Fig. 1. Effect of manganese on the shape of the austenite field[2]

With 50 different effects of manganese on steel, we will look at the first 25 this time and conclude next week.

The effects of manganese[2] can be summarized as:

1. Lowers the temperature at which austenite begins to decompose (Fig. 1)

2. Extends the metastable austenitic region and delays the commencement of all of the austenite decomposition reactions

3. Favors the formation of lower bainite and suppresses the upper-bainite reaction on isothermal transformation

4. Is the most effective alloying addition for lowering the martensite start (Ms) temperature

5. Favors the formation of e-martensite

6. Has little effect on the strength of martensite and on the volume change from austenite to martensite

Fig. 2. Manganese – carbon phase diagram at 1% carbon[2]

7. Has little or no solution-hardening effect in austenite and between 30-40 MN/m2 per wt.% in ferrite (by lowering the stacking-fault energy of austenite, manganese increases the work-hardening rate)

8. By lowering the Ms temperature, manganese prevents the deleterious effects of autotempering

9. Lowers the transformation temperature causing substantial grain refinement

10. In general, lowers the tough-to-brittle-impact transition temperature (due to its grain-refinement action)

11. Increases the propensity for weld cracking due to the effect on hardenability. The severity of its influence depends to a great extent on the type of steel and the welding techniques

12. Does not increase the susceptibility of the steel to delayed fracture due to hydrogen absorption

Fig. 3. Isothermal sections from the iron-manganese-carbon ternary diagram

13. Improves the fatigue limit

14. Reduces the number of cycles to failure under high strain conditions

15. Forms five carbides (Mn23C6, Mn7C, Mn3C2, Mn5C2, and Mn7C3), the dominant one being Mn3C, which forms a continuous range of solid solutions with Fe3C thus reducing the solubility of carbon in a-iron (Fig. Nos. 2, 3)

16. Prevents the formation of an embrittlement at the cementite grain boundary

17. Suppresses the yield extension in deep-drawing steels by virtue of its grain-refinement effect

18. Suppresses strain aging

19. In combination with nitrogen, has a solid-solution-hardening effect and improves high-temperature properties

Fig. 4. Effect of alloying elements on hardenability – Grossmann multiplying factors[2]

20. Extends the range of use of low-carbon steels. Manganese is usually used in amounts varying from 0.5-1.7 wt.% and is valuable to the production process because of the way it combines with oxygen and sulfur

21. Has a strong influence on the pearlite morphology of high-carbon steels

22. Extends the range of use of high-carbon steels through its grain refining and pearlite refining actions

23. Raises strength values in bainitic steels by reducing grain size and increasing dispersion hardening

24. Allows bainitic steels to be produced by air hardening

25. Increases hardenability (Fig. 4)