The failure forms of forging molds mainly include deformation failure, wear failure, cold and hot fatigue failure, fracture failure, etc. In order to ensure the service life of molds, the mold cavity surface is required to have high hardness, wear resistance, corrosion resistance, fatigue resistance and high temperature oxidation resistance. The failure of die occurs mostly on the working face or pressed surface, which is closely related to the mechanical properties of the surface. If the performance of cavity surface or subsurface of mold can be improved effectively, the service life of mold can be improved very well without treating the whole material of mold. Therefore, in addition to the correct selection of mold materials, appropriate heat treatment, to ensure the processing accuracy, the surface of the cavity surface welding strengthened treatment can greatly improve the service life of the mold.

It is a practical work to improve the die life and reduce the die cost.

Failure analysis of hot forging die for step shaft

The step shaft is a long rod shaft with a total length of 322.7mm. The material of the hot forging die is 5CrNiMo, the module specification is 480mm*260mm*160mm, and the forging form is 20MN hot forging press for preforging and final forging. The accumulated axial deviation of the step shaft forgings is 1.6mm, and the production of the forgings is stopped due to local unqualified dimensions due to accumulated tolerance limit. The failure mode of the mold is mainly the expansion of the hot blank surface in contact with the cavity surface, the mold surface is sprayed with coolant and lubricant when the forging is released, and the surface temperature drops and shrinks, resulting in the fatigue failure of the mold surface and the inner layer under the alternating action of tensile and compressive stress. In addition, mold cavity surface high-temperature tempering, mold surface high temperature softening, oxide skin abrasive wear and high temperature oxidation erosion, etc., aggravate the mold cavity wear speed, make the mold surface appear deep stretch mark premature failure. Therefore, the die cavity surface strengthened treatment, can greatly improve the service life of the die.

Surfacing and repairing technology of hot forging die

The repair procedure of hot forging die surfacing is as follows: surface cleaning of die body, condition inspection -- removing surface fatigue layer by carbon arc gouging of die body cavity -- overall surfacing after preheating -- isothermal and tempering after welding.

(1) Carbon arc air gouging.

After the mold cavity size detection is completed, carbon arc air gouging is carried out to cut the cavity surface. Gouging requirements: use 12mm carbon rod, set 70-75V voltage, gouging depth is 5-8mm along the cavity surface depth direction, remove all bumps and cracks, smooth transition at the corner, clean with slag gun and reveal metal luster, require crack source to be completely cleared.

(2) Surfacing process.

Welding materials imported from The United States: DZ1 (base), DZ2 (surface) two kinds of welding wire (DZ1 has good crack resistance, DZ2 is resistant to severe wear and high temperature)

Preheat the mold to the average temperature of 500+5℃. The preheat time is calculated according to the module thickness of 1 inch (25mm) /1 hour. The preheat insulation time is about 6h. After the heat preservation cotton cover insulation show welding position, only use DZ1 wire welding at the bottom of the cavity during the welding process, about 60% of the total depth of welding, welding mold temperature is not less than 380 ℃, the welding process in each layer a red bead knock, immediately with pneumatic pick when asked to round with a blunt end pick cone play thoroughly, in order to reduce shrinkage stress and prevent crack, slag removal gun is used after clean welding slag, and then on to the next layer welding, welding shall not have in the welding defects such as porosity, slag inclusion, incomplete fusion. After the bottom area is welded, the surface DZ2 wire shall be welded until the upper surface of the mold is higher than the machining allowance of 5~8mm.

(3) Heat treatment process.

Post-welding heat treatment process requirements: isothermal - with furnace cooling - a tempering - with furnace cooling - secondary tempering - with furnace cooling. Immediately after welding is complete charging for 500 ℃ isothermal * 4 h, along with the furnace cooling to room temperature after charging for the first time tempering treatment, tempering temperature is 580 + 10 ℃, the tempering time calculated on the basis of the module thickness 25 mm/h, with the secondary tempering furnace cooling to room temperature, the above process, requires cooling rate per hour is not more than 30 ℃, the mold after the second tempering hardness requirements for: 45 HRC ~ 50 HRC.

Economic analysis of die before and after surfacing

The step shaft mould material is 5CrNiMo, and the forging form is pre-forging and final forging. The total manufacturing cost of the new set of moulds is 22,000 YUAN, and the accumulated average life is 5000 pieces/set. The single consumption of the new mould =22000/5000=4.4 yuan/piece. The average manufacturing cost of pre-forging die and final forging die repaired by surfacing of die steel welding materials is 19,000 yuan, the actual average life is 9522 pieces/set, the single consumption of surfacing repairing die = 19,000 /9522=2.0 yuan/piece. The service life of this mould is increased by 90.4% and the single consumption of the mould is reduced by 54.5% after it is repaired by surfacing of steel. It can be seen that the overall surfacing technology of waste die obviously improves the die life and reduces the cost of the die.

conclusion

(1) the "ordinary steel + welding surface strengthening" after the overall welding repair technique of hot die forging, machining quality qualification, can well meet the requirements of forging die designing, shows the whole welding repair, can make the scrap mold reused, and due to the superiority of the overlaying welding technology, module and systemic reciprocating reuse.

(2) Special welding materials are used to repair the cavity of the hot forging die, which can give full play to the working efficiency of the forging die and significantly improve its life.

(3) The application of the scrap mould body integral surfacing technology can save the base material of forging mould, shorten the mould making cycle, reduce the time of mould replacement and improve the effective capacity of forging equipment, which has considerable economic benefits.