20CrMo Steel Hydraulic Cylinder: Insufficient Carburizing and Quenching Hardness Analysis

1. Preface

This article introduces the effect of the surface carbon concentration of 20CrMo steel on its heat treatment properties.

The carbon concentration in the carburizing process must be strictly controlled to ensure that the carbon content in the furnace atmosphere during the whole carburizing process is within the set range and that the surface carbon concentration of the workpiece after carburizing process meets the requirements;

At the same time, the quenching process is properly optimized to finally ensure that the workpiece gets better quenching hardness, thus obtaining good wear resistance.

20CrMo is a low alloy carbon structural steel, which is widely used in the production and application of various types of workpieces, such as gears, shafts and high-strength fasteners.

In the production application of a company, this material is used for the hydraulic cylinder of the hydraulic crusher on the construction machinery.

It is required to have high hardness and high wear resistance after carburizing heat treatment, and good plasticity and toughness, that is, good comprehensive mechanical properties.

During the initial production of the hydraulic cylinder, after the normal process flow, it was found that the surface hardness of the workpiece was generally low.

Even under the control of different process conditions, the problem of the low surface hardness of the workpiece was still not solved.

Therefore, we conducted targeted research on the workpiece.

2. Heat treatment process, technical requirements and problems of hydraulic cylinder

This type of hydraulic cylinder has a large volume, the unit weight of the product is about 365kg, and the effective thickness is 150-200mm.

The actual workpiece is shown in Fig. 1.

20CrMo Steel Hydraulic Cylinder: Insufficient Carburizing and Quenching Hardness Analysis 1
Fig. 1 Physical object of a hydraulic cylinder

After heat treatment, carburizing and quenching, the carburizing layer depth of the workpiece is required to be 1.0-1.4mm, and the overall hardness of the workpiece is controlled at 58-62HRC.

See Table 1 for the chemical composition requirements of 20CrMo steel in GB / T 3077-1999 alloy structural steel.

Table 1 chemical composition of 20CrMo steel (mass fraction) (%)

CMnSiCrMoPS
0.17~0.240.4~0.70.17~0.370.80~1.100.15~0.25≤0.035≤0.035

In actual production, a variety of process methods are used for treatment.

The results show that the surface hardness is less than 50HRC, which is unqualified, and the technical requirements cannot be met by adjusting the carburizing temperature and increasing the quenching temperature.

See Table 2 for the specific heat treatment process.

Table 2 heat treatment process

NO.Process parametersWorkpiece surface hardness (HRC)
1Strong permeability: 920 ℃ × 330min, carbon potential 1.1%;
Diffusion: 920 ℃ x130min, carbon potential 0.85%;
Quenching insulation: 830 ℃ × 30min, carbon potential 0.85%.
45~47
2Strong permeability: 920 ℃ × 350min, carbon potential 1.1%;
Diffusion: 920 ℃ × 140min, carbon potential 0.9%;
Quenching and heat preservation: 840 ℃ x30min, carbon potential 0.9%.
46~47
3Strong permeability: 930 ℃ × 330min, carbon potential 1.2%;
Diffusion: 930 ℃ x 30min, carbon potential 0.9%;
Quenching insulation: 860 ℃ × 40 min, carbon potential 0.9%.
49~50
4Strong penetration: 930 ℃ x450min, carbon potential 1.2%;
Diffusion: 930 ℃ × 250min, carbon potential 0.9%;
Quenching and heat preservation: 860 ℃ x30min, carbon potential 0.9%.
46~48

3. Cause analysis of low surface hardness of hydraulic cylinder

(1) Carburizing temperature

Carburizing temperature is an important technological parameter in carburizing process, and also a factor affecting the ability of austenite to dissolve carbon.

As the temperature rises, the solubility of carbon in austenite increases.

According to the iron-carbon phase diagram, the saturated solubility of carbon in austenite is 1.0% at 850 ℃ and 1.25% at 930 ℃.

The accuracy of carburizing temperature directly affects the quenching quality of the workpiece.

Through the 9-point temperature detection of the equipment, there is no deviation in the temperature, the furnace temperature is normal, and no obvious temperature difference is found.

Therefore, the influence of temperature on the surface hardness of the workpiece is excluded.

(2) Effect of carbon concentration

During the process execution, the furnace test block (25mm×25mm) is used for each process number.

The hardness test results of the test block are better than that of the workpiece body.

See Table 3 for the hardness test results of the carburized test block executed according to process 3, the end face and longitudinal direction of the workpiece.

Table 3 workpiece hardness test results (HRC)

Surface

Core

End face

59

60

58.5

59.6

20

21

Portrait

56.6

57.5

55.2

56

According to the hardness method specified in GB / T 9450-2005 determination and verification of effective hardened layer depth of carburizing and quenching of iron and steel, the hardness gradient of carburizing layer is tested on the furnace test block after heat treatment.

The results are shown in Table 4.

Table 4 hardness gradient test results of workpiece penetration layer

Carburizing layer depth / mmHardness HV1
0.1622.9
0.2747.7
0.3714.4
0.4720
0.5685.8
0.6662.7
0.7635.9
0.8635.9
0.9599.9
1568.8
1.1540

According to the metallographic analysis method, the carburized layer of the test block is observed to determine whether the carbon concentration in the carburized layer meets the specified requirements.

The effective hardened layer depth of the workpiece and the metallographic structure of the surface layer are as shown in Fig. 2.

20CrMo Steel Hydraulic Cylinder: Insufficient Carburizing and Quenching Hardness Analysis 2
Fig. 2 Metallographic Structure of Permeation Layer of Test Block

According to the observation of the metallographic structure of the carburized layer of the test block in Fig. 2, the surface layer is basically needle-like martensite + residual austenite, and no obvious carbide composition is found.

Meanwhile, through the effective hardened layer depth detection, the test block has an obvious “head up” phenomenon after carburizing treatment, which indicates that there is a relatively obvious oxidation atmosphere in the carburized layer, resulting in a low surface hardness and an increase in the step hardness.

In order to better observe the microstructure of the infiltrated layer of the workpiece test block, the test block is annealed.

The annealing process is 860 ℃ × 30min, cool down to 500 ℃ with the furnace and take out air cooling.

Prepare metallographic samples and observe the equilibrium metallographic structure of 20CrMo steel carburized parts, as shown in Fig. 3.

20CrMo Steel Hydraulic Cylinder: Insufficient Carburizing and Quenching Hardness Analysis 3
Fig. 3 Metallographic Structure of Test Block after Annealing

According to the observation of the equilibrium metallographic structure in Fig. 3, the microstructure morphology is quite different from that of the normal low-carbon steel carburizing layer after slow cooling, and the hypereutectoid layer, eutectoid layer and transition layer in the carburizing layer can not be clearly and effectively distinguished.

The microstructure of low carbon steel after carburizing and slow cooling shall be: the surface layer is pearlite + net cementite, the inside is eutectoid structure, the transition zone of subeutectoid structure and the original structure.

According to the observation of the equilibrium structure in Fig. 3, its morphology and structure are more similar to the equilibrium structure obtained after annealing of ordinary medium carbon steel, which is a uniformly distributed pearlite + ferrite structure, and no obvious cementite is found, indicating that the carbon potential of the carburizing atmosphere in the furnace is insufficient and sufficient carbon concentration can not be ensured on the workpiece surface.

Therefore, when the carburizing temperature is normal, it is necessary to increase the carbon potential to obtain sufficient carbon concentration on the workpiece surface and form an effective carbon concentration gradient.

4. Improvement of process methods and conditions

The diffusion of carbon atoms from the surface to the center is necessary for carburizing and obtaining a certain depth of the carburized layer.

The driving force of diffusion is the carbon concentration gradient between the surface and the core.

In order to achieve a better-carburizing effect, it is required that the activated carbon atoms be absorbed in time to ensure the uniform circulation of the atmosphere in the furnace, and the carbon atoms (decomposition rate) provided should be matched with the absorption rate to prevent carbon deposition and insufficient supply.

Through the analysis of the original process links and test blocks, it is considered that the low hardness of the actual workpiece is mainly due to the low carbon concentration on the surface of the carburizing layer caused by the insufficient atmosphere in the furnace, which can not achieve effective carburizing treatment, and can not obtain the ideal carburizing layer structure, and can not achieve sufficient hardness.

In this regard, targeted rectification measures were taken to overhaul the equipment as a whole, replace the carbon potential monitoring equipment, verify the tightness of the furnace body, and conduct carbon determination treatment on the furnace atmosphere again to ensure the uniformity and accuracy of the furnace atmosphere.

After re-evaluating the furnace conditions and re-setting the carburizing and quenching process parameters, production can be carried out.

See Table 5 for the adjusted heat treatment process.

Table 5 adjusted heat treatment process

NO.Process parametersWorkpiece surface hardness (HRC)
1Strong penetration: 930 ℃ x450min, carbon potential 1.3%;
Diffusion: 930 ℃ x 30min, carbon potential 1.0%;
Quenching insulation: 850 ℃ × 30min, carbon potential 1.0%;
Tempering: 150 ℃ x240min
62.6, 623, 62.1, 62.4, 62.9, 62.8
2Strong permeability: 920 ℃ × 450 min, carbon potential 1.3%; 
Diffusion: 920 ℃ x30min, carbon potential 1.0%;
Quenching and heat preservation: 840 ℃ x30min, carbon potential 1.0%;
Tempering: 180 ℃ x240min
59.4, 613, 60.1, 59.4, 60.9, 60.1

The metallographic structure of the infiltrated layer of the test block treated by the adjusted heat treatment process is shown in Fig. 4.

20CrMo Steel Hydraulic Cylinder: Insufficient Carburizing and Quenching Hardness Analysis 4
Fig. 4 Metallographic Structure of Permeation Layer of Test Block

It can be seen from Fig. 4 that since the metallographic structure is mainly composed of fine tempered martensite, fine-grained carbide and a small amount of residual austenite, which is consistent with the normal carburizing and quenching structure, the effective surface hardness is ensured and the overall surface hardness of the workpiece is within the range required by the technical conditions.

In order to better understand the microstructure changes before and after the adjustment of the specific heat treatment process, the test block is annealed with the same process.

Annealing process: 860 ℃ × 30min, cool down to 500 ℃ with the furnace and take out air cooling.

Metallographic samples were prepared to observe the equilibrium structure of 20CrMo steel carburized parts.

Fig. 5 shows the annealed structure after the adjustment process heat treatment.

20CrMo Steel Hydraulic Cylinder: Insufficient Carburizing and Quenching Hardness Analysis 5
Fig. 5 Metallographic Structure of Test Block after Annealing

Pearlite and reticulated cementite can be clearly seen from Fig. 5, and the difference between them can be clearly seen by comparing the surface layer structure in Fig. 3.

The structure in Fig. 3 is closer to the equilibrium structure of ordinary medium carbon steel after annealing, that is, its carbon content (mass fraction) is about 0.5%;

The equilibrium structure in Fig. 5 is the pearlite + network cementite structure after normal carburizing annealing.

This change in structure fully indicates that there is a big problem in the furnace atmosphere under the original process conditions, resulting in the carburizing conditions of the workpieces failing to meet the set requirements, and the workpieces failing to meet the specified technical requirements after the process treatment.

5. Conclusion

1) The slow-cooling solid phase transformation structure of low alloy carbon structural steel after carburizing and quenching can be used to determine the carbon content in the final carburized layer, so as to determine whether the furnace atmosphere meets the set requirements.

2) Although increasing the carbon potential can enhance the carburizing effect to a certain extent, due to the limited saturated solubility of carbon in austenite, it is necessary to flexibly set the carbon potential according to the actual situation to avoid possible carbon deposition.

3) The precision of carburizing treatment equipment will directly affect the final heat treatment effect.

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