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“Can you make this in stainless?” is one of the most common requests a machine shop gets — and the first question back is always “which one?” In episode 2 of Grain Boundary, Sam and Dan explain what makes steel stainless, how 303, 304, 316 and 17-4 differ, why some grades are so much harder to machine, and how to pick the right one for your part.
In this episode
- 00:47 What makes steel “stainless”: chromium and a self-healing oxide layer
- 01:38 The stainless families: austenitic, ferritic, martensitic and precipitation-hardening
- 02:24 304: the everyday default, and why it fights back in the machine
- 03:54 303: the free-machining grade, and what you give up
- 04:55 316: molybdenum, salt water and medical parts
- 05:56 17-4: machine it soft, heat treat it hard
- 07:25 Why the magnet test doesn’t tell you about quality
- 07:58 Galling and passivation
- 09:01 How to pick a stainless grade for your part
Quick reference
| Grade | Family | Key feature | Machining | Typical uses |
|---|---|---|---|---|
| 303 | Austenitic | Added sulfur for short chips | Easiest of the four | Fittings, shafts, screws, small turned parts |
| 304 | Austenitic | About 18% chromium, 8% nickel (“18-8”) | Work-hardens; needs sharp tools and steady feed | Sinks, food equipment, tanks, railings, enclosures |
| 316 | Austenitic | Adds 2–3% molybdenum against chlorides | A bit gummier than 304 | Marine, medical (316L), pharmaceutical and food processing |
| 17-4 | Precipitation-hardening | Heat treats to about 40–45 HRC (H900) | Machine in Condition A, then heat treat | Aerospace fittings, pump shafts, valves, molds, fasteners |
| 430 | Ferritic | Less nickel, magnetic | — | Appliance panels |
| 410 / 440C | Martensitic | Hardenable by heat treatment | — | Knives, surgical tools, bearings |
Need welding? Avoid 303. Near salt water, chemicals or in medical use? Choose 316. Need high strength? 17-4.
Transcript
00:00 Sam: Hey, welcome back to Grain Boundary, the show about what things are actually made of. I’m Sam.
00:05 Dan: And I’m Dan. And last time we did aluminum, which, by the way, people really liked.
00:10 Sam: They did! A lot of you wrote in about the soda can fact.
00:13 Dan: Two alloys in one can. Never gets old.
00:16 Sam: So today we’re moving on to the metal that, honestly, causes more confusion in a machine shop than probably anything else.
00:22 Dan: Stainless steel.
00:24 Sam: Stainless steel. Because people order it like it’s one thing. “Can you make this part in stainless?”
00:30 Dan: And the machinist goes, “Sure. Which one?”
00:33 Sam: Right. And the customer goes, “Uh… the stainless one?”
00:37 Dan: Yeah. So today we’re going to fix that. We’ll talk about three-oh-three, three-oh-four, three-sixteen, seventeen-four, and why some of them are so much harder to machine than others.
00:47 Sam: Okay, let’s start with the basics. What actually makes steel “stainless”?
00:52 Dan: Chromium. To be called stainless, steel needs at least about ten and a half percent chromium. And what the chromium does is form this incredibly thin oxide layer on the surface. You can’t see it, it’s just a few atoms thick, but it seals the metal off from air and water.
01:07 Sam: Wait, that’s the same trick as aluminum, right? From last episode?
01:11 Dan: Exactly the same idea. A protective oxide skin. And the best part is, if you scratch it, it heals itself. The chromium just reacts with oxygen again and the layer grows back.
01:22 Sam: That’s so cool. So it’s not that stainless can’t rust. It’s that it keeps re-sealing itself.
01:28 Dan: Right. And if you overwhelm that layer, with salt, or certain chemicals, or contamination, it absolutely can rust. Which is where choosing the right grade comes in.
01:38 Sam: Okay so, grades. Give us the map, like you did with aluminum.
01:42 Dan: So, there are a few big families. The one you’ll see most is the austenitic family. That’s the three-hundred series. Three-oh-four, three-sixteen, three-oh-three. These have chromium plus nickel. They’re tough, they’re great for corrosion, and they’re usually not magnetic.
01:57 Sam: Usually?
01:58 Dan: Usually. Hold that thought, we’ll come back to it.
02:01 Sam: Okay.
02:02 Dan: Then there’s the ferritic family, like four-thirty. Cheaper, less nickel, magnetic. That’s a lot of appliance panels. Then the martensitic family, like four-ten and four-forty C. These can be hardened with heat treatment, so, knives, surgical tools, bearings. And then the precipitation-hardening grades, like seventeen-four, which we’ll get to. That one’s kind of the superstar.
02:24 Sam: Let’s start with the most common one. Three-oh-four.
02:27 Dan: Three-oh-four is the default stainless. You’ll also hear it called eighteen-eight, because it’s roughly eighteen percent chromium and eight percent nickel. Kitchen sinks, food equipment, tanks, railings, brackets, enclosures. If someone just says “stainless,” they probably mean three-oh-four.
02:44 Sam: And it’s good stuff?
02:47 Dan: It’s great stuff. Corrosion resistance is excellent for most everyday environments. It welds well. It’s tough. The problem shows up when you put it in a machine.
02:56 Sam: Ah. Here we go.
02:58 Dan: So, three-oh-four work-hardens. Meaning, the moment you start cutting it, the surface right in front of the tool gets harder. If your tool is sharp and you’re taking a solid cut, you get under that hardened layer and you’re fine. But if your tool is dull, or you’re taking a timid little cut, or you let the tool dwell and rub…
03:14 Sam: You’re basically making it harder for yourself.
03:17 Dan: Literally. You’re hardening the next layer you have to cut. Machinists have a saying with stainless: don’t be shy. Keep the feed going, keep the tool sharp, don’t rub.
03:25 Sam: And there’s a heat problem too, right?
03:27 Dan: Big one. Stainless doesn’t conduct heat very well. Aluminum is roughly ten times better at carrying heat away. So with stainless, the heat doesn’t go into the chip or the part. It stays right at the cutting edge. And that cooks the tool.
03:41 Sam: So that’s why stainless parts cost more to make.
03:44 Dan: That’s a big part of it. Slower speeds, more tool wear, more care. The same part in stainless can easily take a lot longer to machine than it would in aluminum.
03:54 Sam: Okay. So then what’s three-oh-three?
03:56 Dan: Three-oh-three is basically three-oh-four with a little sulfur added on purpose. The sulfur forms these tiny particles in the metal that make the chips break into small pieces, instead of long, stringy ribbons that wrap around everything.
04:09 Sam: Oh, I’ve seen those. The bird’s nests.
04:12 Dan: The bird’s nests, exactly. So three-oh-three is called a free-machining grade. It machines a lot faster than three-oh-four. For things like fittings, shafts, screws, small turned parts, it’s a shop favorite.
04:24 Sam: Okay, so why doesn’t everyone just use three-oh-three?
04:28 Dan: Because there’s no free lunch. Those sulfur particles are weak spots for corrosion, so three-oh-three doesn’t hold up as well, especially in wet or salty environments. And it doesn’t weld well, it tends to crack. So if your part needs to be welded, or it’s going on a boat, three-oh-three is the wrong choice.
04:45 Sam: So three-oh-three is great to machine, but you give up corrosion resistance and weldability.
04:50 Dan: That’s the trade. And honestly, a good shop will ask you about that before they quote it.
04:55 Sam: Okay. Three-sixteen. The fancy one.
04:59 Dan: The fancy one. Three-sixteen is like three-oh-four, plus about two to three percent molybdenum. And molybdenum is the magic ingredient against chlorides, which basically means salt.
05:09 Sam: So, sea water.
05:11 Dan: Sea water, de-icing salt, swimming pools, a lot of chemicals, even some cleaning products. Chlorides attack stainless by creating tiny pits, little holes that dig into the metal. Three-sixteen resists that much better than three-oh-four. That’s why it’s called marine grade.
05:26 Sam: And it’s used in medical stuff too, right?
05:29 Dan: Yeah. Low-carbon versions, like three-sixteen L, are used in medical devices and surgical implants, pharmaceutical equipment, and food processing where they clean with harsh chemicals.
05:40 Sam: So what’s the catch this time?
05:42 Dan: Price, mostly. More nickel, plus molybdenum, so the material costs more. And it’s a bit gummier to machine than three-oh-four. So, if your part lives indoors, in a dry place, paying for three-sixteen might just be wasting money.
05:56 Sam: Good tip. Okay, you called seventeen-four the superstar. Why?
06:01 Dan: So, everything we’ve talked about so far, three-oh-four, three-sixteen, you can’t make them much harder with heat treatment. Seventeen-four is different. It’s about seventeen percent chromium, four percent nickel, and some copper. And you can heat treat it to get really high strength.
06:17 Sam: Like the T6 thing with aluminum.
06:19 Dan: That’s a great comparison, actually. It’s the same basic idea, it’s called precipitation hardening. You age it at a set temperature, and tiny particles form inside the metal that lock everything in place. The most common hard condition is called H nine hundred, because it’s aged at nine hundred degrees Fahrenheit. And that gets you to about forty to forty-five on the Rockwell C scale.
06:41 Sam: Okay, for people who don’t speak Rockwell, is that hard?
06:44 Dan: That’s hard. That’s way past any of the three-hundred series. It’s in the range of a lot of hardened tool and gun parts. And it still has decent corrosion resistance. Not quite as good as three-sixteen, but good.
06:56 Sam: And here’s what I like about this one, for a machine shop specifically.
07:00 Dan: Yeah, this is the clever part. You can machine it in the softer condition, called Condition A, and then heat treat it afterwards. And the heat treatment is gentle enough that the part barely changes size. So you get the easy machining and the high strength.
07:13 Sam: Where does seventeen-four show up?
07:15 Dan: Aerospace fittings, pump shafts, valve parts, firearm parts, molds, high-strength fasteners. Anywhere you need strength and corrosion resistance at the same time.
07:25 Sam: Okay, I promised we’d come back to the magnet thing.
07:28 Dan: Right. So a lot of people test stainless with a magnet. If it sticks, they think it’s fake or low quality.
07:34 Sam: I’ve totally done that.
07:36 Dan: Everyone has! But it doesn’t really work. Three-oh-four is non-magnetic when it’s annealed, but once you bend it, or machine it, or cold work it, it can become slightly magnetic. And plenty of great stainless grades, like four-thirty and seventeen-four, are fully magnetic.
07:51 Sam: So a magnet tells you something about the type, but nothing about quality.
07:55 Dan: Exactly.
07:58 Sam: Two more shop things I want to hit. Galling, and passivation.
08:01 Dan: Ooh, galling. So, if you’ve ever tightened a stainless nut on a stainless bolt, and it suddenly just locked up and wouldn’t move…
08:08 Sam: Yes!
08:10 Dan: That’s galling. When two stainless surfaces slide under pressure, the protective layer gets scraped off, and the metal basically cold-welds to itself. Sometimes you have to cut the bolt off.
08:21 Sam: How do you avoid it?
08:23 Dan: Anti-seize compound on the threads. Slower tightening. Or using two different grades or hardnesses, so they don’t stick to each other as easily.
08:32 Sam: And passivation?
08:34 Dan: So, when you machine stainless, tiny bits of regular iron from tools and fixtures can get stuck on the surface. And that free iron can rust, which makes a perfectly good stainless part look like it’s rusting.
08:45 Sam: Oh, that’s sneaky.
08:47 Dan: Very sneaky. Passivation is a chemical bath, usually citric or nitric acid, that removes that surface iron and helps the protective layer form cleanly. For medical, food, or aerospace parts, it’s often required.
09:01 Sam: Okay. Let’s wrap it up the same way we did last time. If I’m ordering a stainless part, how do I pick?
09:07 Dan: Couple of questions. First, where’s it going to live? If it’s indoors and dry, three-oh-four is usually perfect. If it’s near salt water, chemicals, or it’s medical, go three-sixteen.
09:18 Sam: Second?
09:20 Dan: Does it need to be welded? If yes, stay away from three-oh-three.
09:24 Sam: Third?
09:25 Dan: Is it a lot of small turned parts that won’t see harsh conditions? Three-oh-three will save you real money on machining time.
09:31 Sam: And if it needs to be really strong?
09:34 Dan: Then seventeen-four. Machine it soft, heat treat it hard.
09:38 Sam: Okay, quick recap. Stainless is stainless because of chromium, which builds a self-healing oxide layer. Three-oh-four is the everyday default. Three-oh-three machines easiest, but gives up corrosion resistance and weldability. Three-sixteen adds molybdenum for salt and chemicals. And seventeen-four can be heat treated for serious strength.
09:57 Dan: And don’t trust the magnet test.
10:00 Sam: And last time it was “always check the temper.” So this time…
10:04 Dan: Always check the grade.
10:05 Sam: There it is. That’s it for this episode of Grain Boundary. Thanks for listening, and we’ll see you next time.
10:11 Dan: Take care, everyone.