Reasons for the defects of sand clamp The sand clamping is another main form of casting inclusions defect, mainly in the molding sand with the molten metal pouring into the cavity and is stuck inside the casting. Sand clip and slag the appearance of the difference is: After machining the sand is the white granular, the size of the sand grains, more found in the casting position of the upper surface of the subcutaneous. The main ingredients of the sand clip are SiO2, such as the coating is broken when pouring, the coating together with the type of sand into the casting of molten iron and the formation of sand clip. Because the sand is the molten iron flowing through the type of broken wall into the sand caused by the pouring edge, so the other parts of the castings can often see the sand clip. In contrast, molten iron slag generally enters the mold at the initial stage of pouring, its distribution is relatively concentrated, and the distribution of sand clip is relatively scattered. Compared with the traditional sand mould casting, the EPC adopts dry sand molding, the sand clamping defect is generally dispersed and the granular, rarely block. The main reasons for the defects of sand clamp are that the adhesive seam is not tight, the casting system has sharp angle, the coating is too thin, the local leakage coating and the low strength of the coatings are caused. In addition, the pouring cup and the top of the direct runner contact the site, such as tight seal, dry sand is easy to enter the straight runner with molten iron into the cavity formation of sand. It is necessary to note that the formation of sand clip is not only the molding use of sand, paint, slag, poured cup knot into the formation of the cavity of the defects, commonly known as sand clip.
Measures to prevent sand clamping defects (1) Guarantee coating continuous, homogeneous, complete: according to our country lost foam casting raw materials, craft and negative pressure pouring in the actual situation, small casting coating thickness general control in 1~2mm advisable. Pouring system due to hot metal scouring time long, momentum, the gate coating is generally thicker than the casting coating, the actual operation can be thickened. Control the entire coating does not crack, not breakage, do not leak white. (2) To improve the appearance of bonding quality: the appearance and the gate of the bonding seam should be tight, there is no openings and gaps, to prevent the penetration of the seams of paint. The appearance and the gate junction should be smooth transition, avoid the runner and the appearance of the adhesive site there is sharp angle, resulting in pouring sand. (3) To improve the coating strength: the appropriate coating strength is conducive to resistance to hot metal scouring, paint ratio, refractory binders and organic binders to match reasonable, so that the coating has a good comprehensive strength, enhance the ability to withstand high temperature scouring. Bentonite, Silica sol, water glass and other inorganic binders are conducive to maintaining the high temperature strength of coatings. (4) Use of special pouring cups and sealing mud: Dry sand molding need to use plastic film sealed sand box, the foam straight runner top cover under plastic film. The above need to place or erect the pouring cup, such as for erection of pouring cup of the iron mouth to have a certain straight segment to make the Tieliu cylindrical whereabouts, the erection should be with the foam straight runner Center to the positive; for placing poured cups under the combination of refractory mud and plastic film with tight, in both cases to master the outlet of the cup is less than the direct runner, to prevent the high temperature metal liquid flow will dry sand rushed into the castings.
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Making injection molded prototypes is both an art and a science. High levels of technical expertise and attention to detail are required to prevent small mistakes from costing companies big money when it comes to mass-production of novel parts.
Preventing such a circumstance is all about highly competent design. This article discusses some of the molding defects that can occur in a part during injection molding, and ways to fix and avoid them. Design shortcomings we will discuss include:
Most mistakes are caused by nescient personnel without the necessary experience or the right tools at their disposal. Conversely, creative solutions and ingenuity abound in personnel with the right experience and the correct combination of hardware and software. Finding the right team of people with relevant expertise is the most important part of the process.
Description: Flow lines are streaks, patterns, or lines - commonly off-toned in color - that show up on the prototype part as a consequence of the physical path and cooling profile of the molten plastic as it flows into the injection mold tooling cavity. Injection molded plastic begins its journey through the part tooling via an entry section called a gate. It then flows through the tool cavity and cools (eventually hardening into a solid).
Causes: Flow line defects are caused by the varying speed at which the molten plastic flows as it changes direction through the contours and bends inside the mold tool. They also occur when the plastic flows through sections with varying wall thickness, or when the injection speed is too low causing the plastic to solidify at different speeds.
Increase injection speeds and pressure to the optimal level, which will ensure the cavities are filled properly (while not allowing the molten plastic time to start cooling in the wrong spot). The temperature of the molten plastic or the mold itself can also be elevated to ensure the plastic does not cool down sufficiently to cause the defect.
Description: Sink marks are small craters or depressions that develop in thicker areas of the injection molded prototype when shrinkage occurs in the inner portions of the finished product. The effect is somewhat similar to sinkholes in topography, but caused by shrinkage rather than erosion.
Causes: Sink marks are often caused when the cooling time or the cooling mechanism is insufficient for the plastic to fully cool and cure while in the mold. They can also be caused by inadequate pressure in the cavity, or by an excessive temperature at the gate. All else being equal, thick sections of the injection molded part take longer to cool than thin ones and so are more likely to be where sink marks are located.
Causes: Vacuum voids are often caused by uneven solidification between the surface and the inner sections of the prototype. This can be aggravated when the holding pressure is insufficient to condense the molten plastic in the mold (and thereby force out air that would otherwise get trapped). Voids can also develop from a part that is cast from a mold with two halves that are not correctly aligned.
Description: Surface delamination is a condition where thin surface layers appear on the part due to a contaminant material. These layers appear like coatings and can usually be peeled off (i.e. delaminate).
Causes: Foreign materials that find their way into the molten plastic separate from the finished product because the contaminant and the plastic cannot bond. The fact that they cannot bond not only has an affect on the appearance of the prototype, but also on its strength. The contaminant acts as a localized fault trapped within the plastic. An over-dependence on mold release agents can also cause delamination.
Description: As the term implies, short shots can be described as a situation where a molding shot falls short. This means that the molten plastic for some reason does not fully occupy the mold cavity or cavities, resulting in a portion where there is no plastic. The finished product becomes deficient because it is incomplete.
Causes: Short shots can be caused by a number of things. Incorrect calibration of the shot or plasticizing capacities can result in the plastic material being inadequate to fill the cavities. If the plastic is too viscous, it may solidify before fully occupying all the cavities and result in a short shot. Inadequate degassing or gas venting techniques can also result in short shots because air is trapped and has no way to escape; plastic material cannot occupy the space that air or gas is already occupying.
Description: Warping (or warpage) is the deformation that occurs when there is uneven shrinkage in the different parts of the molded component. The result is a twisted, uneven, or bent shape where one was not intended.
Causes: Warping is usually caused by non-uniform cooling of the mold material. Different cooling rates in different parts of the mold cause the plastic to cool differently and thus create internal stresses. These stresses, when released, lead to warping.
Causes: Burn marks are caused either by the degradation of the plastic material due to excessive heating or by injection speeds that are too fast. Burn marks can also be caused by the overheating of trapped air, which etches the surface of the molded part.
Description: Jetting refers to a situation where molten plastic fails to stick to the mold surface due to the speed of injection. Being fluid, the molten plastic solidifies in a state that shows the wavy folds of the jet stream on the surface of the injection molded part.
Causes: Jetting occurs mostly when the melt temperature is too low and the viscosity of the molten plastic becomes too high, thereby increasing the resistance of its flow through the mold. When the plastic comes in contact with the mold walls, it is rapidly cooled and the viscosity is increased. The material that flows through behind that viscous plastic pushes the viscous plastic further, leaving scrape marks on the surface of the finished product.
Description: Flash is a molding defect that occurs when some molten plastic escapes from the mold cavity. Typical routes for escape are through the parting line or ejector pin locations. This extrusion cools and remains attached to the finished product.
Causes: Flash can occur when the mold is not clamped together with enough force (a force strong enough to withstand the opposing forces generated by the molten plastic flowing through the mold), which allows the plastic to seep through. The use of molds that have exceeded their lifespan will be worn out and contribute to the possibility of flash. Additionally, excessive injection pressure may force the plastic out through the route of least resistance.
A large number of the defects mentioned above can be prevented in the design process by incorporating proper tooling design into the iterative process. Using moldflow software like Solidworks plastics will help you identify ideal gate locations, anticipate air pockets, flow or weld lines, and vacuum voids. Most importantly, it will help you design solutions to these problems ahead of time, so that when it comes to production you do not have to worry about the defects costing you money.
Introducing various metal casting defects with many pictures by Dandong Foundry in China. These are the common sand casting defects on the surface and inside of cast iron and cast steel parts. 1. Blowhole and Pinhole This is a kind of cavities defect, which is also divided into pinhole and subsurface blowhole. Pinhole is very tiny hole, some could be seen on the surface. Subsurface blowhole only can be seen after machining or grinding.
This defect includes chemical burn-on, and metal penetration. Normally, you could see extra metal materials at the corner positions. This is caused by the poor sand. The metal has penetrated into sand molds.
Sand hole is a type of typical shrinkage cavity defect. You could see the empty holes after sand blasting or machining process. The sand dropped from sand molds, rolling into the liquid metal, then caused sand holes.
It is also called as cold shut. It is a crack with round edges. Cold lap is because of low melting temperature or poor gating system. This is not just surface defect. Normally, this position may cause air leakage, moreover, the material quality at this position will be very poor, so may be fragile.
Joint flash is also called as casting fin, which is a thin projection out of surface of metal castings. Joint flash should be removed during cleaning and grinding process.
Sharp fins and burrs are similar problems as the flash. Actually, large flash is a casting problem, the foundry should improve it by modifying the patterns, but small fins and burrs are not casting defects, foundries just need to grind and remove them.
Shrinkage defects include dispersed shrinkage, micro-shrinkage and porosity. For large porosity on the surface, you could see them easily, but for small dispersed shrinkage, you may see them after machining. The following photo is showing the porosity shrinkage. The metal density is very poor, many small holes could be seen after machining.
These are also called as shrinkage holes, which is a type of serious shrinkage defect, you can see these holes easily on the rough surface of the metal castings. Foundries could improve their gating and venting system, then could solve these shrinkage problems.
This defect is also a type of shrinkage defect, which looks like depressed region on the surface of metal castings. This defect is not serious as shrinkage cavity, but still cause poor surface quality and may have some inside defects, so foundries should try their best to solve or improve it.
It is also called as rat tail, which looks like many small water flow traces on the surface of metal castings. Sometimes, it is because of the low metal temprature melted, or unreasonable gating and venting system.
This mold defect is because of the shifting molding flashes. It will cause the dislocation at the parting line. Near the parting line, the left side may be several milimeters lower or higher than another side. The parting line is not defect, but if the left side has different height as the right side, it will be one casting defect. If there is no special requirement, mismatch smaller than 1mm is allowable. No any mismatch is impossible for sand casting process.
This is not casting defect, but it is a real casting quality problem you may meet. It is the damage during machining or delivery processes. The workers should pay more attention to this problem.
This defect is also called as exogenous inclusion, entrapped slag. Normally, the slag is from melted metal. During metal melting process, foundries should remove the dirt and inclusion completely, otherwise, these inclusions will be poured into the castings.
This is also called as under-nodulizing defect. Because of many reasons, the spheroidization of graphite for ductile iron will be affected, therefore, caused the bad spheroidization rate. By metalloscope, you can see very few graphite balls, and many worm-like graphite.
It means the uneven hardness on the same surfaces. The hardness is not uniform, some postions may have extra high hardness. When machining to harder positions, the machining will become more difficult. The drill bit may be broken.
It is also called as sand crush. Some sand blocks dropped from the sand mold, so they will cause the similar shaped sand holes or incomplete. This is the problem of sand molds. The sand molds may not be tight enough.
This problem will cause the oversized tolerance for flatness and straightness. This is very common defect for long castings, and flat castings with thin wall thickness. The reasons are the natural deformation during cooling process in sand molds, or in air, sometimes, the overly sand blasting also could cause this problem.
After welding repair, even after machining or grinding, the welding marks will still be visual. As for unimportant casting surfaces, if the client allow welding repair, then these marks should be acceptable. But for high pressure-bearing positions, or the client has clearly forbidden any welding repair, then these marks will be taken as defects.
Chill iron could effectively reduce the shrinkage for the key positions, so using chill iron is very common in iron foundries. However, the edges of chill irons could be clearly found by visual inspection. Some clients will not require to grind them if these marks do not affect the appearance. However, the clients could require the casting manufacturer to grind them just for better surface looking. Please clearly notice that these marks should not be judged as the casting defects. Refer to iron-foundry.com.
It is also called as "white iron". The surface of the castings with this defect will be extremely white, shiny and smooth. The defective castings will be fragile and crispy, so during machining, some edges and tips will be broken. This defect was caused by the low temperature of sand molds, and prematurely out of sand molds, so the hot iron become chilled quickly. The proper annealing heat treatment to them could solve this defect.
On metallograph photo, you could see many fish-bone free carbide. This is a serious defect of cast iron material, normally happen to ductile iron. Because of inverse chilling defects and poor inoculation, there will be mass free carbide, which will cause fragile, poor welding property to ductile iron castings. High temperature annealing heat treatment could improve its quality.
This defect is also called as internal sweating. There are iron beans in the castings. This is because of unreasonable gating design, which caused some liquid iron became beans suddenly, then these beans were wrapped by other liquid irons. This is a surface defect, but will cause serous problem if they located in key positions.
There is very thin iron skin on the surface of castings. Two layers. This is because of unreasonable gating system, which caused very thin air layers existed. This defect is a surface defect, so normally it can be grinded off. However, it should be discarded if it is not just on the surface.
This defect is a kind of material problem. It is caused because of low pouring temperature and high content of carbon element. This defect is very harmful, will cause material very fragile. On the broken surface, you can see the obvious black surface caused by this defect.
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To do this, there are three keys to remember: 1. Focus on identifying a casting defect on the basis of its appearance. 2. Be aware of the interactive nature of foundry processes and variables. 3. Use rigorous experimental design methods to study complex causes of defects.
Foundry personnel can be quick to label a defect cause based on a cursory examination. Terms such as slag defect and cold shut are part of the defect process. The International Casting Defect Atlas gives a specific code/category to defects based on appearance. It also suggests foundry personnel be aware that most casting defects are due to the interaction of several process variables rather than one factor, such as temperature or gating system design. This allows the foundry engineer to design experiments to capture the complexity of the defect cause. AFS Corporate Member CWC Textron Foundry (Muskegon, Michigan) used this process of casting defect categorization, identification, cause determination and defect reduction to address an issue with camshaft castings.
The defects began occurring after an upgrade from manual pouring of molds to automatic pouring of molds. Auto pouring systems are considered to be safer reliable and efficient compared to manual systems.
However, in the early tests comparing castings from manually poured molds to castings from automatically poured molds, the casting scrap rate was always significantly higher for the automatically poured molds. Other variables, such as pouring temperature and transfer time impacted the scrap rate.
This gray and ductile iron foundry has been producing alloy and ductile camshafts for many decades. All molds are produced on a horizontally parted, high pressure, tight flask, green sand, automatic molding line. Flasks on this molding line move on a conveyor to the pouring area.
Primary melting is done with a cupola. The base iron is held in a channel induction furnace. MgFeSi is added to a large tundish ladle as it is filled from the induction furnace. In manual pouring, the ladles were filled from the tundish ladle and in-mold inoculation was used. The main sources of casting scrap are were simply listed as dirt or slag.
Now, an automatic pouring system has replaced the manual pouring ladles. The equipment was acquired and installed to make mold pouring safer and more consistent from mold to mold. The automatic pouring system ladle holds significantly more iron than the manual pouring ladles. During the transition to automatic pouring, the manual pouring area was kept intact.
Whenever molds were poured using the automatic pouring system, the camshaft castings exhibited a high frequency of cope-side inclusion defects. These defects were small, but deep enough to cause the castings to be scrapped.
Step 1 Identify the Defect Foundry personnel have a tendency to identify a defect based on cause like slag defect or sand inclusions. While this is an acceptable method, after the diagnosis is done, the International Atlas of Casting Defects recommends that unknown defects be classified based on appearance rather than cause. Using the photos and descriptions given in the atlas, the foundry decided these defects appear to be G121 (inclusions) or B123 (pinholes) (Figures 2 and 3).
Detailed optical micrography confirmed the defects were dross and pinholes related to magnesium vapors. It is essential to verify the casting defects through such detailed analysis, prior to exploring cause and remediation steps. In addition to optical microscopy, SEM (scanning electron microscopy) along with spectroscopy was used.
Step 2 Experimental Design When the castings were poured with the manual ladles, the frequency of inclusion defects was very low. When the castings were poured with the automatic pouring system, the frequency of inclusion defects was very high. Also, it was observed that on a same day at the same time, under similar sets of green sand parameters, frequency of the inclusion defects was still higher with the automatic pouring system compared to manual pour. This would suggest that metal and molding sand were not primary causes of defect. For this reason, no trials were run with modified base iron metallurgy or modified molding sand properties.
Instead, CWC focused on studying the pouring temperature, pouring time and method, Mg-treatment and inoculation method (tablet in manual pour, in-stream in automatic pouring unit). Table 1 summarizes the key variables and factors considered. Detailed fractional factorial design experiments were designed based on well-known statistical methods. Most foundry casting defects are caused by interacting variables (for example, low pouring temperature + specific chemistry).
Factorial design is a tool that allows experimentation on many factors simultaneously. In this case study, researchers ran 2 level factorial design with three factors requiring a total of eight runs. Three factors considered in the experiment were: ladle type (regular ladle and insulated ladle), temperature (lower pouring temperature and higher pouring temperature) and pour cup (D-shaped cup: conical cup with one side flat- and offset-basin cup). Detailed factorial design/experimental design is provided in Table 2.
Step 3 Gating Design and Filtration Review Often, foundry personnel jump to modifying the gating system when they observe slag/dross defects. While turbulence in the gating system may be an important factor, the gating system is one of the few constants in the multi-variable production environment of the foundry. Since this gating system worked well in the manual pouring system, no major modifications were proposed. It was noticed that the cross-sectional area of the sprue base and runner was very large and the pouring time was controlled by manual pouring operation. It is strongly recommended that computer simulations of solidification and flow are conducted to review the performance of the gating system. Oxidation of iron and formation of inclusions will likely increase as the velocity of the metal increases. Slowing down the flow and keeping the gating system full may show a reduction in surface inclusions.
Filters should be considered an insurance policy rather than the main function of keeping external slag and dross away. Inefficient dross removal practice can lead to filter blockages, quickly leading to misruns and slow pours. Filters are often considered flow modifiers as significant dross in castings are related to turbulence in the runner and ingate system.
There are two aspects of filter sizing: the primary sizing is related to ensuring that the filter does not act as the choke. The standard rule of thumb is the cross-sectional area of the filter should be at least 4-6 times that of the choke.
The secondary sizing requirement is related to filter capacity, or the volume of ductile iron that can be passed through the filter prior to blockage. Typically, this capacity is around 20-40 lb./square inch of filter.
Filter pore size can be classified as fine, medium and large. In most applications, medium or large openings are preferred. The filter supplier can provide appropriate sizing sheets for particular castings and filter types.
Step 4 Preliminary Trials It is important that trials be conducted with just a handful of variables. Proper experimental designs are required to ensure interaction effects are captured (effect of pouring time, temperature and chemistry together, for example). Pours were grouped by heat, and at least 10-20 molds were poured per heat. It is important to measure and document all variables related to the casting.
Step 5 Production Trials Production trial volumes and details are important in a quality assurance program. Production trials for automotive applications typically require thousands of castings. Some foundries might review data for a whole shift or for several heats to ensure repeatability and reliability of quality. Data tracking includes pouring temperatures, pouring times, microstructure, chemistry, lab tests related to mechanical properties, and other information.
During the preliminary and production trials, the pouring was done automatically but the metal transfer was manual (by forklift) and, therefore, took more time, resulting in a higher than normal temperature loss. Following the five-step process, key recommendations to reduce the defect included:
Increase the pouring temperatures. Control pouring temperatures (by improving ladle insulation). Reduce magnesium additions. Increase the bismuth addition after MgFeSi treatment. Improve the automatic pouring ability to pour in the center of the cup. Reduce velocity and turbulence of the metal in the mold. Operate the automatic pouring system with fully automatic ladle filling.
The casting defects are inherited by wrought materials, possibly with slightly less effect on scatter depending on the amount of plastic deformation involved. However, this is assuming that the working process does not crack and destroy the cast product on its first pass or other early passes through forges or mills. Ingots of Ni-base alloys poured badly, such as top-poured in a vacuum induction melting furnace, are well known for cracking on the first stroke of the forge.
Assuming the forging work piece survives the forging process, it seems that the deformation process merely pushes the cast bifilm defects around a little, but otherwise has only a minor effect on the defect population. Forgings therefore suffer nearly as much scatter in properties as castings. Even rolled sheet, extrusions, and drawn wire do not completely shed the disadvantages inherited from the original bifilm population. This is a consequence of the stability of the oxides and the residual air in the form of the 1% of insoluble argon which will always remain in association with the oxide inclusion. The residual argon bubbles are volume defects which allow the apparent nucleation of pores and cracks from the inclusion. The so-called alumina stringers seen on polished sections of wrought steels are almost certainly the residues of alumina bifilms (because the stringer on the 2D section has to be a plane in 3D), and porosity (the residual argon bubbles) is nearly always detectable in association with the fragments of oxide (Fig.1.55).
The unbonded interface of the bifilm appears therefore to survive significant plastic deformation. This is perhaps to be expected from such stable ceramic interfaces and the remnants of air between the faces of the films, trapped in rucks and folds, and in the microscopic roughness of the interfaces. This reservoir of oxidizing or nitriding gas will continue to create new film, as a bifilm is extended by plastic working, thus preventing bonding. Only when the reservoir of air is consumed will new extensions of the crack start to bond. Finally, of course, the remaining 1% of argon in the air will remain as a permanent pore because argon appears to be completely insoluble in metals.
These thoughts match the observations of Toda etal. (2009) who studied hot and cold deformation of an AlMg alloy up to 22% strain which failed to permanently close pores. Even for the hot rolling of steel, Joo etal. (2014) observed the closure of some pores, but generally found that bonding was rare or not at all. Given that, on occasions, there will be a generous supply of residual argon in the bifilm, it is perhaps not surprising that the bifilm will have an extended life, if not an effectively infinite life, despite the punishment it may receive.
Al alloys retain their unbonded regions even after the severe extrusion required to produce millimeter-thick window frames, as is evident from filiform corrosion (FFC). This degradation of the surface is clearly seen by eye on unprotected extrusions, in which tens or hundreds of corrosion sites per square centimeter follow the elongated unbonded bifilms that tunnel through the metal, happening to intersect the surface from time to time to create long lines of corrosion sites. The survival of the bifilms during plastic working is probably the result of the reservoirs of air that remain trapped in the rucks and folds between the oxide surfaces. Even after the consumption of the oxygen and nitrogen, the residual argon in the air will, of course, assist to further delay the onset of bonding, as will any residual hydrogen in the alloy.
From much work carried out on the application of pressure during solidification and from compaction during such processes as hot isostatic pressing (HIP) of cast metals, the benefits to properties are almost certainly the result of the closing of defects such as pores and bifilms. There is evidence in many metals that pores and bifilms, especially if the oxides are relatively thick, are reluctant to bond or weld (Staley etal., 2007) or only bond in limited places (Raiszadeh and Griffiths, 2008), which is to be expected from the great stability of some of their surface oxides and nitrides (exceptions to this include those oxides that react at the temperatures of HIP (Lumley etal., 1999). Thus, although the defects will remain weak in tension between their surfaces, the increased strength properties almost certainly arise from the fact that the defects are now closed; their contacting surfaces can now at least resist shear as a result of friction and jogs as shown schematically in Fig.1.36. Whether or not the defects will fully remain unbonded, or will bond, will now depend on the chemistry of the interfaces and the conditions of time and temperature.
Fig. 24.6a and b show casting defects in an Al automobile cast component . Large defects are detrimental to fatigue strength, so it is difficult to set a high allowable stress. High-quality components having small defects are desirable but costly. Inspection of a small number of samples does not guarantee the safety size of defects for mass production. The method of the statistics of extremes is useful for these cases.
Fig. 24.6c shows the data of statistics of extremes for the Al cast materials produced by three casting methods. The role of the design engineer is to choose one from these three materials. Thus, the data of Fig. 24.6c give the information for determining quantitatively the largest size of defects from the numbers of productions.
There are some basic categories of casting defects standardized by International Committee of Foundry Technical Associations, as cited by Kalpakjian . Among them cavities consists of rounded or rough internal or exposed cavities, including blowholes, pinholes, pipes, shrinkage areas, and porosity. Because of their thermal expansion characteristics, metals shrink during solidification, and cooling to ambient temperature. Shrinkage cause dimensional changes and sometimes cracking and cavities; therefore, it is an important parameter to consider when determining the resistance and toughness of the casting mechanical components. Porosity may be caused by shrinkage and/or off gassing during solidification. It causes adverse effects on the mechanical properties of the castings and may affect the fatigue resistance of the component.
The railway couplings chemical composition showed that the cast material is that of a low-alloy manganese steel with 0.31% carbon, 1.7% manganese, 0.37% silicon, and small percentages of additional elements.
Cast steels containing those percentages of manganese and silicon present good mechanical properties that can improved by heat treatment. The sulfur and phosphorus contents are below the minimum imposed by ASTM requirements for this class of steel casting. Furthermore, manganese tends to inhibit the effect of sulfur promoting intergranular weakness.
Tensile test on the mechanical properties of the railway coupling that was obtained for the study presented the following mean values : 0.2% yield stress, Sy=463.95MPa; ultimate tensile stress, Sut=659.58MPa; rupture strain, =28.5%; reduction of area, q=47.9%; Young modulus, E=207GPa; ratio Poisson, =0.3.
In order to evaluate quantitatively the effects of casting defects on the fatigue properties of aluminum die-casting alloy, a Kth for aluminum die-casting alloy containing large casting defects was proposed by means of area method. Fatigue tests were carried out by using a rotating bending fatigue testing machine for the specimen having four artificial drilled holes whose area were ranged from 300 to 2000m. According to the fatigue limits of the hourglass type specimen and of the smooth specimens having four artificial drilled holes, equation for Kth estimation for aluminum die-casting alloy was proposed by means of area method.
Radiography, X-ray, is accepted as the highest grade of internal inspection and the most costly. Radiography is good because a permanent record of the inspection is available at any time for review. It is used mostly for welds and also for critical areas in castings. Components can be taken to fixed X-ray equipment. Large components or assemblies are radiographed using radioactive isotopes. Strict safety precautions must be enforced. In view of its importance for the integrity of high performance fans, this method is described in the greatest detail.
The most technically advanced companies in the fan industry have facilities which by using real time techniques cut the time defects by about two-thirds, enabling production and delivery improved. The system (Figure 17.1) is more sensitive, more t also provide a far more comprehensive and easily accessible s: previous X-ray units. Each moving part is stamped with a all X-ray images are automatically archived onto 50 mm laser. 30 years, providing full component traceability.
Inspection by X-rays is carried out by irradiating one surface of the specimen with X-rays whilst a radiation sensitive electronic imaging sensor is held against the opposite surface. The radiation, in passing through the specimen, is differentially absorbed by discontinuities caused by flaws, voids, changes in thickness or material density, and an image of the variations integrated throughout the sample thickness, is produced on the surface of the electronic sensing screen.
After the electronic image has been noise reduced, it is displayed on screen where variations within the specimen appear as shadow objects of differing half tones, from which information may be obtained about the presence of flaws. The record produced in this way is known as a real time radiograph. Real time because the image display is live and if the specimen is moved the X-ray radiograph changes to show the corresponding incident shadow on the image display. The use of X-rays to produce a radiograph is called X-Radiography. Figures 17.2 and 17.3 show two examples of impeller blade radiographs.
X-rays are a form of electromagnetic radiation which may be generated by causing a stream of fast-moving high energy electrons to strike a metal target. The sudden deceleration of electrons gives rise to radiation of photons (X-rays) with a continuous energy spectrum.
X-rays possess great penetrating power which increases with increasing energy of the waves (increasing frequency or shorter wavelengths). X-ray equipment is defined by the energising voltage, which can typically range from 25 kV to 15 m V. X-rays can be used to examine items varying from layers of paper to steel of thickness up to 0.5 metre. All materials are penetrated by X-rays, but the greater the density, the less the penetration.
Radiation of short wavelength produced by high target potential is said to be of high energy and is described as a hard X-ray, having greater penetrating power. Longer wave radiation produced by lower target potential is said to be low energy and is described as soft X-ray, having lower penetrating power.
For low energy, constant potential X-ray generators, the beam intensity produced by the X-ray tube is determined mainly by the magnitude of the filament current, and to a lesser extent by the target potential. A near linear relationship exists between filament current and beam current so it is customary to express the output capability of such a tube in terms of filament current.
The quality of a real time X-ray radiograph is nearly always quoted in terms of the amount of detail discernible on the image of an image quality indicator (IQI) of the same material as the specimen placed on the surface of the specimen. This IQI sensitivity depends upon the radiographic technique used, the type of IQI and specimen thickness. When radiographing other materials other than steel it is customary to use conversion tables related to the material and radiation energy to obtain approximate equivalent thickness factors.
In the UK two different patterns of IQI are recommended, known as the wire type and the step hole type and one or the other is commonly used in most European countries. In the USA an ASTM-plaque is generally used.
IQI sensitivities are expressed as percentage values, i.e. the size of the minimum discernible IQI details is expressed as a percentage of the specimen thickness, thus a smaller numerical value implies a better sensitivity. Typical radiographic sensitivities range between 0.5 and 2.5 percent depending upon inspection variables.
For example, an X-ray radiograph image showing the existence of lack of fusion commencing 50 mm (2 inches) from the reference mark over a length of 25 mm (1 inch) and the defect repeated 150 mm (6 inches from) the reference mark over a length of 25 mm (1 inch), and also localized porosity for 19 mm (0.75 inch) at a distance of 150 mm (6 inches) from the reference mark, the code would be 2-L-1: 6-PL-0.75: 8.5-L-0.5.
Intermediate stage product inspection or intermediate radiography. As a general rule when the items are cast, an inspection at that stage segregates the good castings and rejects before any value is added to the casting. This minimises wastage of time before the casting is handled and cleaned for burrs and excessive materials.
Radiographic technique is usually defined for different products. This inspection stage can be carried out any number of times before the finished product stage. In this mode product inspection records are not normally required. Good castings are transferred to the next production stage and rejects dealt with as appropriate.
Intermediate radiography with image storage. Quality demands on products may stipulate a minimum acceptance quality for defect sizes and type. Proof of acceptance based on records may be required by independent inspectors. Once the products are accepted for the next stage, radiographic records may be required for short term storage requirements, possibly 6 to 24 months.
Real time radiography with record keeping and digital or analogue long term image storage. Safety critical and sensitive application products normally require inspection records to be held in archives for the duration of the product life. A complete history of the product has to be maintained. Stringent quality control inspection specifications are stipulated and adherence to the specification is mandatory.
It should be noted that subsurface imperfections can only be determined by methods such as radiography and ultrasonic. There have been many instances where an apparently good casting has failed, only to reveal quite massive internal faults.
Shrinkage subsurface: this is often referred to as centreline shrinkage since it occurs near the mid point of the casting wall which is the last to solidify. Since shrinkage is a subsurface condition, it should be evaluated by radiography.
It will be apparent that strength and integrity are closely related to the quality of a component as cast. The criteria for acceptance are generally those described in ASTM Standard E155 together with its reference radiographs. A procedure should be adopted which defines the inspection process. Before X-ray or fluoroscopic examinations are carried out, the following checks should be made:
Blades may be divided into 3 basic categories as shown in Figure 17.4 where areas required to be of high integrity are shown cross-hatched, whilst areas having a lower integrity requirement are shown plain.
There shall be no continuous defect line during X-ray between the insert and aluminium casting. The defects shall not be longer than 3 mm. The total defects shall not be greater than 6 mm and shall not be adjacent to one another.
The techniques described will act as a powerful tool to identify areas for improvement or for process variables to be tailored to improve overall quality of product. It is essential that a design and testing procedure is adopted which recognizes that a major cause of failure especially in axial flow impellers is due to insufficient knowledge of the fatigue criteria and how they are affected by casting quality.
Close co-operation between design and production departments is necessary to ensure that the stated operating life is achieved. Constant vigilance is, nevertheless, indicated with continual research to improve knowledge. By this vigilance, product integrity can be assured.
NCI contains numerous graphite nodules as well as casting defects such as shrinkage cavities with various shapes and sizes. Thus, the fatigue limit is not a fixed material constant and it inevitably shows scatter. Therefore, laboratory testing of a limited number of small-sized specimens is likely to provide a nonconservative fatigue strength in comparison with the actual fatigue strength of high-volume commercial products. From the engineering point of view, therefore, it is essential to predict the lower bound of the scatter of fatigue limits by taking the statistical nature of the microstructure into account. Fig. 11.17 shows the extreme value statistics distribution of the maximum size of graphite nodules, areamax, for NCIs as shown in Fig. 11.11. It is noteworthy that the distribution of areamax has no specific relation to the mean graphite nodule diameter, dm (cf. Fig. 11.11). This means that the fatigue strength estimate based on dm is not only meaningless but also nonconservative in some cases. Based on the same procedure employed for nonmetallic inclusion (cf. Chapter 6: Effects of nonmetallic inclusions on fatigue strength), the lower bound of fatigue limit of NCI can be estimated by assuming the worst-case scenario that a graphite nodule with the maximum size expected for a control volume of specimens or products is located just below the surface. Here, the lower bounds are calculated by substituting areamax expected for five specimens and HV=HVmatrix into Eq. (6.5).
Fig. 11.18 shows the design curves of fatigue limit and the experimentally obtained results. Each curve is composed of a horizontal line for the lower bound for five smooth specimens and a curve for the fatigue limit of the specimen with a detrimental defect. Both curves are calculated using the Vickers hardness of the matrix, HVmatrix, converted from the value of the gross Vickers hardness, HVgross using Eq. (11.2). The value of area at the point of intersection represents the critical size of the nondamaging defect other than graphite, that is the defect with area smaller than this critical size does not decrease the fatigue limit of a smooth specimen. The Vickers hardness values of FCD400-S, -M and -L are similar to each other, and the fatigue limits result in almost the same value for the same area when the defect is greater than the critical size, cf. Fig. 11.18a. A clear difference arises only in the fatigue limit of smooth specimens due to the different size distribution of graphite nodules.
The fatigue strength of smooth specimens can also be determined by casting defects such as microshrinkage cavities. Fig. 11.19 shows an example of cavities observed in FCD400-M as shown in Figs. 11.11b and 11.20 shows its extreme value distribution , in which the distribution of graphite nodules is also shown for comparison. For irregularly shaped cavities, areamax is defined as the square root of the area of a circumscribed circle as shown in Fig. 11.19. The number of cavities is much less than that of graphite nodules. In addition, the size of the cavity that is commonly observed on a small sample is, in most cases, comparable to or smaller than that of the surrounding largest graphite nodule, cf. Fig. 11.20. However, the presence of the microshrinkage cavity cannot be ignored because of the low gradient of the trend line. The value of areamax of the largest cavity expected for five specimens reaches 295m, which gives a lower bound of 162MPa as shown in Fig. 11.18a. This stress level is 26% lower than the experimental value (220MPa). The failure from a cavity will usually not occur under a stress slightly above the predicted lower bound (162MPa) unless a cavity with this order of the size (areamax=295m) exists by chance just below the surface. However, if a larger stress than the predicted lower bound is carelessly applied for the mass product, unexpected failures will occur with a significantly high probability.
The nature of flow during mold filling results in casting defects such as gas entrapment, oxide film, misruns, cold shut, etc., making the analysis of mold filling important in the production of quality castings. The development of these and other models has been reported at a series of conferences on modeling of castings, welding, and advanced solidification processes. A bench mark problem was evolved by Cross and Campbell (62). A successful filling algorithm should be able to handle free surface flow and turbulent flow in the mold filling stage. Of the algorithms that have been used for modeling mold filling, the volume of fluid (VOF) method is computationally more effective than the MAC method. Although the FEM can generate complex gird system more effectively than FDM, it is known to be unsuitable for predicting flows. The semi implicit method for pressure-linked equations (SIMPLE) algorithm coupled with VOF method (63) based on body-fitted coordinate has been reported for modeling the mold filling of thin-walled and curved castings (64).
For a preliminary discussion of transport phenomena, the reader is referred to Ref. (6). The methodology is briefly described here. The integral form of the continuity equation and momentum balance equations are discretized using any of the several methods like MAC, SOLA, or simplified marker-and-cell (SMAC). The discretized equations are combined with MAC or VOF methods and solved to analyze to free surface during mold filling. A description of scheme can be found in Ref. (5), where the free surface tracking by MAC and VOF for three-dimensional (3D) flows is considered.
Some of the effective techniques for modeling the free surface are MAC and VOF methods. The MAC algorithm has the following features: primitive variable and finite difference scheme in a cell structure and use of marker particles. The Eulerian mesh is used for solving velocity field and the instantaneous positions of markers in the Lagrangian coordinate system. As the marker particles move with the fluid, the cells that are empty get to fill over a period of time depending on flow field. An improvement in the MAC method is a simplified MAC or SMAC. As against the MAC, VOF method solves for fractional volume of fluid occupying the cells. The fraction may vary from 0 (empty cell) to 1 (cell filled with fluid), while anything in between represents free surface. A detailed account of these algorithms as applicable to mold filling can be found in Ref. (5). The SIMPLE method can be adopted for body-fitted coordinate systems, which have been developed to mold filling of thin-walled and curved castings (64).
The ASTM standard stating the types and numbers of defect classes is currently the basic method of classifying casting defects; however, a diagnostic decision of a person making the classification of defects in casts according to this standard involves a significant subjective error. A method of analyzing casting defect images obtained in X-ray flaw detection tests of the parts produced of AlSiCu alloys was proposed along with the methods of classifying casting defects using artificial neural networks in order to objectivize the results of casting quality evaluation (60,61).
Casting defects identified in the castings of combustion engine parts were classified in accordance with the ASTM E155 standard. X-ray flaw detection tests were carried out for the castings of six- and eight-cylinder car engine blocks and heads manufactured with the Cosworth method. The group of flaw detection photographs taken was fundamental for the further analysis. The size and scale of the images analyzed were harmonized in the first step. Filtration, decimal-to-binary conversion, and edge detection were next performed. Defects were extracted and their geometric characteristics calculated at the next stage. The analysis of sections of engine blocks and heads allows preparation of a casting image in such a way as to detect a casting image to enable the detection of the edge of the objects provided in the image and then to extract those that are the images of casting defects. The images of casting defects are extracted from all the objects provided on the photograph by calculating the geometric factors used in the paper and then by comparing whether the calculated values are within the assumed confidence range. Figures 13 and 14 illustrate an X-ray image of a combustion engine block section and a binary notation of this image on the basis of which casting defects can be identified.
The interval estimation was applied to estimate the intervals the values of geometric coefficients describing casting defects morphology should be in. Confidence intervals for average values were determined. It was found in this way, based on the calculated geometric coefficients, which objects represent casting defects, and the characteristic values of coefficients were determined for the individual groups and classes of defects. The classification of casting defects into four groups was used: gas cavities, porosity, shrinkage cavities, and microshrinkages. Eight classes were distinguished for each group. An assumption was used that the investigated values of geometric coefficients have normal distribution.
The data set utilized for creating a neural model was divided randomly into three subsets: training, validation, and test. The following was used for classification purposes for evaluating the quality of the model created with a neural network: the share of correct classifications expressed as percentages, neural network response concentration charts, and network prediction error histogram for a given class of casting defects.
the Feret coefficient characterizing defect elongation calculated as a quotient of the maximum casting defect diameter in the vertical direction and the maximum diameter of the casting defect in the vertical direction,
The influence of independent variables on the network response was analyzed while designing an artificial neural network. The purpose of the analysis was to consider only such coefficients describing the casting defect morphology, the influence of which on the classification result is significant. A genetic algorithm was used for selecting input variables. The following independent variables were used for the classification of casting defects according to the calculations made: defect surface area, horizontal Feret diameter of defect, vertical Feret diameter of defect, defect circularity coefficient, and defect roundness coefficient. The number of neurons in the hidden layer (layers) and the training method and parameters were assumed depending on the network type and the influence of such values on the correct classification factor taken for evaluating the classifier quality. A network response was coded with 33 neurons corresponding to the defect group and class. The same method of network response coding consisting of making decisions independently for each class may result in assigning the defect analyzed to several classes or unclassified cases. The highest ratio of correct classifications was obtained for the MLP-type network trained with the backward propagation method and conjugate gradients method with two hidden layers used and 15 neurons in each layer.
Other examples of network response coding are presented in (62,63). The methodology presented enables evaluation of the quality of castings of car engine blocks and heads made of AlSi alloy of the EN AC-AlSi7Cu3Mg type. The calculation methods used in the work were verified with the data not used for neural network training.
The very high investment costs of power installations make it necessary to determine the practical available life and the time of safe operation of equipment or parts thereof after finishing operation within the time resulting from the calculated life. The issue comes down to determining the so-called residual life of a particular device, installation, or part, defined as the difference in time between the practical and calculated available life. Low-alloy chromium-molybdenum steel, in particular 13CrMo4-5, 10CrMo9-10 and 16Mo3, is used most often in the power industry for parts or pressure installations. Changes in the internal structure of steels as a result of long-lasting increased temperature make it necessary to evaluate part condition. The metallographic method is widely used for evaluating the degree of internal damage due to creepage, where an image of metallographic structures is obtained in tests with the light microscope, scanning electron microscope, transmission electron microscope, or confocal microscope. Extensive experience is required for classifying internal damages of steel after long-term exploitation in creepage conditions. It is reasonable to develop a computer-aided internal damage classification system of steels working in creepage conditions to objectivize the evaluation. A methodology was therefore elaborated of analyzing the metallographic images of steel structures obtained as a result of investigations in the scanning electron microscope and a system was created for classifying such damages using artificial neural networks and image analysis. The model was simplified to include five classes:
The image analysis methods were used to separate the images of damages visible in metallographic photographs obtained as a result of tests of low-alloy chromium-molybdenum steel with the scanning electron microscope. The metallographic photographs of steel structures were initially harmonized according to the size, contrast, and resolution. Median filtration, decimal-to-binary conversion, indexation, and binary erosion were next done. A decimal-to-binary conversion threshold was selected experimentally. The surface areas and circumferences of the separated objects were calculated in the next stage along with the minimum and maximum distance between the objects and the shape coefficient values.
The MLP-type network was used for the classification of internal damage conditions. The values of geometric coefficients calculated for the individual internal damages were used as input data. The input variables important for proper classification were selected using the genetic algorithm. The number of neurons in the input layer was successfully decreased and the number of correct classifications increased by employing a genetic algorithm for selecting independent variables. The following geometric coefficients were used as independent variables: surface area, circumference, Malinowska coefficient, circularity coefficient 1, circularity coefficient 2, minimum distance, horizontal Feret coefficient, vertical Feret coefficient, content factor, dimensionless factor. The network output was coded by means of five neurons, each representing an internal damage class. The data set was split into three subsets: a training, validation, and test. Twenty-five percent of training vectors were assigned to each validation and test set. A network with 31 neurons in the hidden layer trained with the backward error propagation and conjugate gradients method yielded the best results. The work of the neural network was evaluated according to the correct classification factor, the average value of which was 93%.
Mould and core materials play an important role in producing serious casting defects as mould erosion, metal penetration, scab, and hot tear. Although certain correlations have been put forward between the properties of steel foundry mould and core materials and casting defects based on investigational laboratory work under controlled conditions, there is a paucity of data from actual shop floor experience. One difficulty in collating information from foundries is, probably, the lack of time because the castings must be produced and dispatched according to very tight schedules. Many foundries have made attempts to mark castings and note the properties of the mould and core materials from which they were manufactured. However, a careful examination of the castings subsequently has not often been possible because of a lack of personnel to follow the components before they were fettled and repaired. It is certainly necessary to establish norms for acceptance tests for mould and core materials, and in conjunction with planned laboratory work valuable empirical data from the shop floor should be obtained to find correlation of properties and casting defects. A possible way is to have suitable inspecting staff who would work in close liaison with the sand controller and records should be kept. This chapter presents some lines along which records should be kept.
This chapter focuses on scab defect. Scabs and such related casting defects as rat-tails and buckles are the result of cracks appearing in the sand mould or core wall when the material is subjected to rapid heating during casting. Cracks are caused by the rapid and considerable expansion of quartz during its transformation to the form at the temperature of 573C. The parts near the metal interface will expand more than those inside the mould wall because of the temperature gradient existing. If a single crack appears, the casting shows defects known as buckles or rat-tails. Scab defects are more common with green synthetic sands than dry or silicateCO2-bonded sands. It is assumed that the moisture evaporated from the heated crust condenses behind forming a wet sandsand interface within the mould. The chapter presents some experimental studies of scab and related defects and possible remedy for scab defect.