Specifications for Cast Iron

Specifications for Cast Irons.

Specifications for Materials can be classified into major sections based on the specifying authority and the field for which the specifications are made. To clarify simple table can be created,

International bodies Customer Supplier / Foundry
Chemistry
Manufacturing process
Heat Treatment
Mechanical properties
Microstructure

Major difference between Cast Irons and Steels and other metals

• For steels and other metals the specifications start with chemistry. The manufacturing process and heat treatment govern the mechanical properties and microstructure. There is a definite correlation between chemistry, process, heat treatment and the properties and microstructure.

• International material specifications are prepared without any specific end component in mind. Hence these become generic, one set giving chemistry, other giving heat treatment cycles and corresponding mechanical properties. The customers use these standards to prepare their requirements. The starting point is chemistry.

Cast Iron Specifications : International

Cast irons do not enjoy such straight forward relationships between chemistry and properties. Hence the International standards specify the grades by tensile strength obtained on a test bar of standard size. And these values mainly indicate the melt quality and not the tensile strength of casting.

International standards also define the generic types of graphite forms, size and shape distribution. They do not define acceptance / rejection limits or the microstructural requirements for a casting.


Cast Iron Specifications : Customer

For the customer, the scope or applicability of the specification is much narrowed down for one specific casting, or a set of component families.

• From the requirement of finish, cleaning of the casting, he specifies the casting process – sand casting, shell casting etc.
• As the size and shape of the casting is known, he has a definite relation between the test bar properties and the casting properties. He therefore can specify the hardness range and the location on the casting where it should be tested.
• With fixed size, shape and properties requirements, and after finalisation of the casting process, he can specify the range of chemistry – which will give consistant properties on finished casting. Though there can be some variation from foundry to foundry, an overall guideline can definitely be specified.
• Based on the service conditions, he specifies additional limits on the acceptance of chemical elements – for example phosphorus content for cylinder liner application, manganese content for SG iron etc.
• If any heat treatment or specific stress relieving operation is required, that is specified.
• Then the microstructure definition comes. The microstructure cannot be consistant through out the casting. Hence to avoid any ambiguity, he specifies in detail the microstructure requirements and the location where it should be checked.
• Lastly the component testing procedures are defined like pressure testing for leakages etc.

It should also be appreciated that these specifications are not prepared at the designers table. They are evolved after thorough interactions with foundry, and after testing on a number of samples.

Cast Iron Specifications : Foundry

When the drawings and specifications are received in the foundry, it becomes a foundryman’s responsibility to translate these into unambiguous process sheets. This calls for similar preparation of specifications for raw materials, consumables and working procedures in the foundry. They also get fine tuned over a period. Typically these should cover following areas of foundry operations,

Raw materials :
• Pig iron
• Ferro alloys
• Scrap
• Inoculants
• Raw material sources
• Storage of raw materials and foundry returns

Sand :
• Moulding / Core Sand Quality
• Sand preparation
• Testing of sand quality

Melting :
• Charge calculations
• Molten metal analysis
• Temperature control
• Wedge test

Pouring :
• Pouring time
• Inoculation method
• Pouring temperature
• No of castings per heat

Fettling :
• Cooling duration – when the boxes are broken
• Runner riser removal process
• Shot blasting
• Heat identification

Testing :
• Sample selection
• Tensile testing
• Hardness testing
• Testing procedures and records
• Analysis of records
• Casting defects records and analysis

To ensure that these systems are initiated at the foundries, customers have started taking keen interest in the foundry operations by auditing the quality systems in the foundry.