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Annealing Carbon Steel Grades: 1018, 1045, A36, and Higher-Carbon Stock

Annealing of plain carbon steels — AISI 1018, 1020, 1045, ASTM A36, and higher-carbon grades like 1060, 1080, and 1095 — follows the same core mechanism (austenitize above Ac3, cool slowly through transformation) but with grade-specific temperatures, soak times, and achievable hardness outcomes that reflect the carbon-content differences across the range. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. Low-carbon steels produce predominantly ferritic microstructures on slow cooling and often respond adequately to simpler subcritical cycles; medium-carbon steels are the core full-annealing application where pearlite fraction is large enough to justify the longer cycle; high-carbon steels approaching or exceeding eutectoid composition are almost always spheroidize-annealed rather than full-annealed for downstream machinability. This article covers grade-specific annealing parameters across the carbon steel range, how structural grades like A36 differ from AISI designations in practice, and the scale and decarburization considerations unique to carbon steel annealing.

Which carbon steel grades are most commonly annealed, and why?​

Plain carbon steels in the AISI 10xx series span a carbon range from approximately 0.05% (1005, 1006) through 0.95% (1095), with annealing applications distributed across that range based on grade and end use. The most commonly annealed carbon steels for industrial work are: AISI 1018 and 1020 low-carbon bar and plate stock, typically annealed before or after cold forming, before heavy machining on sections where pre-existing residual stress would compromise dimensional stability, or to restore ductility after extensive cold drawing; AISI 1045 medium-carbon bar stock, annealed when machinability is the primary driver — typically before rough machining of shafts, axles, and similar components where the as-rolled condition (163–200 HB) is too hard for efficient cutting; ASTM A36 structural plate and weldments, annealed or stress-relieved (the distinction blurs at this carbon content) primarily after welding or heavy cold forming to restore dimensional stability; and high-carbon grades (1060, 1080, 1095), typically spheroidize-annealed rather than full-annealed to produce the machinable microstructure that pearlitic high-carbon steel cannot provide on its own. Carbon steels are generally NOT annealed when: they are purchased hot-rolled and machined as-received (hot-rolled carbon steel is already at the soft end of its condition range); they are in cold-finished standard condition and cold finishing has produced the required surface and dimensional result; or the stock is destined for subsequent austenitize-quench-temper operations where the pre-anneal microstructure is erased anyway. The decision to anneal rests on whether the pre-machining, pre-welding, or pre-forming condition of the steel will interfere with the next operation — if the as-received hardness, ductility, or residual-stress state is acceptable, annealing is an unnecessary cost (ASM Handbook, Vol. 1, ASM International, 1990; ASM Handbook, Vol. 4A, ASM International, 2013; ASTM A29).

How do annealing temperatures differ across the carbon steel range?​

The annealing temperature for a carbon steel is governed primarily by its position on the iron-carbon phase diagram — specifically, where the steel's composition sits relative to the Ac3 line (upper critical temperature for hypoeutectoid steels) and the Acm line (for hypereutectoid steels). For hypoeutectoid plain carbon steels (below 0.8% C — the vast majority of industrial grades), Ac3 decreases as carbon content increases. AISI 1018 at 0.18% C has Ac3 approximately 1,650 °F (900 °C); 1045 at 0.45% C has Ac3 approximately 1,500 °F (815 °C); near-eutectoid 1080 at 0.80% C has Ac3 effectively at the eutectoid temperature of approximately 1,333 °F (723 °C). Full annealing temperatures are set roughly 50 °F above Ac3: about 1,700 °F for 1018, 1,550 °F for 1045, and approximately 1,400 °F for near-eutectoid 1080. For hypereutectoid grades above 0.8% C, the situation reverses — the Acm line rises steeply with carbon content, and a proper full-anneal temperature (above Acm) approaches 1,650 °F or higher for 1095. However, full annealing above Acm is rarely specified for hypereutectoid grades because the resulting microstructure (coarse lamellar pearlite plus grain-boundary pro-eutectoid cementite) is hard and poorly machinable. Instead, hypereutectoid carbon steels are spheroidize-annealed at sub-critical temperatures (1,300–1,380 °F) to produce rounded carbide particles in a ferritic matrix. The practical consequence for specifiers is that a drawing calling "anneal" on 1018 at 1,700 °F is a fundamentally different process from "anneal" on 1080 at 1,380 °F — the temperatures, cycle times, and intended outcomes differ even though the same word describes both (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide: Practices and Procedures for Irons and Steels, ASM International, 2nd ed., 1995).

What are the annealing parameters for AISI 1018 and 1020?​

AISI 1018 and 1020 are the most widely specified low-carbon steels for general industrial use — 1018 at approximately 0.18% C, 1020 at approximately 0.20% C, both with 0.60–0.90% Mn per ASTM A29. Full annealing parameters for these grades: ramp to 1,600–1,700 °F (870–925 °C) at a controlled rate not exceeding 400 °F per hour above 600 °F; soak one hour per inch of section thickness, minimum one hour; furnace cool at 25–50 °F per hour through the transformation range (approximately 1,500 °F down through 1,200 °F); continue slow cooling below 900 °F before removing to still air. The resulting microstructure is predominantly ferrite with small pearlite colonies (low carbon content limits pearlite volume fraction); typical hardness is 111–149 HB. This is essentially the softest condition low-carbon steel can reach — because the pearlite fraction is small, there is little hardening potential in slow-cooled 1018/1020 structure. For many applications where low-carbon steel needs softening, subcritical annealing at 1,050–1,200 °F is adequate and substantially faster and cheaper than full annealing — the low carbon content means ferrite recrystallization dominates the softening mechanism, and supercritical temperatures produce only modest additional benefit. Normalizing of 1018/1020 (1,600–1,700 °F soak, still-air cool) produces hardness 126–163 HB, modestly higher than annealed. Normalizing is more commonly specified than full annealing for 1018/1020 stock when pre-machining conditioning is the driver, because the cycle is faster and the hardness difference is typically irrelevant for subsequent machining operations. When full annealing is specified on 1018/1020, it is usually because the designer wants maximum cold-formability (deep drawing, spinning) or the softest possible starting condition for heavy hand-forming work (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995; SAE J1397).

What are the annealing parameters for AISI 1045?​

AISI 1045 (0.43–0.50% C, 0.60–0.90% Mn) is the medium-carbon grade where full annealing delivers its largest practical benefit, because the pearlite fraction is significant (roughly 50–55% by volume for 0.45% C) and soft pearlite plus pro-eutectoid ferrite is substantially more machinable than the equivalent normalized or as-rolled condition. Full annealing cycle for 1045: ramp to 1,500–1,550 °F (815–845 °C); soak one hour per inch of section thickness; furnace cool at 25–50 °F per hour through the transformation range (1,400 °F down to 1,200 °F) before still-air removal. The resulting microstructure is coarse pearlite plus pro-eutectoid ferrite; typical hardness 149–187 HB — soft enough for efficient machining of all common 1045 geometries. Normalizing of 1045 (1,600–1,650 °F soak, still-air cool) produces hardness 163–202 HB, approximately 15–25 HB higher than full anneal depending on section size and cooling rate. The decision between annealing and normalizing 1045 is driven by the downstream machining application: for heavy rough machining or cold forming, full anneal produces the best machinability; for most production machining on modern CNC equipment, normalizing is adequate and the shorter cycle is preferred. Spheroidize annealing of 1045 is uncommon — the pearlitic structure of full-annealed 1045 is already machinable, and the additional 20–30 HB hardness reduction from spheroidizing rarely justifies the extended cycle. A full-annealing cycle on 1045 bar stock in UTEC Industrial's car-bottom furnace typically runs overnight: a morning load at 1,550 °F soak cools in the furnace through the night and is ready for pickup the following morning, with the furnace chart documenting the complete ramp, soak, and controlled-cool profile as the heat treatment record (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995; SAE J1397).

How is A36 structural steel annealed, and how does it differ from 1018?​

ASTM A36 is the most widely specified structural steel grade in the United States, supplied as hot-rolled plate, bars, and shapes for construction, bridge, and general fabrication work. Its chemistry is defined by ASTM A36 rather than by AISI designation: carbon 0.25% max (for plate ≤ 3/4" thick; lower for thicker sections and certain shapes), manganese 0.80–1.20%, silicon 0.40% max, with typical carbon content running 0.15–0.25%. A36's carbon content places it in the low-carbon range alongside 1018 and 1020, but with higher manganese (averaging 1.0% vs. 0.75% for 1018) — the manganese adds modest hardenability and strength without requiring alloy premium. Annealing parameters for A36 mirror those for 1018: full anneal at 1,600–1,700 °F, soak one hour per inch, slow furnace cool through transformation, final hardness typically 115–160 HB. In practice, A36 is rarely full-annealed — hot-rolled A36 plate arrives in a condition already adequate for welding, cold forming, and machining, and full annealing is specified only when the incoming condition has been compromised (for example, heavily cold-worked A36 that needs ductility restoration, or A36 weldments where heat treatment is required by code or by dimensional stability requirements). Stress relief at 1,050–1,150 °F is far more commonly specified on A36 than full annealing — typical applications include stress relief of welded structural fabrications (bridge plate girders, heavy machine bases), post-weld heat treatment for code-mandated applications such as pressure vessels covered by ASME Section VIII or code-compliant structural welding per AWS D1.1 Clause 5.8, and sub-critical softening of heavily cold-formed A36 shapes. The distinction between "anneal" and "stress relief" on A36 drawings is often blurred by the designer; when a drawing calls for "annealing" of a welded A36 structure, the heat treater's judgment call — and communication with the designer — is whether a full anneal above Ac3 is actually intended or whether stress relief at sub-critical temperature was the designer's actual requirement (ASTM A36; AWS D1.1; ASM Section VIII; ASM Handbook, Vol. 4A, ASM International, 2013).

How should higher-carbon steels be annealed, and why spheroidize rather than full-anneal?​

Higher-carbon plain steels — AISI 1060 (0.60% C), 1080 (0.80% C, near-eutectoid), 1095 (0.95% C, hypereutectoid), and similar grades — are typically produced for applications where high hardness after quench-hardening is the service requirement: springs, knife blanks, wear surfaces, certain fasteners, cutting implements. Pre-machining annealing of these grades is almost always spheroidize annealing rather than full annealing, for two reasons. First, full annealing of eutectoid and hypereutectoid grades produces a microstructure that is difficult to machine: eutectoid 1080 full-annealed is essentially 100% lamellar pearlite with hardness 200–230 HB — harder than most full-annealed structural steels and with chip-control problems under carbide tooling; hypereutectoid 1095 full-annealed has lamellar pearlite plus grain-boundary pro-eutectoid cementite, which is harder and more abrasive still. Second, the spheroidized microstructure — rounded carbide particles in a ferritic matrix — is the only practical way to reach a machinable hardness (typically 175–200 HB for 1080, 180–210 HB for 1095) on these grades. The spheroidize cycle for high-carbon steel: hold at 1,300–1,380 °F (subcritical, below A1) for 8–24 hours, or cycle through small excursions above and below A1 (roughly 1,300 °F to 1,430 °F) for multiple cycles, then slow furnace cool. Total furnace occupancy for a spheroidize anneal of heavy 1080 section can reach 48–60 hours. Because the cycle is so long and spheroidizing is essentially required for downstream machinability, spheroidize annealing is typically incorporated into the mill supply of high-carbon bar stock — annealed high-carbon bar arrives already spheroidized, and additional customer-specified annealing of as-supplied stock is uncommon. When a drawing does call "anneal" on 1060 or 1080 stock that has been heat-treated or work-hardened in prior operations, the heat treater's standard interpretation is spheroidize anneal unless the designer specifically calls for full anneal (which would be unusual and should prompt clarification) (ASM Handbook, Vol. 4A, ASM International, 2013; ASM Handbook, Vol. 1, ASM International, 1990; Heat Treater's Guide, ASM International, 1995).

What scale and decarburization issues affect carbon steel annealing?​

Carbon steels are more vulnerable than alloy steels to two surface-condition issues during annealing: scale formation and surface decarburization. Scale is iron oxide formed at the steel surface by reaction with oxygen in the furnace atmosphere. Scale formation rate increases steeply with temperature above approximately 1,200 °F and with time at temperature. A typical full-anneal cycle at 1,550–1,700 °F for several hours produces noticeable scale on carbon steel exposed to combustion atmosphere — typically 0.003–0.015 inch thickness depending on cycle time, surface area, and atmosphere composition. Scale must be removed before further machining or surface finishing operations (shot blast, wire brush, or pickling for heavy scale); machine shops plan stock allowance to include scale removal in the first roughing pass. Decarburization is the loss of carbon from the steel surface through reaction with oxygen or hydrogen in the atmosphere — the surface layer loses carbon content and becomes softer than the interior. Decarburization is more insidious than scale because it is not visibly obvious but affects the subsequent heat-treatment response: a decarburized 1045 surface may harden to 45 HRC instead of 55 HRC at the same austenitizing and quench parameters, producing a soft skin that wears prematurely in service. The decarburization layer on carbon steel annealed in an oxidizing atmosphere is typically 0.005–0.030 inch depending on cycle time and temperature; hypereutectoid steels (1080, 1095) are particularly vulnerable because their high carbide fraction provides a greater carbon contrast between the core and a decarburized surface. Protective measures: for parts requiring a hardened final condition, leave sufficient stock allowance (typically 0.030–0.060 inch per surface) to machine away the decarburized layer after annealing; alternatively, specify a protective atmosphere (endothermic gas, nitrogen, or partial vacuum) to limit or prevent decarburization. For parts where final surface hardness matters, the specification should either direct the heat treater to a protective-atmosphere process or include stock allowance for decarb removal in subsequent machining — this is upstream of the annealing cycle itself and belongs in the drawing's heat treatment note rather than on the shop floor (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM E1077 — Standard Test Methods for Estimating the Depth of Decarburization of Steel Specimens; Heat Treater's Guide, ASM International, 1995).

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References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASM International. (1990). ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys. ASM International.
  • ASM International. (1995). Heat Treater's Guide: Practices and Procedures for Irons and Steels (2nd ed.). ASM International.
  • Machinery's Handbook (31st ed.). (2020). Industrial Press.
  • SAE J1397: Estimated Mechanical Properties and Machinability of Steel Bars. SAE International.
  • ASTM A29: Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought. ASTM International.
  • ASTM A36: Standard Specification for Carbon Structural Steel. ASTM International.
  • ASTM E1077: Standard Test Methods for Estimating the Depth of Decarburization of Steel Specimens. ASTM International.
  • AWS D1.1: Structural Welding Code — Steel. American Welding Society.
  • ASME Boiler and Pressure Vessel Code, Section VIII Division 1. American Society of Mechanical Engineers.

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