35CrMoA alloy steel pipes are widely used in key mechanical structural components due to their high strength, high temperature resistance, and good hardenability. However, the surface of 35CrMoA alloy steel pipes at the factory often cannot directly meet the final use requirements. Different surface treatment processes for 35CrMoA alloy steel pipes not only change their appearance but also aim to prevent corrosion, improve wear resistance, enhance mechanical properties, or adapt to specific working conditions.
First, basic cleaning and pretreatment of 35CrMoA alloy steel pipes.
Before any deep processing, the surface of the 35CrMoA alloy steel pipe must be cleaned to lay a good foundation for subsequent processes.
1. Sandblasting/Shot blasting
Principle: High-speed jetting of sand or steel shot impacts the surface of the 35CrMoA alloy steel pipe.
Function:
(1) Rust removal: Efficiently removes oxide scale and rust from the surface of the 35CrMoA alloy steel pipe.
(2) Slag Removal: Clean the weld bead and the surrounding area of the 35CrMoA alloy steel pipe after welding.
(3) Surface Roughening: Obtain a uniform and consistent micro-roughness, greatly increasing the surface area and improving the adhesion of subsequent coatings (such as paint).
(4) Stress Relief: Simultaneously relieves slight surface stress on the 35CrMoA alloy steel pipe.
Result: Obtain a matte, uniform silver-gray metallic surface.
2. Pickling
Principle: Immerse the 35CrMoA alloy steel pipe in an acidic solution (such as hydrochloric acid or sulfuric acid), dissolving the oxide scale and rust through a chemical reaction.
Function: Thoroughly removes rust from corners that are difficult to reach with sandblasting, resulting in a very clean metallic surface of the 35CrMoA alloy steel pipe.
Note: Sufficient passivation and neutralization treatment must be performed after pickling to prevent “re-rusting” and eliminate residual acid. For high-strength steels like 35CrMoA, the pickling time must be strictly controlled to prevent the risk of hydrogen embrittlement.
Second, improving the corrosion resistance and aesthetics of 35CrMoA alloy steel pipes.
This is a crucial step in determining the environmental adaptability and lifespan of 35CrMoA alloy steel pipes.
1. Galvanizing
Principle: A zinc layer is plated onto the surface of the 35CrMoA alloy steel pipe, forming a dual physical and electrochemical protection.
(1) Hot-dip galvanizing: The 35CrMoA alloy steel pipe is immersed in molten zinc. The coating is thicker (60-80μm or more), with excellent durability. However, the surface of the 35CrMoA alloy steel pipe is not smooth enough, which may produce “zinc tears”.
(2) Electro-galvanizing: Zinc is plated through an electrolytic reaction. The coating is thinner (5-25μm), and the surface of the 35CrMoA alloy steel pipe is smooth and uniform. However, the corrosion resistance life of the 35CrMoA alloy steel pipe is not as good as that of hot-dip galvanizing.
Applications: 35CrMoA alloy steel pipes are widely used in outdoor structural components such as building scaffolding, guardrails, and power transmission towers.
2. Phosphating
Principle: 35CrMoA alloy steel pipe is immersed in a phosphating solution, and a water-insoluble phosphate crystal film is formed on the surface of the pipe through a chemical reaction.
Functions:
(1) Improved corrosion resistance: 35CrMoA alloy steel pipe has a certain rust-proof ability.
(2) Improved coating adhesion: The porous structure of the phosphate film makes it a perfect base layer for paint. The combination of the two (phosphating + coating) is a classic anti-corrosion system.
(3) Friction reduction: The phosphate film has lubricating properties and is often used for parts such as engine crankshafts and connecting rods that require cold forming or friction reduction.
3. Blue/Blackening
Principle: Through the chemical oxidation of alkaline or acidic solutions, a dense layer of iron(II, III) oxide (Fe₃O₄) film is formed on the surface of the 35CrMoA alloy steel pipe.
Functions: This film is blue or black, aesthetically pleasing, and can improve slight corrosion resistance. However, 35CrMoA alloy steel pipes have relatively weak protective capabilities and usually require additional rust-preventive oil coating to enhance their effectiveness.
Applications: Primarily used in applications where corrosion resistance is not critical but aesthetics and anti-reflective properties are important, such as instruments, firearms, and small hardware components.
Third, Achieving Special Functions and Ultimate Performance of 35CrMoA Alloy Steel Pipes.
These processes aim to further enhance the surface hardness, wear resistance, or impart special properties to 35CrMoA alloy steel pipes.
1. High-Frequency/Medium-Frequency Surface Quenching
Principle: Rapidly heating the surface of the 35CrMoA alloy steel pipe using induced current, followed by rapid cooling (quenching).
Function: This process imparts extremely high hardness (HRC 50-60) and wear resistance only to the surface layer (2-10mm) of the 35CrMoA alloy steel pipe, while the core retains its original toughness and plasticity. This is a typical process that fully utilizes the hardenability advantage of 35CrMoA alloy steel pipes.
Applications: Manufacturing components subjected to friction and impact, such as hydraulic supports, gears, and shafts.
2. Nitriding
Principle: At a specific temperature (480-580℃), active nitrogen atoms are infiltrated into the surface of the 35CrMoA alloy steel pipe.
Function: Forms an extremely hard, wear-resistant, and corrosion-resistant nitride layer. Low processing temperature results in minimal deformation.
(1) Gas nitriding: High hardness, deep layer, but long processing time.
(2) Ion nitriding: Less deformation, environmentally friendly, and currently the mainstream trend.
Applications: For parts requiring high-dimensional accuracy and high wear resistance, such as crankshafts, molds, and precision machine tool spindles.
3. Precision Machining: Achieving Precise Dimensions and Surface Finish
For 35CrMoA alloy steel pipes requiring precision fitting (such as hydraulic cylinder barrels), the following machining processes are performed:
(1) Precision turning/boring: Ensures the dimensional accuracy and cylindricity of the inner/outer circles.
(2) Grinding: Obtains extremely high dimensional accuracy and extremely low surface roughness.
(3) Grinding/Honing: As the final process, it achieves a mirror-like smooth surface (roughness Ra can reach below 0.2μm), while forming a cross-hatching pattern conducive to oil storage, particularly suitable for the inner wall of hydraulic cylinders.
Fourth, Ultimate Protection and Decoration of 35CrMoA Alloy Steel Pipes
1. Paint Spraying: The most common and economical method of protection and decoration. Primer, intermediate coat, and topcoat can be selected as needed to form a composite coating system, providing long-term corrosion protection.
2. Electrostatic Powder Coating: Powder coating is electrostatically adsorbed onto the surface of the 35CrMoA alloy steel pipe, and then cured by baking. The coating is uniform, strong, beautiful, solvent-free, and more durable than ordinary paint.
Surface processing of 35CrMoA alloy steel pipes is a systematic project. The choice of process depends on your careful consideration of corrosion resistance, wear resistance, dimensional accuracy, aesthetics, and cost.
Post time: Mar-31-2026