Steel plate is the starting point for plasma-cut parts — flanges, gussets, brackets, frames, and wear components all originate from flat stock. Selecting the right thickness and grade requires understanding not just what sizes are available, but how ASTM grades differ in yield strength, weldability, and service conditions.
Plate vs Sheet: The 3/16" Threshold
Industry convention in North America places the boundary at 3/16" (4.76 mm). Material below this threshold is produced on a continuous sheet mill and sold in sheet sizes with gauge designation. Material at 3/16" and above is produced on a plate mill and sold by fractional or decimal thickness.
Some mills draw the line at 1/4", classifying 3/16" as heavy sheet. Material between 10 gauge (0.135") and 3/16" can be sourced from either sheet or plate inventory depending on the supplier — check availability before designing around this range.
Use sheet when the part needs to be bent, formed, or rolled. Use plate when the part is flat, load-bearing, or will be plasma-cut to a near-net shape.
Standard Plate Thicknesses
| Fraction | Decimal (in) | Metric (mm) |
|---|---|---|
| 3/16" | 0.1875 | 4.76 |
| 1/4" | 0.2500 | 6.35 |
| 5/16" | 0.3125 | 7.94 |
| 3/8" | 0.3750 | 9.53 |
| 1/2" | 0.5000 | 12.70 |
| 5/8" | 0.6250 | 15.88 |
| 3/4" | 0.7500 | 19.05 |
| 7/8" | 0.8750 | 22.23 |
| 1" | 1.0000 | 25.40 |
| 1-1/4" | 1.2500 | 31.75 |
| 1-1/2" | 1.5000 | 38.10 |
| 2" | 2.0000 | 50.80 |
| 3" | 3.0000 | 76.20 |
| 4" | 4.0000 | 101.60 |
7/16" and 9/16" availability: These are standard at mills but not always stocked by local service centers. Verify before committing — most shops default to 3/8" or 1/2" and adjust accordingly.
Standard sheet sizes: 48×96", 48×120", 60×120", and 60×240" for heavy structural work.
ASTM Grade Comparison
A36 — General-Purpose Mild Steel
The default mild steel plate. When a drawing specifies "steel plate" without a grade, A36 is almost universally assumed.
| Property | Value |
|---|---|
| Yield strength (Fy) | 36 ksi minimum |
| Tensile strength (Fu) | 58–80 ksi |
| Elongation | 20% minimum |
| Carbon content (max) | 0.26% (≤3/4") |
| Weldability | Excellent |
Best for general structural frames, gusset plates, brackets, base plates, and non-critical flat parts. Correct choice for the vast majority of light fabrication work.
A572 Grade 50 — High-Strength Low-Alloy (HSLA)
50 ksi yield at a small cost premium over A36. Allows thickness reduction while maintaining equivalent load capacity.
| Property | Value |
|---|---|
| Yield strength (Fy) | 50 ksi minimum |
| Tensile strength (Fu) | 65 ksi minimum |
| Weldability | Excellent |
| Cost index | 5–10% above A36 |
Weight savings example: A bracket designed in 1/2" A36 could be redesigned in 3/8" A572-50 for similar load capacity — a 25% weight reduction.
Best for trailer frames, crane booms, equipment frames, and any application where strength-to-weight matters. Increasingly common as the cost delta has narrowed.
A516 Grade 70 — Pressure Vessel Steel
Produced with controlled chemistry to maintain toughness at low temperatures. Specified by pressure vessel codes (ASME Section VIII) rather than structural codes.
| Property | Value |
|---|---|
| Yield strength (Fy) | 38 ksi minimum |
| Tensile strength (Fu) | 70–90 ksi |
| Charpy impact | Required testing at temperature |
| Cost index | 15–25% above A36 |
Best for pressure vessels, tanks, boilers, and closed containers subject to internal pressure or cold-temperature service. Avoid for standard structural brackets — A36 is sufficient and cheaper.
AR400 / AR500 — Abrasion-Resistant Steel
Through-hardened to a target Brinell hardness — 400 BHN for AR400, 500 BHN for AR500. These are wear grades, not structural grades. Their purpose is surface abrasion resistance, not load-bearing capacity.
| Property | AR400 | AR500 |
|---|---|---|
| Brinell hardness | ~400 BHN | ~500 BHN |
| Yield strength | ~180 ksi | ~220 ksi |
| Weldability | Requires preheat + low-H | Requires preheat + low-H |
| Cost index | 2–3× A36 | 3–4× A36 |
Welding AR plate: Use low-hydrogen process (E7018 or ER70S-3 MIG with clean joints), preheat to 300–400°F, and slow cool. Standard wire without preheat will crack in the heat-affected zone.
Best for bucket liners, chutes, dump body floors, dozer blade edges, and any surface subject to sliding or impact abrasion.
Surface Finish Options
Mill scale (as-rolled): Hot-rolled plate carries iron oxide scale bonded to the surface. Must be removed before powder coating, painting, or precise welding.
Pickled and Oiled (P&O): Acid pickling removes scale; oil prevents flash rusting. Preferred when painting follows immediately and surface prep time is limited.
Blast and prime: Shot-blasted to SSPC-SP10 (near-white), then primed. Highest cost; specified for structural steel going directly to field erection.
ASTM A6 Thickness Tolerance
ASTM A6 governs thickness tolerances for structural plate. Tolerances are asymmetric — plates may be over nominal thickness but not under.
| Nominal Thickness | Over Tolerance | Under Tolerance |
|---|---|---|
| 3/16" to 3/8" | +0.030" | -0.000" |
| Over 3/8" to 3/4" | +0.030" | -0.000" |
| Over 3/4" to 1" | +0.035" | -0.000" |
| Over 1" to 2" | +0.040" | -0.000" |
| Over 2" to 4" | +0.050" | -0.000" |
Plasma-cut parts achieve ±0.015" overall dimensional tolerance from the cut — well within plate mill tolerance — and this cut variation is the dominant source of dimensional error in flat parts.
Cutting Process Suitability by Thickness
| Thickness | Plasma | Fiber Laser | Waterjet | Oxy-fuel |
|---|---|---|---|---|
| 3/16"–1/4" | Good | Good | Excellent | Possible |
| 3/8"–1/2" | Good | Marginal | Excellent | Good |
| 3/4" | Good | Not practical | Excellent | Good |
| 1"–1-1/2" | Good | Not practical | Good | Good |
| 2"+ | Marginal | Not practical | Good | Best |
Plasma is the most cost-effective process for mild steel from 3/16" through 3/4". Above 3/4", kerf taper increases and oxy-fuel or waterjet becomes preferable for precision cuts.
How to Spec Plate on a Drawing
PL 3/8 × 8 × 12 ASTM A36
Reading left to right: PL = plate, 3/8 = thickness, 8 × 12 = width × length in inches, ASTM A36 = grade.
For A572: PL 1/4 × 6 × 10 ASTM A572 Gr. 50
When grade is unspecified, fabricators default to A36. If A572-50 is intended, it must be called out explicitly — the cost and strength difference are both significant enough that the assumption should never be implicit.