Cast Iron - temeperature limits 2

[kclim]

Two Questions:
1. What factor(s) limit the use of cast iron in higher temperature applications? Is it the debonding of the graphite phase from the matrix?
2. Is there any difference in the max servicable temperature for gray iron vs ductile iron? If so, why?

Questions are regarding use of cast iron cylinders in turbine casings.

Thanks

 

[metengr]

Cast iron for use in large steam turbine casings is normally on the back end (exhaust) of low pressure steam turbines where steam exit temperatures are less than 250 deg F.

Information on elevated temperature strength is shown below

http://www.ductile.org/didata/Section3/3part1.htm#High Temperature Tensile Properties

In high pressure and intermediate pressure steam turbine casings the service temperatures are such that creep strength and creep deformation are the main performance drivers, followed by weldability and thermal fatigue performance (unit cycling). I have seen cast iron exhaust cylinders however these are not in a critical application where you have rotating equipment. Frankly speaking, you have better material selection for turbine casings that using modified low alloy steel castings and wrought products which have been around for many years with proven performance.

 

[tbuelna]

kclim,

Turbocharger turbine housings commonly use a type of ductile cast iron called Ni-Resist.

http://www.ductile.org/didata/Section5/5intro.htm#AUSTENITIC DUCTILE IRONS

Terry