Heat treatment and rust removal methods for boiler tubes

Boiler steel pipes are classified into medium-pressure and high-pressure types, and are predominantly manufactured using seamless processing techniques due to the demanding operating conditions. Welded steel pipes are generally not suitable for boiler applications.

These pipes are extensively used in:

Heat exchangers

Tube bundles

High-pressure boilers

Economizers

Superheaters

Piping systems in the petrochemical industry

Heat Treatment Processes for Boiler Pipes

Heat treatment is a metallurgical process that enhances the mechanical properties of high-pressure boiler pipes through controlled heating and cooling cycles. It improves the pipe’s internal microstructure to achieve desired qualities such as toughness, hardness, and wear resistance. The primary heat treatment methods include:

a. Quenching (Hardening)

Quenching involves heating the pipe evenly to a specific temperature, then rapidly cooling it in water, oil, or air, sometimes even in a cold medium. This process increases the pipe's hardness and strength, essential for high-pressure operation.

b. Tempering

After quenching, the pipe becomes brittle due to internal stress. Tempering is performed to reduce brittleness and improve toughness. While it slightly lowers hardness, it ensures the pipe is less prone to cracking or failure under mechanical stress.

c. Annealing

Annealing is used to relieve internal stress and improve the pipe’s ductility and machinability. This process involves heating the pipe to a critical temperature and then slowly cooling it in insulating materials such as dry ash, lime, or asbestos, or in a closed furnace to achieve uniform cooling.

Rust Removal Methods for Boiler Pipes

Rust removal is a critical step in the surface preparation of boiler pipes, especially before applying any anti-corrosion coatings. Common methods include:

a. Solvent Cleaning

Organic solvents or emulsions are used to remove oil, grease, dust, and lubricants from the surface. However, this method does not remove oxides, rust, or weld residues, and is typically used as a preliminary cleaning step.

b. Mechanical Tool Cleaning

Manual or powered tools (e.g., wire brushes) are used to remove loose rust, oxide layers, and welding slag.

Manual tools can achieve SA2 cleanliness level

Power tools can reach SA3, but may not remove tightly adhered oxide scales or provide sufficient surface roughness for coating adhesion.

c. Chemical Pickling

Chemical or electrolytic pickling is used to remove oxides and rust at a microscopic level. This method is effective but may require proper waste disposal and corrosion control measures post-treatment.

d. Abrasive Blasting (Spray Rust Removal)

Abrasive blasting uses high-speed abrasive impact to clean the pipe surface thoroughly. This method:

Removes all rust, oxides, and contaminants

Produces uniform surface roughness

Increases physical adsorption and mechanical bonding between the protective coating and the pipe surface
Thus, abrasive blasting is considered the most effective and preferred method for corrosion protection preparation.