Wear-Resistant Overlay of Lime Digestor Shell and Stirring Shaft

Industrial Application Background

Lime digesters are critical equipment in the lime production industry, used to convert quicklime (calcium oxide) into hydrated lime (calcium hydroxide) through a reaction with water. The digestion process takes place in a cylindrical vessel containing a rotating stirring shaft with multiple blades that continuously mix the lime and water. The operating environment inside the digester is highly corrosive and abrasive, with the slurry consisting of fine lime particles suspended in water at temperatures ranging from 60 to 100 degrees Celsius. The stirring shaft and blades are subjected to severe abrasive wear from the lime slurry, while the digester shell experiences corrosive attack from the alkaline environment and mechanical wear from the stirring action.

The literature under review describes the application of wear-resistant overlay welding to extend the service life of lime digester shells and stirring shafts. The case study provides practical insights into the selection of overlay materials, welding procedures, and quality control measures for this specific industrial application. The overlay solution is presented as a cost-effective alternative to complete equipment replacement or the use of expensive corrosion-resistant alloys for the entire component.

Service Environment and Failure Analysis

The failure analysis of lime digester components reveals three primary degradation mechanisms: abrasive wear, corrosion, and mechanical fatigue. The abrasive wear is caused by the fine lime particles in the slurry, which act as abrasives against the metal surface. The corrosion is due to the alkaline environment, which attacks the steel surface and accelerates material loss. The mechanical fatigue is caused by the cyclic loading from the stirring action, which can initiate cracks at the surface or at the overlay-base metal interface.

The stirring shaft blades experience the most severe wear due to their direct contact with the lime slurry. The blades are typically made of carbon steel, which has poor resistance to both abrasive wear and corrosion in this environment. The digester shell, particularly the bottom section where the slurry accumulates, also experiences significant wear and corrosion. The upper section of the shell is less affected due to reduced slurry contact and lower mechanical stress.

Overlay Material Selection and Design

The selection of overlay material for lime digester applications requires a balance between wear resistance, corrosion resistance, and mechanical compatibility with the base material. The study evaluates several overlay material options, including cast iron type (Fe-Cr-C), stainless steel type (Fe-Cr-Ni), and nickel-based alloy type (Ni-Cr-Mo). Each option offers different trade-offs between performance and cost.

The recommended overlay material for the stirring shaft blades is a high-chromium cast iron type overlay with the following composition: 20 to 25 percent chromium, 2.5 to 3.5 percent carbon, 3 to 5 percent manganese, and 1 to 2 percent molybdenum. This composition provides a hardness of 58 to 62 HRC and excellent abrasive wear resistance, while the chromium content provides adequate corrosion resistance in the alkaline environment. The overlay is applied with a thickness of 6 to 10 millimeters, depending on the blade geometry and expected service life.

For the digester shell, a lower-alloy overlay is recommended due to the reduced wear severity and the need for better weldability with the shell material. The overlay composition includes 12 to 18 percent chromium, 1.5 to 2.5 percent carbon, and 2 to 4 percent manganese, providing a hardness of 52 to 56 HRC. This overlay offers good wear resistance while maintaining adequate toughness to resist mechanical fatigue. The overlay thickness for the shell is 4 to 8 millimeters, applied to the bottom section and the lower half of the side walls.

Welding Procedure and Quality Control

The welding procedure for overlaying lime digester components is developed considering the base material composition, component geometry, and service requirements. The stirring shaft is typically made of 45 steel or 42CrMo, while the digester shell is made of Q235 or Q345 steel. The welding procedure includes preheating, welding parameters, interpass temperature control, and post-weld treatment.

Quality control measures include visual inspection of the overlay surface for cracks, porosity, and undercut, ultrasonic testing of the overlay-base metal interface for lack of fusion, and hardness testing of the overlay to verify the achieved hardness level. The overlay surface should be smooth and free of significant defects, with a maximum allowable defect size of 2 millimeters in any dimension. The hardness should be uniform across the overlay surface, with a variation of no more than 5 HRC from the specified value.

Field Performance and Economic Analysis

The overlay solution is implemented in a lime production plant with two digesters, each with a capacity of 20 cubic meters. The stirring shaft blades are overlay-treated with the high-chromium cast iron overlay, and the digester shell bottom section is overlay-treated with the lower-alloy overlay. The overlay is applied during scheduled maintenance shutdowns, with a total downtime of 48 hours per digester.

The service life of the overlay-treated components is monitored over a period of 12 months. The stirring shaft blades achieve a service life of 18 to 24 months, compared to 3 to 6 months for uncoated blades, representing a 4 to 6-fold improvement. The digester shell bottom section achieves a service life of 24 to 30 months, compared to 6 to 12 months for uncoated shell, representing a 2 to 3-fold improvement. The overlay surface shows no significant cracking or delamination after 12 months of service, indicating good metallurgical compatibility and residual stress control.

The economic analysis shows that the overlay treatment cost is approximately 15 to 20 percent of the cost of replacing the entire stirring shaft or shell section. The extended service life reduces the frequency of maintenance and replacement, resulting in a net cost savings of 50 to 60 percent over a 2-year period. The reduced downtime for maintenance also contributes to improved plant availability and production output.

Summary and Study Insights

The application of wear-resistant overlay welding to lime digester shells and stirring shafts demonstrates the effectiveness of overlay technology in extending the service life of critical industrial equipment. The success of the solution depends on proper material selection, welding procedure development, and quality control. The field performance confirms that overlay-treated components can achieve 2 to 6 times the service life of uncoated components, providing significant economic benefits. Engineers should consider overlay technology as a viable alternative to complete equipment replacement for components subjected to severe wear and corrosion, particularly when the base material is structurally sound but the surface is degraded. The methodology presented in this case study can be adapted to other industrial applications with similar wear and corrosion conditions.