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As the supporting and pressure-bearing component of a hydraulic cylinder, the cylinder barrel is a key part. For specific operating conditions, the inner bore of the cylinder barrel needs to be electroplated with chromium. In actual electroplating, we encountered cylinder barrels made of 27SiMn material. The pass rate for inner bore electroplating using the boring process was very low, while the pass rate for inner bore electroplating using the honing process reached over 95%. Therefore, we analyzed the characteristics of 27SiMn material, the impact of the two processing methods (boring vs. honing) on the surface condition, and the key factors affecting the adhesion of the electroplated layer.
I. Surface Roughness and Morphology:
1.1 Boring:
The surface texture created by boring is typically a continuous, highly directional spiral or concentric circle. The valleys of this texture are sharp, and the texture direction is unidirectional. During electroplating, the plating metal tends to preferentially deposit and grow rapidly at the sharp valleys and edges, easily leading to stress concentration in these locations. Simultaneously, this continuous, highly directional texture is detrimental to the microscopic “anchoring” (mechanical interlocking) of the plating metal.
1.2 Honing:
The surface formed by honing has a cross-hatched structure. It consists of numerous small, directional, relatively uniformly deep grooves. This structure has the following advantages:
- Larger actual surface area: Provides more adhesion points for the plating.
- Better “anchoring” effect: The cross-hatched grooves act like countless small hooks, firmly gripping the plating metal, significantly enhancing mechanical interlocking force.
- Stress Dispersion: Uniformly distributed trenches help disperse the internal stress of the coating, avoiding excessive stress in local areas that could lead to peeling.
II. Surface Altered Layer and Residual Stress:
2.1 Boring: Boring (especially at high speeds, dry, or with insufficient lubrication) generates significant cutting heat and mechanical stress, particularly since current boring processes primarily employ a combination of boring and roll burning. This easily leads to:
● Work hardening: Severe plastic deformation of the surface metal increases hardness, but brittleness may also increase.
● Microcracks: Microcracks are easily generated in sharp valleys or under the hardened layer.
● Residual tensile stress: Cutting heat and plastic deformation typically generate residual tensile stress on the surface. Tensile stress reduces the fatigue strength of the material, and more importantly, it weakens the adhesion between the plating and the substrate. The electroplated layer itself usually has tensile stress, and the tensile stress on the substrate surface is superimposed on it, making the bonding interface more prone to failure (peeling).
2.2 Honing: Honing is a low-speed, low-pressure, multi-edge grinding process.
- Low cutting heat: Sufficient coolant is typically used, resulting in a lower temperature rise.
- Shallow and slight alteration layer: The degree of plastic deformation and hardening of the surface layer is much lower than in boring.
- Residual compressive stress: The honing process typically introduces beneficial residual compressive stress into the surface layer. This compressive stress helps close microcracks, improves fatigue strength, and is highly beneficial for improving coating adhesion because it resists the tensile stress within the coating itself, resulting in a more stable bonding interface.
III. Surface Cleanliness and Activity:
3.1 Boring:
Bore surfaces may retain more cutting fluid, grease, metal shavings, or a very thin layer of oxidation or deterioration caused by high temperatures. If these contaminants or oxide layers are not thoroughly removed, they will become a barrier between the plating and the base metal, severely affecting adhesion. The sharp texture of the bored surface also makes it easier to trap contaminants, making cleaning more difficult.
3.2 Honing:
The honing process uses a large amount of cooling lubricant, which itself has a certain flushing effect. The cross-hatched structure after honing is relatively easier to penetrate and rinse away by the cleaning fluid. The slight deterioration layer produced is also easier to remove effectively in subsequent pretreatments (such as pickling and activation), exposing a fresh, active metal surface.
IV. 27SiMn Material Properties:
27SiMn is a medium-carbon alloy structural steel containing silicon and manganese. Silicon readily forms silicon oxides (SiO2 or silicates) on the surface. This oxide is very stable, dense, and inert.
4.1 Boring:
High temperatures more readily promote the formation or thickening of this surface silicon oxide. If this inert oxide layer is not thoroughly removed during pretreatment, it will severely hinder the formation of strong metallic bonds between the plating metal and the base metal (which is fundamental to plating adhesion).
4.2 Honing:
Lower temperatures and effective cooling reduce this oxidation tendency. The relatively “fresh” surface and easier-to-clean texture after honing allow pretreatment (especially strong acid pickling activation) to more effectively remove this silicon oxide layer, exposing the active iron substrate.
V. Summary:
Honing, compared to boring, provides a superior surface finish for 27SiMn internal hole electroplating:
- Optimized surface morphology: The cross-hatched structure provides excellent mechanical interlocking (anchoring), increasing the bonding area and dispersing stress.
- Favorable residual stress state: Beneficial residual compressive stress is typically present on the surface, enhancing the stability of the bonding interface.
- Cleaner, more active surface: Contaminants and critical harmful silicon oxides are more easily and thoroughly removed, ensuring a strong metallurgical/chemical bond between the plating and the fresh base metal.
- Less surface damage: Reduces defects such as work hardening and microcracks.
In contrast, boring produces highly directional, sharp-valved textures, potential residual tensile stress, a more difficult-to-clean surface, easier retention of contaminants/oxides (especially silicon oxides), and potential micro-damage, all contributing to insufficient plating adhesion and severe peeling problems.
VI. Recommendations:
6.1 Honing is the preferred method: For 27SiMn internal hole plating requiring high adhesion, honing is the strongly recommended surface finishing method.
6.2 If boring is necessary: If boring is the only option for some reason, the following measures must be taken to improve the situation:
- Optimize boring parameters: Reduce cutting speed and feed rate, use sufficient and effective coolant, minimize cutting heat and plastic deformation, and reduce residual tensile stress and modified layer thickness.
- Fine boring: Achieve a finer surface roughness (although still not as good as honing), reducing sharp valleys.
6.3 Enhanced pretreatment: This is the most critical remedial measure!
- Thorough degreasing: Use strong cleaning agents (possibly including ultrasonic cleaning) to ensure the removal of all grease and cutting fluid residue.
- Intensive acid pickling activation: Due to the silicon oxide problem of 27SiMn, stronger or longer acid pickling (such as hydrochloric acid or mixed acids) is required compared to ordinary carbon steel. Ensure effective removal of surface oxides and altered layers, exposing a fresh, active metal surface. Closely monitor the pickling effect and time to avoid over-corrosion.
- Activation Process: Immediately after pickling, perform activation (e.g., weak acid etching) to remove any thin passivation film that may form, maintain high surface activity, and proceed with electroplating as soon as possible.
- Consider Sandblasting: Fine sandblasting (e.g., using fine glass beads or alumina abrasive) after boring and before electroplating can improve surface texture (increasing roughness and disorder, simulating partial honing effects), remove some altered layers, and may introduce beneficial compressive stress. However, sandblasting parameters must be controlled to avoid excessive roughness or embedding of foreign objects.
6.4 Electroplating Process Optimization: Work closely with the electroplating supplier. Adjustments to pre-plating (e.g., pre-plating nickel) or main plating process parameters (e.g., current density, temperature) may be necessary to suit the characteristics of the boring surface.
In summary, honing has significant advantages in improving the adhesion of electroplating in the inner holes of 27SiMn and preventing peeling, mainly due to the unique and favorable surface physical and chemical states it produces. If boring must be used, extremely strict control of the machining process and subsequent pretreatment processes is required.
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