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Macro-examination of metallographic specimens

Dr. Dmitri Kopeliovich

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:

Three different procedures are used for observation various characters of macrostructure:

1. Surface examination

The sample surface is usually ground (polish is not necessary)

The finish grinding is Grade 320 paper.

Large cracks, shrinkage cavities, gas pockets may be investigated.

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2. Macroetching

Macroetching helps to reveal much information of cast, hot-rolled and forged details.

Sulfur print is a macroetching technique of steel samples, allowing to determine sulfur distribution and segregation in the sample:

The sulphuric acid reacts with the sulphides in the steel specimen, forming hydrogen sulphide, which then reacts with the silver bromide of the photographic paper, forming dark traces corresponding the sulfur segregation;

Macroetching reagents

(quantities of the components are in weight parts)

Crystal structure: Iodine10 parts, Potassium Iodide20 parts, Water100 parts

Flow lines, cracks, porosity: Hydrochloric acid50 parts, Water50 parts, Boiling for 5-15 min.

Dendritic structure: Copper ammonium chloride 9 parts, Water91 parts, Immersion for 0.5-4 hours.

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3. Fracture Examination

Fracture examination 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.

Fracture examination should be supported by other metallography examinations.

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