1, raw material defects

If the ingot or steel has residual shrinkage holes, bubbles, looseness, mixture, etc., it may lead to cracking of forgings. Forging cracks caused by metallurgy are often accompanied by a large number of mixtures of oxides, sulfides, silicates and so on. The raw materials of high carbon and high alloy steel are prone to serious second-phase segregation such as carbide. If the forging is not broken and evenly distributed, the mechanical properties of forgings will be reduced, and heat treatment may lead to cracking or distortion of forgings. If there are scratches, scars, folds and cracks on the surface of raw materials, it will bring defects to forgings. Therefore, raw materials must be inspected in die forging production.

2, heating specification

When forging large die forgings and alloy steel die forgings, if the heating speed is too fast, the temperature difference between the inner and outer layers is large, and cracks will appear in the central part due to temperature stress and organizational stress.

When the heating temperature is too high and the holding time is too long, resulting in slight overheating, shiny, crystalline coarse crystals will break along the crystal. Slightly overheated coarse grains can be treated by annealing or normalizing and corrected by heavy crystals. When it is severely overheated, naphthalene fracture or stone fracture will occur. Naphthalene fracture is characterized by bright spots of fish scales and crystal fracture; The reason of naphthalene fracture is that thick austenite grains form intra-crystal texture, which is extremely stable. When transformed into ferrite during cooling, the texture characteristics will still remain. Stone fracture crystal is obvious, the surface has no metallic luster, the color is dark, and it breaks along the crystal; The reason of stone fracture is that the solubility of nonmetallic mixture increases at overheating temperature. During cooling, nonmetallic mixture precipitates from supersaturated coarse austenite and surrounds austenite grains to form brittle crystal shell. The mechanical properties of severely overheated forging blank are very poor. Naphthalene fracture can be eliminated by normalizing at high temperature, and die forgings are difficult to correct.

When the forging heating temperature is low and unheated, transgranular cracks may appear, the tail end is sharp, there is no subsequent heating process, and there is no oxidation decarbonization on the crack surface.

For alloy structural steel, if the final forging temperature is too high, austenite will continue to grow after the final forging, even exceeding the original grain size. The fracture of coarse crystal can be seen in fracture inspection, and Wei structure appears in high magnification observation. If the final forging temperature is too low, the steel is distributed in the dual-phase region along the blank main deformation direction, and ferrite precipitated from austenite preferentially adheres to the mixed surface, forming a banded structure. Wei structure and banded structure reduce the mechanical properties of forgings, especially the impact toughness. In order to refine the grain, improve the microstructure and improve the mechanical properties, it is necessary to completely anneal to produce recrystallization.

3. Die forging process

Using different die forging deformation methods, such as open die forging, closed die forging, extrusion, jacking, high-speed die forging, rolling, etc., its essence is to apply different forms of thermal and mechanical conditions to the blank through corresponding die forging equipment and dies, so as to produce different physical fields and microstructure and performance evolution processes. No matter whether the selection of deformation method is reasonable or not, the quality of die forgings is very different. The wrong choice may not realize the forming process, and unreasonable choice will make the forming difficult and prone to many quality defects.

These die forging process parameters, such as deformation temperature, deformation speed and deformation degree, are obviously directly related to the quality of die forgings. For example, for ingots or some materials, it is necessary to compact the structure and refine the particles through deformation. If the die forging deformation is small, the expected effect cannot be achieved; Some non-ferrous metals, especially high-strength aluminum alloys and magnesium alloys, need small deformation speed and appropriate deformation degree, which is suitable for forming on a press and helps to avoid cracks.

The quality of forgings is also related to the design of forgings and forgings. The selection of machining allowance and forging tolerance should proceed from the actual situation, and the specified time is too short. Due to surface defects and dimensional errors, it is easy to cause waste products after machining; Whether the parting surface, die forging web, fillet radius, leather size and flash structure of forgings are appropriate will affect the flow filling quality of metal; When the lock setting is ignored, the size of forgings will be out of tolerance due to the wrong upper and lower dies. In addition, the installation, fastening, preheating, cooling and lubrication of forging dies should comply with the specifications, and should be checked at any time to correct violations in time.

Every link in die forging production will have a noticeable influence on the quality of forgings.

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