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What is 45# steel?

What is 45# 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

Note: For steels larger than 80mm, the elongation and reduction of area after fracture are allowed to be reduced by 2% and 5% respectively compared with the above table.

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


  • ① Quenching is 840℃ water quenching.
  • ② 45# steel is qualified if the hardness is greater than HRC55 (up to HRC62) before being tempered after quenching. The highest hardness in practical application is HRC55 (high-frequency quenching HRC58).

45# steel (840±10℃) metallographic diagram

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Figure.1 (100×) Figure.1 (500×)
  • Material: 45# steel
  • Process conditions: heating to (840±10°C), holding for 1h, fast cooling to 600°C, holding for 1h, air cooling
  • Etching method: 4% nitric acid alcohol solution etching
  • Organization description: pearlite and white reticulated ferrite, the grain size is 8.

The normalizing of 45# steel is to heat the steel to 30~50℃ above Ac3, and cool it naturally in the air after heat preservation. The main difference between it and complete annealing is that the cooling rate is faster, and the purpose is to normalize the structure of the steel and make the pearl shine The volume increases and becomes thinner, reducing the amount of ferrite.
If 45# steel is used for ordinary structural parts, normalizing can be used as the final heat treatment. Normalizing can improve the structure after casting or forging, refine the austenite grains, homogenize the structure, and form a fine and uniform ferrite and pearlite structure. The grain size can reach 8 levels, as shown in Figure 1. It can be seen that the layer spacing of pearlite has reached a very fine degree, thereby improving the strength, hardness and toughness of the steel.
45# steel is also the most commonly used quenched and tempered steel. It must be normalized before quenching and high temperature tempering to obtain a uniform and fine structure as an important process of pre-heat treatment.

45# steel (manual arc welding) metallographic diagram

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Figure.1 (200X) Figure.2 (500X) Figure.3 (200X) Figure.4 (200X) Figure.5 (200X) Figure.6 (100X)
  • Material: 45# steel
  • Process situation:
  • Figure 1–2: Manual arc welding
  • Figure 3-6: Manual arc welding, no preheating before welding, no stress relief treatment after welding
  • Etching method: 4% nitric acid alcohol solution etching

Organization description:

Figure 1: The upper part is the weld zone, the microstructure is martensite, bainite, troostite and a small amount of columnar ferrite; the lower part is the overheating morphology of the heat-affected zone: martensite, bainite, bolster Intensity and very small amount of ferrite, black troostite is distributed along the crystal.
Figure 2: Figure 1 The topography of the superheated zone at higher magnification. The weld zone transforms from liquid phase to austenite (rapid cooling, columnar), and during subsequent cooling, ferrite precipitates from the columnar austenite grain boundary to delineate the outline of the columnar crystals, and then generates next to ferrite Non-spontaneous nucleation of bainite transformation and non-spontaneous nucleation and spontaneous nucleation of Torstenite transformation.
Due to the rapid cooling rate, the residual carbon content is higher than the hardenability of the first transformation zone, resulting in martensitic transformation.
Since the base material is No. 45# steel, it has better hardenability than low carbon steel. During the cooling process after welding, a large amount of martensite appears in the superheated zone of the heat affected zone of the base material. The generation of martensitic transformation makes the weld and the heat-affected zone have great structural stress, which is prone to cracking. Especially in the overheated zone, the grains are coarse, the martensite needles are very long, and there is no tempering, the stress is greater, here is the weakest area of the weld, and the sensitivity to cracks is the greatest.
The upper part of Figure 3 shows the weld structure: pearlite and ferrite. The proeutectoid ferrite precipitates along the columnar grain boundaries. The bottom half of the figure is the structure of the semi-melting zone and the superheated zone of the heat-affected zone of the base metal; bright white quenched martensite, black acicular lower bainite and troostite. The welding seam is in good condition with the base metal.
When the weld metal solidifies, it first begins to crystallize at the edge of the molten pool connected to the base metal. Because of the low temperature of the base metal, good thermal conductivity, large temperature gradient, and small crystallization speed, the crystallization of the molten pool near the base metal is Crystals grow directly on the grains of the base material. However, at this time, the edge of the base metal is affected by the welding thermal cycle and has been rapidly heated to a high temperature, resulting in an overheated state, which makes the crystal grains very coarse, and the weld seam continuously grows from these coarse grains and the new weld crystals are also very coarse. In addition, the high temperature of the weld during welding is also the reason why the columnar crystals are easy to grow thick. The semi-melting zone of the base metal is generally very small, and it is difficult to distinguish clearly by metallographic method, while the overheating zone is wide, the grains are very coarse, and the martensite needles are also very coarse after the cooling phase transformation.
Generally speaking, the carbon content of the electrode is lower than that of the base metal, the proportion of ferrite is higher, the expansion coefficient of ferrite is small, the strain stress of crystal shrinkage is small, and the ferrite has good plasticity and is easy to deform, which can reduce stress concentration. , Which can prevent welding cracks. The austenite of low carbon steel is easily decomposed into pearlite and ferrite after air cooling, while the base material of No. 45# steel is a medium carbon steel with a certain degree of hardenability. Air cooling after welding makes the austenite in the recrystallization zone easy Transformed into martensite.
Figure 4 shows the structure of the normal recrystallization zone in the heat-affected zone, which is white quenched martensite, black acicular lower bainite, black troostite, black massive fine pearlite and a small amount of white along the crystal. The mixed structure of ferrite. The heating temperature of the normal recrystallization zone (or normal normalizing zone) is in the temperature range above AC3 to the obvious growth of the grains. In this temperature range, the austenite grains are relatively small, and the length of the martensite needles after transformation Relatively short, the closer to the superheated zone, the higher the austenitizing temperature, the more uniform the chemical composition, the more stable the austenite, and the easier it is to obtain the martensite structure after air cooling. On the contrary, the closer to the incomplete normalizing zone, the lower the temperature, the more uneven the chemical composition, the more unstable austenite, and the easier it is to decompose non-martensitic structure to form the above-mentioned mixed structure.
Figure 5: Incompletely recrystallized area of the base material, the structure is black flake pearlite and white ferrite. Part of the ferrite precipitates out in the form of fine needles to form Widmanstatten structure.
The heating temperature of the base metal during welding in this area is between AC1 and AC3. Because the heating temperature is relatively low, it is not completely austenitized, and a part of the ferrite is not transformed and the band structure of the raw material is retained. In the austenitized part, due to the low temperature, the austenite grains are very small and unstable, and are easily decomposed into finer pearlite and ferrite. Due to the fast cooling rate, part of the ferrite has fine needles. The shape precipitates, forming the Widmanstatten organization state.
Figure 6: The original structure of the base metal, pearlite and ferrite, are distributed in a rolled band. In the picture, the gray elongated inclusions in the middle of the ferrite are sulfides. During rolling, the plastic inclusions are deformed into strips along the rolling direction, and the inclusions can become the foreign non-spontaneous nucleation core where the ferrite precipitates and grows. Therefore, long strips of sulfide distributed in the center of ferrite can often be seen in rolled materials.
45# steel is a medium carbon steel, and its weldability is much worse than that of low carbon steel. Therefore, pre-weld preheating and post-weld stress relief treatment are generally required during welding. In this example, there is no pre-weld preheating and post-welding stress relief treatment. The martensite structure is easy to appear in the heat-affected zone. The martensite has high hardness and high brittleness. It is easy to cause stress concentration caused by strain, and cannot deform to absorb strain energy, thereby inducing cold cracks and fatigue in use. Where cracks are prone to occur. In particular, the coarse martensite in the overheated zone is the weak area of the weld.

Source: China Flanges Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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