SAE1020 vs SAE1045 – Composition, Heat Treatment, Properties, and Applications

Introduction

SAE1020 and SAE1045 are two of the most commonly specified plain-carbon steels in engineering drawings and procurement. The selection dilemma typically arises when designers and procurement specialists must balance manufacturability and cost against required mechanical performance: low-carbon steels are easier to form and weld, while medium-carbon steels offer higher strength and wear resistance but need more careful heat treatment and fabrication controls.

The primary practical difference between the two grades is the carbon content and its downstream effects: SAE1045 has substantially higher carbon than SAE1020, which shifts the balance toward greater strength and hardenability at the expense of ductility and weldability. Because they occupy adjacent points on the carbon-steel spectrum, these grades are often compared when specifying shafts, gears, fasteners, and general mechanical parts where tradeoffs among strength, toughness, and cost must be optimized.

1. Standards and Designations

• SAE/AISI: SAE 1020 (AISI 1020), SAE 1045 (AISI 1045)
• ASTM/ASME: Commonly referenced by SAE/AISI designation in procurement; specific product standards (bars, plates, forgings) may reference ASTM grades with similar chemistry.
• EN: Roughly equivalent EN steels are C20 (for 1020) and C45 (for 1045) in some European standards (designation systems differ by standard).
• JIS/GB: JIS and GB standards use different nomenclature but comparable carbon ranges exist (e.g., JIS S20C / S45C).
• Classification: Both are plain carbon steels (not alloyed steels, not stainless, not HSLA by default). They are not tool steels.

2. Chemical Composition and Alloying Strategy

Notes: - Both grades are plain carbon steels; alloying additions are minimal and primarily incidental. SAE1045 contains a higher carbon and typically higher manganese to help maintain strength and hardenability. - Higher carbon increases tensile strength and potential hardness; manganese aids strength and deoxidation but also increases hardenability. Silicon is a deoxidizer and contributes slightly to strength. - Alloying strategy is simple: control carbon for target strength and use heat treatment to obtain desired microstructure rather than relying on alloying elements.

3. Microstructure and Heat Treatment Response

• As-rolled / annealed: SAE1020 typically shows a ferrite–pearlite microstructure with a higher proportion of soft ferrite relative to pearlite. SAE1045 shows more pearlite and less ferrite because of the higher carbon content.
• Normalizing: Both grades respond to normalizing by a refined ferrite/pearlite microstructure; SAE1045 develops a harder pearlitic matrix and higher strength after normalizing than SAE1020.
• Quenching & tempering: SAE1045 has higher hardenability and achieves significantly higher hardness and strength after quench and temper than SAE1020. SAE1020 is difficult to harden uniformly in thicker sections because of lower carbon and low hardenability.
• Microalloying and thermo-mechanical processing: Neither grade is typically supplied with microalloying unless specifically ordered; thermo-mechanical treatments can modestly refine grain size and slightly improve strength and toughness in both grades, but the carbon level remains the dominant factor.
• Practical implication: SAE1045 offers broader heat-treatment tuning (e.g., higher tensile and yield after quench–temper) while SAE1020 is mainly used in annealed or normalized conditions for its ductility and formability.

4. Mechanical Properties

Notes: - Values are typical ranges. Actual values depend strongly on product form, section size, and heat treatment cycle. - SAE1045 is substantially stronger in most heat-treated conditions; SAE1020 is more ductile and forgiving in forming operations. The increased strength of 1045 comes at the cost of reduced elongation and generally lower as-quenched toughness unless properly tempered. - For impact-critical applications where toughness at low temperature matters, 1020 in an appropriate condition or a low-alloy steel with good toughness might be preferred.

5. Weldability

• Carbon content and hardenability govern weldability. Higher carbon increases the risk of cold cracking and martensite formation in the heat-affected zone.
• Carbon-equivalent formulas are commonly used for qualitative assessment. Example indices:
• $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Interpretation (qualitative): SAE1020 has a low $CE$ and $P_{cm}$ and is readily welded with standard procedures and minimal preheat. SAE1045 has a higher carbon content and, therefore, higher $CE/P_{cm}$ — it typically requires preheat, controlled interpass temperatures, and possibly post-weld heat treatment for critical joints to mitigate hydrogen-induced cracking and HAZ hardening.
• Practical guidance: Use low-hydrogen electrodes, controlled preheat, and tempering for thicker sections of 1045; for 1020, standard welding consumables and procedures are usually adequate.

6. Corrosion and Surface Protection

• Neither SAE1020 nor SAE1045 is stainless; they are susceptible to general atmospheric corrosion and require surface protection where corrosion is a concern.
• Typical protections: solvent cleaning, primer/paint systems, phosphating, hot-dip galvanizing, electroplating (where appropriate), or protective coatings (polymer/epoxy).
• PREN (pitting resistance equivalent number) is not applicable to plain carbon steels; it is used for stainless steels:
• $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Selection note: For corrosive environments, consider stainless steels or corrosion-resistant coatings rather than relying on carbon-steel chemistry.

7. Fabrication, Machinability, and Formability

• Formability: SAE1020 is more ductile and easier to bend, draw, and form in cold operations. SAE1045 will have limited formability in higher-strength conditions and is more prone to cracking during forming.
• Machinability: SAE1045, because of higher strength and hardness, is generally harder to machine than SAE1020. However, neither grade is free-cutting; machinability ratings are moderate unless special free-machining variants are specified.
• Grinding and finishing: 1045 produces higher tool wear and requires more robust tooling or lower cutting speeds to achieve similar tool life compared to 1020.
• Surface hardening: SAE1045 responds well to carburizing, induction hardening, and through hardening treatments to increase wear resistance and fatigue life of components like shafts and gears; 1020 is not a good candidate for significant case-hardening because of low carbon.

8. Typical Applications

Selection rationale: - Choose 1020 when ease of forming, welding, and cost control are primary drivers, and service loads are moderate. - Choose 1045 when component strength, wear resistance, and the ability to achieve higher hardness via heat treatment are necessary.

9. Cost and Availability

• Cost: SAE1020 is generally less expensive per kilogram than SAE1045 because of lower carbon content and simpler processing demands; market prices vary with regional supply and steelmaking fluctuations.
• Availability: Both grades are commonly available worldwide in bar, plate, and coil forms. SAE1020 is very common for sheet and structural products; SAE1045 is widely stocked for shafts, bars, and forgings.
• Product forms: 1045 is commonly supplied as hot-rolled bars and forged blanks, and often specified when post-heat-treatment properties are required. 1020 is frequently used in cold-formed and welded assemblies.

10. Summary and Recommendation

Choose SAE1020 if: - You need a readily weldable, formable, and economical steel for moderate loads. - Parts will undergo significant cold forming or require good ductility and toughness in the as-fabricated condition. - Large welded structures or assemblies require minimal preheat and simple welding procedures.

Choose SAE1045 if: - The design requires higher static strength, better wear resistance, or the part will be surface-hardened or through-hardened. - You are manufacturing medium-duty rotating components (shafts, axles, gears) where greater tensile strength and hardness are necessary. - You can control welding parameters, or welding is minimized in favor of machining/assembly and heat treatment.

Final note: Material selection should always be validated against component design loads, fatigue requirements, joining method, intended heat treatment, surface treatments, and cost constraints. When in doubt for critical or safety-related components, consider specifying properties (e.g., minimum tensile/yield, hardness, or toughness) rather than relying solely on grade name, and consult with heat-treatment and welding specialists to establish appropriate procedures.