2.1 Main Scheme

(1) Determining the content of thixotropic element Mn according to the cast iron mill ball diameter and cooling method, adopting the correct composition design to realize the expected goal of as-cast structure, strengthening the compound modification effect of rare earth to realize the carbide necking, broken net, and bar transformation, adopting the microelement of multi-element alloying to improve the base, toughness, and hardness of the body, to improve and refine the eutectic structure and to form the hardness and high hard structure.

2) Strengthening the effect of refining, deoxidation, and modification to ensure the homogeneity and stability of as-cast microstructure.

3) The centrifugal casting machine is used to refine the grains, compact the solidification structure, change the dendrite’s size and shape, and reduce the inclusion.

4)In the whole process, great attention is paid to reducing the possible internal stress of the grinding ball, taking this as the center, effective technical measures are adopted for the melting, casting, and solidification processes, de-molding and cooling methods, and the technological parameters of the sub-temperature treatment process.

2.2 Test requirements

The furnace is a 20kgs medium frequency furnace, double platinum-rhodium is used, the thermocouple is used to measure temperature, furnace temperature is 1500 ~ 1550 °C, pouring temperature is 1360 ~ 1410 °C, and 80mm grinding ball is made by centrifugal metal and mold pouring machine.

2.3 Composition design

Composition design should consider:

1) To control the total amount of carbides by reasonably selecting the carbon content, to select Si/C according to the cooling rate of cast iron mill balls with different diameters, to decrease the “Network state”of carbides, at the same time, the relative amount of M₇C₃ will be increased due to the precipitation of Cr atoms due to the dissolution of non-carbides SI, to strengthen the effect of carbides off-grid and rod modification by graded re compound modification, and to realize the reasonable phase structure by proper composition distribution.

2) Refining, modifying, and inoculating the grain boundaries, purifying and refining the grains, controlling the quantity of non-metallic inclusions, improving the shape and size of non-metallic inclusions to improve the toughness.

3) On the premise of obtaining high toughness, the martensite matrix’s hardness can be brought into full play. The hardness of the material can be further improved by microalloying.

The leading phase of the eutectic structure of medium chromium white iron is cementite; too low carbon content is not conducive to the increase of hardness, and too high carbon content may increase the volume of carbides in the structure while forming a large network, like carbides, it directly affects the degree and distribution of martensite formation. When the carbon content goes from 3. 2% down to 2. 5%, the amount of eutectic and carbide is reduced by 40% ~ 50%. When the heat treatment process is selected, the amount of cementite is reduced because of the precipitation of dispersed granular secondary cementite and the eutectic. The eutectic carbide is easy to break off then; it is possible to develop further in strip and block direction. The cast iron mil ball diameter and mold condition usually determine the carbon content at 1. 8% ~ 2. 8%. From the chemical composition of the eutectic, there is a certain correspondence between CR and C in the eutectic, and between the total percentage of Fe, CR carbide in the eutectic, and the chemical composition of the eutectic, the corresponding relation can be rough as follows: the percentage of FE-CR carbide in the eutectic (%) = 12. 33 ×% C + 0. 55 ×% CR-15. Two. In general, when the chromium content is about 8%, not only can it dissolve into carbides completely during solidification, but also the alloy iron-chromium carbides formed are quite stable.

The common white iron is obtained mainly by adjusting silicon according to the wall thickness. Silicon is also a very important element for alloy white iron, so the control of silicon must be very careful. It is proved that when the silicon content is 0. 5% ~ 1. 0%, the SI/C value is 0. When the silicon content is too high, it is disadvantageous to the formation of high carbon martensite. Sulfur and phosphorus are controlled at ≤0. 06%.

Most of the alloying elements play a role in delaying austenite transformation and transformation in austenite. While considering the multi-element alloying to realize the desired as-cast microstructure, at the same time, the partial pearlite structure should be avoided, which usually occurs when the extent to which multi-element alloying slows down the austenite transformation is not appropriate, this mixture of martensite and pearlite causes internal stress due to two different transformations, because in austenite the pearlite transformation takes place first and the martensite matrix is transformed later. It is found that the proper combination of thixotropic element Mn with Cu, Ni, Mo, and V can make the cast iron mill ball matrix obtain fine lamellar martensite + the proper amount of austenite. Then at the subsequent sub-temperature, during the treatment, part a residue → m transformation occurs. The matrix gets secondary hardening, and the dispersed fine secondary cementite is distributed in the matrix. The quantity of Mn is usually 2. 0% ~ 3. 5% is fine. The addition of trace elements such as V, Ti, W, and NB forms MC carbides with higher hardness. Secondly, because of MC-type carbides’ high melting point, the eutectic structure is refined by diffusion nucleation in the early stage of solidification. With the extension of solidification time, the number of grains increases, the liquid film decreases gradually, the deformation decreases, the plasticity increases, and the thermal cracking sensitivity decreases significantly.

2.4 The main technical parameters

The disconnection of reticulated eutectic carbides mainly occurs in the weak connecting parts of the network, such as some cross-sections, abrupt changes, and weak branches, etc., the carbide was broken and changed to rod or lump, and the hardness was increased by hard matrix, MC Carbide, and secondary cementite.

1) To strengthen the refinement of primary austenite, the “Two-step method” is adopted to treat their composite modifier, i. e. adding the modifier in Furnace, iron, and water bags. In this way, the austenite dendrite can be refined and controlled, the eutectic can be made, more weak connection parts of carbide can be formed, and the network breaking behavior can be strengthened and accelerated. Si20Al50Fe + SiC was used as the comprehensive deoxidizer, CAC2 was used as the desulfurized, and medium electromagnetic stirring force was used in the medium frequency induction furnace to realize the full stirring of alloy elements and treatment agent.

2) The defects of slag inclusion, shrinkage porosity, and air hole were eliminated completely by using a centrifugal metal mold ball casting machine. The ceramic fiber is used as the base of the mold, and the coating and acetylene smoke are used as the working coating to control the cooling rate of the casting, which makes the solidification structure fine and uniform and the casting stress small.

3) Minis used as the thixotropic element to increase the stability of austenite, the as-cast structure is realized as M + A residue

4) The application of 550 ~ 650 °C sub-temperature treatment process, strict control of heating speed, to achieve slow temperature rise, to prevent the increase of stress.