistance). These are stronger than austenitic steels. Presence of δferritecause grainrefinement of austenite hence increases strength. Furtherrefinement is obtained by using controlled rolling at 900 950°C whichresults in very fine dispersion of ferrite and austenite grains. These steelsexhibit superplasticity (500% elongation at 950°C). Good corrosionresistance similar to Austenitic type. Not susceptible to stress corrosioncracking and free from intergranular corrosion. These have ductile to brittletransition temperature. These suffer from temper embrittlement and sigmaphase embrittlement.

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Precipitation hardenable: these offer combination of properties butexpensive and difficult to hotprocess. These are used particularly for hightemperature applications such as powerplant. Strengthening is because ofprecipitates like Ni3Al, Ni3Ti or Ni3Mo etc. on ageing. These aresusceptible to hydrogen embrittlement. The matrix can be (1) martensitic(2) semiaustenitic (3) austenitic.

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Phase diagramsPhase diagram is a graphical representation of any alloy system which givesrelation between phases in equilibrium in a system as a function oftemperature, pressure and compositions. Generally pressure is assumedconstant at atmospheric value.

When graphical diagrams deals with phases which are in equilibrium withthe surroundings it is called an equilibrium diagram, otherwise it is called aphase diagram. eg: Fe – Fe3C system is phase diagram (or meta stable), Fe– graphite is an equilibrium diagram.

Gibbs phase rule: The changes in the number of phases in alloy underequilibrium conditions are expressed by the Gibbs phase rule F = C – P +2 ; where F – degrees of freedom, C – number of components, P – numberof phases in system. The minimum value for F is zero and this sets an upperlimit on the number of phases that can be exist in a system underequilibrium.

Unary diagrams These are for single component systems and thus there isno composition variable. The variables are pressure and temperature.Temperature on ordinate (Y – axis ) and pressure on abscissa (X – axis). Insingle phase region both temperature and pressure can be variedindependently. In two phase region either pressure or temperature can bevaried independently. Both can’t be varied simultaneously. Three phaseequilibrium exists only for fixed value of pressure and temperature.

Binary diagrams These depict the equilibrium between two components.Two components can be mixed in an infinite number of differentproportions, that is composition also becomes a variable. These are drawnusually at one atmospheric pressure i.e. pressure is made constant, becausethe variation in pressure results in insignificant effect on the equilibrium.Hence F = C – P + 1 : phase rule for condensed phases. Temperature indegrees or Fahrenheit as ordinate ((Y – axis ) and composition in weight oratom percentage as the abscissa (X – axis) Most common Binary diagrams types

1. Two components completely soluble in the liquid statea) completely soluble in solid state (Isomorphous)b) completely insoluble in solid state (Eutectic)c) Partially soluble in solid state (Eutectic)d) Peritectic reactione) Formation of intermediate phases (congruent and

incongruent)2. Partially soluble in liquid state (Monotectic)3. Components insoluble in liquid as well in solid state4. Transformations in the solid state

a) Allotropic changeb) Order – disorderc) Eutectoidd) Peritectoid

Methods for determination of phase diagrams1. Thermal Analysis 2. Dilatometry 3. Metallographic methods 4. Xraydiffraction 5. Electrical – resistivity methods.

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