First, cold heading fasteners
(1) Surface cracking: When the free pier is made of metal material with low coarse plasticity, it will form a single drum shape due to the influence of the surface friction of the punch, which will make its side face bear circumferential tensile stress. When the temperature is too high, the intergranular bonding force is greatly weakened, and the crack direction of the intergranular fracture joint is often perpendicular to the circumferential tensile stress. When the forging temperature is low, the intergranular strength is often higher than the intragranular strength and transgranular fracture occurs.
Solution:
(1) Minimize the circumferential tensile stress caused by workpiece processing.
(2) Reduce the contact friction between the workpiece and the tool, and adopt high-efficiency special cold heading forming oil.
(3) The stress is reduced by using a concave die with a cushion or a movable collar jacket.
(2) Internal fracture: due to the shallow deformation, horizontal tensile stress is applied in the middle of the section. When this stress exceeds the fracture stress of the material, cracks perpendicular to the tensile stress direction will occur.
Solution: punch holes in the bar to make it have grooves and radians to reduce the horizontal tensile stress in the center of the blank, or change the original tensile stress into compressive stress.
Second, cold-rolled processed plates
(1) Surface cracking: When the rolled piece passes through the roll gap, the particles tend to flow laterally. Due to the influence of friction resistance, the width of the central part is much smaller than the width of the edge part, and the thickness of the central part is converted into an increase in length, and the thigh end is round. Because the rolled piece is a whole, the edge is subjected to additional tensile stress, which is easy to produce circumferential cracks. When rolling thin plates, when the roll shape is concave or the blank section is concave, the opposite situation will occur, and in severe cases, periodic cracks will appear in the middle of the plate.
Solution:
(1) must have suitable good roll profile and blank size and shape.
(2) It is to formulate reasonable rolling process regulations.
(3) Use special cold-rolled oil instead of non-special oil such as rapeseed oil and vegetable oil to improve extreme pressure and wear resistance.
(2) Internal crack: When rolling thick slab between flat rolls, the surface deformation is caused by small reduction. The central layer is basically not deformed, so the central layer constrains the surface layer to give compressive stress to the surface layer and the surface layer to give tensile stress to the central layer. When this uneven deformation and tensile stress accumulate to a certain extent, it will cause central cracking and stress relaxation. When the deformation continues, the stress accumulates to a certain extent, which will cause cracks in the center, thus continuing to produce periodic cracks in the center.
Solution: (1) reduce the machining speed and gradually deepen the deformation; (2) Increase the bottom pressure to avoid uneven deformation.
Thirdly, cutting special-shaped parts.
(1) surface cracking: mechanical wear occurs due to the lack of lubricity of the machined part, and the side wear increases sharply. Friction and wear lead to the decrease of tool durability, and excessive stress on workpiece surface leads to cracking.
Solution:
(1) Select special cutting oil instead of water-based cutting fluid;
(2) The severely worn tools should be replaced immediately;
(3) Use materials that meet the requirements of processing technology.
(2) Internal crack: In the cutting process, the workpiece can't bear the stress change caused by repeated deformation, which leads to cracks in the workpiece.
Solution:
(1) Reduce the cutting speed;
(2) Use materials that meet the processing requirements;
(3) Use professional cutting oil instead of water-based cutting fluid.
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