Heat treatment knowledge:stainless steel nitriding process
Heat treatment knowledge: stainless steel nitriding process: two-stage nitriding
Stainless steel 1Crl1Ni2W2MoV is a 12% chromium-type martensitic heat-strength stainless steel. Due to its high room temperature strength and durability, as well as good toughness, oxidation resistance and corrosion resistance, it is widely used in the aviation industry. . The material used for a certain bearing part is 1Cr11Ni2W2MoV. In order to improve the strength and wear resistance of the part, the surface of the part needs to be nitrided and strengthened. The design requires the depth of the nitriding layer to be greater than 0.20mm and the hardness to be greater than 600HV. Since the depth of the nitriding layer using ion nitriding cannot meet the requirements, gas nitriding is now used for nitriding.
Stainless steel 1Cr11Ni2W2MoV contains a large amount of Cr and Ni. When exposed to air, a dense oxide film (CrO3, NiO) is easily formed on the surface, which is also called a passivation film, which prevents the penetration of nitrogen ions. For gas nitriding, removing the passivation film on the surface of stainless steel is a key step to determine the success of nitriding. In the production, anhydrous ammonium chloride is generally sprayed into the nitriding box by chemical methods. During the heating process, the hydrogen chloride decomposed by the ammonium chloride reduces the oxide film on the surface of the workpiece. The amount of ammonium chloride has a great influence on the surface hardness and depth of the nitrided layer. Too much amount will cause pitting on the surface of the workpiece, corrode the nitrided tank, block the pipeline, and use too little to remove the passivation film. The layer will be thin and uneven. Another way to remove the passivation film is mechanical dry sand blowing. The passive film on the surface of the stainless steel can be removed by sand blowing, which can have a good effect. In this process test, sand blowing was used instead of ammonium chloride to remove the passive film on the surface of 1Crl1Ni2W2MoV stainless steel, and the nitriding process of this material was explored at the same time.
1. Process test
The test material 1Crl1Ni2W2MoV, its composition is shown in Table 1.
Table 1 Chemical composition of 1Crl1Ni2W2MoV (mass fraction) (%)
|
Chemical composition |
C |
Cr |
Ni |
W |
Mo |
V |
Mn |
Si |
Fe |
|
Content |
0.14 |
11.2 |
1.62 |
1.68 |
0.46 |
0.22 |
0.52 |
0.48 |
/ |
The material preparation heat treatment adopts quenching: 1000℃×1h oil cooling; high temperature tempering: 680℃×2h air cooling.
Sand blowing process: Use 100-200 mesh quartz sand and compressed air pressure of 0.3-0.5MPa to dry sand the nitriding surface. The parts after sand-blowing are strictly forbidden to be taken with bare hands, and should be packed as soon as possible, and the interval should not be More than 2h.
In the test, a pit-type nitriding furnace was used, and the furnace temperature uniformity reached ±5°C, and an atmosphere circulation device was installed in the furnace to achieve uniform atmosphere. Ammonia is used as the infiltration agent. After the parts are put into the furnace, ammonia is used to exhaust the air in the furnace. And when the ammonia decomposition rate is close to zero when measured at less than 200℃, the furnace temperature is increased to reach the nitriding temperature. When the furnace temperature rises to near the nitriding temperature, the ammonia decomposition rate should be adjusted to the required process value as soon as possible. After the nitriding lasts for a sufficient time, continue to pass ammonia gas to maintain the positive pressure in the furnace and cool down with the furnace. When the furnace temperature drops to ≤150℃, stop the ammonia supply before removing the parts. Rapid cooling should be avoided to prevent the parts from deforming.
According to production experience, two nitriding process tests are used in this nitriding process, namely one-stage nitriding and two-stage nitriding. The ammonia decomposition rate is adjusted by the ammonia flow.
One-stage nitriding process: 580℃×45h, the ammonia decomposition rate is 60%, and it is cooled to ≤150℃ and air-cooled in the original atmosphere.
Two-stage nitriding process: the first stage is 550℃×15h (ammonia decomposition rate is 55%) and the furnace is heated up to the second stage 580℃×30h (ammonia decomposition rate is 65%), and the furnace is cooled to ≤150℃ in the original atmosphere Air cooling out of the oven.
The metallographic method is used to determine the depth of the nitride layer. The metallographic method uses the characteristics of the nitride layer structure and the core structure to determine the depth of the nitride layer. The nitrided sample is etched with 5% CuSO4 solution and measured with a metallographic microscope Depth of nitride layer. The Vickers hardness tester tests the surface hardness of the nitrided layer, and the micro Vickers hardness tester measures the hardness at different positions of the nitrided layer.
2. Test results and analysis
Figure 1 shows the part after nitriding. The surface of the part is not blistered, and the color is uniform and light gray. It shows that it is feasible to use sand blowing process instead of ammonium chloride to remove the passive film on the surface of stainless steel.
Figure 1 Macro view of parts after nitriding
(1) Surface hardness and depth of nitriding parts
Carry out hardness inspection and metallographic method to detect the depth of nitrided parts. Table 2 shows the surface hardness and depth of nitrided parts obtained by two different nitriding processes. Although the hardness of the nitrided layer obtained by a one-stage nitriding process is satisfactory, the depth of the nitrided layer is insufficient. Under the two-stage nitriding process conditions, the depth and hardness of the infiltration layer meet the design requirements for parts. In order to verify that the depth of the nitriding layer under the two-stage nitriding process meets the requirements, a micro Vickers hardness tester is used to test the nitriding layer. Table 3 shows the two-stage nitriding process along the surface to the inside of the substrate, every 0.05 mm, the results of the micro Vickers hardness test, the results also verify that the depth of the two-stage nitriding process is within the range of 0.30 mm, and it also shows that the hardness of the nitriding layer is relatively smooth along the section.
Table 2 Surface hardness and nitriding depth under different nitriding process conditions
|
One stage nitriding |
Two-stage nitriding |
|
|
Surface hardness HV |
620～752 |
726～856 |
|
Infiltration layer depth/mm |
0.15～0.20 |
0.27～0.30 |
Table 3 Hardness of different positions in the nitride layer
|
Distance to surface/mm |
0.05 |
0.10 |
0.15 |
0.20 |
0.25 |
0.30 |
|
Hardness HV |
837 |
736 |
705 |
695 |
662 |
583 |
(2) Metallographic structure of nitriding parts
Before the parts are nitrided, they are subjected to preliminary heat treatment (quenching + tempering) to prepare the structure for nitriding. Figure 2 shows the metallographic structure of the part after the two-stage nitriding process. Figure 2a shows the metallographic picture of the nitrided layer. From this figure, it can be seen that the nitrided layer is uniform and continuous, and the nitrided layer is mainly nitrided sorbite and dispersed distribution Nitride composition, and no bright white layer is found in the nitride layer. Figure 2b The core structure of nitrided parts is mainly composed of tempered sorbite and a small amount of ferrite. The reason why the two-stage nitriding process is better than the one-stage nitriding process is that the surface of the workpiece is formed with large dispersive nitride during the first stage of nitriding, and the second stage of nitriding has a higher temperature than the first stage of nitriding, and the time is longer than that of the first stage of nitriding. One-stage nitriding accelerates the diffusion of nitrogen in the steel, deepens the thickness of the nitriding layer, and makes the hardness distribution of the nitriding layer tend to be gentle. Although the increase in temperature in the second stage will cause the accumulation and growth of nitrides, it is different from a higher temperature nitriding, because the highly dispersed and fine nitrides first formed during the first stage of nitriding will accumulate and grow The coarsening process is much slower than that of the nitride grown directly at high temperature. Therefore, when the stainless steel nitriding depth is required to be deep, the two-stage nitriding process has great advantages.
(A) Surface
(B) Heart
Figure 2 Metallographic structure of nitrided parts
3. Conclusion
(1) The use of sand blowing to remove the passive film on the surface of stainless steel can effectively replace the nitriding process of stainless steel 1Crl1Ni2W2MoV with ammonium chloride.
(2) The nitriding process of 1Crl1Ni2W2MoV stainless steel recommends two-stage nitriding process: 550℃×15h (ammonia decomposition rate 55%) + 580℃×30h (ammonia decomposition rate 65%).
(3) The nitride layer under the two-stage nitriding process is mainly composed of nitrided sorbite and dispersed nitride, and the core is mainly composed of tempered sorbite and a small amount of ferrite.
