When purchasing stainless steel butterfly valves for water treatment, water supply systems, or fluid control projects, the stainless steel grades we are familiar with are typically 304, 316, etc. However, upon arrival at the site, we often find that the valve body nameplate does not display numerical identifiers such as ‘304’ or ‘316,’ but instead shows markings like CF8 or CF8M.
In fact, the term ‘304 stainless steel’ typically only appears on the ‘Product Quality Certification’ or material reports, while the markings such as CF8 or CF8M on the valve nameplate correspond to the casting-grade stainless steel designations. To help clarify the relationship between these two, this article will provide a detailed analysis of the similarities and differences between CF8 and 304, CF8M and 316, as well as their practical application differences in butterfly valves.
Differences Between CF8, CF8M, SS304, and SS316
- CF8: Casting-grade stainless steel, equivalent to 304. Commonly used for casting butterfly valve bodies and covers.
- CF8M: Casting-grade stainless steel equivalent to 316, containing an additional 2-3% molybdenum (Mo) compared to CF8, offering superior corrosion resistance.
- SS304: Rolled or forged stainless steel commonly used in flanges, bolts, pipes, and casting butterfly valve stems and discs.
- SS316: A type of stainless steel used for rolling or forging, containing molybdenum, with better corrosion resistance than 304.
Simple memory: CF8 ≈ 304, CF8M ≈ 316
The main difference between them is whether they are used for casting (CF8, CF8M) or for rolling/forging (304, 316).
What Are the Differences in Chemical Composition?
Material | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Carbon (C) |
SS304 / CF8 | 18–20% | 8–11% | None | ≤ 0.08% |
SS316 / CF8M | 16–18% | 10–14% | 2–3% | ≤ 0.08% |
Mo is key in SS316 / CF8M: it improves resistance to chloride corrosion, such as seawater corrosion.
Recommendations for Matching Materials and Operating Conditions
Material | Corrosion Resistance | Recommended Applications |
304 / CF8 | General corrosion resistance | Potable water systems, HVAC, firefighting, municipal wastewater, and general water treatment |
316 / CF8M | Excellent chloride and seawater resistance | Seawater desalination, marine environments, chemical processing, swimming pools, and high-salinity systems |
For most water systems, 304/CF8 is sufficient; however, if seawater or high-chlorine environments are involved, 316/CF8M is recommended.
Why Do Some Butterfly Valves Use SS316 for the Disc and CF8M for the Body?
This is a very typical engineering combination in butterfly valves at present, for the following reasons:
Component | Material | Reason for Selection |
Body | CF8M (Cast) | Ideal for casting large, complex structures such as valve bodies and flanged ends; cost-effective and easy to produce. |
Disc | SS316 (Forged or Machined) | Offers higher mechanical strength, denser structure, smoother surface finish, and better sealing performance—especially critical for tight shut-off and high-cycle operations. |
Detailed explanation:
- Mechanical property differences: 316 forgings have superior strength and toughness compared to CF8M castings, particularly in applications involving repeated opening and closing cycles and fluid erosion.
- Machining and sealing requirements: The valve disc and seal ring require high precision fit. The 316 surface is smoother and harder, reducing the risk of scratching the sealing surface.
- Cost optimisation: Using 316 forgings for the entire valve is too costly. The CF8M valve body combined with a 316 valve disc achieves the optimal balance between performance and economy.
- In water systems, the CF8M valve body ensures overall corrosion resistance, while the 316 valve disc reduces wear and extends seal life under high flow rates and frequent opening/closing cycles. This is the standard configuration commonly used in high-quality butterfly valves.
Of course, in general, the nameplate or technical documentation on a butterfly valve will not separately specify ‘CF8M for the valve body, SS316 for the valve disc,’ but will uniformly indicate the primary material as CF8M.
Key Recommendations for Material Selection of Water Treatment Butterfly Valves
- Clean Freshwater / Drinking Water (Low Chlorine Environment)
Selecting butterfly valves made of CF8 / 304 material is an economical and reliable choice, sufficient to meet the needs of most residential and municipal water supply systems.
- Chlorinated Water (e.g., disinfected tap water, swimming pool water)
We recommend using CF8M/316 material butterfly valves. Chloride ions can cause pitting corrosion in 304 material, while 316 contains molybdenum (Mo), which significantly enhances resistance to chloride corrosion.
- Slightly saline water / mild seawater / coastal environments
Preferably use CF8M/316 material butterfly valves to better resist corrosion caused by salt and extend service life.
- Industrial wastewater / chemical process water (corrosivity uncertain)
If the water may contain high concentrations of chlorides or other corrosive media, it is recommended to prioritise CF8M/316 material butterfly valves to ensure long-term stability.
Special note on valve stem material
Regardless of whether the valve body is CF8 or CF8M, the valve stem is strongly recommended to be made of SS316 forged stainless steel.
This is because the valve stem, as a moving part, is more prone to wear and directly contacts the medium, thus requiring higher strength and stronger corrosion resistance.
Frequently Asked Questions
The composition is basically the same, but CF8 is a casting, while SS304 is mostly forged or rolled.
The composition is similar, but CF8M is used for castings, while SS316 is used for forgings, with slight differences in performance.
For ordinary municipal water supply, HVAC, fire protection, and non-corrosive water systems, CF8 (304) is sufficient.
However, if used in seawater, saline water, or chemical water, it is strongly recommended to use CF8M (316), otherwise pitting corrosion and leaks may occur.
Because SS316 (and CF8M) contains 2-3% molybdenum (Mo), an element that significantly enhances corrosion resistance, particularly against chloride and seawater corrosion, resulting in a higher price but also greater durability.
Summarize
Properly understanding and selecting the appropriate stainless steel materials—CF8, CF8M, 304, and 316—is the foundation for ensuring the long-term stable operation of butterfly valves. Different operating environments require different material specifications, balancing corrosion resistance with manufacturing processes and cost efficiency. There is no one-size-fits-all solution in engineering selection; only by tailoring solutions to specific operating conditions can the optimal cost-effectiveness be achieved.
As a professional industrial valve manufacturer, TFW Valve offers butterfly valve products with various stainless steel configurations, suitable for complex operating conditions such as municipal water supply, wastewater treatment, desalination, and chemical industries. We not only provide products but also offer material selection and technical support to help you easily tackle various system challenges.
Visit the TFW Valve official website to learn more about selection solutions and customised services.
