The temperature at which austenite ⇔ • pearlite occurs (lower critical temperature) is called A1 • Some elements, notably nickel, manganese, cobalt and copper raise the A4 temperature and lower the A3.
• Therefore these elements when added to a carbon steel tend to stabilize austenite ( γ ) and extend the range of temperature over which austenite exists as a stable phase.
• Most of these elements have an FCC crystal structure like that of the austenite, therefore, dissolve substantially with ease in the austenite and consequently retard the transformation of austenite back to ferrite.
• These elements are termed “ Austenite stabilizers ” • On the other hand, other elements such as chromium, tungsten, vanadium, molybdenum aluminum and silicon tend to stabilize ferrite by lowering the A4 temperature and raising the A3
• Such elements restrict the field over which austenite may exist and, thus, form what is known as “gamma ( γ ) loop”. • “ Ferrite stabilizers ” are principally elements with a BCC crystal structure like that of α -ferrite.
• Suppressing the austenitic range would definitely affect the ability of steels for heat treatment. • Another important effect alloying elements may have is their effect on the formation and stability of carbides within the steel .
• Carbides have a significant hardening effect when present in steels especially when the carbides are harder that the iron carbide (cementite).
Displacement of the eutectoid point • In this respect, all allying elements will lower the eutectoid carbon content
• Austenite stabilizers tend to lower the eutectoid temperature while ferrite stabilizers tend to raise eutectoid temperature
Effect on Transformation rates • Additions of alloying elements especially nickel and chromium tend to retard transformation rates, i.e., shift the TTT and modified TTT curves to the right.
• This has a great beneficial effect in that slower cooling rates (such as oil quenching or air cooling) may be used and still get a completely martensitic structure.
• A minor disadvantage introduced by alloying elements (except cobalt) is the lowering of the MF temperature.
Effect on mechanical properties
• Another important effect of alloying elements on carbon steel is the determination of the final “stable” structure at room temperature.
Exercise : • Determine the final structure of three steels having compositions of: – 4.25% Ni, 1.25 % Cr, 0.25% Mo, 0.45 % Mn, 0.25% Si and 0.3 C.
– 17.5% Cr, 11% Ni, 1.2% Mn, 0.6% Si and 0.05% C. – 14 % Cr, 0.8% Si, 0.5% Mn and 0.03% C.
Chapter Five
Mechanical Properties • Mechanical properties are of special interest, as these determine how a particular engineering part will be performing during its service. • Of a great importance are some of these properties which will be discussed below.
Tensile strength • This property is probably of interest to most engineers who are involved in someway or another with metallic materials. • Strength can be defined as “ The material’s resistance to deformation when subjected to loading ”
• The test by which this property is determined, however, provides much more information than just the tensile strength. This test (common to all engineers) is the “ Tensile test ”. • The test consists, simply, of pulling a standard specimen of the material in interest and recording date such as the load applied and the elongation experienced.
• In order to systematize the discussion the concepts of stress and strain should be defined. • “Stress” is defined as being the amount of applied load per unit area. • The unit of stress is Pascal (1 Pa = 1 N/m2) or MPa.
• The discussion will begin with one type of stress known as “ engineering or nominal stress ”. P σ = A o • Where : σ is the engineering stress • P is the applied load and • Ao is the original cross sectional area
• While engineering strain “e” can be defined as the relative elongation per unit length or: − Δ l l l = = f o e l l o o
• Upon loading, the specimen starts to elongate with the load (stress) increasing. This elongation is elastic (temporary) and the relationship between stress and strain is linear and can be expresses as:
σ = Ee • Where E is known as the modulus of elasticity or Young’s modulus and has units of MPa. • This behavior continues up to a certain stress level known as “ the proportional limit ”. • This range is termed the “ elastic linear range ”
• After the proportional limit, elastic elongation continues up to the “ yield point ” ( Y ), the relationship between stress and strain, however, is no longer linear. • This range is termed the “ elastic non- linear range ”.
• The two ranges above are usually summed up in one range the “ elastic range ”. • In this range deformation is not permanent and if a component is loaded within this range and then the load is released then this component will experience a phenomenon know as” Elastic recovery ”.
• This means that the component will go back to its original shape or dimensions after the load has been released. • The yield point can be determined by the 0.2 % offset method
• An important property relevant to the elastic range is the “ modulus of resilience ” ( MOR ), • MOR can be defined as the “ energy per unit volume the material can absorb elastically ”.
• On the stress strain curve this is equal to the area under this curve in the elastic range. • Assuming linearity up to the yield point gives:
2 Y = MOR 2 E • This property is especially important in design applications where parts are supposed to stay in the elastic range.
• After the yield point, plastic (or permanent) deformation takes place with stress value still increasing until reaching a maximum value known as ” The Ultimate Tensile Stress or UTS ”;
P = max UTS A o • Up to this point the specimen exhibits what is known as “ Uniform elongation ”. • Uniform elongation means that “ The increase in length and the reduction in cross sectional area are uniform throughout the whole length of the specimen ”
• At this stress level the “ necking phenomenon ” (concentrated reduction in the cross sectional area) takes place at the center of the specimen leading to reduction in stress level and final fracture at the “ necked region ”
• It becomes now important to define the terms true stress and true strain as follows: • True stress P σ = A Where A is the true or instantaneous area
• True strain ⎛ ⎞ l ⎜ ⎟ ε = f ln ⎜ ⎟ ⎝ ⎠ l o Where lf and lo are the final and original lengths of the specimen, respectively
• This curve can be represented by a mathematical formula relating true stress and true strain, usually in the form: σ = K ε n Where • K is known as the strength coefficient • n is known as the strain hardening index .
• A very important material property relevant to this curve is “ Toughness ” • This is defined as “ the amount of energy per unit volume the material can absorb before fracture ”
• On true stress-true strain curve toughness is equal to the area under the true stress- true strain curve.
Ductility • Ductility is another important material property which can be defined as “ the materials ability to deform plastically (permanently) before fracture ”. • Ductility can be estimated in two ways:
• Percent elongation (EL%) f − l l = o EL % x 100 % l o Percent reduction in area ( Δ A%) • o − A A Δ = f A % x 100 % A o
Hardness • Hardness is defined as “ the resistance of the material to permanent indentation or localized plastic deformation ” Hardness tests are probably the most frequently performed mechanical tests for the following reasons:
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