
Strictly from a heat treater’s perspective, the purpose of adding alloying elements to steel is to enhance the material’s response to heat treatment, which in turn results in improvement of the mechanical and physical properties of the steel. Alloying additions can be made for one or more of the following reasons:
- To increase hardenability
- To help reduce part distortion
- To produce a finer grain size
- To improve tensile strength without appreciably lowering ductility
- To avoid quench cracking
- To gain toughness
- To achieve better wear resistance
- To improve hot hardness
- To achieve better corrosion resistance

1. Lowers the temperature at which austenite begins to decompose
2. Extends the metastable austenitic region and delays the commencement of all 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 ofe-martensite
6. Has little effect on the strength of martensite and on the volume change from austenite to martensite
7. Has little or no solution-hardening effect in austenite and between 30–40 MN/m2per wt. % in ferrite (by lowering the stacking-fault energy of austenite, manganese increases the work-hardening rate)
8. By lowering the MStemperature, 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
13. Improves the fatigue limit
14. Reduces the number of cycles to failure under high strain conditions
15. Forms five carbides (Mn23C5, Mn15C4, Mn3C, Mn5C2and Mn7C3), the dominant one being Mn3C, which forms a continuous range of solid solutions with Fe3C, thus reducing the solubility of carbon ina-iron
16. Prevents the formation of an embrittling grain-boundary cementite
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
20. Extends the range of use of low-carbon steels
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

27. Assists interphase precipitation
28. Improves austemper and martemper properties
29. Increases temper embrittlement unless the carbon content is very low and trace element impurities are minimal
30. In spring steels, promotes ductility and fracture toughness without undue loss in tensile strength
31. Removes the risk of hot shortness and hot cracking when the ratio of manganese to sulfur is greater than 20:1 by forming a higher melting-point eutectic with sulfur than iron sulphide
32. Has a major influence on the anisotropy of toughness in wrought steels due to the ability to deform manganese sulfides during hot working
33. Forms three manganese sulfide morphologies (Type I, II and III) dependent upon the state of oxidation of the steel
34. Enhances free-cutting steels
35. Increases the stability of austenite
36. Has similar atomic size as iron (Mn = 3.58Å, Fe = 3.44 Å)
37. Lowers the stacking-fault energy of austenite (in contrast to alloying element additions such as chromium or nickel)
38. Allows lower solution temperatures for precipitation-hardening treatments in highly alloyed austenite due to increased carbon solubility
39. Forms intermetallic compounds suitable for precipitation-hardened austenitic steels
40. Plays a major role in controlling the precipitation process that occurs during isothermal transformation to austenite
41. Increases the rate of carbon penetration during carburizing
42. Contributes, in combination with nitrogen, to the performance of work-hardenable austenitic stainless steels
43. Improves hot corrosion resistance in sulfurous atmospheres
44. Enhances wear-resistance in carbon-containing austenitic steels where the manganese content is between 12-14%
45. Improves response of low-alloy steels to thermomechanical treatments
46. Strengthens certain steels by maraging by producing an austenitic structure using manganese-containing compounds
47. Enhances the performance of TRIP steels
48. Promotes ferro-elastic behavior in appropriate steels
49. Less tendency to segregate within the ingot
50. In general, improves surface quality