Metallurgical Analysis - Part 3: A Step-by-Step Look at How Springs Are Tested

Written by Matthew Bridson, Metallurgist, IST

Once a spring has undergone its initial visual and dimensional inspection, the real detective work begins. Metallurgical analysis allows us to understand exactly what is happening inside the material from surface coatings to internal microstructures and ultimately determine why a spring performed the way it did. Below is an inside look at how this process unfolds.

Sectioning the Spring

Before testing can begin, the spring must be prepared. Depending on the spring’s diameter and length, it is sectioned either with hand snips or a precision cut-off machine. Several samples are taken:

The fracture faces
A longitudinal sample
A transverse sample
A sample for chemical analysis (if required)

Each cut is made slowly and under coolant. This prevents overheating, which can alter the material’s properties and compromise the analysis.

The fracture surfaces are then carefully cleaned in an ultrasonic acetone bath. This removes oils, dirt, or other contaminants that could interfere with SEM (Scanning Electron Microscopy) or EDX (Energy Dispersive X-ray) examination. Any powder-coat paint on or near the fracture face is removed so the underlying wire surface can be inspected for defects.

Preparing the coated sections unlike the fracture samples, the powder-coating is intentionally left intact on both the longitudinal and transverse sections. This allows us to measure the average thickness of the coating later in the process.

These directional samples are mounted in Bakelite using a heated mounting press. First, a fine powder is added to improve edge retention. A coarser Bakelite compound is added on top, and the mixture is compressed under heat and pressure to form a solid mount.

Grinding and Polishing to a Mirror Finish

With the samples mounted, the next stage is surface preparation. The goal: create a mirror-like finish suitable for microscopic examination.

We begin with coarse grinding pads, gradually working through progressively finer grit papers until reaching 1200 grit. The samples are then transferred to cashmere polishing pads and treated with diamond paste, finishing at an ultra-fine 1-micron polish.

A flawless surface is essential — any scratches or imperfections could mask or mimic true material defects.

Microscopic Examination: What We Look For

1. Longitudinal Sample

This sample is examined first. Under the microscope, we assess:

Inclusions within the material
Any unusual features, such as long or irregular inclusions
Whether the volume of inclusions is normal or excessive

These observations help us understand the steel’s cleanliness and the manufacturing conditions.

2. Transverse Sample

The transverse section provides multiple critical insights:

Coating Thickness

We measure the powder-coating thickness at five randomly selected points around the spring’s circumference. From these readings, an average thickness is calculated. Any inconsistencies — such as bare patches or unusually thick areas — are recorded.

Surface Defects

The transverse sample is also used to identify the largest surface defect present. These may include:

Corrosion pits
Peening laps
Tooling or manufacturing marks

Such defects can become crack initiation points, making them crucial for failure analysis.

Etching and Studying the Microstructure

After the initial examinations, the samples are etched in an acid appropriate for the material type. Etching reveals the microstructure, which can now be studied in detail.

We examine:

Uniformity from edge to centre
Presence of partial or full decarburisation
Any anomalies that indicate overheating, improper heat treatment, or material defects

This microstructural analysis often provides the deepest insight into the spring’s performance.