Steel Bolts - Cutting, Grinding, Polishing

I am regularly asked to examine the micro structure of steel components, such as nuts, bolts, plates, etc. Often, this is just for quality control purposes, or to evaluate changes in production processes. Occasionally, litigation is involved.

The first step to the analysis is cutting the component (in this case, a mild steel bolt) to a suitable size. This is performed using an automated cutting machine. The bolt is carefully lined up with the cut-off wheel, and clamped in place. The cutting parameters (rotation speed, feed rate, type of wheel, etc) depends on many factors, such as the component thickness, material, and orientation. For this bolt, it is first cut length-wise, along the length of the threaded portion. The bolt is then rotated 90 degrees, and cut again to separate one part from the body of the bolt.

This sample from the steel bolt is now small enough to encase in epoxy resin, a process known as “mounting”. For robust components, such as steel bolts, this is primarily to make the sample easier to manage in subsequent processes. However for components which are delicate or complex, the epoxy reinforces the delicate parts, holding them in place for analysis.

The mounted steel bolt section then undergoes a grinding and polishing process. The cutting process can leave imperfections on the cut surface, and it would be inaccurate to examine or analyse this cut surface as-is. So we must remove this cut surface, and prepare a new surface. To assist in the microscopy analysis, we want this surface to be as perfectly flat and reflective as we can.

The grinding process is performed in a series of steps, using diamonds suspended in a special solution. The surface of the mounted steel bolt is pressed into the solution, and rotated at high speeds. The diamonds scratch into the surface, slowly removing material. Each step of the process uses smaller and smaller diamonds, to give a smoother and smoother finish. The final polishing step uses silica instead of diamonds, to achieve an incredibly smooth and reflective surface.

After that, our bolt sample is ready for the next step: microscopy analysis, which will be covered in the next case study. Subscribe to the news letter below to be notified when it’s available!