Preceding sections have described the influence of the micro structure on strength and toughness using metallurgical mechanisms. Chemical and physical metallurgy can change microstructural characteristics so that optimum strength and toughness requirements may be obtained. By combining the various treatments it is possible to achieve a wide range of steel properties (Figure 13) :
- Chemical metallurgy treatments
Variation of the chemical composition of a steel by adding alloying elements aims to increase strength and/or increase resistance to brittle fracture. Solid solution hardening generally lowers toughness and is not widely employed.
Precipitation hardening also increases strength and decreases toughness. The addition of manganese and nickel produces a small increase in strength due to solution hardening but a more significant reduction is impact transition temperature due to grain refinement (Figure 14).
Alloying with the micro-alloying elements Niobium, (Nb) Vanadium (V) and Titanium (Ti) producing carbides and nitrides simultaneously raises strength by precipitation hardening and toughness by grain refinement. Decreasing the content of elements such as S and P improves the degree of purity, which has positive effects on toughness and weldability.
- Physical metallurgy treatments
The microstructure of a steel can be greatly affected by heat treatment or forming. Correctly chosen temperature, degree of deformation, time between deformation steps and cooling rate can reduce the grain size and control the state of precipitation, thus raising toughness and strength (Figure 15).
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