metal Laboratory Techniques

Guidelines for Shop Inspection Laboratory Techniques

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_________________________________________________________________________________________________________________ Support Documents

Guidelines for Shop Inspection – Laboratory Techniques –

LABORATORY TECHNIQUES Chemical Analysis................................................................................................. 1

Mechanical Testing ............................................................................................... 3

Corrosion and Weathering .................................................................................... 7

Metallography ..................................................................................................... 14

NDT..................................................................................................................... 19

Chemical Analysis

(XRF) X-Ray Florescence

XRF is an emission spectrographic technique which has found wide applications in elemental identification and quantitative determination without regard to form or oxidation state in various solid and liquid materials and compositions. XRF can also be used in the determination of thickness of thin films of metal on various substrates. Sampling depth may range form a few micrometers to a millimetre or more, depending on x-ray energy used and matrix composition of the sample.

(OES) Optical Emission Spectroscopy

These instruments enable the rapid quantitative determination of a wide range of alloys including; carbon/low alloy steels, stainless steels, cast irons, aluminum alloys, nickel alloys, and copper alloys. Relatively simple sample preparation allows rapid turnaround of results using this technique.

(GDS) Glow Discharge

Similar to OES, GDS is used extensively for metal analysis. The straight line calibration similar to ICP makes this technique particularly attractive for the analysis of stainless steel, nickel, aluminum and copper alloys.

"Qualitative depth profiling", is an application providing advanced capabilities in chemical depth profiling and is particularly used for surface analysis. The analysis is conducted at incremental depths starting from the surface of the material. A full spectrum of analysis is then graphically presented which represents exactly the concentrations of all the pertinent


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elements with respect to the depth profile. This technique has many applications and is extremely useful for the analysis of metallurgical surface treatments and coatings. It is also a valuable analytical tool for corrosion investigation work . Typical applications include evaluation of metallic coating problems . Cause of dull or discoloured regions. Phosphorous concentration in electroless nickel. Analysis of electroplated hot-dipped galvanized, galvalume, galfan and galvaneal. Evaluation of heat treated components (surface / case treatment). Profiling of carburized, nitrided and carbonitrided surfaces. Routine coating evaluation, including composition, coating thickness and coating weight, adhesion problems

(EDS) Energy Dispersive Spectroscopy - Microprobe

Electron probe microanalysis combines structural and compositional analysis in one operation. Microanalysis provides information concerning specimen composition on a microscopic scale. This creates the possibility of local analysis of a small region on the specimen surface. Elemental compositions can be mapped for homogeneity and heterogeneity at the micrometer scale. For example, EDS can provide analysis of individual inclusions in steel and other alloys, and can be used as a semi-quantitative method in nuclear applications.

FT-IR

FT-IR is used for the identification of organic or inorganic coatings or contaminations on metal surfaces.

Combustion; Carbon & Sulfur

Combustion carbon and sulfur determination is accepted as the most accurate method for determining carbon and sulfur in metal, ore and powder samples. These samples may be in the form of solid material, drillings or powders. This technique is used mainly to complement ICP or OES analysis for the full chemical analysis of metallic samples.

Gas Analysis; Oxygen, Nitrogen, Hydrogen

Inert gas fusion techniques are offered for quantitative content of these gases in metals. Processed materials are subjected to gas absorption as the material is initially cooled and worked. Also during drawing, rolling, heat treatment or annealed processes. When gas absorption is controlled during these processes, it will minimize their adverse effects on material strength.

(GC/MS) Gas Chromatography / Mass Spectroscopy

This technique is commonly used in the analysis of petroleum oil, coal gasification and liquefaction products, drugs, metalobics, food products, perfume, cosmetics, plasticizers, pesticides, pollutants in air, waste water and solid waste, products and by-products of manufacturing processes, and solvents used in manufacturing processes.


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Mechanical Testing

Introduction

Major branches of engineering depend on the results of mechanical tests for design and/or quality control purposes. Test specimens are prepared for metallic and non-metallic materials in the evaluation of tensile, compression, impact, fracture toughness, fatigue and bend properties. Routine testing of fasteners, chain materials, weld coupons, wire rope, castings, sheet, plate, forgings and other components is done in an expedient manner providing an efficient, quality conscious service. Many fabricators, heat treaters and foundries rely on mechanical testing services to facilitate early production release or production start. Some materials require more in-depth testing, such as dynamic fracture test, or cryogenic and elevated temperature mechanical properties.

Bend

Bend testing is a procedure to determine the relative ductility of metal that is to be formed (usually sheet, strip, plate or wire) or to determine soundness and toughness of metal (after welding, etc.) The specimen is usually bent over a specified diameter mandrel. The four general types of bends are; free bend, guided bend (ASTM E190), semi-guided bend (ASTM E290), and wrap around bend.

Compression

Compression testing is a method for assessing the ability of a material to withstand compressive loads. This test is commonly used as a simple measure of workability of metal, particularly in forging and similar bulk deformation processes. Engine mounts, bolster springs, cast products, and similar components are tested to determine load versus displacement.

Plastic Strain Ratio, ASTM E517

This test method determines the plastic strain ratio, r, of sheet metal intended for deep-drawing applications. “r” is a parameter that indicates the ability of a sheet metal to resist thinning or thickening when subjected to either tensile or compressive forces in the plane of the sheet. It is a measure of sheet steel drawability.

Ring Flattening Test, ASTM A513

This Procedure tests the ability of a section of tube approximately 4” in length to flare (with a tool having a 60° included angle) until the tube at the mouth of the flare has been expanded 15% of the inside diameter, without cracking or showing flaws.


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Tensile Testing, ASTM E8, ASTM A370, ASTM B557

A tensile test measures the resistance of a material to a static or slowly applied force. A machined specimen is placed in the testing machine and load is applied. A strain gage or extensometer is used to measure elongation. The stress obtained at the highest applied force is the Tensile Strength. The Yield Strength is the stress at which a prescribed amount of plastic deformation (commonly 0.2%) is produced. Elongation describes the extent to which the specimen stretched before fracture. Information concerning the strength, stiffness, and ductility of a material can be obtained from a tensile test. Variations of the tensile testing include; Room Temperature, Low Temperature, Elevated Temperature (ASTM E21), Shear, Temperature and Humidity, Combined Tension and Compression, Through Thickness, True Strain, Notched Tensile, and r (ASTM E646) & n (ASTM E517) values.

Nick Break

The principle of this test is to break the sample through the weld metal in order to examine the fractured surface. Applying a three point bend load induces the fracture. The fracture surface is then examined and the type and location of any weld defect are reported.

Fillet Fracture

Fillet weld fracture testing is performed in accordance with various welding specifications. The purpose of this test is to check the weld for soundness (complete fusion, and no major weld anomalies.) The sample is folded over the weld side of the fillet until the sample folds flat upon itself or failure occurs. The fracture is inspected for complete joint fusion and/or weld anomalies.

Hydrogen Embrittlement (ASTM F519)

Industrial plating processes use a number of operations that can produce hydrogen damage in the components being plated. Hydrogen damage can result in loss of strength and/or ductility in the plated component. Hydrogen Embrittlement testing is used to verify that the plating process is being controlled in such a way that hydrogen evolution is kept to a minimum. This test comprises of manufacturing a notched specimen from a susceptible steel and exposing the specimen to the same processes as used in the plating operation. The specimen is then statically loaded to the prescribed tension for an extended period of time. After exposure, the specimen is examined for cracking at the base of the notch. The plating process is deemed in control if no cracks are observed.

Impact Testing

The impact test is a method for evaluating the toughness and notch sensitivity of engineering materials. It is usually used to test the toughness of metals, but similar tests are used for polymers, ceramics and composites. Metal industry sectors include Aerospace, Power Generation, Automotive, and Nuclear.


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The notched test specimen is broken by the impact of a heavy pendulum or hammer, falling at a pre-deterined velocity through a fixed distance. The test measures the energy absorbed by the fractured specimen.

Charpy, ASTM E23 - A test specimen is machined to a 10mm x 10mm (full size) cross-section, with either a “V” or “U” notch. Sub-size specimens are used where the material thickness is restricted. Specimens can be tested down to cryogenic temperatures. Keyhole, ASTM E23 - The steel casting industry uses this type of specimen

more frequently. The notch is machined to look like a keyhole. It is tested in the same manner as the “V” and “U” notch.

Hardness

Brinell, ASTM E10 - This is a simple indentation test for determining the hardness of a wide variety of materials. The test consists of applying a prescribed load, usually between 500kg and 3000kg for a specified time (10-30 seconds) using a 5 or 10mm diameter tungsten carbide ball on the flat surface of a metal sample.

Microhardness, ASTM 3384 - A microindentation is made on the surface of a metal sample. The hardness number is based upon the measurements made of the indent formed in the surface of the test specimen.

Knoop, ASTM E384 - The Knoop indenter has a polished rhombohedral shape with an included longitudinal angle of 172° 30’ and an included transverse angle of 130° 0’. The narrowness of the indenter makes it ideal for testing specimens with steep hardness gradients and coatings. Knoop is a better choice for hardness testing of hard brittle materials.

Vickers - This testing is similar to Brinell in that a defined indenter is pressed into a material. Once the indenting force is removed, the resulting indentation diagonals are measured. Microindentation Vickers is per ASTM E384 and Macroindentation Vickers is per ASTM E92.

Rockwell, ASTM E18 - This test differs from the Brinell test in the shape of the indenter and in the manner that the number is determined. The Rockwell number represents the difference in depth penetration between two loads. There are two types of Rockwell; Rockwell and Superficial Rockwell. The difference between the two are the minor and major loads applied to the specimen. The indenter used may be a diamond cone or a hardened ball depending principally on the characteristics of the material being tested.

Portable Hardness, ASTM E110 - This testing is normally used for on-site applications or on very large samples. The Krautkramer Branson Micro-Dur II unit performs the hardness testing by applying a 5 kg. Vickers load indenter and electronically converts the values in the preferred scale.


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Drop Weight Test / Dynamic Tear Test ASTM E208, E436, E604



Jominy Hardenability, ASTM A255

The Jominy test involves heating a test specimen of steel 25mm diameter and 100mm long to an austenitising temperature and quenching from one end with a controlled and standardized jet of water. After quenching, the hardness is measured at intervals taken form the quenched end. The hardness gradient along the test surface provides an indication of the material’s hardenability

Creep

Many design applications involve materials and components that are subjected to high temperature environments for extended periods of time. Testing of materials in its simplest forms involves subjecting a tensile specimen to constant load in a high-temperature environment and measuring its extension (creep) over time.

Stress Rupture, ASTM E139

This method evaluates elevated temperature durability of the test material in which a tensile specimen is stressed under load until it breaks. The time for rupture is measured. Elongation is measures after failure. Stress rupture curves allows estimation of the expected lifetime of a component for a particular combination of stress and temperature. Industry sectors include Aerospace, Chemical, Power Generation and Nuclear.

Determining the nil ductility temperature of steel using ASTM E208 involves impacting a series of “cracked” samples, to deflection levels that ensure yield conditions at the crack location, at different temperatures so as to determine the highest temperature (usually sub-zero) at which the sample fractures. Below this temperature the sample steel consistently fractures but does not fracture above this temperature.

Drop Weight Tear Tests (DWTT) by ASTM E436 have a very similar objective to nil ductility tests, however, the larger 12 by 3 x t inch specimen provides information about the relative amounts of ductile and cleavage/brittle features present on the fracture surface.

Dynamic Tear (DT) Testing to ASTM E604 measures the energy absorbed during the fracture process of a “pre-cracked” specimen. The test objective is very similar to the tests outlined above, however, the measurement of energy eliminates some of the subjectivity of some of the other tests and allows acceptance levels to be specified by the end users for steels used in critical applications such as pressure vessels.


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Corrosion and Weathering

Salt Spray (Neutral / Fog), ASTM B117

This is the most commonly used salt spray for testing of inorganic and organic coatings, especially where such tests are used for material or product specifications. Government agencies and the automotive industries have developed standard requirements for coating testing. Salt Spray testing is a tool for evaluation the uniformity of thickness and degree of porosity of metallic and nonmetallic protective coatings. A number of samples can be tested at once depending upon their size.

Copper Accelerated Salt Spray, ASTM B368 (CASS)

This test is used for the rapid testing of decorative copper-nickel-chromium or nickelchromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum.

Humidity Test, ASTM D1735

Water and high humidity can cause the degradation of coatings, so the knowledge of how a coating resists water is helpful in predicting its service life. This procedure covers the basic principles for testing water resistance of coatings in an apparatus similar to that used for salt spray testing. Reagent water is used in a test chamber that is kept at a typical temperature of 100 +/-2°F.

Fluid Resistance

Fluid flow rate, or fluid velocity is a complex variable of corrosion. Its influence is dependent on the alloy, fluid constituents, fluid physical properties, geometry, and corrosion mechanism. The presence of fluid flow can sometimes be beneficial in preventing or decreasing localized attack. It can also cause a type of erosion of a surface through the mechanical force of the fluid itself (impingement.)

Cyclic Corrosion Testing

The purpose of cycle corrosion testing is to provide an accelerated lab test which will simulate the results of actual outdoor activities by the changing of environments inside the exposure zone which contains the test samples. Testing can be done to various specific Automotive Specifications. The cycles typically include salt spray, humidity and drying periods.


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Copper Accelerated Salt Spray, ASTM B368 (CASS) This test is used for the rapid testing of decorative copper-nickel-chromium or nickelchromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum.

Intergranular Attack

Corrosion occurs at grain boundaries because grain boundary segregation or precipitation produces local galvanic cells. In austenitic stainless steels and nickel alloys, chromium carbides can precipitate at grain boundaries. The formation of the carbides removes the chromium from the austenite adjacent to the boundaries. The low-chromium austenite at the surface of grains is anodic to carbides and corrodes.

Oxalic Acid Test, ASTM A 262, Practice A

The oxalic acid etch test is a rapid method of screening those specimens of certain stainless steel grades which are essentially free of susceptibility to intergranular attack associated with chromium carbide participates. The test is used for acceptance but not rejection of material.

Ferric Sulfate - Sulfuric Acid, ASTM A262 - Practice B ASTM G28 Method A

This test is based on weight loss determinations and provides a quantitative measure of the relative performance of the material evaluated. The procedure includes subjecting a specimen to a 24 to 120 hour boil in ferric sulfate - 50% sulfuric acid. This procedure measures the susceptibility of stainless steels and nickel alloys to intergranular attack associated with the precipitation of chromium carbides at grain boundaries.

Nitric Acid, ASTM A262, Practice C, (Huey Test)

This procedure includes a boiling nitric acid test to evaluate the heat treatment or “sensitization” of material. It may also be used to check the effectiveness of stabilizing elements and of reductions in carbon content in reducing susceptibility to intergranular attack in chromium-nickel stainless steels.

End Grain Pitting

Some wrought austenitic stainless steel alloys, especially from bar stock, may contain a random pattern of pits. If under microscopic examination the pits appear sharp and deep (black in appearance), it is possible the material may be susceptible to end grain attack in nitric acid and tested to Practice C of ASTM A 262 when specified.


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Copper - Copper Sulfate - 16% sulfuric acid, ASTM A262 - Practice E (Straus Test)

This procedure is conducted to determine the susceptibility of austenitic stainless steel to intergranular attack associated with the precipitation of chromium-rich carbides. Once the specimen has been subjected to the solution boil, it is bent through 180° and over a diameter equal to the thickness of the specimen being bent. This test is based on a visual examination of the bent specimen.

Copper - Copper Sulfate - 50% sulfuric acid, ASTM A262 - Practice F

This test is based on weight loss determination which provides a quantitative measure of the relative performance of the material evaluated. It measures the susceptibility of “as received” stainless steels to intergranular attack.

ASTM A763 Practice W,X,Y, and Z are similar tests for ferritic stainless steels.

Sour Service Corrosion Testing

A variety of corrosion problems can be encountered in industries such as oil and gas production, oil and gas transmission, energy conversion systems, and nuclear power systems. Such problems include weight loss corrosion, pitting corrosion, corrosion fatigue, stresscorrosion cracking, sulfide stress cracking, and hydrogen-induced cracking.

Hydrogen-Induced Cracking (HIC) Test, NACE TMO284

This test method evaluates the resistance of pipeline and pressure vessel plate steels to Hydrogen Inducted Cracking caused by hydrogen absorption from aqueous sulfide corrosion. An unstressed test specimen is exposed to a solution at ambient temperature and pressure and after a specified time, the test specimen is removed and evaluated.

Sulfide Stress Corrosion Cracking (SSCC), ASTM G35

The polythionic acid (sulfurous acid and hydrogen sulfide) environment provides a way of evaluating the resistance of stainless steels and related alloys to intergranular stress corrosion cracking. This practice can be applied to wrought products, castings, weld metal of stainless steels or other materials to be used in environments containing sulfur or sulfides

Stress Corrosion Cracking (SCC) Test

The objective of this test is to predict the service behavior or to screen alloys for service in a specific severe environment for an accelerated time. Stress corrosion specimens can be smooth, or precracked or notched. Smooth SSC specimens allow for the evaluation of the total SCC life, which includes crack nucleation and propagation. The use of precracked or


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notched specimens is based upon the engineering concept that all structures contain cracklike flaws. Precracking can contribute to the susceptibility of SCC of alloys such as titanium which may not be evident from smooth specimens.

C-Ring specimens (ASTM G38 C-Rings) - these are commonly used to determine the susceptibility to SSC of alloys in different product forms such as tubing, rods, and bars in the short transverse direction.

Tensile specimens, per ASTM G49 - This is one of the most versatile methods of SCC testing because of the flexibility permitted in the type and size of the test specimen, the stressing procedures, and the range of stress level. It allows the simultaneous exposure of unstressed specimens with stressed specimens and subsequent tension testing to distinguish between the effects of true SCC and mechanical overload.

Bend Specimens - Bent Beam specimens are used to test sheets, plate and flat extruded material or wires and extrusions with circular cross sections. Three-point bend specimens are used because of the ease of load application and the ability to use the same loading rigs for different stresses. Four-point bends provide a uniform tensile stress over a relatively large area of the specimen. (ASTM G30 U-Bends) U-Bend specimens are prepared by bending a strip 180° around a mandrel with a predetermined radius. This method is used to qualitatively evaluate the susceptibility of alloy and heat treatment to SCC.

Typical tests performed include

ASTM G36 (Boiling Magnesium Chloride) ASTM G123 (Boiling Acidified Sodium Chloride) ASTM G44 (Alternate Immersion)

ASTM C692 (Influence of Thermal Insulations on external SCC tendency of Austenitic Stainless Steel)

Pitting Corrosion Test, ASTM G46

This Procedure is used to assist in the selection of test methods that can be used in the identification and examination of pits and in the evaluation of pitting corrosion to determine the extent of its effect. The importance of this evaluation is to be able to determine the extent of pitting, either in a service application where it is necessary to predict the remaining life in a metal structure, or in laboratory test programs that are used to select the most pitting-resistant materials for service. ASTM G48 Method A and ASTM A923 Method C are typical pitting corrosion tests performed.

Crevice Corrosion Tests, ASTM G78, ASTM G48 Method B

This is a form of localized corrosion that primarily affects passive-type alloys Stainless steels, particularly with little or no molybdenum, are especially prone to crevice corrosion. Marine


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applications and process industries such as pulp and paper have requested development of more resistant alloys. Crevice corrosion test selection is primarily based on two accepted phases; initiation and propagation. Test selections include ASTM G78, Immersion Tests, Spool Specimen Test Racks, ASTM G48, Materials Technology Institute Tests, Multiple-Crevice Assembly testing and Electrochemical tests.

ASTM G78 provides guidance to conducting crevice corrosion tests for stainless steels and related nickel-based alloys in seawater and other chloride-containing environments. Another procedure used is ASTM G48 Method B, Ferric Chloride Test. This involves exposing a specimen to a highly oxidizing acid chloride environment. The crevices are created at sites of contact with TFEfluorcarbon blocks.

Passivation Testing, ASTM A380, F86, A967

The practice of passivation is used on metallic surgical implants and stainless steel parts, equipment or systems. Passivation is the process by which a stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. Passivation is the removal of exogenous iron or iron compounds form the surface of a stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself. There are a number of methods used to test for passivation. Per ASTM A 967, there is Practice A, which is a water immersion test. Practice B is a High Humidity Test. Practice C is a salt spray test. Practice is a copper sulfate test, and Practice E is a Potassium Ferricyanide-Nitric Acid Test.

Electrochemical Corrosion Testing

The Electromotive Force (EMF) Series in which elements are listed in order of their potential relative to a Standard Hydrogen Electrode is replaced in the corrosion field with the Galvanic Series. In the Galvanic Series, elements are listed in order of the potential they develop in a solution relative to a reference electrode, usually the Saturated Calomel Electrode (SCE). The solution used for the practical galvanic series is flowing seawater, however, since the potential developed is a function of the solution, galvanic series can be developed for different solutions.

The potential an electrode develops relative to a reference electrode in a solution is a function of the surface condition of the metal and the composition of the solution and is referred to as the Corrosion Potential or the Open Circuit Potential. It is this potential and the effect of variations from it that is central to the electrochemical techniques described here.

Potentiodynamic polarization curves (ASTM G5, ASTM G61)

To produce a potentiodynamic polarization curve, the potential of an electrode (relative to a standard reference electrode) made from the metal of interest is varied at a controlled rate over a range of potentials from cathodic to anodic values. The resulting current, which is


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related to the corrosion rate and the controlled potential, are plotted on a graph such as the one illustrated.

The potential at which the current reaches its maximum, the current in the region where there is little change in current with potential and the potential at which the current begins to increase again are used in materials selection of design of corrosion control systems. Reversing the scan to determine the potential at which the reverse scan crosses the forward scan provides information on the tendency of the metal to pit.

Linear Polarization (ASTM G59)

Scanning the potential from about 20 mV cathodic to 20 mV anodic to the corrosion potential produces a linear polarization (also known as polarization resistance) scan. The slope of the resulting approximately straight line is proportional to the corrosion rate.

The Linear Polarization technique is useful for determining the effect on the corrosion rate of a number of possible corrosion inhibitors added at different concentrations. Information can be obtained from this technique relatively quickly to allow decision to be made on what to include in more time consuming and complicated laboratory or field tests.

Corrosive Gas Accelerated Corrosion Tests

Accelerated corrosion tests to assess the resistance of samples to industrial atmospheres are performed. Some of these use sulphur dioxide (SO ) to accelerate the corrosion. 2 These tests use a relatively high concentration of SO and are normally used to test the 2 resistance of coatings. Other tests use very low concentrations of corrosive gases and are used primarily to test electronic components.

Humid Sulphur Dioxide Tests (ASTM G87, DIN50018, BS EN ISO 32311998, Kesternich Test)

This test consists of exposing parts to the environment of a cabinet operated at an elevated temperature and high humidity with the addition of sulphur dioxide. There are variations in the test conditions made to adjust the severity of the conditions. These variations include the concentration of SO added and either leaving the test pieces in the cabinet for eight 2 hours and in the ambient laboratory atmosphere for 16 hours (most common) or leaving the test pieces in the cabinet continuously. In both approaches the water in the bottom of the cabinet and the SO are changed daily. The number of cycles performed usually varies 2 between five and 25.

Salt/ SO Spray (Fog) Testing (ASTM G85 A4) 2

This test consists of a standard salt spray with the addition SO to the fog for a period of 2 one hour in every six hours.


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Mixed Flowing Gas Tests (ASTM B827, ASTM B845)

These test consist of exposing the samples (usually electronic components) in a cabinet through which flows humid air with low concentrations of hydrogen sulphide, sulphur dioxide, nitrogen dioxide and chlorine.


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Metallography

Grain Size Determination

In order to establish a scale for grain size, ASTM E112 shows charts with outline grain structures at various dimensions. This has led to a universally accepted standard by which grain sized range form 1 (very coarse) to 10 (very fine). A material's grain size is important as it affects its mechanical properties. In most materials, a refined grain structure gives enhanced toughness properties and alloying elements are deliberately added during the steel-making process to assist in grain refinement. Grain size is determined from a polished and etched sample using optical microscopy at a magnification of 100X.

High Temperature Oxidation Analysis

The oxide film on aluminum provides a high degree of resistance to oxidation in normal atmospheres. The presence of water vapor during heat treatment can result in a disruption of the oxide film and this condition can lead to deterioration of properties in aluminum alloys. The most common manifestation of high temperature oxidation is blistering, but occasionally the only manifestations are internal discontinuities or voids.

Intergranular Attack

Intergranular Attack is a form of intergranular corrosion. Its morphology is characterized by a uniform or relatively uniform attack of all grain boundaries over the surface of the specimen. Stress does no contribute to the morphology of intergranular attack.

Intergranular Oxidation

Intergranular oxidation is the formation of isolated particles of corrosion products beneath the metal surface, between the crystals or grains. This occurs as the result of preferential oxidation of certain alloy constituents by inward diffusion of oxygen.

Macro Examination, ASTM E381, A561, A604, E340

Macroetching is the procedure in which a specimen is etched and evaluated macrostructurally at low magnifications. It is a frequently used technique for evaluating steel products such as billets, bars, blooms, and forgings. There are several procedures for rating a steel specimen by a graded series of photographs showing the incidence of certain conditions and is applicable to carbon and low alloy steels. A number of different etching reagents may be used depending upon the type of examination to be made. Steels react differently to etching reagents because of variations in chemical composition, method of manufacturing, heat treatment and many other variables.

Macro-Examinations are also performed on a polished and etched cross-section of a welded material. During the examination, a number of features can be determined including weld run


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sequence, important for weld procedure qualifications tests. As well as this, any defects on the sample will be assessed for compliance with relevant specifications. Slag, porosity, lack of weld penetration, lack of sidewall fusion and poor weld profile are among the features observed in such examinations. It is normal to look for such defects either by standard visual examination or at magnifications of up to 50X. It is also routine to photograph the section to provide a permanent record. This is known as a photomacrograph.

Micro Examination

This is performed on samples either cut to size or mounted in a resin mold. The samples are polished to a fine finish, normally one micron diamond paste, and usually etched in an appropriate chemical solution prior to examination on a metallurgical microscope. Micro-examination is performed for a number of purposes, the most obvious of which is to assess the structure of the material. It is also common to examine for metallurgical anomalies such as third phase precipitates, excessive grain growth, etc. Many routine tests such as phase counting or grain size determinations are performed in conjunction with micro-examinations.

Weld Examination

Metallographic weld evaluations can take many forms. In its most simple form, a weld deposit can be visually examined for large scale defects such as porosity or lack of fusion defects. On a micro scale, the examination can take the form of phase balance assessments from weld cap to weld root or a check for non-metallic or third phase precipitates. Examination of weld growth patterns is also used to determine reasons for poor mechanical test results. For example, an extensive central columnar grain pattern can cause a plane of weakness giving poor charpy results.

Solderability Evaluation, ASTM B678

Metallic coatings are frequently used to provide solderable surfaces. But, an improperly produced coating may not yield the required solderability. There are many coating defects that cause poor solderability including porosity, codeposited impurities, incorrect thickness, and surface contamination. This method provides a procedure for evaluating the solderability of metallic-coated products and test specimens to assure satisfactory performance in manufacturing processes requiring soldering with soft (Sn/Pb) solder and rosin flux.

Coating / Plating Evaluation (ASTM B487, ASTM B748) A coating or plating application is used primarily for protection of the substrate. The thickness is an important factor in the performance of the coating or plating. A portion of the specimen is cut, mounted transversely, a prepared in accordance with acceptable or suitable techniques. The thickness of the cross section is measured with an optical microscope. When the coating or plating is thinner than .00020", the measurement should be taken with the aid of the scanning electron microscope. Cross-sectioned metallographic examinations of substrates with platings, surface evaluations, thickness measurements, weight per volume, and even salt spray testing can aid in the evaluation of platings.


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Case Depth

Case hardening may be defined as a process for hardening a ferrous materials in such a manner that the surface layer (known as the case), is substantially harder than the remaining materials (known as the core). This process is controlled through carburizing, nitriding, carbonitriding, cyaniding, induction and flame hardening. The chemical composition, mechanical properties, or both, are effected by these practices. Methods for determining case depth are either chemical, mechanical or visual and should be selected on the basis of specific requirements.

Decarburization Measurement

This method is designed to detect changes in the microstructure, hardness, or carbon content at the surface of the steel sections due to carburization. The depth is determined as the depth where a uniform microstructure, hardness, or carbon content, typical of the interior of the specimen is observed. This method will detect surface losses in carbon content due to heating at elevated temperatures, as in hot working or heat treatment.

Fractographic Examination

This involves the study of the fracture surfaces of broken components and can help determine the cause of the failure. In most cases, the appearance of a failed surface illustrates certain features or patterns which offer clues leading to a reason for failure. Sequential striations initiating at a change in section, for example, may suggest a fatigue mechanism to have been in force. Distortion at the fracture surface may suggest ductile overload, etc. Fractographic examination can be performed on a macro scale by visual of low magnification, stereoscan examination, or at much higher magnifications in a Scanning Electron Microscope (SE).

Surface Evaluation

Surface inspection includes the detection of surface flaws and he measurement of surface defects and roughness. One method includes the use of a laser light. When the scattered light is reflected off the surface of a sample, it can be analyzed and measure. Another method is the use of a motorized stylus (profilometer). The stylus is placed on the surface and the texture of the material is measured in micro-inches or milimeters

Surface Evaluation

Surface inspection includes the detection of surface flaws and he measurement of surface defects and roughness. One method includes the use of a laser light. When the scattered light is reflected off the surface of a sample, it can be analyzed and measure. Another method is the use of a motorized stylus (profilometer). The stylus is placed on the surface and the texture of the material is measured in micro-inches or millimeters.


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Inclusion Counts, ASTM E45, AMS 2300

Most metallic materials have some form of inclusion content. In steels, for example, nonmetallic inclusions exist in the form of compounds such as manganese sulphide. Inclusion counts are performed to assess their type, shape, quantity and distribution (ASTM E45). Cleanliness of materials can be an important factor in many applications and current high quantity steel-making processes ensure that inclusion content is kept to a minimum. The presence of inclusions in a material can be used to determine the rolling direction of a plate. Three mutually perpendicular axes are polished and examined. Inclusion stringer direction can be used to assess the direction of rolling. Certain bearing and/or aerospace applications may require a "frequency / severity" determination of inclusions per AMS 2300. This requires a specific test specimen to be machined, hardened, and inspected by wet magnetic particle technique.

Particle Size

Many research programs require the microstructural evaluation of small particles. The particles can be examined morphologically in the imaging mode. Structure can be examined using electron diffraction and the composition of the particles can be examined using EDS.

Specific Surface Area (SSA)

The specific surface area of the ceramic powder used in the development of investment casting molds becomes key to the ultimate casting quality. Each powder particle used in the ceramic slurry must be uniform and without defect. For titanium medical implant and aerospace castings, the surface condition is critical.

Porosity Determination Porosity is a characteristic of being porous, with voids or pores resulting from trapped air or shrinkage in a casting, and is considered a defect. The loss in casting properties measured by a tensile test may reflect the amount of porosity in a casting. Because imperfections become areas of higher stress concentration, the percentage of property loss becomes greater when the strength requirement is higher. A metallographic examination can determine if porosity exists in a casting. X-ray techniques are also used for non-destructive evaluations of porosity in castings.

Alpha Case, Alloy Depletion, ASTM F136

The alpha case is the oxygen, nitrogen or carbon enriched alpha stabilized surface resulting from elevated temperature exposure. In certain Titanium alloys, the microstructure is strongly influenced by the processing history and heat treatment, and the alpha case can effect fatigue strength and corrosion resistance. (ASTM F136), In certain Titanium alloys there should not be any continuous alpha network at prior grain boundaries and there should not be any coarse, elongated alpha platelets.


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Eutectic Melting Examination

Conducted primarily for the aerospace industry, this particular service can be offered under Boeing approval. Eutectic melting is a metallographic examination conducted on aluminum alloy materials and the phenomenon can occur whenever the eutectic melting temperature is exceeded. If the temperature during a thermal operation rises beyond the eutectic melting temperature, solid solution melting takes place. This drastically effects the mechanical properties of the aluminum and can quench cracking.

Phase Volume Fraction Determination

Many metallographic structures are dual or multi-phase. Examples are austenitic or duplex stainless steels which exhibit both austenite and Delta ferrite Phase. It is often important to quantitatively establish the phase balance as it can affect the mechanical and corrosion properties of the material. Two methods are mainly used, the preferred being manual point counting (ASTM E562). This method requires a square grid to be superimposed on a metallurgical microscope image at a suitable magnification and counts taken of underlying structures at the grid intersections. Depending on the expected volume fraction of a phase, a specific number of random counts are performed to give a good statistical population. Alternatively, volume fraction phase counts can be done automatically by image analysis.


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NDT

Dye Penetrant

This method employs a penetrating liquid, which is applied over the surface of the component and enters the discontinuity or crack. Subsequently, after the excess penetrant has been cleared from the surface, the penetrant exudes or is drawn back out of the crack is observed. Liquid penetrant testing can be applied to any non-porous clean material, metallic or non-metallic, but is unsuitable for dirty or very rough surfaces. Penetrants can contain a dye to make the indication visible under white light, or a fluorescent material that fluoresces under suitable ultra-violet light. Fluorescent penetrants are usually used when the maximum flaw sensitivity is required. Cracks as narrow as 150 nanometers can be detected.

Magnetic Particle Testing

The Magnetic Particle Inspection method of Non-Destructive testing is a method for locating surface and sub-surface discontinuities in ferromagnetic material. It depends for its operation on the face that when the material or part under test is magnetized, discontinuities that lie in a direction generally transverse to the direction of the magnetic field, will cause a leakage field, and therefore, the presence of the discontinuity, is detected by use of finely divided ferromagnetic particles applied over the surface, some of these particles being gathered and held by the leakage field, this magnetically held collection of particles forms an outline of the discontinuity and indicates its location, size, shape and extent.

Ultrasonic Inspection

Ultrasonic methods of NDT use beams of sound waves (vibrations) of short wavelength and high frequency, transmitted from a probe and detected by the same or other probes. Usually, pulsed beams of ultrasound are used and in the simplest instruments a single probe, hand held, is placed on the specimen surface. An oscilloscope display with a time base shows the time it takes for an ultrasonic pulse to travel to a reflector (a flaw, the back surface or other free surface) in terms of distance travelled across the oscilloscope screen. The height of the reflected pulse is related to the flaw size as seen from the transmitter probe. The relationship of flaw size, distance and reflectivity are complex, and a considerable skill is required to interpret the display. Complex mutiprobe systems are also used with mechanical probe movement and digitization of signals, followed by computer interpretation are developing rapidly.

Visual

Non-Destructive visual inspections can be preformed on-site or at the laboratory facility, and are based upon the requirements of the client or specification.


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metal Laboratory Techniques

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