1. Cut diagonally
Slant cutting is when the bar is not pressed tightly when cutting the material on a sawing machine or a punching machine, the inclination of the end surface of the blank with respect to the longitudinal axis exceeds the specified allowable value. Severe cuts may cause folds during the forging process.
2. The end of the blank is bent and has burrs
When cutting the material on the cutting machine or punching machine, because the gap between the blades of the scissors or the cutting die is too large or the cutting edge is not sharp, the blank has been bent before it is cut. As a result, part of the metal is squeezed into the blade or die. In the gap, drooping burrs are formed at the ends.
Blanks with burrs are likely to cause local overheating and overburning when heated, and are easy to fold and crack during forging.
3. The end face of the blank is recessed
When cutting materials on the shearing machine, because the gap between the scissors blades is too small, the metal section and the bottom cracks do not overlap, resulting in secondary shearing. As a result, part of the end metal is pulled off and the end surface becomes concave. Such blanks are prone to folding and cracking during forging.
4. End cracks
When cold shearing large-section alloy steel and high-carbon steel bars, cracks are often found at the ends 3 to 4 hours after shearing. The main reason is that the unit pressure of the blade is too large, so that the blank of the circular section is flattened into an ellipse. At this time, a large internal stress is generated in the material. While the flattened end face strives to restore its original shape, and under the action of internal stress, cracks often appear within a few hours after cutting. When the material hardness is too high, the hardness is uneven and the material segregation is serious, it is easy to produce shear cracks.
For blanks with end cracks, the cracks will further expand during forging.
5. Gas cutting crack
Gas cutting cracks are generally located at the end of the blank, which is caused by the fact that the fasteners are not preheated before gas cutting, and the tissue stress and thermal stress are generated during gas cutting.
For blanks with gas cutting cracks, the cracks will further expand during forging. Therefore, it should be removed before forging.
6. Convex core cracking
When the lathe is unloading, a convex core is often left in the center of the end face of the bar. During the forging process, because the section of the convex core is small and the cooling is fast, its plasticity is low, but the base of the blank has a large section, slow cooling, and high plasticity. Therefore, the junction of the abrupt cross-section becomes a place where the stress is concentrated, and the plastic difference between the two parts is large, so under the action of the hammering force, the periphery of the convex core is prone to cracking.
The good quality of fasteners is a prerequisite for ensuring the quality of forgings. If the fasteners have defects, they will affect the forming process and final quality of forgings.
If the chemical element of the fastener exceeds the specified range or the content of impurity elements is too high, it will have a greater impact on the forming and quality of the forging. For example: S, B, Cu, Sn and other elements are easy to form a low melting point Forgings are prone to hot brittleness. In order to obtain the intrinsically fine-grained steel, the residual aluminum content in the steel needs to be controlled within a certain range, such as Al acid 0.02% to 0.04% (mass fraction). If the content is too small, it will not play the role of controlling the growth of the grains, and it is often easy to make the essential grain size of the forgings unqualified; if the aluminum content is too much, it is easy to form wood grain-like fractures under the condition of forming the fibrous structure during pressure processing. Tear-shaped fractures, etc. For another example, in 1Cr18Ni9Ti austenitic stainless steel, the more Ti, Si, Al, Mo content, the more ferrite phase, the easier it is to form band cracks during forging, and make the parts magnetic.
If there are defects such as shrunken tube residue, subcutaneous blistering, severe carbide segregation, and coarse non-metallic inclusions (slag inclusions) in the fasteners, cracks are likely to occur in the forgings during forging. Defects such as dendrites, serious looseness, non-metallic inclusions, white spots, oxide film, segregation bands and heterogeneous metals in the fasteners can easily cause the performance of forgings to decline.
Surface cracks, folds, scars, and coarse crystal rings of fasteners can easily cause surface cracks on forgings.
