20# vs 45# – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners frequently choose between 20# and 45# when specifying plain carbon steels for shafts, gears, fasteners, and general mechanical parts. The decision typically balances manufacturability (weldability, formability, and machining), required mechanical performance (strength, hardness, and toughness), and cost constraints.
The primary technical distinction between the two grades is their carbon content and the resulting differences in mechanical behavior and heat-treatment response. Because 45# contains significantly more carbon than 20#, it achieves higher strength and hardenability after heat treatment but sacrifices ductility and becomes more demanding for welding and some forming operations. These trade-offs make the two grades complementary for different application classes and processing routes.
1. Standards and Designations
- Chinese GB/T: 20# and 45# (commonly used domestic designations).
- JIS: S20C (≈ 20#) and S45C (≈ 45#).
- AISI/ASTM: AISI 1020 (≈ 20#) and AISI 1045 (≈ 45#).
- EN: C20 and C45 (EN 10083 family; note detailed spec depends on product form and heat treatment).
Classification: both 20# and 45# are plain carbon steels (unalloyed/low-alloy carbon steels), not stainless, HSLA, or tool steels. They are commonly supplied in hot-rolled, normalized, annealed, or quenched-and-tempered conditions depending on application.
2. Chemical Composition and Alloying Strategy
Table: Typical chemical composition ranges (wt%) for 20# and 45#. Values are representative ranges used for specification comparison; actual supplier certifications should be consulted for precise chemistry.
| Element | 20# (typical wt%) | 45# (typical wt%) |
|---|---|---|
| C | 0.17–0.24 | 0.42–0.50 |
| Mn | 0.25–0.60 | 0.50–0.80 |
| Si | 0.03–0.35 | 0.15–0.35 |
| P | ≤ 0.035 | ≤ 0.035 |
| S | ≤ 0.035 | ≤ 0.035 |
| Cr | ≤ 0.25 (trace) | ≤ 0.30 (trace) |
| Ni | ≤ 0.30 (trace) | ≤ 0.30 (trace) |
| Mo | ≤ 0.08 (trace) | ≤ 0.08 (trace) |
| V | ≤ 0.03 (trace) | ≤ 0.03 (trace) |
| Nb | ≤ 0.03 (trace) | ≤ 0.03 (trace) |
| Ti | ≤ 0.03 (trace) | ≤ 0.03 (trace) |
| B | ≤ 0.001 (rare) | ≤ 0.001 (rare) |
| N | ≤ 0.012 (typical) | ≤ 0.012 (typical) |
Alloying strategy and effects: - Carbon (C) is the primary strength and hardenability control. Higher C increases achievable hardness and strength after quenching but reduces ductility and weldability. - Manganese (Mn) increases hardenability and tensile strength and counteracts sulfur embrittlement; 45# typically has higher Mn to improve strength and hardenability. - Silicon (Si) is a deoxidizer and contributes modestly to strength. - Trace alloying elements (Cr, Ni, Mo, V) are usually minimal in these grades but, if present, increase hardenability, strength, and sometimes toughness.
3. Microstructure and Heat Treatment Response
- 20#: In common as-rolled or normalized conditions, microstructure is predominantly ferrite + pearlite (coarse or fine depending on cooling and normalization). Low carbon content yields a larger ferrite fraction and relatively low pearlite content. Normalizing/refining reduces pearlite spacing and improves uniformity and strength modestly.
- 45#: In as-rolled or normalized states, microstructure is ferrite + pearlite with a higher pearlite fraction and finer interdendritic spacing compared with 20#. Because of higher carbon and Mn, 45# has greater hardenability and can form martensite or bainite when quenched, followed by tempering to produce quench-and-tempered microstructures (tempered martensite) with significantly higher strength and wear resistance.
Heat-treatment effects: - Normalizing (air cooling from above Ac3) refines grain and produces a uniform ferrite-pearlite structure; both grades benefit, but strength increase is more pronounced in 45#. - Quenching and tempering is commonly applied to 45# to achieve high strength and fatigue resistance. 20# has limited hardenability — quenching will produce low martensite content and limited benefit. - Annealing (full anneal) softens material for machining or forming; 20# is readily annealed. 45# annealed is machinable but will still have higher strength than normalized 20#.
4. Mechanical Properties
Table: Typical mechanical property ranges by common condition (representative ranges; exact values depend on product form and heat treatment).
| Property | 20# (annealed/normalized) | 45# (annealed/normalized/quench–tempered) |
|---|---|---|
| Tensile strength (MPa) | ≈ 350–550 (annealed→normalized) | ≈ 500–900 (annealed→QT) |
| Yield strength (MPa) | ≈ 200–350 | ≈ 300–700 |
| Elongation (A%) | ≈ 20–30% | ≈ 10–25% (decreases with hardening) |
| Impact toughness (Charpy V) | Moderate—good at room temp when normalized | Good when properly tempered; lower in hardened condition if not tempered |
| Hardness (HB or HRC) | ≈ 120–200 HB (annealed→normalized) | ≈ 150–300 HB (annealed→QT depending on temper) |
Interpretation: - 45# offers substantially higher achievable tensile strength and hardness after appropriate heat treatment, due to higher carbon and hardenability. - 20# is more ductile and forgiving in forming; elongation and toughness in 20# are generally higher for comparable processing. - Impact toughness depends heavily on processing—adequate tempering after quenching is critical for 45# to avoid brittle behavior.
5. Weldability
Weldability of plain carbon steels is strongly influenced by carbon content and alloying elements via carbon equivalent metrics. Common indices:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 20# (low C) typically has a low carbon equivalent, giving good weldability with low preheat and limited risk of hydrogen cracking or hard martensite formation in the heat-affected zone (HAZ). Post-weld heat treatment (PWHT) is rarely required for standard thicknesses under normal conditions. - 45# (higher C and slightly higher Mn) yields a larger carbon equivalent; weld procedures often require preheating, controlled interpass temperatures, and consideration of PWHT, especially for thick sections or restrained joints. The probability of HAZ hardening and cold cracking increases if proper welding controls are not used.
Welding practice: - Use matched filler metals and appropriate preheat/PWHT as per CE/Pcm guidance. - For critical welded assemblies made of 45#, consider using lower-hydrogen electrodes and conducting PWHT to temper the HAZ and reduce residual stresses.
6. Corrosion and Surface Protection
- Both 20# and 45# are non-stainless plain carbon steels and do not possess inherent corrosion resistance. Typical protection methods include painting, powder coating, oiling, phosphate treatments, and galvanizing (hot-dip or electro-galvanizing) depending on the environment and service life requirements.
- PREN (Pitting Resistance Equivalent Number) is used for stainless alloys and is not applicable to plain carbon steels. Example formula for stainless assessment:
$$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
- For corrosion-prone environments, choose protective coatings or consider stainless or corrosion-resistant alloys instead of 20#/45#.
7. Fabrication, Machinability, and Formability
- Forming and bending: 20# is easier to cold-form and bend because of its lower strength and higher ductility. 45# requires more force and may need annealing before extensive forming.
- Machinability: Both grades machine well in annealed conditions. 45# in annealed state machines predictably and can produce better surface finish for certain turning/grinding operations due to higher carbon improving chip formation; however, harder or quenched-tempered 45# is more difficult and causes faster tool wear. 20# offers lower tool wear in general machining.
- Surface finishing: 45# can be hardened and ground to tight tolerances (e.g., for shafts and wear surfaces) after quench and temper. 20# is suitable where tight tolerances are not coupled with high hardness requirements.
- Cold working: 20# tolerates deeper cold drawing or cold heading compared with 45# which may crack if not annealed.
8. Typical Applications
| 20# – Typical uses | 45# – Typical uses |
|---|---|
| General structural components, cold-headed fasteners, welded assemblies, low-load shafts, axles for light loads, formable stamped parts | Shafts, axles, gears, machine parts requiring quench-and-temper strength, connecting rods, pins, wear components where higher hardness and fatigue resistance are needed |
| Fabricated frames, general-purpose forgings where weldability and formability are priorities | Heat-treated parts for power transmission, crankshafts in light applications, hardened dowel pins and bearing seats |
Selection rationale: - Choose 20# when weldability, formability, and lower cost are prioritized and when strength requirements are moderate. - Choose 45# when the design requires higher tensile strength, hardness, or fatigue life and when parts will be heat-treated to achieve these properties.
9. Cost and Availability
- Both grades are commodity steels with wide availability in bar, plate, and forgings. 20# typically has the lowest raw material cost because of lower carbon content and simpler processing for many applications.
- 45# commands a slightly higher price point mainly because of higher carbon and more frequent use in heat-treated and ground finished products. Total component cost should account for required heat treatments and machining — parts made from 45# often incur additional processing cost (quenching, tempering, grinding).
- Lead times are generally short for standard shapes; specialty forms or specific heat treatments can increase lead time.
10. Summary and Recommendation
Table: Quick comparison
| Attribute | 20# | 45# |
|---|---|---|
| Weldability | Excellent (lower C, lower CE) | Moderate to challenging (higher C, higher CE) |
| Strength–Toughness balance | Moderate strength, higher ductility/toughness | Higher achievable strength, reduced ductility if hardened |
| Cost | Lower raw material cost; simpler processing | Slightly higher material cost; often higher processing cost |
Final recommendations: - Choose 20# if: your design requires good weldability and formability, moderate strength is sufficient, cost sensitivity is high, or components will be joined by welding with minimal preheat and PWHT. - Choose 45# if: the part requires higher strength, hardness, or fatigue resistance achievable by quenching and tempering (e.g., shafts, gears, pins), and you can accommodate stricter welding controls, preheat, and possible PWHT or additional machining operations.
Concluding note: Always verify actual chemical and mechanical certificates from suppliers and qualify weld procedures based on carbon equivalent and part geometry. For critical components, perform heat-treatment trials and mechanical testing representative of the final manufacturing route to ensure the selected grade meets functional and lifecycle requirements.