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Gas Nitriding Technology of 34CrNi3MoA Steel

34CrNi3MoA steel is a high-strength alloy steel widely used in the steam turbine industry. After quenching and tempering, it has good comprehensive mechanical properties. Especially after nitriding, it is often used to manufacture important parts with high strength and good plasticity, such as gears. , shaft and other components.

In a steam turbine unit, the design requires nitriding of 34CrNi3MoA steel, the hardness after nitriding is 550~750 HV, the layer depth is not less than 0.7mm, and the brittleness is not more than grade 3. When nitriding the material according to the traditional gas nitriding process, the infiltration rate is slow and the hardness of the nitriding layer is low, so it is difficult to meet the design requirements. Therefore, a new type of nitriding process must be adopted for nitriding production of 34CrNi3MoA steel.

1 Test materials and equipment

The chemical composition of 34CrNi3MoA steel used in this paper is shown in the following table. All experiments in this paper were carried out in a PIT pit-type gas nitriding furnace.

2 Test results and discussion

2.1 Kinetic curves of conventional nitriding and nitriding with rare earth catalyst at 510℃

It can be seen from Figure 1 that under the same nitriding process and the same nitriding time, the nitriding layer of the nitriding sample added with rare earth catalyst is 10%~20% deeper than the nitriding layer of conventional nitriding.

2.2 Influence of different nitriding temperature on the hardness of nitriding layer

It can be seen from Figure 2 that under the same nitriding process and the same nitriding time, the surface hardness of the nitriding layer of the nitriding sample added with rare earth catalyst is 40-50 higher than that of the conventional nitriding nitriding layer. At the same time, as the nitriding temperature increases, the surface hardness of the nitriding layer decreases.

Due to the relatively strong affinity between rare earth elements and nitrogen atoms, a “Coriolis air mass” with rare earth elements as the core is formed in the matrix. At the same time, it is beneficial to the diffusion to the periphery, especially to the intragranular diffusion, which increases the diffusion coefficient, which can effectively increase the depth of the nitrided layer.

2.3 Nitriding test process of 34CrNi3MoA steel

The 34CrNi3MoA steel was subjected to rare earth gas nitriding using the process parameters shown in Fig. 3, and the furnace was installed at room temperature. #, 2#, 3# samples, check the surface hardness, nitriding layer depth and brittleness of the samples.

The brittleness of the 1#~3# samples are all first-class, which meets the technical requirements. The depth of the nitriding layer of the l# sample is shallow, about 0.4 mm, which does not meet the technical requirements, while the depth of the nitriding layer of the 2# and 3# samples is greater than 0.8 mm, which meets the technical requirements, and the nitriding layer of the 3# sample is Deeper than the 2# sample. The hardness value of the sample was measured by the hardness gradient method, and the hardness value was measured every 0.05 mm. The results are shown in Figure 4.

In the first stage of nitriding (500℃), the hardness of the nitrided layer is higher but the nitriding layer is shallow, and the hardness distribution is steep; with the increase of the temperature of the second stage (550℃), the depth of the nitrided layer increases but the hardness will increase with The hardness of the nitrided layer is increased again in the third stage of low-temperature nitriding (510℃); the hardness distribution of the second and third stages after nitriding is relatively smooth, and there is no phenomenon of land drop in hardness. The hardness of the 2# sample and the 3# sample is greater than 550HV when the distance is 0.2 mm from the surface, and the infiltrated layer meets the technical requirements. In the actual production process, the amount of grinding on one side of the nitriding surface is less than 0.2 mm, and the use of this process can meet the technical requirements.

3 Conclusion

1) The method of using rare earth catalyst in the nitriding process of 34CrNi3MoA steel can effectively improve the surface hardness of the infiltrated layer by about 40~50 HV, and at the same time, it can increase the infiltration rate by 10%~20%, greatly shorten the process time, and effectively reduce the production cost.

2) The three-stage nitriding process of 34CrNi3MoA steel with rare earth catalysts can meet the technical requirements of nitrided layer ≥0.7 mm and surface hardness of 550~750HV.

3) The nitriding process method can be promoted as a new process for nitriding 34CrNi3MoA steel.

Selection of vacuum nitriding equipment: RVN-T series gas nitriding furnace produced by SIMUWU is an excellent choice for completing this type of process. Its process performance can fully meet the needs of such hot processing, with good temperature control accuracy, temperature uniformity and gas quenching uniformity.