431 Stainless Steel: How It Compares to Martensitic 416 and 420, and Austenitic 304 and 316?

As you have learned, 431 SS is famous for:

• the best corrosion resistance among martensitic stainless steels due to its higher composition of Chromium (15.0–17.0%) and Nickel (1.25–3.0%). 
• and is also valued for its hardness and strength after heat treatment, which is preferred by many manufacturers for making lots of CNC machined components.

But if you are also seeking 431 SS and hesitating over whether it is the right steel for your project or not—you could be confusing it with other stainless steels because:

• Very common martensitic steels 416 and 420 are also good at hardness and strength after heat treatment.
• Very famous austenitic steels 304 and 316 are quite outstanding at corrosion resistance as well.

These are what this blog will focus on, and you will finally figure out 431 SS properties at what level among these steels.

Comparison 1: Martensitic SS 431 vs 416 vs 420

These three stainless steels have a martensitic structure, meaning they all can be strengthened by quenching and tempering and are also naturally strongly magnetic (while austenitic steels can’t do this, or you can refer to our previous blog: austenitic-vs-martensitic-stainless-steel).

Although they can all be made into precision parts like gears, shafts, and pins, their biggest differences are:

• 431 SS is the only one that can be used for corrosion-resisting parts like marine pump shafts and outboard engine drive shafts. It offers not only good corrosion resistance but also higher toughness to withstand high torque and tensile forces. You might need to consider its higher price and processing costs—as it contains more expensive elements like Chromium and Nickel. Its toughness also makes the CNC machining process more likely to experience "spring back" if high dimensional stability is required (when directly machining the strengthened 431 SS).
• 416 SS is the best for machining into various parts because of its added sulfur content—allowing for the fastest production during CNC machining. For materials that are easy to process, have high yield, and are suitable for use in dry environments: you can select the economical 416.
• 420 SS is the most wear-resistant one, and its corrosion resistance and cost are in the middle. For applications requiring exceptionally high hardness, wear resistance, and use as cutting edges, molds, or heavy-duty gears: you can select 420.

Comparison 2: Martensitic SS 431 vs Austenitic 304 vs Austenitic 316

These three stainless steels represent a choice between "strength" and "better corrosion resistance."

SS 304 and 316 are austenitic steels, which means they are non-magnetic in their annealed state and cannot be hardened by heat treatment (they can only be hardened by cold working). For strength and hardness comparisons, SS 431 is far superior.

For SS 431, while its corrosion resistance is top tier for a martensitic steel, it still ranks below 304 and 316 in salty or acidic environments.

Another major difference is that while these three have similar machinability ratings, they behave differently during CNC machining. In practice, SS 431 is easier to cut, drill, turn, and mill, mainly because it isn’t as "gummy" as the austenitic grades (FCC structure) like SS 304 and SS 316.

Conclusion

Choosing between 431, 416, 420, 304, and 316 depends entirely on your project’s environment and mechanical requirements:

• Choose 431 SS if you need the high strength and toughness combined with good corrosion resistance for marine or high-torque mechanical parts.
  
• Choose 416 SS for high-volume, low-cost components used in dry environments where machinability is the priority.
  
• Choose 420 SS when extreme hardness and wear resistance (like blades or molds) are required.
  
• Choose 304/316 SS when maximum corrosion resistance or food-grade safety is needed, and high mechanical strength is not a primary concern.

Get a Professional Quote from VMT CNC Machining Factory

Are you still unsure which stainless steel grade is best for your specific application? Or do you already have a design ready for production?

At VMT, we specialize in precision CNC machining services for a wide range of stainless steel grades, including the high-toughness 431 SS. With our advanced equipment and decades of expertise, we manage the challenges of "gummy" austenitic steels and the high-hardness requirements of martensitic steels with ease.

Why partner with VMT?

• Material Expertise: We help you select the most cost-effective material without sacrificing performance.
  
• Precision Quality: High dimensional stability, especially for heat-treated components.
  
• Fast Turnaround: Efficient CNC turning and milling to keep your project on schedule.
  

Contact us for a free technical consultation and a competitive quote!

FAQs

Is 431 stainless steel magnetic?

Yes, 431 stainless steel has a martensitic structure, making it strongly magnetic in all conditions. Unlike 300-series austenitic steels, which are non-magnetic, 431 retains its magnetism even after heat treatment.

Can 431 stainless steel be used in marine environments?

Yes, 431 is the best performer among martensitic grades for marine use. It is widely used for outboard engine drive shafts and pump components. However, for stagnant seawater or extreme chemical exposure, austenitic Grade 316 is still preferred due to its higher Molybdenum content.

What is the equivalent of 431 stainless steel in other standards?

431 SS is commonly known as 1.4057 under the EN/DIN standard and SUS431 in the Japanese JIS standard. These grades share similar chemical compositions, featuring high Chromium and low Nickel.

Beyond these, it is also designated as UNS S43100 in the Unified Numbering System, BS 970 431S29 in British Standards, and AFNOR Z15CN16-02 in the French system. It is also frequently referred to by the aerospace grade AMS 5628, which specifies the material for high-strength applications like aircraft components.

Why is 431 stainless steel harder to machine than 416?

416 stainless steel is a "free-machining" grade because it contains added sulfur, which acts as a lubricant and breaks chips easily. 431 lacks this sulfur but offers much higher toughness and corrosion resistance, which makes the CNC machining process more demanding on cutting tools.

Does 431 stainless steel require pre-heating before welding?

Yes. Due to its high carbon content and hardenability, 431 stainless steel is prone to cracking if welded cold. It generally requires pre-heating to 200-300°C and post-weld annealing to restore ductility.

What is the typical hardness of 431 SS after quenching and tempering?

After proper heat treatment (quenching and tempering), 431 stainless steel typically reaches a hardness range of 30 to 45 HRC (approx. 285-450 HB). This makes it ideal for components requiring a balance of high tensile strength and impact resistance.