Metallographic examination

It is possible to confuse SCC with other brittle cracking phenomena. Confirmation of SCC typically requires a metallographic examination. On thin-walled components, the surface from which the cracking originates may not be apparent. In these cases, a formal metallographic examination may be required to assure positive identification of the surface from which the cracks originate. 

Cautions. Precise determination of the cause of weld-root cracks can be difficult. Determining correct repair procedures and preventive measures may require metallographic examinations and analyses. 

Several of the welded junctions were removed from the system for metallographic examination (Fig. 15.20). As can be seen from Fig. 15.20, the internal surface was covered with reddish and tan deposits and corrosion products. The metal surface itself retained a bright, metallic luster. 

Metallographic examination of other areas revealed similar degrees of corrosion on the two blades. At no point on the coated blade had the corrosion penetrated to the base metal, although in the two areas on the coated blade about 0.002 inch of the original 0.003 inch coating had been oxidized. 

Little data are available on hot corrosion behaviour. Figure 3.35 indicates maraging steel to have better resistance to air exposure at 535°C than a 5% Cr tool steel . Metallographic examination indicates that exposure to... 

The characteristic mode of corrosion of some alloys may be the formation as a corrosion product of a redeposited layer of one of the alloy constituents, as in the case of the brasses that dezincify, or of a residue of one of the components, as in the case of the graphitic corrosion of cast iron. Particularly in the case of the dezincified brass, the adherent copper is not likely to be removed with the other corrosion products, and therefore the mass-loss determination will not disclose the total amount of brass that has been corroded. This is especially important because the copper layer has very little strength and ductility and the extent of weakening of the alloy will not be indicated by the mass loss. In these cases, also, the mass-loss determinations must be supplemented by, or replaced by, mechanical tests or metallographic examination, or both, to reveal the true extent of damage by corrosion. Difficulties in obtaining accurate mass losses of heavily graphitised specimens have been reported... 

Table VII shows the data on the effect of the low-temperature irradiation on the tensile properties of cast 98-2 solder (98% lead-2% tin). These data indicate that the radiation had no effect on the tensile properties of the commercial solder which is used for the side seam of tinplate containers. Metallographic examination confirmed the absence of change in the microstructure of the solder after irradiation. (Figure 2).

Numerous compounds are observed in the Li-Ag phase diagram. The alloys are heated under Ar and cast in mild steel crucibles for metallographic examination, with homogeneity achieved by remelting under vacuum. Similar procedures were employed in an earlier study, except that H2 was used in place of Ar. An Ar cover gas was also employed to prepare the brasslike yj phase in the Li-Ag system for structural study. The silver and lithium were melted together in an iron crucible for 15-30 s before cooling without quenching to minimize the loss of lithium-. ... 

Farnsworth, M., C. S. Smith, and J. L. Rodda (1949), Metallographic examination of a sample of metallic zinc from ancient Athens, Hesperia (Suppl. 8), 126-129. 

Figure 5.30. Schematic drawing showing the construction of an isothermal transformation diagram from measurements of the progress of the transformation at various constant temperatures. This may be done, for instance, by metallographic examination of several specimens, quenched from the 7-field quickly enough to miss the nose of the C-curve and then isothermally annealed for various length of time. Notice that curves for the transformation of different samples may be shown on the same diagram and that more complex trends may be observed in real diagrams of specific alloys. In the example reported, Ms is the temperature at which the alloy will begin to show the martensitic transformation, Mf indicates the temperature below which no additional martensite forms.

Particular techniques and instruments may, of course, be required for special purposes for instance the preparation of specimens for specific examination (for example, metallographic examination) and tests of their polishing, cutting, drawing, rolling, etc. 

Under steady-state conditions, the concentration of interstitially absorbed hydrogen can be established at any depth of the membrane (dotted line in Fig. 25). By subsequent metallographic examination of the mem-... 

The acid is rather feeble, but it reddens litmus,2 decomposes carbonates and neutralises alkalis with the formation of salts. The baser metals are also slowly attacked, and the liquid finds frequent use as an etching reagent in the metallographic examination of alloys.3... 

Figure 3.13 displays a backscattered electron image of the Ni-Zn transition zone after a 2 h anneal at 400°C. Upon superficial metallographical examination, four intermetallic layers appear to be distinguishable in the microstructure of the Ni-Zn transition zone, giving an impression of the formation of all the compounds possible according to the equilibrium phase diagram by M. Hansen and K. Anderko.142 The same applies to Co-Zn reaction couples (Fig. 3.14a). However, upon more close examination this first impression tumes out to be quite erroneous, with only two of the four intermetallic compounds actually occurring.

With time, the Ni3Zn22 phase must be consumed in the course of the latter reaction. However, if the experiment is interrupted before its full consumption, then the layers of all the intermatallic compounds of the Ni-Zn binary system, stable at a given temperature, will be present between nickel and zinc. Moreover, metallographic examination of the cross-section surface after repeated anneals in the as-received condition may well show a greater number of distinquishable layers in the Ni-Zn transition zone than the number of those compounds because some will have duplex structures. 

The adopted methods were x-ray radiography (RT-X), especially concerning circumferential and longitudinal joints magnetic particle testing (MT) ultrasonics (UT) tensile and cold-bend test macroscopic sections hardness test metallographic examinations microanalysis (EDS) SANS. 

Hydrogen homogeneity was controlled by metallographic examination. Metallography of hydride structure on radial-axial and radial-transverse sections shows a uniform hydride distribution with hydrides elongated in the longitudinal direction (Fig. 1). From the hydrided pressure tube material curved compact toughness (CTT) specimens were machined. Except for the thickness and the curvature of the tube, the in-plane dimensions of specimens were in proportion described for compact specimen in ASTM standard test method (E-399). 

Welding Control. Plugs in the inner and outer capsules for the sales packages and industrial sources are seal-welded with an argon-shielded or helium-shielded tungsten electrode DC arc. The capsule is rotated under the automatically controlled arc to produce a minimum weld penetration of 1.27 mm. Each weld bead is visually inspected by periscope or by Questar telescope, and imperfectly formed welds are rejected. Weld quality is controlled by periodic metallographic examination of dummy capsules welded in the in-cell equipment. 

As in the case of corrosion failures, the sequence of steps involved in analyzing wear failures are initial examination of the failed component including service conditions to establish the mode or combination of modes of wear failure, metallographic examination to check if the microstructure of the worn part met the specification, both in the base material and in the hardened case or applied surface coatings, existence of localized phase transformations, shear or cold worked surfaces, macroscopic and microscopic hardness testing to determine the proper heat treatment, X-ray and electron diffraction analysis to determine the composition of abrasives, wear debris, surface elements and microstructural features such as retained austenite, chemical analysis of wear debris surface films and physical properties such as viscosity and infrared spectral determination of the integrity of lubricants and abrasive characteristics of soils or minerals in the cases of wear failures of tillage tools. 

The first and foremost step in failure analysis of ceramics consists of identifying the fracture origin and the type of cracking, which throws light on the type of failure such as failure due to impact, residual stress combined with load, thermal shock, improper machining, oxidation and corrosion. This is aided by micro- and macrofracto-graphy, examination of microstructure by SEM, chemical analysis and metallographic examination. 

Metallographic examinations of a cross-section through the failed clamp material confirmed it was an austenitic stainless steel and that extensive branched transgranular cracks were present throughout the material, as illustrated in Figure 7.49. 

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