The Influence of Manganese on the Properties of Superalloys

What is Manganese

Manganese, the chemical symbol known as Mn and the atomic number as 25, is a grayish-white, hard-brittle and shiny transition metal. Pure manganese is slightly softer than iron while impure manganese containing a small amount of impurities is firm and brittle; moist will oxidize the latter. Manganese abounds in nature. Soil contains 0.25% manganese. Tea, wheat and hard-shell fruits also contain much manganese. Manganese-related works include gravel, mining, electric welding, production of dry batteries, and dye industry. In 1774, Gann separated metal manganese; Bergman named it manganese. Manganese can be obtained by aluminothermic reduction of pyrolusite. In this article, we will discuss in detail the role and influence of manganese in superalloys from 5 aspects.

	I																	O
1	1 H	II											III	IV	V	VI	VII	2 He
2	3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg	III	IV	V	VI	VII	VIII			I	II	13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Gc	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba	57-71 La-Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra	89-103 Ac-Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Uub

La-Lu		57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu
Ac-Lr		89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr

Item Name: Manganese
Element Symbol: Mn
Atomic Number: 25
Atomic Weight: 54.938
Atomic Radius: 127pm
Density: 7.3g/cm3
Melting Point: 1244°C
Boiling Point: 1962°C
Electronic Layout: [Ar]3d54s2

Figure 1
The Atomic Structure of Manganese

The Role of Manganese in Superalloys

Stabilize Austenite

The addition of manganese to the iron-chromium-nickel alloy can expand the γ zone and stabilize the austenite. In some early superalloys, a certain amount of manganese was added as an alloying element to stabilize the γ matrix. Such as Nitronic 50 (manganese content 4% ~ 6%) and Nitronic 60 (manganese content 7% ~ 9%).

Improve Oxidation Resistance

Manganese can increase the bonding force between the oxide film and the substrate, thereby improving the anti-stripping ability and improving the oxidation resistance. In some alloys with relatively few chromium elements, some manganese content can be added appropriately. For example, in Hastelloy B-3, insufficient chromium content will lead to limited oxidation resistance. Therefore, in alloy B-3, the manganese content is adjusted to below 3%, which can improve the oxidation resistance to a certain extent.

B-3

Improve Welding Performance

Manganese segregation in the welding melting zone can reduce the tendency of hot cracking. This allows some alloys that can be used as welding materials (such as Inconel 718 and Hastelloy X) to add a certain amount of manganese to improve welding performance.

718

As a Refining Agent

A small amount of manganese added to superalloys can be used as a refining agent. It can chemically react with sulfur to generate manganese sulfide, which can reduce the adverse effects of sulfur on the alloy.

The Influence of Manganese on Superalloys

In most superalloys, manganese is regarded as a harmful element.

Like other impurity elements, manganese also concentrates on the grain boundary, which weakens the bonding force of the grain boundary and significantly reduces the endurance strength.

Figure 2
The Effect of Chromium on Alloy Durability

It can be seen from the above figure that the increase of manganese has a significant effect on the durability of the alloy, but it has little effect on the instantaneous performance.

The manganese enriched on the grain boundary may also promote the precipitation of harmful phases, reduce mechanical properties, and produce obvious persistent gap sensitivity.

Therefore, for most superalloys, the content of manganese must be strictly controlled.

Conclusion

Manganese has both good and bad effects on superalloys.

Manganese can stabilize austenite, improve oxidation resistance, improve welding performance, and can also be used as a refining agent.

At the same time, manganese will also segregate in the grain boundaries, reducing the durability.

Most superalloys regard manganese as a harmful element and need to control the content.

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2021.08.29