A carbon steel valve body (for ball valves) be more suitable or equal to a ductile iron body for a glycol-water based coolant system for cooling a large diesel engine?
Both the carbon steel and ductile iron could be used. Under equal circumstances the best (material-wise) should be used, which is the nodular cast iron.
The question is actually if you have any other parameters present in this case to be considered, creating 'un-equal' circumstances, giving preference to one or the other, or perhaps even better materials or products.
Other parameters on the most obvious side could be price, demand (short time, one off replacement up to fabrication and use of larger numbers over a longer time periode), general fabrication quality of the two valves compared, inner surface and surface quality, possible discounts and dependability and quality of supplier etc.The carbon steel body is better (and cost a lot more) than nodular cast iron.  

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