Q235 Steel vs 45 Steel vs 40Cr Steel

In engineering practice, steel selection directly influences performance, fabricante, confiabilidade, and cost of components.

Three commonly referenced steels in Chinese and international standards — Q235, 45 aço, e 40Cr — cover a broad spectrum of design requirements, from basic structural support to high‑strength mechanical parts.

Although each is based on iron‑carbon metallurgy, their alloying strategies, microstructural behavior, desempenho mecânico, and optimal applications differ substantially.

This article provides a multi‑perspective, autoritário, and in‑depth comparison to guide material selection and engineering decision‑making.

1. Metallurgical Identity and Classification

Q235 Steel

Q235 is a low-carbon structural steel widely used in general engineering and construction applications.

It is the most common Chinese aço carbono nota, equivalent to ASTM A36 e EN S235JR. Q235 offers a balance of strength, ductilidade, e soldabilidade, making it suitable for bridges, edifícios, ship structures, oleodutos, and machinery frames.

Características

• Composição química: Carbon ≤ 0.20–0.25%, Mn 0.30–0.70%, trace S and P.
• Propriedades mecânicas: Yield strength ≈ 235 MPa, tensile strength ≈ 375–500 MPa.
• Weldable and formable: Pode ser facilmente cortado, soldado, and cold-formed.
• Econômico: Economical option for general structural applications.
• Aplicativos: Construction beams, quadros estruturais, construção naval, vasos de pressão.

45 Aço (also known as C45 or 1.1191)

45 steel is a medium-carbon steel widely used in China and internationally for mechanical parts requiring higher strength and hardness than low-carbon steels.

It corresponds roughly to AISI 1045. It is suitable for shafts, engrenagens, and fasteners that are mechanically loaded and can be heat-treated.

Características

• Composição química: Carbon ≈ 0.42–0.50%, Mn 0.50–0.80%, S/P <0.05%.
• Propriedades mecânicas (recozido): Tensile strength ≈ 570–700 MPa, yield strength ≈ 330–500 MPa.
• Heat-treatable: Can be quenched and tempered to achieve higher hardness and wear resistance.
• Good machinability and moderate toughness: Balances strength and processability.
• Aplicativos: Eixos, engrenagens, parafusos, eixos, bielas, and mechanical parts under moderate loads.

40Cr Steel (também conhecido como 1.7035)

40Cr is a medium-carbon, cromo-alloyed steel widely used in applications requiring maior força, dureza, e resistência ao desgaste than ordinary medium-carbon steels.

Chromium improves hardenability, resistência à corrosão, e resistência à fadiga. It is roughly equivalent to AISI 5140.

Características

• Composição química: Carbon ≈ 0.37–0.44%, Chromium ≈ 0.80–1.10%, Mn 0.50–0.80%, S/P <0.035%.
• Propriedades mecânicas (normalizado): Tensile strength ≈ 745–930 MPa, yield strength ≈ 435–600 MPa.
• Excellent hardenability: Can be quenched and tempered to achieve high hardness (até HRC 50) Para peças resistentes ao desgaste.
• Good fatigue resistance and toughness: Suitable for critical mechanical components.
• Aplicativos: Eixos, engrenagens, virabrequins, eixos pesados, fusos, and other high-strength mechanical parts.

2. Comparação de composição química: Q235 Steel vs 45 Steel vs 40Cr Steel

The chemical composition of steel directly determines its phase transformation behavior and mechanical properties.

The following table presents the standard composition ranges (per Chinese national standards) and the functional mechanisms of key elements for the three steels:

Key differences:

Q235 has low carbon and no intentional alloying elements, focusing on processability; 45 steel has higher carbon and stricter impurity control, enabling heat treatment;

40Cr adds chromium to optimize hardenability and mechanical properties, bridging the gap between carbon steel and high-alloy steel.

3. Características microestruturais: From As-Delivered to Heat-Treated States

Microstructure is the link between chemical composition and mechanical properties.

The three steels exhibit distinct microstructures in different states, directly affecting their performance:

As-Delivered State (Hot Rolled)

• Q235 Steel: Consists of ferrite (α-Fe) + Pearlita (lamellar mixture of ferrite and cementite). Ferrite is the main phase (70–80%), ensuring good ductility and weldability.
  Pearlite content (20–30%) provides moderate strength. The structure is coarse-grained due to low alloy content and simple hot rolling process.
• 45 Aço: Ferrita + Pearlita, with higher pearlite content (40–50%) than Q235 due to higher carbon content.
  The structure is finer and more uniform (Aço de alta qualidade), with fewer inclusions, leading to better strength and toughness balance.
• 40Cr Steel: Ferrita + Pearlita + trace chromium-rich carbides. Chromium refines the grain size, making the pearlite lamellae thinner than 45 aço.
  The presence of chromium carbides (Cr₃C) lays the foundation for subsequent heat treatment strengthening.

Heat-Treated State (Têmpera + Temperamento, P&T)

• Q235 Steel: Poor hardenability; têmpera (refrigeração a água) only forms martensite in the surface layer, with the core remaining ferrite-pearlite.
  Heat treatment is rarely used, as it cannot significantly improve overall performance and may cause deformation/cracking.
• 45 Aço: Depois de temperar (840–860℃ water/oil cooling), the structure transforms into lath martensite (hard but brittle).
  Tempering at 200–300℃ (low tempering) produces tempered martensite, improving toughness while maintaining high hardness.
  Tempering at 500–600℃ (medium tempering) forms sorbite, achieving a balance of strength (σᵤ≥600 MPa) e ductilidade (δ≥15%).
• 40Cr Steel: Excellent hardenability; oil cooling (instead of water cooling) can achieve full martensite transformation even for workpieces with diameter ≤50 mm.
  After medium tempering (520–560℃), the structure becomes tempered sorbite (fine-grained sorbite + carbonetos dispersos), with higher strength and toughness than 45 aço. Chromium stabilizes the martensite structure, reducing temper brittleness.

4. Mechanical Properties Comparison — Q235 Steel vs 45 Steel vs 40Cr Steel

5. Heat Treatment Characteristics: Hardenability and Process Adaptability

Heat treatment responsiveness (Hardenabilidade, temper stability) determines the scope of application of steel. The three steels differ significantly in this regard:

Hardenability

• Q235 Steel: Very poor hardenability. The critical cooling rate is high; only thin workpieces (≤5 mm) can form a small amount of martensite after water cooling, while thick workpieces remain ferrite-pearlite.
  Heat treatment is not economically viable, so it is used in the as-delivered state.
• 45 Aço: Moderate hardenability. Workpieces with diameter ≤20 mm can achieve full martensite by water cooling; for thicker workpieces (20–40mm), oil cooling leads to incomplete hardening (core is sorbite).
  It is suitable for medium-sized, medium-load parts requiring heat treatment.
• 40Cr Steel: Excellent hardenability. Chromium reduces the critical cooling rate, enabling full martensite transformation in workpieces with diameter ≤50 mm by oil cooling (avoiding water cooling-induced deformation/cracking).
  For workpieces up to 80 milímetros, water-oil quenching can achieve uniform hardening, making it suitable for large, heavy-load parts.

Common Heat Treatment Processes and Effects

• Recozimento: Q235 annealing (600–650℃) relieves rolling stress; 45/40Cr annealing refines grains and reduces hardness for machining. 40Cr annealing also dissolves chromium carbides, preparing for quenching.
• Normalizando: Q235 normalizing (880–920℃) improves structure uniformity; 45/40Cr normalizing enhances strength and toughness, used as a pre-treatment for complex parts.
• Têmpera + Temperamento: The core process for 45/40Cr. 45 steel uses water quenching + medium tempering; 40Cr uses oil quenching + medium tempering, achieving better comprehensive performance and lower deformation.
• Endurecimento da superfície: 45/40Cr can undergo induction hardening or carburizing (45 aço) to improve surface hardness (HRC 50–60) Para peças resistentes ao desgaste.
  40Cr’s chromium content enhances surface hardening effect and wear resistance.

6. Processing Performance: Fundição, Forjamento, Soldagem, e Usinagem

Processing performance directly affects manufacturing efficiency and cost, and is a key factor for material selection in mass production:

Casting Performance

• Q235 Steel: Poor castability. Low carbon and alloy content lead to poor molten fluidity and high shrinkage rate, prone to shrinkage cavities and porosity. Rarely used for casting; mainly for rolling and forming.
• 45 Aço: Moderate castability. Higher carbon content improves fluidity compared to Q235, but still prone to hot cracking. Used for small to medium-sized cast parts with low precision requirements.
• 40Cr Steel: Better castability than 45 aço. Chromium refines the cast structure, reducing shrinkage and hot cracking tendency.
  Suitable for precision cast parts requiring heat treatment, but casting cost is higher than rolling.

Forging Performance

• Q235 Steel: Excellent forging performance. Forging temperature range (1150–850℃) is wide, with good plasticity and low deformation resistance. Suitable for hot forging of simple shapes (por exemplo, parafusos, colchetes).
• 45 Aço: Good forging performance. Forging temperature (1100–800℃); requires uniform heating to avoid cracking. Forged parts have refined grains, improving heat treatment effect.
• 40Cr Steel: Moderate forging performance. Chromium increases deformation resistance, requiring higher forging force and stricter temperature control (1100–820℃).
  Post-forging annealing is necessary to eliminate internal stress and prepare for heat treatment.

Desempenho de soldagem

• Q235 Steel: Excellent welding performance. Low carbon content avoids martensite formation in the heat-affected zone (HAZ), with no preheating or post-weld heat treatment (Pwht) required for thin workpieces. Compatible with all welding methods (SMAW, GMAW, GTAW).
• 45 Aço: Poor welding performance. High carbon content leads to hard martensite in the HAZ, prone to cold cracking.
  Pré-aquecimento (150–200℃) and PWHT (tempering at 600–650℃) are mandatory. Welding is only used for repair, not for load-bearing welds.
• 40Cr Steel: Worse welding performance than 45 aço. Chromium increases HAZ hardenability, making cold cracking and temper brittleness more likely.
  Strict preheating (200–300℃), low heat input welding, and PWHT are required. Welding is generally avoided; mechanical joining (bolting, fascinante) é preferido.

Usinagem Desempenho

• Q235 Steel: Excellent machining performance. Low hardness and good plasticity make cutting easy, with low tool wear.
  Suitable for high-speed machining and automated production lines (por exemplo, machining of brackets, pratos).
• 45 Aço: Good machining performance in the as-delivered state (HBW 190–230). Após tratamento térmico (hardness ＞ HRC 30), machining difficulty increases, requiring hard alloy tools. It is a typical “machinable heat-treated steel”.
• 40Cr Steel: Moderate machining performance in the as-delivered state. Chromium increases cutting resistance, so tool wear is higher than 45 aço.
  After Q&T (HBW 280–320), machining requires higher cutting speed and feed rate control, with machining cost 15–20% higher than 45 aço.

7. Resistência à corrosão

All three steels are carbon/alloy structural steels without intentional corrosion-resistant alloying elements (Cr content in 40Cr is too low for passive film formation), so their corrosion resistance is generally poor, with slight differences:

• Q235 Steel: Fraca resistência à corrosão. High impurity content (S, P) and low alloy content accelerate atmospheric and freshwater corrosion, with a corrosion rate of 0.1–0.3 mm/year in industrial atmospheres. Must be protected (pintura, galvanização) for outdoor service.
• 45 Aço: Slightly better corrosion resistance than Q235. Lower impurity content and finer structure reduce corrosion initiation sites.
  Corrosion rate is 0.08–0.25 mm/year in industrial atmospheres, still requiring protection for long-term service.
• 40Cr Steel: Best corrosion resistance among the three. Chromium forms a thin oxide film on the surface, inhibiting corrosion.
  Corrosion rate is 0.05–0.20 mm/year in industrial atmospheres, and it has better resistance to mild acids/bases than Q235 and 45 aço.
  No entanto, it still suffers from pitting corrosion in high-chloride media, requiring anti-corrosion treatment (chromizing, pintura).

8. Application Scenarios Q235 Steel vs 45 Steel vs 40Cr Steel

The application of the three steels is strictly based on their performance and cost, covering different industrial fields:

Q235 Steel

Low-cost, general-purpose structural steel. As aplicações incluem:

• Building and construction: Steel frames, vigas, colunas, steel plates, and reinforcement bars for ordinary buildings, pontes, and workshops.
• Mechanical manufacturing: Non-load-bearing parts (colchetes, bases, capas), parafusos, nozes, and washers for low-load equipment.
• Pipeline and container: Low-pressure water pipelines, tanques de armazenamento, and brackets for non-corrosive media.

45 Aço

Medium-strength, heat-treatable carbon steel. As aplicações incluem:

• Mechanical parts: Eixos de engrenagem, bielas, virabrequins, parafusos, and nuts for medium-load equipment (por exemplo, small motors, bombas, and agricultural machinery).
• Tool components: Lâminas, socos, and dies for low-speed, low-wear tools (after surface hardening).
• Indústria automotiva: Non-critical parts (por exemplo, pedais de freio, arejando as juntas) for low-end vehicles.

40Cr Steel

Alta resistência, alloy structural steel. As aplicações incluem:

• Mechanical transmission parts: High-load gear shafts, eixos de acionamento, engrenagens, and bearings for heavy machinery (por exemplo, engineering machinery, máquinas-ferramentas).
• Automotivo e aeroespacial: Critical parts (por exemplo, engine crankshafts, árvores de cames, transmission gears) for high-end vehicles and light aircraft.
• Petrochemical industry: High-pressure pipeline flanges, válvulas, and pump shafts for medium-corrosion, high-load environments.

9. Cost and Cost-Effectiveness Comparison

Cost is a key factor in large-scale production. The relative cost (taking Q235 as the baseline) and cost-effectiveness of the three steels are as follows:

10. Conclusão

The comparative analysis of Q235 steel, 45 aço, and 40Cr steel highlights how teor de carbono, liga, e tratamento térmico influence mechanical performance, fabricante, e adequação do aplicativo.

• Q235 steel é um low-carbon structural steel with excellent ductility, soldabilidade, e conformabilidade.
  Its cost-effectiveness makes it ideal for general structural and fabrication applications, but it has limited strength and requires corrosion protection.
• 45 aço é um medium-carbon, heat-treatable steel offering higher strength and hardness than Q235.
  Quando extinto e temperado, it achieves significantly improved tensile strength and wear resistance, tornando-o adequado para mechanical parts such as shafts, engrenagens, and axles.
• 40Cr steel é um medium-carbon chromium-alloy steel projetado para high-strength and fatigue-resistant applications.
  Isso é deep hardenability and wear resistance allow it to perform under heavy cyclic loads, como visto em virabrequins, bielas, and high-load machinery components.

Bottom line: Material selection should balance força, resistência, usinabilidade, soldabilidade, e custo against service requirements.
Q235 suits structural and low-load applications, 45 steel covers moderate-load mechanical parts, and 40Cr steel excels in high-strength, high-fatigue, and wear-critical components.

 

Perguntas frequentes

What is the main difference between Q235, 45, and 40Cr steels?

• Q235 is low-carbon structural steel; 45 steel is medium-carbon and heat-treatable; 40Cr is a medium-carbon chromium-alloy steel with high strength and hardenability.

Can Q235 steel be heat-treated to improve strength?

• Não, Q235’s low carbon content limits heat-treatment hardening. Strength improvements rely on cold working or design optimization.

Which steel is best for shafts and gears?

• 45 steel is suitable for moderate-load shafts and gears; 40Cr is preferred for high-strength, high-fatigue, and wear-resistant mechanical components.

Is 40Cr steel corrosion-resistant?

• Not inherently. Revestimentos de proteção, chapeamento, or design considerations are needed for corrosive environments.

How does heat treatment affect 45 and 40Cr steels?

• Quenching and tempering significantly improve tensile strength, dureza, e resistência à fadiga, making them suitable for mechanically demanding components.