Material discontinuities and their Types

Material discontinuities and their Types

The aim of an inspection is to determine if the material being inspected is to be accepted or rejected. During the inspection, the inspector looks for the discontinuities in the material and identifies their nature and size. Then, those discontinuities are evaluated according to an acceptance criterion to determine whether the material is to be accepted or rejected.

A discontinuity is an imperfection or interruption in the normal physical characteristics or structure of a material (e.g. crack, porosity, and inhomogeneity etc.). On the other hand, a defect is a flaw or flaws which by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications. Defect normally results into material rejection.

A discontinuity is not necessarily a defect. Any imperfection which is found by the inspector is called a discontinuity until it can be identified and evaluated as to the effect it is going to have on the service of the part or to the requirements of the specification. A certain discontinuity can be considered to be a defect in some cases and not a defect in some other cases since the definition of the defect changes with the type of component, its construction, its materials and the specifications or codes being used.

Discontinuities are generally classified according to the stage of the manufacturing or use in which they initiate. Hence, discontinuities are classified into four categories namely (i) inherent discontinuities, (ii) primary processing discontinuities which include welding discontinuities, (iii) secondary processing discontinuities, and (iv) service discontinuities.

Inherent discontinuities

Inherent discontinuities (Fig 1) refers to the discontinuities which originate during the initial casting process of the liquid metal such as casting of ingots, continuous casting of semi finished products, and casting of parts of any given shape in the foundries. Some of the initial casting discontinuities are removed by chopping or grinding but some of them remain and further change their shape and nature during the subsequent processing of the material. The different types of inherent discontinuities are described below.

Fig 1 Types of inherent discontinuities

Cold shut – Cold shut occurs normally during the casting of parts because of imperfect fusion between two streams of liquid metal which converged together. It can be on the surface or subsurface. It can be attributed to sluggish liquid metal, surging or interruption in pouring, or any factor which prevents the fusion of two meeting streams.

Pipe – During solidification the liquid metal shrinks causing an inverted-cone shaped cavity in the top of the cast product (normally in ingots). It can be on the surface or subsurface. If this defective portion is not cut out completely before further processing such as rolling or forging, it shows up in the final product as an elongated subsurface discontinuity. Also, pipe can occur during extrusion when the oxidized surface of the billet flows inwards toward the centre of the extruded bar.

Shrinkage cavities – Shrinkage cavities are subsurface discontinuities which are found in the cast materials. These cavities are caused by the lack of enough liquid metal to fill the space created by shrinkage. These are similar to pipe in an ingot.

Micro-shrinkage cavities – Micro-shrinkage cavities are aggregates of subsurface discontinuities which are found in the cast materials. These cavities are normally found in the foundry castings close to the gate. These cavities occur if metal at the gate solidifies while some of the metal beneath is still liquid. Also, micro-shrinkage can be found deeper in the part when the liquid metal enters from the light section into heavy section where metal can solidify in the light section before the heavy section.

Hot tears – Hot tears occurs when low melting point materials segregate during the solidification and thus when they try to shrink during solidification cracks then tears develops since the surrounding material has already solidified. Also, hot tears occur at the joining of thin sections with larger sections because of the difference of the cooling rate and thus solidification.

Blowholes and porosity – Blowholes and porosity are small rounded cavities found at the surface or near surface of the cast products and these are caused by the entrapped gasses which are not able to escape during solidification. Blowholes are caused by gases released from the mould itself (external gases) while porosity is caused by gases entrapped in the liquid metal (internal gases). During subsequent processing of the material these gas pockets get flattened or elongated or fused shut.

Non-metallic inclusions – Non-metallic inclusions are normally oxides, sulphides or silicates which remained in the liquid during casting. The properties of these inclusions are different from the metal and normally they have irregular shapes and discontinuous nature and hence they serve as stress raisers which limit the ability of the material to withstand stresses.

Segregation – Segregation is localized differences in material composition and hence the mechanical properties caused by the concentration of some alloying elements in limited areas. Some of these compositional differences can be equalized during subsequent hot working processes but some remain after the subsequent processing.

Primary processing discontinuities

Primary processing discontinuities (Fig 2) refers to the discontinuities which originate during hot or cold forming processes such as rolling, forging, extrusion, drawing, and welding etc. Also, some of the inherent discontinuities in the material can propagate and become significant. The different types of primary processing discontinuities are described below.

Fig 2 Types of primary process discontinuity

Seams – Seams are elongated surface discontinuities which occur in materials during rolling or drawing operations. These result due to under-filled areas which are closed shut during rolling passes. The under-filled areas can result because of blowholes or cracks in the material. Also seams can result from the use of faulty, poorly lubricated or oversized dies.

Lamination -Laminations are thin flat subsurface separations which are parallel to the surface of the material. These can result from inherent discontinuities such as pipe, inclusions, and porosity etc. which are flattened during the rolling process.

Stringers – Stringers are elongated subsurface discontinuities which are found in bars. These run in the axial direction. These result from the flattening and lengthening of non-metallic inclusions during the rolling process.

Cupping – Cupping is a subsurface discontinuity which can occur in bars during extrusion or severe cold drawing. It is a series of cone shaped internal ruptures which happen because the interior of the material cannot flow as fast as the surface where it causes stress buildup and thus rupture.

Cooling cracks – Cooling cracks can occur on the surface of bars after rolling operations due to the stresses developed by uneven cooling. These run in the axial direction (similar to seams) but unlike seams, they do not have surface oxidation.

Rolling and forging laps – Laps are elongated surface discontinuities which occur during rolling or forging operations due to the presence of some excessive material (fin) that is folded over. They can result because of oversized passes and blanks or improper handling of the material.

Internal or external bursts – Internal bursts are found in bars and forgings formed at excessive temperatures due to the presence of inherent discontinuities which are pulled apart by the tensile forces developed during the forming operation. External bursts occur when the forming section is too severe or the sections are thin.

Slugs – Slugs are surface discontinuities found on the inner surface of seamless (extruded) pipes. These occur when some metallic pieces which are stuck on the mandrel, are torn and fused back on the inner surface of the pipe.

Gouging – Gouging is surface tearing found on the inner surface of seamless (extruded) pipes and it is caused by excessive friction between the mandrel and the inner surface of the pipe.

Hydrogen flakes – Hydrogen is available during the various process operations (from decomposition of water vapour, hydrocarbons, or atmosphere etc.) and it dissolves in material at temperatures above 200 deg C. Hydrogen flakes are thin subsurface discontinuities which develop during cooling of large size sections produced by rolling or forging because of the entrapment of hydrogen resulting from rapid cooling.

Welding discontinuities

Welding discontinuities (Fig 3) are also the primary processing discontinuities. Several types of discontinuities result from welding operations. Some of the discontinuities associated with fusion welding processes (arc welding, gas welding, etc.) are described below.

Fig 3 Types of welding discontinuities

Cold cracks – Cold cracks, also known as delayed cracks, are hydrogen induced surface or subsurface cracks which appear in the heat affected zone or in the weld metal during cooling or after a period of time (hours or even days). The sources of hydrogen which leads to this type of cracks can include moisture in the electrode shielding, the shielding gas or base metal surface, or contamination of the base metal with hydrocarbon (oil or grease).

Hot cracks – Hot cracks include several types of cracks which occur at high temperatures in the weld metal or heat affected zone. In general, hot cracks are normally associated with steels having high sulphur content. The common types of hot cracks include solidification cracks and liquidation cracks.

Solidification cracks occur near the solidification temperature of the weld metal. These are caused by the presence of low melting point constituents (such as iron sulphides) which segregate during solidification then the shrinkage of the solidified material causes cracks to open up. Centerline crack is a longitudinal crack along the centerline of the weld bead. It occurs because the low melting point  impurities move to the centre of the wield pool as the solidification progresses from the weld toe to the centre, then shrinkage stresses of the solidified material causes cracking along the centerline. The likelihood of centerline cracking increases when the travel speed is high or the depth-to-width ratio is high. Crater crack occurs in the crater formed at the termination of the weld pass. Crater cracks are mostly star shaped and they are caused by three dimensional shrinkage stresses. The likelihood of the crater cracks increases when welding is terminated suddenly.

Liquidation cracks are also known as hot tearing which occur in the heat affected zone. When the temperature in that region reaches to the melting temperature of low melting point constituents causing them to liquidate and segregate at grain boundaries. As the weld cools down, shrinkage stresses causes the formation of small micro-scale cracks which later can link up due to the applied stresses to form a continuous surface or subsurface crack.

Lamellar tearing – Lamellar tearing is a subsurface discontinuity which occurs in rolled material having high content of non-metallic inclusions. These inclusions have low strength and they are fattened during roiling, thus they can be torn underneath the welds because of shrinkage stresses in the through thickness direction.

Lack of fusion – Lack of fusion is the failure of the filler metal to fuse with the adjacent base metal (or weld metal from previous pass) because the surface of base metal has not reached to the melting temperature during welding. This typically occurs when welding large components which can dissipate heat quickly especially when it is at a relatively low temperature before welding. Lack of fusion is frequently seen at the beginning of the first pass and in such case it is normally called a cold start. Also, lack of fusion can occur when the surface of the previous pass is not properly cleaned from slag where slag reduces the heating of the under-laying surface.

Lack of penetration – Lack of penetration is insufficient (less than specified) penetration of the weld metal into the root of the joint. This is mostly caused by improper welding parameters such as low amperage, oversized electrode, improper angle, high travel speed, or inadequate surface pre-cleaning. Also, lack of penetration can happen when the root face is too large, the root opening is too narrow, or the bevel angle is too small.

Porosity – Porosity is small cavities or bores which are found on the surface of the weld or slightly below surface. Porosity occurs when some constituents of the liquid metal vapourize causing small gas pockets which get entrapped in the liquid metal as it solidifies. These small cavities or bores can have a variety of shapes but mostly they have a spherical shape. The distribution of cavities and bores in weld metal can be linear (linear porosity) or theses can be clustered together (cluster porosity). In general, porosity can result from the presence of dirt, rust or moisture on the surface of base or filler metal. Also, it can result from high sulphur content in the base metal or excessive arc length.

Inclusions – Inclusions refer to the presence of some material, which is not supposed to be present, in the weld metal. Inclusions can be slag inclusions, tungsten inclusions, or oxide inclusions. Slag inclusions mostly happens in shielded metal arc welding (SMAW) and it occurs when the slag cannot float to the surface of the liquid metal and get entrapped in the weld metal during solidification. This can happen when the solidification rate is high, the weld pool viscosity is high, an oversized electrode is used, or slag on the previous pass has not been properly removed. Tungsten inclusions can be found in weld metal deposited by gas tungsten arc welding (GTAW) as a result of allowing the tungsten electrode to come in contact with the liquid metal. Oxide inclusions results from the presence of high melting point oxides on the base metal which mixes with the liquid material during welding.

Undercut – Undercut is a reduction in the base metal thickness at the weld toe. This is caused by an oversized liquid weld pool which can result from excessive amperage or oversized electrode.

Overlap – Overlap is the protrusion of the weld metal over the weld toe (due to lack of fusion). This can be caused by insufficient amperage or travel speed.

Secondary processing discontinuities

Secondary processing discontinuities refer to the discontinuities which originate during grinding, machining, heat treatment, plating and other finishing operations. The different types of secondary processing discontinuities are described below.

Grinding cracks – Grinding cracks develop at locations where there is a localized heating of the base metal and they are normally shallow and at right angle to the grinding direction. Such cracks can be caused by the use of glazed wheels, inadequate coolant, excessive feed, or grinding depth.

Pickling cracks – Pickling is chemical surface cleaning operation (using acids) used to remove unwanted scale. Picking cracks are hydrogen induced cracks caused by the diffusion of the hydrogen generated at the surface into the base metal. Such cracks mostly occur in materials having high residual stresses such as hardened or cold worked metals.

Heat treatment (quenching) cracks – Heat treatment cracks mostly occur during quenching especially when harsh media is used for quenching (such as cold water, oil quenching is less harsh). During quenching the material at the surface cools immediately upon contacting the liquid while the material inside take relatively longer time. This difference in cooling rate causes residual stresses in the component and can also result in cracks at the surface if the residual tensile stress is higher than the strength of the material. In steels, austenite gets converted into martensite upon quenching. This transformation results in volume increase and thus causes tensile stresses at the surface layer since the material at the surface transformed and solidified before material at the core.

Machining tears – Machining tears result from the use of machining tools having dull or chipped cutting edges. Such discontinuities serve as stress raisers and can lead to premature failure of a component especially when it is subjected to fatigue loading.

Plating cracks – Plating cracks are surface discontinuities which can develop due to the penetration of hydrogen or hot plating material into the base metal. Also, some plating materials (such as chromium, copper, or nickel) produce residual tensile stress which can reduce the fatigue strength of a component.

Service discontinuities

Service discontinuities (Fig 4) occur when the material is subjected to severe conditions after it is placed into service. These discontinuities originate or develop while the material is in service. The service conditions (loading, mechanical and chemical environment, maintenance) of the material affect its expected life. Although most of service discontinuities can look somehow similar but they are caused by different failure mechanisms. The different types of service discontinuities are described below.

Fig 4 Types of service discontinuities

Fatigue cracks – When the material is subjected to fatigue stress (cyclically applied stress), fatigue cracks can develop and grow and this eventually leads to failure. Fatigue cracks can happen even if the magnitude of the stress is smaller than the ultimate strength of the material. Fatigue cracks normally originate at the surface but in some cases can also initiate below the surface. Fatigue cracks initiate at location with high stresses such as discontinuities (hole, notch, scratch, sharp corner, porosity, crack, or inclusions etc.) and can also initiate at surfaces having rough surface finish or due to the presence of tensile residual stresses.

According to ‘linear elastic fracture mechanics’ (LEFM), fatigue failure develops in three stages. During the stage 1, development of one or more micro cracks takes place due to the cyclic local plastic deformation at a location having high stress concentration. During stage 2, the cracks progress from micro cracks to larger cracks (macro cracks) and keep growing making a smooth plateau-like fracture surfaces which normally have beach marks that result from variation in cyclic loading. The geometry and orientation of the beach marks can help in determining the location where the crack has originated and the progress of crack growth. The direction of the crack during this stage is perpendicular to the direction of the maximum principal stress. The stage 3 occurs during the final stress cycle where the remaining material cannot support the load, thus resulting in a sudden fracture. The presence of the crack can be detected during the crack growth stage (stage 2) before the component suddenly fails.

Creep cracks – When a metal is at a temperature greater than 0.4 times to 0.5 times of its absolute melting temperature and is subjected to a high enough value of stress (lower than the yield strength at room temperature but it is actually higher than the yield strength at the higher temperature), it keep deforming continuously until it finally fractures. Such type of deformation is called creep and it is caused by the continuous initiation and healing of slipping dislocation inside the grains of the material.

According to the rate of progress of the deformation, three stages of creep deformation can be distinguished. During the initial stage (or primary creep), the strain rate is relatively high but slows with the increasing time due to work hardening. In the second stage (or steady-state creep), the strain rate reaches a minimum and becomes steady due to the balance between work hardening and annealing (thermal softening). The characterized ‘creep strain rate’ typically refers to the rate in this secondary stage. During the third stage (or tertiary creep), the strain rate exponentially increases with stress because of necking phenomena and finally the material ruptures.

Creep cracks normally develop at the end of the second stage (the beginning of third stage) and they eventually lead to failure. However, when a component reaches to the third stage, its useful life is over and thus creep is to be detected (by monitoring the deformation) during the second stage which takes the longest time period of the three stages. For steels, adding some alloying elements such as molybdenum and tungsten can enhance creep resistance. Also, heat treatments which produce coarse grains (such as annealing) can also increase life under creep conditions.

Corrosion – Corrosion is attack and loss of metal due to an electrochemical process which involves an anodic reaction and at least one cathodic reaction. Iron ore is an oxide of iron in chemical balance with the environment. When this iron ore is converted to iron, the chemical balance is changed and the iron becomes active (i.e. it corrodes on contact with the natural environment and tries to revert back to its natural state). The natural environment normally contains moisture, which provides an electrolyte for a corrosion cell to form.

The two major types of corrosion are pitting and inter-granular. Pitting is a localized corrosion which extends into the metal surface. Pitting corrosion appears as pin holes on the surface in varying degrees.

The susceptibility to inter-granular corrosion (particularly in aluminum and some types of corrosion resistant steels) is caused by improper heat treating, or in-service use. The material then corrodes inter-granular from the surface under certain conditions. This condition can appear as fine cracks.

Stress corrosion cracks – Stress corrosion cracks are small sharp and normally branched cracks which result from the combined effect of a ‘static’ tensile stress and a corrosive environment. The stress can either be resulting from an applied load or a residual stress. Stress corrosion cracks can lead to a sudden failure of ductile materials without any previous plastic deformation. The cracks normally initiate at the surface due to the presence of pre-existing discontinuity or due to corrosive attack on the surface. Once the cracks initiate at the surface, corrosive material enters the cracks and attacks the material inside forming corrosion products. The formation of the corrosion products (which have a larger volume than original metal) inside the tight cracks causes a wedging action which increase the stress at the crack tip and causes the crack to grow. The corrosive environment varies from material to material. As an example salt water is corrosive to aluminum and stainless steel, ammonia is corrosive to copper alloys, and sodium hydroxide is corrosive to mild steel. The resistance to corrosion can be improved by plating the surface of a component by appropriate material which does not react with the environment.

Hydrogen cracks – Hydrogen cracking, also known as hydrogen embrittlement results from the presence of hydrogen medium and normally occurs in conjunction with the presence of applied tensile stress or residual stress. Hydrogen can be already present in the metal due to previous processes such as electroplating, pickling, and welding in moist atmosphere or the melting process itself. Also, hydrogen can come from the presence of hydrogen sulphides, water, methane or ammonia in the work environment of a material. Hydrogen can diffuse in the metal and initiate very small cracks at subsurface cites (normally at the grain boundaries) subjected to high values of stress. The presence of such cracks at several locations causes ductile materials to show brittle fracture behaviour.

Wear – Wear is the loss of material from the surface due to a mechanical action. Wear can normally be recognized by visual examination of the surfaces involved. Specific terminologies used to describe various types of wear are (i) abrasive wear, (ii) adhesive wear, (iii) fretting wear, (iv) gouging wear, and (v) erosive wear.

Abrasive wear occurs when two surfaces move or slide against each other producing an abrasive or mechanical cutting action. Heat is normally generated during this abrasive action. Adhesive wear occurs when two surfaces move against each other and generate sufficient heat to cause localized intermittent welding or bonding and continued sliding fractures one side of the bond. Scuffing, galling, scoring, and seizing are all the results of adhesive wear. Fretting wear occurs when two surfaces constantly impact each other without significant sliding movement. It is frequently seen on fasteners such as cotter pins, bolts, rivets, and sometimes in bearings which are static but subject to vibration. Fretting wear can appear as numerous small indentations. Gouging wear occurs when large fragments are removed from the surface by high energy impact from large pieces of material. The crushing of hard abrasive products such as rock and ores produces rapid surface damage. Erosive wear is due to the erosion which occurs when particles in a fluid rub against a surface at high velocities and remove material from that surface. Erosive wear occurs in nozzles, pumps, impellers tubes, pipes, and valves etc.

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