A36, 1018, and 1045 cover the three jobs most shops actually need from a plain carbon steel bar or plate: welded structures, screw-machine parts, and heat-treated shafts. The grades sit at 0.26%, 0.18%, and 0.45% carbon respectively, and that single number drives almost every downstream decision on welding preheat, machinability, achievable hardness, and price per pound [S1][S4].
For a buyer or engineer choosing between them, the practical split is this: A36 is an ASTM structural shape and plate standard bought hot-rolled, 1018 is an AISI/SAE flat-bar and round-bar standard bought cold-drawn for machining, and 1045 is the same AISI/SAE family pushed into medium-carbon territory where flame or induction hardening becomes viable [S1][S2][S4].
Chemistry and the three carbon tiers that separate them
Carbon content is the single spec that cascades into everything else: 1018 nominally contains 0.18% C with 0.6–0.9% Mn, A36 is capped at 0.26% C with 0.75% Mn, and 1045 sits at 0.45% C nominal with a similar manganese range [S1][S3][S4]. Phosphorus and sulfur are both held to 0.04% and 0.05% maximums across the low-carbon grades, which is why all three weld and form without exotic procedures [S1].
That 0.27-percentage-point carbon gap between 1018 and 1045 is what pushes 1045 out of the "low-carbon, weld-any-which-way" bucket and into medium-carbon territory, where the American Welding Society forum guidance is to take "the usual precautions when welding this kind of steel," meaning controlled preheat and interpass temperature to avoid martensite and hydrogen cracking [S3]. For comparison context, ASTM A36 sits in the same low-carbon weld-friendly family as 1018, which is why AWS practitioners report no significant difference when joining either to 1045 [S3].
Mechanical properties: yield, tensile, and what cold drawing does to 1018
Assuming the most common supply condition (A36 hot-rolled, 1018 cold-drawn), the headline numbers line up like this: A36 delivers 36,300 psi yield and 58,000 psi tensile with about 20% elongation in 50 mm; 1018 cold-drawn jumps to 53,700 psi yield and 63,000 psi tensile at the cost of elongation dropping to roughly 15%; 1045 in the as-rolled or cold-drawn condition is widely reported as roughly 40% stronger than 1018 once you account for its higher carbon and similar manganese content [S1][S6].
The cold-drawing step on 1018 is doing real work: it is the difference between 36,000 psi yield (A36 hot-rolled) and 53,700 psi yield (1018 cold-drawn), a near-50% jump with no change in chemistry worth mentioning [S1]. For more demanding structural applications, ASTM A572 Grade 50 is the next step up at 50,000 psi minimum yield, about 39% stronger than A36 with 0.23% carbon maximum, and it is often dual-certified with A36 on the same plate [S5]. The relevant point for a specifier is that A36 is the floor, 1018 cold-drawn is the next tier for non-structural parts, and 1045 is where you go when wear or surface hardness starts to dominate the design.
Weldability, preheat, and the 1045 problem
All three grades are weldable, but they are not equivalent. A36 and 1018, both under 0.30% C, can be joined with any standard process and generally no preheat on thin sections, which is why they dominate structural fabrication and welded assemblies [S1][S2][S5]. 1045 at 0.45% C requires the operator to control cooling rate, typically with preheat in the 300–400 °F range depending on section thickness and restraint, to keep the heat-affected zone from forming hard, crack-prone martensite [S3].
This is the real reason shops keep 1018 and A36 in stock for general fabrication and reach for 1045 only when they intend to harden the part later. A welded 1045 shaft that is then flame- or induction-hardened only at the wear surface is a textbook use case, whereas a welded 1045 structural bracket that will never be heat-treated is a textbook mistake, because the weld HAZ will be hard and brittle while the base metal stays tough. A side-by-side weld of A36 to 1045 is treated as routine by experienced welders, while 1045-to-1045 in a thick section is the configuration that demands procedure qualification [S3].
Machinability, formability, and where each grade wins on the shop floor
1018 is the machinist's default: cold-drawn bar stock with a consistent surface, tight dimensional tolerance, and a chip that breaks cleanly enough for high-volume screw-machine work on shafts, spindles, pins, and rods [S1][S2]. A36 is easier to bend and form because of its lower yield strength, which is why it is the default for building frames, bridges, equipment bases, gusset plates, and general structural fabrication where the part will not see a tool changer [S1][S5]. 1045 sits between them on machinability (still good, but tool wear is higher than 1018) and is the right pick when the finished part will be hardened at the wear surface for gears, arbors, axles, and spline shafts [S4][S7].
For formability-led jobs like brake-formed brackets or roll-formed sections, A36's lower yield strength (36,300 psi versus 53,700 psi for 1018 cold-drawn) is a real production advantage, and springback is more predictable [S1]. For screw-machine throughput, 1018 cold-drawn wins on chip control and surface finish. For wear parts that need a 50–60 HRC case on a tough core, 1045 (or 4140 if you need through-hardening and higher alloy content) is the correct starting point, and it is why 1045 is "often heat treated for enhanced performance in wear-heavy" service per mill-side guidance [S4][S5][S7].
Cost, availability, and supply condition traps
Hot-rolled A36 plate, sheet, angle, channel, I-beam, and wide-flange sections are the cheapest carbon steel product on the market in North America, which is why it dominates construction [S1][S5]. Cold-drawn 1018 bar carries a price premium over hot-rolled product because cold drawing is a labor-intensive secondary operation, but that premium buys tighter tolerance, better finish, and the higher mechanical properties already discussed [S1]. 1045 falls in the middle on raw bar cost but climbs quickly once heat treatment is added to the routing, and it is commonly stocked in both hot-rolled bar and cold-drawn bar so the buyer can choose between as-rolled ductility and cold-drawn strength [S4].
The supply condition trap is real: specifying "1018 bar" without naming hot-rolled or cold-drawn leaves the mechanical properties undefined, because the same grade can land at 36,000 psi yield or 53,700 psi yield depending on how it was finished [S1]. The same applies to A36 plate versus A36 hot-rolled bar versus cold-drawn A36 bar; always pin the supply condition, the ASTM or AISI/SAE designation, and the applicable mechanical-property table on the print. For buyers comparing per-piece cost, the rule of thumb is A36 HR bar as the baseline, 1018 CD bar at a noticeable premium, and 1045 (in either condition) priced above 1018 once heat treatment is included [S1][S4].
Decision matrix: which grade to specify for which job
For welded structures, building frames, and base plates where the priority is weldability, formability, and cost, A36 hot-rolled is the correct call at 36,300 psi yield, 0.26% C maximum, and universal availability in plate and structural shapes [S1][S5]. For machined pins, spacers, bushings, studs, and general screw-machine parts where surface finish and tight tolerance matter, 1018 cold-drawn at 53,700 psi yield and 0.18% C is the default, with case-hardening available for wear surfaces that do not need through-hardening [S1][S5]. For shafts, gears, splines, and wear-heavy components that will be flame- or induction-hardened to 50+ HRC at the surface, 1045 at 0.45% C and roughly 40% higher strength than 1018 is the right starting stock, with the explicit understanding that any welding on 1045 needs preheat and procedure control [S3][S4][S6].
None of these three is the right answer for through-hardened alloy parts, impact-resistant structures at sub-zero service, or any application needing corrosion resistance beyond a light oil coat; for those, the conversation shifts to 4140 chrome-moly, A572 Grade 50 for higher strength-to-weight structural work, or a stainless steel family [S5]. If the part needs both high strength and welded seismic-grade reinforcement, the A706 vs A615 rebar spec map covers the next decision point; if the part is a precision shaft coupling rather than a raw bar, the beam helical vs bellows coupling encoder shaft spec map is the relevant adjacent read.
The trackable signal for the next sourcing cycle is ASTM A36 plate and A572 Grade 50 dual-cert pricing relative to 1018 cold-drawn bar premium, since that ratio sets the economic crossover between hot-rolled structural and cold-finished machining stock on every RFQ; the second signal is 1045 bar lead time, which tightens whenever the heat-treated bar and billet capacity runs hot [S5].
Spec-level background on the components involved: carbon fiber.