Ductile Dross Formation Monitoring

Prévia do material em texto

DUCTILE IRON
DROSS 
FORMATION
MONITORING
Ir G HENDERIECKX
GIETECH BV
CONTENT
1. INTRODUCTION
2. DROSS
3. FACTORS
3.1 Mg CONTENT
3.2 S-CONTENT
3.3 O2-CONTACT 
3.4 TURBULENCE
3.5 MOULD MATERIAL
4. ACTIONS
4.1 CHEMICAL COMPOSITION
4.2 MOULD DESIGN
4.3 POURING & POURING SYSTEM
4.4 DROSS COLLECTORS
5. CONCLUSION
1. INTRODUCTION
Every ductile iron foundry, especially the ones producing large and or complex castings, does know the dross problem, which is for most of them the main scrap cause.
It is important to realise how dross is made and if it can be avoided or at least decreased to an acceptable level.
The factors that are increasing the dross presence should be controlled and understood. It is clear that a correct monitoring, taken in account the mutual interference of several of the factors, can master the problem.
After this investigation, some of the actions will be discussed in detail.
2. DROSS
Dross is the inclusion material in ductile iron that consists of all reaction products with Mg (magnesium) and as an enlargement with RE (Rare Earths, especially Cerium).
The most common products and their reactions are:
Mg 
+
O
--->
MgO
Mg
+
S
--->
MgS
Mg 
+
S
+
O
---->
MgO
+
S
2 Mg
+
SiO2
---->
Si 
+
2MgO
2 MgS
+
SiO2
---->
Si
+
2MgO
+
S
Ce 
+ 
S 
--( 
CeS
2 Ce 
+ 
2 O 
+ S 
--( 
Ce2O2S
The question which of the reaction will be preferentially occurring can be answered from the “free energy” chart and the presence of the elements, one compared to the other. See figure for oxygen on next page.
The reactions will be decreased or even stopped if the availability of the reacting elements is low or not existing, which of the elements it will be.
This indicates that the following elements are important:
1. Mg
2. RE (especially Ce)
3. S
4. O2
5. SiO2.
 
3. FACTORS
3.1 Mg CONTENT
3.2 S-CONTENT
3.3 O2-CONTACT 
3.4 TURBULENCE
The factors are mainly the magnesium and sulfur as well as oxygen content and the way they can act to perform the reaction.
3.1 Mg content
The magnesium content, and to a lesser degree the Cerium content, are basic for the formation of dross. The magnesium content should be minimum level.
The magnesium is needed for the formation of the correct free graphite shape. In all literature it can be found that the content should be > 0,03 % at the time of solidification. See figure.
This is the traditional way of evaluation.
Later the influence of RE (Rare Earths) and especially Ce (cerium) is also taken in account. It is the Mg + Ce content that is important.
On top of this, it is clear that the Mg- and Ce-content is not constant in time, from the nodulising till pouring and solidification. During this time, both elements can come in contact with S and O2, which will lead them to:
1. decrease in residual Mg- and Ce-content
2. new reaction and formation of dross.
To assure that the Mg- and Ce- content is sufficient at the time of solidification; a fading rate is set, depending on the type of covering and transport and pouring.
During the transport of the ladle the contact is depending on the time and cover of the ladle; during pouring in the pouring box, it depends on the size of ladle and pouring box as well as the height of the ladle compared to the pouring box; during the mould filling depending on the pouring system and turbulence.
The most popular formulae are:
Fading in % / min
Mg
Rare Earths
RE
Ce
 
La
Covered full ladles
0,0006 
0,00013
Open ladle (reheated in furnace)
0,0033
0,0002
0,00025
Other literature is indicating:
% Mg-loss = (t / 1000) x (T / 1450)2
t : time in minute
T : temperature in °C
Another literature tells about tests that reveal:
0,0001 % / min in pressurized (N2) pouring device
0,0007 % / min as the metal surface is covered very well
0,003 % / min in a non-covered metal surface situation.
All these figures are fairly close one to another to use them as guideline for the fading effect of Mg and Ce (RE).
An example of the result of fading is given in next figure (fading 0,003 % Mg / min).
 
The minimum amount of Mg + Ce is according to this theory:
Minimum residual magnesium, combined with rare earths (RE) content to obtain 
100 % nodular (spheroidal) free graphite
Mg
0,03
0,025
0,020
0,018
0,016
RE
0,000
0,005
0,010
0,015
0,020
 In a figure, this shows the following data:
The residual magnesium can be calculated by using the following formula:
% Mgt = (0,75 % Sb + % Mgr) / (
Mgt
: added amount of magnesium
Sb
: percentage sulphur previous to treatment
Mgr
: percentage magnesium present in the metal test 
after nodulising
(
: efficiency of the magnesium-input
The efficiency of the treatment, (, depends on:
1. treatment temperature
2. type and shape of treatment ladle
3. type and size of treatment alloy
4. condition of treatment alloy.
As a conclusion, it is clear that most of the foundries do use too high residual magnesium content that will lead to:
1. exploded graphite (Mg > 0,07 %)
2. carbides formation (Mg > 0,06 %)
3. large dross formation.
3.2 S-content
Up to last year, most of the foundries and labs did indicate that the S-content should be minimised to the utmost.
Now, it is discovered that sulfur does play an important role to the nuclei and free graphite formation. It is found that a too low S-content did lead to a low NC (nodule count), which is an important factor for producing porosity free castings with high mechanical strength and ductility.
When the initial S-content is too high (> 0,020 %), it is preferred to perform a desulfurization treatment. The slag should be removed very carefully.
The Mg-addition during the nodulising treatment is calculated according to the formula on previous page. The residual S-content will be at a level, after the treatment, which ensures the presence of correct free graphite and a high nodularity and nodule count.
The best level is
S = 0,010 % +/- 0,002 %.
At level:
* S < 0,008 %
lower nodule count
* S > 0,012 %
more dross formation until pouring.
It is preferred to remove the dross after the nodulising treatment very carefully because it is possible that the sulfur will re-enter the metal.
2 MgS
+
SiO2
---->
Si
+
2MgO
+
S
The magnesium-oxides also appear as slag.
3.3 O2-CONTACT
It is clear that, after a correct nodulising treatment and careful removal of the slag from the metal surface, the S-content will be low and it will be difficult for the residual Mg to react with sulfur (both are present at a low concentration).
The reaction with oxygen on the other hand will never stop. The metal surface, if not or not carefully covered, is in contact with the air. During pouring in the pouring box, there is, depending on the way of pouring, contact with the air as well as during the filling of the mould.
For this reason, oxygen will be the most important factor for the dross formation. Due to the fact that ductile iron castings are not poured under a protection gas (nitrogen, argon…) or vacuum (all this is too expensive or even physically not possible for larger castings), there will always be contact of liquid metal and air and consequently dross formation.
The conclusion is that the foundry has to decrease the dross formation to the maximum, but it will always happen!
3.4 TURBULENCE
Before the metal can solidify in the mould cavity, it has been flown several times:
1. from ladle to pouring box
2. from pouring box into the pouring system
3. from the pouring system into the mould cavity
4. filling the mould cavity.
During all these activities, the liquid metal is in contact with the air. Minimising the reaction of magnesium with the oxygen is a matter of:
1. restricting the time of contact
2. restricting the contact surface between metal and air.
A laminar flow has very few contact, a turbulent flow has a lot of contact area. The monitoring will be to control and minimise the turbulence of the liquid metal flow.
Turbulence means a chaoticmetal flow or chaos that leads to non controlled and unpredictable results. It is proven that:
1. metal speed > 0,5 m /s
risk for turbulence
2. metal speed > 2 m / s
turbulence and air entrapment
3. thin sections < 5 mm
have less problems with turbulence.
Where do we have the suspicious flow?
3.4.1 Ladle to pouring box
Depending on the type of ladle (lip, T-pot or bottom pouring ladle) and the 
height of the ladle above the pouring box and its metal level, dross will be 
formed.
In the next figure, the dross formation and even the dross formation plus the 
air inclusion (in liquid metal) is indicated. It is clear that nearly always the 
distance between ladle and metal level in the pouring box will be too high. But 
anyhow, the lower, the smaller the effect.
 
This means that at least dross will be mixed with the metal in the pouring box and it is fact to keep it out of the sprue. This can be done by keeping it, with the plug in the sprue opening, during 30 s (at least) to enable the dross to float to the surface.
If not possible, it enters the pouring system, which will have to bloc it.
3.4.2 Pouring box - sprue
The metal will fall into the sprue and its speed will increase till it hits the bottom. The higher the speed, the higher the impact on the more splashing will occur. The speed can be calculated as (one section sprue without holds):
Ssprue
Schoke
NON-PRESSURISED POURING SYSTEM
v = (2*g*H)0,5
The impact at the bottom will be:
m * v² = 2 * m * g * H
It is important to minimise the falling height (location of casting in mould) and the impact (use of sprue-pit to let the metal fall into metal).
In a pressurised pouring system the sprue will fill fairly quick, depending on the pressurising (1 / 2 / 0,7 – 0,85). The time for splashing is restricted.
In a non-pressurised system the sprue will fill together with the casting and the metal impact in the sprue is during a longer time (1 / 2 / 2). This can be avoided by locating the choke section in the start of the runner (1,2 / 1 / 2 / 2).
 
Ssprue
Schoke
NON-PRESSURISED POURING SYSTEM
 
A second item is the shape of the sprue. The sprue must be down tapered or at least straight. If not, a lot of air (especially if no refractory sprue pipes are used) will be sucked in the falling metal.
The diameter of the sprue should be as small as possible for 2 reasons:
1. less siphon effect in the pouring box
2. avoid that metal comes loose from the sprue wall and sucks air / gas.
A third item is the connection of the pouring box with the sprue. No liquid can bridge 90° edges and air sucking will occur again.
 
RUNNER
INGATE
PRESSURISED
NON PRESSURISED
SLAGPIT
Every not completely filled (with metal) part of the pouring system will entrap or suck air and gas (if chemical bounded sand is used) into the metal.
The dross formed in this stage of filling is important, but the following parts of the pouring system should be able to bloc it.
3.4.3 Runner
The runner is the part of the pouring system that transports the metal horizontally. It is the part that is suited for separating the metal and dross. This can be done by decreasing the speed of the metal to that extend that the slag can float up from the metal. The speed should be that low that the flow is not turbulent anymore.
For this reason, the section is always about the double the choke section and if necessary even more (but it does decrease the casting yield). The ratio’s are pressurised and non-pressurised: 1 / 2 / 0,7 – 0,85 and 1 / 2 / 2.
The runner must fill as quick as possible and even before the metal enters the mould cavity. This is possible for the non-pressurised pouring system (ingates are on top of the runner) but not possible for the pressurised system. Depending on the pressurising (ratio of ingate section to the sprue section), the runner will fill.
Anyhow, the first metal, which is not the best because:
1. splashed on the bottom of sprue
2. cleaned the pouring system from dust
3. cold metal due to contact with could mould material.
It should be directed in a location that avoids that the metal can enter the mould cavity. The possibilities are given on next page.
 
 
NON PRESSURISED
RUNNER
INGATE
RUNNER SLAGPIT
SLAG BLOCKING CORE
The dirty metal falls into the slag pit and is hold in that location because the next metal is falling on top of it.
The lower type is the best!
The first design is the most common although the lower one is much more effective. 
The longer the runner is, the more the speed will be reduced. 
A broken runner, with 90° direction changes, is better than a curved one because it reduce the speed more and the dross (less dense than metal) will not enter the mould cavity.
The runner is not creating dross but is the item that should bloc all the previous formed dross and prevent that it can enter the ingates and the mould cavity.
3.4.4 Ingates
The ingates are the last part of the pouring system and bring the metal into the mould cavity. What is passing here is entering the mould cavity and will be a potential inclusion in the casting. Nothing can prevent the entering!
The earlier formed dross can be blocked by the runner; so it is a matter of not forming new one. This will not be done if the:
1. metal speed is low
2. ingate is filled completely.
In the pressurised pouring system, the metal speed in the ingates is very high, depending on:
1. ratio of ingate section to the sprue section
2. ferrostatic metal height.
For this reason, a high mould should not be poured pressurised!
In the non-pressurised pouring system, the metal speed in the ingates is low but the ingate is not completely filled until the metal level in the mould reach the top level of the ingate.
HORIZONTAL
VERTICAL
INCLINED
It is also important to have a homogeneous flow in the mould cavity to ensure that dross (or other inclusions) is not captured or oxide film will form a type of cold flaw.
3.4.5 Vents
The vents and as an extension the risers are very important in 2 ways:
1. decreasing the pouring rate (increasing pouring time) if the section is lower than the choke section
2. collecting to a certain extend the dross and air.
By decreasing the pouring rate (due to the hindered escape of the mould cavity air), the vertical metal speed will be lower and the dross can float up more.
The venting (and risers) can collect dross and remove it from the casting. To enable this, they must be located correctly, which means there were the dross is floating up and or taken with the metal flow.
3.4.6 Filters
The filters are the most popular tool to avoid that dross can enter the mould cavity. But to be able to do this, they should be:
1. correctly calculated
2. located properly.
A filter with a too low flow capacity will not hold the dross and may be even break. A filter with a too large opening pattern, will not bloc the small particles. A filter in a pressurised pouring system will not work.
A filter located in the connection sprue – pouring box is not very effective. The filter should be located as close as possible to the mould cavity, preferentially the ingates.
But pay attention because incorrect filters can also create dross.
3.4.7 Casting shape and location
It is clear that the shape of the casting is important, combines with its location in the mould. There are 2 important features:
1. vertical speed of mould filling
2. connection and difference of sections.
It is clear that the speed of the metal decides if the dross can float up from the liquid metal. A high speed will avoid the segregation of dross and a metal with very low speed will show the dross on top of its flow. 
It is important to realise that the floating up speed is dependant on the size of the inclusion: a large inclusion has a higher floating tendency.
The location in the mould can take advantage of these factors.
 
Depending on the fact if it is located horizontally, vertically or inclined, the vertical filling speed of the liquid metal is totally different.The connection of sections with totally different value will influence the speed very much. It is known that, if this is too large, the metal will jump or fall dead completely with a lot of turbulence.
 
The flow capacity is equal all over the sections (with correction of the ferrostatic height in the pouring system compared to the metal level in the mould); the vertical metal speed will change a lot in sections AA and BB.
(Section * speed) is constant.
In AA the metal accelerates very much and will come loose from the walls and can include air.
In BB the speed has to reduce very much, which is impossible and the metal will spout out like a fountain, which is totally turbulent and including a lot of air as well as mixing the dross with the metal.
To float, the dross needs a very low vertical speed of the metal level.
3.5 Mould material
The mould material, especially chemical bounded sand that uses S-containing catalysts, can react with the magnesium in the liquid metal.
This reaction will increase if:
· Mg content is high
· Wall thickness of casting is large
· Coating layer is thin
· LOI, especially S-content, of the mould sand is high
· High metal temperature (pouring temperature).
The dross that is formed is different from the previous discussed one (MgO) and is of the type MgS. It will also be seen as a surface layer with dross and flake free graphite. Depending on the factors, the layer can be 1 to 3 or 4 mm.
 
 
The free graphite appears, depending on the residual magnesium content and the casting wall thickness, in the contact surface (metal – sand), goes over to compacted free graphite somewhat deeper and ends in nodular free graphite in the section.
It is commonly accepted that the sulfur is diffusing to the metal in a gas phase, which exist during a short time. The diffusion will increase if:
1. increased amount of sulfur in sand
2. higher pouring temperature
3. increased wall thickness.
It is less dependant or not at all of:
1. coating layer thickness
2. water or alcohol based coating.
Depending on the coating, which can be sulfur absorbing or sulfur blocking, the layer will be larger or smaller. The type of coating is important.
Zircon based coating is decreasing the contaminated metal layer, much more than graphite based. Graphite based is even dangerous if it contains also sulfur!
An increasing cooling capacity of the coating will decrease the thickness of the contaminated layer.
It has been discovered by some coating suppliers that the presence of magnesium-carbonate and calcium-carbonate, will decrease the contaminated layer because it can react (at higher temperature) with the penetrating sulfur.
Especially the calcium carbonate is very effective.
S-content in casting
0
0,02
0,04
0,06
0246810
% in coating
S-content in 
casting
Calcium-carbonate
Magnesium-carbonate
S-content in casting
0
0,04
0,08
0,12
0,16
0,2
012345678910
Calcium-carbonite addition (%)
S-content in casting (%)
No coating
graphite coating
graphite / calcium-carbonate coating
zircon coating
calcium-carbonate - graphite coating
Coating cannot avoid the metal will have a surface layer with degenerated free graphite and dross.
4. ACTIONS
As already indicated several times, it is impossible to avoid dross formation in ductile iron, due to the fact that there is a need of a residual magnesium content, which will react with available sulfur and oxygen.
It is a matter of decreasing the dross formation and to try to collect the (inevitable) dross, present in the casting, in extra casting-parts that will be removed later.
4.1 Chemical composition
The first item is the Mg, which should be properly calculated in combination with the RE (Rare Earths). The residual content depends on the time needed for transport of ladle to the mould, pouring time and wall thickness of the casting; it depends on the Mg- and RE-fading.
It is also very important to remove the dross from the metal surface after nodulising, which will take some time (dross must be able to float to the surface). It is preferred to remove the slag again just before pouring.
The sulfur content should be controlled before nodulising (< 0,025 %) and after nodulising because too low will decrease the inoculation effect (< 0,008 %) and too high will create a lot of dross (> 0,012 %). 
4.2 Mould design
It is fact to locate the casting in a way that the ferrostatic height is limited to the maximum. 
On the other hand, the vertical speed in the casting should be controlled to avoid turbulence and to control the area where the dross can float to the surface. The possibility to incline the casting in the mould (complicated for the pattern – split line) or even position the mould inclined (assure a constant inclination) is a cheap but powerful tool.
Let it be clear that dross will tend to stick to the mould and or core wall if there is contact and the metal (dross) speed is very low.
4.3 Pouring & pouring system
Ensure that the pouring is done at the proper height above the mould and or pouring box. This calls for a free area around the mould and appropriate ladles (content and type). It is clear that the dross removal before pouring is important because dross has float up during the transport from the nodulising area to the mould pouring area. Bottom and T-pot ladles have fewer problems with this phenomenon.
The pouring box must be designed properly to ensure that the dross cannot enter the sprue. This can be done with a slag dam (especially for small and medium castings) and or a sprue-plug, which will be removed after the pouring box is filled and the dross has floated up.
The pouring system should be properly designed concerning shape and sections. The sprue-pit and the slag-pit at the end of the runner are very important. It is very important to set rules about the use and decision making of pouring system.
To decrease the ferrostatic height, the sprue can be split in 2 parts. But this will only work if there is an extra runner and ingate present. It can even be that a pressurised and a non-pressurised system is combined, but both with the proper design.
4.4 Dross collectors
Once the dross has entered or is formed in the mould cavity, it is necessary to get it out of the casting (dross collector) or to collect it in the machining stock.
The cheapest solution is to collect it in the machining stock. But it is difficult to control because the amount of dross will vary from casting to casting and mostly the customer does not like a too high machining stock. It must be accepted that machining material with a lot of dross will be more difficult than clean material.
The next solution is to remove the dross by the risers and vents. This is possible if they are correctly located but has also some danger, especially the risers. If the amount of dross is too high, compared to the riser metal amount, it can be that the riser has a lower activity for feeding or that the feeding material is not a proper metal (low nodularity and dross inclusions).
Another, more costly solution, is the design of slag collectors as extra casting detail. If this collector is properly located, it can collect the dross and can be removed later in the grinding and fettling operation. This is mostly costly because it is a hand performed operation.
Dross collectors only work if they are properly located, which means in area where the slag is still taken by the metal flow and the speed is sufficient to avoid the sticking of dross to the mould walls. 
4.5 Mould material
For chemical bounded sand, especially reclaimed sand, the sulfur content must be kept to a minimum. The larger the wall thickness, the lower the sulfur content of the sand should be.
Maximum sulfur content in sand is 0,10 to 0,12 % for steel and ductile iron, for all other materials, maximum 0,5 % is admitted. The combination of sulfur with oxygen and nitrogen is worse. 
This is sometimes solved by the use of “new sand” around the pattern and for the cores. The sulfurcontent is than minimum. But this has the disadvantage that the sand grains will have the silica-expansion by heating above 600 °C, which increase the tendency to finning and penetration.
It is also necessary to use the correct type of coating and apply it with a correct thickness of the layer, to bloc the contact liquid metal – mould material to the maximum extend. 
The best method is to apply two layers of coating:
· first layer on the sand containing zircon and calcium- or magnesium-carbonate
· second layer of graphite coating with a low content of sulfur.
Pay attention on the thickness because a too thick layer can crack easily and give way to sulfur penetration of the metal.
5. CONCLUSION
The production of ductile iron is connected with the presence of dross. Dross is mostly present in the casting surface and if it is not removed by machining (or grinding), it can reduce the fatigue life of the casting drastically.
Because it is not avoidable, the dross formation should be reduced as much as possible. This can be done by controlling the chemical composition, especially the magnesium, Rare Earth and sulfur contents.
The preferred residual contents, after nodulising, are depending on the time for transporting the metal, pouring and the solidification time of the casting.
The pouring system is important to avoid turbulence during filling (turbulence increase the dross formation) and to bloc the present dross by avoiding it to enter the mould cavity. The ingates should not create dross by a too high speed.
Top pouring, falling down of the metal, especially more than 500 mm, will always have dross and air inclusions.
If the dross enters the mould cavity, it can be collected in the risers, vents, machining stock and or dross collectors. This is an action to correct a none or less controlled dross formation in previous stages of the metal pouring process.
Dross can be detected very easily by NDT tests, especially UT and MT (fluorescence to a high extend) and PT. It is very much present in large castings.
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Ir G.D HENDERIECKX GIETECH BV