Flat bar is the unglamorous hero of the old catalogue: 304 or 316, No. 1 finish, dull on purpose. It is how Australian fabricators make brackets, straps, bent ears, and the odd ledger that an engineer has actually blessed.
It is not a No. 4 bench. If you need cosmetic sheet, buy sheet. If you need an L, buy angle. If you need a U, buy channel. Flat bar is the rectangle that still does most of the honest work in a stainless shop.
This page sits with the other custom-cut and sections essays. It will not sell you a length.
What you are buying
Widths and thicknesses live in merchant lists — 20 × 3, 25 × 6, 40 × 10, 50 × 12 and up. Lengths are typically 4–6 m, then custom-cut to the job. Edges are mill edges, not furniture. Deburr if a hand will find them.
Grade follows the environment. Indoor plant: 304. Coastal strap on a balustrade: 316. Fasteners: match with A2 or A4.
Finish is usually HRAP / No. 1: hot-rolled, annealed, pickled. Grey, matte. That is not rust. If the bar has to sit next to a brushed splashback, you are asking a mill product to impersonate sheet. Polish is extra work — name it on the order.
Bar, strip and sheet slit
Flat bar is a bar-mill product. It has bar tolerances, bar edges, and a bar identity on a certificate. Strip is often a slit-sheet or narrow-coil cousin: thinner, wider in the sheet sense, different camber, different edge. A 50 × 3 “flat” cut from 3 mm sheet is a blank, not a bar.
Do not mix the words on a structural drawing. An engineer who wrote flat bar meant the section in the software. Substituting slit sheet changes stiffness and edge. If you meant a custom-cut blank, write blank.
Sizes the shop actually says
Lists change. These are the rectangles Australian trades keep in their mouths. Confirm availability. This is not a stock list and it is not a price.
| Width × thickness (mm) | Everyday use | Notes |
|---|---|---|
| 20 × 3 | Light straps, small clips, tags | Bends easily; not a load story |
| 25 × 3, 25 × 5, 25 × 6 | Brackets, ties, small frames | The default small bar |
| 32 × 6, 40 × 5, 40 × 6 | Cleats, ledgers, bent ears | Punch or drill before you argue about finish |
| 40 × 10, 50 × 6, 50 × 8 | Heavier brackets, wear strips | Edges need a thought if a palm can find them |
| 50 × 10, 50 × 12 | Engineered cleats, simple frames | HRAP will show every weld |
| 75 × 10, 100 × 12 | Ledgers, base plates, guards | Still bar, still dull, still not a bench |
Cut on a dedicated stainless saw. A carbon-steel blade leaves iron in the cut. Protecting stainless during install starts on the rack: timber or plastic bearers, no rusty chain.
Brackets, straps and the jobs that fit
- Brackets and cleats. Two holes, a bend or a weld, a known thickness. This is the product.
- Straps and ties. 20 × 3 and 25 × 3 around a duct, a pipe, a light cover. Match alloy to the thing you are hugging. A 304 strap on a coastal 316 rail is a brown stripe.
- Bent ears. Heat or press. Austenitics work-harden; a tight cold bend on thick bar can crack the outer fibre if you rush it.
- Wear strips. 316 if the wear is wet and salty. Still HRAP unless you paid to dress it.
- Simple frames. Welded rectangles. Design the corner; a flat-bar frame is not automatically square.
- Shims that will stay wet. Then 316, and accept that a shim is a crevice.
Uses that need a conversation: anything a palm will slide on, a food-zone lug that cannot be washed, anything asked to be a spring. Flat bar is a rectangle, not a leaf spring. See the fabrication and care hub before you hide a bracket where a hose cannot reach.
Fabrication
Flat bar bends on a press. Hole-punching work-hardens austenitics; drill if the finish matters. Welding is ordinary 304/316 practice — welding essay — plus the reminder that a dull bar will show a bright weld until you pickle or dress it.
Do not grind it with a carbon-steel wheel and then complain about rust freckles. Protection during install applies to bar sitting on a rusty rack.
Bending
Inside radius should respect thickness. A 3 mm strap will take a tight ear. A 10 mm cleat will not take a 3 mm internal and stay pretty. Warming thick austenitic bar is a procedure, not a rosebud and hope. If the bend is structural, the engineer already had a radius in mind — use it.
Springback is larger than mild steel. Check the first piece. A batch of 40 × 6 ears at 85° is a site argument with a mill certificate attached.
Holes
Punching is fast and common. It leaves a rollover on one face and a break on the other. Deburr. On a seen face, drill and countersink if you must, or punch and dress. Slots for adjustment are drill-and-file or mill work; a chain of punched holes is a tear waiting for a load.
Spacing from the edge still matters. A 6 mm hole in a 20 × 3 strap with 4 mm of edge is a stamp, not a connection. Follow the drawing.
Welding and the bright bead
304/304L to 308L filler, 316/316L to 316L filler. 304 versus 316 does not take a day off because the part is a cleat. A 308 bead on a coastal 316 strap is a line of weaker alloy.
Heat tint on HRAP is obvious: a rainbow on a grey bar. Remove it on outdoor and hygiene work, then passivate. A pretty grind that smears carbon steel is worse than the tint you started with.
Distortion: a 3 mm strap will banana if you run a long fillet. Stitch. Clamp. A 12 mm bar will move less and still move.
Finish, edges and hands
HRAP will show fingerprints less than a polish and dirt more than a 2B sheet. Wash it like the rest of the stainless: neutral detergent, no bleach, no steel wool. Cleaning without chlorides is the same chemistry on a bar as on a bench.
Edges cut people. A 50 × 10 mill edge on a handrail-height member is a design problem. Break the arris, or specify a dressed edge, or stop calling it a rail. Coastal care will not save a sharp heel that also ponds salt.
If you polish bar to match an adjacent No. 4, use dedicated abrasives with the grain. A wheel that already cut carbon steel is a rust tool.
Alloy and the wet strap
Indoor plant and dry frames: 304. Coastal straps and pool-fence cleats: 316, and still wash it. 316 tea-stains on a horizontal strap that holds water. Tea staining is not proof the merchant swapped the alloy; it is proof the strap is a shelf. A2 into 316 outdoors is a brown halo. Isolate carbon-steel structure behind a stainless strap or the rust will print through.
For a true L or U, go back to angle and channel. For the weld, the film and the wash, stay in fabrication and care.