"Sealing Surface Components for Methylamine Pump Inlet Valves via Laser Cladding"

Enhancement of Methylamine Pump Valve Sealing Surfaces Using Laser Cladding Technology

Abstract:
Laser cladding technology is employed to enhance the sealing surfaces of the inlet and outlet valves of methylamine pumps used in urea production. A 5kW cross-flow CO2 laser was utilized to apply Co-based self-fluxing alloy powder onto the sealing surfaces of valve bonnet components, which are made from Cr18Ni12Mo2Ti stainless steel. The resulting laser-melted layer is characterized by a smooth surface, measuring 2mm in thickness. This process effectively eliminates common defects such as pores, cracks, and inclusions, resulting in a dense microstructure with fine grain size, improved hardness, and toughness. A metallurgical bond between the melt layer and the substrate was achieved.

Keywords: laser cladding; valve sealing surface; surface enhancement

1. Introduction
Methylamine solutions used within urea production lines exhibit significant corrosive properties, necessitating the use of acid-resistant stainless steel for internal components of main equipment pipelines and control systems. Strengthening these components or their working surfaces is crucial for enhancing performance, extending service life, and improving the reliability and safety of the overall system. This paper details the application of high-power laser technology to enhance the sealing surfaces of methylamine pump inlet and outlet valves, along with subsequent analysis and verification of the results.

2. Structure and Operating Conditions of Methylamine Pump Valves
The inlet and outlet valves of the methylamine pump operate under specific conditions: the working medium is ammonium carbamate, with a pressure range of 1.7 to 20 MPa and temperatures between 188 to 200°C. The sealing surfaces of the valve bonnet are subjected to impacts of up to 20 MPa at frequencies of 50 to 70 revolutions per minute. The valve body features a complex design, including numerous through holes and blind holes, with two concentric annular sealing surfaces. Both the body and bonnet are made from Cr18Ni12Mo2Ti stainless steel. Traditional strengthening methods like flame or arc surfacing are challenging due to the narrow sealing surfaces, leading to deformation and internal stress, which compromises the lifespan of the valves.

3. Laser Cladding Process
The sealing surfaces were coated with commercially available Co-based powder, using phenolic resin as a binder mixed with alcohol and applied to the processing surface to achieve a thickness of 2mm. Initial tests were conducted on similar material blocks to establish parameters. A 5kW cross-flow CO2 laser was employed, utilizing a wavelength of 10.6μm. The focused laser beam was scanned across the pre-coated surface, with parameters set to laser power between 2-3 kW, scanning speeds of 6-10 mm/s, and a spot size of 6mm. The CNC rotary table facilitated controlled movement. Smaller components did not require pre-heating, while larger parts were pre-heated and underwent annealing to prevent cracking.

4. Results and Analysis
4.1 Microstructure of the Cladding Layer
The microstructure of the laser cladding layer near the bonding surface is categorized into three regions: the melting zone, the mutual fusion zone, and the substrate. The mutual fusion zone, which is only 10-30μm wide, indicates a strong bond between the melt layer and the substrate with minimal thermal impact. The dominant dendritic microstructure of the melt layer is fine and uniform, with a grain size of 10-12μm, enhancing the toughness and corrosion resistance of the cladding layer.

4.2 Thickness and Quality of the Melt Layer
The laser cladding achieved a melt layer thickness of 2mm with a smooth surface. The flatness of the sealing surface was controlled to below 0.4mm, and surface roughness measured at Ra 6.3μm. Testing indicated that the melt layer exhibited no defects, with a yield exceeding 95%.

4.3 Composition and Hardness of the Melt Layer
Due to the rapid heating and cooling during laser cladding, the thermal effect on the substrate was minimized, ensuring minimal dilution of the melt layer. The hardness of the sealing surface was measured between 40 and 48 HRC, influenced by the powder composition and processing parameters.

4.4 Wear and Corrosion Resistance
Wear resistance tests revealed that the laser-cladded layer exhibited over ten times the wear resistance compared to the substrate material. Corrosion tests in various solutions confirmed that the corrosion resistance of the cladded layer was at least equal to that of the substrate.

5. Conclusion
The implementation of high-power laser cladding technology effectively enhances the sealing surfaces of methylamine pump inlet and outlet valves. The resulting melt layer demonstrates superior hardness, toughness, and corrosion resistance, significantly extending the service life under harsh operational conditions. This method offers distinct advantages for surface enhancement, particularly for components that are challenging to treat with traditional techniques, thereby improving system reliability and safety. The high energy concentration of the laser and the minimal thermal effects on surrounding areas allow for precise control, making laser cladding a superior option for critical component enhancement.