Field of the Invention
The present invention relates to a spheroidal graphite cast iron alloy.
Description of the Related Art
In the state of the art, gear rims are known which for example are used for transmitting a drive torque to a milling machine. These rims are in spheroidal graphite cast iron or in steel.
In the state of the art, spheroidal graphite cast iron gear rims are calculated either according to the AGMA 6014 (6114 respectively) standard or according to the ISO 6336 standard.
According to the ISO 6336 standard, the maximum admissible stresses are given according to the curves of part 5 of this same standard, curves of σHlim (pressure stress) and σFlim (root flexural stress of the gear tooth), versus hardnesses. The higher the hardness, the higher are the maximum admissible stresses and therefore the larger is the power which may be transmitted by the gear rim.
In present curves from ISO 6336, the hardness range extends up to 300HB, the produced grades are according to the EN 1563 standard—spheroidal graphite cast iron grades—in which grades with a tempered ferritic, pearlitic and martensitic matrix are only taken into consideration.
For calculations according to the AGMA 6014 (6114 respectively), references are made to the material standards ASTM A536 and ISO 1083. The curves giving admissible stresses versus hardness are given up to about 340HB. But for high hardnesses, there are no corresponding grades in the standards.
The present cast iron grades give the possibility of obtaining at best hardnesses of 320HB on gear rims. For very large powers, they reach their limit of use and presently the only solution is to change the material by passing to cast steel. The 320HB hardnesses of present cast irons are obtained by quenching followed by tempering.
There also exist grades according to EN 1564—spheroidal graphite cast iron grades obtained by staged quenching, so-called ADI cast irons—for which the values of σHlim and σFlim are also defined depending on hardness intervals. Staged quenching is achieved in a bath of salts. In order to produce gear rims, it will be necessary to be equipped with pans of large dimensions.