DFM

1. Form Design of Steel Castings
• Cast steel has greater strength than cast iron
• Used when the loading conditions are severe
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• Properties of material is most important
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Properties:
Physical properties:
• Made in four ordinary grades useful for most applications
• For special duty special grades
For special duty special grades
Hardness: Extreme hardness useful in crusher jaws, edge mill
components, etc.

2. Form Design of Steel Castings
Tensile and compressive strength:
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• Strength properties same as ordinary machine steel
• Tensile strength, however, is two to four times greater
than that of cast iron
• The compressive strength is approximately the same as
• The compressive strength is approximately the same as
the tensile strength
• No need of worry about avoiding tensile stress as cast
iron.

3. Form Design of Steel Castings

4. Form Design of Steel Castings
Bending strength:
• Twice as high as that of cast iron so greater strength compared to cast iron
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• Re-designing of a component from cast iron to cast steel results in smaller
dimensions and therefore lighter weight
dimensions, and, therefore, lighter weight
Strength in torsion:
• The difference between cast iron and cast steel is not as pronounced as with
tensile and transverse loading but 50 % higher than cast iron can be used
• By the addition of copper, manganese, nickel, strength can be improved.

5. Form Design of Steel Castings
Elasticity :
• Twice that of cast iron, So suited than cast iron for structure requiring
greater stiffness
• Cast iron have better damping capacity
• By suitable form design it is possible to give steel castings fatigue
resistance required in vehicle components
Elongation at fracture:
• Elongation at fracture for cast steel is 12 to 20 per cent, Therefore very
suitable for parts which when severely loaded shall only take on an elastic
or permanent deformation and shall not suddenly fracture.

6. Form Design of Steel Castings
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Heat resistance:
• Growth Phenomenon does not occur in cast steel because there is no free
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graphite present therefore can be used temperature more than 400 degree
Celsius.
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Wear resistance:
• Cast steel is given greater toughness and wear resistance when alloyed with
manganese
• Used where resistance to wear is required components like mechanical
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shovels and crushing machines
• Cast steel is unsuitable as a bearing material, but is used for backing shells
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for bearings which are then lined with white metal

7. Form Design of Steel Castings
Chemical properties :
• There is a special grade of cast steel which is rustless, Its resistance to acids
and bases is still not widely enough known
Technological properties:
• Most properties are same as cast iron but special care to be considered
where the properties change
• Cast steel has a higher melting point (1450 degree c) than cast iron
• It flows sluggishly, tends to form bubbles more readily, and has twice as
much contraction, so more careful treatment is required when compare to
cast iron
• Critical factors of design are accuracy of size, and provision for cleaning

8. Form Design of Steel Castings
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• Since the risk of shrinkage-cavity formation is greater with cast steel than
with cast iron, designs must have the greatest possible uniformity of wall
thickness
thickness
• The main thing is to avoid constrictions by using suitably chosen radiusing
(fig. 72)
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• The use of feeders is extremely important wherever concentrations of metal
occur these can be removed after casting (fig. 73b)
• Steel casting calls for risers on a much larger scale than is needed for cast
iron if shrinkage cavities are to be avoided and bubbles and slag carried
away
• Risers often amount to 50 to 100 per cent of the weight of the work piece.

9. Form Design of Steel Castings

10. Form Design of Steel Castings
• Surfaces intended to take risers must be made sufficiently large (fig. 74b) otherwise
there is a risk of shrinkage cavities forming as at “a”
• Surfaces required to be dense when cast will be placed at the bottom as with cast
iron
• If this is impracticable, the alternative is to provide a slag bead which can be
trimmed off after casting.

11. Form Design of Steel Castings
• To obtain a dense cast structure in the fork shown in fig. 75 it is
necessary to cast solid to allow bubbles to escape upward so that
shrinkage cavities cannot form
• The check circle method has established itself as a means of
detecting concentrations of metal.
• A concentration of metal always occurs at the point where two walls
of equal thickness come together (fig. 76a)
• Such concentration can be reduced, however, if one of the walls is
made thinner (version b).
• This is why, for steel castings, ribs are always made thinner than the
wall in the ratio

13. Form Design of Steel Castings
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• Fillets having a radius of one-third to one-quarter of the wall
thickness are used
• Rib which enters a wall at an angle gives a bigger concentration of
metal than one which enters perpendicularly (fig. 77). The former
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should therefore be avoided wherever possible
• The fact that cast steel contracts twice as much as cast iron raises the
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risk of hot cracks occurring while in the mould
• For a length of 500mm the contraction is as much as 10 mm
• If the mould will not yield to this extent there will be a risk of cracks
occurring at the critical points (fig.78a).

15. Form Design of Steel Castings
• One method of preventing this is to reinforce the weakest point by
what are known as contraction ribs which can be trimmed off
afterwards if necessary or the critical points may be strengthened by
ribs (fig. 79 b)
• Owing to the large amount of contraction, the size variation also is
larger with cast steel than with cast iron
• Designer must aim for maximum uniformity in the contraction
which occurs (by using uniform wall thicknesses, avoiding
which occurs (by using uniform wall thicknesses, avoiding
concentrations of metal, etc.)
• Castings can often be straightened while in the red hot condition
• Castings can often be straightened while in the red-hot condition.

16. Form Design of Steel Castings

17. Form Design of Steel Castings
• Fettling also is more difficult with cast steel than with
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cast iron.
• The large amount of contraction causes cores to be held
more tightly in the casting so that they are more
difficult to remove
• The designer must allow for this by making the cores
readily accessible and easily removable
readily accessible and easily removable
• Guide lines set for form design of cast iron also apply
to cast steel

18. Form Design of Steel Castings
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Rules
1. Take great care to avoid concentrations of metal.
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2. Use risers generously.
3. Allow sufficient area for risers to stand on.
4. Provide for convenient removal of risers.
5. Avoid constrictions by making proper use of radiusing.
6. Aim at maximum uniformity of wall thicknesses.
7. Always make ribs thinner than walls
8. Prevent hot cracks by providing contraction ribs.
9. Ensure proper core support
10. Ensure easy removability of cores.