Production of ductile iron castings without feeders

Transcription

1 Production of ductile iron castings without feeders Possibilities of producing ductile iron castings in green sand mould without feeders by means of "ingate" feeding and control of the graphite precipitation pattern by means of advanced thermal analysis. Rudolf Val. Sillén* * NovaCast Technologies AB, Sweden Abstract The paper focuses on the mechanisms behind shrinkages in cast iron and how they can be avoided. Grey and ductile iron are unique casting alloys due to their solidification behaviour. During solidification the alloys initially contract when the temperature drops from the pouring temperature to the liquidus temperature. This contraction must always be compensated for by supply of feed metal from the gating system and often also from a feeder (riser). The paper will demonstrate that this type of feeding can only be functional until the amount of solid phase reaches a certain level. This usually happens slightly after reaching the low grey eutectic temperature. When this stage has been reached a balanced precipitation rate and amount of graphite will make it possible to compensate for contraction of remaining liquid and the eutectic austenite. The proper balance is crucial as the precipitation of graphite is associated with a volume expansion, which if too high will cause mould wall movement and increase shrinkage tendency. Too low precipitation will lead to micro-shrinkages. The paper describes how optimization of the gating system in many cases makes it possible to eliminate feeders for supply of the initial feed metal. After the low eutectic temperature is reached the contraction can be eliminated by careful control of graphite precipitation using a combination of chemical and adaptive thermal analysis. Through proper control of the feeding sequences it is possible to produce certain ductile iron castings in green sand moulds without feeders! Key words Feederless ductile iron, green sand 217/1

2 Introduction Shrinkages are one of the most common casting defects. The main cause of shrinkage cavities is that all commercial alloys contract when a casting cools from the pouring temperature to solidus. The contraction is usually between 1-5 % depending on the type of alloy and the pouring temperature. The main contraction occurs between the pouring temperature and the liquidus temperature. If the contraction is not compensated for by feed metal, either by supplying feed metal or by forming a depression on the outer surface, a shrinkage cavity will occur. Cast iron alloys, which solidify with a precipitation of graphite, represent a more complex behaviour, the reason being that the dissolved carbon partly precipitates as graphite with a lower density than the base iron. The precipitation is therefore associated with an increase in volume, which in some cases partly might offset the contraction of the liquid and the austenite. By careful control of the mould filling and the precipitation of graphite it should be possible to produce ductile iron castings in green sand moulds without the use of feeders. In this paper we will study what happens during solidification, as well as the mechanisms behind shrinkages and how to avoid them. What happens during solidification of cast iron? When liquid metal cools off, the temperature is reduced, causing energy to be released. The temperature represents the total amount of thermal energy in a melt and is related to the kinetic energy of the molecules. When a melt cools down, the thermal agitation of its molecules is reduced. This energy is expressed as specific heat e.g. as KJ/ Kg and degree C. When the liquid reaches a temperature called liquidus, the bond between the atoms becomes more rigid on a macro scale level. More energy is then released until the liquid is transformed into solid state. The energy released at this stage is called latent heat of fusion, fusion enthalpy or just latent heat for short. Theoretically, the temperature stays constant until the transformation is completed. Latent heat is measured as KJ/Kg. The temperature where the metal or a precipitated phase is completely solid is called solidus. The precipitation of various phases in grey and ductile iron is to a large extent dependent on factors such as nucleation that can not be estimated using the chemical composition. In this investigation we have therefore used advanced thermal analysis as a tool to understand and control the progression of the solidification. Thermal analysis is based on recording temperatures at certain time intervals during the solidification process. Cooling curves can thereby be constructed and used to analyse and classify an alloy. A cooling curve is a plot of the temperature as a function of time for a sample of an alloy poured into a standardized mould with a thermocouple, usually positioned in the center. Arrest temperatures such as liquidus and solidus in a cooling curve, as well as cooling rates during various phases of the solidification can be used as metallurgical attributes to classify a melt and to correlate it to the behaviour when poured in a mould. 217/2

3 When a casting cavity has been filled with liquid cast iron the temperature is reduced until the liquidus temperature (TL) is reached, then austenite crystals start to form if the alloy is hypoeutectic. If the alloy is eutectic both austenite and graphite are precipitated from the melt. If the alloy is hypereutectic then the initial phase is graphite. The latent heat for austenite is fairly low, about 200 KJ/kg. Therefore almost no recalescence (R) occurs at TL. The latent heat for graphite is very high, about 3600 KJ/kg. Thus when graphite precipitates, heat is released, which causes the temperature to increase and causes recalescence. Precipitation of graphite is also associated with a volume expansion as the density of graphite is about 2.2 g/cm 3 versus about 6.9 for the liquid melt. Let us study the progression of solidification using a hypereutectic alloy as an example. The cooling curve shows what happens at each moment in the centre of the sample cup. A typical cooling curve and its first derivative looks as follows: When the liquidus temperature is reached the cooling curve shows a horizontal plateau. The length of the horizontal plateau is a function of the time it takes for austenite to grow from the walls of the cup to the centre where the thermocouple is located. The melt contracts first in the liquid state and then during the crystallisation of primary austenite, which continues until the low eutectic point (TElow) is reached. At that time the eutectic reaction where simultaneously austenite and graphite are precipitated has just started. The temperature increases due to release of latent heat until the high eutectic temperature (TEhigh) is reached. The increase in temperature is called recalescence (R). The eutectic solidification then continues until no more liquid remains and the solidus temperature (TS) is reached. The GRF1 factor measures the eutectic behaviour during the second phase of eutectic. GRF2 is a marker that 217/3

4 indicates the effect of the eutectic at the very end of freezing. During the eutectic phases (S2 and S3) the liquid can expand its volume provided a sufficient amount of eutectic graphite is precipitated. The available amount of eutectic graphite can be estimated using the formula: Eutectic Graphite % = Carbon% - ( *Si%) (1) Thus if Si is 2% and C is 3.8% then the maximum amount of eutectic carbon is about 2%. Assuming that the density for the liquid is 6.9 g/cm 3 and 2.2 for graphite, the volume expansion from the graphite is about 6.2%. This expansion can, if carefully controlled, compensate for the contraction of remaining liquid and austenite in a casting. The solidification progression can be illustrated as follows: The illustrations show the situation in a section through a cup used for thermal analysis. Figure A shows the melt at position 1 when the cup has just been filled. At position 2 the temperature has reached liquidus. The temperature gradient is then zero due to thermal currents. The zone with metal at the liquidus temperature is gradually reduced, which is shown in 217/4

5 positions 2 to 4. Note that the temperature in the liquidus zone is constant. Thus there is no contraction in this zone! In position 3 austenite has started to grow inwards. In position 3+ the low eutectic temperature has been reached at the walls and a zone which is expanding has been created. In position 4 also secondary eutectic is formed and expanding. In positions 5-7 a solid phase also appears. In a casting several of these zones appear at the same time. If an initial amount of feed metal can be supplied until position 3+ is reached then it is likely that the casting can be made without any feeders. The challenge is to be able to supply the initial need for feed metal from the gating system and to control the nucleation and thereby the precipitation rate of graphite so that it can balance the contraction of austenite and remaining liquid. Accodring to the author this is possible by means of simulation systems for optimizing gating systems and by using a combination of chemical and advanced thermal analysis for controlling the solidification process. Which variables can be monitored by thermal analysis? Thermal analysis can be used to monitor the metallurgical variables that influence the development shrinkages. The ATAS thermal analysis system (developed by NovaCast in cooperation with the Swedish Foundry Association) is specially designed to monitor and interpret cooling curve data and predict the potential risk for shrinkages, as well as other problems. The system is based on analysis of "grey" cooling curves. The illustration shows a typical cooling curve for a hypoeutectic iron and the time-position for various types of shrinkages. Shrinkage mechanisms In the following we have classified shrinkages in four basic types. We will discuss how they occur and how they can be avoided. The four basic types are Outer sunks (pull downs), Macro shrinkages, Micro shrinkages and Porosities. A. Outer Sunks A1 Definition and location 217/5

6 Outer sunks (pull downs, sinks) can be seen on the outside of the casting, usually as a smooth depression in the casting surface. They are normally located on thick sections of the casting and on surfaces located on the top of the casting during pouring. Outer sunks are also referred to as pull downs and they can occur not only on horizontal top faces of the casting but sometimes also on vertical surfaces. A2 The basic mechanism behind outer sunks The metal starts to solidify at the surface of the mould cavity and a thin skin is formed. The temperature drops further, which causes the liquid and the semi-liquid metal inside the casting cavity to contract further. If no feed metal is available either from the gating system or from a feeder then contraction will cause a negative pressure inside the cavity. In order to equalise the pressure difference between the atmosphere and the interior the solid outer skin will be pulled inwards. The effect is that the contraction is compensated for by a reduction of the volume of the casting. Thus outer sunks develop at an early stage of the solidification process before the massive eutectic freezing has commenced. A3 Variables that influence the creation of outer sunks The major variables that influence the likelihood for development of outer sunks and their effect are as follows: 1. Too high pouring temperature. The volumetric contraction is about 1.4% per 100 C for cast iron. 2. Insufficient amount of feed metal available at early stages of the solidification. In order to be able to produce ductile iron castings in green sand mould without feeders it is essential that feed metal can be supplied from the pouring cup through the gating system during the initial contraction in liquid state and until major parts of the casting have reached a temperature close to the eutectic temperature. B. Macro shrinkages B1 Definition and location Macro shrinkages are usually found inside the casting and close to heat centres. They appear as larger holes, usually with rough surfaces, often dendritic and are often larger than 5 mm. If the iron is hypereutectic then the shrinkage might be rounded with smooth walls. Macro shrinkages are usually not revealed unless the casting is machined or deliberately cut through sections of heat centres. They can also appear in or close to ingates. B2 The basic mechanism behind macro shrinkages Macro shrinkages develop after the initial solidification on the surface. A shell, that can not be deformed by the pressure difference to the 217/6

7 atmosphere, has then been established. If no more feed metal is available at that point in time then the contraction of the liquid and semi liquid metal as well as contraction of the already solidified parts result in a cavity. Gas dissolved in the metal might also diffuse into the shrinkage cavity. B3 Variables that influence the creation of macro shrinkages Shrinkages are influenced by the behaviour both of the alloy and the mould. The dominating variables for macro shrinkages are: 1. Insufficient supply of feed metal too small feeder modulus or feeder neck or feeder does not pipe due to too large ingate modulus or height position. 2. Feeding path closed too early wrong position of feeder. More dendrites than usual at an early stage. Too low active carbon equivalent (ACEL= the true carbon equivalent measured with ATAS). For ductile iron, ACEL should be eutectic or slightly hypoeutectic and carbon minimum 3.6%. 3. Mould hardness during solidification. Soft moulds that favour mould wall movement. A green sand mould, hard at room temp can be soft during solidification due to the high water level in the condensation zone. 4. Mould weighting or clamping insufficient to stand the pressure during solidification. Eutectic pressures up 50 kg/cm 2 have been claimed! 5. Higher liquidus temperature (TL) than normal. In hypoeutectic irons this means too much primary austenite and more difficult feeding. In hypereutectic it means more primary graphite, which reduces the amount of eutectic graphite. 6. Higher amount of primary austenite (S1) than normal. Too low ACEL. 7. Too high recalescence and recalescence rate causes expansion from graphite to occur too early. (In ductile - too much late inoculation with low ACEL). 8. Too low eutectic temperature might cause some primary carbides to form, which reduces the amount of eutectic graphite. B4 - How can macro shrinkages be avoided? The remedies are basically the same as for outer sunks. However, to avoid macro shrinkages the feed metal must be available longer than for Outer sunks because macro defects occur at a later stage during solidification. A solidification simulation e.g. using NovaSolid is highly recommended to ensure that feed metal can reach critical areas and that feed metal is available during the initial contraction period. In order to produce ductile iron castings without feeders the active carbon equivalent should be adjusted to the castings modulus so that the solidification is eutectic. A low gas content in the metal is also essential. 217/7

8 C. Micro shrinkages C1 Definition and location Micro shrinkages are smaller cavities with irregular surfaces, often with signs of dendrites. Sizes are often less than 3 mm. The defects are usually located close to heat centres in the casting. Micro shrinkages are often referred to as porosity or leakage as castings with this type of defect often leak during a pressure test. Micro shrinkages are usually not revealed unless the casting is machined or deliberately broken. C2 The basic mechanisms behind micro shrinkages The defects occur at the latest stages of solidification. It is therefore more difficult to solve micro shrinkage problems by changing the gating or feeding system. Micro shrinkages are more of a metallurgical problem. There are three basic mechanisms behind micro shrinkages: The first and most common mechanism behind micro shrinkages is that the contraction of the austenite (primary and eutectic) at the end of freezing can not be fully compensated for by the precipitation of carbon into graphite, which is associated with an increase in volume. The precipitation pattern of graphite from the start of eutectic freezing until the end of freezing is therefore very important. C3 Variables that influence the creation of micro shrinkages 1. Low mould stability which might cause mould wall movement. Too soft moulds due to high moisture content or low compression during moulding,. 2. Too low levels of sea coal addition in the green sand. 3. Bentonite (Calcium) with low wet compression strength in green sand. Sodium bentonite are optimal for reducing shrinkages. 4. Insufficient weighting or clamping of moulds. 5. Too little eutectic graphite especially at the end of freezing; its precipitation can not compensate for the shrinkage of the austenite. A true eutectic composition is. (Note that the eutectic point is a function of the thermal modulus!). Often a high C/Si ratio can reduce the risk for shrinkages. 6. Too high hypereutectic composition in ductile iron. If too high then some of the dissolved carbon will be precipitated as primary graphite and the amount of eutectic graphite might be insufficient. Evidenced by too many large size nodules which have been growing early in the liquid. 7. Too much magnesium in ductile iron. Levels of Mg and RE must be consistent with the thermal modulus of the casting. Low levels of Ce and high levels of La ( ) can be very effective in reducing shrinkages in ductile iron. 217/8

9 8. Too high amount of phases that exhibit solidus temperatures below 1100 C. e.g. Fe 3 P. 9. Too high recalescence and recalescence rate causes expansion too early and consumes a high amount of the carbon so that the expansion at the end of freezing is unsufficient. 10. Too low eutectic temperature might cause some primary carbides to form. A low eutectic temperature also means that the contraction in liquid state increases. 11. Too small Graphite Factor 1 (GRF1) indicating too low amounts of eutectic graphite during the second part of the eutectic. 12. Too high Graphite Factor 2 (GRF2) indicating too little eutectic graphite precipitation at the end of freezing. 13. Too low solidus (TS) which may induce carbides at the last portions to freeze. Too high levels of Mg, Nb, V or similar elements tend to segregate to the grain boundaries and form carbides that contract during their solidification. For ductile iron TS should be above 1100 C. 14. Too high silicon will increase segregation of carbide forming elements, which increases the tendency for micro shrinkage. C4 - How can micro shrinkages be avoided? The first condition is to have a hard mould. A green sand mould is compressible, however hard at room temperature, because of the formation of a condensation zone. In the condensation zone, which travels from the surface of the cavity and inwards, the moisture content can be up to 3 times higher than the initial value. This means that the bentonite layer becomes semi-fluid and can easily be compressed. It is recommended to use a sodium bentonite, sea coal as additive and a low moisture Metallurgically, the most important factor is to ensure that a sufficient amount of carbon is precipitated as graphite during solidification. It is important that the initial growth rate is not too high. The metallurgical factors can be influenced by selection of charge materials, charging sequence, the melting cycle (temp/time steps) as well as type and amount of alloying materials, inoculants and FeSiMg. It is also important to avoid phases that are liquid below the main solidus temperature. D. Porosities D1 Definition and location Porosities are small, dispersed cavities with irregular or rather smooth surfaces, often less than 1 mm in size. The defects are usually located close to heat centres and in grain boundaries. They are more dispersed than micro shrinkages. Typically, the defect is not discovered until the casting is subjected to a leakage test with water or air. D2 The basic mechanism behind porosities The defects occur at the very latest stages of solidification. Therefore it is not possible to solve porosity problems with changes in the gating or 217/9

10 feeding system (unless a very steep temperature gradient is maintained). Porosities are a metallurgical problem mainly depending on the chemical composition. The main mechanism is that due to the composition of the iron, one or more phases solidify at a lower temperature than the austenite-graphite eutectic. In ductile iron too high magnesium levels can cause similar problems as magnesium segregates to the rest melt and can induce formation of carbides which contracts and creates porosity. Production of ductile iron castings without feeders in green sand. By applying the principles outlined in this paper the author believes that ductile iron castings even with high modulus can be produced in green sand mould without feeders. The first condition is that the mould is sufficiently compressed and that that the sand properties are optimized. The second condition is that the modulus of all parts of the gating system must be higher than about 40% of the dominating modulus of the casting. This ensures that most of the feed metal needed to compensate for contraction of the metal between the pouring temperatures down to liquidus can be supplied from the pouring cup. The third condition is that the metal expands sufficiently during solidification in order to match the contraction of austenite and remaining liquid phase in all parts of the casting. The expansion that comes from precipitated graphite must not only be sufficient in volume, the precipitation must also be balanced in order to avoid mould wall movement and exhibit an expansion pattern until the end of freezing. The control of the solidification progression can be achieved by using a combination of chemical and thermal analysis of grey samples. The essential metallurgical factors to consider are:?? The active carbon equivalent must be selected as a function of the modulus of the casting. Basically the active carbon equivalent should be eutectic. Truly hypereutectic solidification must be avoided.?? The carbon/silicon ratio should be high.?? The nucleation level in the base iron must be sufficient so that the low eutectic temperature is high, preferably higher than 1140 C.?? The recalescence must be less than 5 C.?? The magnesium level must be as low as possible.?? The graphite precipitation pattern in the final iron must be controlled so that a sufficient amount of eutectic graphite is precipitated after reaching the high eutectic temperature (GRF1 should be high and GRF2 should be low). These factors usually are at their peak about 3 minutes after Mg-treatment. rudolf.sillen@novacast.se 217/10