Forging cracks are typically formed at high temperatures, when the crack is deformed, the crack is expanded and contacted with air, so it can be observed under a microscope of 100X or 500X, which can be seen in the crack, and the oxidation of the oxide, and both sides are deprophydric, tissue is ferric body. The morphological feature is that the crack is relatively strong and generally in a plurality of forms, non-tip ends, more round purity, non-fine directionality, except for the above typical form, sometimes some forging cracks are quite fine. The crack is not full trip carbon and is semi-free.
The cracks formed by the quenched heating process and the crack formed by the forging heating process have a significant difference in properties and morphology. For structural steel, the heat treatment temperature is generally much lower than the forging temperature, even if high-speed steel, high-glutinous steel is heated to heat insulation time is far less than the forging temperature. Since the heat treatment heating temperature is high, the holding time is too long or rapidly heating, it will produce early cracking during heating. Generate cracks along a coarse crystalline border; slightly decarburizing tissue on both sides of the crack, the parts heating speed is too fast, and the early cracking is produced, and there is no obvious decarburization on both sides of the crack, but the cracked and its tail charge There is an oxide. Sometimes due to the failure of the high temperature instrument, the temperature is very high, causing the tissue of the part to be extremely thick, and its crack is distributed along the coarse grain boundary. Structural steel common flaws:
1. Forging Defects
(1) Overheating, over-fever: main feature is the crystal grain, there is obvious Weili tissue. Over-burning indicates high heating temperature, large fracture grain, unevenness, no metal gloss, and oxidized carbon oxide around the grain boundary.
(2) Forging cracks: often produced in the coarse, stress concentration or alloy element segregation, and the interior of the crack is often filled with oxidized skin. The forging temperature is high, or the terminal temperature is low, it is prone to cracks. Also a crack is formed after watering after watering.
(3) Folding: punching, cutting, blade wear, roughing rough, etc. Causes surface defects, in subsequent forging, the defects such as surface oxidation are wound into the forgings body and form a seam. When viewed on the microscope, it can be found that there is obvious decarburization around the fold.
2. Heat Treatment Defect
(1) Quenching: The characteristics are just right, showing the pendant distribution, the starting point is wider, and the tail is elongated. After this crack produces a martensite transition, the microstructure surrounding the crack has no significant difference from other regions, and no cleaning.
(2) Overheating: The microstructure is thick. If it is lightweight, it can be saved by quenching.
(3) Over-burn: The crystal grain has been melted, and the grain boundary is extremely thick.
(4) Soft point: Microstructure has a block or mesh flexor and an unlimed iron body. Insufficient heating, insufficient holding time, and uniform cooling is uneven.
Causes of Forging Cracks and Heat Treatment Cracks
Forging Cracks: steel in the forging process, due to the existence of surface and internal defects in steel, such as hairline, sand eyes, cracks, inclusions, subcutaneous bubbles, shrinkage, white spots and interlayer, etc., may become the cause of forging cracking. In addition, due to poor forging process or improper operation, such as overheating, overburning or final forging temperature is too low, forging cooling speed is too fast, etc., will also cause forging cracking.
Heat Treatment Cracks: quenching cracks are macro cracks, mainly caused by macro stress. In the actual production process, the steel workpiece is often due to unreasonable structural design, improper selection of steel, incorrect quenching temperature control, quenching and cooling speed is not appropriate and other factors, on the one hand, increase the quenching stress, will make the formation of quenching microcrack expansion, the formation of macroscopic quenching cracks, on the other hand, due to increase the sensitivity of the microcrack, increase the number of microcracks, reducing the steel brittle fracture resistance Sk , thus increasing the possibility of quenching crack formation.
Heat Treatment Crack Generation Mechanism and Grinding Crack Generation Mechanism
Heat Treatment Cracking
Generation mechanism: Cracking after quenching of the workpiece is caused by internal stress.
The internal stress is divided into thermal stress and phase change stress.
(1) The workpiece in heating or cooling, due to the existence of temperature differences in different parts of the thermal expansion or contraction caused by inconsistent stress called thermal stress.
(2) Quenching workpiece in the heating, ferrite and carburizing body transformation into austenite, cooling and from austenite to martensite. Due to the different organizations of the specific volume, so the heating and cooling process is bound to occur during the volume change. The heat treatment process due to the workpiece surface and the heart of the temperature difference between the various parts of the organization transformation is not simultaneous and the resulting stress is called phase change stress.
Grinding Cracking
Generation mechanism: The high grinding temperature tends to burn the surface of the workpiece, anneal the surface of the quenched steel parts and reduce the hardness. Even if secondary quenching may occur due to the pouring of cutting fluid, it will produce tensile stress and micro cracks on the surface of the workpiece, reducing the surface quality and service life of the part.
