Q235 čelik vs 45 Čelik vs 40Cr čelik

U inženjerskoj praksi, izbor čelika direktno utiče na performanse, proizvodnost, pouzdanost, i cijenu komponenti.

Three commonly referenced steels in Chinese and international standards — Q235, 45 čelik, i 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, Mehaničke performanse, and optimal applications differ substantially.

This article provides a multi‑perspective, mjerodavan, 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 Carbon čelik razred, equivalent to ASTM A36 i EN S235JR. Q235 offers a balance of strength, duktilnost, i zavarljivost, making it suitable for bridges, zgrade, ship structures, cjevovodi, and machinery frames.

Karakteristike

• Hemijski sastav: Carbon ≤ 0.20–0.25%, Mn 0.30–0.70%, trace S and P.
• Mehanička svojstva: Yield strength ≈ 235 MPa, tensile strength ≈ 375–500 MPa.
• Weldable and formable: Može se lako rezati, zavaren, and cold-formed.
• Isplativ: Economical option for general structural applications.
• Aplikacije: Construction beams, strukturni okviri, brodogradnja, Plodovi pod pritiskom.

45 Čelik (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, zupčanici, and fasteners that are mechanically loaded and can be heat-treated.

Karakteristike

• Hemijski sastav: Carbon ≈ 0.42–0.50%, Mn 0.50–0.80%, S/P <0.05%.
• Mehanička svojstva (žaljenje): 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.
• Aplikacije: Osovine, zupčanici, vijci, osovine, Povezivanje šipki, and mechanical parts under moderate loads.

40Cr Steel (također poznat kao 1.7035)

40Cr is a medium-carbon, hrom-alloyed steel widely used in applications requiring veća snaga, tvrdoća, i otpornost na habanje than ordinary medium-carbon steels.

Chromium improves hardenability, Otpornost na koroziju, i snagu zamora. It is roughly equivalent to Aisi 5140.

Karakteristike

• Hemijski sastav: Ugljik ≈ 0,37–0,44%, Krom ≈ 0,80–1,10%, Mn 0.50–0.80%, S/P <0.035%.
• Mehanička svojstva (normalizovano): Vlačna čvrstoća ≈ 745–930 MPa, granica tečenja ≈ 435–600 MPa.
• Odlična stvrdljivost: Može se kaliti i temperirati kako bi se postigla visoka tvrdoća (do HRC-a 50) za dijelove otporne na habanje.
• Dobra otpornost na zamor i žilavost: Pogodno za kritične mehaničke komponente.
• Aplikacije: Osovine, zupčanici, radilice, osovine za teška opterećenja, vretena, i druge mehaničke dijelove visoke čvrstoće.

2. Poređenje hemijskog sastava: Q235 čelik vs 45 Čelik vs 40Cr čelik

Hemijski sastav čelika direktno određuje njegovo fazno transformacijsko ponašanje i mehanička svojstva.

Sljedeća tabela predstavlja standardne opsege sastava (prema kineskim nacionalnim standardima) i funkcionalni mehanizmi ključnih elemenata za tri čelika:

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. Mikrostrukturne karakteristike: 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) + perlit (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 Čelik: Ferita + perlit, with higher pearlite content (40-50%) than Q235 due to higher carbon content.
  The structure is finer and more uniform (visokokvalitetni čelik), with fewer inclusions, leading to better strength and toughness balance.
• 40Cr Steel: Ferita + perlit + trace chromium-rich carbides. Chromium refines the grain size, making the pearlite lamellae thinner than 45 čelik.
  The presence of chromium carbides (Cr₃C) lays the foundation for subsequent heat treatment strengthening.

Heat-Treated State (Gašenje + Kaljenje, Q&T)

• Q235 Steel: Poor hardenability; gašenje (vodeno hlađenje) 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 Čelik: Nakon gašenja (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) i duktilnost (δ≥15%).
• 40Cr Steel: Odlična stvrdljivost; 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 + dispergovanih karbida), with higher strength and toughness than 45 čelik. Chromium stabilizes the martensite structure, reducing temper brittleness.

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

5. Heat Treatment Characteristics: Hardenability and Process Adaptability

Heat treatment responsiveness (otvrdnjavanje, 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 Čelik: Moderate hardenability. Workpieces with diameter ≤20 mm can achieve full martensite by water cooling; for thicker workpieces (20-40 mm), oil cooling leads to incomplete hardening (core is sorbite).
  It is suitable for medium-sized, medium-load parts requiring heat treatment.
• 40Cr Steel: Odlična stvrdljivost. 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 mm, water-oil quenching can achieve uniform hardening, making it suitable for large, heavy-load parts.

Common Heat Treatment Processes and Effects

• Žarljivost: 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.
• Normalizacija: Q235 normalizing (880–920℃) improves structure uniformity; 45/40Cr normalizing enhances strength and toughness, used as a pre-treatment for complex parts.
• Gašenje + Kaljenje: The core process for 45/40Cr. 45 steel uses water quenching + medium tempering; 40Cr uses oil quenching + medium tempering, postizanje boljih sveobuhvatnih performansi i manje deformacije.
• Otvrdnjavanje površina: 45/40Cr može biti podvrgnut indukcijskom kaljenju ili karburizaciji (45 čelik) za poboljšanje površinske tvrdoće (HRC 50–60) za dijelove otporne na habanje.
  40Sadržaj Cr hroma poboljšava efekat površinskog očvršćavanja i otpornost na habanje.

6. Performanse obrade: Livenje, Kovanje, Zavarivanje, i Mašinska obrada

Performanse obrade direktno utiču na efikasnost proizvodnje i troškove, i ključni je faktor za odabir materijala u masovnoj proizvodnji:

Casting Performance

• Q235 Steel: Loša sposobnost livenja. Nizak sadržaj ugljika i legure dovodi do loše fluidnosti rastaljene i visoke stope skupljanja, sklona skupljanjem šupljina i poroznosti. Rijetko se koristi za lijevanje; uglavnom za valjanje i oblikovanje.
• 45 Čelik: Umjerena sposobnost livenja. Veći sadržaj ugljenika poboljšava fluidnost u poređenju sa Q235, ali i dalje sklon vrućem pucanju. Koristi se za male i srednje livene dijelove sa niskim zahtjevima za preciznošću.
• 40Cr Steel: Bolja sposobnost livenja nego 45 čelik. Krom oplemenjuje livenu strukturu, 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 (E.g., vijci, nosači).
• 45 Čelik: 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.

Performanse zavarivanja

• 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, Zasjeniti, GTAW).
• 45 Čelik: Poor welding performance. High carbon content leads to hard martensite in the HAZ, prone to cold cracking.
  Prethodno zagrevanje (150–200℃) and PWHT (tempering at 600–650℃) su obavezne. Welding is only used for repair, not for load-bearing welds.
• 40Cr Steel: Worse welding performance than 45 čelik. 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, zakivanje) preferira se.

Obrada Performans

• 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 (E.g., machining of brackets, ploča).
• 45 Čelik: Good machining performance in the as-delivered state (HBW 190–230). Nakon termičke obrade (hardness ＞ HRC 30), povećava se težina obrade, zahtijeva alate od tvrde legure. To je tipičan „čelik koji se može mašinski obraditi“.
• 40Cr Steel: Umjerene performanse obrade u stanju isporuke. Krom povećava otpornost na rezanje, pa je habanje alata veće od 45 čelik.
  Nakon Q&T (HBW 280–320), strojna obrada zahtijeva veću brzinu rezanja i kontrolu posmaka, sa troškovima obrade 15–20% višim od 45 čelik.

7. Otpornost na koroziju

Sva tri čelika su ugljični/legirani konstrukcijski čelici bez namjernih legirajućih elemenata otpornih na koroziju (Sadržaj Cr u 40Cr je prenizak za formiranje pasivnog filma), pa je njihova otpornost na koroziju općenito loša, sa malim razlikama:

• Q235 Steel: Slaba otpornost na koroziju. Visok sadržaj nečistoća (S, Str) i nizak sadržaj legure ubrzavaju atmosfersku i slatkovodnu koroziju, sa stopom korozije od 0,1-0,3 mm/godišnje u industrijskoj atmosferi. Mora biti zaštićen (slikanje, pocinčavanje) za usluge na otvorenom.
• 45 Čelik: Nešto bolja otpornost na koroziju od Q235. Niži sadržaj nečistoća i finija struktura smanjuju mjesta iniciranja korozije.
  Stopa korozije je 0,08-0,25 mm/god u industrijskoj atmosferi, još uvijek zahtijeva zaštitu za dugotrajnu službu.
• 40Cr Steel: Najbolja otpornost na koroziju među ova tri. Krom stvara tanak oksidni film na površini, inhibiranje korozije.
  Stopa korozije je 0,05-0,20 mm/god u industrijskoj atmosferi, i ima bolju otpornost na blage kiseline/baze od Q235 i 45 čelik.
  Međutim, još uvijek pati od pitting korozije u medijima s visokim sadržajem klorida, zahtijevaju antikorozivni tretman (hromiranje, slikanje).

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

Primjena tri čelika striktno je zasnovana na njihovim performansama i cijeni, pokriva različite industrijske oblasti:

Q235 Steel

Niska cijena, konstrukcijski čelik opće namjene. Aplikacije uključuju:

• Izgradnja i izgradnja: Čelični okviri, grede, stubovi, čelične ploče, and reinforcement bars for ordinary buildings, mostovi, and workshops.
• Mechanical manufacturing: Non-load-bearing parts (nosači, baze, pokriva), vijci, matice, and washers for low-load equipment.
• Pipeline and container: Low-pressure water pipelines, Spremnici za skladištenje, and brackets for non-corrosive media.

45 Čelik

Medium-strength, heat-treatable carbon steel. Aplikacije uključuju:

• Mechanical parts: Osovine zupčanika, Povezivanje šipki, radilice, vijci, and nuts for medium-load equipment (E.g., small motors, pumpe, and agricultural machinery).
• Tool components: Blades, udarci, and dies for low-speed, low-wear tools (after surface hardening).
• Automobilska industrija: Non-critical parts (E.g., pedale kočnice, zglobovi upravljača) for low-end vehicles.

40Cr Steel

Visoka čvrstoća, alloy structural steel. Aplikacije uključuju:

• Mechanical transmission parts: High-load gear shafts, pogonska vratila, zupčanici, and bearings for heavy machinery (E.g., engineering machinery, Mašinski alati).
• Automobilski i vazduhoplov: Critical parts (E.g., engine crankshafts, bregaste osovine, transmission gears) for high-end vehicles and light aircraft.
• Petrochemical industry: High-pressure pipeline flanges, ventili, 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 (uzimajući Q235 kao osnovnu liniju) i isplativost tri čelika su kako slijedi:

10. Zaključak

Komparativna analiza Q235 čelik, 45 čelik, i 40Cr čelik ističe kako Sadržaj ugljika, legiranje, i toplotni tretman utiču na mehaničke performanse, proizvodnost, i prikladnost za primjenu.

• Q235 čelik je a low-carbon structural steel sa odličnom duktilnošću, zavabivost, i formibilnost.
  Njegova isplativost čini ga idealnim za opšte konstrukcijske i proizvodne primene, ali ima ograničenu snagu i zahtijeva zaštitu od korozije.
• 45 čelik je a medium-carbon, termički obrađen čelik nudi veću čvrstoću i tvrdoću od Q235.
  Kada ugašena i kaljena, postiže značajno poboljšanu vlačnu čvrstoću i otpornost na habanje, čineći ga pogodnim za mehaničkih dijelova kao što su osovine, zupčanici, i osovine.
• 40Cr čelik je a srednje ugljični čelik od legure hroma dizajniran za aplikacije visoke čvrstoće i otpornosti na zamor.
  To duboka kaljivost i otpornost na habanje omogućavaju mu da radi pod velikim cikličnim opterećenjima, kao što se vidi u radilice, Povezivanje šipki, i komponente mašina visokog opterećenja.

Zaključak: Izbor materijala treba da bude uravnotežen snaga, žilavost, obratnost, zavabivost, i trošak protiv zahtjeva usluge.
Q235 odgovara strukturalnim aplikacijama i aplikacijama sa malim opterećenjem, 45 čelik pokriva umjereno opterećene mehaničke dijelove, i 40Cr čelik se ističe visokom čvrstoćom, visok zamor, i komponente koje su kritične za habanje.

 

FAQs

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

• Q235 je niskougljični konstrukcioni čelik; 45 čelik je srednje ugljični i termički obrađen; 40Cr je čelik od legure hroma srednjeg ugljika visoke čvrstoće i kaljivosti.

Can Q235 steel be heat-treated to improve strength?

• Ne, Nizak sadržaj ugljika Q235 ograničava stvrdnjavanje toplinskom obradom. Poboljšanja čvrstoće se oslanjaju na hladnu obradu ili optimizaciju dizajna.

Which steel is best for shafts and gears?

• 45 čelik je pogodan za osovine i zupčanike umjerenog opterećenja; 40Cr je poželjan za visoku čvrstoću, visok zamor, i mehaničke komponente otporne na habanje.

Is 40Cr steel corrosion-resistant?

• Ne inherentno. Zaštitni premazi, oblaganje, ili su potrebna razmatranja dizajna za korozivna okruženja.

How does heat treatment affect 45 and 40Cr steels?

• Kaljenje i kaljenje značajno poboljšavaju vlačnu čvrstoću, tvrdoća, i otpornost na zamor, što ih čini pogodnim za mehanički zahtjevne komponente.