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).

References :

  1. Griffith, A.A., Phil. Trans. Royal Society A221 (1921).
  2. Wells, A.A., Unstable Crack Propagation in Metals: Cleavage and Fast Fracture, Proc. Symp. Crack Propagation, Cranfield 1961, Vol. 1.
  3. E 813-81 Standard Test Method for JIC, A Measure of Fracture Toughness, ASTM 1981.
  4. Method for crack opening displacement testing, BS5762, British Standard Institution, London 1979.
  5. Methods of tests for plain strain fracture toughness (KIc) of metallic materials, BS5447, British Standard Institution, London 1977.
  6. Milne, I. et al, Assessment of the integrity of structures containing defects, CEGB-R/H/R6-Rev. 3, Central Electricity Generating Board, London, 1986.
  7. Guidance on Methods for assessing the acceptability of flaws in fusion welded structures, PD 6493: British Standards Institution, London 1991.
  8. Kumar, V. et al, An Engineering Approach for Elastic-Plastic Fracture Analysis, Electric Power Research Institute (EPRI), NP 1931, Project 1237-1, Final Report, General Electric Company, New York.
  9. Dahl, W. et al, Application of Fracture Mechanics Concepts to the Failure of Wide Plates, Nuclear Engineering Design 1985.

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