The choice of valve material is crucial to its performance, applicable scenarios, and service life. The following details the differences between cast iron, carbon steel, and cast steel based on material properties, performance differences, and application scenarios:
Material Property Comparison
Cast iron has a carbon content of 2%-4%, a high silicon content, and many impurities. It is directly cast after melting, making the process simple. It is brittle, wear-resistant, and has high compressive strength, but has low tensile strength, poor shock and vibration resistance, and average corrosion resistance.
Carbon steel has a carbon content of 0.02%-2.11%, primarily composed of iron and carbon. It is formed from ingots through pressure processes such as rolling and forging. It offers good overall performance, with a balance of strength, ductility, and toughness, good weldability and machinability, but poor corrosion resistance.
Cast steel has a carbon content of 0.15%-0.6%, has few impurities, is cast from molten steel, and requires heat treatment to improve properties. It has high strength, good toughness, and impact resistance. Alloying (such as adding chromium or nickel) can enhance corrosion and high-temperature resistance.
Key Performance Differences
1. Mechanical Properties
Strength and Toughness
Cast Iron: High compressive strength (e.g., gray cast iron has a compressive strength of 200-400 MPa), but low tensile strength (only 100-250 MPa), poor toughness, and prone to fracture under impact.
Carbon Steel: Tensile strength increases with increasing carbon content (e.g., Q235 steel has a tensile strength of approximately 370-500 MPa, while 45 steel can reach over 600 MPa). It also has good plasticity and toughness, and can withstand some deformation.
Cast Steel: Tensile strength is typically 450-800 MPa, and its toughness is significantly superior to cast iron (e.g., ZG230-450 cast steel has an elongation of up to 22%), making it suitable for bearing dynamic loads.
Wear Resistance
Cast iron (especially wear-resistant cast iron) contains graphite, which has a low coefficient of friction and superior wear resistance to ordinary carbon steel and cast steel. It is often used in applications requiring friction reduction (e.g., valve body sealing surfaces).
2. Corrosion Resistance
Cast Iron: Graphite in the matrix easily forms microcells, resulting in average corrosion resistance. Surface coatings (such as paint or galvanizing) or corrosion-resistant cast iron (such as high-silicon cast iron) are required.
Carbon Steel: Rusts easily and corrodes rapidly in humid, acidic, or alkaline environments. Additional corrosion protection treatments (such as electroplating or epoxy coating) are required.
Cast Steel: Ordinary cast steel has similar corrosion resistance to carbon steel, but stainless steel cast steel (such as CF8 and CF3M) can be created by adding alloying elements (such as Cr and Ni) to significantly improve corrosion resistance.
3. High-Temperature Resistance
Cast Iron: Gray cast iron has poor heat resistance (long-term operating temperature ≤ 300°C). Ductile iron can have slightly higher heat resistance (≤ 400°C), but it is susceptible to oxidation at high temperatures.
Carbon Steel: Ordinary carbon steel has a long-term operating temperature ≤ 450°C. Its strength decreases significantly at high temperatures, so heat-resistant steel (such as 15CrMo) is required.
Cast steel: Through alloying, it can withstand high temperatures (e.g., ZG1Cr18Ni9Ti stainless steel cast steel can withstand temperatures exceeding 800°C), making it suitable for high-temperature media (e.g., steam and hot oil).
4. Processability
Cast iron: It offers excellent casting properties and good fluidity, allowing for complex molding (e.g., valve bodies with heat sinks). However, it has poor machinability (hardness and brittleness) and weldability (prone to cracking).
Carbon steel: It offers excellent forging, welding, and machinability, making it easy to form into various shapes (e.g., flanges and threaded connections), making it suitable for applications requiring frequent assembly and disassembly.
Cast steel: The casting process is more complex than cast iron (requiring controlled casting temperature and cooling rate), but its properties can be improved through heat treatment (e.g., normalizing and tempering), resulting in superior weldability compared to cast iron.
Application Comparison
1. Cast Iron Valves
Application Scenario:
Low-pressure, static, or small-diameter pipelines (e.g., DN ≤ 300mm), with non-corrosive or mildly corrosive fluids such as water, steam, and oil.
Typical valves: Gate valves, globe valves, and check valves (commonly used in water supply and drainage, and heating systems).
Advantages: Low cost, easy casting, and good wear resistance.
Disadvantages: Cannot be used in high-pressure, impact, or corrosive environments.
2. Carbon Steel Valves
Applicable Applications:
Medium- and high-pressure pipelines (e.g., PN ≤ 16 MPa), media such as air, steam, oil, and gas, that must withstand certain pressures and vibrations (e.g., industrial pipelines and thermal systems).
Typical valves: Ball valves, butterfly valves, and safety valves (e.g., valves made of Q235 and 20# steel).
Advantages: High strength, balanced overall performance, and moderate processing costs.
Disadvantages: Inadequate corrosion resistance, requiring regular corrosion protection.
3. Cast Steel Valves
Applicable Applications:
High-pressure, high-temperature, high-impact, or complex operating conditions (e.g., PN ≥ 25 MPa, temperature ≥ 450°C), or applications requiring corrosion resistance (e.g., stainless steel and cast steel).
Typical valves include power plant valves (such as gate valves and globe valves), chemical valves (such as stainless steel ball valves), and offshore engineering valves.
Advantages: High strength, good toughness, and adaptability to extreme environments through alloying.
Disadvantages: High cost and complex casting process.
Summary and Selection Recommendations
Cast Iron: Low Cost, Low Pressure: Inexpensive, with excellent casting properties, it meets basic sealing and pressure-bearing requirements.
Carbon Steel: Suitable for medium- and high-pressure applications, conventional media. It offers a balance of strength and toughness, is easy to process, and offers a high cost-effectiveness.
Cast Steel (Alloy): Suitable for high pressure, high temperature, and corrosion applications. Alloying can improve strength, high-temperature resistance, and corrosion resistance, making it suitable for complex operating conditions.