For general fabrication — base plates, brackets, weldments, shafts, gears, structural frames — the shortlist collapses to four workhorses: ASTM A36 / A572 structural plate, ASTM A516 pressure-vessel plate, 4140 and 4340 bar, and 8620 for case-hardened parts. Cr-Mo forging grades from ASTM A182 (F1, F5, F9, F11, F12, F21) belong on the shelf but are not the default for ambient-temperature fabrication [S2].
The hard rule of thumb in any shop: pick the cheapest grade that meets the design margin in yield, impact, and weldability. Over-spec'ing Cr-Mo is the single most common cost mistake in custom fabrication; under-spec'ing A36 into a dynamic-loaded part is the most common failure-mode mistake. The chemical and tensile data for ASTM A182 grades F1 (C ≤0.28, Mo 0.44–0.65; yield ≥275 MPa, tensile ≥485 MPa) and F5 (C ≤0.15, Cr 4.0–6.0, Mo 0.45–0.65; yield ≥275 MPa, tensile ≥485 MPa) confirm these grades are alloyed for high-temperature pressure vessel service [S2].
Common Alloy Steel Grades and What They Actually Buy You
ASTM A36 remains the default for general-purpose structural plate with a minimum yield of 250 MPa and tensile 400–550 MPa, designed for ambient-temperature welding. ASTM A572 Grade 50 lifts yield to 345 MPa at near-identical weldability, and is the natural upgrade when deflection or weight drives the design. ASTM A516 Grade 70 (carbon steel, not alloy) is the go-to for pressure-vessel plate up to ~370°C, with tensile 485–620 MPa per typical spec sheets — note that A516 is a carbon-steel grade, not an alloy, and is referenced only as a weldability/strength benchmark [S2].
For bar stock, AISI 4140 (Cr-Mo, ~0.40% C, 0.8–1.1% Cr, 0.15–0.25% Mo) is the workhorse for shafts, gears, and pins above 25 mm section. Through-hardened 4140 reaches tensile 850–1,000 MPa with Q&T; pre-hardened 4140 at 28–32 HRC is supplied at ~850 MPa tensile for direct machining. AISI 4340 (Ni-Cr-Mo, ~0.40% C, 1.8% Ni, 0.8% Cr, 0.25% Mo) buys an extra 10–15% toughness over 4140 at the same hardness, at roughly 1.4–1.7× the price. AISI 8620 (Ni-Cr-Mo, ~0.20% C) is the default case-hardening grade — carburize, quench, get ~60 HRC case with tough core for gears, splines, and bearing journals. Read a broader definition in the alloy steel reference page for context on what counts as "alloy" versus carbon [S2].
Selection Criteria: Match the Grade to the Constraint
Three constraints decide the grade, in this order: (1) weldability vs. required strength, (2) section thickness vs. hardenability, (3) service environment. For thin-to-medium welded assemblies up to 25 mm in mild service, A36 or A572 wins on cost and ease — no preheat below 19 mm at ambient. Move to 4140 only when wear or fatigue demands a through-hardened bar; expect preheat to 150–200°C for welding 4140 above ~25 mm section, with PWHT at 595–650°C to restore toughness in the HAZ. [S1]
Hardenability is the second gate. 4140 has a Jominy end-quench distance to 50 HRC of roughly 12–15 mm in oil — meaning core hardness drops fast beyond that section. 4340 extends that to ~25–30 mm because of its nickel content; that's why 4340 dominates large landing-gear, crankshaft, and high-stress shaft applications. For heavy plate or thick-walled pressure parts, ASTM A387 Grade 11 (1.25 Cr-0.5 Mo) and Grade 22 (2.25 Cr-1.0 Mo) step in — both designed for elevated-temperature service with tensile 515–690 MPa, mandatory PWHT, and impact testing at -29°C or per design code. For an applied view of how this maps to upstream service, the alloy steel for oil and gas working map is a useful parallel [S2].
Comparison Table: Common Fabrication Grades Against 4 Decision Criteria
The four decision criteria for general-fabrication grade selection are: (a) minimum yield strength, (b) as-welded strength without PWHT, (c) through-section hardenability, (d) relative cost. Lining up the common grades against those axes gives: A36 plate — 250 MPa yield, weldable as-rolled, not hardenable, baseline cost. A572 Gr.50 — 345 MPa yield, weldable as-rolled, not hardenable, ~10–15% cost premium. A516 Gr.70 — 260 MPa yield, weldable with moderate preheat above 25 mm, not hardenable, cost premium ~20%. 4140 bar — 655 MPa yield (Q&T), requires preheat + PWHT for welding, ~12–15 mm through-hardenability, ~3–4× cost. 4340 bar — 745 MPa yield (Q&T), requires preheat + PWHT for welding, ~25–30 mm through-hardenability, ~5–6× cost. A387 Gr.11 plate — 310 MPa yield, weldable with mandatory PWHT, not hardenable for through-section, ~2× cost [S2].
The pattern is clear: cost tracks alloy content and required heat treatment, not raw strength. If a design needs 550 MPa yield in a welded assembly under 30 mm thick, the cheapest route is usually quenched-and-tempered A514 (S690QL equivalent) plate — not 4140 bar welded into a built-up shape. Built-up weldments in 4140 are expensive, distortion-prone, and a PWHT logistics problem on anything larger than a workbench.
Real Use Cases: Which Grade Goes Where
Industrial base frames, equipment skids, conveyor supports, and building-structure members default to A36 / A572 plate with fillet welds using E7018 electrodes; no preheat under 19 mm at ambient. Conveyor shafts under 80 mm diameter with keyways are commonly 1045 cold-drawn (carbon steel) — not alloy — for static loading; the moment dynamic loading or spline length pushes the design, 4140 Q&T enters. A 500-foot bridge designed, fabricated, painted and erected in a single lift by Alliance Industrial Group illustrates the scale A36/A572 welded plate construction can reach when detailing and welding procedure are controlled [S3].
For gears and splines, 4140 pre-hardened to 28–32 HRC is standard for industrial gearboxes; AGMA-class service above 200 hp typically shifts to 4320 or 8620 case-hardened for surface durability. For wear plates and chute liners, AR400 / AR500 (carbon-manganese, through-hardened abrasion-resistant) is cheaper and tougher than 4140 in actual sliding-abrasion service. For fasteners in marine, pool, and outdoor-furniture fabrication, stainless dominates for corrosion reasons — but where the spec calls for an alloy steel fastener, ISO 898-1 property classes 8.8, 10.9, and 12.9 govern the chemistry and temper, and a U.S. supplier holding ISO 9001:2015 certification publishes REACH, California Prop 65, RoHS, and Conflict Minerals statements as the compliance baseline for spec-in [S1].
Limitations, Failure Modes, and What to Avoid
Three failure modes dominate general-fabrication alloy-steel work: (1) weld cracking in 4140/4340 from cold cracking (hydrogen-assisted) — prevented by preheat ≥150°C, low-hydrogen electrodes (E7018 or E11018 baked per AWS A5.1), and PWHT 595–650°C for 1 h per 25 mm thickness. (2) Stress-corrosion cracking in plain-carbon fasteners exposed to chlorides — solved by stepping up to 316 stainless or hot-dip galvanizing, not by going to higher-strength alloy. (3) Quench cracking from in-shop heat treatment of 4140 without proper fixturing and tempering within 30 minutes of quench — the most common cause of scrap on small batches of custom parts. [S1]
What to avoid: do not specify ASTM A182 F11 / F22 / F5 / F9 for ambient-temperature fabrication. The Cr-Mo chemistry and elevated-temperature tensile/creep ratings in ASTM A182 (F11 Class 2: 275 MPa yield min, 485 MPa tensile min, 143–207 HB; F22 family; F5 with Cr 4.0–6.0% Mo 0.45–0.65%; F9 with Cr 8.0–10.0% Mo 0.9–1.1%) are engineered for creep resistance at 300–600°C, not for shop welding at room temperature [S2]. The cost premium is real (typically 3–6× over A36 plate), preheat/PWHT windows are stricter, and you gain nothing at ambient service. Cross-check material against the titanium alloy and aluminum alloy reference pages only when weight or corrosion, not strength, is the constraint — at which point alloy steel is the wrong family.
Standards, Sourcing, and Compliance Baseline
The default standards bench for general fabrication: ASTM A36 / A572 / A516 for plate; ASTM A108 / A29 for bar; AISI-SAE 4140 / 4340 / 8620 grade designations; ASTM A182 for high-temperature pipe fittings and forgings when the spec does drift into that territory (Cr 0.45–0.65% Mo for F1, Cr 4.0–6.0% for F5, Cr 8.0–10.0% for F9 — chemistry per published spec sheets) [S2]. For welding procedure, AWS D1.1 (steel) governs shop and field welding above 5 mm thickness. For pressure-boundary work, ASME BPVC Section VIII + Section IX (welding procedure qualification) replaces AWS D1.1.
On the supplier side, ISO 9001:2015 certification is the minimum QMS baseline for any distributor shipping into OEM, marine, or structural fabrication — Alloy Fasteners, a U.S. stainless and alloy fastener distributor with 65+ years in business, publishes ISO 9001:2015 plus REACH, California Prop 65, RoHS, and Conflict Minerals statements as the documentation pack spec-in engineers should expect [S1]. For general-fab shops like Alliance Industrial Group, the 2013 Oregon Manufacturing Award (medium company category) is the kind of third-party credential that matters when a general contractor is qualifying a structural-fab supplier for tonnage work — Alliance publicly cites 10,000 tons of steel erected in 6 months and a 500-foot bridge single-pick as documented scale evidence [S3].
Closing signal to track: the next time a fabrication RFQ lands on the desk, run the grade through three filters in order — weldability, required strength, then environment. If all three land on A36/A572/4140, do not over-spec. If environment pushes you past 200°C sustained service or into sour (H₂S) service per NACE MR0175, switch the reference set to the alloy-steel oil-and-gas working map instead of treating it as a general-fab problem. The two failure modes in this trade are not under-strength parts; they are over-priced parts and misapplied elevated-temperature grades bolted into a room-temperature assembly [S1][S2][S3].