Abstract: The sealing surfaces of inlet and outlet valves in methylamine pumps used in urea production have been enhanced using laser cladding technology. A 5kW cross-flow CO2 laser was employed to apply Co-based self-fluxing alloy powder onto the sealing surfaces of valve bonnet components made from Cr18Ni12Mo2Ti stainless steel. The resulting laser-melted layer exhibited a thickness of 2mm and a smooth surface. The laser cladding process effectively eliminated defects such as pores and cracks, resulting in a dense microstructure with fine grain size, increased hardness, and enhanced toughness. A metallurgical bond was achieved between the melt layer and the substrate.
Keywords: laser cladding; valve sealing surface; surface enhancement
1. Introduction
In urea production, methylamine solution is highly corrosive, necessitating the use of acid-resistant stainless steel for internal components of the main equipment. Strengthening these components or their working surfaces is crucial for improving performance and extending service life, thereby enhancing system reliability and safety. This study explores the application of high-power laser cladding to enhance the sealing surfaces of methylamine pump inlet and outlet valves, alongside subsequent detection and analysis.
2. Structure and Operating Conditions of Methylamine Pump Valves
The methylamine pump's inlet and outlet valves operate under specific conditions: the working medium is ammonium carbamate, with a pressure range of 1.7 to 20 MPa and temperatures between 188 and 200°C. The sealing surface experiences an impact pressure of 20 MPa at a frequency of 50 to 70 revolutions per minute. The valve body features a complex structure with numerous through and blind holes, and the sealing faces are narrow concentric annular surfaces. Both the body and bonnet are constructed from Cr18Ni12Mo2Ti stainless steel. Traditional strengthening methods such as flame or arc surfacing are challenging due to the intricate geometry and risk of deformation, leading to short service life and increased maintenance.
3. Laser Cladding Process
The sealing surface was treated with commercially available Co-based powder, mixed with phenolic resin and alcohol, and applied to the processing surface with a thickness of 2mm. Initial tests were conducted on similar material blocks. The cladding was performed using an HGL-90 type 5kW cross-flow CO2 laser, with specific parameters including laser power, scanning speed, and spot size. The laser beam, focused onto the surface, allowed for a single scan to create the melt layer. Pre-heating and post-processing were applied to larger components to prevent cracking.
4. Results and Analysis
4.1 Microstructure of the Cladding Layer
The microstructure of the laser cladding layer consists of three distinct zones: the melting zone, the mutual fusion zone, and the substrate. The fusion zone, which is narrow, indicates a strong bond between the melt layer and the substrate, minimizing thermal effects. The dendritic structure observed in the melt layer contributes to its density and fine grain size, enhancing corrosion resistance and toughness.
4.2 Thickness and Quality of the Melt Layer
The laser cladding process produced a melt layer with a thickness of 2mm and a smooth surface. The flatness of the sealing surface was controlled to below 0.4mm, with a surface roughness of Ra6.3μm. Testing revealed no defects in the melt layer, achieving a yield of over 95%.
4.3 Composition and Hardness
The rapid cooling during the cladding process resulted in minimal diffusion between the melt layer and the substrate, maintaining the integrity of the alloy composition. The hardness of the sealing surface was measured at 40 to 48 HRC, influenced by the powder composition and processing parameters.
4.4 Wear and Corrosion Resistance
Comparative wear tests indicated that the laser-cladded layer exhibited over ten times the wear resistance of the substrate. Corrosion tests in various solutions demonstrated that the corrosion resistance of the cladded layer was at least equivalent to that of the base material.
5. Conclusion
The application of high-power laser cladding effectively strengthened the sealing surfaces of the inlet and outlet valves in methylamine pumps. The resulting melt layer achieved a reliable metallurgical bond with the substrate, exhibiting minimal heat impact and deformation. The dense, fine-grained structure of the cladded layer contributed to improved hardness and toughness. Under conditions of high corrosion, impact, temperature, and pressure, the service life of the laser-cladded valves was extended by two to three times compared to untreated valves, significantly enhancing system reliability and safety. The advantages of laser processing, including high energy concentration and minimal thermal effects, make it a superior choice for enhancing critical components where traditional methods may be inadequate.
