35# vs 45# – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
35# and 45# are two widely used medium‑carbon steel grades encountered in mechanical components, shafts, fasteners, and forgings. Engineers, procurement managers, and manufacturing planners commonly weigh tradeoffs between cost, machinability, weldability, and load‑bearing performance when choosing between them. Typical decision contexts include whether to prioritize higher as‑delivered strength and wear resistance versus easier forming, lower heat‑treatment sensitivity, and simpler joining procedures.
The principal difference between the two grades is their carbon content and the resulting higher strength and hardness potential of the 45# material compared with 35#. That basic shift in composition affects microstructure, heat‑treatment response, hardenability, and downstream operations — which is why these grades are frequently compared for components that require a balance of strength, toughness, and manufacturability.
1. Standards and Designations
- GB (China): 35# and 45# (commonly used nomenclature in Chinese standards).
- EN / European: C35, C45 (EN 10083 family for heat‑treatable steels).
- SAE/AISI: roughly equivalent to 1035 (≈0.35%C) and 1045 (≈0.45%C).
- JIS (Japan): S35C, S45C.
- ASTM/ASME: not direct one‑to‑one single designation, but comparable to medium‑carbon steels covered by broader specifications (bars, forgings).
Classification: both 35# and 45# are plain carbon steels (not stainless, not HSLA by default). They may be supplied as basic carbon steels for heat treatment; alloying elements beyond C, Mn, and Si are typically minimal unless the product is deliberately specified as alloyed or microalloyed.
2. Chemical Composition and Alloying Strategy
Table: typical composition ranges (wt%). These are representative ranges used in practice for carbon‑steel grades with the 35 and 45 designations. Actual supplied material should be verified against mill certificates.
| Element | 35# (typical, wt%) | 45# (typical, wt%) |
|---|---|---|
| C | 0.32 – 0.38 | 0.42 – 0.50 |
| Mn | 0.25 – 0.65 | 0.50 – 0.80 |
| Si | 0.15 – 0.35 | 0.15 – 0.35 |
| P | ≤ 0.035 (max) | ≤ 0.035 (max) |
| S | ≤ 0.035 (max) | ≤ 0.035 (max) |
| Cr | typically ≤ 0.25 (trace) | typically ≤ 0.25 (trace) |
| Ni | typically ≤ 0.30 (trace) | typically ≤ 0.30 (trace) |
| Mo | typically ≤ 0.08 (trace) | typically ≤ 0.08 (trace) |
| V, Nb, Ti | usually not specified (trace in some routes) | usually not specified (trace) |
| B, N | trace levels if controlled | trace levels if controlled |
How alloying affects properties: - Carbon: primary driver of strength and hardenability. Higher C (45#) increases achievable hardness and tensile strength after hardening and tempering but reduces ductility and weldability. - Manganese: raises hardenability and tensile strength and counteracts brittleness from sulfur; common in both grades at moderate levels. - Silicon: deoxidizer and contributes modestly to strength. - Trace alloying elements (Cr, Mo, V): if present intentionally, improve hardenability, wear resistance, and tempering stability; but typical 35#/45# are not deliberately alloyed to high levels unless specified.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As‑rolled or normalized states: a mixture of ferrite and pearlite. 35# (lower carbon) will present a greater fraction of ferrite and coarser pearlite; 45# has a higher pearlite/cementite fraction (more lamellar pearlite) and may show some proeutectoid cementite depending on cooling. - After quenching: martensite formation is more pronounced in 45# for the same quench severity because of higher carbon. 35# will form martensite but to a lower extent (and may require deeper quench or alloying for equivalent hardness). - After tempering: tempered martensite, with tempering responses differing by carbon — higher carbon steels retain higher hardness at equivalent tempering temperatures.
Effect of processing routes: - Normalizing (austentitizing and air cooling) refines grain size and produces a relatively uniform ferrite+pearlite matrix; 35# tends to be more ductile afterward. - Quenching & tempering can elevate strength and toughness: 45# can achieve higher strength/hardness windows but requires careful tempering to avoid brittleness. - Thermo‑mechanical processing and microalloy additions (V, Nb, Ti) can refine ferrite grain size and produce stronger ferrite/pearlite or bainitic matrices, increasing strength without solely relying on carbon; such treatments are usually specified rather than inherent in standard 35#/45#.
4. Mechanical Properties
Table: typical mechanical property ranges for common conditions (normalized or quenched & tempered ranges overlap). These are representative values — verify with mill certificates and heat‑treatment records for design.
| Property | 35# (typical) | 45# (typical) |
|---|---|---|
| Tensile strength (Rm) | ~500 – 700 MPa | ~600 – 800 MPa |
| Yield strength (Rp0.2 / Re) | ~300 – 500 MPa | ~350 – 600 MPa |
| Elongation (A) | ~16 – 25% | ~10 – 18% |
| Impact toughness (Charpy V notch, as‑normalized) | moderate; higher than 45# under same heat treatment | lower than 35# for same treatment; improves with tempering |
| Hardness (HB) | ~150 – 220 HB | ~180 – 260 HB |
Interpretation: - 45# is stronger and capable of higher hardness and wear resistance due to higher carbon; it typically shows reduced ductility and lower notch toughness in equivalent conditions. - 35# is more ductile and forgiving to forming and weld thermal cycles; it is often preferable where toughness, bending, or cold forming are important. - Final properties depend strongly on heat treatment: a quenched & tempered 35# can approach or exceed the strength of a normalized 45#, but hardness and wear resistance capacity remain constrained by carbon content.
5. Weldability
Weldability depends primarily on carbon content, carbon equivalent (CE), and alloying. Two common empirical indices are useful to interpret relative weldability:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
and
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 45# with higher carbon will have a higher $CE_{IIW}$ and $P_{cm}$ than 35# (all else equal), indicating greater propensity to form hard martensitic heat‑affected zones and a higher cold‑crack risk. This necessitates preheat, controlled interpass temperatures, and post‑weld heat treatment (PWHT) more often for 45#. - 35# is more weldable in typical shop practice: lower preheat requirements, less distortion risk, and fewer cracking issues. - When alloying elements (Cr, Mo) are present, hardenability increases and weldability decreases further for both grades; that effect is magnified in 45# due to its higher base hardenability from carbon.
6. Corrosion and Surface Protection
- Neither 35# nor 45# are stainless steels; both are susceptible to general atmospheric corrosion and localized attack in aggressive environments.
- Common protective strategies: hot‑dip galvanizing, electroplating, conversion coatings, painting/primer systems, or polymer overlays. Selection depends on service environment, geometry, and cost.
- PREN (Pitting Resistance Equivalent Number) is not applicable to plain carbon steels because it is used for stainless alloys:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Use corrosion‑resistant alloys (stainless steels) or coatings when corrosion resistance is a design requirement; for plain carbon steels, focus on barrier and cathodic protection methods.
7. Fabrication, Machinability, and Formability
- Machinability: 35# typically machines more easily than 45# because lower hardness and less tool wear; however, machinability depends on heat treatment and microstructure. Free‑cutting modifications (sulfur additions) are a separate category.
- Formability and bending: 35# has higher ductility and better cold formability. 45# can be cold‑formed but has reduced allowable strain before cracking; hot forming or annealing may be required for tight bends.
- Grinding, finishing, and surface hardening: 45# responds better to surface hardening processes (induction hardening, carburizing followed by quench & temper when combined with appropriate carbon profiles) to improve wear resistance.
- Dimensional stability: both grades require attention to residual stresses introduced by machining and heat treatment; 45# may need stress‑relief annealing depending on final application.
8. Typical Applications
| 35# — Typical uses | 45# — Typical uses |
|---|---|
| Shafts and axles for moderate loads, pins, studs, and bolts where ductility and toughness are important | Heavily loaded shafts, gears, crankshafts, camshafts, heavy bolts, wear parts requiring higher hardness |
| Forgings and components that will be tempered for moderate strength and good toughness | Components that require higher quenched & tempered strength and surface wear resistance |
| Parts that are frequently welded or require more aggressive forming | Parts requiring higher fatigue strength and wear resistance; components subjected to bending fatigue |
Selection rationale: - Choose 35# when forming, welding ease, and toughness are priorities or when cost is a constraint. - Choose 45# when higher as‑delivered strength, surface hardness, and wear resistance are primary design drivers and when the fabrication process can accommodate the steel’s higher hardenability.
9. Cost and Availability
- Cost: 35# is generally slightly less expensive than 45# on a per‑kg basis because of lower carbon content and similar production routes; the difference is modest. Heat treatment and finishing costs can outweigh material price differences.
- Availability: both grades are ubiquitous in bar, plate, and forging stock. 45# may be more commonly stocked for heavy‑duty components and applications requiring quenched & tempered deliveries, whereas 35# is common for general‑purpose parts.
- Product forms: both are widely available as hot‑rolled bars, cold‑drawn bars, forgings, and plate. Specially microalloyed or controlled‑chemistry variants are produced to order.
10. Summary and Recommendation
Summary table (qualitative):
| Aspect | 35# | 45# |
|---|---|---|
| Weldability | Better (lower CE) | More sensitive (higher CE) |
| Strength–Toughness balance | More ductile, better toughness at equal heat treatment | Higher achievable strength/hardness; lower ductility if over‑hardened |
| Cost | Slightly lower (material cost) | Slightly higher (material + potential processing) |
Recommendations: - Choose 35# if: - You need better as‑fabricated ductility, simpler welding processes, and reduced risk of cracking. - The part will undergo significant forming or require higher notch toughness at design loads. - Cost and ease of manufacturing are prioritised over maximum hardness or wear resistance.
- Choose 45# if:
- Higher strength, greater hardness potential, or improved wear resistance are required (e.g., heavily loaded shafts, gears).
- The manufacturing process can include appropriate preheat/PWHT and controlled quenching/tempering to manage toughness.
- Fatigue life, surface hardness through induction hardening, or quenched & tempered properties are design drivers.
Final note: both 35# and 45# are general‑purpose medium carbon steels. The best choice depends on the finished component’s load case, fabrication route, and cost constraints. For critical applications specify required heat treatment, mechanical property targets, and acceptance testing (UT/MT, hardness mapping, impact testing) on purchase documents to ensure the mill and heat‑treatment route deliver the intended performance.