Manganese Friend or Ennemy

8/3/2019 Manganese Friend or Ennemy

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MANGANESE: FRIEND OR ENNEMYGRAY IRON

Ir G.D HENDERIECKX GIETECH BV MARCH 2009 1

Manganese is one of the most popular alloying elements in gray iron, even that muchthat it is mostly not considered as an alloy. But it is because it does increase thepearlite content and as a consequence the tensile strength of gray iron.

How did it become so popular?First it is a cheap element with a high availability. It is mostly added as a ferro-alloywith about 5 to 7 % of carbon. This alloy has a low melting point and this is anadvantage when it was used in the cupola melting in earlier days.It also did show a tensile and hardness increase and additions even up to 1,00 %became common, especially when surface hardening (flame hardening) was locallyrequired (slides of a tool bed casting).

Let s systematically examine all influence of these element.


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1. Segregation

It is known that manganese has a fairly high segregation behaviour, which meansthat the real manganese content at the surface layer is less than the content of the

core of a section.

The segregation tendency is ranked as follows (segregation factor), stating that afigure < 1,00 indicates an element that stays maximum in the first freezing liquidand a figure > 1,00 indicates an element that segregates to the last freezing liquid.

Element Mo Ti V Cr Mn PFactor 25,3 25,0 13,2 11,6 1,7 3,5 2,0

Element Si Co Ni CuFactor 0,7 0,4 0,3 0,1

The higher the factor, the more important the cooling time. A slow cooling (up to

solidification), will increase the segregation.

The segregation effect isshown in next figure.

If the casting has small thickness (less than 25 mm), the segregation cannot cause anoticeable difference but if the sections are large or there is a hot spot (sectionthickness over 75 mm), the difference is measurable.

So the foundries, pouring large castings did start getting porosity problems due to thefact that higher manganese content does increase the volume shrinkage duringsolidification.


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2. Increase pearlite content and strengthThe second influence is the increase of the pearlite content.

The ranking for the pearlite promoting effect is as follows:

Element Sn Mo P Cu Ti Mn Ni CrEffectiveness 39 7,9 5,6 4,9 4,4 0,44 0,37 0,37

Increasing the pearlite content has the consequence that tensile strength andhardness should increase. This was not always achieved, unless the content wasvery high. What is the reason?

In the absence of manganese, sulfur in iron forms iron sulfide which has a lowmelting point of 1193 C and thus will segregate to the eutectic ceIl boundaries,where the last areas to solidify are located.

Manganese reacts with sulfur to form manganese sulfides. The ratio is 1,7 as shownin next formula.

% Mn = 1,7 x % S

So this amount of manganese is bounded and cannot increase the pearlite. On thecontrary, it also removes sulfur from the matrix, which also is a pearlite promoter.So the real effect is, as shown in next figure, that first the tensile strength andhardness are decreasing and increasing with higher contents. The bottom point isobtained with about:

% Mn = 1,7 x %S + 0,3

The effect of manganese is largely dependent on the sulfur content and it is clear thatcupola iron will require much higher manganese content than electrical melted iron,due to its higher sulfur content (introduced by the cokes).

This leads to the following formula and picture for calculating the expected tensilestrength and hardness.

Rm = 1120 MPa + 2865 / D 150 (% C) 422 (% Si) 73 (% Mn 1,7 % S) - 181 (% Cu)2164 (% Mo)2 + 95 (% Cr) + 14 (% Ni) + 211 (% Cu) + 275 (% Mo) + 98 (% Si)2

D is the diameter of the test coupon in mm

It is clear that the effect of manganese on tensile is not very high.


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For the hardness, there is the following formula and graph.

HB = 444 71,2 (% C) 13,9 (% Si) + 170 (% S) - 61 {(% Mn) 1,7 (% S) -0,3}+ 58 {(% Mn) 1,7 (% S) -0,3}

2

HB = 444 71,2 (% C) 13,9 (% Si) + 170 (% S) - 61 {(% Mn) 1,7 (% S) -0,3}+58 {(% Mn) 1,7 (% S) -0,3}

2+ 20,5 (% Cr + % Cu) + 7 (% Ni) + 22 (% Mo)

The effect of manganese on the hardness is complex and first decrease strongly,after it will increase.

To use this formula the following limitations are valid (in plus to the above limitationsfor C, Si, Mn, P and S):

% C 2,90 4,00 % % P 0,05 1,00 %% Si 1,00 3,30 % % S 0,03 0,18 %% Mn 0,25 1,00 %% Cr 0,20 0,60 % % Cu 0,50 1,50 %% Mo 0,20 1,00 % % V 0,10 0,20 %% Ni 0,60 1,00 % % Sn 0,04 0,08 %


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3. Lining wear and slag

Another problem occurred, especially with electrical melting, that the manganese wasreacting heavily with the silicon of the ladle lining, causing a lot of slag and quickwearing of the lining. The slag consists of manganese-silicide compounds with a highmelting point and is difficult to remove. This caused that, electrical melted iron that issupposed to be clean, was not the clean at all and more slag inclusions appeared inthe castings. The slag also appeared in cupola melted iron but the foundries wereaware of it and had mostly a much better slag removing procedure.

4. Blowholes

The last problem that occurred was an increase in blowhole presence and frequency.This was surprising because blowholes are mostly caused by entrapped air or by COinclusions.

The formation can be explained as follows:

Manganese sulfide freezes at 1620 C, highest of any constituent in the melt.Manganese sulfide (has a specific gravity of 4,0), will segregate by flotation duringsolidification. The quantity of manganese sulfide in commercial irons has no influenceon either casting or use properties, but cope surface blowholes (mainly sub-surface)may be encountered when a critical value of the product of the manganese timessulfur content is exceeded, particularly in combination with low pouring temperatures.


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These blowholes occur as a result of the reaction of a fluid oxide-rich surface slagand graphite precipitated during solidification of the iron to yield carbon monoxidegas. Manganese sulfide serves to aggravate this situation as it floats to the surfaceand dissolves in the iron silicate/ manganese silicate oxidation slag, and lowers its

melting point, creating a very fluid slag even at the eutectic temperature. Only a veryfluid slag can be brought into intimate contact with graphite at eutectic temperatures.

Evolution of steam from improperly dried refractory can also produce a very fluid andoxide-rich surface slag. Ladle surface slag produced by oxidation and chemicalaction is very important gas hole producing agents.This type of gas defect can occur in all types of moulds and is aggravated by thepresence of high sand moisture contents. However, researchers showed that thechief contributing factor is low pouring temperature associated with high sulfur andmanganese contents of metal (Figure above).

In the reaction Mn + FeS =MnS + Fe an increase in concentration of either

manganese or sulphur will cause manganese sulfide to be precipitated. The reactionproceeds to the right with falling temperature.

It is also recognized that the following maximum is blowhole free:

% Mn = 1,7 x %S + 0,6

BLOWHOLES IN GRAY IRON

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0 0,2 0,4 0,6 0,8 1 1,2 1,4

Mn (%)

S(%)

NO BLOWHOLES

BLOWHOLES

DUE TO MnS


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Conclusion

The addition of manganese must be done very carefully and after evaluation of theeffects due to the chemical composition, melting furnace lining and casting section.

If carefully calculated and controlled, there will be a benefit at low cost. If just added,it can cost a lot of disappointments and extra scrap.

Sometimes it will be much more cost effective to add copper in stat of manganesedue to the low segregation tendency, high pearlite promoting effect and no interactionwith the ladle lining.

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Manganese Friend or Ennemy