Surfacing Process for Catalyst Flue Gas Turbine Blade Room Outer Sleeve

Literature Overview

The 2009 paper by Luan Jiangfeng, Ding Qimin, and Na Qingling from Liaoning Petrochemical University, published in Welding Technology (Vol. 38, Issue 10, pp. 70-72), documents the engineering implementation of arc surfacing on the inner surface of a ZG1Cr18Ni9Ti cast stainless steel outer sleeve for a catalytic flue gas turbine blade room in a petroleum refining unit. This case study represents a practical repair engineering solution addressing a critical component in high-temperature catalytic cracking operations.

Application Background

Catalytic flue gas turbines (also known as waste heat boilers or catalytic cracking waste heat recovery units) operate under severe conditions:

The blade room outer sleeve, typically made of ZG1Cr18Ni9Ti (equivalent to ASTM CF8Ti), is exposed to the full severity of these conditions. The inner surface is subject to:

• Solid particle erosion from catalyst fines carried in the flue gas
• Thermal fatigue from cyclic temperature variations
• Oxidation and sulfidation corrosion at elevated temperatures
• Thermal stress cracking due to differential expansion

Welding Material Selection

The selection of E0-19-10Nb welding consumable follows the principle of matching the base metal composition while considering the service environment:

The use of Nb as a stabilizer instead of Ti is deliberate: Nb forms more stable carbides at elevated temperatures, providing superior resistance to intergranular corrosion during high-temperature service. The slightly higher Ni content in the consumable ensures full austenitic structure in the weld, preventing the formation of brittle delta ferrite.

Advanced Process Techniques

Variable Current Welding

The application of variable current (alternating between higher and lower current levels) serves multiple purposes:

1. Thermal input management: Prevents excessive heat accumulation in the local area, reducing the risk of thermal cracking and excessive grain growth.
2. Stress relief: The alternating thermal cycles promote stress relief through cyclic heating and cooling.
3. Microstructure refinement: The thermal cycling promotes recrystallization and grain refinement in the heat-affected zone.

External Edge Water Cooling

Water cooling applied to the external edge of the sleeve during welding serves to:

• Control the maximum temperature in the heat-affected zone
• Reduce the thermal gradient between the weld zone and the bulk material
• Minimize residual stress and distortion
• Prevent sensitization of the austenitic stainless steel

Segmented Reverse Welding

The segmented reverse welding technique involves:

• Dividing the circumference into segments (typically 60-90° each)
• Welding each segment in a specific sequence (e.g., opposite segments welded alternately)
• Reversing the welding direction for each segment

This approach:

• Balances thermal input around the circumference
• Minimizes circumferential distortion (ovalization)
• Controls the direction of residual stress vectors

Heat Input Control for Elliptical Deformation

The control of welding heat input to manage elliptical deformation is a sophisticated approach that considers:

• The geometric asymmetry of the sleeve (inner vs. outer surface)
• The differential thermal expansion between the weld zone and the base metal
• The constraint conditions of the component (typically clamped or supported)

By carefully controlling the heat input, the welding process can be designed to produce controlled, predictable deformation rather than random distortion.

Quality Assurance and Inspection

For this type of repair welding, the following quality assurance measures are essential:

1. Visual inspection: Verification of weld appearance, bead profile, and absence of surface defects.
2. Dye penetrant testing (PT): Detection of surface cracks, particularly at the weld toe.
3. Magnetic particle testing (MT): Not applicable to austenitic stainless steel; alternative methods required.
4. Ultrasonic testing (UT): Detection of subsurface cracks, lack of fusion, and porosity.
5. Dimensional verification: Confirmation that the sleeve maintains its geometric specifications after welding.

Engineering Practice Integration

This case study demonstrates several principles applicable to repair welding of high-temperature components:

1. Material matching with service consideration: The consumable is selected not only to match the base metal but also to improve performance in the specific service environment.
2. Process innovation for distortion control: The combination of variable current, water cooling, and segmented welding represents a sophisticated approach to managing thermal effects.
3. Repair as an opportunity for improvement: The repair welding can be designed to produce a surface with improved properties compared to the original casting.

Critical Reflections

The success of this repair welding depends on several factors that are not always clearly documented in case studies:

• Surface preparation: The condition of the inner surface before welding (removal of oxide scale, cleaning of contaminated areas) is critical but often underemphasized.
• Preheating and interpass temperature: While not explicitly mentioned, controlled preheating and interpass temperature are essential for preventing cold cracking in stainless steel welding.
• Post-weld heat treatment: For austenitic stainless steel, a solution treatment or stress relief may be required to restore full mechanical properties and remove residual stresses.

The case study provides valuable practical insights into the repair of critical high-temperature components in the petroleum refining industry, where downtime costs are extremely high and the consequences of failure can be severe.