Material composition and microstructure:
Cast iron: The main components are iron, carbon (usually>2%), and silicon. Carbon mainly exists in the form of flake graphite (gray cast iron) or spherical graphite (ductile iron). The presence of flake graphite reduces the strength and toughness of the material, making it brittle.
Cast steel: The main components are also iron and carbon, but the carbon content is significantly lower than cast iron (usually around 0.1% -0.5%). Carbon mainly exists in the form of compounds (iron carbide) in the ferrite or pearlite matrix. This gives it higher strength and toughness. Cast steel can also achieve special properties by adding other alloying elements such as chromium, nickel, molybdenum, etc.
Strength and toughness (the most crucial difference):
Cast iron:
Low tensile strength: Grey cast iron is particularly low, while ductile iron has a strength close to the lower limit of cast steel.
Poor toughness: This is the main drawback of cast iron. Gray cast iron is highly sensitive to impact loads, with high brittleness, and is prone to brittle fracture (without plastic deformation) when subjected to impact, bending, or sudden temperature changes. Ductile iron has much better toughness than gray cast iron, but it is still inferior to cast steel.
Relatively high compressive strength (close to or better than cast steel): This makes cast iron perform better under compression (but valves usually bear internal pressure, mainly tensile stress).
Cast steel:
High tensile strength: significantly higher than gray cast iron, usually also higher than the average value of ductile iron.
Good toughness: It has good plasticity and impact resistance, can withstand certain deformation and impact loads without breaking, and usually undergoes plastic deformation before fracture. This makes cast steel valves safer and more reliable in stress concentration, vibration, or impact environments.

Pressure resistance (pressure bearing capacity):
Cast iron: Due to its lower tensile strength and brittleness, it has lower pressure bearing capacity. Gray cast iron valves are typically only used for low-pressure applications (such as PN10, PN16). The pressure bearing capacity of ductile iron valves is higher than that of gray cast iron, and can reach medium pressure (such as PN25, PN40 or even higher).
Cast steel: Due to its high strength and toughness, it has a high pressure bearing capacity. Widely used in medium pressure, high pressure, and ultra-high pressure conditions. The pressure rating range is much wider than that of cast iron valves.
Corrosion resistance:
Cast iron: Gray cast iron has poor corrosion resistance in strong acids, certain salt solutions, and oxidizing environments. It has certain corrosion resistance in alkaline environment, weak acid, atmosphere, water and steam (especially after forming protective oxide film). The corrosion resistance of ductile iron is similar to that of gray cast iron.
Cast steel: The corrosion resistance of ordinary carbon steel cast steel is comparable to or slightly inferior to that of gray cast iron, especially prone to rusting in fresh water and steam. The advantage lies in the ability to obtain excellent corrosion resistance through alloying: Austenitic stainless steels (such as CF8/304, CF8M/316), duplex stainless steels, alloy steels, etc. formed by adding elements such as chromium, nickel, molybdenum, etc., can resist harsh corrosive environments such as strong acids, strong alkalis, various salt solutions, and high-temperature oxidation. This is incomparable to cast iron valves.

Temperature resistance performance:
Cast iron: significantly brittle at low temperatures, not suitable for low temperature applications (usually>0 ° C, ductile iron has a slightly lower lower lower limit). At high temperatures, the strength and oxidation resistance are limited. Gray cast iron is typically used for a long time at temperatures below 300 ° C, while ductile iron is slightly higher. Under high temperature, the morphology of graphite will change and its performance will decrease.
Cast steel: suitable for a wide temperature range.
Low temperature: Low carbon steel cast steel can be used for low-temperature conditions after appropriate treatment (such as LCB can reach -46 ° C, LCC can reach lower temperatures).
High temperature: Carbon steel cast steel (such as WCB) can be used for a long time at 425 ° C or even higher (depending on the grade and standard). Alloy steel cast steel (such as WC6, WC9, C5, C12) and stainless steel cast steel (such as CF8, CF8M) can withstand higher temperatures (500 ° C, 600 ° C or even higher).
Cost and processability:
Cast iron:
Low cost: The raw material cost is low, the casting fluidity is good, the shrinkage rate is small, the production process is relatively simple and mature, and the manufacturing cost is significantly lower than that of cast steel.
The machinability is acceptable, but graphite can easily cause tool wear when cutting gray cast iron.
Cast steel:
High cost: The raw material cost is high, the casting fluidity is poor, the shrinkage rate is large, and defects are prone to occur. The production process requirements are high (such as heat treatment having a huge impact on performance), and the manufacturing cost is much higher than that of cast iron.
The machinability is average, and the high strength requires greater power for cutting.