304, 316 and 310 Stainless Steel Hex Bars: Grades, Sizes and Selection

Three Austenitic Grades, One Bar Family

304, 316 and 310 stainless steel bars are austenitic, non-magnetic in the annealed condition and produced as round, square, flat and hexagonal sections. What separates them is alloy content, corrosion resistance and elevated-temperature behaviour: 304 covers general engineering, 316 adds molybdenum for chloride service, and 310 or 310S trades pitting resistance for oxidation resistance at high temperature.

Because all three are supplied from the same mills in the same profiles, the grade question is usually decided by the environment the finished part will see, and only then by size availability. Getting that order of decisions right avoids paying for molybdenum that a dry indoor application does not need, or discovering pitting on a coastal installation that was ordered as 304.

Standards and Supply Conditions

Stainless bar is normally ordered against one of the following specifications, and the specification should be quoted in full on the purchase order because each carries its own test and tolerance requirements:

ASTM A276 - hot finished or cold finished bars for general corrosion-resisting service.

ASTM A479 - bars for pressure vessels and high-temperature service, with additional mechanical and grain-size requirements.

EN 10088-3 - stainless steel semi-finished products, bars, rods and sections for general purposes.

GB/T 1220 - stainless steel bars for domestic engineering projects.

Surface condition is quoted alongside the standard: bright annealed for machined components, peeled or ground where concentricity matters, polished for visible parts, black hot rolled for parts that will be machined all over, and pickled for fabrications that will be welded.

Size Ranges by Profile and Process

Process route controls the practical size window. Cold drawing holds tight tolerance and good straightness on small sections, hot rolling reaches the middle range economically, and forging covers the heavy end.

Profile	Cold drawn (mm)	Hot rolled (mm)	Forged (mm)
Round bar	1 - 80	10 - 200	80 - 600
Square bar	4 x 4 - 80 x 80	10 x 10 - 200 x 200	80 x 80 - 500 x 500
Hexagon bar - across flats	14 - 80	Larger sizes to drawing	To drawing
Flat bar - width x thickness	5 x 1 - 50 x 5	50 x 6 - 200 x 20	To drawing

Flat bar dimensions are always quoted as width by thickness, because the two figures are frequently transposed on enquiry sheets and a transposed order can be impossible to roll. Hexagon bar is measured across the flats, which is the dimension that matches the wrench or the mating nut, and not across the corners.

Mechanical Properties in the Annealed Condition

Grade	Tensile strength (min)	Yield strength 0.2% (min)	Elongation (min)
304	515 MPa	205 MPa	40%
304L	485 MPa	170 MPa	40%
316	515 MPa	205 MPa	40%
316L	485 MPa	170 MPa	40%
310S	515 MPa	205 MPa	40%

These are minimum annealed values. Cold drawing raises both tensile and yield strength considerably, so a cold-drawn bar can be materially stronger than the same grade in the annealed condition; where strength is the design driver, the drawn condition and the required mechanical properties must both be stated on the order.

Choosing Between 304, 316 and 310

304 is the workhorse: shafts, studs, fasteners, food and dairy equipment, architectural fittings and general machined parts in indoor or mildly polluted atmospheres.

316 and 316L add molybdenum for chloride-bearing environments such as coastal structures, marine hardware, chemical plant and process piping components.

310 and 310S are chosen for high-temperature service, where resistance to scaling and creep matters more than chloride pitting resistance, and they should not be substituted for 316 in a chloride application.

A useful check is to write down the environment before the part: atmospheric, immersed, chloride-bearing or high temperature. Chloride means molybdenum; sustained heat means the 310 family; everything else usually means 304, with the low-carbon variant when welding is involved.

Machining, Finishing and Inspection

Austenitic grades work harden rapidly, so machining practice matters more than the grade label. Heavy feeds with modest speeds keep the tool below the work-hardened layer, sharp tooling avoids rubbing that raises surface hardness, and generous coolant carries heat away from the cut. For hexagonal and flat bar that will be milled, bright drawn stock with consistent straightness reduces setup time on automatic machines.

Inspection documentation for bar typically covers chemical composition, mechanical properties, dimensional check and, where specified, intergranular corrosion or ultrasonic testing. Mill test certificates should be reviewed against the purchase order before the bar is released to production, especially for pressure and high-temperature applications where A479 requirements apply.

Frequently Asked Questions

Q: Is 304 bar suitable for outdoor use?
Yes in rural, urban and most industrial atmospheres. It becomes unsuitable close to the sea or where de-icing salt accumulates, because chlorides cause pitting and the molybdenum-bearing 316 family is then required.

Q: What is the difference between 316 and 316L bar?
316L has a lower carbon maximum, which reduces carbide precipitation during welding. Mechanical minimums are slightly lower, and 316L is preferred whenever the finished part will be welded.

Q: Why is hexagon bar measured across flats?
Across-flats is the dimension that mates with a wrench or a hex socket, so ordering in any other dimension risks a bar that cannot be gripped or fitted.

Q: Can these bars be supplied cut to length?
Yes. Bars are supplied in random lengths or cut to fixed length, and sawn ends can be deburred or chamfered when the bar feeds directly into a machining cell.

Q: How are the bars packed for export?
Bundles are strapped and usually wrapped for sea freight, with each bundle tagged by heat number so the certificate can be matched to the physical bar on arrival.

Q: When should the 310 family be selected?
When the part runs hot, for example furnace furniture, heat-treatment fixtures or burner components, where oxidation resistance at temperature outweighs chloride corrosion resistance.