EXAMINATION METALLOGRAPHIC

EXAMINATION METALLOGRAPHIC
By Scribe Valdemir Mota de Menezes

May provide data as part was produced, their properties and their homogeneity, since it consists in viewing the macro and / or microstructure.

Macrography

Examination of the macrostructure of the part or sample is made to the naked eye or with maximum magnification of 10x.
Allows to obtain general information about the piece as:
Uniformity of the workpiece material
Analysis of the distribution of impurities
Analysis of macro-defects in manufacturing
Analysis of grain size (parts coarse-grained), ...

Micrography

The examination of the microstructure of the part or sample is performed with the aid of a microscope (optical, electronic, atomic force ...)
Allows to obtain more detailed information such as:
Analysis of the type, quantity, distribution and shape of the various constituents
Analysis of micro-manufacturing defects (microcracks)
Grain size analysis
Determining the presence or absence of inclusions
Analysis of the fracture surface, ...

TECHNICAL metallographic

1.Técnicas macrographic: procedure for observation of the macrostructure
2.Técnicas micrographs: procedure for observation of the microstructure

1 - Techniques macrographic

PREPARATION OF THE BODY OF EVIDENCE FOR Macrography
A.Escolha location to be examined and cutting of the specimen (if necessary)
B.Preparação the specimen to obtain a flat and polished
C.Ataque appropriate chemical

RECOMMENDATIONS:

Before the court of CP, to survey the history of the material:
Preliminary careful visual inspection, as the surface of fracture or other signs of corrosion and surface
If possible, determine the manufacturing process in which the specimen was subjected (forgings, powder metallurgy, casting, machining, etc. ...), although Macrography can provide information on the manufacturing process,
Check if the number passed or not by heat treatment, thermochemical, chemical or mechanical later.
Check if the part has gone through welding or assembly processes
Checking that the special marks have, for example punching,
Often it is necessary to photograph and / or draw the part before making the cut.
Hardness measurements can give indications of the processes used in manufacturing the part.
The prior determination of the chemical composition may be important in some cases

A-Choose the location and section to be examined for the cutting of the specimen (if necessary)
The most common types of cuts are: longitudinal and transverse
(See examples of cuts more suitable for certain purposes)

B-Preparation of the specimen to obtain a flat and polished

Court: In general it is used with abrasive saws (diamond) cut outside.
Polishing: general uses of water are abrasive grit SiC
COMMENTS:
The cut must be as slow as possible and with cooling, to avoid changes in the macro and microstructure of the material and hardening the surface.
The polishing is not to be specular, as it makes it more difficult to attack since the reactive does not wet the surface evenly. Moreover it is difficult to photograph a specular surface.

C-Attack appropriate chemical
The attack highlighted the chemical heterogeneities present in the material (such as differences in chemical composition and crystalline structure).
The etching can be done:
By immersion of the specimen or reagent reactive in attack (by immersion)
Using a cotton swab or brush to extend the reactive section of interest on the attack (by application)
The chemical attack can last from a few seconds until the same day.
Immediately after the attack the specimens should be washed in fresh water and dry.
The chemical attack at some times can be quite deep and can hinder subsequent analysis, as the micrograph)
Polishing and oversights in the attack may lead to misinterpretation of results
- Examples of reactive chemicals for steel:

Direct Printing Baumann used to reveal Lüdders lines (lines of deformation).
Common uses a photographic paper (with AgBr), the paper is immersed in a solution of 1-5% H2S04 and applied on the polished surface. After five minutes the paper is loaded in a fastener of sodium hypochlorite for 10 min. And then washed in running water.
Mechanism for disclosure of lines Lüdders:
The sulphide-rich regions that react with H2S04 reacts with AgBr producing silver sulfide (brown) which is impregnated into the paper.