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Heat Treating AISI 1018 and 1020 Low-Carbon Steel: Normalizing and Stress Relief

AISI 1018 and 1020 are the two most widely specified low-carbon steels in general industrial manufacturing — plain-carbon grades with approximately 0.15–0.20% carbon used for weldments, shafts, pins, machine bases, fixtures, and parts where weldability, cold formability, and cost dominate over strength or wear resistance. 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. Both grades have limited through-hardening response because their low carbon content and absence of alloy additions produce very low hardenability; the realistic heat treatment options are normalizing for grain refinement, stress relief after welding or cold work, process annealing for cold-forming ductility, and case hardening (carburizing) for parts that require a hard surface on a tough core. This article covers parameters, achievable properties, and the practical reasons low-carbon steel is heat treated differently from medium-carbon or alloy grades.

What are the composition and key characteristics of 1018 and 1020?​

AISI 1018 per ASTM A29 is a plain-carbon steel with nominal composition 0.15–0.20% carbon, 0.60–0.90% manganese, 0.040% maximum phosphorus, and 0.050% maximum sulfur; 1020 is nearly identical at 0.18–0.23% carbon with the same manganese range. The two grades are essentially interchangeable for most heat treatment purposes, and mills often produce bar stock that meets both specifications simultaneously. The defining characteristic is the carbon content: below approximately 0.30% C, the steel cannot form useful quantities of martensite during quenching because too little carbon is available to lock the body-centered-tetragonal structure; the result of a severe quench on 1018 is a mixed ferrite-pearlite structure with hardness typically 120–170 HB, not the 55+ HRC that medium- and high-carbon steels produce. As-rolled properties are modest: tensile strength 55–65 ksi, yield 32–48 ksi, elongation 25% or better. In normalized condition the values improve slightly (tensile 58–70 ksi, yield 32–50 ksi) with finer grain. These properties are adequate for the dominant use cases — weldments, structural brackets, low-stress shafts, fixtures, and machine components where cost and weldability drive the grade choice (ASM Handbook, Vol. 1, ASM International, 1990; ASTM A29; SAE J1397).

What are the normalizing parameters for 1018 and 1020?​

Normalizing is the most common thermal treatment applied to 1018 and 1020 because it refines as-rolled grain structure, homogenizes microstructure across mixed heats, and produces a uniform, reproducible starting condition for subsequent machining or welding. The cycle: ramp to 1,650–1,700 °F (900–925 °C) at a rate not exceeding 400 °F per hour above 600 °F; soak one hour per inch of section thickness with a one-hour minimum; remove from the furnace and cool in still air. The resulting microstructure is fine-grained pro-eutectoid ferrite plus pearlite with ASTM grain size typically 7–9; hardness falls in the 111–149 HB range depending on section size (thin sections cool faster and sit at the upper end of the range). Normalizing 1018 and 1020 is commonly specified before precision machining, before welding of critical weldments where variable starting microstructure would affect heat-affected-zone properties, and as a conditioning treatment for parts that will subsequently receive surface hardening. A car-bottom furnace is well-suited to low-carbon normalizing loads at any section size within its envelope, and programmable ramp-and-soak control produces the same cycle whether the load is a single large weldment or a batch of smaller parts (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).

What are the stress relief parameters for 1018 and 1020?​

Stress relief of 1018 and 1020 uses the standard sub-critical cycle applied to carbon and low-alloy steels: heat to 1,100–1,200 °F (595–650 °C), soak one hour per inch of section thickness (minimum one hour), cool at a controlled rate not exceeding 400 °F per hour down to 600 °F, then remove to still air. The resulting residual stress reduction is typically 70–85% of the initial stress magnitude, with microstructure and hardness essentially unchanged. Low-carbon weldments are the largest-volume application: welding deposits molten metal at 2,800 °F-plus into a cold base metal, and the differential contraction on cooling produces tensile residual stresses that can approach the yield strength of the base metal in the weld region. Left untreated, these stresses cause distortion during subsequent machining, dimensional instability over time, and (in service) reduced fatigue life and increased susceptibility to stress-corrosion cracking. Stress relief at 1,150 °F for a 1-inch-thick weldment — ramp at 400 °F per hour, soak one hour, furnace cool below 600 °F — reduces the stress magnitude enough to eliminate most distortion issues while preserving the weld's mechanical properties. For machined parts with residual stress from heavy cutting, the same cycle applies, though many shops use a lower-temperature process anneal (see the process annealing question below) when dimensional stability rather than full stress relief is the goal (ASM Handbook, Vol. 4A, ASM International, 2013; AWS D1.1).

Can 1018 or 1020 be through-hardened by quench and temper?​

1018 and 1020 cannot be usefully through-hardened because their hardenability is too low to suppress pearlite formation at any practical cooling rate in anything beyond paper-thin sections. Jominy end-quench data for 1018 shows that even at the quenched end (J1 position, 1/16 inch from the quenched face) the hardness reaches only approximately 37–40 HRC in the as-quenched condition — this is the maximum hardness the steel can achieve, and it occurs only where the cooling rate is essentially infinite. Moving just 1/8 inch away from the quenched end, hardness drops to 20–25 HRC as pearlite dominates the microstructure. For production parts, this means that even a severe water quench of a 1/4-inch 1018 bar produces a through-section hardness in the 18–25 HRC range — far below the 45–58 HRC that a medium-carbon or alloy steel would deliver with the same cycle. When a drawing specifies "quench and temper 1018 to 30 HRC," the specification is not achievable and signals either a grade error (the designer meant 1045 or 4140) or an unrealistic hardness target. The practical rule is that hardness above approximately 25 HRC through-section in carbon steel requires carbon content of 0.35% or higher; below that, case hardening (carburizing) is the only viable path to a hard surface. A competent intake review flags these specifications at order entry so the customer can confirm the grade and target hardness before processing begins (ASM Handbook, Vol. 1, ASM International, 1990; SAE J406; SAE J1268).

How is case hardening (carburizing) used on 1018 and 1020?​

Case hardening by carburizing is the traditional path to a hard, wear-resistant surface on a tough low-carbon steel core. The process diffuses carbon into the surface layer at 1,650–1,750 °F (900–955 °C) in a carbon-rich atmosphere (gas, pack, or vacuum), enriching the surface carbon content to approximately 0.8–1.0% over a controlled depth — typically 0.020 to 0.060 inch for general service. After carburizing, the part is quenched (either directly from the carburizing temperature or after a reheat cycle), producing a hardened case of 58–62 HRC on a 1018 or 1020 core that retains its original 20–25 HRC ferrite-pearlite structure. Applications include small pins, bushings, cams, and gear teeth in light-to-medium service where a hard wear surface on a tough core is required and the part is small enough for commercial carburizing furnaces. Note that 8620 alloy steel is the preferred grade when core toughness and deeper cases are required because its nickel-chromium-molybdenum chemistry improves core hardenability and case quality — 1018 and 1020 are specified for carburizing mainly when cost is the deciding factor or the part is a non-critical general component. UTEC Industrial does not perform carburizing, carbonitriding, or nitriding — these processes require atmosphere-controlled or vacuum furnaces outside the facility's equipment set; buyers with carburizing requirements should work with commercial heat treaters that operate gas-carburizing or vacuum-carburizing furnaces, and the upstream normalizing or downstream stress relief steps can run locally if single-facility processing on the non-carburizing steps is beneficial (ASM Handbook, Vol. 4A, ASM International, 2013; AMS 2759/7 for carburizing when specified).

When is process annealing specified instead of full annealing for 1018 and 1020?​

Process annealing is a sub-critical cycle (heat to 1,000–1,250 °F, soak, slow cool) used to restore ductility in cold-worked low-carbon steel without fully transforming to austenite — an important distinction from full annealing, which heats above the upper critical temperature and produces maximum softness. For 1018 and 1020, process annealing at 1,200–1,250 °F (650–675 °C) is the standard softening treatment before additional cold forming operations (deep drawing, heavy bending, cold heading). The cycle recrystallizes the cold-worked ferrite grains, restoring elongation and reducing the yield strength increase that cold work produces, but without the cycle time and cost of full annealing. Typical hardness after process annealing is 120–150 HB, which is adequate for subsequent forming operations. Spheroidize annealing — a longer sub-critical cycle at approximately 1,300–1,350 °F for extended time (6–24 hours) — is rarely specified for low-carbon steels because they contain too little carbide to benefit from spheroidization; the process is more useful for medium- and high-carbon grades where spheroidal carbides substantially improve machinability over the pearlitic as-rolled structure. For 1018 and 1020 in most production contexts, the choice is between normalizing (for a pre-machining or pre-welding conditioning treatment) and process annealing (for parts that will see further cold work); full annealing and spheroidizing are not typically part of the low-carbon-steel workflow (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).

How does 1018/1020 compare to A36 and 1045 for heat treatment selection?​

Direct comparison helps clarify when each grade is the correct choice and what heat treatment to expect. 1018 and 1020 are bar stock grades per ASTM A29 — produced to chemistry specification, typically available cold-drawn or hot-rolled in rounds, squares, flats, and hex. A36 is a structural steel grade per ASTM A36 specified primarily by mechanical properties (36 ksi minimum yield, 58–80 ksi tensile) with a wider, looser chemistry envelope (up to 0.26% C, up to 1.2% Mn); A36 is the standard for structural shapes (W-beams, channels, angles, plates) and for structural plate in weldments. From a heat treatment standpoint, the three grades behave similarly: all are low-carbon, low-hardenability, treatable only by normalizing, stress relief, process annealing, or case hardening. A36 is the most common input to PWHT (post-weld heat treatment) at a heat treater because structural weldments dominate the weldment volume. 1045 is the next step up in carbon content (approximately 0.45%) and offers substantially different heat treatment options — surface hardening to 55–58 HRC by induction or flame, modest through-hardening in thin sections, higher core strength. For a designer choosing between 1018/1020 and 1045, the decision is driven by whether the part requires surface hardness for wear: if yes, 1045 (or a grade further up the carbon content ladder); if the part is primarily structural, weldable, or decorative, 1018/1020 are more weldable, more cold-formable, and less expensive. For a designer choosing between 1018/1020 and A36, the decision is typically driven by shape and supply: bar stock is 1018/1020; structural shape and plate is A36 (ASTM A29; ASTM A36; SAE J1397).

What are common specification errors with 1018 and 1020?​

Specification errors that cause problems with low-carbon steel heat treatment: Specifying quench and temper or through-hardness on 1018 or 1020 — the grade cannot reach meaningful through-hardness, so the specification is either a grade error or a hardness error. The correct resolution is to confirm with the designer whether the grade should change (to 1045 or 4140) or the hardness target should change (to an achievable 20–25 HRC range for 1018/1020). Specifying 1018 for a wear surface without calling out case hardening — leaves the heat treater without guidance on how to produce surface hardness. The correct specification is either "1018 case hardened, 0.030 inch case depth minimum, 58–62 HRC surface" or a grade change to 1045 with induction hardening. Specifying "harden 1018" without process definition — ambiguous; the heat treater cannot infer whether normalizing, stress relief, case hardening, or through-hardening is intended. Specifying 1018 for a part with section thickness above approximately 4 inches where welding is required but PWHT is not called out — for thick-section low-carbon weldments, PWHT should be specified per the applicable code (AWS D1.1 Clause 7 for structural, ASME Section VIII Div 1 UW-40 for pressure vessels). Conflating 1018 with A36 — the grades are not equivalent; A36 allows higher carbon (up to 0.26%) and higher manganese, which affects weldability assessment and heat-affected-zone hardness. For heat treatment purposes, the grades respond similarly, but for welding procedure qualification they are distinct. Intake review at order entry should catch these issues before production begins (ASM Handbook, Vol. 4A, ASM International, 2013; AWS D1.1; ASTM A29).

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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 J406: Methods of Determining Hardenability of Steels. SAE International.
  • SAE J1268: Hardenability Bands for Carbon and Alloy H Steels. SAE International.
  • 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.
  • AWS D1.1: Structural Welding Code — Steel. American Welding Society.

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