Replacing Sintered Alumina Surface on Plug Valve Heads with Hardfacing

Literature Overview

This technical paper published in Petrochemical Equipment Technology (1991, Vol. 12, No. 3, pp. 58–59) by Wang Qingsheng and Wei Jun from China Petrochemical Corporation's Linyuan Refinery documents a failure analysis and repair solution for plug valves in a heavy oil co-axial catalytic cracking unit. The case study provides valuable insights into tribological failure mechanisms in petrochemical equipment and demonstrates the practical application of hardfacing technology as a replacement for ceramic overlay solutions.

Failure Analysis

The Linyuan Refinery operated a 60 × 10⁴ t/a heavy oil co-axial catalytic cracking unit equipped with two plug valves controlled by pneumatic motors. These valves were installed at a lower position, facilitating easy operation and maintenance. However, within less than two months of trial production, operators discovered that the catalyst circulation rate could not be controlled properly — even with the plug valve in the fully closed position, a significant flow of catalyst continued.

Upon inspection during a unit shutdown, the following damage was found:

Component	Damage Description
Plug valve head	Sintered alumina surface largely loosened and detached
Contact ring of valve head	Alumina completely lost at the sealing contact area
Valve seat	Partial detachment of alumina lining
Result	Gap between plug and seat allowing catalyst flow

The root cause of failure was identified as the mechanical degradation of the sintered alumina coating under the combined effects of:

Abrasive wear from circulating catalyst particles (silica-based with sharp angular morphology).
Thermal cycling from the hot catalyst stream (temperatures typically 500–700 °C).
Mechanical impact during valve opening and closing cycles.
Thermal expansion mismatch between the alumina coating and the metal substrate.

Hardfacing Solution Design

The engineering team decided to replace the sintered alumina surface with a hardfacing overlay deposited directly onto the plug valve head and valve seat. The key design considerations included:

Material selection — A high-chromium, high-carbon martensitic hardfacing alloy was selected for its excellent resistance to dry abrasive wear from ceramic catalyst particles. The overlay hardness target was ≥ 55 HRC.
Geometric design — The hardfacing overlay was applied to the sealing contact surfaces of both the plug and the valve seat, ensuring a metal-to-metal sealing interface with controlled clearance.
Welding process — Submerged arc welding or flux-cored wire welding was employed to achieve uniform overlay thickness and minimize defects.

Process Parameter	Specification
Hardfacing material	High-Cr, high-C martensitic alloy
Overlay hardness	≥ 55 HRC
Overlay thickness	3–5 mm
Welding process	SAW or FCAW
Preheat temperature	200–300 °C
Interpass temperature	≤ 250 °C
Post-weld treatment	Stress relief at 550–600 °C

Performance and Reliability Assessment

The hardfaced plug valve heads and seats demonstrated superior performance compared to the original sintered alumina solution:

The metal-to-metal sealing interface maintained its integrity over extended operating periods.
The hardfacing overlay resisted abrasive wear from catalyst particles without spalling or delamination.
The thermal expansion compatibility between the hardfacing alloy and the base steel prevented the coating detachment observed with the ceramic solution.
Maintenance intervals were significantly extended, reducing unplanned shutdowns.

Key Engineering Lessons

This case study provides several important lessons for engineers working with abrasive service equipment:

Ceramic coatings are not universally superior — While sintered alumina offers excellent hardness (90+ HRA), its brittleness and poor thermal shock resistance can lead to catastrophic failure in dynamically loaded applications.
Metal hardfacing offers better toughness — The ductile nature of the martensitic overlay allows it to accommodate thermal cycling and mechanical impacts without cracking or delaminating.
System-level thinking is essential — The failure was not simply a materials selection issue but resulted from the interaction of multiple factors including thermal cycling, mechanical loading, and tribological conditions.
Repairability is a critical design consideration — Hardfaced surfaces can be ground and re-welded during maintenance, whereas damaged ceramic coatings often require complete component replacement.

Conclusion

The replacement of sintered alumina surfaces with hardfacing overlays on plug valve heads and seats in a heavy oil catalytic cracking unit represents a successful engineering solution to a tribological failure problem. The hardfacing approach provided superior durability, thermal compatibility, and repairability compared to the original ceramic solution. This case study underscores the importance of holistic failure analysis and the practical advantages of metal hardfacing technology in petrochemical equipment maintenance.