Causas das manchas negras na superficie do barel de 38CrMoAlstel para máquina de moldeo por inxección

38CrMoAl steel after the child nitriding, the surface hardness can reach more than 1200HV, nitriding layer depth of 1mm or more, commonly used in the manufacture of high fatigue strength, high wear resistance parts, such as the main shaft of the lathe bed, fine dense screw, barril, etc.. [1]. The production process of 38CrMoAl steel is as follows: die casting ingot → hot forging → blanking forging → central cutting → quenching and tempering treatment → nitriding treatment → machining. A series of physical and chemical test and analysis were carried out to find out the cause of the black spots on the surface of the cylinder.

1 physical and chemical test 1.1 macro observation

Como se pode ver na figura 1, there are dense black spots on the surface of the barrel.Irregular spot size and distribution

1.2 Chemical composition analysis

The sample was taken at the black spot on the surface of the plastic injection machine, and the chemical composition was analyzed. The results are shown in Table 1, which shows that its chemical composition is full of GB / T 3077-2015 < Standard Pair of Gold Bonded Steel

Technical requirements of 38CrMoAl steel

1.3 SEM analysis

The sample was taken at the black spot on the surface of the material barrel of the plastic injection molding machine, and after cleaning by ultrasonic wave, the FEIQ U AN T A 400F was used. The appearance of the black spots was ice sugar concave pit, and the surface morphology was observed by SEM. Como se pode ver na figura 2, the barrel surface mouth features, no mechanical processing marks.

The cross-section interview samples were intercepted at the black spot on the surface of the injection molding machine cylinder, and the cross section morphology was observed. It can be seen from Fig. 3 that there are many cracks in the section of the cylinder, the cracks in different regions are unevenly distributed, there are no oxidation phenomena and non-metallic inclusions around the cracks, and the nitride layer on the surface of the local area falls off.

It can be seen from fig. 4 that the thickness of the white bright layer (compound layer) in the nitrided layer on the surface of the cylinder is uneven, and the cracks in the thicker area of the nitrided layer are also more, and the cracks crack along the grain boundary. The matrix structure of the cylinder is bainite and the grain size is relatively large.

2 Analysis and discussion

The chemical composition of the material barrel for the plastic injection molding machine conforms to the technical requirements of 38CrMoAl steel according to the GB / T3077-2015 standard. Barrel table

There are black spots. By scanning electron microscope analysis, we can see that the black spots on the surface are crystal sugar – like pits. The cracks propagating along grain boundaries can be seen in the cross section of the barrel, and there are no oxygenated products and non-metallic inclusions around the cracks, indicating that the cracks have nothing to do with inclusions in the steel, and the cracks are formed after heat treatment. The results of metallographic examination show that there are more cracks in the thicker white light layer in the nitriding layer, and the thickness of the local white bright layer exceeds 50 μ m. The structure of the cylinder substrate is Bayesian and the grains are coarse.

Studies have shown that after the nitriding of the workpiece, the white bright layer in the surface nitriding layer is composed of one or two of the xi phase, ε phase, γ ‘phase, also known as the compound layer, because it is not easy to be corroded by ordinary etchant, so it is white bright under the gold phase display micro mirror [2-3]. Kong Dequn et al. [5] study shows that the matrix structure before nitriding has a significant impact on the thickness of the white bright layer after nitriding. The coarser the grain size of the matrix structure, the more uneven the white bright layer after nitriding, and prone to loose phenomenon. Uniform and small recycled sorbite fabrics can be obtained from 38 CrMoAl steel after tempering treatment [6] , but the batch of work after quenching and tempering processing, the grain size is large, leading to the formation of a layer of uneven thickness of the surface of the workpiece after nitriding white bright layer. The white bright layer formed on the surface of the barrel after nitriding has high hardness and can enhance the wear resistance of the workpiece. Non obstante, the white light layer often has a larger brittleness, and with the increase in the thickness of the white light layer, the white light layer will be loose (not dense), such as fruit sandwich loose, in the workpiece polishing will appear peeling, peeling and other phenomena like [4]. Related research shows that [7-9], for the surface hardness of the workpiece, such as surface carburizing or nitriding treatment of the workpiece, in machining, such as control of the grinding process, can avoid the generation of grinding cracks.

In summary, the barrel surface nitriding uneven, in the grinding process, nitriding layer in the white bright layer thick region intergranular cracking and nitriding layer spalling phenomenon, which is the barrel surface black spots like defects the main reason.

Conclusion and recommendation

(1) uneven nitriding on the surface of 38 C r M o A l steel cylinder for injection molding machine

Uniform, in the process of machining, the thicker area of the nitrided layer on the surface of the cylinder cracks along the crystal and peeling off the nitrided layer, forming a punctate pit, which is the main reason for the black spot on the surface of the cylinder.

• it is suggested that the uniform and fine sorbite structure can be obtained by adding and adjusting the texture to ensure the uniform and fine sorbite structure of the cylinder. For the workpiece with high surface hardness, attention should be paid to the clamping mode of the workpiece in the process of machining, so as to avoid grinding defects caused by the clamping of the workpiece.

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