Types of Iron
- Grey Iron
- Having mechanical properties that are established by the tensile strength and hardness with chemical properties adding variations.
- SG (Spheroidal Graphite) or Ductile Iron
- Having properties that are established by the tensile strength and ductility based on the number and type of spheres of graphite formed which generally relates to the chemical composition.
- Having mechanical properties of Hardness that generally relate to the chemical composition.
- Having corrosion resistance generally related to the chemical analysis.
Cast Iron is Fe (Iron) with a proportion of:
- Carbon (2.5 to 4.5%) makes the iron easier to cast and machine.
- Silicon (1.0 to 5%) softens the iron and helps grain size Higher %'s give heat stability.
- Manganese (0.8%) Counteracts some impurities and strengthens & hardens.
- Sulphur in normal iron but not wanted in Spheroidal Graphite Iron.
- Phosphorus used to assist the flow of metal in drainpipe production, weakens the iron.
- Copper strengthens the metal and forms finer grains.
- Magnesium converts flake graphite to spheroidal graphite giving strength & ductility.
- Nickel strengthens and in larger (18-22%) quantities provides corrossion resistance.
- Chrome hardens and weakens, mixed with Ni 2% and Cr22-28% very hard wearing.
- Aluminium detrimental producing gasses within structure.
Testing for cast iron falls into 4 categories: Visual, Mechanical, Chemical, Non Destructive.
|Wedge sample||Grey Iron||Taken from the furnace the sample is wedge shaped. Poured into the mould the wedge is cooled rapidly and broken to see the 'depth of chill'. This appears as a silver area as opposed to the normal grey of the iron. The length of chill is measured from the front of the wedge and can be used as an indicator by referral to the mechanical properties.|
|Microscopic Analysis||Grey Iron, SG, Ni-Hard, Ni-Resist||Requires specimen polishing to show grain boundaries, size, shape and type. Specimen is etched with acid to accentuate grain types. Used to count the number of, size and shape of graphite formations.|
|Hardness Test||Using an impact tester, where a force is used to indent the surface of the casting. The depth and size of the indentation in measured against known parameters and the hardness of several tests is used. The results can be given in various units such as BHN (Brinell Hardness Number)|
|Tensile Test||A sample piece is prepared of known dimensions. The piece is clamped in a tensometer and stretched until fatigue causes the sample to break. The force used is calculated to give a tensile strength in Newtons per square millimetre.|
|Ductility||A sample is prepared of known dimensions. The sample is bent to destruction and the percentage bend is the Ductility represented as a percentage. Grey Iron has very little whilst SG can be as much as 20%|
|Wet and dry (more expensive)||Where the materials are ground , weighed and tested by their chemical reactions to other chemicals to determine the percentage of each element present.|
|Coin Sample (modern way)||A sample of the metal is taken in a graphite mould giving a small disc of iron, the sample disc is polished and 'sparked' in a spectrograph at high temperatures. The resulting vapour is analysed for the proportion of each element present.|
|Ultrasonics||Sound waves are used to look through the casting, holes provide an anomaly to the echo and the results indicate porosity or shrinkage. However, cast iron and especially SG iron do not ultrasonic particularly well. This is due to the carbon/graphite within the iron appearing as holes and the sensitivity of the testing equipment. The equipment is adjusted for the carbon and will not pick up small areas of porosity.|
|MPI||Magnetic Particle Inspection. The casting is painted with a whitewash, iron filings are painted, in a loose wash, onto the area to be to be tested and a magnetic charge is induced in the casting. Areas of cracks or sub-surface shrinkage causes a disruption in the magnetic field and a concentration of filings around the defect. Efficient and simple.|
|Radiography||The casting is subject to x-rays and the resultant photographic plates analysed visually for defects.|