Strengthening and Toughening Mechanism of Steel - Lab Report Example

Eight different TM conditions per alloy can be obtained. The main focus of this experiment was to investigate the characterization of austenite structure before the transformation. This investigation is carried out concerning the grain-size distribution. The outcome of this investigation shows that different ratios of picric and hydrochloric acids were required by different alloys and different thermomechanical treatments. This can be controlled by carefully measuring the pH- value. Etching techniques and quantitative metallography is used to characterize transformation structure.

Apart from this, it is also necessary to investigate the precipitation behavior during thermomechanical treatment. This helps explain the structural changes caused due to recrystallization.
The mechanical properties of these specimens are investigated by performing tensile tests and Charpy V notch tests.

The results indicate that steel A, B, and D are of the same strength, whereas steel C demonstrates less strength. This different behavior is due to the structural differences. It is also found that the transformed structure consists of bainite and ferrite. Where the ferrite content in steel A, B, and D is 40%, the ferrite content in steel c is 50%.  The ferrite grains are much coarser which is inherited from the coarser austenite structure.

Now the steel samples are compared concerning low-temperature toughness.

High impact energy values are found with steel A and B up to -90·C. This is caused by small and homogeneous austenite grains. There are differences in recrystallization behavior which makes it necessary to investigate the precipitation process and its influence on recrystallization.

The examination of steel B exhibits the following results: some Al-rich particles were found after a short holding time. At longer holding V-rich particles are found. In V-Nb-Ti steel, the Nb precipitates appear later and in lesser amounts compared to Ti-free steel. The different recrystallization of steel D and B indicates a strong tendency to recrystallization of V-Nb-Ti steel leading to an inhomogeneous austenite grain structure. 

Summary:

All four steel specimens had undergone TM rolling treatment with varying conditions. The mechanical properties are related to the microstructures. The formation of these structures can be explained with the help of the precipitation and recrystallization behavior of the austenite. Steel A and B gained the best overall properties in terms of strength and low-temperature ductility. Whereas, steel D exhibits good strength but low impact energy values at low temperatures when compared to steel A and B. The result of steel C is low in terms of strength and ductility due to its relatively coarse austenite structure.

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