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What is carbon steel

What is carbon steel?

Carbon steel is an iron-carbon alloy with a carbon content of 0.0218% – 2.11%. Generally, it also contains a small amount of silicon, manganese, sulfur, and phosphorus. The higher the carbon content in carbon steel, the greater the hardness and strength, but the lower the plasticity.

20220801115211 64416 - What is carbon steel

Classification of carbon steel

  • (1) According to the purpose, carbon steel can be divided into carbon structural steel, carbon tool steel, and free-cutting structural steel. Carbon structural steel can be divided into engineering construction steel and machine manufacturing structural steel;
  • (2) According to the smelting method, it can be divided into open hearth steel and converter steel;
  • (3) According to the deoxidation method, it can be divided into rimming steel (F), killed steel (Z), semi-killed steel (B), and special killed steel (TZ);
  • (4) According to the carbon content, carbon steel can be divided into low carbon steel (WC ≤ 0.25%), medium carbon steel (WC0.25% – 0.6%), and high carbon steel (WC>0.6%);
  • (5) According to the quality of steel, carbon steel can be divided into ordinary carbon steel (higher phosphorus and sulfur), high-quality carbon steel (lower phosphorus and sulfur), high-quality steel (lower phosphorus and sulfur), and super high-quality steel.

Types of carbon steel

Carbon structural steel
Brand: example Q235-A · F, indicating σ s=235MPa.
Brand Note: q is the yield strength of a quality grade (with ABCD grade IV), f rimming steel.
Features: low price, excellent process performance (such as weldability and cold formability).
Application: general engineering structures and common mechanical parts. For example, Q235 can be used to make bolts, nuts, pins, hooks, and less important mechanical parts, as well as deformed steel bars, section steel, steel bars, etc. in building structures.
High-quality carbon structural steel
Brand: e.g. 45, 65Mn, 08F.
Brand Note: directly represents the 10000 fractions of carbon content in the metal.
Application: nonalloy steel used for manufacturing important mechanical parts is generally used after heat treatment.
Common steel grades and uses:
08F, low mass fraction of carbon, good plasticity, and low strength, used for stamping parts such as automobile and instrument shells;
20#. Good plasticity and weldability, used for parts with low strength requirements and carburized parts, such as a hood, welding container, small shaft, nut, washer, and carburized gear;
45#, 40Mn, with good comprehensive mechanical properties after quenching and tempering, is used for mechanical parts with large stress, such as gears, connecting rods, machine tool spindles, etc;
60#, 65Mn steel has high strength; It is used for manufacturing various springs, locomotive rims, and low-speed wheels.
Carbon tool steel
Grade: for example, T12 steel represents carbon tool steel with wc=1.2%.
Brand Note: t plus thousands of carbon content of the metal.
Features: it belongs to eutectoid steel and hypereutectoid steel, with high strength, hardness, and good wear resistance. It is suitable for manufacturing all kinds of low-speed cutting tools.
Common steel grades and uses:
T7, T8: make parts that bear a certain impact and require toughness. Such as sledgehammers, punches, chisels, woodworking tools, and scissors.
T9, T10, T11: manufacturing tools with low impact and high hardness and wear resistance. Such as tap, small drill bit, die, and hand saw blade.
T12, T13: making tools free from impact. Such as a file, scraper, razor, and measuring tool.
Cast steel
Brand: for example, zg200-400, indicating σ s=200MPa, σ B = 400MPa cast steel.
Properties: the casting properties are worse than cast iron, but the mechanical properties are better than cast iron.
Application: it is mainly used to manufacture important mechanical parts with complex shapes and high mechanical performance requirements, which are difficult to be formed by forging and other methods in technology, such as automobile gearbox shells, locomotive couplers, coupling, etc.

Chemical Properties of Carbon Steel

ASTM A53 Composition, max, %
Elt C Mn P S
Type S (seamless pipe)
Gr. A 0.25 0.95 0.05 0.045
Gr. B 0.3 1.2 0.05 0.045
Type E (electric-resistance-welded)
Gr. A 0.25 0.95 0.05 0.045
Gr. B 0.3 1.2 0.05 0.045
Type F (furnace-welded pipe)
Gr. A 0.3 1.2 0.05 0.045
Composition, max, %
Elt Cu (1) Ni (1) Cr (1) Mo (1) V (1)
Type S (seamless pipe)
Gr. A 0.4 0.4 0.4 0.15 0.08
Gr. B 0.4 0.4 0.4 0.15 0.08
Type E (electric-resistance-welded)
Gr. A 0.4 0.4 0.4 0.15 0.08
Gr. B 0.4 0.4 0.4 0.15 0.08
Type F (furnace-welded pipe)
Gr. A 0.4 0.4 0.4 0.15 0.08
Notes:
The total composition for these five elements shall not exceed 1.00%.
ASTM A105 Composition, %
Elt C Mn P S Si
0.35 0.6 0.035 0.04 0.1
max 1.05 max max 0.35
Composition, %
Elt Cu Ni Cr Mo V
0.4 0.4 0.3 0.12 0.08
max (1) max (1) max (1-2) max (1-2) max
Notes:
The sum of Copper, Nickel, Niobium, Molybdenum, and Vanadium shall not exceed 1.00%.
The sum of Niobium and Molybdenum shall not exceed 0.32%.
Note. For each reduction of 0.01% below the specified carbon maximum (0.35%), an increase of 0.06% Manganese above the specified maximum (1.05%) will be permitted up to a maximum of 1.35%.
ASTM A106 Composition, %
Elt C Mn P S Si
max max max min
Gr. A 0.25 (1) 0.27 0.035 0.035 0.1
0.93
Gr. B 0.30 (2) 0.29 0.035 0.035 0.1
1.06
Gr. C 0.35 (2) 0.29 0.035 0.035 0.1
1.06
Composition, %
Elt Cr Cu Mo Ni V
max (3) max (3) max (3) max (3) max (3)
Gr. A 0.4 0.4 0.15 0.4 0.08
Gr. B 0.4 0.4 0.15 0.4 0.08
Gr. C 0.4 0.4 0.15 0.4 0.08
Notes:
For each reduction of 0.01% below the specified Carbon maximum, an increase of 0.06% Manganese above the specified maximum will be permitted up to a maximum of 1.35%.
Unless otherwise specified by the purchaser, for each reduction of 0.01% below the specified Carbon maximum, an increase of 0.06% Manganese above the specified maximum will be permitted up to a maximum of 1.65%.
These five elements combined shall not exceed 1%.
ASTM A216 Composition, % max
Gr. C Mn P S Si
WCA 0.25 (1) 0.70 (1) 0.04 0.045 0.6
UNS J02502
WCB 0.30 (2) 1.00 (2) 0.04 0.045 0.6
UNS J03002
WCC 0.25 (3) 1.20 (3) 0.04 0.045 0.6
UNS J02503
Composition, % max
Gr. C Mn P S Si
WCA 0.25 (1) 0.70 (1) 0.04 0.045 0.6
UNS J02502
WCB 0.30 (2) 1.00 (2) 0.04 0.045 0.6
UNS J03002
WCC 0.25 (3) 1.20 (3) 0.04 0.045 0.6
UNS J02503
Notes:
For each reduction of 0.01% below the specified maximum Carbon content, an increase of 0.04% manganese above the specified maximum will be permitted up to a maximum of 1.10%.
For each reduction of 0.01% below the specified maximum Carbon content, an increase of 0.04% Mn above the specified maximum will be permitted up to a maximum of 1.28%.
For each reduction of 0.01% below the specified maximum Carbon content, an increase of 0.04% manganese above the specified maximum will be permitted to a maximum of 1.40%.
ASTM A234 Composition, %
Gr. C Mn P S Si
max max
WPB 0.3 0.29 0.05 0.058 0.1
(1 2 3 4 5) max 1.06 min
WPC 0.35 0.29 0.05 0.058 0.1
(2 3 4 5) max 1.06 min
WP1 0.28 0.3 0.045 0.045 0.1
max 0.9 0.5
WP12 CL1 0.05 0.3 0.045 0.045 0.6
0.2 0.8 max
WP12 CL2 0.05 0.3 0.045 0.045 0.6
0.2 0.8 max
WP11 CL1 0.05 0.3 0.03 0.03 0.5
0.15 0.6 1
WP11 CL2 0.05 0.3 0.04 0.04 0.5
0.2 0.8 1
WP11 CL3 0.05 0.3 0.04 0.04 0.5
0.2 0.8 1
WP22 CL1 0.05 0.3 0.04 0.04 0.5
0.15 0.6 max
WP22 CL3 0.05 0.3 0.04 0.04 0.5
0.15 0.6 max
WP5 CL1 0.15 0.3 0.04 0.03 0.5
max 0.6 max
WP5 CL3 0.15 0.3 0.04 0.03 0.5
max 0.6 max
WP9 CL1 0.15 0.3 0.03 0.03 1
max 0.6 max
WP9 CL3 0.15 0.3 0.03 0.03 1
max 0.6 max
WPR 0.2 0.4 0.045 0.05
max 1.06
WP91 0.08 0.3 0.02 0.01 0.2
0.12 0.6 0.5
WP911 0.09 0.3 0.02 0.01 0.1
0.13 0.6 0.5
Composition, %
Gr. Cr Mo Ni Cu Others
WPB 0.4 0.15 0.4 0.4 V 0.08
(1 2 3 4 5) max max max max max
WPC 0.4 0.15 0.4 0.4 V 0.08
(2 3 4 5) max max max max max
WP1 0.44
0.65
WP12 CL1 0.8 0.44
1.25 0.65
WP12 CL2 0.8 0.44
1.25 0.65
WP11 CL1 1 0.44
1.5 0.65
WP11 CL2 1 0.44
1.5 0.65
WP11 CL3 1 0.44
1.5 0.65
WP22 CL1 1.9 0.87
2.6 1.13
WP22 CL3 1.9 0.87
2.6 1.13
WP5 CL1 4 0.44
6 0.65
WP5 CL3 4 0.44
6 0.65
WP9 CL1 8 0.9
10 1.1
WP9 CL3 8 0.9
10 1.1
WPR 1.6 0.75
2.24 1.25
WP91 8 0.85 0.4 V 0.18
9.5 1.05 max 0.25
Nb 0.06
0.1
N 0.03
0.07
Al 0.02 max (6)
Ti 0.01 max (6)
Zr 0.01 max (6)
WP911 8 0.9 0.4 V 0.18
9.5 1.1 max 0.25
Nb 0.06
0.1
N 0.04
0.09
Al 0.02 max (6)
B 0.0003
0.006
W 0.9
1.1
Ti 0.01 max (6)
Zr 0.01 max (6)
Notes:
Fittings made from a bar or plate may have 0.35 max carbon.
Fittings made from forgings may have 0.35 max Carbon and 0.35 max Silicon with no minimum.
For each reduction of 0.01% below the specified Carbon maximum, an increase of 0.06% Manganese above the specified maximum will be permitted, up to a maximum of 1.35%.
The sum of Copper, Nickel, Niobium, and Molybdenum shall not exceed 1.00%.
The sum of Niobium and Molybdenum shall not exceed 0.32%.
Applies both to heat and product analyses.

Elements affecting carbon steel

Manganese
About 0.25% – 0.80%. Solid solution strengthening; Remove FeO and reduce the brittleness of steel; Synthesis of MNS with sulfuration can reduce the harmful effect of sulfur. Beneficial.
Silicon
About 0.10% – 0.40%, solid solution strengthening; In addition to the adverse effect of FeO on steel quality, it is beneficial.
Sulfur
FES and Fe form low melting point eutectic (melting point: 985 ℃), which leads to steel embrittlement and cracking during hot processing at 1000-1250 ℃, “hot embrittlement”. Harmful.
Phosphorus
The strength and hardness are increased, but the plasticity and toughness are reduced, “cold brittleness”. Harmful.

Common structures in carbon steel

Ferrite: carbon dissolved in α- The interstitial solid solution in Fe is white after nitric acid corrosion, with low strength and hardness, but it has good plasticity and toughness.
Pearlite: a mechanical mixture of ferrite and cementite, fingerprint-shaped, the white matrix is ferrite, strength and hardness are significantly higher than ferrite, plasticity and toughness are worse than ferrite but much better than cementite. The distance between the centers of two adjacent pieces of cementite (or ferrite) in flake pearlite is called the flake spacing of pearlite. Pearlite can be divided into pearlite, sorbate, and troostite according to the distance between photos, and the distance between troostite slices has been difficult to observe. The lamellar spacing is related to the undercooling, which increases continuously, and the lamellar spacing of pearlite formed by transformation decreases continuously. The undercooling is the difference between the pearlite transformation temperature and the critical point A1. The hardness and strength of the three pearlites are from low to high.
Martensite: (m) is carbon soluble α- The supersaturated solid solution of Fe is generally obtained by quenching medium and high carbon steel. The hardness of martensite mainly depends on its carbon content of martensite. The hardness of martensite increases with the increase of carbon mass fraction. When the carbon mass fraction reaches 6%, the hardness of quenched steel approaches the maximum value, and the carbon mass fraction further increases. Although the hardness of martensite will increase, the hardness of steel will decrease due to the increase in the amount of residual austenite. Toughness is very poor, which is characterized by hardness and brittleness. Lath martensite is common in low carbon steel, maraging steel, and stainless steel, while sheet (needle) martensite is common in high and medium carbon steel.
Tempered martensite, tempered troostite, and tempered sorbate: these three kinds are heating products, and the corresponding processes are called low temperature, medium temperature, and high-temperature tempering respectively. With the increase of tempering temperature, the hardness, strength, and wear resistance will generally show a downward trend, but the plasticity and toughness are improving, especially the tempered troostite has a high elastic limit and toughness, so it is the main structure of spring steel. Tempered sorbate has good comprehensive mechanical properties, so it is generally used as the final heat treatment of structural axial components.

Heat treatment of carbon steel

The heating process, cooling process, heating time, heating temperature, cooling time, cooling method, constant temperature, etc. of heat treatment have a great impact on the mechanical properties of materials after heat treatment.
1. Quenching: heat carbon steel above AC3 (hypereutectoid steel) or AC1 (eutectoid steel or hypereutectoid steel), 30 ° c-50 ° C, and keep it for a period of time before quenching in water or oil, which can increase the hardness and strength of the material.

  • ➀ Quenching liquid with high specific heat, high thermal conductivity, low viscosity, and low volatility has a strong quenching ability and high cooling rate.
  • ➁ The higher the carbon content of steel, the lower the quenching temperature.
  • ➂ When high carbon steel is quenched with water, if heat quenching crack occurs, oil quenching is used to improve it.
  • ➃ Quality effect refers to the difference between the internal and external hardness of steel after quenching. The smaller the quality effect is, the greater the hardening energy is, the greater the hardening depth is, and the heart is easier to harden.
  • ➄ Hemp quenching: after hemp quenching, the structure will completely change into hemp field loose iron, and it will hardly crack.

2. Tempering: eliminate internal stress, increase the toughness and stabilize the internal structure.

  • ➀ Temper brittleness: the toughness of steel decreases after tempering.
  • ➁ Low-temperature tempering: the temperature is 150-200 ° C, which can reduce the residual stress and stabilize the residual wastage of iron. It is commonly used for cutting tools, measuring tools, gauges, etc. made of high carbon steel.
  • ➂ Medium temperature tempering: the temperature is 350-500 ° C. After medium temperature tempering, high yield strength, the elastic limit of tensile parts, and high toughness can be obtained. It is mainly used for the treatment of various springs and hot working dies.
  • ➃ High-temperature tempering: the temperature is 400-650 ° C, which can turn the martensite loose iron into tempered martensite loose iron. It is often used for tempering mechanical structure steel with strong toughness.
  • ➄ Constant temperature tempering: increase the toughness and make the structure stronger and tougher, also known as worth tempering.
  • ➅ Hemp tempering: after hemp tempering, the tempered hemp field loose iron and toughened steel can be obtained.

3. Annealing: soften the steel and eliminate internal stress.

  • ➀ Process annealing: heat the steel to about 600 ° C-650 ° C below A1 or AC1, keep it for a period of time, and then slowly cool it in the air, which can soften the material and eliminate work hardening.
  • ➁ Constant temperature annealing: it can soften the material in a short time. It is commonly used in high carbon tool steel and alloy tool steel.
  • ➂ Complete annealing: heat carbon steel to 30 ° c-50 ° C above AC3 (hypereutectoid steel) or AC1 (eutectoid steel or hypereutectoid steel), keep it for a period of time, and then slowly cool it in the furnace to refine the crystalline structure and completely soften the material. It is commonly used for steel to be machined.
  • ➃ Spheroidizing annealing: spheroid the reticular Shiraz carbon iron, and improve the ductility and mechanical properties of the material.
  • ➄ Homogenization annealing: heat the steel to about 1000 ° c-1200 ° C, keep it for a long time, and then slowly cool it, which can eliminate segregation.
  • ➅ Relaxation annealing: it can eliminate the residual stress caused by forging, casting, machining, welding… And will reduce the hardness of steel, also known as stress relief annealing, which is a kind of low-temperature annealing.

4. Normalization: improve the material structure, refine the grains, and eliminate the processing stress.

  • (1) Heat the steel early to 30 ° c-50 ° C above A3 or ACM temperature, keep it for a period of time to make the structure become uniform worsted iron, and then place it in the air for cooling. Normalization can improve the material structure, refine the grains, and eliminate the processing stress.
  • (2) Constant temperature normalization: after the constant temperature normalization of steel, fine wave ferrostructure can be obtained.

5. Supercooling treatment: eliminate residual worsted iron, stabilize the rounding, and increase the hardness of the workpiece after quenching.
6. Quenching and tempering treatment: after the material is processed and manufactured, quenching + tempering treatment shall be carried out.
7. During the cooling process of heat treatment, if the cooling rate of carbon steel is faster, the temperature at which allergy occurs will be lower.

What is mild steel?

Mild steel is carbon steel with a carbon content of less than 0.25%. It is also called mild steel because of its low strength and hardness. It includes most ordinary carbon structural steels and some high-quality carbon structural steels. Most of them are used for engineering structural parts without heat treatment, and some are used for mechanical parts requiring wear resistance after carburization and other heat treatment.
The annealed microstructure of low carbon steel is ferrite and a small amount of pearlite, which has low strength and hardness and good plasticity and toughness. Therefore, its cold formability is good, and cold forming can be carried out by crimping, bending, stamping, and other methods. This steel also has good weldability. Low carbon steel generally refers to steel with a carbon content between 0.10 and 0.25% This kind of steel has low hardness and good plasticity, which is easy to adopt the cold plastic deformation forming process, welding, and cutting. It is often used to manufacture chains, rivets, bolts, shafts, etc.
Low carbon steel is generally rolled into angle steel, channel steel, I-beam, steel pipe, steel strip, or steel plate, which is used to make various building components, containers, boxes, furnace bodies, agricultural machinery, etc. High-quality low-carbon steel is rolled into thin plates, and deep drawing products such as automobile cabs and engine hoods are made; It is also rolled into bars for making mechanical parts with low strength requirements. Low carbon steel is generally not subject to heat treatment before use. Those with a carbon content of more than 0.15% are carburized or cyanidated for shafts, bushings, sprockets, and other parts requiring high surface temperature and good wear resistance.
The use of low-carbon steel is limited due to its low strength. Properly increasing the manganese content in carbon steel and adding trace amounts of vanadium, titanium, niobium and other alloy elements can greatly improve the strength of steel. If the carbon content in steel is reduced and a small amount of aluminum, boron, and carbide forming elements are added, the ultra-low carbon bainite group can be obtained, which has high strength and good plasticity and toughness.

What is medium carbon steel?

Medium carbon steel is carbon steel with a carbon content of 0.25% – 0.60%. It includes most high-quality carbon structural steels and some ordinary carbon structural steels. This kind of steel is mostly used to make various mechanical parts, and some are used to make engineering structural parts.

Medium carbon steel belongs to hypereutectoid steel, and its annealing structure is pearlite and ferrite. With the increase of carbon content in steel, the amount of pearlite in the structure increases, while the number of ferrite decreases. The quenching structure of steel with a carbon content greater than 0.40% is martensite; When the carbon content is greater than 0.40%, there is a small amount of retained austenite in addition to martensite, and the amount of retained austenite increases with the increase of carbon content in the steel.

Main uses of medium carbon steel

High-strength medium carbon quenched and tempered steel has certain plasticity, toughness, and strength, good machinability, good comprehensive mechanical properties after quenching and tempering, poor hardenability, easy to crack, low welding performance, good preheating before welding, and heat treatment after welding.
Medium carbon steel is mainly used to manufacture high-strength moving parts, such as air compressors, pump pistons, steam turbine impellers, heavy machinery shafts, worms, gear, etc., surface wear-resistant parts, and crankshaft, machine tool spindle, roller, fitter tools, etc.

Final heat treatment method of medium carbon steel

The final heat treatment methods of medium carbon steel include quenching and tempering, low-temperature tempering after quenching, low-temperature tempering after high-frequency quenching, isothermal quenching, and medium temperature tempering after quenching.

  • (1) Conditioning. The microstructure is tempered sorbate. This structure has good comprehensive mechanical properties, high strength, good plasticity, and toughness. The steel used for quenching and tempering should have good hardenability, to ensure the uniformity of microstructure and properties on the whole section of the quenching and tempering part. Compared with alloy steel, carbon steel has poor hardenability, so it is only suitable for quenching and tempering medium carbon steel parts with a small section size.
  • (2) Low-temperature tempering after quenching. The structure is tempered martensite, which has high strength and appropriate plasticity and toughness.
  • (3) Low-temperature tempering after high-frequency quenching. The microstructure of the high-frequency quenching layer is very fine cryptoneedle martensite. Tempered martensite is obtained after low-temperature tempering. After this treatment, the effect is similar to that of carburizing treatment. Quenching and tempering or normalizing are generally carried out before high-frequency quenching. Therefore, after high-frequency quenching and tempering, the core strength of the parts is high, and the plasticity and toughness are good, while the surface hardness is high and the wear resistance is good. In addition, the surface of high-frequency quenched parts produces compressive stress, which has high fatigue limit and long service life.
  • (4) Isothermal quenching. The structure is bainite, which has high strength and good plasticity and toughness.
  • (5) Medium temperature tempering after quenching. The microstructure is tempered sorbate.

The heat treatment process of medium carbon steel (45# steel, 40Cr steel)

Heat treatment of steel: it is a process that solid steel is heated, insulated, and cooled in an appropriate way to obtain the required structure and properties. Heat treatment can be used not only to strengthen steel and improve the service performance of mechanical parts but also to improve the process performance of steel. Their common point is that they only change the internal organizational structure, and do not change the surface shape and size.
The heat treatment process can significantly improve the mechanical properties of steel, increase the strength, toughness, and service life of parts, and improve the hardness and wear resistance. Therefore, important machine parts and tools should be heat treated. Heat treatment can also improve the processing performance of the workpiece, thereby improving productivity and processing quality. Therefore, heat treatment plays a very important role in the machinery manufacturing industry. Take 45# steel and 40Cr steel as examples.
High-temperature tempering after quenching is called “quenching and tempering” in production. The parts after quenching and tempering have good comprehensive mechanical properties and are widely used in various important structural parts, especially those connecting rods, bolts, gears, and shafts working under alternating loads. However, the surface hardness is low and not wear-resistant. Quenching and tempering + surface quenching can be used to improve the surface hardness of parts.
1. 45# steel – high-quality medium carbon structural steel
45# steel, which is called in GB, is called S45C in JIS, 1045080m46 in ASTM, and C45 in DIN; 45# steel is a high-quality carbon structural steel. Its chemical composition: carbon (c) content is 0.42-0.50%, Si content is 0.17-0.37%, Mn content is 0.50-0.80%, Cr content < =0.25%. Cold and hot processing performance is good, mechanical performance is good, and the price is low, a wide range of sources, so it is widely used. Its biggest weakness is that it is not suitable for workpieces with low hardenability, large section size, and high requirements.
The recommended temperature for heat treatment of 45 Steel: normalizing 850, quenching 840, tempering 600.

  • ① 45# steel is qualified if its hardness is greater than HRC55 (up to HRC62) after quenching and before tempering. The highest hardness in practical application is HRC55 (high-frequency quenching hrc58).
  • ② 45# steel does not adopt the heat treatment process of carburizing and quenching.

Quenching and tempering of 45 Steel: the quenching temperature of 45# steel is A3 + (30-50) ℃. In practice, the upper limit is generally taken. Higher quenching temperatures can accelerate the heating speed of the workpiece, reduce surface oxidation, and improve work efficiency. To homogenize the austenite of the workpiece, sufficient holding time is required. If the actual charging amount is large, it is necessary to extend the holding time appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, the defects of coarse grains and serious oxidation and decarburization will also appear, which will affect the quenching quality. We believe that if the charging amount is greater than the provisions of the process documents, the heating and insulation time needs to be extended by 1/5. Because 45# steel has low hardenability, 10% saline solution with a high cooling rate should be used. After the workpiece is immersed in water, it should be quenched thoroughly, but not cold thoroughly. If the workpiece is cooled thoroughly in salt water, it may crack the workpiece. This is because when the workpiece is cooled to about 180 ℃, austenite rapidly transforms into martensite, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature area, the method of slow cooling should be adopted. As it is difficult to master the outlet water temperature, it must be operated by experience. When the shaking of the workpiece in the water stops, the outlet water can be cooled by air (if it can be cooled by oil, it is better). In addition, the workpiece should move rather than static when entering the water and should move regularly according to the geometric shape of the workpiece. The static cooling medium and the static workpiece lead to uneven hardness and stress, resulting in large deformation and even cracking of the workpiece.
After quenching, the hardness of 45# steel quenched and tempered parts should reach hrc56-59, and the possibility of a large section is lower, but it should not be lower than hrc48. Otherwise, it means that the workpiece has not been completely quenched, and sorbate or even ferrite structure may appear in the structure. This structure is still retained in the substrate through tempering, which cannot achieve the purpose of quenching and tempering. 45# steel is tempered at high temperature after quenching. The heating temperature is usually 560-600 ℃, and the hardness requirement is hrc22-34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawing has hardness requirements, the tempering temperature should be adjusted according to the drawing requirements to ensure hardness. For example, some shaft parts require high strength and high hardness; However, some gears and shaft parts with keyways require lower hardness because they have to be milled and inserted after quenching and tempering. The tempering holding time depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be re-penetrated. Generally, the tempering holding time of the workpiece is always more than one hour.
If 45# steel is carburized, hard and brittle martensite will appear in the core after quenching, and the advantage of carburizing treatment will be lost. At present, the carbon content of materials using carburizing process is not high, and the core strength of 0.30% can reach very high, which is rare in the application. 0.35% have never seen an example, which is only introduced in the textbook. The process of quenching and tempering + high-frequency surface quenching can be adopted, and the wear resistance is slightly worse than carburization.
2. 40Cr steel – alloy structural steel
40Cr belongs to gb3077 “alloy structural steel”. The carbon content of 40Cr steel is 0.37% – 0.44%, which is slightly lower than that of 45# steel. The content of Si and Mn is equivalent, and the content of cr0.80% – 1.10%. In the case of hot rolling supply, this 1% Cr basically does not work, and their mechanical properties are roughly the same. Since the price of 40Cr is about half higher than that of 45# steel, those who can use 45# steel do not need 40Cr for economic reasons.
Quenching and tempering treatment of 40Cr Steel: the main role of Cr in heat treatment is to improve the hardenability of steel. Due to the improvement of hardenability, the strength, hardness, impact toughness, and other mechanical properties of 40Cr after quenching (or quenching and tempering) treatment are also significantly higher than those of 45# steel. However, due to the strong hardenability, the internal stress of 40Cr during quenching is also greater than that of 45# steel. Under the same conditions, the cracking inclination of 40Cr material is also greater than that of 45# steel. Therefore, to avoid workpiece cracking, 40Cr quenching mostly uses oil with a low thermal conductivity as the quenching medium (sometimes double liquid quenching method is also used, as the saying goes, water quenching oil cooling), while 45# steel uses water with a high thermal conductivity as the quenching medium. Of course, the choice of water and oil is not absolute, and it is also closely related to the shape of the workpiece. 40Cr parts with simple shapes can also be water quenched, while 45# steel parts with complex shapes may have to be oil quenched or even salt bath.
The quenching and tempering of the 40Cr workpiece are regulated by various parameter process cards. Our experience in actual operation is:

  • (1) 40Cr workpiece should be oil-cooled after quenching. 40Cr steel has good hardenability. Cooling in oil can harden, and the deformation and cracking tendency of the workpiece is small. However, in the case of tight oil supply, small enterprises can quench workpieces with uncomplicated shapes in the water, and no cracking is found, but the operator should strictly master the temperature of entering and exiting water based on experience.
  • (2) The hardness of the 40Cr workpiece is still high after quenching and tempering, and the second tempering temperature should be increased by 20-50 ℃, otherwise, it is difficult to reduce the hardness.
  • (3) After the 40Cr workpiece is tempered at high temperature, the complex shape is cooled in oil and simply in water, to avoid the influence of the second kind of tempering brittleness. The workpiece after tempering and rapid cooling shall be subject to stress relief treatment when necessary.

The maximum hardness of medium carbon steel after heat treatment is about HRC55 (hb538), and σ B is 600 – 1100mpa. Therefore, medium carbon steel is the most widely used in various applications with medium strength. In addition to being used as building materials, it is also widely used to manufacture various mechanical parts. As long as the temperature of medium carbon steel is enough and the holding time is enough, it is generally possible to reach this hardness value. If it does not deform, it is impossible. It is suggested that there should be machining allowance and then grinding on the grinding machine, and the second is surface quenching.

What is high carbon steel?

High carbon steel is often called tool steel, with carbon content ranging from 0.60% to 1.70%, which can be quenched and tempered. Hammers, crowbars, etc. are made of steel containing 0.75% carbon; Cutting tools such as drills, taps, reamers, etc. are made of steel with a carbon content of 0.90% to 1.00%.
After proper heat treatment or cold drawing and hardening, high carbon steel has high strength and hardness, high elastic limit and fatigue limit, and acceptable cutting performance, but poor welding performance and cold plastic deformation ability. Due to the high carbon content, cracks are easy to occur during water quenching, so double liquid quenching is often used, and oil quenching is often used for small section parts. This kind of steel is generally used after quenching through medium temperature tempering or normalizing or in a surface quenching state. It is mainly used to manufacture springs and wear-resistant parts. Carbon tool steel is a kind of high carbon steel without alloying elements. It is also a kind of tool steel with low cost, good cold and hot workability, and a wide application range. Its carbon content ranges from 0.65 to 1.35%, and it is steel specially used for making tools. High carbon steel density 7.81g/cm³. It can be used in the production of fishing gear.

Performance characteristics of high carbon steel

Advantage:

  • 1. High hardness (hrc60-65) and good wear resistance can be obtained after heat treatment.
  • 2. Under annealing conditions, the hardness is moderate and has good machinability.
  • 3. Raw materials are easily available and production costs are low.

Disadvantages:

  • 1. The thermal hardness is poor. When the working temperature of the tool is greater than 200 ℃, its hardness and wear resistance drop sharply.
  • 2. Low hardenability. The diameter of complete quenching in water quenching is generally only 15-18mm; During oil quenching, the maximum diameter or thickness of complete quenching is only about 6mm, and it is easy to deform and crack.

The hardness and strength of high carbon steel mainly depend on the amount of solid solution carbon in the steel and increase with the increase of the amount of solid solution carbon. When the amount of solid solution carbon exceeds 0.6%, the hardness will not increase after quenching, but the amount of excess carbide will increase, the wear resistance of steel will increase slightly, and the plasticity, toughness, and elasticity will decrease. Therefore, different steel grades are often selected according to the service conditions and the strength and toughness matching of the steel. For example, 65 steel with lower carbon content can be selected for manufacturing springs or spring-loaded parts with little stress. Generally, high carbon steel can be produced by an electric furnace, open hearth furnace, and oxygen converter. When high quality or special quality is required, electric furnace smelting plus vacuum self-consumption or electro slag remelting can be used. During smelting, strictly control the chemical composition, especially the content of sulfur and phosphorus. To reduce segregation and improve isotropic properties, ingots can be subjected to high-temperature diffusion annealing (especially important for tool steel). During hot processing, the stop forging (rolling) temperature of hyper eutectoid steel is required to be low (about 800 ℃), and the precipitation of coarse network carbide should be avoided after forging and rolling. Slow cooling should be paid attention to below 700 ℃ to prevent cracks caused by thermal stress. During heat treatment or hot working, the surface decarburization shall be prevented (especially for spring steel). During hot working, there should be a sufficient compression ratio to ensure the quality and service performance of steel.

Welding of high carbon steel

1. When the carbon content of high carbon steel is greater than 0.60%, the hardening and crack sensitivity after welding is greater, so the weldability is extremely poor and cannot be used to manufacture welded structures. It is often used to manufacture components and parts that need more hardness or wear resistance, and its welding work is mainly welding repair.
2. Since the tensile strength of high carbon steel is mostly above 675mpa, the commonly used welding rods are e7015 and e6015. E5016 and E5015 welding rods can be selected when the structural requirements of components are not high. In addition, chromium-nickel austenitic steel electrodes can also be used for welding.
3. Welding process

  • (1) To obtain high hardness and wear resistance of high carbon steel parts, the material itself needs to undergo heat treatment, so annealing should be carried out before welding.
  • (2) The weldment shall be preheated before welding, and the preheating temperature is generally above 250 – 350 ℃. During the welding process, the interpass temperature must be kept not lower than the preheating temperature.
  • (3) After welding, the weldment must be kept warm and cooled slowly, and immediately sent to the furnace for stress relief heat treatment at 650 ℃.

4. High carbon steel has a relatively high carbon content, so its weldability is relatively poor. Preheating is required during welding, slow cooling, or low-temperature tempering treatment of 350 degrees after welding. The specific length of heat treatment is determined by the thickness of the workpiece. If you can’t preheat, you have to use welding materials with good welding performance and crack resistance, but the welding speed must be reduced.
5. High carbon steel belongs to the category with poor welding performance. If welding is to be carried out, it should be welded under preheating conditions, and stress relief heat treatment must be carried out after welding.

Ultra-high carbon steel

Ultra-high carbon steel is an iron-based alloy material with a carbon content of 1.0-2.1%. The earliest industrial case of ultra-high carbon copper is Damascus. Its carbon content is 1.5%. In the mid-1970s, Stanford University first carried out superplasticity research on ultra-high carbon steel, and then the National Laboratory of the United States also carried out research on ultra-high carbon steel and obtained a series of patents. In addition, Japan and other countries have also carried out research on ultra-high carbon steel. In the 21st century, China has also carried out research on ultra-high carbon steel. Ultrafine-grained ultra-high carbon steel without network carbide was obtained by the appropriate preparation process. It not only has superplasticity with a high deformation rate at medium and high temperatures. And it has good comprehensive mechanical properties at room temperature. Ultrafine grained ultra-high carbon steel is expected to not only replace some medium and high carbon steel to make tools and dies, steel wires, and structural parts, to significantly improve its service life, but also make use of the good solid-state connection characteristics at medium and high temperatures, and can also be connected with itself or other metal-based materials to prepare new high-performance layered composites. It is a new type of material with great industrial application prospects.

Source: China Piping Solutions Provider – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(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, and 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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

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