About electronic crane scale, electronic crane scale price | quote --- analysis of the cause of hook cracking
2017-11-8 Zhongyuan Weighing Apparatus Factory
The electronic crane scale ring cracks during the quenching and tempering process. (The failure of the parts was analyzed by scanning electron microscopy, spectrometer, hardness tester and metallographic microscope. The results show that the high inclusion content in the material and the improper use of the cooling medium are the main reasons for the cracking of the surface of the ring.
1. Introduction
The 3t ring is produced by a factory for Changzhou Electronic Scale Co., Ltd. Made of 40Cr steel. A total of 18 pieces were produced, and after the quenching and tempering, 18 pieces of bolts were all cracked, and the cross-section diameter of the plug was 20 mm. The quenching and tempering process is 860T: the cooling medium is water. The production process of the lifting ring is as follows: steel into the factory for chemical analysis and mechanical properties re-examination → blanking - heating - die forging → grinding - bolting machine processing - magnetic particle testing → quenching and tempering - bolt processing thread → assembly - tensile test - magnetic powder flaw detection.
Analysis of Causes of Surface Cracks on Rings
From the above inspection results, the chemical composition meets the standard requirements, indicating that the chemical composition is not caused by the deviation of the chemical composition and the cracking of the heat treatment process; the hardness test shows that the hardness of the surface and the hardness of the core are almost the same, indicating that the bolt is completely hardened. The macroscopic inspection shows that the crack cracks along the longitudinal direction and is cracked from the surface to the core; the metallographic examination shows that the metallographic structure is normal, the surface is the same as the core structure and there is no obvious decarburization on both sides of the crack; the fracture analysis shows that the crack is along the crystal Expanded, the fresh fracture has a dimple feature with MnS inclusions in the dimple.
Since the diameter of the bolt part of the ring is 20mm, the water is used as the cooling medium during quenching and cooling, and the cooling speed is large, so that the bolt part is completely hardened, and the center and the surface are transformed into martensite, but the center and the surface are The tissue transformation is not simultaneous, the surface cools quickly, and the transformation of austenite to martensite occurs first. When the transformation has been completed to form a strong martensite shell, the center begins to perform austenite to the horse. The transformation of the body. Because the specific volume of martensite is accompanied by expansion with volume; this expansion of the central portion causes the surface layer to be subjected to tensile stress of outward expansion, while the center is subjected to compressive stress. When the tensile stress value of the surface layer reaches or exceeds the strength of the steel, a longitudinal crack from the surface to the inside may be formed. In addition, since the material itself has a large amount of manganese sulfide inclusions, the diameter of the plug portion is relatively small, and even a cooling medium having a slow cooling rate may cause longitudinal cracking. Therefore, the internal cause of the cracking of the lifting ring is that the material itself has more inclusions, and the external cause is improperly used due to the cooling medium.
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