Executive Summary
Readers have seen a variety of interesting surface defects in their work. Solutions include strategies like material selection and surface treatments, as well as testing and choosing the correct lubricant or metalworking fluid (MWF). Scanning electron microscopy (SEM) is a frequent choice for analyzing surface defects, but readers have a range of tools they use depending on the application.
Q.1. Related to metal forming, what are the most interesting surface defects you have solved and how?
Pitting.
A client was experiencing failure of a diecasting plunger used for tin components. They moved to higher temperature tool steel only to find the problem worsened. SEM revealed a Swiss-cheese surface of the cobalt alloy. My father-in-law (a metallurgist) saw the image in my desk and told me the problem—dissolution of the cobalt by the molten tin. He also came up with the solution—use cast iron plungers. It worked perfectly!
Asperities on the inside of aluminum beverage cans. If they are too large, the coating applied to the inside does not cover them. At those points, the can may eventually leak. The lubricant being used needs more lubricity. A new type of semisynthetic coolant is designed to cover variations in the aluminum from batch to batch.
During my years as a mechanical engineer at a petroleum storage plant situated in Puerto Rico, I encountered various surface defects in metal forming processes. One of the most challenging yet interesting defects I dealt with was pitting corrosion among others on the storage tanks, pipes and pumps. To address these issues, we implemented a multi-faceted approach as: material selection, surface treatments (specialized coatings and cathodic protection were particularly effective), regular maintenance and inspections (rigorous maintenance schedule using non-destructive testing [NDT] methods such as ultrasonic testing).
Basic stuff like tears or oil dents.
Roll steel caused dulling and rougher metal surface, which didn’t allow our inspection lasers and autocollimators to get the proper reflected light signals for measurements during our processing. After finding out the rolled steel manufacturer changed their rolling process to new lubricant, we had them switch back and problem solved.
I had an interesting experience with stretching oils used in steel forming.
Lube stains on hot surfaces. It was solved by identifying the staining molecule and then adjusted the oil formulation.
Tears and scoring.
Removing sand residues after sandblasting of forgings along the production line.
Burst corners. Solved with lubricity additives.
Surface cracks and cavitations are some of the metal forming defects. Mark them using dye penetrants, then cut them out where the defect is or polish the surface if the defect is minor.
Poor surface finish.
Cracks and fractures. Implement proper material selection and heat treatment processes to enhance material ductility and toughness.
Scratches and seizure on stainless steel tubes when doing tube drawing with a chlorinated paraffins-free drawing fluid. The problem was solved by increasing the extreme pressure and lubricity properties of the MWF.
In your experience, issues with lubricants in metal forming are mostly related to:
Base lubricants
42%
Additives package
67%
Emulsifier package
17%
Based on an informal poll sent to 15,000 TLT readers. Total exceeds 100% because respondents were allowed to choose more than one answer.
With the help of semi-solid lubricant, grease and its type. Sometimes it didn’t work but with the help of carving methods and grease afterward.
1.) Raw material defects, 2.) chatter of the machine tool (on lathe or mill) or 3.) an inaccurate surface finish specification (or no surface finish specification). 1.) It is rare (we did not change vendors), 2.) change feed or speed or depth of cut or tool and 3.) change the drawing.
The tendency for formed parts to rust later. It’s solved with a better additive package.
In a previous job—steel foil surface “flap”-like defects that caused electrical shorts in solar cells.
Not so much a specific defect but a defect that suddenly occurs in what was a stable process. What suddenly caused the process to go wrong.
Q.2. When you encounter a new surface defect that you need to analyze, what is your favorite piece of analytical equipment?
High powered microscope.
SEM followed by 3D non-contacting profilometry. The SEM gives great 3D imaging with the ability to identify elements to confirm metal transfer particles or the presence or absence of a coating. Interferometry gives you the overall contact at low magnification with ability to quantify roughness parameters along lines of choice.
Microscope. I find that viewing the defects makes me think about what I am really dealing with.
When you hear about a new surface defect in a production line, what do you usually do first?
Literature review
24%
Simulation or analytical experiments
44%
Data analysis
52%
Seek information from others
28%
Based on an informal poll sent to 15,000 TLT readers. Total exceeds 100% because respondents were allowed to choose more than one answer.
Definitely the white light interferometer. It measures large lateral areas down to sub nanometer surface finishes ,which couldn’t have been solved with a stylus profilometer.
I was encountered with a press machine whose application speed had to be adjusted so as not to tear the metal surface.
Zoom-in approach: naked eye, microscope, SEM. The most relevant equipment is often the energy dispersive spectroscopy (EDS) probe in the SEM.
1.) SEM and 2.) X-ray diffraction for residual stress.
Optical microscope with a portable head.
Microscope.
Dye penetrant is the simplest effective and economical. Zinc additive in lubricating oil can leach soft metals such as copper, bronze and others.
Simulated tribotesting coupled with X-ray photoelectron spectroscopy (XPS) analysis.
Imaging techniques, spectroscopic techniques.
First of all, I use a microscope to identify the nature of the surface defect. In a second step I try to simulate the tribological conditions of the process by a suitable test machine.
Body and machine.
Microscope.
Analysis starts with a magnifying glass and the mind’s eye.
Confocal laser scattering microscopy.
If you can cut up the part small enough, a scanning electron microscope/energy dispersive X-ray spectrometer can provide an elemental analysis of a contaminant causing the surface defect.