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Championed for corrosion resistance and unmatched polishability, stainless steel is one of the most versatile, widely adopted metal alloys in manufacturing today. But it may also be the most misunderstood.

When stainless steel is needed for applications that require a specific hardness (HRC), there’s a huge risk involved with procurement and manufacturing because of how this material is listed and sold in the marketplace.

To help you avoid costly material mishaps, we’ve created a guide on the fundamentals of stainless steel and what you need to know before your next project.

What is Stainless Steel Made of?

Stainless steel comprises varying amounts of chromium, carbon, nickel, and iron depending on what is needed for a given application.

Chromium content creates the “stainless” effect by providing a protective chromium oxide armor. This protective layer is what gives stainless steel its hallmark shine and resistance to rust. These are the constants of stainless. Then there are varying levels of carbon content, which create the different stainless steels available today.

Carbon is Key

Stainless steel comes in a wide array of grades for different applications—such as 410, 420, or 440—each one representing different ranges of carbon content. 400-series steels usually denote ferritic or martensitic stainless steel, which contain less nickel than austenitic stainless steels (300 series).

Carbon content is a crucial factor when determining which grade is suitable for your application. However, the exact level of carbon content can vary depending on your material supplier, and it’s far too easy to wind up with a full supply of newly purchased material that ultimately can’t be used to reach your desired specs—all due to a lack of carbon in the steel.

Austenitic, Ferritic, and Martensitic Stainless Steels

Three of the most common categories of stainless steels are austenitic, ferritic, and martensitic. Austenitic typically has the lowest carbon content, and martensitic can have some of the highest, with ferritic falling in the middle. The chart below illustrates the differences in each type of stainless steel and common applications.

Austenitic SteelsFerritic Stainless SteelsAustenitic Steels
304, 316, A286430410, 416, 420, 440C

Most corrosion-resistant of all stainless steel

Low carbon content
High heat resistance

Can be demagnetized
Economical stainless steel option

Can be annealed or heat treated
to improve corrosion resistance, but not hardness

Can have the highest carbon content

Very dimensionally stable

Surgical instruments

Car exhaust components 
Appliance parts
Bathroom fixtures
Plastic molding steel
Firearm components

Know Your Materials Supplier

400 series stainless steel (most of which are martensitic) is an ideal material for creating plastic molds and cutting tools because it’s assumed that all steels that fall in this category are sufficiently hardenable—but this is a common misconception. In fact, some martensitic steels have a wide variance in carbon content, and some don’t contain enough carbon for heat treatment processes such as through hardening, annealing, or precipitation hardening.

As a rule, if you require a certain HRC, you’ll need a precise carbon level to help you achieve that. If you mistakenly purchase low carbon grade martensitic, you may never reach the results you want.

Compositions will vary from supplier to supplier, and some will even tailor carbon levels to a specific application. For materials like 410 and 416, low carbons (anything less than .08%), are used for machined parts that will not be hardened. Parts requiring heat treatment should contain .10% carbon or more.

Below is a chart of common stainless steels and the carbon ranges that each grade is most commonly found.

Steel Grade410, 416420440C
Carbon Range.15% maximum.15% minimum1% carbon
Hardenability35 – 43 HRC40 – 52 HRCUp to 62 HRC

You’ll notice that 410 and 416 are sold with a maximum .15% carbon level, and there is no minimum specified.

420 is offered at a minimum of .15% (which achieves low 40s HRC) up to “mold-grade” .35% carbon (which achieves low 50s HRC). A wide range of stainless steel grades fall under the 420 category. This makes procurement for mold applications especially risky when searching for a new supplier.

440C is the premier tool stainless steel. Thanks to its 1% carbon content, this material provides good corrosion resistance, specifically for food processing, cutting, and grinding equipment. 

Working With a Stainless Steel Supplier

Since the grade names of stainless steel are not application specific, it’s important to know the composition of the steel ahead of procurement. Informed buying decisions are key for limiting risk. You may also find that some material suppliers offer steel carbon levels tailored to specific applications to provide a more functional understanding of the product and to help guide you in the buying process. 

Stainless Steel Heat Treatment

It’s important to work closely with your materials supplier and heat treater to ensure that the right material and approach to heat treatment are being used to achieve the desired results. Due diligence on prospective suppliers can save you a lot of money down the line. Plus, it will give you a greater understanding of the material and subsequently make working with a heat treater much easier.

It’s always important that you notify your heat treater of any supplier or material changes. It’s also beneficial to find a heat treater that has demonstrated expertise in working with stainless steel and understands the nuances found in the stainless industry today. Without guidance from knowledgeable heat treaters, you might be more likely to end up with a material that doesn’t have sufficient carbon content to achieve the hardness required for your parts. 

Partner with Experts in Stainless Steel

Our team of metallurgists are experts in the heat treatment of stainless steel and are ready to help solve your biggest stainless challenges. Reach out to a Paulo expert today about your next project.

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