What are the surface treatments 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 35CrMoA alloy 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, 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 of 35CrMoA alloy steel pipes.

Principle: Using high-speed jets of sand or steel shot to impact the surface of the steel pipe.

Functions:

(1) Rust removal: Efficiently removes oxide scale and rust from the surface.

(2) Weld slag removal: Cleans the weld bead and surrounding area after welding.

(3) Surface Roughening: Achieves a uniform, fine roughness, significantly increasing surface area and improving the adhesion of subsequent coatings (such as paint).

(4) Stress Relief: Simultaneously relieves minor surface stress.

Result: Obtains a matte, uniform silver-gray metallic surface.

2. Pickling of 35CrMoA Alloy Steel Pipes

Principle: Immersing the 35CrMoA alloy steel pipe in an acidic solution (such as hydrochloric acid or sulfuric acid) dissolves oxide scale and rust through a chemical reaction.

Function: Thoroughly removes rust from corners difficult to reach with sandblasting, resulting in a very clean metal surface.

Note: After pickling, the 35CrMoA alloy steel pipe must undergo sufficient passivation and neutralization treatment to prevent "re-rusting" and eliminate residual acid. For high-strength steels like 35CrMoA, pickling time must be strictly controlled to prevent hydrogen embrittlement risk.

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 of 35CrMoA Alloy Steel Pipe

Principle: A zinc layer is plated onto the surface of the 35CrMoA alloy steel pipe, forming a dual protection of physical and electrochemical properties.

(1) Hot-dip galvanizing: The 35CrMoA alloy steel pipe is immersed in molten zinc. The coating is thicker (over 60-80μm) and has excellent durability, but the surface of the 35CrMoA alloy steel pipe is not smooth enough, which may cause "zinc tears".

(2) Electro-galvanizing: Zinc is plated through an electrolytic reaction. The coating is thin (5-25μm), and the surface is smooth and uniform, but the corrosion protection life 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 of 35CrMoA Alloy Steel Pipe

Principle: The 35CrMoA alloy steel pipe is immersed in a phosphating solution. Through a chemical reaction, a layer of water-insoluble phosphate crystal film is generated on the surface of the 35CrMoA alloy steel pipe. Functions:

(1) Improved corrosion resistance: 35CrMoA alloy steel pipes have a certain degree of rust prevention.

(2) Improved coating adhesion: The porous structure of the phosphate film makes it a perfect base layer for paint; the combination of the two (phosphate + coating) is a classic anti-corrosion system.

(3) Reduced friction: 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 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 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, Achieving Special Functions and Ultimate Performance in 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 of 35CrMoA Alloy Steel Pipes

Principle: Rapidly heating the surface of the 35CrMoA alloy steel pipe using induced current, followed by rapid cooling (quenching).

Function: Only the surface layer (2-10mm) of the 35CrMoA alloy 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 subjected to friction and impact, such as hydraulic supports, gears, shafts, etc.

Nitriding

Principle: At a specific temperature (480-580℃), active nitrogen atoms are diffused into the surface of the 35CrMoA alloy steel pipe.

Function: Forming an extremely hard, wear-resistant, and corrosion-resistant nitride layer. Low processing temperature, minimal deformation.

(1) Gas nitriding: High hardness, deep layer depth, but long cycle time.

(2) 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.

Machining: Achieving precise dimensions and surface finish.

For 35CrMoA alloy steel pipes requiring a precision fit (such as hydraulic cylinder barrels), the following machining processes are performed:

Precision turning/boring: Ensuring the dimensional accuracy and cylindricity of the inner/outer circles.

Grinding: Obtaining extremely high dimensional accuracy and extremely low surface roughness.

Grinding/Honing: As a final process, it can obtain a mirror-like smooth surface (roughness Ra can reach below 0.2μm), while forming a cross-hatching pattern that is beneficial for oil retention, especially suitable for the inner wall of hydraulic cylinders.

Fourth, Ultimate Protection and Decoration of 35CrMoA Alloy Steel Pipes

1. Paint Spraying of 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 of 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.