Alloy casting performance concept: Casting performance refers to the ability of alloy casting to obtain high quality castings. Casting Properties of the Alloy: Filling capacity (fluidity) , shrinkage, oxidation, segregation and gettering, etc. . The casting properties of the alloy have a significant influence on the casting process, casting quality and casting structure design. Therefore, in the selection of casting parts of the material, should be in the premise of ensuring the use of performance, as far as possible to choose good casting performance of materials. However, in actual production, in order to guarantee the service performance, some alloys with poor casting performance are often used. At this point, more attention should be paid to the design of casting structure, and to provide appropriate casting process conditions, in order to obtain good quality castings. Therefore, it is necessary to fully understand the casting properties of the alloy. Filling capacity of Alloy 01 definition of filling capacity of alloy 01—— The ability of a liquid alloy to fill a mold and obtain a casting of correct size and clear outline, is called filling capacity of a liquid alloy. The filling process of liquid alloy is the first stage of casting formation. There are a series of physical and chemical reaction phenomena, such as the flow of liquid alloy and the heat exchange between the liquid alloy and the casting mold, accompanied by the crystallization of the alloy. Therefore, the filling capacity depends not only on the flow capacity of the alloy itself, but also on external conditions, such as casting properties, pouring conditions, casting structure and other factors. 02 The influence of the casting quality on the casting quality -- The liquid alloy is easy to obtain the thin-walled and complicated casting, and is not easy to appear the defects such as unclear outline, insufficient pouring, cold separation, etc It is beneficial to the floating and discharging of gas and non-metallic inclusion in liquid metal, reducing the defects such as air hole and slag inclusion, improving the feeding ability and reducing the tendency of shrinkage hole and porosity. 03 influencing factors on filling ability of alloy and its technological countermeasures

(1) definition of fluidity of alloys -- fluidity is the fluidity of liquid alloys. It is an inherent property of the alloy, depending on the alloy type, crystallization characteristics and other physical properties (such as the viscosity, the heat capacity, the smaller the thermal conductivity, the greater the crystallization latent heat; the lower the surface tension, the better the flow) . Methods of measurement-in order to compare the fluidity of different alloys, standard spiral specimens are usually cast to determine. The fluidity of the tested alloy can be represented by the length of the flow sample obtained under the same casting mold (usually sand mold) and pouring conditions (such as the same pouring temperature or the same superheat temperature) . The fluidity of gray cast iron and silicon brass is the best, while that of cast steel is the worst. For the same kind of alloy, the influence of various casting process factors on the filling capacity can also be investigated with the flow specimen. The resulting length of the flow sample is the product of the time of the liquid metal from the start of pouring to the end of the flow and the flow velocity. So anything that affects the two factors above will have an impact on liquidity (or filling capacity) . The chemical composition of the alloy determines its crystallization characteristics, and the influence of the crystallization characteristics on the fluidity is in a dominant position. The alloys with eutectic composition (such as FE-C alloy with 4.3% carbon) are solidified at constant temperature, the inner surface of solidified layer is smooth, and the flow resistance to subsequent liquid metal is small, in addition, the EUTECTIC alloy has a low solidification temperature, and it is easy to obtain a large degree of superheat, so it has good fluidity, there are two-phase zone of liquid and solid coexisting in the casting cross section. The flow resistance of the first dendrite to the subsequent liquid metal is larger, so the fluidity decreases. The more the alloy component deviates from the EUTECTIC component, the bigger the solidification temperature range is, the worse the fluidity is. This is the reason why alloys with near eutectic composition are often used as casting materials.

(2) Mold Property 1. The heat storage coefficient of a mold. It represents the ability of the mold to absorb and store heat from the molten metal in the mold. The greater the heat conductivity, specific heat capacity and density of the casting material, the stronger its heat storage capacity, the stronger the chilling energy to the liquid metal, the shorter the time that the liquid metal keeps flowing, and the worse the filling capacity. For example, mold casting is more likely to produce defects such as insufficient pouring and cold insulation than sand casting. 2 preheating the mold with mold temperature can reduce the temperature difference between the mold and the liquid metal, reduce the heat transfer intensity, thus improve the filling capacity of the liquid metal. For example, when aluminum alloy castings are cast in permanent mold, the mold temperature is raised from 340 °C to 520 °C, and the length of spiral sample is increased from 525 mm to 950 mm at the same pouring temperature (760 °c) . Therefore, preheating mold is one of the technological measures that must be taken in metal mold casting. 3 The gas mold in the mold has certain gas generating ability, can form the gas film between the liquid metal and the mold, can reduce the flow resistance, is advantageous to the mold filling. However, if the gas volume is too large, mold exhaust is not free, in the cavity generated gas back pressure, it will hinder the flow of liquid metal. Therefore, in order to improve the permeability of mold (core) sand, it is necessary and often used to open a vent hole in the mold. (3) pouring condition 1 pouring temperature pouring temperature has a decisive influence on the filling capacity of liquid metal. The higher the pouring temperature, the lower the viscosity of the alloy, and the longer the flow time, the higher the filling capacity. For thin-walled castings or alloys with poor fluidity, it is often convenient to improve filling capacity by raising pouring temperature. However, with the increase of casting temperature, the gas absorption and oxidation of the alloy become serious, and the total shrinkage increases. Therefore, in principle, the pouring temperature should be as low as possible under the premise of ensuring sufficient liquidity.

2 filling pressure metal liquid in the direction of flow pressure, the greater the flow rate, the better filling capacity. Therefore, it is often used to increase the sprue height or artificial pressure methods (such as: Pressure Casting, low-pressure Casting, etc.) to improve the filling capacity of liquid alloy. (4) when the wall thickness of the casting is too small, the wall thickness changes rapidly or there is a large horizontal plane structure, it will make the alloy liquid filling difficult. Therefore, when designing the casting structure, the wall thickness of the casting must be larger than the minimum allowable value. This is not only beneficial to the smooth filling of alloy liquid, but also can prevent sand inclusion defects. The casting property of the alloy -- Segregation of the alloy -- the phenomenon of non-uniform chemical composition in the casting. Segregation makes the properties of the castings uneven, which will result in scrap if it is serious. Segregation can be divided into two categories: micro-segregation and macro-segregation. Intragranular segregation (also known as dendrite segregation)-refers to the phenomenon of non-uniform chemical composition in each part of the grain, is a kind of micro-segregation. In the crystallization process of any solid solution-forming alloy, uniform grains of chemical composition can be obtained only when the atoms are sufficiently diffused under very slow cooling conditions. Under the actual casting condition, the solidification speed of the alloy is faster than the full diffusion of the atoms, so the chemical composition of the dendrimers is not uniform. In order to eliminate the intragranular segregation, the casting can be reheated to high temperature, and after a long period of heat preservation, so that the full diffusion of atoms. This method of heat treatment is called diffusion annealing. Density Segregation (formerly called specific gravity segregation)-refers to the chemical composition of the upper and lower parts of the casting is not uniform, is a kind of macro-segregation. When the density of alloy elements is very different, when the casting is fully solidified, the elements with low density are mostly concentrated in the upper part, while the elements with high density are mostly concentrated in the lower part. In order to prevent the density segregation, it is necessary to stir or speed up the cooling of liquid metal sufficiently, so that the elements with different density can not be separated in time. There are many kinds of macrosegregation, besides density segregation, there are positive segregation, reverse segregation, v-shaped segregation and zonal segregation.

CASTABILITY OF ALLOYS -- gettering of alloys -- gettering of alloys -- gettering of gases by alloys during melting and pouring. The gettering property of the alloy increases with the increase of temperature. The solubility of gas in alloy solution is much higher than that in solid. The higher the superheat of the alloy, the higher the content of the gas. There are three forms of gas in the casting: solid solution, compound and gas hole. (1) blowholes in castings can be classified into three categories according to the sources of gases in the alloy. In the process of condensation, the gas dissolved in the alloy solution is precipitated because of the decrease of the gas solubility, which can not be removed in time. The most common precipitating pores in aluminum alloy are less than 1mm in diameter. It not only affects the mechanical properties of the Alloy, but also seriously affects the air tightness of the casting. 2 The invasive STOMATA are formed when the gas collected in the surface layer of the sand mould penetrates into the alloy liquid. Reactive blowhole is called reactive blowhole, which is formed by chemical reaction between reactive blowhole and mould material, core support, water content of chill iron, rust corrosion and molten slag. Reactive stomata have various types and shapes. Such as the alloy liquid and sand mold interface due to chemical reaction of the pores, mostly distributed in the casting surface 1 ~ 2mm, the surface after processing or cleaning, exposed a lot of small holes, so called subcutaneous pores. The porosity destroys the continuity of the alloy, reduces the effective bearing area and causes stress concentration near the porosity, so the mechanical properties of the casting, especially the impact toughness and fatigue strength, are reduced significantly. The dispersed porosity can also promote the formation of microporosity and reduce the gas tightness of the casting. (2) measures to prevent porosity 1. Reduce the gas emission of molding sand (core sand) and increase the exhaust capacity of casting mold. Control the temperature of alloy liquid, reduce unnecessary superheat, reduce the original gas content of alloy liquid. 3 pressure condensation, to prevent gas precipitation. Because the pressure changes directly affect the gas precipitation. For example, when liquid aluminum alloy is crystallized in a pressure chamber of 405 ~ 608 KPA (4 ~ 6 atmospheres) , the casting without gas hole can be obtained. When melting and pouring, try to reduce the chance of contact between alloy liquid and gas. Such as in the alloy liquid surface coating agent protection or the use of vacuum melting technology. 5 degassing the alloy liquid. If chlorine is introduced into the liquid, when the insoluble chlorine bubbles float up, the hydrogen atoms dissolved in the liquid are diffused to the chlorine bubbles and carried out of the liquid. Cold Iron, core support and other surfaces must not have rust, oil, and should be kept dry, etc. .

Cold Iron, core support and other surfaces must not have rust, oil, and should be kept dry, etc. . Solidification and shrinkage of Alloy (1) definition of solidification and shrinkage -- the process by which a substance changes from liquid to solid. SHRINKAGE-A reduction in the volume of a casting during solidification and cooling. (2) if the solidification and shrinkage can not be controlled reasonably, the defects such as shrinkage cavity, porosity, casting stress, deformation and crack will appear in the casting. 02 casting solidification mode and influencing factors (1) during the solidification process, there are generally three zones on the cross section of the casting, namely, solid phase zone, solidification zone and liquid phase zone. The width and width of solidification zone of liquid phase and solid phase are the main factors that affect casting quality. Casting “Solidification mode”according to the width of the solidification zone to be divided into the following three categories.

Cold Iron, core support and other surfaces must not have rust, oil, and should be kept dry, etc. . Solidification and shrinkage of Alloy (1) definition of solidification and shrinkage -- the process by which a substance changes from liquid to solid. SHRINKAGE-A reduction in the volume of a casting during solidification and cooling. (2) if the solidification and shrinkage can not be controlled reasonably, the defects such as shrinkage cavity, porosity, casting stress, deformation and crack will appear in the casting. 02 casting solidification mode and influencing factors (1) during the solidification process, there are generally three zones on the cross section of the casting, namely, solid phase zone, solidification zone and liquid phase zone. The width and width of solidification zone of liquid phase and solid phase are the main factors that affect casting quality. Casting “Solidification mode”according to the width of the solidification zone to be divided into the following three categories.

(1) layer-by-layer solidification of pure metals or eutectic alloys (for example, component a in Fig. 1) in which there is no coexisting zone of liquid and solid phases during solidification (Fig. 2) , thus the solid in the outer layer and the liquid in the inner layer of the section are clearly separated by a boundary (the solidification front) . As the temperature decreases, the solid layer thickens, the liquid layer decreases, and the solidification front advances to the center. This form of solidification is called layer-by-layer solidification. Pasty solidification if the crystallization temperature range of the alloy is wide (for example, the C component in Fig. 1) and the temperature distribution curve in the casting (the t casting curve in Fig. 1) is relatively flat, then during a certain period of solidification, there is no solid layer on the surface of the casting, and the solidification zone of liquid and solid phase coexists throughout the whole section (Fig. 1(C)) . Because this kind of solidification way and cement is similar, namely present pasty then solidify, therefore is called pasty solidification. Intermediate solidification most alloys, such as the B in Fig. 1, are intermediate in their solidification, called intermediate solidification. Generally speaking, the layer-by-layer solidification is beneficial to the filling and feeding of the alloy, and is convenient to prevent the shrinkage cavity and porosity. (2) the main factor influencing the solidification mode of castings 1. The smaller the crystallization temperature range, the narrower the solidification zone, and the more inclined to layer by layer solidification. For example: Sand Casting, low carbon steel for layer-by-layer solidification; High Carbon Steel because crystallization temperature range is very wide, for pasty solidification. 2 The temperature gradient of the casting section is determined by the temperature gradient of the casting section under the condition that the range of the alloy crystallization temperature is fixed (see figure 2 t 1→ t 2) . If the gradient of casting increases from small to large, the corresponding solidification zone becomes narrow from wide.

The temperature gradient of castings is mainly determined by A. The lower the solidification temperature, the higher the temperature conductivity, the higher the latent heat of crystallization, the more uniform the internal temperature of the casting, the greater the melting capacity and the smaller the temperature gradient (for example, most aluminum alloys) . The bigger the heat storage coefficient of the mold, the stronger the chilling capacity and the bigger the temperature gradient of the casting. The higher the pouring temperature is, the more heat is brought into the mold, the smaller the temperature gradient of the casting is. The larger the wall thickness of the casting, the smaller the temperature gradient. From the above discussion, it can be concluded that the alloys which tend to be solidified layer by layer (such as gray cast iron, al-si alloy, etc.) are easy to cast and should be selected as far as possible, appropriate process measures (E. G. , permanent mold casting) may be considered to reduce the solidification zone. 03 alloy shrinkage and its influencing factors (1) the principle and process of alloy shrinkage the structure of liquid alloy is composed of atomic group and “Hole”. The atoms within the atomic cluster are arranged in order, but the spacing between the atoms is greater than in the solid state. After pouring the liquid alloy into the mold, the temperature decreases, the cavity decreases, the atomic spacing shortens, and the volume of the alloy liquid decreases. When the alloy liquid solidifies, the holes disappear and the atomic spacing is further shortened. The distance between atoms is also reduced as the atoms are cooled to room temperature after solidification. The shrinkage of the alloy from pouring temperature to room temperature experienced the following three stages: (1) liquid shrinkage, i. e. the shrinkage of the alloy in liquid state between pouring temperature and liquidus temperature at the beginning of solidification. It lowers the level of fluid in the cavity. The solidification shrinkage is that the alloy is in the process of solidification from the beginning temperature to the end temperature. In general, the solidification shrinkage is still mainly shown as the decline of the liquid level. 3 solid state shrinkage is the shrinkage of the alloy in the solid state from the end-of-solidification temperature to room temperature. The shrinkage at this stage is a reduction in the linear size of the casting. Liquid shrinkage and solidification shrinkage are the main causes of shrinkage cavity and porosity, while solid shrinkage is the basic cause of casting stress, deformation and crack, and directly affects the dimensional accuracy of casting. (2) the main factors affecting the shrinkage 1. The shrinkage of the alloy varies with the chemical composition. In common alloys, the shrinkage of cast steel is the largest and that of gray iron the smallest. The shrinkage of gray cast iron is very small because most of carbon is in graphite state and the specific volume of graphite is large. The volume expansion of graphite precipitation during the crystallization process counteracts the part shrinkage of the alloy.

2. The higher the pouring temperature is, the bigger the liquid shrinkage of the alloy is. 3 the actual shrinkage of the casting is different from the free shrinkage of the alloy, which is hindered by the mold and core, and because of the complex structure and uneven wall thickness of the casting, the retraction is also hindered by the retraction of the parts as they cool down. Shrinkage and porosity in castings -- definition of shrinkage and porosity -- if the liquid shrinkage and solidification shrinkage of the alloy are not supplemented by the liquid alloy, it creates a hole where it finally solidifies. The volume is large and concentrated is called shrinkage cavity, small and scattered is called shrinkage loose. HAZARD -- Shrinkage and porosity can reduce the effective bearing area of the casting and cause stress concentration at this location, thus reducing mechanical properties. For parts requiring air tightness, shrinkage cavity, shrinkage porosity will also cause leakage and seriously affect its air tightness. Therefore, shrinkage cavity and porosity is one of the big casting defects. 01 shrinkage cavity and porosity 1 shrinkage cavity is formed by pouring the liquid alloy into the cylindrical cavity. Because of the cooling effect of the mold, the temperature of the liquid alloy decreases gradually and its liquid shrinkage proceeds continuously, the cavity is always full (see Fig. 3(a)) ; as the temperature drops, the surface of the casting is first solidified into a hard shell with the ingate closed (see Fig. 3(b)) ; the liquid metal in the hard shell continues to contract as it cools further, as the liquid shrinkage and the solidification shrinkage are much greater than the solid shrinkage of the hard shell, the liquid level drops and detaches from the top of the Shell (see Fig. 3(c)) , when the metal is fully solidified, an inverted conical shrinkage cavity is formed in the upper part of the casting (see Fig. 3(d)) . As the casting continues to cool to room temperature, the volume of the shrinkage cavity is reduced and the volume of the shrinkage cavity is slightly reduced (see Fig. 3(e)) . If a riser is placed at the top of the casting, the shrinkage cavity is moved to the riser.

2 The shrinkage cavity appears in the final solidification zone of the casting, such as the upper part or the center of the casting, the larger wall thickness of the casting and the vicinity of the Ingate.

3 shrinkage porosity is caused by the failure to make up for the shrinkage in the final solidification zone of the casting or by the failure to make up for the liquid zone separated by dendrite crystals due to the pasty solidification of the alloy. Shrinkage porosity can be divided into macro-porosity and micro-porosity. Macroporosity is a kind of small hole which can be seen by naked eye or magnifier. It is mostly distributed in the center axis of casting or under the shrinkage hole (Fig. 4) . Microporosity is a kind of micro-porosity between grains, which can only be seen by a microscope. This porosity is more widely distributed, sometimes throughout the entire cross-section. It is difficult to avoid microporosity completely, and most castings are not treated as defects, but the castings which require high air tightness, mechanical properties, physical properties or chemical properties must be reduced. Different casting alloys have different tendency to form shrinkage cavity and porosity. The tendency of shrinkage cavity of layer-by-layer solidified alloys (pure metals, eutectic alloys or alloys with narrow crystallization temperature range) is large, and the tendency of shrinkage porosity is small, while the tendency of shrinkage cavity of pasty solidified alloys is small, but it is easy to produce shrinkage porosity. The shrinkage cavity and porosity can be transformed to each other in a certain range because some technological measures can be used to control the solidification mode of castings. In order to prevent shrinkage and porosity, the castings should be solidified according to the principle of “Sequential solidification”. The principle of “Sequential solidification”refers to the establishment of an increasing temperature gradient from the part far from the risers to the risers by means of various technological measures, and the solidification starts from the part far from the risers and proceeds gradually towards the risers, finally, the riser itself solidifies. In this way, good feeding can be achieved, allowing the shrinkage hole to move to the riser, thus obtaining a compact casting. For this purpose, risers shall be placed at the thickest and highest points of the casting and shall be of sufficient size. If possible, the Ingate should be opened on the riser, so that the hot metal filling, liquid flow through the riser first. At the same time, some parts of the casting can be placed on the cold iron (Fig. 5) , to speed up the cooling, in order to give full play to the riser feeding role. The disadvantage of sequential solidification is that the temperature difference of each part of the casting is larger, the thermal stress is larger, and the casting is easy to deform and crack. In addition, because of the riser, increased metal consumption and cleaning costs. Sequential solidification is generally used for alloys with large shrinkage and narrow solidification temperature range (such as cast steel, malleable iron, brass, etc.) , as well as castings with large wall thickness difference and high air tightness requirement.

Pressure feeding the mold will be placed in the pressure chamber, after pouring, quickly close the pressure chamber, so that the casting solidification under pressure, can eliminate the porosity or shrinkage hole. This method is also called “Autoclave casting”. The infiltration technology is used to prevent the leakage, which is caused by shrinkage and porosity, from permeating into the pores of the castings in the form of Gel, and then the impregnating agent is hardened and integrated with the inner wall of the pores of the castings, so as to stop up the leakage. (3) the determination of shrinkage cavity and porosity in order to prevent the occurrence of shrinkage cavity and porosity, the position of shrinkage cavity and porosity in the casting must be judged correctly when making the casting process plan so as to take necessary technological measures. The location of shrinkage cavity and porosity is usually determined by isothermal method or inscribed circle method. According to the heat dissipation of each part of the casting, the points which reach the solidification temperature at the same time are connected into the ISOTHERMS and drawn inwards layer by layer until the isotherms on the narrowest section are in contact with each other. In this way, the position of the final solidification of the casting, i. e. the shrinkage cavity and porosity, can be determined. Figure 6(A) shows the location of the shrinkage cavity determined by the isotherm method, and figure 6(B) shows the actual location of the shrinkage cavity on the casting.

2 inscribed circle method this method is often used to determine the location of shrinkage cavity on the intersecting wall of the casting, as shown in Fig. 7(a) . In the largest part of the inscribed circle diameter (called the “Hot spot”) , there is more metal accumulation, often the final solidification, prone to shrinkage and POROSITY (Fig. 7(b)) .

Solidification and shrinkage of alloys —— Casting stress, deformation and cracking -- Classification and formation of casting internal stress -- The stress caused by the block of solid state shrinkage of castings is called casting stress. Casting stresses can be divided into three types. Mechanical stress this stress is caused by mechanical obstruction to the shrinkage of the casting and is temporary. As soon as the mechanical obstruction is removed, the stress is removed. The causes of mechanical obstruction are: high-temperature strength of core sand, poor yielding; obstruction of sand box belt and core bone, etc. . 2 thermal stress is called thermal stress because of the different cooling speed of each part of the casting, which results in the different shrinkage during the same period and the restraint between the parts. After the casting is cooled to room temperature, the thermal stress still exists, so it is also called residual stress. The volume change of 3 phase transformation stress alloy is caused by the phase transformation in the elastic state. If the cooling rate of each part of the casting is different and the phase transformation is not carried out at the same time, the resulting stress is called phase transformation stress. Casting stress is the ALGEBRAIC sum of thermal stress, mechanical stress and phase change stress. Depending on the situation, the three stresses sometimes overlap and sometimes cancel each other out. The existence of casting stress will bring a series of bad effects, such as making casting produce deformation, crack, reduce bearing capacity, affect machining accuracy, etc. . Ways to reduce and eliminate casting stress 1 technological aspect A. The casting is solidified according to the principle of “Simultaneous solidification”(as shown in figure 8) . Therefore, the Ingate should be placed in the thin wall and the chill should be placed in the thick wall so that the temperature difference of each part of the casting is small and the solidification is carried out at the same time, so that the thermal stress can be reduced to the minimum. It should be noted that at this time, the casting center often appear shrinkage porosity, organization is not dense enough. B. The casting stress can be reduced by improving the yield of mould and core, dropping sand early and unpacking the box to eliminate the mechanical obstruction, and putting the casting into the heat preservation pit to slow the cooling. In the aspect of structure design, we should try our best to make the structure simple, the wall thickness uniform, the transition between thin and thick wall gradually, in order to reduce the temperature difference of each part, and make each part shrink freely. The thermal stress of the casting can be eliminated by natural aging and artificial aging.

3 deformation and crack 1 deformation casting with casting stress is in an unstable state, and the stress is reduced by deformation and tends to be stable. Obviously, the stress in the casting can be reduced or eliminated only if the part subjected to elastic tension is shortened and the part subjected to elastic compression is lengthened.

The direction of deformation of the t-shaped casting is shown by the dotted line in figure 9(a) . This is because when the t-shaped castings are cooled, the thick walls are stretched and the thin walls are compressed, which are equivalent to two springs of different lengths (Fig. 9(b)) , elongating the short spring above and pressing the long spring below to maintain the same length (Fig. 9(c)) . But such a combined spring system is unstable and attempts to return to its original equilibrium, in which the upper spring shortens and the lower one elongates, resulting in a bending deformation similar to the one described above (Fig. 9(D)) . The basic measures to prevent casting deformation are to reduce casting internal stress, for example, when designing, the thickness of casting wall should be uniform, when making casting technology, all parts of casting should be cooled at the same time, and the yielding of mould (core) sand should be increased. In order to compensate the deformation of the casting, the reverse deformation method can be used to make the mould in advance. Figure 10 shows the machine bed, because the guide rail is thicker, the side wall is thinner, the flexure deformation occurs after casting. If the pattern with a double-dotted line expression of the anti-deflection, casting will make the guide rail become straight. It should be pointed out that the casting deformation can only reduce but not eliminate the casting stress completely. After machining, the stress in the part is out of balance and causes the deformation again, which will affect the machining accuracy. Therefore, for important castings, stress relief annealing should be performed prior to machining.

2 crack when the casting stress exceeds the strength limit of the material at that time, the casting will produce crack. The crack can be divided into hot crack and cold crack. Hot Cracking, which is formed at high temperature, is one of the most common casting defects in the production of steel castings, malleable iron billets and some light alloy castings. It is characterized by the irregular shape of the crack, the oxidation color on the surface of the crack (black on the surface of the cast steel and dark gray on the surface of the aluminum alloy) , and the passage of the crack along the grain boundary. Hot Cracks often appear in the final solidification of the casting or the surface of the casting prone to stress concentration. Cold cracking-this is caused by low temperatures. Alloys with poor plasticity, high Brittleness and low thermal conductivity, such as white cast iron, high carbon steel and some alloy steels, are prone to cold cracking. The crack is characterized in that the shape of the crack is a continuous straight line or a smooth curve, usually through the grain. The surface of the crack is clean with a metallic luster or a slight oxidizing color. Cold cracks often appear in the casting under tension, especially in the stress concentrated parts, such as the inner corner, shrinkage cavity and near the non-metallic inclusion. The factors such as reducing casting stress or reducing alloy Brittleness, such as reducing the content of harmful elements such as sulfur and phosphorus, etc. . 

Source: Caitong