If you are about to choose the appropriate stainless steel material for your project, you must distinguish the differences among the stainless steels. CF8 and CF8M are two of the most commonly used materials in the industry due to their exceptional performance. While similar, they are distinct from one another. This article provides a comprehensive overview of the differences between CF8 and CF8M in terms of composition, properties, and application areas, while summarizing their respective suitable scenarios to help you make an informed choice for your next project.
What is CF8
CF8 is an austenitic stainless steel, also known as 18/8 stainless steel due to its composition of 18% chromium and 8% nickel. It is renowned for its exceptional versatility, offering excellent strength, corrosion resistance, ductility, weldability, and machinability. CF8 is widely used in the foundry industry and can be found in numerous applications. Post-processing heat treatment helps restore its optimal corrosion resistance.
What is CF8M
CF8M, like CF8, is a casting grade of austenitic stainless steel. Its primary composition is similar to CF8 stainless steel, but the addition of molybdenum gives it superior corrosion resistance, making it an upgraded version of CF8 stainless steel. CF8M exhibits excellent strength and ductility, and is easy to machine and weld, making it highly suitable for applications requiring both toughness and formability. When welding CF8M, note that post-weld annealing is essential to restore its optimal corrosion resistance.
CF8 vs CF8M, Composition
Except for the three special elements molybdenum, nickel, and chromium, the composition of CF8 and CF8M differs only slightly. The specific composition ratios are as follows.
| Element (%) | CF8 | CF8M |
| Carbon (C) | ≤ 0.08% | |
| Chromium (Cr) | 18% – 21% | |
| Iron (Fe) | 63.8% – 74% | 60.3% – 71% |
| Manganese (Mn) | ≤ 1.5% | |
| Molybdenum (Mo) | – | 2.0% – 3.0% |
| Nickel (Ni) | 8.0% – 11% | 9.0% – 12% |
| Phosphorus (P) | ≤ 0.04% | |
| Silicon (Si) | ≤ 2.0% | |
| Sulfur (S) | ≤ 0.04% | |
CF8 vs CF8M, Corrosion Resistance
Both CF8 and CF8M exhibit excellent corrosion resistance. However, CF8M demonstrates superior corrosion resistance across all aspects due to the addition of molybdenum. CF8 stainless steel offers good resistance to ordinary atmospheric conditions and freshwater environments, withstanding corrosion from weak acids and alkalis. Yet it remains susceptible to pitting corrosion in chloride-containing environments such as seawater or salt spray. Molybdenum forms a denser oxide film on the surface of CF8M stainless steel, not only providing superior resistance to chloride pitting and crevice corrosion but also offering enhanced protection against stress corrosion cracking (SCC).
CF8 vs CF8M, Heat Resistance
Both CF8 and CF8M stainless steels can be used at high temperatures without losing their properties, though their performance differs slightly. Both can operate in intermittent service at temperatures between 800°C and 870°C, but CF8M exhibits superior creep resistance at high temperatures compared to CF8’s deformation resistance. However, when operating continuously between 425°C and 815°C, both materials require attention to the risk of intergranular corrosion caused by carbide precipitation. If you intend to use the material immediately after welding (without annealing), consider using a low-carbon version (CF3 or CF3M).
CF8 vs CF8M, Mechanical Property
CF8 and CF8M both belong to the austenitic stainless steel category. While their mechanical strengths are similar, they exhibit differences in ductility and other properties. Below is a detailed comparison:
Tensile Strength
Both exhibit comparable tensile strength, capable of withstanding significant stress before permanent deformation occurs. This makes them suitable for components operating under high-stress conditions. However, in actual measurements, CF8M stainless steel typically demonstrates slightly higher tensile strength than CF8.
Yield Strength
CF8M exhibits a slightly higher yield strength than CF8. Under identical pressure conditions, CF8 is more prone to deformation. Consequently, CF8M is the ideal casting material for high-pressure working environments.
Hardness
CF8 stainless steel generally possesses higher hardness, making it commonly used in components subject to friction. While CF8M’s hardness is lower than CF8’s, it remains sufficient for most corrosive environments.
Ductility (Elongation)
CF8 exhibits a fracture elongation of approximately 55%, offering excellent ductility that enables it to withstand significant external impacts—a critical property for withstanding fluctuating environments. CF8M possesses relatively weaker ductility, but its outstanding corrosion resistance often proves more vital in harsh conditions.
Fatigue Strength
The addition of molybdenum enhances the matrix strength of CF8M, thereby improving its fatigue resistance. This makes it an ideal casting material for components subjected to cyclic loading. In contrast, CF8 exhibits slightly inferior fatigue strength and is unsuitable for parts requiring repeated stress cycles.
| CF8 | CF8M | |
| Tensile Strength | ≥ 485 MPa (70 ksi) | ≥ 485 MPa (70 ksi) |
| Yield Strength | ~ 260 MPa | ~ 290 MPa |
| Hardness | ~ 140 HB | ~ 170 HB |
| Elongation | ~ 55% | ~ 50% |
| Fatigue Strength | Fair | Strong |
CF8 vs CF8M, Weldability
CF8 exhibits excellent weldability and ranks among the most weldable austenitic stainless steels. In comparison, CF8M’s weldability is slightly less favorable, though it remains quite good. Due to its molybdenum content, CF8M stainless steel has a relatively higher susceptibility to hot cracking during welding. However, this concern is fully controllable under proper welding procedures. It is important to note that if you do not plan to perform solution treatment on the material post-welding, or if it is not feasible, but your product will operate in a corrosive environment, it is recommended to prioritize CF3 or CF3M stainless steel (with lower carbon content) to prevent weld rusting.
CF8 vs CF8M, Machinability
Compared to CF8, machining CF8M is relatively more challenging due to its slightly higher hardness and more pronounced work-hardening tendency, making it approximately 10%-15% more difficult to machine than CF8. Therefore, cutting speeds should be appropriately reduced during machining to prevent accelerated tool wear.
CF8 vs CF8M, Magnetic Permeability
Many people mistakenly believe that stainless steel is completely non-magnetic. However, during the casting process, to prevent hot cracks, we typically control its composition to retain a small amount of “ferrite.” Therefore, CF8 castings usually exhibit weak magnetism. CF8M, due to the presence of molybdenum and its specific composition ratio, is typically near non-magnetic. If your project requires completely non-magnetic products, specialized solution treatment or composition adjustments will be necessary.
CF8 vs CF8M, Polishing
Stainless steel generally possesses a smooth and bright appearance. However, when compared, CF8 is more easily polished to achieve a mirror finish with a brighter, whiter luster. CF8M exhibits a slightly duller sheen after polishing, though the difference is barely noticeable to the naked eye.
CF8 vs CF8M, Cost
Due to the inclusion of expensive molybdenum and a higher nickel content in CF8M, its raw material costs are undoubtedly higher than those of CF8. The specific premium price depends on the prevailing market fluctuations of molybdenum, iron, and nickel plates, typically ranging between 15% and 25% above the CF8 price. Additionally, because CF8M is more challenging to process than CF8, its processing costs (such as tooling) are also slightly higher than those for CF8 stainless steel.
CF8 vs CF8M, Application
CF8 Application
Food Processing: Components for food processing equipment that come into contact with ingredients not containing high salt or acidic substances.
Construction: Building hardware, bearings, brackets, joints, etc.
Machinery Parts: Flanges, bearings, rollers, sleeves, bushings, etc.
Water Treatment Equipment: Components for freshwater treatment equipment, etc.
Pumps and Valves: Pump and valve assemblies in industrial equipment, transporting media including water, gas, oil, etc.
CF8M Application
Marine Vessels: Marine hardware, pumps, valves, propellers, anchors, etc.
Petrochemical Industry: Pumps, valves, impellers, transport pipelines, heat exchangers, etc.
Food Processing: Agitators, components in direct contact with food ingredients, etc.
Medical & Pharmaceutical: Pharmaceutical equipment, medical devices, surgical implants, etc.
Automotive: Brackets, housings, engine components, exhaust system parts, etc.
When to Choose CF8 for Your Project
Cost-Sensitive: If your project has a limited budget and the target operating environment does not demand high corrosion resistance from the material, CF8 is the better choice.
Mild Environment (No Strong Corrosive Media): If your product operates solely in freshwater, atmospheric, or weakly acidic/alkaline environments, CF8 will fully meet your requirements.
Cryogenic Conditions: If your project requires the product to maintain material toughness without embrittlement at extremely low temperatures, CF8 performs better than CF8M.
Non-Extreme High Temperatures: For non-prolonged exposure to extreme heat, CF8 is fully capable.
Complex Machining Requirements: If your product requires extensive cutting, drilling, or intricate welding processes, CF8 offers greater time and labor savings compared to CF8M.
Balanced Performance: CF8’s equilibrium of strength, ductility, and corrosion resistance makes it an ideal choice for projects without specialized performance demands.
When to Choose CF8M for Your Project
Chloride Exposure: For harsh chloride environments like seawater and salt spray, CF8M is the optimal choice (effectively resisting pitting and crevice corrosion).
Chemical and Acidic Media: For systems requiring contact with sulfuric acid, phosphoric acid, acetic acid, or chlorine-based bleaches, CF8M’s long-term reliability in these environments is critical.
Sustained High-Temperature Environments: When components operate under prolonged extreme heat and external stress, CF8M’s outstanding creep resistance provides a solution.
Long-Term Cost Efficiency: While CF8M carries a higher upfront cost, its lower maintenance expenses and extended service life result in more favorable total lifecycle costs over time.
Conclusion
Overall, if your project does not have particularly high material performance requirements and operates in a relatively mild environment, CF8 stainless steel is a more cost-effective option. However, if your target operating conditions are extremely harsh, involving high temperatures or strong corrosion, CF8M stainless steel will be your optimal solution.
For professional casting services, Pingheng Machinery is your trusted partner. We possess extensive experience in investment casting of both CF8 and CF8M stainless steel, offering comprehensive technical support. At Pingheng, you can customize your castings to exact specifications. We deliver your orders on time with tight tolerances and exceptional surface finishes.
FAQ
What is the difference between CF8 and CF8M compared to the commonly mentioned 304 and 316 stainless steels?
304/316 typically refers to forged or rolled materials (such as plates, bars) that comply with AISI standards. CF8/CF8M, however, refers to cast materials that comply with the ASTM A351 standard. Their chemical compositions are very similar, but CF8/CF8M undergoes slight adjustments (e.g., increased silicon content for improved fluidity) to accommodate the casting process. However, both grades are equivalent in corrosion resistance and mechanical properties.
What is the relationship between CF3 and CF3M stainless steel and CF8 and CF8M stainless steel?
CF3 stainless steel is the low-carbon version of CF8 stainless steel, corresponding to AISI 304L stainless steel; CF3M stainless steel is the low-carbon version of CF8M, corresponding to AISI 316L stainless steel. Due to their lower carbon content, CF3 and CF3M stainless steels exhibit superior resistance to intergranular corrosion and better weldability.
When must I choose CF8M over CF8?
If your project involves marine environments, high chloride ion concentrations, or strong acidic chemical media, CF8M must be selected. CF8M’s resistance to pitting and crevice corrosion ensures the long-term reliability of your components.
Why is CF8M a mandatory requirement for non-standard components in the pharmaceutical industry?
In the pharmaceutical sector, manufacturing equipment frequently undergoes CIP (Clean-in-Place) and SIP (Sterilization-in-Place) processes, necessitating the use of potent (chlorine-containing) cleaning agents. Under repeated exposure to such cleaning cycles, CF8 is prone to pitting corrosion, which may lead to bacterial growth or contamination of pharmaceuticals with excessive metal ions. Consequently, CF8M has become the compliance standard within the pharmaceutical industry.
When comparing CF8 and CF8M materials, which exhibits superior fluidity during investment casting and is more suitable for casting thin-walled or complex non-standard parts?
In reality, both CF8 and CF8M demonstrate adequate formability for complex thin-walled components in investment casting. When deciding between the two, the ultimate choice should be influenced by your project’s specific performance requirements for the parts.
