What are the surface requirements for 35CrMoA alloy steel pipes

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 the steel pipes at the factory often cannot directly meet the final use requirements. Different surface treatment processes not only change their appearance but also aim to prevent corrosion, improve wear resistance, enhance mechanical properties, or adapt to specific working conditions.

First, what is the purpose of basic cleaning and pretreatment of 35CrMoA alloy steel pipes?
Before any deep processing, the surface of 35CrMoA alloy steel pipes must be cleaned to lay a good foundation for subsequent processes.
1. Sandblasting/Shot blasting treatment of 35CrMoA alloy steel pipes
Principle: High-speed jetting of sand or steel shot impacts the surface of 35CrMoA alloy steel pipes.
Functions:
(1) Rust removal: Efficiently removes oxide scale and rust from the surface of 35CrMoA alloy steel pipes.
(2) Weld slag removal: Cleans the weld bead and the surrounding area of 35CrMoA alloy steel pipes after welding. 
(3) Surface roughening: Obtains a uniform and consistent micro-roughness, greatly increasing the surface area of the 35CrMoA alloy steel pipe and improving the adhesion of subsequent coatings.
(4) Stress relief: Simultaneously eliminates slight surface stress.
Result: Obtains a matte, uniform silver-gray metallic surface.
2. Pickling treatment of 35CrMoA alloy steel pipe
Principle: Immersing the 35CrMoA alloy steel pipe in an acidic solution (such as hydrochloric acid or sulfuric acid) dissolves 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 metal surface.
Note: Sufficient passivation and neutralization treatment must be performed after pickling to prevent "rust return" 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, what are the requirements for improving the corrosion resistance and aesthetics of 35CrMoA alloy steel pipe?
This is a key step that determines the environmental adaptability and lifespan of 35CrMoA alloy steel pipe. 
1. Galvanizing Treatment of 35CrMoA Alloy Steel Pipe
Principle: A zinc layer is plated onto the surface of the steel pipe, forming a dual protection of physical and electrochemical properties.
Hot-dip galvanizing: The steel pipe is immersed in molten zinc, resulting in a thicker coating (60-80μm or more) with excellent durability, but the surface is not smooth enough and may produce "zinc tears".
Electro-galvanizing: Zinc is plated through an electrolytic reaction, resulting in a thinner coating (5-25μm) with a smooth and uniform surface, but its corrosion resistance is not as good as hot-dip galvanizing.
Applications: Widely used in outdoor structural components such as building scaffolding, guardrails, and power transmission towers.
2. Phosphating Treatment of 35CrMoA Alloy Steel Pipe
Principle: The steel pipe is immersed in a phosphating solution, and a layer of water-insoluble phosphate crystal film is formed on its surface through a chemical reaction.
Functions:
(1) Improves corrosion resistance: It has a certain rust-preventing ability. (2) Improved coating adhesion: The porous structure of the phosphate coating makes it a perfect base layer for paint. The combination of the two (phosphate coating + painting) is a classic anti-corrosion system.
(3) Friction reduction: The phosphate coating has lubricating properties and is often used for parts such as engine crankshafts and connecting rods that require cold forming or friction reduction.
3. What are the functions of the blue/blackening of 35CrMoA alloy steel pipes?
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 steel pipe.
Function: This film is blue or black, aesthetically pleasing, and can improve slight corrosion resistance. However, its protective ability is relatively weak, and it usually requires additional application of anti-rust oil to enhance its effect.
Applications: Mainly used in situations where corrosion resistance requirements are not high, but aesthetics and anti-reflective properties are desired, such as instruments, firearms, and small hardware parts.

Third, what are the special functions and ultimate properties 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 hardening treatment of 35CrMoA alloy steel pipes
Principle: Rapidly heating the surface of the steel pipe using induced current, followed by rapid cooling (quenching).
Function: Only the surface layer (2-10mm) of the steel pipe acquires extremely high hardness (HRC 50-60) and wear resistance, while the core retains its original strength, toughness, and plasticity. This is a typical process that fully utilizes the hardenability advantage of 35CrMoA material.
Applications: Manufacturing components that withstand friction and impact, such as hydraulic supports, gears, shafts, etc.
2. Nitriding treatment of 35CrMoA alloy steel pipes
Principle: At a specific temperature (480-580℃), active nitrogen atoms are diffused into the surface of the steel pipe.
Function: Forming an extremely hard, wear-resistant, and corrosion-resistant nitride layer. Low processing temperature, minimal deformation.
Gas nitriding: High hardness, deep layer depth, but long cycle time.
Ion nitriding: Less deformation, environmentally friendly, and currently the mainstream trend.
Applications: Parts requiring high-dimensional accuracy and wear resistance, such as crankshafts, molds, and precision machine tool spindles.
3. What are the finishing processes for 35CrMoA alloy steel pipes?
For 35CrMoA steel pipes requiring precision fit (such as hydraulic cylinder barrels), the following processes are performed:
Precision turning/boring: Ensures the dimensional accuracy and cylindricity of the inner/outer diameters.
Grinding: Achieves extremely high dimensional accuracy and extremely low surface roughness.
Grinding/honing: As a final process, it achieves a mirror-like smooth surface (roughness Ra can reach below 0.2μm), while forming a cross-hatched pattern conducive to oil retention, particularly suitable for the inner walls of hydraulic cylinders.

Fourth, what are the ultimate protection and decoration options for 35CrMoA alloy steel pipes?
1. Paint spraying for 35CrMoA alloy steel pipes: 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 for 35CrMoA alloy steel pipes: Powder coating is electrostatically adsorbed onto the surface of the 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.