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Vacuum Heat Treatment Furnace

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Application example of vacuum heat treatment technology in mold

Strengthening and toughening heat treatment technology for large mold

Large-scale molds adopt strengthening and toughening heat treatment processes, such as high-temperature quenching, vacuum quenching, etc., to obtain higher mechanical properties and service life. After the large mold is vacuum quenched, the lower bainite structure is obtained, which has high hardness and toughness, and at the same time, the mold has little distortion and is not easy to crack.

Mold vacuum heat treatment application case:

Automobile front beam mold, the size is 800mm×1200mm×550mm, the mold material is 5CrNiMo steel, and the weight of a single piece is 5t. Technical requirements: the hardness of the mold body is 32 ~ 37HRC, and the hardness is 27 ~ 30HRC.

1) For molds produced by conventional vacuum heat treatment (vacuum quenching and vacuum tempering), after forging 200 to 400 pieces on site, the upper mold is intact, but the spring seat of the lower template is severely collapsed, resulting in early failure of the mold. For this, a vacuum quenching process is used.

2) High temperature austenitization, bainite vacuum quenching (new process). Vacuum quenching process: 560°C×4h preheating +860°C×4h average temperature +900～920°C×12h heating, precooling to 780～800°C in a vacuum heat treatment furnace, then adding oil, cooling to a mold body surface temperature of 200°C Out of the oil left and right, immediately carry out two vacuum tempering. Primary tempering: 280～300℃×8h preheating + 530℃×14h heating, air cooling. Secondary tempering: 280～300℃×2h preheating +510℃×10h heating, air cooling. Finally, vacuum annealing is performed.

3) Mold life. After using the above process, the hardness of the mold surface is 35-36HRC, and the hardness of the leaf spring seat is 37HRC. Since the new process prolongs the isothermal time in the vacuum tempering furnace at 280°C (from 3h to 8h in the original process), the core structure of the mold is fully transformed into the lower bainite structure, thereby obtaining lath martensite and lower bainite. The composite structure of bainite improves the comprehensive mechanical properties of the mold, and the life of the mold is increased by 20%.

Vacuum Heat Treatment Technology for Large Die

Vacuum heat treatment can enable large molds to obtain high comprehensive performance, good surface quality, small vacuum heat treatment distortion, and high surface hardness, thus meeting the performance requirements. For example, the vacuum heat treatment of large blanking dies and die-casting dies uses a vacuum furnace with 1. 5MPa nitrogen gas quenching. The vacuum furnace can be used for large-scale modular vacuum heat treatment of about 5t to obtain superior mechanical properties and minimal distortion.

Mold vacuum heat treatment application case:

An extra-large aluminum die-casting mold with a size of 1660mm×1500mm×465mm and a mass of 5600kg is processed from 9800kg electroslag remelted 4Cr5MoSiV1 (H13) steel forging billet.

The mold is convectively heated uniformly and gradually to 750°C and then (radiantly with flat graphite rod heating elements) to 1000°C. After a sufficient holding time, uniform cooling is achieved with 1.5 MPa nitrogen and changing the airflow pattern controlled by the cooling distribution on the module (equipped with thermocouples). Stop cooling at 400°C and enter the isothermal maintenance stage, so that a large amount of heat from the core is transferred to the cooler surface for uniform temperature, avoiding mold distortion or even cracking.

Chemical Heat Treatment Technology for Large Die

After proper chemical heat treatment, large molds can generally obtain higher hardness, better wear resistance and fatigue strength than ordinary quenching of the steel, and the core still has good ductility and high strength. At present, vacuum nitriding technology has become one of the main means for large (cast iron) new molds or maintenance of old molds to prolong life and improve mold processing quality.

Mold vacuum heat treatment application case:

Vacuum nitriding repair technology for drawing die of car body panels. The mold around the left side of the FAW Jetta sedan has an overall size of 4300mm×2300mm×600mm and a mass of 9.5t. It is made of ordinary gray cast iron. The chemical composition (mass fraction) of the material is C3.06%, Mn0.85%, Si1.64 %, Cr0. 04%, Mo <0. 01%. The mold surface has been vacuum quenched. After welding repair treatment, the surface hardness reaches 320 ~ 350HV1, and vacuum nitriding treatment is adopted after repair again. A large vacuum nitriding furnace of ϕ3000mm × 5000mm (furnace size) is adopted, and the process is automatically controlled and completed automatically. In order to improve the uniformity of vacuum nitriding of the mold and save electric energy, a special insulation screen is set in the vacuum nitriding furnace.

(1) Mold vacuum nitriding heat treatment

1) Mold pretreatment. The mold is heated at 200-300°C to remove surface grease and paint, then the surface is cleaned and polished, and finally the mold is fully cleaned and dried before being loaded into the furnace.

2) Vacuum nitriding process. Vacuumize for 2 hours, and the initial vacuum reaches 50Pa; 300°C and 400°C are heated up in stages (56h in total), and ammonia gas is heated; 540°C × 24h ion nitriding, nitrogen-hydrogen mixed gas is used for vacuum nitriding, the air pressure is 350Pa, and the furnace is cooled To 99 ℃ (about 25h), the mold is out of the oven. The total process cycle is 107h.

3) Test results. After the mold furnace is cooled to room temperature, the surface of the mold cavity is lightly polished, and the surface hardness is measured with an ultrasonic hardness tester. The surface hardness is 525-585HV1 according to the vertical position of the mold in the furnace, and the hardness is increased. 200HV1 and above. 4mm。 Inspection with furnace test block hardness is 538HV1, nitriding layer depth 0. 4mm. The installation test shows that the mold distortion is extremely small. For the soft spots (450~520HV1) that appeared in the original welding spot, in order to achieve the effect of ion nitriding, welding treatment can be carried out.

(2) Mold use effect

After installation and use, the surface quality of the stamping parts is significantly improved, the downtime of the stamping parts for roughening is reduced, and the life of the die is extended.