1. Introduction

If you are sourcing stainless steel investment castings for valves, pumps, or industrial components, you have almost certainly encountered two of the most common casting grades: CF8 and CF8M.

At first glance, they look similar. Both are austenitic stainless steels used in the as-cast condition. Both offer good corrosion resistance and mechanical properties. Both are widely available from investment casting foundries around the world.

But they are NOT interchangeable. Choosing the wrong grade can lead to premature corrosion failure, unexpected costs, or rejection by the end customer.

This guide will help you understand exactly when to specify CF8, when to upgrade to CF8M, and why the decision matters for your application.

2. What Are CF8 and CF8M?

CF8 and CF8M are designations from ASTM A351 / A351M, the standard specification for austenitic steel castings for pressure-containing parts. These grades are the cast equivalents of wrought stainless steels:

CF8 is the cast equivalent of wrought 304 stainless steel.

CF8M is the cast equivalent of wrought 316 stainless steel.

The key difference: CF8M contains molybdenum (Mo), while CF8 does not. This single element addition has significant implications for corrosion resistance, mechanical properties, and cost.

3. Chemical Composition Comparison

The fundamental difference lies in the chemistry. Here is the actual composition per ASTM A351:

Key observation: CF8M has a lower chromium range (16-18% vs 18-20%) but adds 2-3% molybdenum and a higher nickel content (10-14% vs 8-11%). The molybdenum is what gives CF8M its superior corrosion resistance.

Wrought Equivalents

4. Mechanical Properties

Both CF8 and CF8M meet similar minimum mechanical property requirements per ASTM A351, but there are subtle differences:

Note: CF8 has a slightly higher minimum elongation (35% vs 30%), making it somewhat more ductile. CF8M's molybdenum content can provide slightly better high-temperature strength.

5. Corrosion Resistance: The Critical Difference

This is the single most important factor in the CF8 vs CF8M decision.

The 2-3% molybdenum in CF8M provides significantly enhanced resistance to:

Pitting corrosion in chloride-containing environments

Crevice corrosion

Stress corrosion cracking (SCC) in certain environments

Sulfuric acid and other reducing acids

In practical terms:

CF8 (304) is suitable for fresh water, food processing, and mild chemical environments

CF8M (316) is required for seawater, marine environments, chemical processing, and pharmaceutical applications

Industry reality: When Flowserve, KSB, or Sulzer specify materials for pump casings handling seawater or aggressive chemicals, CF8M (or higher grades like duplex) is almost always the minimum requirement. CF8 is generally reserved for fresh water and general industrial service.

Corrosion Resistance Comparison

6. Typical Applications

CF8 (304) is commonly used for:

Gate valve bodies and bonnets for water and steam service

Check valve components in non-aggressive media

Food machinery components (pump housings, conveyor parts)

Architectural hardware and handrail fittings

General industrial castings where corrosion risk is low

Glass fittings and spigot components

CF8M (316) is specified for:

Chemical pump casings and impellers

Valve bodies handling aggressive or chloride-containing fluids

Marine hardware and seawater system components

Pharmaceutical and bioprocess equipment

Paper and pulp industry components

Heat exchangers and condenser components

Offshore oil and gas instrumentation

Real-world example: A European valve manufacturer sourcing valve bodies for chemical processing plants will typically specify CF8M as the minimum. The same manufacturer producing gate valves for municipal water supply may use CF8. The decision is driven entirely by the end application and the fluid being handled.

7. Weldability and Machinability

Weldability: Both CF8 and CF8M offer good weldability. However, CF8M requires matching filler metal (ER316L) to maintain corrosion resistance in the weld zone. CF8 uses ER308L filler.

Machinability: CF8 is slightly easier to machine due to its lower work-hardening rate. CF8M's molybdenum content increases work-hardening, which can lead to faster tool wear. In practice, the difference is manageable with proper tooling and speeds.

Both grades are routinely machined in our facility using 4-axis and 5-axis CNC machining centers, with no special challenges beyond standard stainless steel machining practices.

8. Cost Comparison

CF8M is more expensive than CF8 due to higher nickel and molybdenum content. The price difference fluctuates with commodity markets, but the general range is:

Premium: CF8M typically costs 25-40% more than CF8 on a per-kg basis, depending on current LME nickel and molybdenum prices.

9. How to Choose: A Practical Decision Framework

When deciding between CF8 and CF8M, ask yourself these three questions in order:

Question 1: What fluid or media will the part handle?

This is the single most important factor.

Fresh water, air, steam, food → CF8 is sufficient

Seawater, chlorides, chemicals, acids → CF8M is the minimum

If unsure, consult the NACE MR0175 / ISO 15156 guidelines for oil and gas, or the relevant API standard

Question 2: What is the operating temperature?

Both grades perform well from cryogenic to 800°F (425°C)

For intermittent service up to 1500°F (815°C), CF8M has a slight edge due to molybdenum

For continuous high-temperature service, consider HK or HP grade heat-resistant steels

Question 3: What is the cost-benefit trade-off?

For a simple valve body in water service, CF8 is the economical choice

For a chemical pump casing, the 25-40% premium for CF8M is insignificant compared to the cost of premature failure

Consider the total cost: CF8M typically adds 5-15% to the total part cost, but failure in service can cost 100x that

Decision Matrix:

10. Frequently Asked Questions

Q: Can I substitute CF8M for CF8?

A: Yes, but it is usually over-specification. CF8M costs 25-40% more. However, if your supply chain prefers to standardize on CF8M to reduce complexity, there is no technical issue using CF8M where CF8 was specified.

Q: Can I substitute CF8 for CF8M?

A: Generally no, especially in corrosive environments. The lack of molybdenum in CF8 makes it vulnerable to pitting and crevice corrosion in chloride-containing environments. Always check with your engineering team before downgrading.

Q: What is the difference between CF8M and 316?

A: CF8M is the cast equivalent of wrought 316. The chemical compositions are very similar, but CF8M (as a casting) may have slightly different mechanical properties due to the cast microstructure. ASTM A351 CF8M and ASTM A276 316 are not directly interchangeable without design review.

Q: What NDT is recommended for CF8 and CF8M castings?

A: Standard NDT includes visual inspection, dimensional inspection (CMM), and pressure testing. For critical applications, dye penetrant (PT) for surface defects and radiographic (RT) or ultrasonic (UT) for internal defects are recommended. As an IATF 16949 certified foundry, we can support all levels of NDT per your specifications.

Q: Can CF8M be used for cryogenic service?

A: Yes. Both CF8 and CF8M retain good impact toughness at cryogenic temperatures. CF8 is commonly used for LCB (low carbon) valve body applications down to -46°C. CF8M performs similarly.

Q: What is the PREN (Pitting Resistance Equivalent Number)?

A: PREN = Cr% + 3.3 × Mo% + 16 × N%. For CF8: approximately 19. For CF8M: approximately 24-26. The higher PREN of CF8M confirms its superior pitting resistance, which is why Flowserve and KSB specify CF8M for seawater and chemical pump applications.

Need Help Choosing the Right Grade?

Selecting the correct material grade for your investment casting project can be complex. Our engineering team offers free DFM and material selection support to help you make the right decision.

Send your drawing and specifications to: sales@wdshunye.com

What you will receive within 24 hours:

Material grade recommendation based on your application

Free DFM analysis

Factory-direct quotation

Technical data sheet for your specified grade

WUDI COUNTY SHUN YE STAINLESS STEEL PRODUCTS CO., LTD.

A direct manufacturer with 16 years of export experience, IATF 16949 certification, and partnerships with OEM buyers across Europe, America, and Asia.

Phone: +86-17754343500 | Email: sales@wdshunye.com | Website: https://www.shunyecasting.com