Cold rolled vs hot rolled steel: which bar should a machined part be made from?

What is the real difference between cold rolled vs hot rolled steel for machining?

The difference is the last operation at the mill: hot rolled bar is sold as it came off the hot mill, while cold finished bar has had its scale removed and its section reduced or turned cold. That one step changes size accuracy, surface, strength and internal stress at the same time.

In the US system these are separate purchasing specifications, not two conditions of one product. Hot-wrought carbon bar for machining, forging and heat treating is bought to ASTM A576 (special quality) or ASTM A575 (merchant quality, for noncritical parts). Cold-finished bar is bought to ASTM A108, whose scope names bars “for machining into components” and for shafting. The general requirements for both bar families, including the grade chemistry tables, sit in ASTM A29/A29M; its size tables cover hot-wrought bar, while the size tables for cold-finished bar now sit in A108 itself.

A36 is different again. ASTM A36/A36M covers carbon steel shapes, plates and bars “of structural quality” for riveted, bolted or welded construction, and its bars take their dimensional rules from ASTM A6/A6M, the general requirements for structural products. It is a strength specification for steelwork that happens to be available as round and flat bar, not a machining-bar specification.

A note on words: “cold rolled” is exact for sheet and strip, where ASTM A1008 covers cold-rolled sheet and ASTM A1011 covers hot-rolled sheet and strip. Cold-finished round bar is cold drawn, turned and polished, or ground and polished, not literally cold rolled. Old drawings that say only “CRS” or “HRS” leave the grade, the specification and the finishing route open, and a supplier will fill that gap with whatever is cheapest locally.

How do mill scale, decarburization and seams affect a machined part?

Mill scale is a hard, abrasive iron oxide that shortens tool life on the first pass, and the decarburized layer below it is softer than the core. Cold drawn bar avoids both, but it can carry longitudinal seams that only show after plating or heat treatment.

On hot rolled stock, the first cut should get under the scale in one pass rather than rubbing along it, because a light cut that rides in the scale wears the insert fastest. Scale left on any surface also has to come off before finishing: zinc plating, powder coat and paint need pickled or blasted steel, and a part with patches of original scale on unmachined faces will show them through the finish.

Decarburization matters most when the surface has to be hard or carry fatigue load, because the decarburized skin is softer and weaker than the core beneath it. If an unmachined hot rolled surface sits in the load path, specify a maximum decarburization depth or require that surface to be machined. ASTM A576 offers supplementary requirements for purchasers who need superior surface quality or special metallurgical characteristics; they apply only when named on the order.

Cold drawn bar has a clean, bright surface that machines and plates predictably, but drawing does not remove seams already in the hot rolled feedstock. For parts that are fatigue loaded, crimped or plated bright, turned and polished bar, which has had the original surface machined off, removes that risk at a higher material cost.

When does A36 make sense, and when should you specify 1018 bar?

Use A36 when the part is structural steelwork that gets some machining, such as a welded bracket, base plate or gusset where a building or structural code asks for A36. Use 1018 when the part is machined from bar for its function, needs consistent machining from lot to lot, or will be carburized, and choose its cold drawn form when size accuracy and strength matter.

The reason is what each specification controls. Per ASTM A36/A36M the purchaser is guaranteed a minimum yield of 36 ksi (250 MPa) and a tensile range, while carbon carries only a maximum, with no minimum, and most other elements are capped rather than held to a narrow window. Two heats of A36 can therefore machine and weld differently, and some mill product is dual-graded, meeting A36 and a higher-strength grade at the same time. That is fine for a welded frame; it is a nuisance for a turned part that runs on a fixed program and tooling.

1018 is a grade designation with a defined carbon and manganese range in the grade tables of ASTM A29/A29M, so its machining, welding and carburizing response is predictable. As cold drawn bar it is a common default for pins, spacers, shafts and fixture components, and its low carbon makes it a standard carburizing grade when a part needs a hard wear surface over a tough core. Its weakness is machinability: plain low-carbon steel is gummy, forms long stringy chips and gives a poorer as-cut finish than a resulfurized grade.

That leads to the other options worth comparing. Where machinability matters more than weldability, resulfurized grades such as 1117 or the free-machining 12L14 and 1215 cut faster and leave a better finish; the 12L14 vs 1215 guide covers their limits. Where strength matters more, 1045 or an alloy steel chosen through ASTM A400, the practice for selecting steel bars by section and required mechanical properties, is the usual next step. A36 in round bar is rarely the best answer for a lathe part; when it appears on one, it is usually because it was the bar on hand, not because the part needs it.

How should the drawing and RFQ call out hot rolled or cold drawn bar?

Name the grade, the product specification and the condition in one line, then say which surfaces may stay as-supplied and what certification you need. A callout like “AISI 1018 cold drawn bar per ASTM A108, stress relieved” tells a shop exactly what to buy.

Examples that leave no room for substitution:

• Machined shaft, size-critical: “MATERIAL: AISI 1018 COLD DRAWN BAR PER ASTM A108. MILL TEST CERTIFICATE REQUIRED.”
• Part with a long one-sided slot: “MATERIAL: AISI 1018 COLD DRAWN, STRESS RELIEVED, PER ASTM A108” or “AISI 1018 HOT ROLLED PER ASTM A576, MACHINE ALL OVER.”
• Welded base with machined pads: “MATERIAL: ASTM A36/A36M PLATE.” Add a grade such as 1018 only if the machined features need predictable chemistry.
• Equivalents: if you will accept an EN, ISO or JIS grade, name it; if not, write “NO SUBSTITUTION WITHOUT WRITTEN APPROVAL.”

On the RFQ, add the annual volume, which surfaces are functional, any heat treatment or finish, and whether you need a mill certificate per lot. The EN 10204 certificate guide explains which document type proves what, and the RFQ package guide covers the rest.

For turned parts, the CNC turning page lists the steel bar running at our lead CNC plant today as 52100, C1117 and 12L14, plus ASTM A513 DOM tube; 1018 and A36 are not on that list, so if your drawing names them, say in the RFQ whether C1117 or another listed grade is acceptable. The CNC milling page is explicit that milling here is a second operation on castings, not a general billet-machining shop, so a part cut entirely from A36 plate is better placed with a dedicated machining house. Material certification is available on request, as listed on the quality page, and zinc plating, blue trivalent chromate and oil-dip rust protection are on the surface finishing menu our partner plants apply, with the quote naming where each step is done. When the drawing is ready, send it for a quote.