Vacuum Heat Treatment and Nitrocarburizing of Hot Work Die Steel 4Cr5MoSiV1

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On the plastic mold, the push rod (referred to as EPN) made of 4Cr5MoSiV1 hot work mold steel works under the condition of alternating mechanical load and working temperature. In order to improve its fatigue resistance, it must have high thermal fatigue resistance, high yield point, high fracture toughness, oxidation resistance and thermal wear resistance through heat treatment. After vacuum heat treatment, the core hardness is 40-45HRC, and the surface hardness after gas nitrocarburizing is more than 900HV. The actual service life of EPN can reach more than 500,000 times.

The vertical vacuum furnace is used for treatment. Since the EPNs are all long rods, the quenching and tempering are cooled by blowing up and blowing out. The pressure during pressurized cooling is 0.16MPa, and the entire operation process is automatically completed from the beginning to the end of the process. For 4Cr5MoSiV1 steel, in order to reduce the thermal stress of the mold and promote the homogenization of the austenite, before reaching the austenitizing temperature, it is necessary to use segmental preheating, which is determined by the heating characteristics of the vacuum furnace at the low temperature stage. There is preheating, so two stages of preheating at 550°C×50min and 850°C×40min are used for heating. In addition, in order to increase convective heat transfer and prevent the evaporation of chromium under high vacuum, the furnace must be recharged with purity. More than 99.999% nitrogen. For EPN, the furnace pressure for backfilling N2 during the heating process is 266Pa to improve the heating efficiency.

Usually, the austenitization temperature of 4Cr5MoSiV1 steel ranges from 1000 to 1050 °C, with 1030 °C being the best. The grain size obtained at this temperature is ASTM grade 9″, the quenching process we use is 550℃×50min+ 850℃×40min+ 1030℃×90min, the nitrogen pressure used during quenching is 0.16MPa, and the cooling process curve is shown in the figure. Since 4Cr5MoSiV1 Steel contains more chromium and has excellent hardenability and hardenability. The hardness after tempering at 620-650℃ can still maintain the level of 45HRC.

We use two tempering, the tempering process is 550℃×220min+625℃×220min, the pressure used for tempering is 0.045MPa, and the tempering process curve is shown in the figure. After quenching and tempering, the quenching hardness of EPN is 54-55.5HRC, and the tempering hardness is 42-43.4HRC.

As we all know, the advantages of vacuum furnace processing are small deformation of the workpiece and long service life. The final core hardness of the push rod EPN is 40-45HRC, which has a good combination of strength and toughness. At the same time, 4Cr5MoSiV1 steel EPN has high tempering resistance, thermal fatigue resistance, yield point and fracture toughness.

Nitrocarburizing is carried out in a pit gas nitriding furnace. The surface color of the workpiece after nitriding is uniform silver gray, the thickness of the effective nitriding layer is 0.03-0.04mm, the thickness of the compound layer is 0.002-0.003mm, and the surface hardness is above 900HV. After the nitrocarburizing process is completed, start the blower and blow cold air into the furnace to accelerate the cooling of the workpiece, so as to reduce the brittle phase and reduce the brittleness of the infiltrated layer. The furnace is fully automatic.

The nitrocarburizing medium of EPN is ammonia and methanol, and the process is 565℃×1.5h. The process curve is shown in the figure. After EPN is nitrocarburized, the surface hardness is >900HV and the wear resistance is high.

RVGQ vacuum air quenching furnace produced by SIMUWU company, RVN series vacuum nitriding furnace is an excellent choice to complete this kind 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. The whole process is controlled by PLC, and the production is convenient and fast.