materials, processes and defects

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MATERIALS, PROCESSES AND DEFECTS K. SURESH SENANAYAKE

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Material Process and defects

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MATERIALS, PROCESSES AND DEFECTS

MATERIALS, PROCESSES AND DEFECTSK. SURESH SENANAYAKEBasic PropertiesThe qualities of materials that are of practical interest to manufacturing are determined from various properties.Chemical PropertiesPhysical PropertiesMechanical PropertiesProcessing Properties

Chemical PropertiesThe chemical properties (reaction with other materials) are of interest for all material mainly because of the almost universal need for resistance to corrosion.Physical PropertiesPhysical properties for each material are constants associated with the atomic structure.

These properties include density, crystalline type, atomic spacing, specific heat, cohesive strength, and melting point.

Mechanical PropertiesThe mechanical properties are of prime importance in design considerations for determining sizes and shapes necessary for carrying loads.Processing PropertiesDescribe the ability of the material to be processed in definite ways. Hardnessstrength etc.Castability,WeldabilityMachinability, andBendingSignificance of properties to designA designer is necessarily interested in properties because he must know material strengths before he can calculate sizes and shapes required to carry loads, chemical properties to meet corrosive conditions, and other properties to satisfy other functional requirements.Loading Systems and Material FailurePhysical loading of materials is a result of applying force under one or more simple, basic loading systems.Normal stressesShear stressesBendingEffects of stressesTestingDirect testingThe only test that supplies absolute information about a work piece or a material.Indirect testingThe use of such a correction, such that accurate knowledge of the relationship between the two factors must exist.Destructive testingThe methods of testing that are applied to material or component in order to determine the characteristic properties by subjecting it to either partial damage or total failure.Non Destructive Testing (NDT)The test used for characterising a material or part properties/conditions that are not subjecting it to severe damage or destruction.TestingStandardised testsMany properties are defined only by the test procedure that has been developed for their measurement.Tensile testingIn the tensile test the sample is elongated in a uni-axial tension at a constant rate and the load necessary to produce a given elongation is measured as a dependant variable.Stress Strain diagram

AB: Elastic deformation B: Elastic Limit C: Yield point E: Ultimate strength F: Breaking (rupture) strength

TestingCompression testingCompression test is usually used to test brittle materials, which are often weak when subjected to tension.These materials can nevertheless be quite strong in compression.Brittle materials are chiefly used in compression, where their strength is much higher.Hardness testingBrinell testRockwell testVickers testFatigue testingA metal may fail under sufficient cycles of repeated stress, even though the maximum stress applied is considerably less than the strength of the material determined by the static test.Fatigue strengthThe stress that can be applied for some arbitrary number of cycles without failure when the endurance limit cannot be determined, or it is impractical to carry on a test long enough for this determination.

TestingCreep testingContinuous deformation of a material under constant load, producing unit stresses below those of the elastic limit.As operating temperature increase, the deformation by slow plastic flow becomes very important in the design and use of material.

Discontinuities, Defects and IndicationsWhat are the differences between discontinuities, defects and indications?Discontinuity:Discontinuity is defined as any imperfection or interruption in the normal physical structure of an article.A discontinuity in metal may be a hole, crack, inclusion or anything else that breaks the continuity of the metal.Discontinuities may be found on the surface of the metal or within the metal itself.The discontinuity may or may not affect the usefulness of the product.Defect:Defect is defined as a discontinuity, which is judged as unacceptable to the codes, standards or specification used.Indication:Indication used in non-destructive testing is to denote the presence of a discontinuity.

Metal DiscontinuitiesShould identify the types of metal manufacturing and service discontinuities (i.e. to know what causes the defects)Inherent Processing Defects:The refining stage where metals are extracted from oresPrimary Processing Defects:Metal ingots are worked into usable forms such as billets or blooms by wrought processing or casting.Secondary Processing Defects:Final stages of parts manufacturingService Defects:Occurs during the use of the partInherent Processing DefectsThese discontinuities are related to the melting and solidification of the original ingot before it is formed into slabs, blooms and billets.Non metallic inclusions (Slag)are caused by impurities accidentally included in the molten metal.Porosity (Blown Holes) is caused by entrapped gas in the molten metal.Pipeis caused by molten metal shrinking when it cools and solidifiesSegregation non uniform distribution of alloying elements or impurities

Processing DefectsCastings are made by pouring liquid metal into a mould.These moulds are formed close to the shape of finish part.When the metal solidifies it is remove from the mould.Casting moulds are usually made from sand, clay and water.Casting and Casting DefectsThe basic steps of casting process are;Preparation of pattern, diePreparation of mould cavityHeating and melting of metalPouring of molten metal into the mould cavityRemoval of the part, further cleaning and shapingCasting and Casting DefectsCold laps / shutsPorosityBlow holes / gas holesAir locksHot tearsShrinkage cavitiesCracks / stress cracksSand inclusionsSlag and other inclusionsCasting defectsCasting defectsCold shutsCold shuts are produced when there is an interruption in casting process.i.e. when molten metal is poured over solidified metal.PorosityThe entrapped gas causes porosity.Blow holesBlow holes are caused by steam or gas produced from the mould.

Casting defectsHot tearsHot tears are usually occur at the junction of the light and heavy sections due to unequal cooling rate.Shrinkage cavitiesShrinkage cavities are caused by shrinkage just as pipe is formed in an ingot.This is the property which metal has of occupying more space when it is liquid than when it is solid.Air locksAir locks are cavities formed by air trapped in the mould during pouring of the casting.

Casting defectsCracks / stress cracksDiscontinuities due to the fracture of metal during or after solidification.These are approximately straight and form when metal has become completely solid.Sand inclusionsIrregular holes on or near the surface of casting.Usually have rough sides and sand adhering to them.SlagAppear as flaws or irregular surfaces where foreign body has been enveloped with the cast metal.

Forging is the working of metal into a desired shape by hammering or pressing the ,metal while it is hot and in soft condition.There are two primary types of discontinuities formed as a result of discontinuities formed as a result of forging.Forging and Forging DefectsForging lapsForging burstForging defects

Forging defectsForging lapsForging lap is a discontinuity caused by folding of metal in a thin plate on the surface of the forging.Forging burstsForging burst is a rupture caused by forging at improper temperature.

After primary ingot casting, the crapped ingot is then rolled into a slab or billet or bloom.Inherent discontinuities such as pipe, inclusion and porosity in the ingot will now become lamination, stringers and seam (crack).Rolling and Rolling DefectsLapsSeams Lamination Rolling defects

Welding and Welding defectsA welding process is a materials joining process which producers coalescence of materials by heating them to suitable temperature, with or without the application of pressure or by the application of pressure alone and with or without the use of filler metal.Welding discontinuities can be classified into two categories dimensional and structural discontinuities.Dimensional discontinuities are discrepancies resulting from the completed weld not being within specified dimensions of size and/or shape.For example excessive reinforcement, underfill, undercut, overlap, misalignment, incomplete penetration, etc.

Structural discontinuities are discrepancies that interrupt the uniform composition of a weld joint such that its mechanical properties are reduced.Common discontinuities found in the weld are porosity, slag inclusion, crack, lack of fusion, incomplete penetration, excessive penetration, etc.

Welding and Welding defects

Grinding and Grinding DefectsGrinding is a process used to produce smooth surface or remove small amount of metal.Typical discontinuities found after this process are grinding cracks.Grinding cracks are usually oriented perpendicularly to the direction of rotating wheel (grinder).

Heat Treating DefectsHeat-treating is basically the process of hardening or softening metals by controlled heating and cooling.Typical heat treating discontinuities are heat treat cracks, which have no specific direction.Drawing DefectsIt is a process where a cross-section of solid rod, wire, or tubing is reduced or changed in shape by pulling it through a die.SeamsSimilar to seams produced during rollingCentre burstSimilar to centre burst produced during extrusionExtrusion DefectsExtrusion is a process where a billet is forced through a die.Typical products of extrusionSliding doors, tubing having various cross-sections, structural and architectural shapes and door and window frames.CrackingBlisters

Service induced defectsService induced defects are defects found in the materials which are subjected to various service conditions such as corrosion, fatigue cracks, stress corrosion cracking, embrittlement (loss of ductility) material losses, etc.

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