A Comprehensive Guide 20Cr Steel

What is 20Cr steel?

20Cr is a low-hardenability carburized steel. Most of them are used to manufacture carburized parts with high core strength requirements, surface wear, cross-section below 30 mm or complex shape, and low load. It is widely used in real life. 20Cr is the Chinese GB standard, and the standard is GB/T 3077-2015.

Calculation formula for 20Cr steel

• 20Cr flat bar: weight per meter (kg)=0.00785 × thickness × Edge width
• 20Cr pipe: weight per meter (kg)=0.02466 × wall thickness × (Outer diameter wall thickness)
• 20Cr bar: diameter * diameter * length * 0.00623 = kg/meter (weight per meter)
• 20Cr plate: length * width * height * 0.00785=kg/meter (weight per meter)

Characteristics of 20Cr steel

Compared with 15Cr steel, 20Cr carburized steel has higher strength and hardenability. The critical hardenability diameter in oil is 4-22mm, and the critical hardenability diameter in water is 11-40mm, but the toughness is poor. This steel still tends to grain growth during carburizing, and direct quenching after cooling greatly influences impact toughness. Therefore, secondary quenching is required after carburizing to improve the toughness of the core of the part without tempering brittleness. The cold strain plasticity of the steel is high, and the wire can be drawn in the cold state; the machinability is good under high temperature normalizing or quenching and tempering but poor after annealing.

After quenching and low temperature tempering, 20Cr steel has good comprehensive mechanical properties: low temperature impact toughness and tempering brittleness are not obvious. During carburizing, the steel grains tend to grow, so secondary quenching is needed to improve the core’s toughness, which is unsuitable for cooling and quenching. When the hardness after normalizing is 170-217HB, the relative machinability is about 65 %. The 20Cr steel is pearlite and has good weldability. It generally does not need heat treatment after welding, but the parts with a thickness of more than 15mm need to be preheated to 100-150 °C before welding, and tempering heat treatment can be avoided after welding.

Advantages & Disadvantages of 20Cr Steel

Advantages	Disadvantages
High strength	Limited hardenability
Good toughness	Moderate corrosion resistance
Wear resistance	Lower weldability compared to carbon steels
Good machinability	Susceptible to temper brittleness
Cost-effective	Limited availability in some regions
Versatile application range

Equivalent Grades of 20Cr steel

20Cr steel equivalent to US ASTM AISI, European Germany DIN EN, British BS EN, France NF EN, Japanese JIS and ISO standard.

China		USA		Germany		Japanese		ISO
Standard	Grade	Standard	Grade	Standard	Grade	Standard	Grade	Standard	Grade
GB/T 3077	20Cr	AISI SAE 5120;  ASTM A29/A29M	5120	DIN 1652-3	20Cr4	JIS G4503	SCr420	ISO 683-17	20Cr3

Chemical Composition of 20Cr Steel

Steel grade	Specification number	National standard code	(C)%	(Si)%	(Mn)%	(P)%	(S)%	(Cr)%	(Ni)%	(Cu)%
SCr420	3231	CNS	0.18-0.23	0.15-0.35	0.6-0.85	0≦0.03	0≦0.03	0.9-1.2	0≦0.25	0≦0.3
20Cr	3077	GB	0.18-0.24	0.17-0.37	0.4-0.7	0≦0.035	0≦0.035	0.7-1	0≦0.3	0≦0.3
SCr420	G4104	JIS	0.18-0.23	0.15-0.35	0.6-0.85	0≦0.03	0≦0.03	0.9-1.2	0≦0.25	0≦0.3
5120	–	AISI	0.17-0.22	0.15-0.3	0.7-0.9	0≦0.035	0≦0.04
SCr420H	11999	CNS	0.17-0.23	0.15-0.35	0.6-1	0≦0.03	0≦0.03	0.85-1.2	0≦0.25	0≦0.3
SCr420H	G4052	JIS	0.17-0.23	0.15-0.35	0.6-1	0≦0.03	0≦0.03	0.85-1.2	0≦0.25	0≦0.3
5120H	A304	ASTM	0.17-0.23	0.15-0.35	0.6-1	0≦0.035	0≦0.04	0.6-1
5120H	–	AISI	0.17-0.23	0.15-0.35	0.6-1	0≦0.035	0≦0.04	0.6-1
20Cr4	17210	DIN	0.17-0.24	0≦0.4	0.5-1.37	0≦0.035	0≦0.035			0≦0.25
20MnCr5	17210	DIN	0.17-0.22	0≦0.4	1-1.4	0≦0.035	0≦0.035	1-1.3

Mechanical Properties of 20Cr steel

• Tensile strength: ≥835 N/mm²
• Yield strength: ≥540 N/mm²
• Elongation after break: ≥10%
• Rate of reduction in area: ≥40%
• Impact absorbing energy: ≥47 J
• Material Brinell hardness: ≤179 (annealing or high temperature tempering state)
• Test sample size: 15mm

Physical Properties of 20Cr steel

Physical Properties	Metric	Imperial
Density	7850 kg/m3	490 lb/ft3

Thermal Properties of 20Cr steel

Thermal Properties	Metric	Imperial
Thermal Conductivity	47 W/(m·K)	27 BTU/(h·ft·°F)
Coefficient of Thermal Expansion	1.2×10-5 °C-1	6.7×10-6 °F-1
Specific Heat Capacity	0.470 J/(g·°C)	0.112 Btu/(lb·°F)

Electrical Properties of 20Cr Steel

Electrical Properties	Metric	Imperial
Electrical Conductivity	4.18×106 S/m	1.27×106 S/ft
Electrical Resistivity	2.4×10-7 Ω·m	7.9×10-7 Ω·ft

Heat treatment of SAE/AISI 5120

SAE/AISI 5120 steel can be easily heat treated through normalizing or annealing processes depending on the desired results. Normalizing involves heating the material until it reaches approximately 860°C (1580°F), then allowing the material to cool slowly at room temperature until it reaches ambient temperature, whereas annealing involves heating the material until it reaches approximately 790°C (1450°F). Both processes help improve the material’s ductility while still retaining its strength properties.
Heat treatment specification

• First quenching heating temperature: 880 °C (Cooling: oil, water)
• Second quenching heating temperature: 780-820 °C (Cooling: oil, water)
• Tempering heating temperature: 200 °C (Cooling: water, air)

Heat treatment system for 20Cr

Project	Temperature (℃)	Cooling	Hardness HBS
Annealing	860-890	Inside the furnace	≤179
Normalizing	870-900	Air	≤270
High Temperature Tempering	700-720	Air	≤179
Quenching	860-880	Oil or water
Tempering	450-480	Oil or air	≤250
Nitriding	890-910	Direct oil quenching
One-Time Quenching	860-890	Oil or water
Secondary Quenching	780-820	Oil or water
Tempering	170-190	Oil or air	Surface HRC 56-62
Carburization	890-910	Air
Quenching	Induction heating	As needed
Tempering	150-170	Air	Surface HRC 58-65

Corrosion resistance of 20Cr steel 

20Cr steel has excellent corrosion resistance and is very suitable for use in humid environments or areas with high humidity. It is resistant to acidic and alkaline solutions and is an ideal choice for industrial applications that require exposure to corrosive elements. The addition of chromium makes this type of steel more resistant to corrosion, so it can withstand long-term use without damage or weakening.

Heat resistance of 20Cr steel 

20Cr has excellent heat resistance and is ideal for high temperature applications. It can withstand temperatures up to 650 °C (1200 °F) without losing strength or becoming brittle. This makes it an excellent choice for hot working processes such as forging or welding applications.

Machining of 20Cr steel

20Cr has excellent machinability and can work easily with standard machine tools such as lathes and milling machines. Due to its low carbon content, no special cutting tools or techniques are required during the operation, and no additional cooling is required, which helps to reduce warping when processing large parts with this material.

Finish turning 20Cr material products choose what alloy blade?

When turning 20Cr material, if the hardness is high after carburizing and quenching, it is recommended to use the CBN tool. If processed before quenching, an ordinary high-speed steel turning tool and cemented carbide blade can be selected for processing.
20Cr is a steel grade, low hardenability carburized steel. Most of them manufacture carburized parts with high core strength requirements, surface wear, cross-section below 30 mm or complex shape, and low load. 20Cr steel parts have excellent processing performance after quenching and carburizing, but it is also necessary to solve the problem of high hardness and difficult processing after quenching. 
20Cr material selection tool should be selected according to its specific use. Before quenching, ordinary blades can be selected. After quenching, boron nitride turning blades can be selected for processing. Boron nitride cutting tools are designed for high hardness and difficult-to-machine materials.

Weldability of 20Cr steel 

Welding 20Cr requires preheating before welding because of the high melting point of this metal material (about 1360 °C). Preheating helps to prevent cracks in the welding process by reducing the thermal stress of the welded joint itself. However, welding should always use a low current setting to avoid overheating of the metal, which may lead to porosity problems inside the weld.

Uses of of 20Cr Steel

20Cr steel is mostly used to manufacture carburizing parts (oil quenching) with high core strength requirements, surface wear, cross-section below 30 mm or complex shape, and low load, such as machine tool gearbox gears, gear shafts, cams, worms, piston pins, claw clutches, etc.; for parts with small heat treatment deformation and high wear resistance, high-frequency surface quenching should be carried out after carburizing, such as gears, shafts, and spline shafts with modulus less than 3. This steel can also be used in the quenched and tempered state to manufacture parts with high working speed and medium impact load. This steel can also be used as a low-carbon martensitic quenching steel to increase further yield and tensile strength (about 1.5-1.7 times). 
20Cr steel combines strength, toughness, and wear resistance and can be used in various industries. The following are some common applications of 20Cr steel:

• Gears and gear components: The high strength, wear resistance, and good toughness of 20Cr steel make it suitable for manufacturing gears, gear shafts, and other gear components.
• Axles and shafts: 20Cr steel is used to produce axles and shafts for automotive, mechanical, and equipment applications due to its excellent strength and toughness.
• Bolts, nuts, and fasteners: 20Cr steel has good machinability and high tensile strength, suitable for manufacturing various bolts, nuts, and fasteners.
• Spring: The mechanical properties of 20Cr steel, including good toughness and moderate corrosion resistance, make it suitable for manufacturing springs.
• Structural components: In some cases, 20Cr steel is used to produce structural components that require strength and toughness.
• Machining and tools: 20Cr steel can manufacture various machining tools due to its good machinability.

What is the difference between 20Cr and 40Cr steel?

The difference between 20Cr and 40Cr steel is as follows:
(1) The composition of the composition content is different.
20Cr, 40Cr is alloy steel, the number in front of the sign that the average carbon content of a few ten thousandths, followed by the number of alloying element content of a few percent. 20Cr, and 40Cr composition content is different.
20Cr:
Carbon C: 0.18-0.24; silicon Si: 0.17-0.37; manganese Mn: 0.50-0.80; chromium Cr: 0.70-1.00. (Mass fraction) (%)
40Cr:
Carbon C: 0.37-0.44; silicon Si: 0.17-0.3; manganese Mn: 0.50-0.80; chromium Cr: 0.80-1.10. (Mass fraction) (%)
(2) Different uses.
1). 20Cr steel: this steel is mostly used in the manufacture of high strength requirements of the heart, the surface to withstand wear and tear, the cross-section of the following 30mm or the shape of the complex and the load is not too carburized parts (oil quenching), 20Cr for carburized parts, its carburization, hardenability, and heart strength are better, commonly used in the manufacture of gears, clutches, spindles, worm gears and so on.
For heat treatment deformation of small and high wear-resistant parts, carburizing should be high-frequency surface quenching, such as modulus less than 3 gears, shafts, spline shafts, etc.
This steel can also be used in the tempered state for the manufacture of large working speed and bear moderate impact load parts; this steel can also be used as a low carbon martensitic quenching steel, further increasing the yield strength and tensile strength of steel (about 1.5-1.7 times).
2). 40Cr steel: 40cr steel for high hardness, good wear resistance, and deformation of small parts, commonly used to manufacture spindles, worm gears, gears, valves, and oil pump rotors.
This steel is manufactured after tempering to withstand medium loads and medium speed mechanical parts, such as automotive steering knuckles, rear half shafts, and machine tools on the gears, shafts, worm gears, spline shafts, top sets, etc.
After quenching and medium temperature tempering to manufacture high load, impact, and medium speed parts, such as gears, spindles, oil pump rotor, slider, rings, etc.
After quenching and low-temperature tempering for the manufacture of heavy loads, low-impact and wear-resistant, a cross-section of the entity thickness of 25mm or fewer parts, such as worm, spindle, shaft, rings, etc.
After tempering and high-frequency surface quenching for the manufacture of high surface hardness and wear resistance without great impact, parts, such as gears, sleeves, shafts, spindles, crankshafts, mandrels, pins, connecting rods, screws, nuts, intake valves and so on.
In addition, this steel is suitable for manufacturing carbonitriding treatment of various transmission parts, such as large diameter and low-temperature toughness of gears and shafts.
(3) Different grades of steel.

• 1). 20Cr is a steel grade for low hardenability carburizing steel.
• 2). 40Cr is China’s GB standard steel, 40Cr steel is one of the most widely used steel in the machinery manufacturing industry. Tempering treatment has good mechanical properties, low temperature impact toughness, and low notch sensitivity.

(4) Different characteristics.

• 1). 20Cr steel: compared with 15Cr steel, it has higher strength and hardenability, in the oil-critical quenching diameter of up to 4 -22mm, in the water-critical quenching diameter of up to 11-40mm, but the toughness of the poor; this steel carburization still tends to grain growth, direct quenching at reduced temperatures on the impact toughness of the impact of a greater impact, so carburization needs to be secondary quenching to increase the toughness of the parts of the heart, no tempering brittleness.
• 2). 40Cr steel: medium carbon tempered steel, cold heading die steel. The steel is moderately priced and easy to process, and after appropriate heat treatment, it can obtain a certain toughness, plasticity, and wear resistance. Normalizing can promote organization refinement close to the equilibrium state, improving the blank’s cutting performance.

Experimental study on the effect of heat treatment process on the surface roughness of 20Cr steel

Different surface roughness of 20Cr steel can be obtained by normalizing, annealing, quenching and tempering, quenching + medium temperature tempering, normalizing + high temperature tempering after rough machining, semi-finishing and finishing. The effect of heat treatment process on the surface roughness of 20Cr steel was studied. The test method was introduced and the test data were given. It is confirmed by experiments that the surface roughness of 20Cr steel corresponding to the annealing process during rough machining is the smallest, and the surface roughness of 20Cr steel corresponding to the normalizing process is the largest. The surface roughness of 20Cr steel corresponding to the quenching + medium temperature tempering process during finishing and semi-finishing is the smallest, and the surface roughness of 20Cr steel corresponding to the annealing process is the largest.

1. Experimental research background

In the process of large-scale mechanized and automated production, it is necessary to ensure that the surface roughness of the parts tends to be consistent or stable within the set range. The author solves the problem from the aspect of heat treatment process, consults the relevant literature, and confirms that the surface roughness of the part is related to the internal structure of the part, and the internal structure of the part is related to the heat treatment process of the material used. In this regard, the commonly used 20Cr steel is taken as the research object. Under the premise of unchanged machining conditions, the relationship between the heat treatment process of 20Cr steel and the surface roughness of the parts is studied. By improving the heat treatment process of 20Cr steel, the goal of controlling and stabilizing the surface roughness of the parts is achieved, which provides technical support and process guarantee for related production enterprises.

2. Test method

20Cr steel is a kind of steel commonly used in enterprises, which belongs to low carbon and low alloy carburizing steel. It is commonly used in the manufacture of gears, gear shafts, spline shafts, pin shafts, plastic molds, drilling molds, special measuring tools, connecting rods, screws, high strength bolts and other parts.
The 20 Cr steel used in the experiment was taken from the 30 mm × 4000 mm bar produced by Angang. Its chemical composition includes 0.19 %-0.21 % C, 0.87 %-0.91 % Cr, 0.65 %-0.70 % Mn, 0.18 %-0.21 % Si, 0.026 %-0.029 % P, 0.022 %-0.027 % S, 0.021 %-0.025 % Ni, 0.018 %-0.020 % Cu. The original bar was processed into a short round bar with a specification of 30mm × 150mm, which was divided into five groups of test blanks. The five groups of billets were subjected to five heat treatment processes : normalizing, annealing, quenching and tempering, quenching + medium temperature tempering, normalizing + high temperature tempering. A test blank was taken out from each group, and a number of thin discs with specifications of 30mm × 8mm were processed. The thin wafer is then cut by a line and processed into a cylindrical metallographic sample with a specification of 10 mm × 8 mm. The remaining test blanks were left as machined samples, which were processed by rough machining, semi-finishing and finishing respectively.
The SX-2.5-10 box-type resistance furnace was used for heat treatment. The metallographic structure was observed and photographed by 53 X positive metallographic microscope. The surface roughness was measured by TR200 needle-type roughness meter. The Brinell hardness was measured by HB-3000 Brinell hardness tester, and the Rockwell hardness was measured by HR-150 Rockwell hardness tester.
CA6140 lathe is used for cutting. In rough turning, the feed rate is 2mm, the feed rate is 0.483mm/r, the spindle speed is 650r/min, and the turning length is 50mm. In the semi-precision turning, the feed is 1mm, the feed is 0.241mm/r, the spindle speed is 700r/min, and the turning length is 50mm. In the fine turning, the feed rate is 0.5mm, the feed rate is 0.157mm/r, the spindle speed is 870r/min, and the turning length is 50mm.

3. Test situation

3.1 Normal fire
The surface roughness, hardness and microstructure of 20Cr steel after normalizing treatment are shown in Table 1.
3.2 Annealing
The surface roughness, hardness and microstructure of 20Cr steel after annealing treatment are shown in Table 2.
3.3 Tempering
The surface roughness, hardness and microstructure of 20Cr steel after quenching and tempering are shown in Table 3.
3.4 Quenching + medium temperature tempering
The surface roughness, hardness and microstructure of 20Cr steel after quenching and medium temperature tempering are shown in Table 4.
3.5 Normalizing + high temperature tempering
The surface roughness, hardness and microstructure of 20Cr steel after normalizing and high temperature tempering are shown in Table 5.
Tab.1 Situation of 20Cr steel after normalizing treatment

Heat treatment process	Turning method	Surface roughness			Brinell hardness (HBS)	Microstructure
		Ra/μm	Rz/μm	Rsm/μm
Heating at 920 °C, insulation for 40 minutes, air cooling after discharge	Precision turning	2.307	14.225	0.1605	205	Fine ferrite+elongated pearlite
	Semi precision lathe	3.966	16.036	0.2666
	Rough cart	12.33	48.33	0.4750

Table.2 Situation of 20Cr steel after annealing treatment

Heat treatment process	Turning method	Surface roughness			Brinell hardness (HBS)	Microstructure
		Ra/μm	Rz/μm	Rsm/μm
Heating at 920 °C, insulation for 40 minutes, air cooling after discharge	Precision turning	5.087	25.59	0.1962	165	Fine ferrite+elongated pearlite
	Semi precision lathe	6.086	30.957	0.2333
	Rough cart	8.009	40.818	0.447

Table.3 20Cr steel after quenching and tempering treatment

Heat treatment process	Turning method	Surface roughness			Brinell hardness (HRC)	Microstructure
		Ra/μm	Rz/μm	Rsm/μm
Heat at 920 °C first, hold for 40 minutes, and cool with water after discharge; Heating at 580 °C, holding for 40 minutes, and air cooling after discharge	Precision turning	1.398	7. 770	0.0885	33.5	Tempered sorbite
	Semi precision lathe	3.713	16.518	0.2505
	Rough cart	10. 246	39.243	0.5

Table.4 20Cr steel after quenching + medium temperature tempering treatment

Heat treatment process	Turning method	Surface roughness			Brinell hardness (HRC)	Microstructure
		Ra/μm	Rz/μm	Rsm/μm
Heat at 920 °C first, hold for 40 minutes, and then cool with water after discharge; Heating at 400 °C, insulation for 50min, and air cooling after discharge	Precision turning	1.066	6.812	0.1484	45.5	Tempered troostite
	Semi precision lathe	3.491	13.408	0.2488
	Rough cart	11.493	42.88	0.5

Table.5 20Cr steel after normalizing + high temperature tempering treatment

Heat treatment process	Turning method	Surface roughness			Brinell hardness (HBS)	Microstructure
		Ra/μm	Rz/μm	Rsm/μm
First, heat at 920 °C, hold for 40 minutes, and cool with water after discharge; Heating at 580 °C, insulation for 40min, and air cooling after discharge	Precision turning	2.932	15.24	0.144	185	Fine ferrite+coarse short film pearlite
	Semi precision lathe	3.9875	19.253	0.1977
	Rough cart	10.067	42.168	0.479

4. Experimental analysis

The surface roughness obtained by the above five heat treatment processes is listed in Table 6. From Table 6, it can be seen that the surface roughness value measured by annealing treatment during rough turning is the smallest, the surface roughness value measured by normalizing treatment is the largest, and the surface roughness value measured by other process treatments is between the two. When finishing and semi-finishing, the surface roughness value measured by quenching + medium temperature tempering treatment is the smallest, the surface roughness value measured by annealing treatment is the largest, and the surface roughness value measured by other process treatments is between the two.
When rough turning, the hardness and microstructure of the material have a significant effect on the surface roughness of the material : the higher the hardness of the material, the greater the surface roughness value, the lower the hardness of the material, the smaller the surface roughness value ; the more the lamellar structure of the material, the greater the surface roughness value, the less the lamellar structure of the material, and the smaller the surface roughness value. The influence of the hardness and microstructure of the material on the surface roughness of the material is different from that of the rough car. The higher the hardness of the material, the smaller the surface roughness ; the lower the hardness of the material, the greater the surface roughness. The surface roughness value of the fine lamellar structure material is greater than the surface roughness value of the point or granular structure material, and is lower than the surface roughness value of the coarse lamellar structure material.
Table.6 Surface roughness results of different heat treatment processes Unit:μm

Heat treatment process	Precision turning			Semi precision lathe			Rough cart
	Ra	Rz	Rsm	Ra	Rz	Rsm	Ra	Rz	Rsm
Normalizing	2.307	14.225	0.1605	3.966	16.036	0.2666	12.33	48.33	0.4750
Annealing	5.087	25.59	0.1962	6.086	30.957	0.2333	8.009	40.818	0.4470
Tempering	1.398	7.77	0.0885	3.713	16.518	0.2505	10.246	39.243	0.5000
Quenching+medium temperature tempering	1.066	6.812	0.1484	3.491	13.408	0.2488	11.493	42.88	0.5000
Normalizing+high-temperature tempering	2.932	15.24	0.144	3.9875	19.253	0.1977	10.067	42.168	0.4790

The surface roughness value of semi-finishing is greater than that of finishing, and less than that of rough turning. In rough turning, the surface roughness of the material depends on the wear degree of the material to the blade, and the hardness and microstructure of the material have a significant effect on the wear degree of the blade. The higher the hardness of the material, the greater the wear of the blade and the greater the surface roughness. The lower the hardness of the material, the smaller the wear of the blade and the smaller the surface roughness. Therefore, the surface roughness value measured by annealing treatment during rough turning is the smallest. In the case of the same material structure, the more the lamellar structure, the more serious the influence on the wear degree of the blade, and the greater the surface roughness value. In the case of different material structures, the effect of lamellar structure on the degree of blade wear is greater than that of point or granular structure. Therefore, the surface roughness value measured by the normalizing treatment is the largest, and the surface roughness value measured by the other process treatments is between the annealing and normalizing.
When finishing and semi-finishing, the surface roughness of the material depends on the chip breaking ability of the material, and the chip breaking ability of the material depends on the hardness of the material itself. The higher the hardness of the material, the greater the brittleness of the material, the stronger the chip breaking ability, and the smaller the surface roughness value. Therefore, when finishing and semi-finishing, the surface roughness value measured by quenching + medium temperature tempering treatment is the smallest, the surface roughness value measured by annealing treatment is the largest, and the surface roughness value measured by other process treatments is between the two.
The cutting amount during semi-finishing is much smaller than that during rough turning, but higher than that during finishing. The influence of semi-finishing on the wear degree of the blade is less than that of rough turning, but greater than that of fine turning. Therefore, the surface roughness value of semi-finishing is greater than that of finishing, and less than that of rough turning.

5.Conclusion

Through experimental research, it can be seen that for 20Cr steel, the surface roughness value measured by annealing treatment during rough turning is the smallest, the surface roughness value measured by normalizing treatment is the largest, and the surface roughness value measured by other process treatments is between the two. When finishing and semi-finishing, the surface roughness value measured by quenching + medium temperature tempering treatment is the smallest, the surface roughness value measured by annealing treatment is the largest, and the surface roughness value measured by other process treatments is between the two.
Author: Hao Shaoxiang