10# vs 20# – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
10# and 20# are two widely used plain-carbon steel grades in manufacturing and construction, frequently specified in Chinese and regional supply chains. Engineers, procurement managers, and manufacturing planners commonly weigh trade-offs between cost, weldability, formability, and strength when selecting between them. Typical decision contexts include whether a component requires high ductility and easy joining (favoring the lower-carbon option) or higher strength and wear resistance (favoring the higher-carbon option).
The primary technical distinction between these grades is their carbon content: the lower-carbon grade yields a softer, more ductile material with superior weldability and forming behavior, whereas the higher-carbon grade increases strength and hardenability at the expense of some ductility and ease of welding. Because both are plain carbon steels with similar trace elements, they are commonly compared for parts that must balance machinability, heat-treatment response, and cost.
1. Standards and Designations
- GB (China): 10# and 20# are conventional plain-carbon steel designations in Chinese standards (often used in GB/T product specifications).
- JIS (Japan): Equivalent nominal grades in JIS use numeric carbon-based names (e.g., S10C, S20C are common comparable grades).
- ASTM/ASME: US standards generally use A36, A106, or SAE grade nomenclature; direct one-to-one equivalents do not always exist because ASTM grades specify mechanical property requirements rather than simple “10#” naming.
- EN (Europe): EN grades classify steels by chemical and mechanical requirements (e.g., C10, C20 are conceptually comparable but must be cross-referenced).
- Classification: Both 10# and 20# are plain carbon steels (not alloy, not stainless, not HSLA, not tool steels). They are typically used as low-carbon (10#) and medium-low carbon (20#) steels in manufacturing.
2. Chemical Composition and Alloying Strategy
| Element | Typical range for 10# (approx.) | Typical range for 20# (approx.) |
|---|---|---|
| Carbon (C) | 0.06–0.12 wt% | 0.17–0.24 wt% |
| Manganese (Mn) | 0.25–0.60 wt% | 0.25–0.65 wt% |
| Silicon (Si) | 0.02–0.35 wt% | 0.02–0.35 wt% |
| Phosphorus (P) | ≤ 0.035 wt% (max) | ≤ 0.035 wt% (max) |
| Sulfur (S) | ≤ 0.035 wt% (max) | ≤ 0.035 wt% (max) |
| Chromium (Cr) | typically ≤ 0.30 wt% | typically ≤ 0.30 wt% |
| Nickel (Ni) | typically ≤ 0.30 wt% | typically ≤ 0.30 wt% |
| Molybdenum (Mo) | typically ≤ 0.08 wt% | typically ≤ 0.08 wt% |
| Vanadium (V) | trace if any | trace if any |
| Niobium (Nb) | trace if any | trace if any |
| Titanium (Ti) | trace if any | trace if any |
| Boron (B) | not typically added | not typically added |
| Nitrogen (N) | trace | trace |
Notes: These are typical composition windows for conventional commercial 10# and 20# plain-carbon steels; product standards and manufacturers may specify slightly different limits. The alloying strategy for both grades focuses on low levels of alloying elements: carbon predominantly governs strength and hardenability, manganese modifies strength and deoxidation, and silicon is a deoxidizer that slightly increases strength.
How alloying affects properties: - Carbon: primary control of tensile strength, hardness, and hardenability; higher carbon increases strength and wear resistance but reduces ductility and weldability. - Manganese: increases tensile strength, hardenability, and tensile toughness; mitigates the harmful effects of sulfur (forms MnS). - Silicon: minor solid-solution strengthening and deoxidizer; excessive Si can impair surface quality in some processes. - Trace elements (Cr, Ni, Mo): when present in small amounts, they marginally increase hardenability and strength but are not characteristic of these grades.
3. Microstructure and Heat Treatment Response
Typical microstructures: - 10#: In as-rolled or normalized condition the microstructure is predominantly ferrite with a fine dispersion of pearlite. The low carbon content limits pearlite fraction and prevents large amounts of martensite after moderate cooling rates. - 20#: Higher carbon content produces a larger pearlite fraction in the as-rolled state and increases the potential for forming martensite or bainite under faster cooling or quenching.
Heat treatment responses: - Annealing/Full anneal: Both grades will soften, reduce residual stresses, and produce coarse ferrite–pearlite mixtures. 10# reaches a softer state easier and faster due to lower carbon. - Normalizing: Produces a finer ferrite–pearlite microstructure and improved toughness compared with as-rolled; benefits both grades but yields higher normalized strengths for 20# because of its higher carbon. - Quenching and tempering (Q&T): 20# responds more to Q&T because its higher carbon and marginally higher hardenability allow development of higher strength martensitic structures after quench and controlled tempering. 10# has limited hardenability—producing high hardness martensite uniformly is more difficult and typically not cost-effective. - Thermo-mechanical processing: Controlled rolling and accelerated cooling can increase strength for both grades; the carbon content determines the achievable combination of strength and toughness.
4. Mechanical Properties
| Property (typical, processing dependent) | 10# (approx. ranges) | 20# (approx. ranges) |
|---|---|---|
| Tensile strength (MPa) | ~320–470 | ~380–540 |
| Yield strength (MPa) | ~140–310 | ~200–370 |
| Elongation (%) | ~20–40 | ~12–30 |
| Impact toughness (Charpy V-notch, J) | generally higher at low temperature for 10# (process dependent) | lower than 10# at same treatment, but can be improved by processing |
| Hardness (HB or HRC) | lower in nominal condition (softer) | higher in nominal condition (harder) |
Notes: Values are indicative and strongly influenced by product form (plate, bar, rod), thickness, and thermal-mechanical history. In general, 20# exhibits higher tensile and yield strengths because of its higher carbon content and larger pearlite fraction; 10# offers greater ductility and typically superior low-temperature toughness when processed similarly.
Interpretation: - Strength: 20# > 10# due to higher carbon and increased pearlite/microstructural strengthening. - Toughness/Ductility: 10# > 20# in comparable conditions because lower carbon reduces brittle phases and allows more ferrite. - Hardness/Wear resistance: 20# typically harder and better for wear-prone parts after minimal heat treatment.
5. Weldability
Weldability of carbon steels depends mainly on carbon content, equivalent carbon (hardenability) and residual elements. Two commonly used empirical indices are the IIW carbon equivalent and the Pcm parameter:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 10# will have a lower $CE_{IIW}$ and $P_{cm}$ due to lower carbon, indicating generally excellent weldability with low preheat requirements and minimal risk of cold cracking when appropriate procedures are used. - 20# has higher carbon, increasing $CE_{IIW}$ and $P_{cm}$ and thus raising the risk of martensitic hardening in the heat-affected zone (HAZ). For thicker sections or restrained joints, preheat, controlled interpass temperatures, and suitable post-weld heat treatment (PWHT) may be recommended. - Microalloying and higher manganese modestly affect hardenability; however these plain-carbon grades rarely contain enough alloying to drastically change weld procedures beyond those determined by carbon content and section thickness.
Best practice: use lower heat input and controlled cooling for 20# in thicker sections, and specify preheat or PWHT per welding procedure specifications when carbon equivalent thresholds are exceeded.
6. Corrosion and Surface Protection
- Both 10# and 20# are non-stainless carbon steels; intrinsic corrosion resistance is limited and similar between the grades. Selection between them should assume the need for protective measures in corrosive environments.
- Typical mitigation: galvanizing, protective coatings (paints, powder coat), plating, or application-specific inhibitors. For outdoor structural parts, hot-dip galvanizing or robust paint systems are common.
- PREN (Pitting Resistance Equivalent Number) is not applicable to plain carbon steels; it is relevant for stainless grades:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Corrosion allowances, cathodic protection, and coating system selection should be based on environment, expected life, and inspection regime rather than small differences in composition between 10# and 20#.
7. Fabrication, Machinability, and Formability
- Cutting and machining: 20# is harder and typically requires higher cutting forces and may reduce tool life relative to 10#. However, differences are modest for low-carbon range; tooling strategies and speeds should be adjusted for increased carbon content.
- Forming and bending: 10# is superior for cold forming, deep drawing, and tight-radius bending because of higher ductility. 20# is more prone to springback and requires larger bend radii or more force.
- Joining and fastening: 10# is easier to cold-form threads and to join by welding; 20# allows higher-strength fastening but may require post-join heat treatment in critical, high-strength applications.
- Surface finishing: both grades accept typical finishing operations (grinding, polishing, coating); 20# may exhibit higher work hardening which influences final finish quality for very tight tolerances.
8. Typical Applications
| 10# (low carbon) | 20# (higher carbon) |
|---|---|
| Cold-formed components, general structural parts, automotive inner panels, light brackets, welded fabricated assemblies where ductility and weldability matter | Shafts, axles, studs, bolts (where higher strength or wear resistance is needed), structural members with higher load requirements, machine parts that benefit from hardenable steel |
| Low-stress shafts, riveted or bolted connections, low-pressure piping (non-critical) | Parts intended for heat treatment to increase strength (quenched/tempered or normalized) |
| Drawn wire, formed tubes, fasteners that require significant forming | Wear-prone parts (after appropriate heat treatment), medium-strength forged components |
Selection rationale: - Choose 10# when forming, welding, and surface finishing ease are priorities and strength requirements are modest. - Choose 20# when higher as-manufactured strength or the possibility of increasing strength through heat treatment is required.
9. Cost and Availability
- Cost: Both grades are commodity plain-carbon steels and are among the lowest-cost options. 10# is typically marginally cheaper due to lower carbon and slightly simpler processing, but market prices are dominated by steel mill product mix and demand rather than intrinsic grade cost.
- Availability: Both are widely available in bars, rods, plates, and wire forms. 10# and 20# have extensive supply chains in regions using GB/JIS nomenclature. Lead times are usually short for standard sizes; specialty forms (large diameter, tight-tolerance bars, or certified mill test reports) may increase cost and lead time.
10. Summary and Recommendation
| Attribute | 10# | 20# |
|---|---|---|
| Weldability | Excellent (lower CE) | Good to moderate (requires preheat/PWHT in thicker sections) |
| Strength–Toughness balance | Lower strength, higher ductility/toughness | Higher strength, less ductile unless specially processed |
| Relative cost | Slightly lower or comparable | Slightly higher or comparable |
Recommendations: - Choose 10# if you need superior formability, easier welding, higher ductility, lower risk of HAZ cracking, or cost-sensitive high-volume fabrication where post-weld treatment is undesirable. - Choose 20# if you require higher as-manufactured tensile and yield strength, improved wear resistance, or the option to heat-treat components to higher strength levels and the fabrication plan accommodates preheat, controlled cooling, or PWHT.
Final note: Material selection should consider part geometry, section thickness, expected service loads, joining method, and any required post-manufacturing heat treatments. When in doubt, specify material property requirements (tensile, yield, elongation, toughness) and manufacturing constraints rather than grade name alone, and consult mill certificates or qualified metallurgical data for critical applications.