The first two factors underscore the need for careful selection of the
material, and working with a forger who has the capability of producing
the forgings with the required repeatability. It is axiomatic, if the
materials and processes are maintained within the proper limits, the properties
of the forging will be consistent.
When it is desirable or necessary to verify mechanical properties, standard
inspection methods are available. Some of the most commonly employed test
methods are briefly described below. A more detailed discussion, including
references to relevant ASTM specifications, is available in FIA publications.
- Chemical Analysis Control of the chemical composition of
the forging stock is essential to attaining consistent properties
in the forging. The supplier of forging stock routinely furnishes
certified reports of the chemical analysis of the melt from which
the material was produced. The analysis is verified by the forge plant
either by comparative methods or analysis.
- Hardness Testing is widely used for steel forgings for several
reasons.
- The procedure, including sample preparation, is simple
- Hardness correlates closely with tensile strength
- It is nondestructive
- Portable equipment is available for forgings that are too large
to test with stationary equipment
- Tensile Tests are used to establish:
- ultimate strength
- yield strength
- ductility
They are performed on specimens taken from the forging according to
guidelines, mutually agreed between the forger and purchaser, for location
and orientation, and usually constitute destructive testing. In some cases,
particularly for open die forging and ring rolling, specimens may be taken
from a prolongation, which is an area of the forging at an agreed location
that is used for test, and is not part of the finished part. When prolongations
are used, the tensile test is nondestructive.
- Notched-Bar Impact Tests are also performed on specimens
taken from the forging. The tests are dynamic, and are used to define:
- impact strength
- notch toughness
- fracture mode (brittle, ductile or combination)
- Fracture Toughness Testing measures the resistance of a given
material in a given condition to catastrophic failure in the presence
of a pre-existing crack.
- Bend Tests are performed on samples taken from the forging
to test for ductility by bending the specimen through a specified
angle to a specified inside radius of curvature. The criterion for
failure is whether cracks form on the tension surface after bending.
- Ultrasonic Testing is one of the most reliable and widely
used methods of nondestructive testing for steel forgings. It provides
a means for evaluating the internal quality of forgings using a portable
instrument. The process yields information on the size, location and
orientation of discontinuities.
- Eddy Current Testing is a non-destructive test method for
detecting surface imperfections in bar and rod stock, cold forgings
and fully cooled warm and hot forgings. It can be used for 100% automatic
production testing.
- Magnetic Particle Testing is one of the easiest nondestructive
methods for detecting surface and near-surface discontinuities in
ferro-magnetic materials.
- Liquid Penetrant Testing is used to detect discontinuities
that are open to the surface. For ferrous parts a permeable red dye
is used with a white developer. For non-ferrous parts, a fluorescent
die penetrant is most commonly used, viewed under black light. Any
discontinuities are outlined and are readily detected.
- Metallography is the technique of microscopic examination
of polished or etched sections of metal specimens. It is employed
to evaluate:
- micro cleanliness
- grain size
- microstructure
Return to Table of Contents