20# vs 25# – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
20# and 25# are two common designation labels used in several regional standards (notably Chinese GB/GB/T practice) to identify plain carbon steels frequently employed in general engineering, shafts, fasteners, and welded structures. Engineers, procurement managers, and manufacturing planners often face a selection dilemma between the two: lower-cost, more ductile material versus a slightly higher-carbon grade that delivers greater strength and wear resistance. Choices are typically driven by required load capacity, forming or welding needs, machinability, and cost constraints.
The principal distinction between 20# and 25# is their carbon content and the metallurgical consequences of that carbon difference — which control strength, ductility, and hardenability. Because the two grades are otherwise chemically simple (low-alloy or plain-carbon steels), designers compare them directly when specifying material for parts where strength and toughness trade-offs must be balanced with fabrication ease.
1. Standards and Designations
- GB / GB/T (China): 20# (also written as 20G in some contexts), 25# (25G etc.) — plain carbon steels used for general structural and mechanical purposes.
- JIS (Japan): Comparable to JIS-SC or SCr grades depending on exact chemistry and intended use, but direct one-to-one mapping is not exact.
- ASTM/ASME (USA): No direct ASTM grade with the “#” notation; comparable steels would be low-carbon mild steels such as A36 or AISI 1020/1025 depending on exact C and Mn levels and property requirements.
- EN (Europe): Equivalent grades fall into non-alloy structural steels such as S235/S275 families depending on mechanical property levels.
- Classification: Both 20# and 25# are plain carbon steels (non-stainless, non-high-strength low-alloy) in typical practice; they are not tool steels, stainless steels, or HSLA in their standard forms.
2. Chemical Composition and Alloying Strategy
| Element | Typical 20# (approx. range) | Typical 25# (approx. range) |
|---|---|---|
| C (Carbon) | 0.17 – 0.24 wt% | 0.22 – 0.30 wt% |
| Mn (Manganese) | 0.25 – 0.60 wt% | 0.25 – 0.70 wt% |
| Si (Silicon) | ≤ 0.35 wt% (often 0.02–0.30) | ≤ 0.35 wt% (often 0.02–0.30) |
| P (Phosphorus) | ≤ 0.035 wt% (max) | ≤ 0.035 wt% (max) |
| S (Sulfur) | ≤ 0.035 wt% (max) | ≤ 0.035 wt% (max) |
| Cr (Chromium) | usually ≤ 0.30 wt% | usually ≤ 0.30 wt% |
| Ni (Nickel) | trace / not specified | trace / not specified |
| Mo, V, Nb, Ti, B, N | not intentionally added in standard grades; trace levels possible | same as 20# |
Notes: Values are typical ranges seen in common GB-style specifications and mill practice. Exact limits and additional elements vary with the standard, mill, and whether the product is intended for forging, heat treatment, or special service.
How alloying affects properties - Carbon: Primary control of strength and hardness. Higher carbon raises tensile and yield strength and hardness, but reduces ductility and weldability. - Manganese: Improves hardenability, tensile strength, and deoxidizes steel. Mn also offsets sulfur embrittlement to some degree. - Silicon, Cr, Ni, Mo: When present in small amounts they modify hardenability and toughness; in these grades they are usually at low levels and not the primary hardening strategy. - Impurities (P, S): Kept low to preserve toughness and fatigue resistance; sulfur can improve machinability if intentionally elevated in free-cutting variants.
3. Microstructure and Heat Treatment Response
Typical as-rolled microstructures: - 20#: Predominantly ferrite with dispersed pearlite. Lower carbon level means larger ferritic matrix fraction and a softer, more ductile microstructure. - 25#: Increased pearlite fraction compared with 20#, producing a finer lamellar structure when cooled under similar conditions; higher dislocation density after deformation.
Heat treatment response: - Normalizing: Both grades respond to normalizing with grain refinement; 25# will produce a higher volume fraction of pearlite and somewhat higher strength after normalizing versus 20#. - Quenching and tempering: Because both are low-alloy plain carbon steels, their hardenability is limited. 25# achieves higher hardness after quenching than 20# under equivalent quench severity due to higher carbon, but neither achieves the hardenability of medium-alloy steels without alloy additions. - Thermo-mechanical processing: Controlled rolling and accelerated cooling can increase strength and refine microstructure; 25# will typically reach higher strength levels through such processing but with some loss in ductility.
4. Mechanical Properties
| Property | Typical 20# (as-rolled/normalized) | Typical 25# (as-rolled/normalized) |
|---|---|---|
| Tensile strength (MPa) | ~350 – 500 MPa (typical working range) | ~400 – 560 MPa (higher on average) |
| Yield strength (MPa) | ~215 – 315 MPa | ~245 – 370 MPa |
| Elongation (%) | ~26 – 40% | ~18 – 30% (reduced vs 20#) |
| Impact toughness (J, qualitative) | Generally good at ambient; better low-temperature toughness than 25# | Good at ambient; lower than 20# in similarly processed condition |
| Hardness (HB) | ~110 – 160 HB | ~130 – 180 HB |
Notes: These are representative ranges for hot-rolled or normalized conditions. Specific values depend on exact chemistry, section thickness, and thermal/mechanical processing. 25# is generally stronger and harder due to higher carbon and increased pearlite, while 20# is generally more ductile and tougher in comparable conditions.
5. Weldability
Weldability of plain carbon steels is primarily controlled by carbon content and hardenability. Higher carbon increases the risk of forming hard, brittle martensite in the heat-affected zone (HAZ) and thus increases susceptibility to cold cracking.
Useful assessment formulas (qualitative guidance): - Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (more conservative): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - 20# (lower C) will typically show lower $CE_{IIW}$ and $P_{cm}$ values than 25#, implying easier weldability, reduced preheat requirements, and lower risk of HAZ cracking. - 25# (higher C) often requires more careful welding practice — preheating, controlled interpass temperatures, and post-weld heat treatment in critical applications — especially for thicker sections. - Microalloying can elevate hardenability even at low total alloy content; since both grades are generally plain carbon, alloying effects are limited compared to HSLA steels.
6. Corrosion and Surface Protection
- Neither 20# nor 25# is stainless; corrosion resistance is similar and poor compared with stainless steels. Protection strategies include painting/coating, hot-dip galvanizing, electroplating, oiling, or corrosion-inhibiting primers.
- PREN (pitting resistance equivalent number) is not applicable to these non-stainless steels, but the formula for stainless comparison is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Selection note: If corrosion resistance is required rather than cost-optimized carbon steel, specifying a stainless grade or corrosion-resistant alloy is the correct approach rather than relying on 20# or 25# with coatings when long-term or aggressive-environment performance is needed.
7. Fabrication, Machinability, and Formability
- Forming and bending: 20# is more forgiving in forming operations due to lower yield strength and higher elongation; less springback and lower formation forces are typical.
- Cold working: 25# work-hardens faster and may require higher forming forces and tighter tooling controls.
- Machinability: Increasing carbon and pearlite content in 25# usually increases strength and hardness; machinability depends on microstructure and sulfur/phosphorus levels. In general, 25# can be slightly harder to machine (higher cutting forces, faster tool wear) than 20# under identical conditions.
- Surface finishing: Both grades can be surface ground, polished, or plated. Higher hardness of 25# may require more aggressive grinding tool selection.
8. Typical Applications
| 20# (common uses) | 25# (common uses) |
|---|---|
| Structural components, small-diameter pipes, general fabrication where ductility and weldability are prioritized | Shafts, axles, studs, bolts, and parts where higher strength or wear resistance is needed |
| Machine frames, welded assemblies, general-purpose forgings | Medium-duty gears and lightly loaded wear parts (when no alloying required) |
| Low-cost fabricated parts, painted or coated components | Components requiring higher hardness after heat treatment or work-hardening |
Selection rationale: - Choose 20# when forming, welding, ductility, and cost are primary concerns. - Choose 25# when higher strength, moderate increase in wear resistance, or smaller cross-sections requiring higher load capacity are important and controlled welding practices can be implemented.
9. Cost and Availability
- Cost: 20# is generally slightly less expensive than 25# on a per-ton basis because of lower carbon (production cost differences are small but market pricing typically favors lower-C grades). The cost delta is modest.
- Availability: Both grades are widely available in bars, plates, sheets, and forgings in regions where GB/Chinese steels are sold. 20# may be more commonly stocked in general-purpose inventories; 25# is commonly available for mechanical applications.
- Product forms: Round bar, hot-rolled plate, and welded pipes are common. Lead times and availability depend on mill production priorities and regional demand.
10. Summary and Recommendation
| Attribute | 20# | 25# |
|---|---|---|
| Weldability | Good (lower C, easier) | Moderate (higher C, more care) |
| Strength–Toughness balance | Better toughness/ductility at lower strength | Higher strength, lower ductility relative to 20# |
| Cost | Lower / economical | Slightly higher |
Choose 20# if: - The design demands good weldability and high ductility (e.g., extensive welded structures, complex forming). - Cost sensitivity and ease of fabrication are priorities. - Parts are subject to impact loads or require better low-temperature toughness in simple carbon-steel applications.
Choose 25# if: - Higher strength or hardness is required without moving to a medium-alloy steel. - Component size or geometry demands higher load-carrying capacity with limited increase in weight. - The fabrication plan includes controlled welding procedures (preheat, suitable consumables) or the part will be heat-treated/tempered to achieve a target property.
Concluding note: The 20# vs 25# decision is principally a carbon-content trade-off: modestly higher strength and hardness at the expense of ductility and weldability. For critical applications where both higher strength and reliable toughness are required, consider specifying an engineered medium-alloy grade or HSLA material with documented properties rather than relying solely on increased carbon.