Metal Selection

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 or machine.

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 etc
• 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, machinability etc.

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 stress.

Heat and Wear Applications

Three questions arise in selecting for these applications:

• 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 relieved.
  

Corrosion Applications

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.