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Senin, 16 Februari 2009
Grey Iron
Cast irons are alloys of iron, carbon, and silicon in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. In gray cast iron, the carbon that exceeds the solubility in austenite precipitates as flake graphite.
Gray irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of manganese, depending on the desired microstructure (as low as 0.1% Mn in ferritic gray irons and as high as 1.2% in pearlitics). Sulphur and phosphorus are also present in small amounts as residual impurities.
The composition of gray iron must be selected in such a way to satisfy three basic structural requirements:
1. The required graphite shape and distribution
2. The carbide-free (chill-free) structure
3. The required matrix
For common cast iron, the main elements of the chemical composition are carbon and silicon. High carbon content increases the amount of graphite or Fe3C. High carbon and silicon contents increase the graphitization potential of the iron as well as its castability. The combined influence of carbon and silicon on the structure is usually taken into account by the carbon equivalent (CE):
CE = %C + 0.3x(%Si) + 0.33x(%P) - 0.027x(%Mn) + 0.4x(%S)
Although increasing the carbon and silicon contents improves the graphitization potential and therefore decreases the chilling tendency, the strength is adversely affected. This is due to ferrite promotion and the coarsening of pearlite.
The manganese content varies as a function of the desired matrix. Typically, it can be as low as 0.1% for ferritic irons and as high as 1.2% for pearlitic irons, because manganese is a strong pearlite promoter.
The effect of sulfur must be balanced by the effect of manganese. Without manganese in the iron, undesired iron sulfide (FeS) will form at grain boundaries. If the sulfur content is balanced by manganese, manganese sulfide (MnS) will form, which is harmless because it is distributed within the grains. The optimum ratio between manganese and sulfur for a FeS-free structure and maximum amount of ferrite is:
%Mn = 1.7x(%S) + 0.15
Other minor elements, such as aluminum, antimony, arsenic, bismuth, lead, magnesium, cerium, and calcium, can significantly alter both the graphite morphology and the microstructure of the matrix.
In general, alloying elements can be classified into three categories.
1. Silicon and aluminum increase the graphitization potential for both the eutectic and eutectoid t
transformations and increase the number of graphite particles. They form colloid solutions in the matrix.
Because they increase the ferrite/pearlite ratio, they lower strength and hardness.
2. Nickel, copper, and tin increase the graphitization potential during the eutectic transformation, but
decrease it during the eutectoid transformation, thus raising the pearlite/ferrite ratio. This second effect is
due to the retardation of carbon diffusion. These elements form solid solution in the matrix. Since they
increase the amount of pearlite, they raise strength and hardness.
3. Chromium, molybdenum, tungsten, and vanadium decrease the graphitization potential at both
stages. Thus, they increase the amount of carbides and pearlite. They concentrate in principal in the
carbides, forming (FeX)nC-type carbides, but also alloy the aFe solid solution. As long as carbide formation
does not occur, these elements increase strength and hardness. Above a certain level, any of these elements
will determine the solidification of a structure with Fe3C (mottled structure), which will have lower
strength but higher hardness.
Generally, it can be assumed that the following properties of gray cast irons increase with increasing tensile strength from class 20 to class 60:
* All strengths, including strength at elevated temperature
* Ability to be machined to a fine finish
* Modulus of elasticity
* Wear resistance.
On the other hand, the following properties decrease with increasing tensile strength, so that low-strength irons often perform better than high-strength irons when these properties are important:
* Machinability
* Resistance to thermal shock
* Damping capacity
* Ability to be cast in thin sections. Successful production of a gray iron casting depends on the fluidity of the
molten metal and on the cooling rate, which is influenced by the minimum section thickness and on section
thickness variations.
Casting design is often described in terms of section sensitivity. This is an attempt to correlate properties in critical sections of the casting with the combined effects of composition and cooling rate. All these factors are interrelated and may be condensed into a single term, castability, which for gray iron may be defined as the minimum section thickness that can be produced in a mold, cavity with given volume/area ratio and mechanical properties consistent with the type of iron being poured.
Scrap losses resulting from missruns, cold shuts, and round corners are often attributed to the lack of fluidity of the metal being poured.
Mold conditions, pouring rate, and other process variables being equal, the fluidity of commercial gray irons depends primarily on the amount of superheat above the freezing temperature (liquidus). As the total carbon content decreases, the liquidus temperature increases, and the fluidity at a given pouring temperature therefore decreases. Fluidity is commonly measured as the length of flow into a spiral-type fluidity test mold.
The significance of the relationships between fluidity, carbon content, and pouring temperature becomes apparent when it is realized that the gradation in strength in the ASTM classification of gray iron is due in large part to differences in carbon content (~3.60 to 3.80% for class 20; ~2.70 to 2.95% for class 60). The fluidity of these irons thus resolves into a measure of the practical limits of maximum pouring temperature as opposed to the liquidus of the iron being poured.
The usual microstructure of gray iron is a matrix of pearlite with graphite flakes dispersed throughout. Foundry practice can be varied so that nucleation and growth of graphite flakes occur in a pattern that enhances the desired properties. The amount, size, and distribution of graphite are important. Cooling that is too rapid may produce so-called chilled iron, in which the excess carbon is found in the form of massive carbides. Cooling at intermediate rates can produce mottled iron, in which carbon is present in the form of both primary cementite (iron carbide) and graphite.
Flake graphite is one of seven types (shapes or forms) of graphite established in ASTM A 247. Flake graphite is subdivided into five types (patterns), which are designated by the letters A through E. Graphite size is established by comparison with an ASTM size chart, which shows the typical appearances of flakes of eight different sizes at l00x magnification.
Type A flake graphite (random orientation) is preferred for most applications. In the intermediate flake sizes, type A flake graphite is superior to other types in certain wear applications such as the cylinders of internal combustion engines.
Type B flake graphite (rosette pattern) is typical of fairly rapid cooling, such as is common with moderately thin sections (about 10 mm) and along the surfaces of thicker sections, and sometimes results from poor inoculation.
The large flakes of type C flake graphite are formed in hypereutectic irons. These large flakes enhance resistance to thermal shock by increasing thermal conductivity and decreasing elastic modulus. On the other hand, large flakes are not conducive to good surface finishes on machined parts or to high strength or good impact resistance.
The small, randomly oriented interdendritic flakes in type D flake graphite promote a fine machined finish by minimizing surface pitting, but it is difficult to obtain a pearlitic matrix with this type of graphite. Type D flake graphite may be formed near rapidly cooled surfaces or in thin sections. Frequently, such graphite is surrounded by a ferrite matrix, resulting m soft spots in the casting.
Type E flake graphite is an interdendritic form, which has a preferred rather than a random orientation. Unlike type D graphite, type E graphite can be associated with a pearlitic matrix and thus can produce a casting whose wear properties are as good as those of a casting containing only type A graphite in a pearluic matrix. There are, of course, many applications in which flake type has no significance as long as the mechanical property requirements are met.
Jumat, 13 Februari 2009
Grey Cast Iron DIN EN 1561
Grey cast iron is casting alloy, iron and carbon based, the latter element being present mainly in the form of lamellar graphite particles.
The properties of grey cast iron depend on the form distribution of the graphite and the structure of the matrix
This European standard deals with the classification of grey cast iron accordance with the mechanical properties of the material, either tensile strength hardness.
1. Scope
This standard specifies the properties of unalloyed and low alloyed grey cast iron used for castings, which have been manufactured in sand moulds with comparable thermal behavior.
This standard specifies the characterizing properties of grey cast iron by
Tensile strength of separately cast samples, or if agreed by manufacturer and the purchaser by the time of acceptance of the order, of cast-on samples or samples cut from casting
If agreed by the manufacturer and the purchaser by the time of acceptance of the order, the hardness of the material measured on castings or on cast-on knob
This standard does not apply to grey cast iron used for pipes and fittings according to EN 877-1.
This standard specifies six grey cast irons according to the tensile strength and six grey cast irons according to the brinell hardness
2. Definitions
2.1 Grey Cast iron
Iron-carbon material in which the free carbon is present as graphite, mainly in lamellar form
2.2 Relative hardness
Quotient of measured hardness to the hardness calculated from the measured tensile strength by means of an empirical relationship
2.3 Relevant wall thickness
Wall thickness for which the mechanical properties apply
Note : The relevant wall thickness is twice the modulus or twice the volume/surface ratio
3. Manufacture
The method of manufacturing of grey cast iron and its chemical composition shall be left to the discretion of the manufacturer, who shall ensure that the requirements defined in this standard are met for the material grade specified in the order
4. Requirements
4.1 Tensile properties
(2) If by the time of acceptance of the order proving of the tensile strength has been agreed, the type of the sample is also to be stated on the order. If this is lack of agreement type of sample is left to the discretion of the manufacturer
(3) For the purpose of acceptance the tensile strength of a given grade shall be between its nominal value n (position 5 of the material symbol) and (n+100) N/mm2.
(4) This column gives guidance to the likely variation in tensile strength for different casting wall thickness when a casting of simple shape and uniform wall thickness is cast in a given gray cast iron material. For castings of non-uniform wall thickness or castings containing cored holes, the table values are only an approximate guide to the likely strength in different sections, and casting design should be based on the measured tensile strength in critical parts of the casting.
(5) These values are guide-line values. They are not mandatory
(6) This value is included as the lower limit of the relevant wall thickness range
(7) The values relate to samples with an as-cast casting diameter of 30 mm, this corresponds to a relevant wall thickness 0f 15 mm
4.2 Hardness properties
If it is not possible to use the Brinell test method (EN 1003-1), alternative test methods may be used, which shall have correlated values with Brinell test.If a casting is ordered on the basis of hardness, the relevant wall thickness and the position of the test shall be agreed upon by the time of the acceptance of the order
Table 2
(2) By agreement between the manufacturer and the purchaser a narrower hardness range maybe adopted at agreed position on the casting, provided that this is not less than 40 Brinell Hardness units. An example of such a circumstance could be castings for long series production
5.1 Tensile test
5.1.1 Separately cast samples
The separately cast samples to establish the material grade shall be cast vertically. The moulds shall be either sand moulds or moulds with comparable thermal diffusivity. The moulds may be made for casting several samples simultaneously.

Common Metallurgical Defect on Grey Iron

1. Use of high steel scrap content in cupola melted iron with high coke charges
4. Insufficient of Ti or Zr contens to neutralise free nitrogen

1. Soft moulds or not properly cured binder
2. Insufficient clamping or weighting
1. Inadequate slag removal during melting and pouring
Undercooled Graphite
Possible Causes :
1. Too low content of volatiles in greensand moulds
Kamis, 12 Februari 2009
Common Defects in Ductile Cast Iron
Introduction
carbides and gas. These problem areas are described to aid recognition of the defect and causes are discussed
There are many causes of shrinkage in ductile iron, experience globally has shown that about 50% of shrinkage defects are related to sand systems, feeding and gating. The other 50% may be attributed to metallurgical factors such as carbon equivalent, temperature, inoculation or high magnesium residuals.

Firstly, the geometry of the casting should be examined to determine whether the location of the defect is close to a sharp radius or a potential hot spot. At the same time, the sand in the region of the shrink should be examined to look for any soft spots. Sand integrity accounts for a high proportion of shrinkage defects and a worn seal on the moulding machine, for example, resulting in a lower sand compaction can often be the cause of an unexplained sudden outbreak of shrinkage.
The second avenue of investigation should be the gating / runner designs and the feeding of the casting. Whilst many foundries have computer aided design systems, patterns are often altered slightly over the years at shop floor level and can be significantly different from the original design. Also, changes to the feeder specification can lead to different burn characteristics and metal solidification patterns. This can affect the amounts of feed metal available to different parts of the casting.
Metallurgically, there are many factors that can affect the shrinkage tendency.
Figure 2: Effect of magnesium content on shrinkage
Magnesium, apart from being one of the most powerful carbide stabilizers, has a marked effect on the shrinkage tendency of ductile irons. Foundries operating at the higher end of the magnesium range, 0.05% or above, will find that the iron is more prone to shrink than foundries operating at lower, but very acceptable, levels, say 0.035-0.04%. Both under-inoculation and over-inoculation can cause shrinkage. In the case of under- inoculation, not enough dissolved carbon is precipitated as graphite. Graphite nodules have a far lower density than the matrix and to precipitate the low density, high volume graphite has an overall expansion effect, which helps to counter the natural tendency of the iron to shrink. With over-inoculation, too many nucleation points are active early in the solidification, resulting in an early expansion and sometimes large mould wall movements. Later in the solidification, when feeders become inactive and contraction takes place, there is no graphite coming out from solution to counteract the contraction and the result is shrinkage between the eutectic cells. In many foundries, the microstructure shows even sized nodules (accounting for the fact that the section cuts through nodules in 2-dimensions). Many foundry men still consider this to be a good structure, even though the iron is prone to shrinkage. Nodularizers and specialist inoculants are available these days, which help to counter shrinkage by giving a skewed nodule distribution.
Figure 3: The same base iron treated with two different nodularisers resulting in a) Skewed nodule distribution b) Unskewed nodule distribution
A skewed nodule distribution indicates that some nodules are being created late in the solidification process and the drawing of graphite from solution at this stage is a very effective way to counter shrink. Most inoculants act almost instantaneously and this gives the even nodule size effect. Once the potency of the inoculant has gone, then there is no driver to create nodules late in the solidification and shrinkage can be the result. More recently, nodularisers have been developed by Elkem that have the same effect of producing the skewed and shrink reducing nodule distribution curve.
A low carbon equivalent, or metal that has been held for some time at temperature, due to a mechanical breakdown, for example, is also prone to shrinkage. In these cases, the inherent nuclei within the melt will be low and some preconditioning may be necessary to achieve a good level of nucleation.
Compacted Graphite within the structure. There are several causes of this, the most common being that the nodularisation process has partly failed. Incorrect weighing of the nodulariser or the use of the wrong nodulariser are possible reasons for the failure, although a long holding time in the ladle or excessive temperatures can be contributory factors.

Low Nodule Count



Graphite floatation



Figure 10 Sample with spiky graphite present in the matrix due to too elevated level of Pb.
Flake Graphite on the Casting Surface This is commonly seen in foundries, however many ignore the flake graphite on the surface as it forms part of the machining allowance. The defect is illustrated in Figure 11 and clearly shows the thin layer of flake graphite adjacent to the mould. This is found mainly in greensand systems and is caused by a build up of sulphur in the sand, which reacts with the magnesium in the iron to form magnesium sulphides and effectively de-nodularise the iron. A higher Mg or Re in the nodulariser can overcome this, subject to shrinkage restrictions discussed earlier, but the most common remedy is to use an inoculant containing cerium. This has the effect of re-nodularising the iron locally.
Figure 11 Sample with flake graphite on the surface of the casting due to high sulphur content in he moulding sand.
Carbides In the production of ductile iron, it must be remembered that magnesium is one of the most powerful carbide promoters. Coupled with this, the violence of the magnesium reaction during the nodularisation process tends to destroy nuclei. For these reasons, inoculation requirements are heavier than for grey irons and under-inoculation or the use of the wrong inoculant are amongst the most common causes of chill or carbides in ductile iron. Poor inoculation is not the only cause of carbides, however, and all the potential reasons need to be explored to determine the reason behind carbide formation.