Forged into the internal flow pattern and performance of the grinding ball

2023-03-25 09:21

Gao Yuhe, president of Elite Energy Equipment Co., Ltd., a grinding balls expert with 30 years of experience, proficient in technology, good at management, and also knows sales, the love for grinding balls is from the bottom of his heart and overflowing, the most common sentence is: "grinding balls are round, steel balls are spiritual!" ”


Hello everyone, this issue of "Gao Yu and Say grinding balls" is shared with you. I am Zhang Tianwen, head of the quality inspection department of Elite wear-resistant media division, graduated from Shenyang University with a master's degree in materials science and engineering, and has been engaged in steel ball manufacturing and heat treatment for nearly 7 years. I was fortunate to join the Elight wear-resistant media team and become the apprentice of Mr. Gao, learning steel ball quality control and related work under his guidance.


When forging wear-resistant grinding balls, the brittle impurities of the round steel are broken and distributed in the form of crushed granules or chains along the main elongation direction of the metal. Plastic impurities are distributed in bands along the main elongation direction with metal deformation, so that the metal structure after hot forging has a certain directionality, which is often called forging streamline, also known as flow pattern. When the forged grinding balls reaches a certain forging ratio, due to the obvious formation of the forging streamline, the mechanical properties of the grinding ballsv along the longitudinal direction of the streamline, especially the plasticity and toughness, will be significantly higher than that of the streamline transverse.


At present, the common wear-resistant grinding balls forming processes mainly include casting, forging and rolling. For large-diameter grinding balls, the main production process is forging and casting, of which forging grinding balls has the best prospects.


Forged grinding balls have good surface quality, good impact resistance, strong toughness, good wear resistance, and are not easy to break and lose roundness. It is often produced by forging, which is a processing method that heats the metal to 1050ºC (±50ºC) and uses forging machinery to apply pressure to the metal to produce plastic deformation to obtain forgings with certain mechanical properties, certain shapes and sizes. Through forging, defects such as loose casting of metals during the smelting process can be eliminated, and the microstructure can be optimized. At the same time, due to the preservation of complete metal streamlines, the mechanical properties of forgings are generally better than castings of the same material.


The billet used in forging grinding balls is hot-rolled round steel, and there is a relatively large amount of plastic deformation during the forging process, so that the internal structure of the forging ball is dense, the internal structure and grain size are more uniform and fine, so that the hardness of the forging ball is high, the toughness is good, and it is not easy to break.


There are also two different forming processes for forged grinding balls, namely free forging and die forging. Free forging is a forging process in which workers manipulate the free hammer based on experience, and use the impact force of the hammer from top to bottom to deform the blank to make the round bar obtain the required external shape and internal properties; Die forging, on the other hand, is a forging process in which the blank is fixed in a mold on a die forging equipment for one-time forming.





Figure 1 Wear-resistant grinding balls forming process


Taking φ120mm forging ball as an example, die forging process: the bar blank with a diameter of Φ80mm is sawed to a cylindrical bar with a length of 185mm, and then heated and the die forging hammer is die-forged once. Free forging process: the bar blank with a diameter of Φ80mm is heated and forged 25~35 times by 750kg air hammer.


In order to analyze the metal flow law of grinding balls under different forming processes, the metal streamline experiment was carried out after the two groups of grinding balls were prepared. Metal streamline experiment: take the entire round surface or half a circular surface of the specimen along the longitudinal direction, polish the surface and use 1:1 hydrochloric acid aqueous solution to heat to 60 °C ~ 80 °C for a period of corrosion, you can see the metal streamline of the longitudinal section of the sample. Figure 2 below is the metal streamline diagram of the four specimens, which are the result of corrosion by hydrochloric acid along a circular piece with a radius of 3mm cut down longitudinally. It can be seen that the streamlined lines of the free forged specimen are more detailed than the die forged specimen. This is due to the fact that the number of forging hammers in the free forging specimen is more, and compared with die forging, the amount of deformation in all directions fluctuates less up and down, so that the distribution of metal streamlines formed by interspirated debris and segregation is more uniform.


From the perspective of lateral distribution, the streamline of die forging is blurred and relatively sparse, and the streamline is measured to be about 30mm away from the edge of the specimen, while the streamline of the free forging specimen is about 35mm from the edge. This will cause the grinding balls to be more prone to wear failure due to the rough and large range of die forging specimens after the surface hardened layer is ground off.