303 Stainless Steel: Built for Machining, Not for Marine
28 Aug 2025
If you’re after stainless steel that machines like a dream but doesn’t need to brave the elements, 303 stainless steel is your go-to. Also known as AISI 303, this grade offers the best machinability of all austenitic stainless steels.
But be warned – 303 stainless is built for workshop performance, not salty sea air. The addition of sulphur, which makes it easy to cut, also makes it more prone to corrosion. If you're planning on long-term outdoor use – especially near water – this probably isn’t your guy.
Let’s get into 303 stainless steel properties, including what it’s great at, where it falls short and where to use it (or avoid it).
Article in brief
Excellent machinability thanks to added sulphur
Reduced corrosion resistance compared to 304
Very difficult to weld – requires specialist rods and post-weld treatment
Can’t be hardened by heat treatment
Performs well at high temps in dry environments
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What is 303 stainless steel?
If we’re being technical, the material 303 stainless steel is an austenitic chromium-nickel alloy modified with sulphur to improve machining. Often called type 303 stainless steel, it’s part of the 18-8 family (18% chromium, 8% nickel) – like 304, but sulphur is added to enhance machinability.
This makes 303 steel ideal for high-speed machining processes like CNC milling and turning. It's widely used for producing shafts, bolts, bushings, gears and fasteners.
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Key properties of 303 stainless steel
Let’s talk about what makes 303 stainless stand out:
- Machinability: This is the main reason to choose it. You’ll get a smoother finish with less wear on your tools.
- Corrosion resistance: Decent in dry environments, but weaker than 304.
- Weldability: Limited. The sulphur content interferes with fusion and increases the risk of cracking.
- Hardenability: Can’t be hardened by heat treatment – only by cold working.
- Magnetism: Generally non-magnetic in the annealed state, but 303 stainless steel magnetic response can appear after cold working.
- Oxidation resistance: Can tolerate up to 870°C (continuous) and 760°C (intermittent) in dry air.
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303 stainless steel material properties
Here’s a quick look at the 303 ss material properties you need to know:
Mechanical properties of 303 stainless steel
Property | Value
Tensile strength | 750–800 MPa
303 stainless yield strength | 450–650 MPa
Elongation (in 50 mm) | 34%
303 ss hardness (Rockwell B) | Up to 95
Brinell hardness | Max 262
You’ll see different sources call these “303 cres material properties” or “cres 303 material properties,” but it’s all the same spec.
Tensile strength | 750–800 MPa
303 stainless yield strength | 450–650 MPa
Elongation (in 50 mm) | 34%
303 ss hardness (Rockwell B) | Up to 95
Brinell hardness | Max 262
You’ll see different sources call these “303 cres material properties” or “cres 303 material properties,” but it’s all the same spec.
Physical properties of 303 ss
Property | Value
303 stainless steel density | 7,900 kg/m³
Elastic modulus | 193 GPa
Specific heat (0–100°C) | 500 J/kg·K
Electrical resistivity | 720 nΩ·m
Thermal conductivity (100°C) | 16.3 W/m·K
Thermal conductivity (500°C) | 21.5 W/m·K
303 stainless steel density | 7,900 kg/m³
Elastic modulus | 193 GPa
Specific heat (0–100°C) | 500 J/kg·K
Electrical resistivity | 720 nΩ·m
Thermal conductivity (100°C) | 16.3 W/m·K
Thermal conductivity (500°C) | 21.5 W/m·K
So if you’re after the density of 303 stainless steel, that’s your number – 7,900 kg/m³.
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303 stainless steel composition
Here’s what goes into the 303 stainless composition (max values unless noted):
- Carbon (C): 0.15%
- Manganese (Mn): 2.00%
- Silicon (Si): 1.00%
- Phosphorus (P): 0.20%
- Sulphur (S): 0.15%
- Chromium (Cr): 17.0–19.0%
- Nickel (Ni): 8.0–10.0%
This sulphur addition is the secret sauce behind its superb machinability – but it’s also why ss 303 is more likely to pit in aggressive environments.
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When 303 works best (and when it doesn’t)
SS 303 isn’t built for every job, and that’s OK. It’s ideal when:
- Your priority is machining speed and tool life
- The part won’t be welded
- It’s going in a clean, dry indoor space
- You want consistent finishes across production runs
Think of 303 ss as the right choice for things like:
- Screws, bolts, and studs
- Electrical switchgear
- Control panel components
- Bushings and bearings
- Watch parts
- Gears and connectors
But skip it if your job involves:
- Marine environments
- Harsh chemicals or cleaning agents
- Structural welding
- Long-term outdoor use
Welding 303 stainless: Proceed with caution
Here’s the deal – 303 stainless steel can be welded, but only if you’ve got no other choice.
- Use 308L or 309 welding rods. 312 can be a good option due to its high ferrite content, but it’s generally more expensive.
- Always anneal after welding to restore corrosion resistance
- Expect brittleness and cracking if not done properly
If you need stainless steel for welding, steer clear of 303. Look at 304 instead – it’s much more forgiving.
Is 303 stainless magnetic?
303 stainless magnetic response is generally low in its annealed state, just like most austenitic steels. But after cold working, some magnetism can appear. If magnetic performance is critical, test it in the final form or consider an alternative.
Related: Is Stainless Steel Magnetic?
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Final word: Should you use 303?
Choose 303 stainless steel if you’re:
- Machining at high speeds
- Looking for smoother finishes
- Not welding
- Keeping the part indoors or in a mild environment
Avoid 303 cres material for outdoor, marine, or chemical-heavy jobs. It’s just not built for that.
Order 303 stainless steel cut to size
Need 303 stainless steel for your next job? We’ll sort it – cut to your exact size, delivered on the day you choose, and no extra charge for cutting.
The Metal Store is the go-to supplier of stainless steel for customers up and down the UK. Whether you’re a fabricator, engineer, or just someone who likes to tinker in the shed, we’ve got what you need. No faff. Just metal made easy.
