Metallographic services are a core competence for
Exova Materials Testing. The Group employs a large
number of experienced and qualified metallurgists who provide
this service daily in support of routine testing, failure analysis
and other testing services.
All Exova mechanical testing laboratories have facilities for
sample preparation and metallographic examination. Large
numbers of samples can be handled daily through the use of
automatic sample preparation equipment.
Digital imaging is now being widely used by Exova in
metallography, facilitating email distribution of images and
reports.
Specialist Service Offerings for
Macro - Examination
Macro-examination is normally 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 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 x5. 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 mould. 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 micro–examination. |
Inclusion Count
Most metallic materials have some form of inclusion content. In
steels, for example, non-metallic 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 quality steel-making processes ensure that inclusion
content is kept to a minimum.
The presence of inclusions in a material can be used to
determining 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.
Grain Size Determination
In order to establish a scale for grain size, ASTM E112 shows
charts with outline grain structures of various dimensions. This
has led to a universally accepted standard by which grain
sizes range from 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 X100.
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. |
