What are tek screws made of?
Tek screws, the self-drilling screws that go into nearly every window frame you build, come in three materials. They look close enough on a shelf that the wrong one gets ordered constantly, and the quote usually doesn't say which you're getting.
The three are 18-8 stainless, 410 stainless, and bi-metal. They carry the same description and the same nominal size. They drill differently, they're priced very differently, and they age differently once the unit is in a wall.
The three materials
18-8 stainless. The austenitic grade, and the one you're picturing when you write stainless on a purchase order. Good corrosion resistance. It cannot be hardened.
410 stainless. A martensitic grade, so it can be heat treated and hardened. It drills what 18-8 can't. Less corrosion resistance in exchange.
Bi-metal. A 300-series stainless body with a hardened carbon steel drill point and first threads joined to it. The carbon steel drills and taps, then the stainless follows into the hole and stays there facing the weather.
Why 410 drills and 18-8 doesn't
A drill point has to be harder than the material it's cutting.
18-8 is austenitic, which means it doesn't respond to heat treatment the way carbon steel does. It work-hardens as you deform it, but it can't be hardened into a cutting edge. Going into aluminum or vinyl that's no obstacle. Going into steel it wears, rounds over, stops cutting, and then the screw either spins in place burnishing the hole or snaps when the operator leans on it.
410 is martensitic, so heat treatment hardens it and the point holds an edge. That's why 410 exists as a tek screw material at all.
The trade-off is corrosion. 410 carries less chromium and no meaningful nickel, so it doesn't resist corrosion the way 18-8 does. It still beats a zinc-plated case-hardened carbon steel screw, which is rated for dry interior work, while 410 holds up to light outdoor exposure. But it's a step down from the stainless you had in mind when you wrote stainless on the order.
What we'd put on the order
For most window work the decision runs on exposure, not on what you're drilling.
18-8 for exterior. Anything on the weather side of the unit. Window framing is mostly aluminum and vinyl, and 18-8 drills both without complaint, so you rarely pay a drilling penalty for taking the better corrosion resistance. That's the whole reason it works out: on a typical frame you don't have to choose.
410 for interior. Inside the unit, behind the glazing, anywhere the fastener stays dry. You get the harder point and you're not paying for corrosion resistance the joint will never use.
Bi-metal for coastal and highly oxidative environments. Salt air, marine exposure, chemical plants, anywhere the corrosion load is genuinely severe.
That third one comes with a caution.
Bi-metal is the exception, not the upgrade
Bi-metal is expensive. Not slightly more than 410, meaningfully more, and enough that it changes the cost of a unit if you spec it as a default.
It's easy to read the description and conclude it's the have-it-both-ways answer: hardened point, stainless body, no compromise. On paper it is. On a purchase order it's a premium you're paying on every screw, most of which are going into joints that never needed it.
Reach for it when the environment actually demands it. Coastal installations, anything that will see salt spray, and highly oxidative settings are the cases where 18-8 alone gives up too much and 410 gives up far too much. Outside those, it's cost without a matching benefit.
The case where the rule breaks
The exposure rule assumes you're drilling aluminum or vinyl, which covers most window framing. It stops working when there's steel in the assembly.
If an exterior unit has a steel reinforcement channel, a heavier gauge, or a steel subframe, an 18-8 point may not get through it. Now the rule and the metal are in conflict: exposure says 18-8, the substrate says you need a hardened point.
That's the real decision, and there are only three ways out of it. Pilot the hole and use 18-8 as a tapping screw rather than asking it to drill. Accept 410 on that joint and know you've traded some corrosion life. Or spend on bi-metal for that joint specifically, which is one of the few places the price is genuinely earned.
Whichever way you go, decide it once at the print rather than letting the line work it out with whatever's in the bin.
What to specify
Name the material. "Stainless tek screw" is not an order. 18-8, 410, and bi-metal are three different quotes, and if you don't say which, you'll get whatever that supplier stocks.
Say whether the joint is interior or exterior. That one word does most of the work on this decision.
Say what you're drilling and how thick. If there's steel anywhere in the assembly, that's the detail that decides whether 18-8 is even viable.
Ask about head and drive, and standardize those separately from the material. Mixing drives across a build costs your line more time than the screws cost you.
And if the frames are aluminum, say so. Aluminum sitting against a dissimilar metal with moisture reaching the joint is a galvanic question, and it's the aluminum that gives ground, not the screw. A plain carbon steel fastener rusts and stains the finish on top of that.
We keep the tek screws we supply in both 18-8 and 410 across the common window sizes, and when a build needs a head, drive or finish you can't find in a catalog, we get it run to a print. If you've got steel in an exterior assembly and you're weighing the three, tell us what you're drilling and we'll tell you which way we'd go.
For help choosing the right tek screw material, head style, or finish for your window assembly, contact us at [email protected].
