Heat-treated Materials & Metallographic Sample Preparation

Originating in the Bronze Age, heat treatment was further developed to process iron (blacksmithing) around 800 B.C., reaching a first peak with the production of Damascus steel in the beginning of the 10th century A.D. .

Iron was converted into steel by adding carbon, thus giving the alloy new properties like increased hardness. A large number of iron alloys and various elements have been investigated resulting in the development of new steel types suitable for many different applications. Another way to change the properties of steel is heat treatment, like annealing. As time plays a crucial role in this process, temperature-time diagrams have been developed. Today, many temperature-time diagrams have been evaluated to manufacture the best steel for a specific application. These diagrams show the correlation between time and heat and the respective structure (austenite, martensite, bainite) of the treated steel. Heat treatment is primarily performed in hardening shops. The materials treated in hardening plants are mostly the same and reproducible, reliable, and comparable results are mandatory. For effective process control a large number of samples has to be tested. Due to this high throughput, the sample preparation process should be simple and fast. Moreover, the preparation must result in excellent sample flatness to ensure easy and error-free surface inspection.

Modern Damascus steel knife

Preparation Methods

Method 1

This method has been well established for a long time. Due to the hardness of the heat-treated samples, the lifetime of the SiC Paper is quite short, resulting in relatively high costs for consumable consumption.

The preparation is as follows:

Method 2

Planar grinding with a stone offers the advantage of a fast and cost-efficient preparation. But this involves an investment for a special machine. Nevertheless, the overall preparation time is reduced as no SiC papers need to be changed. Additionally, diamond lapping discs can be used, which have a long lifetime.

To avoid the investment for a new machine, diamond grinding discs for planar grinding may be utilized offering the advantage of using standard grinding and polishing equipment.

Method 3

The Galaxy grinding disc is very stiff and yields plane surfaces even if the hardness of the sample surface varies.

Ferrite-Pearlite, etched with 3% Nital, brightfield 20x

Martensitic structure after heat treatment, etched with 3% Nital, brightfield 20x