At the Metalaser Laboratory, we developed a process for manufacturing a rotary cutter for the pelletizing process in petrochemical companies. Rotary cutters are mainly subjected to wear and cyclic fatigues due to harsh and wear-full environments in which they are utilized. Using laser directed energy deposition (L-DED), a wear-resistance material deposited on the base metal. This process decreased the overall cost following lower material consumption and enhanced the part’s life cycle intensively.
Research Highlights
- Adding extremely hard and brittle stellite layers, up to 770 hardness vickers, to build up to 6 side-by-side tracks and 8 layers on the stainless-steel base metal.
- Due to low heat input nature of laser directed energy deposition method and optimized process parameters, all deposited layers on the rotary cutter teeth are crack-free.
- Negligible bending tolerances, as low as 0.02 mm, on the cutter’s shaft was resulted because of the laser’s low heat input and small melt pool size.
Challenges
The most important challenge in coating wear-resistant materials is brittleness. It is very challenging to build up a material with a hardness of approximately 700 hardness vickers (HV) by conventional additive manufacturing methods due to their high heat inputs. In some occasions, Hot Isostatic Process (HIP) could be utilized; however, HIP will require a sealed chamber, which will add to the manufacturing barriers during the processing of large parts.
Our Solution
Laser directed energy deposition (L-DED) method for manufacturing rotary cutters ensures small heat affected zone (HAZ) and enhanced microstructure of the deposited layers with hardness of 700 HV. However, it is essential to have a comprehensive understanding on the L-DED and HIP processes in order to decide which is the best process for manufacturing of a rotary cutter.
L-DED Vs HIP
In this application, laser directed energy deposition process is comparable to process called HIP. To ensure an even distribution of a neutral gas pressure on the material’s surface, it is essential to employ a sealed chamber during the HIP process. The wear-resistant layer metallurgically bonds with the base material and creates a low dilution layer, as shown in Figure 2-A. Major advantages of this process are 1) lower heat input in comparison with other processes, which will induce 2) low deflection in long shafts. Because of the even distribution of pressure on the surface of part during the process, the hardness profile in the added layer is the same along the geometry as is shown in the Figure 2-B.
For a better comparison, the hardness profile of a cutter’s tip coated with HIP and L-DED processes, consequently, is depicted in Figure 3. In another words, the L-DED process inheriting all the HIP advantages, but will result in a higher average hardness. Therefore, L-DED will resist more in wear-full applications like the rotary cutter.
Stellite deposition process
As discussed in the previous section, it is concluded that the L-DED process can make significant advantages over the HIP. By this means, a brittle material with a hardness of 640 to 770 HV can be coated on any surface, and it would be crack-free. Moreover, because of L-DED’s low heat input, no bending deflection will be induced on the rotary cutter shaft. Stellite family, cobalt-based superalloys, is a suitable candidate for the rotary cutter wear-full working condition. By depositing an 8-millimeter stellite coating on every tooth of a stainless-steel cutter (cross section view in Figure 4), the lifetime of the part will increase dramatically. Utilizing the L-DED process resulted in a twofold increase in the cycle life of the rotary cutter, leading to a substantial reduction in the overall maintenance cost of the main mechanism.
After adding stellite layers on top of each tooth, the part will be precisely machined to meet the exact tolerances. The Figure 5 shows the cutter after machining and during the final grinding step. Before this step, the layers were analyzed by Penetration Test (PT) to ensure a crack-free part.
Outcomes
- Stellite is successfully deposited on the surface of the stainless-steel with a hardness of 640 to 770 HV without any cracks.
- Bending less than 0.02 mm in the main shaft achieved due to the low heat input nature of laser directed energy deposition process.
- The life cycle of the rotary cutter was doubled compared to other additive manufacturing methods like Hot Isostatic Press (HIP).