45# carbon steel

45# steel is a carbon structural steel with a carbon content of 0.45%. It is characterized by low price, good cutting performance, high hardness after quenching, and good toughness and wear resistance after quenching and tempering. 45# steel is widely used in the manufacture of structural parts and low-end plastic molds.

“45# steel” is a popular name, and the symbol is generally recorded as “45#”. In fact, the GB standard steel number is “45#”, it is not a serial number, and it is not very accurate to pronounce “45# steel”.

Composition specification:
Similar grades of 45# steel are S45C (JIS) and 1045 (AISI). In addition, there is SM45 grade in my country’s metallurgical technology standards that specifically indicates the use of plastic molds. Compared with 45# steel, SM45 has lower phosphorus and sulfur content and better steel purity.

Standard	GB/T 699	Grade	45
C	0.42~0.50	Cr	≤0.25
Si	0.17~0.37	Ni	≤0.30
Mn	0.50~0.80	Cu	≤0.25
Classified by phosphorus and sulfur		P	S
(45)		≤0.035	≤0.035
High quality steel (45A)		≤0.030	≤0.030
Super high quality steel (45E)		≤0.025	≤0.020
Standard	YB/T 094	AISI	JIS G4051
Grade	SM45	1045	S45C
C	0.42~0.48	0.43~0.50	0.42~0.48
Si	0.17~0.37	—	0.15~0.35
Mn	0.50~0.80	0.60~0.90	0.60~0.90
P	≤0.030	≤0.030	≤0.030
S	≤0.030	≤0.050	≤0.035

Mechanical properties (GB/T 699-1999):

Sample blank size (mm)		25
Recommended heat treatment (℃)	Normalizing	850
	Hardening	840
	Tempering	600
Mechanical properties	Tensile strength Mpa	≥600
	Yield strength Mpa	≥355
	Elongation %	≥16
	Reduction of area %	≥40
	Impact energy J	≥39
Delivery hardness HBS	Unheated steel	≤229
	Annealed steel	≤197

Heat treatment and hardness:
Recommended quenching process specification: quenching temperature is 820~860℃, water cooling or oil cooling, hardness ≥50HRC.
Recommended tempering process specification: tempering temperature is 500~560℃, air cooling, hardness is 25~33HRC.
Tempering in this temperature range is quenched and tempered. Quenching and tempering makes 45# steel have a good balance of strength, plasticity and toughness, with good overall performance, and can adapt to the alternating load environment.
The surface hardness of 45# steel is low after quenching and tempering, and it is not wear-resistant. Therefore, quenching and tempering + surface quenching are used to improve the surface hardness of parts.

The relationship between tempering temperature and hardness is shown in the following table:

Tempering temperature	After quenching	Unit (℃)
		200	300	400	500	550	600
Hardness HRC	57	55	50	41	33	26	22

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