25# vs 35# – Composition, Heat Treatment, Properties, and Applications

Introduction

Engineers and procurement teams frequently choose between 25# and 35# when specifying carbon steels for shafts, pins, bushings, and general structural components where cost, machinability, and mechanical performance must be balanced. Typical decision contexts include whether to prioritize easier forming and welding for large fabrications, or higher as-forged/heat-treated strength in components that will carry higher static or dynamic loads.

The primary distinction between the two grades is their carbon content and the resulting balance between strength and ductility: the higher-carbon grade exhibits greater strength and hardness potential at the expense of ductility and some weldability. Because both are plain carbon steels widely used in similar product forms, designers compare them directly to decide whether additional strength (and potentially heat treatment) justifies trade-offs in formability, toughness, and fabrication complexity.

1. Standards and Designations

• Common national and international standard systems may reference equivalent plain carbon steels, but the literal designations “25#” and “35#” are most commonly encountered in Chinese material nomenclature.
• Typical relevant standard families:
• GB (China): 25#, 35# (plain carbon steels)
• ASTM/ASME: comparable plain carbon steel grades (selection by composition/property rather than literal “#” designation)
• EN: steels in EN 10025/10083 families or EN equivalents selected by carbon and tensile requirements
• JIS: Japanese plain carbon steel equivalents listed by C-content and mechanical properties

Classification: Both 25# and 35# are plain carbon steels (non-alloy). They are not stainless, HSLA, or tool steels in their standard forms. Heat treatment can be applied to modify properties but does not change the base classification.

2. Chemical Composition and Alloying Strategy

Notes: - The most consequential compositional difference is carbon. Small adjustments to Mn and Si influence tensile properties and deoxidation. Other alloying elements are generally absent in standard 25#/35#; if present, they indicate a different specified grade. - Alloying strategy for these grades is minimal: keep chemistry simple, control impurities, and use heat treatment or microalloying only when specific property boosts are required.

3. Microstructure and Heat Treatment Response

Microstructure: - Both grades in the as-rolled or normalized condition typically consist of a ferrite–pearlite microstructure. The volume fraction of pearlite increases with carbon content. - 25#: higher ferrite fraction, coarser/finer pearlite depending on cooling, generally more ductile and tougher in the as-rolled condition. - 35#: higher pearlite fraction and potentially finer pearlite if processed to accelerate cooling, yielding higher strength and hardness in the normalized condition.

Heat-treatment response: - Normalizing: Refines grain structure and produces a more uniform ferrite–pearlite distribution. Both grades respond well to normalizing; 35# will attain higher normalized strength than 25# because of its higher carbon. - Annealing: Softens and improves machinability or formability for both grades; 25# will become more ductile relative to 35# after full anneal. - Quenching and tempering: Both can be hardened, but hardenability is limited relative to alloy steels. 35#, with higher carbon, achieves higher as-quenched hardness but also greater risk of quench-induced cracking and reduced toughness unless carefully tempered. - Thermo-mechanical processing: Controlled rolling and accelerated cooling improve strength and toughness, but dramatic hardenability changes require alloy additions not present in standard 25#/35#.

4. Mechanical Properties

Interpretation: - 35# is the stronger/harder option in equivalent thermo-mechanical states; 25# offers better ductility and typically better impact resistance for components expected to undergo forming or dynamic loads. - For components needing high toughness and large plastic deformation, 25# is generally preferable unless post-processing (e.g., tempering) is planned for 35#.

5. Weldability

Weldability depends primarily on carbon content, combined alloying, and section thickness. For plain carbon steels like 25# and 35#, carbon equivalent indices are widely used to estimate preheat/post-heat needs.

Common carbon-equivalent formulas: - Display example for international guidance: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - A more detailed formula used for predicting cold cracking susceptibility: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - 35# has higher $C$, so calculated $CE_{IIW}$ and $P_{cm}$ will be higher than for 25#, indicating increased tendency for hardening in the heat-affected zone (HAZ) and higher risk of hydrogen-assisted cold cracking. Therefore, 35# typically requires more conservative welding procedures: preheat, controlled interpass temperature, low-hydrogen electrodes, and post-weld heat treatment when thickness and restraint are significant. - 25#, with lower $C$, is more forgiving for welding, easier to join without preheat for moderate thicknesses, and generally requires less rigorous hydrogen control.

6. Corrosion and Surface Protection

• Both 25# and 35# are non-stainless carbon steels and rely on coatings and barriers for corrosion protection. Common strategies:
• Hot-dip galvanizing for outdoor structural components.
• Paint systems (epoxy primers, polyurethane topcoats) for atmospheric protection.
• Cathodic protection or coatings in buried or immersed applications.
• Stainless indices like PREN are not applicable to plain carbon steels. For illustration, PREN is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but this is relevant only for stainless alloys; neither 25# nor 35# should be evaluated by PREN.
• Selection note: If corrosion resistance is a primary design driver, choose a stainless or corrosion-resistant alloy rather than relying on 25# or 35# plus surface treatment.

7. Fabrication, Machinability, and Formability

• Formability and bending: 25# is easier to bend and form cold because of higher ductility; 35# is more prone to springback and may crack if bent beyond recommended radii.
• Machinability: As-received, 25# produces easier machining conditions when softer; however, mildly higher carbon can improve chip formation for some operations. In general, higher-carbon 35# requires higher cutting forces and may shorten tool life if in a hardened condition.
• Cutting, grinding, and finishing: Both respond to standard machining practices, but operations on 35# that are quenched or tempered should be planned as for higher-strength steels (slower speeds, harder tooling, coolant).
• Surface treatments (plating, coating) behave similarly for both grades, though surface preparation for welding or coatings may be more critical on higher-strength quenched surfaces.

8. Typical Applications

Selection rationale: - Choose 25# when weldability, ductility, and forming ease are key and extreme strength is not required. - Choose 35# when higher baseline strength or hardenability is needed and the design can tolerate reduced ductility or additional heat-treatment/welding controls.

9. Cost and Availability

• Cost: Both are commodity carbon steels; 25# is typically marginally less expensive than 35# due to lower carbon content and fewer processing constraints. The price difference is usually small relative to alloy or specialty steels.
• Availability: Both grades are widely available in common product forms: bars, plates, billets, and forgings, especially in regions where the “#” designation is common. Lead times are generally short for standard hot-rolled or normalized product; quenched-and-tempered deliveries take longer.
• Procurement note: Specify required heat treatment and mechanical properties in purchasing documents; plain designation alone may lead to variability in delivered properties.

10. Summary and Recommendation

Recommendations: - Choose 25# if you need good formability, easier welding, better impact resistance in as-rolled condition, and the component does not require high strength or heavy post-processing. - Choose 35# if baseline higher tensile/yield strength is important, or if the part will be heat treated to achieve specified wear or strength targets and you can apply suitable welding and fabrication controls.

Concluding practical guidance: - For welded fabrications with large plate thicknesses or where hydrogen cracking risk must be minimized, default to lower-carbon 25# or specify preheat/post-heat procedures if 35# is required. - For machined components that will be hardened or run under cyclic loading, consider 35# with a defined quench-and-temper schedule, or better yet, evaluate a low-alloy steel with superior hardenability and toughness if high performance is required. - Always specify the exact material standard, required mechanical properties, and any heat treatment or inspection requirements in procurement documentation to avoid ambiguity between “25#” and “35#” deliveries.