Characteristics and Standards of Chromium Alloy Steel Mill liners

High chromium alloy cast iron generally refers to the alloy white cast iron with the chromium content of 12% – 30% and carbon content of 2.4% – 3.6%. It is characterized by the microhardness hv1300 ～ 1800 of M7C3 eutectic carbide, which is dispersed on the matrix of martensite (the hardest structure in the metal matrix) in the form of a broken network and isolated shape, which reduces the splitting effect on the matrix. Therefore, the high chromium alloy liner has high strength, toughness, and high wear resistance, and its performance represents the highest level of metal wear-resistant materials. It has been widely used in large-scale cement ball mills abroad. The service life of the first bin is 6-8 years, and that of the second and third bin is more than 12 years.

Chemical composition and main mechanical properties of chromium alloy steel mill liners
Grades Chemical Composition Hardness(HRC) impact strength (J/cm²)
C Si Mn Cr Mo Ni Cu S P
Cr15 1.5-4.5 ＜1.5 ＜1.5 10.0-18.0 ＜3.0 ＜1.5 ＜1.2 ＜0.06 ＜0.1 ＞56 ＞5
Cr20 1.5-4.5 ＜1.5 ＜1.5 18.0-23.0 ＜3.0 ＜1.5 ＜1.2 ＜0.06 ＜0.1 ＞58 ＞7
Cr26 1.5-4.5 ＜1.5 ＜1.5 23.0-28.0 ＜3.0 ＜1.5 ＜1.2 ＜0.06 ＜0.1 ＞58 ＞7
Cr30 1.5-4.5 ＜1.5 ＜1.5 28.0-32.0 ＜3.0 ＜1.5 ＜1.2 ＜0.06 ＜0.1 ＞48 ＞5

In practical application, low carbon high chromium cast iron (steel) lining plate is generally used for the working condition with large impact load, and high carbon and high chromium cast iron lining plate is used for working condition with small impact load, so as to give full play to the wear resistance of the material. For example, Cr15 liner with 2.4% ～ 2.65% C can be used for mills below 4.2m (one bin); Cr20 and Cr26 liners with 2.65% ～ 2.85% C can be used for mills above 4.2m; when there is pre pulverizing system before grinding, such as “roller press + cement mill”, the average ball diameter is less than 60mm, and the C content can be higher, which can guarantee the service life (one bin) of 8-10 years, and the service life of bin 2 and 3 is longer. The grinding head plate can be made of medium (low) carbon high chromium cast iron; the discharge grate plate can be made of medium and high carbon high chromium cast iron; the partition board composed of blocks can be made of low carbon high chromium cast iron.

 

Practical application

The chromium alloy steel mill liners produced by Qiming Machinery includes Cr13, Cr15, Cr20, and Cr26, which are used for more than 200 sets of ball mills with different specifications and models of ø1.83M, ø2.2M, ø3M, ø3.5m, ø3.8m, ø4.2m, ø5M, and ø5.4m. For example, The Cr15 mill liner has been used in the first bin of ø2.2m × 6.5m mill in Yanshi cement for 13 years; The Cr15 mill liner has been used in the first bin of ø 3m×9m mill in Shunbao Cement for 7 years; In May 2020, the Cr26 mill liners and grinding head plate used on the ø5.4m × 15.5m super large mill of Dayu cement (Shandong) Co., Ltd. have been running continuously for more than 6 years. According to the customer’s report, the service life of the grinding head plate has exceeded 2 times of the original imported grinding head plate, and it is expected to be used for about 2 years.

 

Chromium alloy steel mill liners casting production

Improving impact toughness of high chromium alloy steel mill liners

When choosing high chromium cast iron material to prepare the mill liners, the production enterprise should give priority to its wear resistance, at the same time, it should also pay special attention to the combination of strength and toughness. Only when the organic matching of high strength, strong toughness, and high wear resistance can the excellent performance of high chromium cast iron to be brought into full play.

In order to improve the properties of high chromium cast iron, the fracture toughness KIC value of high chromium cast iron should be increased first. When CR > 13%, M7C3 eutectic carbides of high chromium cast iron can be broken and isolated. When the eutectic carbides remain unchanged and the CR / C ratio is between 6.6 and 7.1, the fracture toughness KIC value of high chromium cast iron is the highest, that is to say, the crack propagation resistance is the strongest. The results show that the morphology, distribution, and size of M7C3 eutectic carbides can be improved by high-temperature quenching or prolonging the holding time,

Increase toughness. The simplest and economical way to change the morphology, quantity, size and distribution of eutectic carbides in high chromium cast iron and improve its strength and toughness is to modify the molten iron, control its solidification, and more importantly, refine the eutectic carbide, reduce its splitting effect on the matrix, refine the matrix and improve the toughness.

Safe use of high chromium alloy steel mill liners

  • Fracture is the main failure mode of high chromium alloy liner. The main causes of fracture are casting defects, such as casting porosity, shrinkage cavity, porosity, inclusion, etc
  • High chromium alloy steel mill liners should be small and thick, single bolt hole, or no bolt hole. It is not allowed to use a large and thin or double bolt hole lining plate.
  • During the installation of high chromium lining plate, the lining plate with bolt holes shall be firmly fixed with bolts. The installation surface and cylinder surface should be in contact with each other. It is better to choose a rubber cushion as a cushion. It is strictly forbidden to install and use high chromium liner without any bedding. During the initial (Trial) operation after installation, the mill must be filled with a certain amount of materials before the operation. It is strictly prohibited to run the mill without load.