Abstract: By comparing to the traditional technology, white cast iron grinding ball manufactured by the new technology is characterized by better tenacity, stronger hardness, and smaller stress. The working life of the ball and its adaptability to various conditions were greatly improved.

Keywords: scope production; medium chrome white cast iron; grinding ball

The microstructure of medium chromium white cast iron grinding ball can be regarded as a mixture of spongy reticulated M₃C type cementite inlaid with austenite and alloy carbide transformation mainly M₇C₃ type distributed in a three-dimensional open framework. The grinding balls’ strength and toughness depend on the brittle M₃C carbide phase’s volume percentage and distribution. According to most service conditions, the medium chromium white and the cast iron grinding ball should be further improved with better toughness as the guiding principle of process design. Therefore, the production of medium chromium cast iron grinding balls should not only control the quantity and size of carbides but also obtain as many M₇C₃-type alloy carbides as possible through composition design, modification and inoculation, and cut off the net and refine the M₃C-TYPE carbides, to change the shape and distribution of non-metallic inclusions. Of course, there is no internal stress, micro-cracks, and other discontinuities in the parts. The grains of the structure are refined and the fine and dispersed hard structure is formed, and the fine lamellar martensite is obtained, the process design goal is extremely advantageous. The cooling rate in the phase transformation and temperature range is more significant for the tissue transformation. The opening temperature shall be selected accordingly, and the cooling method shall be determined accordingly. In the process of control, cooling rate + composition design + metallurgical treatment + heat treatment, the guiding principle of process design should be embodied in a high and consistent degree.

1.1 Structure expect

Cast structure: Martensite + proper amount of retained austenite + M₇C₃ + net breaking and strip granular M₃C + MC

The microstructure of sub-temperature treatment: martensite + a small amount of retained austenite + M₇C₃+ broken network and granular M₃C + MC

This is mainly based on the following considerations:

• the wear resistance and toughness of white cast iron grinding ball are improved with the variety of carbide arranged in M₃C, M₇C₃, and MC. The thermal conductivity and thermal diffusivity of M₃C carbide with coarse and large mesh are the worst, and the percentage of carbide should be controlled from 15% to 30%.
• The addition of V, TI, W, and NB can form MC carbides such as VC, TiC, WC, and NBC with higher hardness than M₃C type chromium carbide and refine the eutectic structure.
• The specific volume change of austenite to martensite transformation results in volume expansion and internal stress. However, if an appropriate amount of retained austenite is retained in the matrix because of its small specific volume, this expansion can be partially offset and the internal stress reduced. Besides, a certain amount of retained austenite is beneficial to resist abrasive wear. Its functions are: to prevent crack propagation, to combine austenite with carbide better than martensite, to absorb energy, and consume external work.
• The corrosion resistance of martensite and austenite is higher than that of pearlite.

1.2 Performance expectations

Hardness: 54 ~ 60 HRC, impact value (no notch) αk > 5. 0/jcm²,3.5 m ball drop test times: 15000 times when the diameter of the ball is less than 80 mm, and 10000 times when the diameter of the ball is more than 80 mm.

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