35CrMoA alloy steel pipes are widely used in critical 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 does not 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
Principle: Using high-speed jets of sand or steel shot to impact the surface of the 35CrMoA alloy steel pipe.
Functions:
1) Rust removal: Efficiently removes surface oxide scale and rust.
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
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: Sufficient passivation and neutralization treatment must be performed after pickling to prevent “rust re-emergence” and remove 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
Principle: A zinc layer is plated onto the surface of 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 (60-80μm or more), with excellent durability, but the surface is not smooth enough and may produce “zinc tears”.
2) Electro-galvanizing: Zinc is plated through an electrolytic reaction. The coating is thin (5-25μm), with a smooth and uniform surface, but the corrosion protection life is not as good as hot-dip galvanizing.
Applications: Widely used in outdoor structural components such as construction scaffolding, guardrails, and power transmission towers.
2. Phosphating
Principle: The 35CrMoA alloy 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 degree of rust prevention.
2) Improved Coating Adhesion: The porous structure of the phosphate coating makes it a perfect base layer for paint. The combination of phosphate and coating (phosphating + coating) is a classic anti-corrosion system.
3) Friction Reduction: The phosphate coating has lubricating properties and is commonly used for parts requiring cold forming or friction reduction, such as engine crankshafts and connecting rods.
3. Blue/Blackening
Principle: Through the chemical oxidation of alkaline or acidic solutions, a dense layer of iron(II, III) oxide (Fe₃O₄) is formed on the surface of 35CrMoA alloy steel pipe.
Function: This film is blue or black, aesthetically pleasing, and provides slight corrosion resistance. However, its protective ability is relatively weak, and an additional coating of rust-preventive oil is usually required to enhance its effect.
Applications: Primarily used in applications 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 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
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.
2. 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 high wear resistance, such as crankshafts, molds, and precision machine tool spindles.
Machining: Achieving precise dimensions and surface finish.
For 35CrMoA alloys requiring precision fit (such as hydraulic cylinder barrels), the following machining processes are performed:
1) Precision turning/boring: Ensuring dimensional accuracy and cylindricity of inner/outer circles.
2) Grinding: Obtaining extremely high dimensional accuracy and extremely low surface roughness.
3) Lapping/honing: As a final process, it achieves a mirror-like surface finish (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, Ultimate Protection and Decoration for 35CrMoA Alloy Steel Pipes
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.
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.
Q1: Why is surface finishing important for 35CrMoA alloy steel pipes?
A: Surface finishing is essential for improving corrosion resistance, wear resistance, and overall product appearance. Proper surface treatment also helps enhance coating adhesion, reduce surface defects, and extend the service life of 35CrMoA alloy steel pipes in demanding industrial environments.
Q2: What surface finishing methods are commonly used for 35CrMoA alloy steel pipes?
A: Common surface finishing methods include shot blasting, sand blasting, polishing, pickling, oil coating, painting, black painting, varnish coating, and anti-corrosion coating. The suitable finishing method depends on the application environment and customer requirements.
Q3: How does shot blasting benefit 35CrMoA alloy steel pipes?
A: Shot blasting effectively removes rust, oxide scale, and surface impurities while improving surface cleanliness and roughness. This process enhances coating adhesion and prepares the steel pipe surface for further anti-corrosion treatment or painting.
Q4: What should be controlled during the surface finishing process?
A: Key factors such as surface cleanliness, roughness, coating thickness, drying conditions, and environmental humidity should be strictly controlled. Improper handling may lead to coating peeling, uneven surfaces, or reduced corrosion resistance.
Q5: What inspections are required after surface finishing?
A: After finishing, the steel pipes should undergo visual inspection, coating thickness testing, adhesion testing, surface roughness inspection, and anti-corrosion performance checks to ensure the final product meets project specifications and international quality standards.
Post time: May-28-2026