Choosing the Right Stainless Steel for CNC Milling
This blog covers 9 key grades of stainless steel, including 303, 304, 316, 416, 440C, SAF 2205, and 17-4 PH, and explains their qualities, applications, and suitability for precision CNC milling parts.
In this article we want to explore the major categories and terminology related to nine common varieties of stainless steel used when custom CNC milling production parts.
If you’re an engineer, designer, or buyer in product custom manufactuing industry, this article will help you avoid costly mistakes and choose the right grade for your next project. You’ll learn how alloy selection affects tool wear, corrosion resistance, weldability, and cost.
First, let’s clarify what stainless steel is.
What’s the CNC machining materials Difference Between Mild and Stainless Steel?
Mild, or “carbon”, steel contains iron as the major alloying element, along with smaller trace amounts of carbon. Stainless steel, on the other hand, uses 10% or more chromium, along with nickel, molybdenum, and other elements.
Between the two, mild steel is easier to weld, form, and machine. It’s usually less expensive but it also rusts easily when exposed to air or water. While excellent for heavy industrial applications where toughness is a priority, it’s not especially attractive and doesn’t have as nice a surface finish.
In stainless steel, chromium helps to form a layer of chromium oxide on the material surface, preventing further oxidation (rust), although not all grades are equal in this regard. Also be aware that chromium and other alloying elements are very hard, and this can adversely affect machinability and weldability.
Is Stainless Steel Stronger Than Mild Steel?
Because there are many kinds of both mild and stainless steel, it’s not universally true that one type is stronger than another. For example, there are some heat-treated high carbon superalloys that are much stronger than 17-4 PH, which is a high grade of stainless steel. But in general, the alloying elements of stainless steel give it the strength advantage.
From a manufacturer’s point of view, what’s most important is how difficult the material is to fabricate, and that’s why they reference the machinability index.
H2: What is a Machinability Index?
The machinability index is a subjective measurement that CNC milling service providers and machinists use to determine how easy a material is to machine, relative to carbon steel AISI 1212, which is rated at 100%.
Lower numbers are more difficult, so a value of 20% or less represents a real challenge when manufacturing complex custom parts.
The machinability index is important in several ways. If a metal has a low index number, the manufacturer must use expensive cutters that can wear out or break quickly. It requires very controlled cutting conditions during processing, and that might lead to a higher scrap rate and longer lead times.
In addition, stainless steel tends to overheat quickly, which can damage both the material and the cutting tools. To control this, some stainless steel grades are engineered for improved machinability, often by adding elements like sulfur which helps to break the metal chips.
Now let’s look at the two main series of stainless steel.
What Are The Two Main Types of Stainless Steels?
Stainless steel can be divided into two main groups or series: Series 3XX (austenitic) and Series 4XX (martensitic). They are classified by their molecular structure which affects their performance significantly.
Austenitic stainless steel, or Series 3XX, is distinguished by a high nickel content, with some additional manganese. It’s usually non-magnetic, has medium hardness, and is generally more difficult to machine.
On the positive side, it’s weldable and has excellent corrosion resistance. Austenitic steels exhibit excellent toughness and improved tensile strength at temperatures below zero.
Martensitic stainless steel, on the other hand, has less nickel and more chromium and carbon. This combination means they can be heat treated to increase their hardness a great deal. It’s magnetic and easier to machine, but offers slightly less resistance to water and other chemicals.
Remember that these are generalizations only. Chemists and engineers are always tweaking their alloying formulas to achieve the most desirable characteristics with the fewest trade-offs.
The importance of material testing
Within these categories there may be minor fluctuations in metallurgy from one supplier to another. In addition, there are many international classification systems that have slightly different parameters for material composition.
Reputable CNC machining companies will therefore always positively identify all stainless steels before they are used to make high tolerance CNC milled parts.
This is necessary to ensure they apply the right CNC machining parameters to achieve the highest quality part. More importantly, many critical applications in aerospace, nuclear energy, or chemical processing absolutely must meet their regulatory and safety requirements. Any compromise on materials could spell disaster.
A Closer Look at Series 300 Stainless Steel
300 series stainless steels are the most common grade used for a wide variety of consumer and industrial parts. They have the right balance of corrosion protection, strength, appearance, and affordability. Here are the basic characteristics of 300 series stainless steels that product developers should consider for CNC milling projects.
SS 303
With a machinability index of 70%, SS 303 is considered a free machining stainless steel because of the addition of sulfur. However, sulfur also makes it difficult to weld and form.
Because of its machinability, SS 303 is good for high-volume production of general purpose hardware such as fittings, valve bodies, connectors, bushings, and brackets. However, it shouldn’t be used in marine or other corrosive environments.
Properties of SS 303
- Density (g/cm3): 7.85
- Hardness (Brinell): 170~190
- Tensile Strength (MPa): 620
- Weldability: Poor
- Machinability (%): 70
When to use SS 303
Choose stainless steel 303 when:
- Machinability is a priority, especially for large production runs.
- High aesthetic finish is desirable without extensive post-processing.
- Welding is not required in the final assembly.
- Moderate corrosion resistance is acceptable in dry or low-aggression environment
SS 304
Stainless steel 304 contains little or no sulfur, which make it much harder to machine but improves its corrosion resistance. Another feature of 304 is that it work hardens. That means the surface reacts to the pressure of the cutting tool, causing heat build-up and excessive tool wear.
To get good results when CNC milling requires flood coolant, using very hard carbide cutters that attack the workpiece slowly. For this reason, many specialty cutting tools also have proprietary coatings that improve their heat resistance and durability.
304 stainless steel is easily formed, welded, and drawn. It’s commonly used in kitchen cutlery, storage vessels and piping, medical and surgical instruments, and general hardware exposed to the environment, but it can be vulnerable to chlorides and salts.
Properties of SS 304
- Density (g/cm3): 7.93
- Hardness (Brinell): 195~215
- Tensile Strength (MPa): 505
- Weldability: Excellent
- Machinability (%): 45
When to use SS 304
Choose stainless steel 304 when:
- Corrosion resistance is essential, especially in mildly acidic or wet environments.
- Cleanability and hygiene are important (e.g., food or medical).
- Welding or forming is part of the production process.
- Cosmetic appearance matters.
SS 316
Stainless steel 316 is a bit more difficult to CNC mill than SS 304 because of the addition of molybdenum. However, this improves corrosion resistance in marine environments or when exposed to acids and chlorides.
SS 316 is more expensive, but the expense is worth it due to increased strength and durability. SS 316 is ideal for cleanroom and medical equipment because it’s biocompatible. It can also readily be used in many marine and aerospace applications where chemicals are present.
Properties of SS 316
- Density (g/cm3): 7.99
- Hardness (Brinell): 125~155
- Tensile Strength (MPa): 515
- Weldability: Excellent
- Machinability (%): 35
When to use SS 316
Choose stainless steel 316 when:
- Chloride or chemical exposure is present, including saltwater, bleach, or acidic compounds.
- Superior corrosion resistance is required over SS 304 or SS 303.
- Welding and hygiene are important (e.g., pharma, food).
- Long-term durability is worth the added material cost.
- You’re designing for outdoor, marine, or chemical-rich environments.
SS 316L
An important variation of stainless steel 316 is SS 316L, meaning low carbon. It has most of the same characteristics as SS 316 but is even more corrosion resistant while being slightly less strong.
It’s mainly used for applications that require heavy weldments, because the surface resists intergranular corrosion. Used in biotech, chemical and food processing equipment, surgical trays, and orthopedic instruments.
Properties of SS 316L
- Density (g/cm3): 7.99
- Hardness (Brinell): 125~155
- Tensile Strength (MPa): 485
- Weldability: Excellent
- Machinability (%): 35
When to use SS 316L
Use stainless steel 316L when:
- Welding is required, and you want to avoid carbide precipitation (which weakens corrosion resistance).
- You’re building pressure vessels, tanks, or pipe systems that will be cleaned or sterilized regularly.
- The environment involves chlorides, acids, or harsh chemicals, especially with cyclic temperature changes.
- You need compliance with hygienic or biomedical standards (e.g., ASTM F138 for medical instruments).
Schedule a discovery call with our engineering team, we’ll help you avoid common pitfalls when choosing a stainless steel for your next CNC milling project.
A Closer Look at Series 400 Stainless Steel
The 400 series stainless steels are typically more expensive than the 300 series, because they’ve been more carefully engineered to offer higher levels of performance for more demanding applications and environments.
Let’s take a closer look at the most common ones used in high tolerance CNC milling.
SS 416
The first on our list of martensitic stainless steels is SS 416. It was specifically formulated to be ‘free machining’, with an index rating of 85~90%. Stainless Steel 416 is known as the most machinable stainless steel.
This is due to a high content of sulfur which, as we’ve seen with the other stainless steels, improves chip forming but interferes with welding and corrosion resistance.
Like other martensitic steels it can be heat treated to improve hardness, and it maintains excellent dimensional stability during CNC milling. Often found in automotive steering systems, gears, hardware, and non-critical aerospace components.
Properties of SS 416
- Density (g/cm3): 7.75
- Hardness (Brinell): 170~190
- Tensile Strength (MPa): 540
- Weldability: Poor
- Machinability (%): 85
When to use SS 416
Use stainless steel 416 when:
- Machinability is the #1 priority – especially in high-volume production.
- You need a moderately corrosion-resistant alternative to carbon steel, with better surface finish and strength.
- The part will be heat treated for hardness but not welded or heavily formed.
- You are making CNC high tolerance mechanical components where cost and speed matter.
SS 420
420 stainless steel has a machinability index of 55%, making it slightly better than SS 304 but worse than SS 416. The high concentration of carbon also means that it’s not easily welded or formed and has only moderate corrosion resistance.
The advantage of SS 420 stainless is that it can be heat treated to high hardness and has excellent edge retention. This makes it a good choice for knives, scissors and other cutting instruments, as well as CNC tooling and dies used for plastic injection molding inserts.
Properties of SS 420:
- Density (g/cm3): 7.75
- Hardness (Brinell): 201~255
- Tensile Strength (MPa): 700
- Weldability: Poor
- Machinability (%): 55
When to use 420
Choose stainless steel 420 when:
- You need a hardenable stainless steel with high strength and wear resistance.
- Edge retention or surface hardness is more critical than corrosion resistance.
- The part can be CNC machined in annealed condition and then hardened after.
- You’re producing blades, tools, or dies that must be polished and sharpened.
SS 440C
Stainless steel 440C is the hardest high-carbon variant in the 400 series. It’s designed to be extremely wear resistant and holds a fine edge. It has a medium machinability index of about 40%, with relatively poor anti-corrosion performance and weldability.
The high carbon content means it can be heat treated to greatly increase its strength. It excels at making fine surgical cutting tools and razors, as well as mechanical components like bearings, pump shafts, and valves.
Properties of SS 440C
Density (g/cm3): 7.75
Hardness (Brinell): 190 ~ 240
Tensile Strength (MPa): 760
Weldability: Poor
Machinability (%): 40
When to use 440C
Use stainless steel 440C when:
- Maximum hardness and wear resistance are required.
- You’re producing high tolerance precision parts under mechanical stress or abrasion.
- The part can be CNC machined in a soft (annealed) state and then hardened.
- Moderate corrosion resistance is acceptable.
- Welds or formed shapes are not part of the design.
Specialty stainless steels
There are other alloys that, although technically “stainless” because of a chromium-oxide layer, don’t fall within the same series classification. They contain proprietary blends of trace elements designed to enhance specific characteristics.
SS-SAF2205
Stainless steel SAF 2205 is known as a duplex steel, meaning it has a roughly 50/50 microstructure of austenite and ferrite. This combination is great for strength and corrosion resistance, especially in chloride-rich environments.
With a machinability index of 35% it’s still difficult to CNC machine, but it has excellent weldability and medium formability. It excels in corrosion resistance, so it’s often found in chemical processing, oil and gas, refining and mining, marine, and other challenging industrial environments.
Properties of SAF 2205
Density (g/cm3): 8
· Hardness (Brinell): 293
· Tensile Strength (MPa): 620~880
· Weldability: Good
· Machinability (%): 35
When to use SAF 2205
Choose SAF 2205 / Duplex 2205 when:
- You require excellent resistance to chloride-induced pitting, crevice corrosion, or stress corrosion cracking.
- The application demands high strength to reduce wall thickness or structural weight.
- You need better performance than 316 in aggressive environments (saltwater, acids, industrial).
- Welded joints will be subjected to stress + corrosion, where 304/316 may crack.
- Budget doesn’t permit super-duplex or exotic alloys like Hastelloy or 904L.
17-4 PH
Finally, we have 17-4 PH, or precipitation hardenable stainless 17-4. It combines high strength with corrosion resistance, making it useful in many industrial components. It has medium machinability, good weldability, but rather poor formability.
It’s often found in levers and shafts, structural and frame components, surgical tools, and dies for plastic injection molding.
Properties of 17-4 PH
- Density (g/cm3): 7.75
- Hardness (Brinell): 311
- Tensile Strength (MPa): 1100
- Weldability: Fair
- Machinability (%): 45
When to use 17-4 PH
- Use 17-4 PH when:
- You need high mechanical strength + corrosion resistance in the same package.
- The part must be precision-machined and distortion-free after hardening.
- Welded structures require strength and stability without post-weld heat treatment.
- You want to replace alloy steels where corrosion resistance is needed.
- Application involves high static or cyclic loads.
Best Practices for CNC Machining Stainless Steel
CNC machining stainless steel presents unique challenges due to its toughness, work hardening tendencies, and low thermal conductivity. To achieve the best results and maintain high precision, consider these proven best practices:
Select the Right Tooling
Use high-performance carbide end mills and drills specifically designed for machining stainless steel. Carbide tools offer superior wear resistance and maintain sharp cutting edges, which is essential for clean, precise cuts in tough stainless alloys.
Optimize Cutting Parameters
Stainless steel requires slower cutting speeds and moderate feed rates to prevent excessive heat buildup and tool wear. Always consult manufacturer recommendations for optimal speeds and feeds, and adjust based on the specific stainless steel grade and part geometry.
Apply Ample Coolant
Effective cooling is critical when CNC milling stainless steel. Use flood coolant or high-pressure coolant systems to dissipate heat, reduce thermal expansion, and flush away chips. This helps prevent work hardening and extends tool life.
Use Rigid Workholding
Secure the workpiece firmly to minimize vibration and movement during machining. Rigid fixturing ensures consistent dimensional accuracy and surface finish, especially when producing complex stainless steel parts.
Minimize Tool Engagement
When possible, use climb milling and light, consistent cuts to reduce tool engagement and heat generation. Avoid deep cuts or aggressive plunging, which can accelerate tool wear and cause work hardening.
Regular Tool Inspection
Frequently check tools for wear or chipping. Dull or damaged tools can lead to poor surface quality, increased heat, and even part rejection. Replace or regrind tools as needed to maintain optimal machining performance.
Chip Control
Stainless steel produces tough, stringy chips that can interfere with the machining process. Use chip breakers, peck drilling cycles, or high-pressure coolant to manage chip evacuation and prevent recutting.
Monitor Part Quality
Continuously inspect machined parts for dimensional accuracy, surface finish, and signs of thermal damage. Early detection of issues allows for quick adjustments to machining parameters, ensuring consistent quality throughout the production run.
By following these best practices, professional manufacturers overcome the challenges of machining stainless steel and achieve reliable, high tolerance results.
Whether you’re producing precision components for aerospace, medical, or industrial applications, attention to detail at every stage of the CNC milling process will pay off in part performance and durability.
Get Stainless Right the First Time
Selecting the right stainless steel for a CNC manufacturing isn’t just a materials decision — it’s a strategic one. The wrong choice can result in excess tooling costs, production delays, poor corrosion performance, or field failure. Reliable CNC machining companies always make the right choice to ensure clean machining, compliance, durability, and long-term value.
By understanding how each grade behaves, you can confidently balance performance, cost, and manufacturability — and build better parts from the start.
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