The microhardness of the reinforced layer is related to the chemical composition of the steel. According to different processing specifications, for medium carbon steel, the microhardness can reach 8. 0. These reinforced layers have high wear resistance.
In practice, it is often encountered that the surface of the workpiece must have high hardness and wear resistance, and its core should maintain the necessary toughness reserve. However, the hardened layer that achieves high hardness on the surface of the steel part cannot but adversely affect the structural strength of the entire part. This is due to the brittleness of the strengthened layer and the low crack initiation work and crack propagation in the layer. This can be confirmed by the results of the static and dynamic tests on the steel parts after the opposite strengthening, including the steel parts after chemical heat treatment.
The mechanical properties of steel after surface strengthening. When this class was proposed, there were certain uncertainties, because when testing the surface-strengthened samples, it was not the mechanical properties of the strengthening layer itself that were quantitatively evaluated, but the strengthening layer and the original appearance in a certain organizational state. The combined performance of the materials. For this reason, the mechanical test of this combination can only give the concept of its strength and plasticity when the proportional relationship between the quenched part and the non-reinforced part of the sample section under study is timed, that is to say, only the evaluation test Similar structural strength.
In order to solve the problems raised, the method proposed when studying the effect of chemical heat treatment on the strength of high hardness steels was used. Normally smooth steel samples were used. The sample is made of high-quality steel grade 70.
After grinding, the size and degree are ± 0.05 claws, some samples are in the original annealed state, and some samples are subjected to standard oil and water quenching, and tempered at 4503. The plasma surface strengthening of the samples is at The arc current on the experimental device is 200 according to the following specifications, and the voltage of the electric sister is 257. The flow rate of the plasma gas is 1.317. During the sub-surface strengthening process of the plasma generator, the sample is in a special fixture that can prevent its edge fusion. Internally processed because the scanning amplitude is greater than the width of the sample. The strengthening layer is longitudinally arranged on the width side 1 of the sample cross section after being strengthened on the plasma surface. Some samples were tempered at 3, 1,035,035, and 450.0 with a tempered 3,1 center plasma plane to strengthen the depth of metallographic and micro hardness evaluation. For this treatment specification, the strengthening depth is 2 mm. After plasma heating, water spray spray S is used.
The static bending test of the sample was carried out on a 4 machine with a scale of 2, 1 using a jig that can reproduce the concentrated bending method. The distance between the two supports is 25. 1. Bending force and deflection. For comparison, the reinforcement layer under tensile and compression conditions was evaluated for comparison.
Tests have been conducted on specimens where the reinforcement layer is located above the loading force. For each treatment specification, test 5 samples. Then statistical processing of the results = levy.
After quasi-standard strengthening treatment and different combination with plasma surface strengthening, the bending strength limit of No. 70 steel specimen, annealing; water quenching; oil quenching 10 oil quenching + 45, tempering + plasma strengthening + plasma strengthening + 450 tempering; oil quenching + equal strength, plasma surface strengthening f⑧ equal strength +150, tempering at C l⑨ equal strength + 250X tempering; ⑩ tempering at equal strength ten 350, tempering at equal strength +450; plasma Surface strengthening, tempering at equal strength +150; equal strength +250, by mole, equal strength + 350, tempering at C, tempering at equal strength + 450C, oil quenching + equal strength, oil Feng ten 450, hour Tempering + equal strength is as expected. Compared with the annealed state, the overall water quenching of No. 70 steel greatly reduces the bending strength limit. 3. After oil quenching, this effect is relatively small, because it can be guaranteed in martensite Softer cooling conditions in the temperature range. From the bending of these treatment specifications, if the annealed samples have good plasticity and do not break at a deflection of 18,1, they will brittlely break at a deflection of 4,1 after oil quenching. After the whole oil is quenched, tempering at 450 can significantly improve the bending strength.
After the plasma surface is strengthened, the hardness of 8.6, 9.0, 3 brittle bending strength does not drop much. This is because the crack initiation work in the strengthened zone becomes smaller. However, unlike overall oil quenching, specimen failure occurs before large plastic deformation 2.
As the tempering temperature increases after the plasma surface is strengthened, the effect of the brittleness effect of the strengthening layer gradually decreases, and the bending strength limit will increase significantly. There is also a plastic deformation that occurs before failure 2.
After combining with them, the characteristic bending of No. 70 steel sample is annealed oil quenched 1 plasma, tempered at equal strength + 350; tempered at Po 350 + 350 tempered, equal tempered at 250, etc. It takes only 350 times for the oil to be quenched at 350 times. When the oil is quenched at 450, the flexural strength of the fire is equal to 10. The higher the value is, the sample treated in this way is pre-oil quenched and tempered at 350 and 4500. Plasma surface strengthening 3. It must be pointed out that, in this case, when called 2000MPa, it can also ensure a high plasticity = 10, mm, together, which is different from other specifications and has good results. What promotes various performances to achieve this good combination is that during plasma heating, the tempering zone close to the quenching zone is caused by the temperature gradient in the heat affected zone that does not exceed the point. The existence of this area on the microhardness and depth curve of the sample is an effective obstacle to the development of microcracks in the strengthened area during the test of the sample.
It can be derived from the bending that 2 occurred before the failure to explain.
The reason why the strength limit of the reinforced layer drops significantly during the stretching work is already acceptable in the foreign rolling stock industry. Although tempering after plasma surface strengthening can significantly increase the strength of plasticity 1 and 2, 13, these indicators are only a few of the indicators obtained earlier. Under these test conditions, the specifications applicable to the turbidity quenching of compression work and the strengthening of the 450 tempered ten plasma surface cannot achieve significant results. Under the tensile stress, the low crack initiation work and propagation work in the brittle layer after the plasma surface is strengthened will substantially reduce the working cross section of the specimen, so the small loading force will be in the entire depth direction of the strengthened layer. Cracks are initiated, and only the remaining section of the specimen can be used to resist progressive damage.
Conclusion The spot bending test was carried out on the No. 70 steel sample by scanning gas plasma arc heating to quench the surface layer, and the structural strength was compared and evaluated.
The quenched layer on the sample surface, if it works under compression, the bending strength limit does not drop much.
The steel structure with the highest flexural structural strength is subjected to surface quenching by plasma arc heating after the overall quenching and 350, 4501 tempering in advance. This is due to the tempering zone formed between the quenched layer and the original structure before the plasma surface is strengthened. This tempering zone hinders the propagation of microcracks during the initial failure stage.
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