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Vacuum Nitriding Heat Treatment-Increase Part Performance
Issues to be noted in vacuum nitriding
(1) Vacuum nitriding is the last process. After vacuum nitriding, the workpiece can only be finely ground or grinded at most, and no other processing is required. This is because the nitriding layer is relatively thin, generally 0.3~0.5mm. If it is machined again, the hardened layer obtained by nitriding will be lost.
(2) The nitrided parts are used in complex stress environments and have high strength requirements for the core. It is necessary to quench and temper the nitrided parts before vacuum nitriding to obtain tempered martensite structure. The tempering temperature of quenching and tempering is generally higher than the nitriding temperature.
(3) For nitrided parts with strict deformation requirements, 1~2 stress relief heat treatments are required before vacuum nitriding to eliminate the internal stress generated during the machining process.
(4) For local non-nitrided parts, it is not appropriate to use the method of leaving processing allowance, but to take measures to prevent nitriding to protect them. There are several protective measures: a. Tin plating method – plating a 10~15μm tin layer on the anti-seepage surface. If the anti-seepage layer is too thin, the effect is poor, and if it is too thick, the tin will easily flow. b. Coating method – mix tin powder, lead powder and chromium oxide powder in a ratio of 3:1:1, and use zinc chloride bath to make a thin paste and apply it to the anti-seepage surface of the parts, or use water glass (mass fraction of 10%~15%) and graphite powder to make a paste and brush it, then slowly dry it. c. Tooling method – make a special tooling to seal the parts that do not need nitriding.
Application examples of vacuum nitriding
(1) Experimental materials. P20 (3Cr2Mo), Cr12MoV, 3Cr2W8V, 38CrMoAIA and H13 (4Cr5MoSiV1) steels are selected as experimental materials. These five steels are typical alloy mold steels and nitriding steels. The matrix of these steels has a certain hardness after pre-heat treatment. It mainly relies on the formation of coherent alloy nitrides to further improve the hardness and wear resistance. Considering the influence of nitriding temperature on the matrix, the temperature of 500~560℃ is taken for process experiments.
(2) Experimental process. Experimental process parameters: vacuum nitriding temperature, furnace pressure, vacuum nitriding time, nitrogen flow rate.
After the workpiece is loaded into the furnace, it is pre-evacuated and heated for a period of time (0.5~2h) according to the amount of furnace loading, and then nitrogen is introduced for vacuum pulse nitriding. Before leaving the furnace, the temperature is vacuumed and cooled to 500℃ (the furnace temperature may be incorrect) and then oil-cooled.
(3) Effect of experimental parameters on nitriding layer
a.Effect of temperature on nitriding layer. If the vacuum pulse nitriding temperature is too high, the alloy compounds will be coarse; if the nitriding temperature is too low, the nitriding layer will be shallow, less alloy compounds will be formed, and the hardness will be low. When the vacuum pulse nitriding temperature is in the range of 510~570℃, the effect on the depth and hardness of the nitriding layer is not obvious. Therefore, different processes can be used according to the use of different mold materials and tempering temperature conditions. For example, the purpose of nitriding cold working die steel such as Cr12 type (Cr12Mo, Cr12MoV, Cr12Mo1V1, SAED2, D6) is to further improve wear resistance and increase service life. The vacuum pulse nitriding temperature of this type of steel is generally 510~520℃, and the time is 8~12h. The depth of the nitriding layer is 0.08~0.12mm, and the hardness is about 1000HV1. For vacuum pulse nitriding of hot working die steel (3Cr2W8V, H13, 4CrW2VSi), one-stage (pulse) nitriding can be used (530~540℃), (12~16h), or two-stage (pulse) nitriding can be used (520~545℃), 8h+(550~560℃), (4~6h).
b.The influence of furnace pressure on the nitriding layer. The higher the upper limit of furnace pressure, the better the depth and hardness of the carburized layer; the lower limit of furnace pressure has an impact on the carburized layer. The higher the vacuum, the better the hardness and thickness of the carburized layer.
c.The effect of nitriding time on the carburized layer. As the nitriding time increases, the hardness increases, and a compound layer appears. The hardness increase is more obvious, and the carburized layer deepens.
d.The effect of NH3 flow on the carburized layer. The more NH3 flow, the higher the hardness and the deeper the carburized layer. For example, 1600 grids are much better than 1000 grids. If the pulse time is too long, the carburized layer becomes thinner and the exhaust gas cannot be fully burned; if the time is too short, the surface brittleness increases.
(4) Final process parameters and experimental results. Based on the process experimental results, the process parameters are formulated as follows: ammonia flow 2500 grids, furnace pressure upper limit -0.01MPa, lower limit -0.08MPa, nitriding time 6h, pulse time 2min, nitriding temperature 550℃.
After vacuum pulse nitriding, the surface hardness of the five experimental samples P20, Cr12MoV, 38CrMoAIA, 3Cr2W8V and H13 all exceeded the national standard. After X-ray phase analysis, brittleness test and hardness method measurement, the brittleness of the nitrided layers of the five materials were rated as level 1, the compound layer was relatively thin, and there was no obvious looseness.
Vacuum nitriding furnace