Metal selection is basically a matter
of quality and cost. The properties of the material must be
adequate to meet design requirements and service conditions.
The cost must be the lowest for the materials available to meet
those conditions. Cost here means final cost rather than the
casting purchase price. It includes machining, assembly, finishing
and other charges incurred in producing the final component
Certain applications involve complex service conditions. These
present problems in metal selection. Examples are applications
needing a combination of resistance to thermal shock with high
strength; or where both heat and corrosion resistance are essential.
In some cases, completely incompatible properties are needed.
An example would be easy machinability with high hardness. The
relative importance of the conflicting demands must then be
studied. In some cases, special treatments must be used, for
example heat treatment, surface hardening etc.
Three Steps in Metal Selection
- Select the main group of Meehanite
metals according to basic requirements and general type of
application; for example, General Engineering, Wear, Heat
- Select the section of that main group
offering properties best suited to the service conditions
for the application; for example, heat with thermal shock;
lubricated wear; heat with corrosion.
- Select the Meehanite type within
that section offering properties nearest to those required
for the application, based on casting sections, stress calculations,
General Engineering Applications
Where strength and machinability are
the main considerations, select from the Engineering Group Flake
Graphite Types. These are all machinable and selection should
be on the basis of specification values for casting sections
and service stresses involved.
Where strength and ductility, or a high degree of toughness
are the main considerations, select from the Nodular Graphite
Types, according to strength values, casting section and service
Heat and Wear Applications
Three questions arise in selecting for these
- Must the casting be machined - if
so, then the hard unmachinable types (HR, WH) are eliminated
- Is a hard, unmachinable structure
required - if so, and machining is also necessary, then the
hard structure must be obtained by special treatment.
- What are the strength requirements
- this will determine the choice of type within a section,
or it may determine the type of treatment given.
Where both high hardness and machining are
required, select from the following treatments:
- Hard but machinable type (eg WA) machined,
heat-treated for maximum hardness, ground to final dimensions.
- Hard but machinable type, machined,
surface hardened (flame or induction) on areas subject to
wear, ground to final dimensions.
- Engineering type giving adequate
strength, cast against chill at parts subject to wear. Stress
The austenitic Flake and Nodular Graphite
Types of Meehanite will withstand reasonably severe corrosion
conditions. For less severe conditions, three Flake Graphite
Types of Meehanite are available. All are machinable.