Metallurgy is the study of structure property relationships of metallic materials.
All metal alloys have a unique microstructure. (See sample B) This microstructure is a direct function of the fabrication processes that the material has been subjected to. A qualified metallurgist can examine a microstructure and gain considerable information about how the metal has been processed. For example, if EN 19 (AISI 4130) has been heat treated, martensite will be seen under the microscope. If it has not been heat-treated, the ferrite and pearlite will reflect in the microstructure.
Duplex stainless steels should have a specific balance in austenite and ferrite in order for it to deliver on both the strength requirements and corrosion resistance requirements. See 1B3T-4
Only a qualified professional metallurgist can interpret microstructures correctly.
Microstructure is altered by welding for example. The localised melting at the joint, and development of the weld metal and the effect on the heat affected zone can all be interpreted by analysing the microstructure of the weld. In the case of duplex stainless steels, the austenite – ferrite phase balance must be maintained. In the case of carbon steels martensite formation should be limited and the heat input associated with welding should be controlled so that any martensite that does form is properly tempered by the process.
All metallic materials follow similar rules and principles in respect of alloying and heat treatment or process treatment. It the professional understanding of these principles that is the crux of metallurgical expertise and the skill of the metallurgist. An understanding that the structure of metal alloys can be changed and that an alloy can exists in many different forms is key to applying metals to engineering applications.
Further, a key to root cause failure investigation of metallic materials is a professional understanding of how these materials are affected at a structural level by processing or by application. There are many structural forms of metallic materials that should be avoided, that make the material brittle or unfit for purpose. This is the key role of the metallurgist is to guide engineers to avoid the material being in this form.
An example of a material having a microstructure that is not desired is shown below. The structure shows EN 24 (a low alloy steel) which has been heat treated for an extended period of time at a temperature which was too high. In this case the prior austenite grains have grown too large. On quenching and tempering the prior austenite grain size is still apparent making the material extremely brittle and nullifying the effect of the quench and temper heat treatment intended to make the steel strong but also tough.