Macro Structure of Metals

INTRODUCTION

            Mechanical engineers are primarily interested in the mechanical behavior of metals because they have to consider about the quality of material before it use for a final product. The final product is used by the customer, therefore they have to make sure that there are no any defects and final product is in the exact quality.

            There are four types of defects. They are,

1.      Point defects

2.      Line defects

3.      Planner defects

4.      Bulk defects

            Within this experiment we were expected to examine the bulk defects affected in given some samples which has been undergone,

v  Ductile failure

v  Brittle failure

v  Fatigue failure

v  Creep Failure

 Ductile failure

            Metal that deform plastically under the tension are called ductile materials, plastic deformation take place by ‘slip’ of atoms on the planes that are of densest atomic packing and along the direction of highest linear density therefore practically in f.c.c metals which are of high ductility plastic deformation, and then final facture of a cup and cone type occurs.

                                  

Brittle failure

            Brittle fracture is a rapid run of cracks through a stressed material. The cracks usually travel so fast that you can't tell when the material is about to break. In other words, there is very little plastic deformation before failure occurs. In most cases, this is the worst type of fracture because you can't repair visible damage in a part or structure before it breaks. In brittle fracture, the cracks run close to perpendicular to the applied stress.

                                                            

Fatigue failure

            Fatigue is the fracture that occurs in components that are subjected to replaced (cyclic/alternating) loads.

σ _max= maximum stress

σ _min= minimum stress

Fatigue failure occurs in three stages,

1. Crack initiation - A crack has to initiate on the surface of a component, for it to fail by        fatigue generally a crack initiates at a point of high stress concentration

2. Crack propagation - When the crack reaches a critical crack length, the crack will be            propagating continuously at each maximum stress cycle.

3. Final failure - When the remaining section cannot withstand the load applied brittle type     of fracture occurs

 Creep failure

             Creep occurs at temperature well above room temperature under constant load. This deformation is plastic and occurs at stresses even below the yield stress of  the material. The temperature at which materials start to creep depends on their melting point.

            In this experiment we use some tests to examine the bulk defects. They are,

1.      Macro-structure Examination

      Macro- (Macrography, Macrostructure) Examination is a method of examination of large regions of the specimen surface or fractured section with the naked eye or under low magnification. The following macrostructure details may be studied:

·         Macro-segregation of the alloying elements or impurities (sulfur in steel, antimony in lead base Babbitt)

·         Large non-metallic inclusions such oxides, sulphides or slag

·         Forging flow lines

·         Cast grain structure

·         Physical defects such gas pockets, shrinkage cavities, cracks.

2.      Fracture Examination

          This is a method relating to observation of the broken surfaces of the failed part. The appearance of the surfaces may be a result of brittle fracture, fatigue with its characteristic fracture, intercrystalline fracture, segregation, slag particles, cracks, starting from physical defects, rough surface, corrosion and other causes. Fracture examination should take into account the working condition, the history of the part, possible overloads, applied to the part, misalignments and other working factors.

3.      Non- Destructive Testing

As the term ‘Non- Destructive’ implies there is no impairment of the properties of the article consequent to testing. There is variety of non-destructive tests. Most important methods are,

1.      Liquid Penetrant Testing

          Liquid penetrant inspection, also called Dye penetrant inspection, is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic-particle inspection is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

 Below are the main steps of Liquid Penetrant Inspection:

·         Pre-cleaning:

            The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor degreasing, or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination.

·         Application of Penetrant:

The penetrant is then applied to the surface of the item being tested. The penetrant is allowed time to soak into any flaws (generally 10 to 30 minutes). The soak time mainly depends upon the material being testing and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature one must be careful not to apply visible red dye penetrant to a sample that may later be inspected with fluorescent penetrant.

·         Excess Penetrant Removal:

            The excess penetrant is then removed from the surface. Removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrant to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this cans the remove the penetrant from the flaws. This process must be performed under controlled conditions so that all penetrant on the surface is removed (background noise), but penetrant trapped in real defects remains in place.

·         Application of Developer:

            After excess penetrant has been removed a white developer is applied to the sample. Several developer types are available, including: non-aqueous wet developer, dry powder, water suspendible, and water soluble. Choice of developer is governed by penetrant compatibility (one can't use water-soluble or suspedible developer with water-washable penetrant), and by inspection conditions. When using non-aqueous wet developer (NAWD) or dry powder the sample must be dried prior to application, while soluble and suspendible developers are applied with the part still wet from the previous step. NAWD is commercially available in aerosol spray cans, and may employ acetone, isopropyl alcohol, or a propellant that is a combination of the two. Developer should form a thin, even coating on the surface.

The developer draws penetrant from defects out onto the surface to form a visible indication, a process similar to the action of blotting paper. Any colored stains indicate the positions and types of defects on the surface under inspection.

·         Inspection:

            The inspector will use visible light with adequate intensity (100 foot-candles is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after a 10 minute development time. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye, but this should not be done when using fluorescent penetrant. Also of concern, if one waits too long after development the indications may "bleed out" such that interpretation is hindered.

·         Post Cleaning:

            The test surface is often cleaned after inspection and recording of defects, especially if post-inspection coating processes are scheduled.

2.      Eddy Current Testing

          In this method we use electromagnetic induction to detect flaws in conductive materials. Eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity. Eddy current testing is an electromagnetic technique and can only be used on conductive materials. When we do this practical the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of penetration into the material is limited, and flaws that lie parallel to the probe may be undetectable.

          Eddy currents are electrical currents induced in a conductor of electricity by reaction with alternating magnetic field. This form of testing relies on the attraction of magnetic particles to the flux leakage when an eddy current is passed through the material, this is an indication of the flaws existence, this flux leakage is caused by the flaw in the ferromagnetic material for which is being tested. Changes in the geometry & homogeneity of the test object will change the magnitude & distribution of the eddy currents. By monitoring these changes, the presence of cracks & other flaws can be detected. Commonly this method used in the aerospace, automotive, marine, and manufacturing industries.

3.      Magnetic Particle Testing

          Magnetic particle inspection processes are non-destructive methods for the detection of surface and sub-surface defects in ferrous materials. They make use of an externally applied magnetic field or half-wave DC (rectified AC) current through the material, and the principle that the magnetic susceptibility of a defect is markedly poorer (the magnetic resistance is greater) than that of the surrounding material.

          The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. Thus the size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of the tiny particles that are attracted to the leakage fields.

          The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light (sometimes referred to as black light). The suspension is sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a buildup of particles.

          This inspection can be applied to raw material in a steel mill (billets or slabs), in the early stages of manufacturing (forgings, castings), or most commonly to machined parts before they are put into service. It is also very commonly used for inspecting structural parts (e.g., landing gear) that have been in-service for some time to find fatigue cracks.

          Usually tested pieces needs to be demagnetizated by a degaussing tool before use. Parts are demagnetized by applying AC current through the part which scrambles the magnetic domains causing it to demagnetize

          It is a quite economic non-destructive test because it is easy to do and much faster than ultrasonic testing and testing. There are two different ways of magnetizing a part Longitudinal and Circular magnetization. Longitudinal Magnetization passes current through a coil and the magnetic flux lines go through the part. Circular magnetization passes current through the part and establishes a magnetic field around the part. The two different methods are used because cracks can only be seen 45 to 90 degrees to the magnetic flux lines. Magnetic Particle Inspection cannot be used for non-ferrous materials and austenitic stainless steels. In such cases, other methods such as dye penetrant inspection are used.

4.      Radiographic Testing

          This method of weld testing makes use of X-rays, produced by an X-ray tube, or gamma rays, produced by a radioactive isotope.  The basic principle of radiographic inspection of welds is the same as that for medical radiography.  Penetrating radiation is passed through a solid object, in this case a weld rather that part of the human body, onto a photographic film, resulting in an image of the object's internal structure being deposited on the film.  The amount of energy absorbed by the object depends on its thickness and density.  Energy not absorbed by the object will cause exposure of the radiographic film.  These areas will be dark when the film is developed.  Areas of the film exposed to less energy remain lighter.  Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film.  Inclusions of low density, such as slag, will appear as dark areas on the film while inclusions of high density, such as tungsten, will appear as light areas.  All discontinuities are detected by viewing shape and variation in density of the processed film.

          Radiographic testing can provide a permanent film record of weld quality that is relatively easy to interpret by trained personnel.  This testing method is usually suited to having access to both sides of the welded joint (with the exception of double wall signal image techniques used on some pipe work).  Although this is a slow and expensive method of nondestructive testing, it is a positive method for detecting porosity, inclusions, cracks, and voids in the interior of welds.  It is essential that qualified personnel conduct radiographic interpretation since false interpretation of radiographs can be expensive and interfere seriously with productivity.  There are obvious safety considerations when conducting radiographic testing.  X-ray and gamma radiation is invisible to the naked eye and can have serious health and safety implications.   Only suitably trained and qualified personnel should practice this type of testing

DISCUSSION

         Different types of methods are used to find the material defects. By examining the structure of the material and modifying the structure of material through heat treatment or other means, can control the properties of material such as the strength, hardness and ductility.

           The following samples are tested in macro structure examination.

·         Longitudinal section through a cast ingot

·         Longitudinal section of a forge. Steel bolt

·         Transverse section of a carburized low-carbon steel bar

           We examined, there are some impurities gather to the center of the casted object. It may cause to fracture the cast ingot.  In forging, the metal undergoes in heavy tensile loads therefore the metal molecules are arranged in different order we can see flow lines. These molecular arrangements are caused to fracture the steel bolt. When we examine the carburized low Carbon steel bar, we saw in surface Carbon content is high because in the carburize process only the surface of the metal is carburized.

           In Fracture surface examination, the samples provided are tested. By this test, the kind of the fracture can be identified. If the fracture surface,

·          Has cup and cone shape                  : - ductile fracture by tensile loading.

·          Is sharp                                            : - brittle fracture. From Chevron patterns in surface,

We can decide the cracks propagation direction

·         Have layers of fracturing stages     : - fatigue failure. This beach marks called striations

Modification of structure in order to achieve desired properties

           For many kind of testing, it is needed to modify the specimen before doing the test. The methods like Radiographic methods do not want this because in those we use Electromagnetic radiations. Some methods like Dry penetrant test require such structure modification to reveal the material properties.

           In Dry penetrant method, the surface of the element should be cleaned well. Otherwise when there is an oil, grease or dust particle on the surface, the penetrant should not work. If we dyed, we have to use a good lighting system and if used fluorescent we have to use UV light. In magnetic particles, testing the specimen should be a ferrous metal. However, there should be different kind of to magnetize the component. More accuracy can obtain by an initial non-ferrous layer if exist.

           In eddy, current testing the specimen should be conductive. So we can use this method to ferrous and non ferrous metals. There should be a good touch between the specimen and the instruments. The voltage of the instruments varies with the dimension of the specimen.

Application of liquid penetrant test

·         Aircraft maintenance

·         Heat affect zone cracks

·         Poor weld penetration

·         Gas porosity

·         Cold shuts

·         Stress corrosion cracks

·         Heat treatment cracks

·         Fatigue cracks

·         Micro shrinkage

·         Grinding cracks

·         Hydrogen cracks

·         Inclusions

·         Hot tears

·         Laminations

Limitations of liquid penetrant test

v   Can detect very small surface discontinuity.

v   Frequently used to confirm suspected defects.

v   Area to be cleaned before and after check.

v   Generally restricted to ‘nonporous’ engineering materials.

v   Fluorescent penetrates are used in critical areas for more sensitive evaluation

v   Cracks not open to the surface cannot be detected.

Applications of eddy current test:-

Ø    Detecting surface-breaking or near-surface cracking and variations in material composition.

Ø   Can be used to measure the thickness of non-electrically conductive coatings on electrically conductive substrates.

Ø   Used in plant inspection for non-ferrite materials (where eddy current penetration is deeper) or for special applications, such as in the inspection of heat exchanger tubes for cracking or corrosion thinning.

Comparison of Eddy Current test with Magnetic Inspection test:-

	Eddy current test	Magnetic particle test
Advantages	Use for all conductive materials	Effective for defects of any shape
	portable	Easily portable
	Sensitivity is high
Disadvantages	Trained operator required	For Ferrous metals only
	Separate probes required for variation of materials	Demagnetization procedure is required

 USED OF THE TEST IN QUALITY CONTROL

Ø The eddy current method can be used as a quality control system. Because it can easily detect defects in the surface. If we can introduce an automated system, the defects are removed quickly.

Ø The Dye penetrant method is normally used for small components. In industrial level, it is often automated with series of and an inspection boot in with mechanized handling, timing solution, agitation etc.