Machining of high temperature wear-resistant alloy cast iron rollers
High-temperature wear-resistant alloy cast iron rollers are widely used in high-temperature, heavy-load equipment in the metallurgical, building materials, and mining industries, such as rolling mill conveyor rollers and sintering machine rollers. Their excellent high-temperature wear resistance and oxidation resistance stem from their unique composition design (containing alloying elements such as chromium, nickel, silicon, and molybdenum) and casting process, resulting in an austenite or bainite matrix with a large distribution of hard carbides (such as Cr23C6 and SiC). Operating temperatures can reach 600-1000°C. However, this structure also presents numerous challenges in turning, such as severe tool wear, cracks on the machined surface, and large fluctuations in cutting forces, necessitating the use of targeted machining techniques.
The difficulty in turning high-temperature, wear-resistant alloy cast iron rollers lies primarily in the selection of tool materials. Their base hardness typically ranges from HB300 to 450, while their carbide hardness reaches as high as HV1500 to 2500. These hardnesses can be maintained even at high temperatures, making ordinary carbide tools ineffective due to abrasive wear in a short period of time. Therefore, ultra-hard tool materials are essential, with cubic boron nitride (CBN) tools being the preferred choice. These tools can reach hardnesses of HV3000 to 5000 and maintain high hardness and wear resistance at high temperatures (above 1000°C), making them suitable for high-speed cutting. For rollers requiring intermittent cutting or those with casting defects, metal ceramic tools (such as TiC-based ceramics) can be used. These tools offer superior impact resistance to CBN tools, but offer slightly lower wear resistance. Tool coatings also require special selection. AlCrN coatings offer excellent high-temperature wear resistance and maintain stable performance above 600°C, effectively extending tool life.
Tool geometry must be designed to balance wear resistance and impact resistance. A negative rake angle (-5° to -10°) should be used to enhance cutting edge strength and resist impact and scratching from carbides. A clearance angle of 6° to 10° reduces friction between the flank and the workpiece, minimizing tool wear. The lead angle should be between 45° and 75°. A smaller lead angle increases tool tip strength and reduces cutting force fluctuations, but increases radial cutting forces. Therefore, this should be adjusted based on the rigidity of the roller. A 45° lead angle is recommended for rollers with good rigidity, while a 75° lead angle is recommended for rollers with poor rigidity. The tool tip radius should be between 0.8 and 1.5 mm. A larger radius disperses cutting forces, reduces stress concentration at the tool tip, and prevents chipping. Furthermore, the tool cutting edge should be properly passivated, with a radius of 0.05 to 0.1 mm to prevent the sharp edge from cracking under impact loads.
Optimizing cutting parameters requires balancing machining efficiency and tool life. The choice of cutting speed is closely related to the tool material. CBN tools can be used at 100-200 m/min, leveraging their high-temperature wear resistance to achieve high-speed cutting. Metal ceramic tools can be used at 60-120 m/min to avoid tool overheating due to excessive speeds. The feed rate should be 0.1-0.25 mm/min. A feed rate that is too low will prolong the friction between the tool and the workpiece, exacerbating abrasive wear. A feed rate that is too high will increase cutting forces and impact, leading to tool chipping or cracking of the workpiece surface. The depth of cut should be determined based on the roller’s machining allowance and surface quality requirements. For roughing, the depth of cut should be 1-3 mm to remove most of the casting allowance; for finishing, the depth of cut should be 0.3-0.8 mm to ensure surface roughness (generally requiring Ra 3.2-1.6 μm). The selection of cutting fluid needs to take into account the high-temperature processing environment. Extreme pressure cutting oil with good high-temperature stability should be used. It has strong lubrication properties and can effectively reduce the friction between the tool and the workpiece. At the same time, it has a moderate cooling effect to avoid thermal cracks in the workpiece due to rapid cooling.
The clamping and machining process significantly impact the turning quality of high-temperature, wear-resistant alloy cast iron rollers. Since rollers are typically cylindrical parts with a relatively large length-to-diameter ratio, sufficient rigidity must be ensured during clamping. Center supports or center rests can be used at both ends to prevent vibration during machining. For heavier rollers, specialized fixtures are required to evenly distribute the clamping force and prevent workpiece deformation or pinching. The machining sequence should adhere to the principle of “separate roughing and finishing, exterior first, interior later.” Roughing should remove surface scale and casting defects to prepare for finishing. Aging treatment is required after roughing to eliminate internal casting stresses and prevent cracking during subsequent machining. Finishing should be completed in a single pass to minimize work hardening and stress concentration caused by multiple passes. For shaft neck surfaces requiring suitable fit, honing can be performed after turning to further improve surface quality and dimensional accuracy.
Quality control and tool wear monitoring are key to ensuring the turning quality of high-temperature wear-resistant alloy cast iron rollers. During the machining process, the surface quality of the workpiece must be checked regularly to observe whether there are defects such as cracks and chipping. Magnetic particle inspection or penetrant inspection can be used to detect surface microcracks. Dimensional accuracy can be measured using tools such as dial indicators and micrometers to ensure compliance with the drawing requirements. The wear status of the tool can be judged by changes in cutting force, chip color, and machined surface roughness. If the cutting force suddenly increases, the chip color changes to blue-black, or the surface roughness value increases, it indicates that the tool is severely worn and needs to be replaced in time. After replacing the tool, the tool must be re-calibrated to ensure the consistency of the machining dimensions. Only through strict quality control and tool management can the machining quality of high-temperature wear-resistant alloy cast iron rollers be ensured to meet their use requirements in high-temperature and heavy-load environments.
