Bar stock is solid metal in standard cross-sections — flat, round, square, or hexagonal — sold in standard lengths and cut to size at the service center or machine shop. It is the raw material for machined parts, pins, shafts, weld-on tabs, and structural connectors. Selecting the correct grade and form requires understanding both the dimensional options and how alloy composition affects machinability, weldability, and heat-treat response.
Bar Stock Cross-Sections
Flat Bar
Specified as thickness × width: a "1/4 × 2 flat bar" is 1/4" thick and 2" wide. Flat bar differs from plate in origin — it is narrow enough to roll-mill directly and has better straightness than plate strips. Both long faces carry mill edges.
Standard thicknesses: 1/8", 3/16", 1/4", 5/16", 3/8", 1/2", 5/8", 3/4", 1", 1-1/4", 1-1/2", 2"
Standard widths: 1/2", 3/4", 1", 1-1/4", 1-1/2", 2", 2-1/2", 3", 4", 6", 8", 10", 12"
| Thickness | Commonly Stocked Widths |
|---|---|
| 1/8" | 1/2", 3/4", 1", 1-1/2", 2", 3" |
| 3/16" | 3/4", 1", 1-1/2", 2", 3", 4" |
| 1/4" | 3/4", 1", 1-1/2", 2", 2-1/2", 3", 4", 6" |
| 3/8" | 1", 1-1/2", 2", 3", 4", 6" |
| 1/2" | 1", 1-1/2", 2", 3", 4", 6" |
| 3/4" | 1-1/2", 2", 3", 4", 6" |
| 1" | 2", 3", 4", 6" |
Round Bar
Specified by diameter. Full range from 1/8" through 6" at most service centers; sizes above 4" are specialty items at some centers.
Common diameters: 1/8", 3/16", 1/4", 5/16", 3/8", 7/16", 1/2", 9/16", 5/8", 3/4", 7/8", 1", 1-1/8", 1-1/4", 1-3/8", 1-1/2", 1-3/4", 2", 2-1/4", 2-1/2", 3", 4", 5", 6"
Square Bar
Specified by side dimension: 1/4", 5/16", 3/8", 1/2", 5/8", 3/4", 7/8", 1", 1-1/4", 1-1/2", 2", 3"
Hex Bar
Specified by across-flats (AF) — the distance between two parallel flat faces (what a wrench engages). Do not confuse with across-corners (AC), which is larger by a factor of approximately 1.155.
| AF (in) | AC (in) |
|---|---|
| 1/4" | 0.289" |
| 3/8" | 0.433" |
| 1/2" | 0.577" |
| 5/8" | 0.722" |
| 3/4" | 0.866" |
| 7/8" | 1.010" |
| 1" | 1.155" |
| 1-1/4" | 1.443" |
| 1-1/2" | 1.732" |
| 2" | 2.309" |
Common AF sizes: 1/4", 5/16", 3/8", 7/16", 1/2", 9/16", 5/8", 3/4", 7/8", 1", 1-1/4", 1-1/2", 1-3/4", 2", 2-1/2", 3"
Always include "AF" in callouts to eliminate across-corners ambiguity.
Material Grades
1018 Cold-Rolled — General-Purpose Machining Bar
Low carbon (0.18% max), good toughness and ductility. Cannot be through-hardened — only case hardening (carburizing) is practical. Clean surface, tight dimensional tolerances (±0.002" on most diameters).
Best for: Pins, bushings, spacers, general turned parts where strength and hardness are not design drivers.
Avoid when: The part needs through-hardening or medium-high strength. Use 1045 or 4140 instead.
1045 Medium-Carbon — Moderate Strength and Wear
Medium carbon (0.45% max) gives a meaningful step up in strength. Can be flame or induction hardened to approximately HRC 54 at the surface.
Best for: Shafts, axles, gears, sprockets, and components that need moderate strength and surface hardness.
Welding: Higher carbon increases HAZ hardening risk. Preheat to 150–300°F for sections over 1" thick. Use low-hydrogen filler (E7018 or ER70S-3).
4140 Chromoly — The Structural Shaft Grade
Chromium-molybdenum alloy with outstanding hardenability. Through-hardenable in sections to 2.5"+ depending on quench media. The standard for structural shafting, tooling, and applications requiring heat treatment to HRC 30–50.
Available conditions:
- Annealed: ~60 ksi yield, machines well, heat treat after machining
- Pre-hardened: ~100 ksi yield, HRC 28–32, skip heat treat if RC 28–32 is sufficient
Welding 4140: Requires preheat (400–600°F for heavy sections), low-hydrogen filler, controlled interpass temperature. Not a first-choice material for welded assemblies — design to weld in mild steel and pin or bolt the 4140 component.
1144 Stressproof — High Strength, No Heat Treat
Resulfurized, rephosphorized free-machining steel. Cold-drawn under controlled tension to achieve high strength without heat treatment.
| Property | Value |
|---|---|
| Yield strength | ~100 ksi |
| Tensile strength | ~115 ksi |
| Machinability rating | ~83% |
Best for: High-volume CNC turned parts needing good strength and excellent finish without a heat treat step.
Weldability: Marginal — sulfur reduces weld quality. Use for machined components only.
12L14 — Maximum Machinability
The highest machinability rating of any common carbon steel bar (~160–170% relative to 1212). Lead content (0.15–0.35%) lubricates the cutting tool and promotes chip breakage.
Best for: High-volume CNC turning where cycle time and tool life are the primary cost drivers.
Do not use when:
- Welding is required (lead fumes are toxic; weld quality is very poor)
- Cold forming or bending is needed (low ductility)
- Lead content is restricted (RoHS, California Prop 65)
- Food contact or potable water applications
Hot-Rolled vs Cold-Finished Bar
| Property | Hot-Rolled (HR) | Cold-Finished (CF) |
|---|---|---|
| Surface | Mill scale | Bright, smooth, clean |
| Diameter tolerance | ±1–2% | ±0.001"–0.003" |
| Straightness | L/240 | L/480 |
| Cost | Baseline | 10–20% above HR |
| Use for | Weldments, rough fabrication | Machined parts, precision shafts |
Residual stress in CF bar: Cold-finished bar carries residual compressive stress on the outer surface. A heavy roughing cut that removes this layer can cause the bar to bow. For long, slender shafts, rough-turn first, allow stress relief, then finish-turn.
Mill Length and Yield Optimization
Bar stock is typically sold in 12-ft random lengths (11–13 ft). Optimize part length to minimize scrap (saw kerf ≈ 1/8"):
| Part Length | Pieces per 12-ft Bar | Scrap per Bar |
|---|---|---|
| 6" | 22 | ~1.25" |
| 8" | 17 | ~6.9" |
| 12" | 11 | ~8.6" |
| 18" | 7 | ~11.1" |
| 24" | 5 | ~23.4" |
Adding or removing 1" of length can eliminate an entire bar from a material order at high quantities.
How to Call Out Bar Stock on a BOM
Flat: FB 1/4 × 2 × 12-1/4, A36 HR
Round: RD 1.500 × 24, 1045 CF
Square: SQ 1 × 6, 1018 CF
Hex: HEX 3/4 AF × 9-3/8, 12L14 CF
When grade is omitted, A36 hot-rolled is typically assumed for flat bar. For round and square bar, the shop may default to 1018 CF or A36 HR depending on their stock — specify explicitly for any application where grade matters.