What are the main differences in microstructure between cast steel and stainless steel control valve castings

Material Basics of Control Valve Castings
Control valve castings play a critical role in regulating and controlling industrial piping systems, and their performance directly impacts system safety and stability. Common valve body materials are primarily categorized as cast steel and stainless steel. Cast steel control valve castings are widely used in high-pressure, high-intensity applications due to their excellent mechanical properties and cost-effectiveness. Stainless steel control valve castings, with their superior corrosion resistance and toughness, are often used in applications involving highly corrosive media and requiring high cleanliness. The differences in their microstructures directly determine their performance and application areas.

Microstructural Characteristics of Cast Steel Control Valve Castings
Cast steel is an iron-based alloy with a carbon content typically between 0.2% and 0.8%. The microstructure of cast steel control valve castings primarily consists of ferrite, pearlite, bainite, and martensite. Ferrite and pearlite: Ferrite and pearlite are the most common microstructure combinations in low- to medium-carbon cast steels. Ferrite provides good toughness and ductility, while pearlite enhances strength and wear resistance.
Bainite: Proper heat treatment can produce a bainite structure, resulting in castings with both high strength and good toughness, making them suitable for use in high-pressure control valves.
Martensite: In the quenched state, cast steel may form a martensite structure. While this significantly increases strength, it also reduces toughness, requiring tempering.
This mixed structure, primarily ferrite and pearlite, gives cast steel control valve castings excellent overall mechanical properties under impact loads and high pressures. However, due to a lack of sufficient alloying elements, corrosion resistance is relatively limited, requiring external coatings or lining materials to enhance corrosion resistance.

Microstructural Characteristics of Stainless Steel Control Valve Castings
The microstructure of stainless steel control valve castings exhibits diverse characteristics depending on their alloy composition. They are primarily categorized as austenitic, ferritic, martensitic, and duplex stainless steels.

Austenitic stainless steel: Microscopically, it exhibits a uniform austenitic structure with a low carbon content and a high proportion of alloying elements such as chromium and nickel. It possesses excellent corrosion resistance and toughness. It is commonly used in control valve castings for the food, pharmaceutical, and chemical industries.

Ferritic stainless steel: Microstructure is primarily ferrite, with an extremely low carbon content and a high chromium content. Its corrosion resistance is superior to that of ordinary carbon steel, but its toughness is inferior to that of austenitic stainless steel. It is suitable for use in moderately corrosive media.

Martensitic stainless steel: Microstructured after heat treatment, it exhibits martensite, exhibiting high hardness and strength, but relatively low corrosion resistance and toughness. It is typically used for valve body components requiring high strength and a certain degree of corrosion resistance. Duplex stainless steel: Its microstructure is composed of austenite and ferrite, typically in a ratio between 40% and 60%. It combines high strength with excellent corrosion resistance, making it particularly suitable for highly corrosive environments such as seawater and those containing chloride ions.

Stainless steel control valve castings, due to their high content of alloying elements such as chromium, nickel, and molybdenum, form a stable passive film within their microstructure, resulting in excellent corrosion and pitting resistance. This is their core advantage in applications under specialized conditions.

Performance Comparison Due to Microstructural Differences
The differences in the microstructures of cast steel and stainless steel control valve castings directly determine their performance characteristics:
Mechanical Properties: Cast steel excels in high-temperature strength and pressure-bearing capacity. Its microstructure, after appropriate heat treatment, achieves an excellent balance of strength and toughness. Stainless steel, while slightly weaker at high temperatures, possesses excellent low-temperature toughness and crack resistance.
Corrosion Resistance: Cast steel lacks a stable passive structure within its microstructure, making it susceptible to oxidation and corrosion. The austenite, duplex structure, and alloying elements in stainless steel's microstructure maintain its stability in acidic, alkaline, and chloride environments.

Regarding structural stability, cast steel structures are susceptible to pearlite spheroidization or grain growth under prolonged high temperatures, resulting in performance degradation. While the austenite structure of stainless steel is relatively stable within a certain temperature range, intergranular corrosion or σ phase precipitation may occur under certain conditions.