Machining of high chromium and high silicon cast iron
High-chromium cast iron and high-silicon cast iron are two types of wear-resistant cast iron with unique properties. They are widely used in metallurgy, mining, chemical engineering, and other fields. High-chromium cast iron contains 12% to 30% chromium, forming a large amount of Cr7C3 carbide, which has extremely high hardness (HRC50-65) and wear resistance. High-silicon cast iron contains 10% to 18% silicon, forming a dense SiO2 oxide film, which has excellent corrosion resistance, but is also brittle and has a high hardness (HB400-500). However, these properties also make it a typical difficult-to-machine material. Machining can lead to rapid tool wear, poor surface quality, and cracking, requiring specialized machining processes and tool technology.
The difficulty in turning high-chromium cast iron stems primarily from its high hardness and the abrasive wear caused by carbides. Cr7C3 carbides have a hardness of up to HV 1800-2800, far exceeding the hardness of common tool materials. This causes severe scratching and impact on the tool during cutting. Therefore, tool materials must be ultra-hard materials with high hardness and excellent wear resistance. Cubic boron nitride (CBN) tools are the preferred choice for machining high-chromium cast iron. With hardness reaching HV 3000-5000, they can withstand the abrasive wear of carbides and offer tool life 10-50 times that of cemented carbide during continuous cutting. For intermittent cutting or high-chromium cast iron with casting defects, ceramic tools (such as Al2O3-based ceramics) can be used. These tools offer slightly better impact resistance than CBN tools, but offer slightly lower wear resistance. The tool geometry parameters should adopt a negative rake angle (-5°~-10°) and a small back angle (5°~8°) to enhance the edge strength. The main deflection angle should be 45°~60° to reduce the impact of the cutting force on the tool. The tool tip arc radius should be 0.8~1.2mm to avoid tool tip cracking.
Machining high-silicon cast iron faces challenges due to its high brittleness and prone to edge chipping. Its elongation is typically less than 1%, making it prone to chipping during cutting, leading to cracks and notches on the machined surface. Furthermore, the presence of silicon reduces the material’s thermal conductivity (approximately one-third that of gray cast iron), concentrating cutting heat in the cutting area and exacerbating tool wear. Tool materials used for machining high-silicon cast iron are primarily CBN and ceramic. CBN tools are suitable for finishing, while ceramic tools are suitable for roughing. However, the low thermal conductivity of high-silicon cast iron can increase the thermal shock of CBN tools, so cutting speeds should be kept to a minimum. A large negative rake angle (-10° to -15°) should be used to utilize the tool’s extrusion to induce brittle fracture and minimize plastic deformation. A clearance angle of 6° to 10° reduces flank friction. A lead angle of 75° to 90° reduces radial cutting forces and prevents workpiece chipping. Furthermore, tool edges should be passivated, with a radius of 0.05 to 0.1mm to prevent chipping from brittle chips.
Cutting parameters should be adjusted based on the characteristics of high-chromium and high-silicon cast iron, respectively, to balance machining efficiency and tool life. The cutting speed for high-chromium cast iron should be determined by the tool material: 80-150 m/min for finishing with CBN tools and 50-100 m/min for roughing with ceramic tools. The feed rate should be 0.1-0.2 mm/r. A feed rate that is too low increases tool-workpiece friction and accelerates wear, while a feed rate that is too high increases impact loads. The depth of cut should be 0.5-2 mm, which can be increased for roughing, but high impact during interrupted cutting should be avoided. The cutting speed for high-silicon cast iron should be slightly lower than that for high-chromium cast iron: 60-120 m/min for CBN tools and 40-80 m/min for ceramic tools. The feed rate should be 0.08-0.15 mm/r. A lower feed rate reduces chip impact on the surface and reduces the risk of edge chipping. The depth of cut should be 0.3-1.5 mm, and layered cutting should be used to reduce cutting force fluctuations. Both types of cast iron require cutting fluid. High-chromium cast iron should use extreme pressure emulsion to enhance lubrication and cooling effects; high-silicon cast iron can use oily cutting fluid to reduce frictional heat and avoid thermal cracking of the workpiece.
The machining process and clamping method significantly impact the turning quality of high-chromium and high-silicon cast iron, and measures must be taken to reduce vibration and impact. Workpiece rigidity must be maintained during clamping. For irregularly shaped castings, specialized fixtures or auxiliary supports are required to prevent vibration during machining. The clamping force should be evenly distributed, and excessive clamping force, which can cause cracking in the workpiece, is particularly important for high-silicon cast iron. The machining sequence should follow the principle of “roughing first, then finishing, from the outside in.” Roughing removes the casting skin and excess material, eliminating the effects of surface defects on finishing. Before finishing, inspect the workpiece for cracks to avoid scrap. When high-chromium cast iron requires high surface roughness, honing or grinding can be performed after turning to leverage its high wear resistance to achieve lower roughness values. Finishing high-silicon cast iron requires a single pass to avoid surface fatigue cracks caused by multiple passes.
Tool wear monitoring and life management are crucial aspects of turning high-chromium and high-silicon cast iron. Tool condition can be assessed by observing chip color, machined surface quality, and changes in cutting force. During high-chromium cast iron machining, if the chip color changes to blue-black, it indicates excessive cutting temperature and increased tool wear. During high-silicon cast iron machining, if continuous edge chipping or cracking appears on the surface, it may be due to tool edge fracture. When tool wear reaches 0.3-0.5mm, the tool should be replaced promptly to avoid excessive tool wear, resulting in workpiece scrap and machine vibration. Furthermore, tool grinding quality significantly impacts machining results. CBN tools should be sharpened with a diamond grinding wheel to ensure a sharp edge free of microcracks. Ceramic tools should be adequately cooled during grinding to prevent thermal shock and tool cracking. Efficient and high-quality turning of high-chromium and high-silicon cast iron is only possible through scientific tool management and process control.
