Research progress of carburizing and carbonitriding surface heat treatment technology

In recent years, Japan has carried out in-depth research on the surface heat treatment technology of carburizing and carbonitriding. Its research progress is described below.

 

1. Carburizing

 

1) The mechanism of gas carburization under high concentration CO atmosphere was studied experimentally. The samples were carburized in a high-concentration CO atmosphere for 20 minutes, and the same surface carbon concentration as the conventional gas carburization time of 90 minutes could be obtained, which could shorten the carburization time by 70 minutes. In addition, gas carburizing in a high-concentration CO atmosphere also has a good effect on deep pore carburizing.

 

2) The effect of surface oxidation of Si-Cr added steel on carburization was studied. Adding Si and Cr to the parent steel, changing the oxidation treatment temperature, generating different oxide films on the surface of the sample, and then carburizing the sample. The test result is that in the atmosphere, the carbon concentration on the surface of the oxidized sample at 873K×30min is very high, the heating temperature is low, and the Si and Cr concentrations are suppressed. The test results show that when the oxidation heating temperature is different, the properties of the Fe oxide film formed are different.

 

3) Stress, strain and deformation prediction of carburizing quenching and nitriding treatment were studied. The results show that transformation plasticity is the main factor affecting the deformation behavior of carburizing and quenching, and creep is the main factor affecting the deformation behavior of nitriding treatment.

 

 

4) The relationship between the surface modification behavior of shot peening vacuum carburizing material and gas carburizing material and the shot peening hardness was studied. The test was carried out with shot peening with a hardness higher than that of carburizing and quenching (hardness greater than 1020HV). The results show that the surface depth of the vacuum carburized material produced by shot peening is 9.3-14.4nm, and the range is 2.6-3.5μm. The surface depth of the gas carburized material treated by shot peening produces nanocrystalline grains in the range of 0.9 μm. The residual compressive stress of the surface layer of the processed material varies with the shot peening hardness. However, the greater the shot peening hardness, the greater the surface residual compressive stress value. When the shot peening hardness is greater than 1200HV, the surface residual compressive stress reaches saturation.

 

5) The relationship between the abnormal carburized layer shape and peeling resistance of Si and Cr-added steel was studied. The test steel is a steel in which Si and Cr are added to the base steel SCr420 to reduce the Mn content. Roll peel test on test steel. The test result is that with the increase of Si content, the depth of grain boundary oxide layer of the sample decreases. When the amount of Cr increases, the surface structure of the sample becomes a soft and incompletely quenched structure. This structure can inhibit the propagation of cracks, thereby improving the peeling resistance of the material.

 

6) Research on environmental protection carburizing treatment methods. The main content is the carburizing treatment of selective hydrogen desorption in the furnace. This method has the effect of energy saving and resource saving.

 

2. Carbonitriding technology

 

1) The inhomogeneity of the phase transition of atmospheric pressure plasma nitriding is experimentally studied. The steel used in the study is SPCC steel plate. The quenching characteristics of the test steel were studied by changing the plasma irradiation interval and irradiation width. The test results show that the quenching characteristics are independent of the irradiation interval, and the surface layer of the sample undergoes martensitic transformation at a treatment temperature of 600℃. The plasma beam is irradiated at the center of the irradiation width, and the center portion is hardened.

 

2) The process of creating voids in the composite layer during nitriding and quenching was investigated by 3D photography. The steel used in the research is 590MPa grade tensile strength steel. The test steel samples were oil quenched after being treated at 640℃×5.4ks in a mixed atmosphere of N2 and NH3. A series of 150 photographs were taken of the sample, and 3D images were made using the continuous section method to study the process of creating voids in the composite layer. The test results show that the voids in the composite layer have nothing to do with the added Ti, Nb, Mn and other elements, but are related to the fine grains of the composite layer. The voids are generated and grown starting from the grain boundaries. The finer the grains, the easier it is for the voids to combine into large voids.

 

3) The use of a hydrogen sensor to control the nitriding atmosphere is studied. Gases such as N2, ammonia, and NH3 decomposition gas were introduced into the furnace, and the samples were nitrided by changing the treatment temperature and time while controlling the nitrogen potential. The nitrogen concentration distribution of the sample was measured and the structure of the nitrided layer was observed. The test results show that the use of thermally conductive hydrogen sensor to linearly control the nitrogen potential has strong reproducibility and can produce high-quality nitrided parts.