When the surface of cold-headed parts shows signs of scuffing, operators often request: "For the next batch of material, please make the phosphating coating a bit thicker."
Is this judgment reasonable? Yes. An insufficient coating can indeed reduce resistance to scuffing. However, attributing all scuffing issues solely to a "thin coating" may lead to misguided improvements.
The key is not how much protective layer was applied at the factory, but rather how much effective protection remains on the surface when it reaches the specific process step prone to scuffing.
I. How Does Scuffing Actually Occur?
The "scuffing" mentioned on-site may encompass various phenomena such as adhesive wear and scratches caused by hard particles; one cannot simply determine the cause based solely on a few grooves.
A typical case of adhesive wear occurs when the protective layer between the material and the die locally fails, resulting in direct metal-to-metal contact. Under high pressure and relative sliding, adhesion takes place, followed by tearing and detachment, leaving behind rough scuff marks.
Therefore, the first question should not be "How thick is the coating?" but rather: Why did the protection fail specifically at that location? The cold forging lubrication layer must both reduce friction and adhere firmly to the material while withstanding subsequent deformation. Research on Cold Forging Lubrication Layers
II. Phosphating and Lubrication Are Not the Same Thing
This discussion focuses on the common steel wire phosphating-soaping system.
Phosphating forms a conversion coating that provides an adhesion base for subsequent lubrication and helps isolate the material from the die; the metal soaps formed during the soaping treatment, along with the adhered soap layer, contribute to reducing friction. These two processes work together, and it's crucial not to evaluate only one component.
Making the phosphating layer thicker does not necessarily mean adequate soaping; having abundant soap on the surface does not guarantee its retention during deformation. Relevant studies also indicate that an excessively thick soap layer may accumulate on the die, affecting dimensional accuracy. Therefore, "making it a bit thicker" is not an unlimited solution. Research on Phosphating-Soaping Systems
III. Even with Qualified Coating Weight, Scuffing Can Still Occur
First, it's important to distinguish: coating weight refers to the mass per unit area, typically expressed in g/m², whereas coating thickness denotes geometric thickness, usually measured in μm. While these two parameters are related, they are distinct indicators.
A single coating weight measurement cannot simultaneously answer the following three questions:
Thus, assessing phosphating quality requires considering coverage, crystal morphology, and adhesion performance, rather than relying solely on a single weight value. Research on Evaluating Phosphating Layers for Cold Forming
- Is the coverage uniform? Meeting the average coating weight requirement does not ensure complete coverage across all areas.
- Is the adhesion strong enough? Having a coating does not guarantee it will remain intact during subsequent deformation.
- Is the lubrication effective? A qualified phosphating layer does not automatically imply proper soaping or sufficient lubrication coordination.
IV. What Is Most Often Overlooked Is the Difference Between "Condition During Testing" and "Condition During Use"
If the steel wire undergoes drawing after phosphating and soaping, then the coating weight measured immediately after these treatments does not directly reflect the surface condition when it enters the cold heading machine.
Similarly, multi-station cold heading involves multiple steps, with each station contributing to the overall deformation. Earlier stations stretch the surface and damage the coating, while subsequent stations continue pressing and sliding. Just because there was no scuffing in earlier stages does not mean sufficient protection remains for later operations.
This explains why the same surface treatment might perform adequately for one part type, yet prove inadequate for another part where surface expansion is greater and sliding distances longer. Studies evaluating multi-station lubrication performance specifically link early-stage surface stretching with late-stage scuff resistance. Evaluation of Lubrication Performance in Multi-Station Cold Forging
V. Does thickening the film really work?
It does, but only if the problem is indeed due to an insufficient effective film layer. We shouldn’t dismiss thickness as unimportant just to oppose the “thickness-only” argument.
A test conducted by Nippon Steel found that, under their testing conditions, a thicker residual phosphating-soap film exhibited better anti-adhesion performance; however, the friction coefficient did not decrease significantly with increasing thickness, and contact temperature and tool surface roughness also had a marked impact on performance.
This shows that “less prone to galling” and “lower friction” are not entirely the same thing, and thickness is not the sole determining factor. These experimental results cannot be directly translated into universal film requirements applicable to all steel wires and all parts. Nippon Steel Cold Forging Technology Report, Section 4.
VI. When galling occurs, how should material suppliers investigate?
We recommend focusing verification on three specific stages rather than simply extending the phosphating time:
Verification Stage What Needs to Be Clarified
Wire on the Production Line Whether the phosphating film weight, soap layer condition, coverage, and adhesion are abnormal under actual operating conditions
Samples from Each Station Determine at which stage galling first appears and what deformation processes the part underwent prior to that point
Normal Batch vs. Defective Batch Under comparable processing conditions, identify any repeatable differences in surface treatment status
If localized areas of incomplete coverage are detected, address the coverage issue first; if the film layer tends to peel off, check the pretreatment and film-forming quality; if the soap layer condition is abnormal, adjust the corresponding process steps. Only when evidence points to insufficient film weight should increasing the film weight become the primary focus of verification.
Verification should not merely involve “testing a few pieces to see if they turn out well.” Under comparable conditions, record the timing of galling onset, defect rates, and die adhesion during continuous production to confirm that improvements are sustainable.
Conclusion
Galling in cold heading can sometimes be caused by an insufficiently thick phosphating film, but this is not a conclusion we can draw simply upon observing galling.
As material suppliers, we need to deliver not just a qualified film weight value, but a surface treatment state that matches our customers’ forming requirements and functions reliably over time.
If the film needs to be reinforced, reinforce it; if coverage, adhesion, or lubrication issues need to be resolved, address those specific problems. The film must be correct—not just thick.