SAND CASTING EQUIPMENT.
FOUNDRY
Businesses we serve
In the production of gray cast iron parts, common casting defects are: porosity, composition and performance failure, hot and cold cracking, shrinkage and shrinkage, slag eyes and iron beans, cold isolation and under-pouring, sand eyes and sand trapping, succulent and wrong spokes, deformation, etc.. Usually, the cause of these defects is not only molding core problems, sometimes there are melting and pouring, sand quality, sand cleaning and many other production processes, so we must analyze the specific in order to take the appropriate reasonable measures to solve.
1. Sand core and sand rigidity
After sand casting, the expansion force caused by the static pressure or solidification of the molten iron often leads to the movement of the wall and the collapse of the core, which will cause internal shrinkage and surface shrinkage of the casting. Therefore, in order to make the casting size stability, to maximize the casting tight. In order to save molding materials, core making is widely used hollow sand core, which is lighter than the solid core, so the heat capacity is small, slow solidification speed, which can lead to sand expansion or sand core collapse. In addition, the iron liquid may penetrate into its hollow part through the cracks on the core head or sand core, which will also cause defects in the casting. In order to improve the rigidity of the hollow sand core, wet sand or water glass sand can be used to fill; the shell core can also be made into two halves, and its internal reinforcement bars can be set, and the core can be bonded to obtain a hard sand core.
2. Correct selection of pouring temperature
2.1 The defects that may be formed when the pouring temperature is too low
2.11 manganese sulfide pore This kind of pore is located in the casting skin below and more above, often after processing revealed, pore diameter of about 2 ~ 6 mm. sometimes the hole contains a small amount of slag, metallographic research shows that this defect is by MnS segregation and slag mixture, the reason is the low casting temperature, while the high amount of Mn and S in the molten iron.
2.12 liquid slag After processing, a single small hole will be found under the skin of the casting, the diameter of the hole is generally 1~3mm. in individual cases there are only 1~2 small holes. Metallographic studies show that these small holes appear together with a small amount of liquid inclusion slag, but no segregation of S is found at this location
2.13 sand core gas caused by porosity Porosity and multi-hollow porosity are often caused by poor sand core exhaust. Because the sand core is mostly hardened in the core box during core making, this often makes the number of sand core exhaust holes insufficient. In order to form air venting holes, the core can be hardened in the core to supplement the drilling. The most common reason for low pouring temperature is that before pouring, the molten iron is transported and stays in the open pouring ladle for a long time and dissipates heat. Using the lid of the pouring ladle with insulation material can significantly reduce the heat loss.
2.2 The pouring temperature is too high; the high pouring temperature will cause the sand mold to rise, especially for the castings with complex sand cores, and the scrap increases when the pouring temperature is ≥1420℃, and the scrap reaches 50% when the pouring temperature is 1460℃. In production, the use of induction furnace melting can better control the temperature of the molten iron.
3. The formation of nuclei
3.1 The effect of breeding breeding treatment sometimes also increases the casting defects, because of strong breeding and sharp nucleation of cast iron parts, the tendency to form carbide increased. So it is recommended that the amount of breeding agent when breeding treatment can prevent white mouth is enough, and the nuclei in sound castings are much less than in castings with shrinkage.
3.2 the role of sulfur due to the majority of scrap steel contains low S, so the electric furnace melting scrap steel, can only obtain a low S content (≤ 0.05%) of cast iron. This cast iron does not work for many breeding agents, the reason is that the breeding recession is very fast, so when melting with scrap steel in the electric furnace, often produce white mouth in the casting.
3.3 The holding and superheating temperature of the iron liquid, people tend to melt and hold the iron liquid with an electric furnace, but increasing the superheating temperature and increasing the holding time will reduce the formation of nuclei, so there is a risk of white mouth. Considering the economic and material performance reasons, the temperature should be as low as possible when holding for a long time.
4. Casting size error caused by sand core
When the sand core is heated, it first expands and then deforms plastically. This change caused by high temperature is related to the sand used and the decomposition of the binder.
4.1 Influence of hardening temperature and hardening time The initial expansion and thermoplasticity of resin sand are related to the hardening temperature and time. If the core is hardened for a long time, the primary and secondary expansion will be large and may bring deformation problems; while when the hardening time is short, the core expansion will be small and decomposition will be fast.
4.2 The effect of sand core coating Most of the sand core surface to brush coating, the test found that the depth of refractory coating penetration into the sand core surface has a great impact on the core deformation, when coated with a coating containing surfactant, because of the depth of penetration and prevent the secondary expansion.
5. Analysis of the causes of defects in gray cast iron
The main factors affecting the mechanical properties of gray cast iron: cooling rate, overheat treatment of the iron liquid, breeding treatment, furnace characteristics, etc.
5.1 About the effect of cooling speed
There are more factors affecting the cooling speed of castings: the wall thickness and weight of the casting, the type of casting material, the gate and weight, etc. As the wall thickness, weight and structure of the casting depends on the working conditions, can not be changed at will, so in the choice of chemical composition should take into account their impact on the organization.
5.2 On the influence of the superheat treatment of the molten iron
Increasing the superheat temperature of the ferrofluid can
5.21 increase the chemistry carbon content and reduce the graphite carbon content accordingly
5.22 refine graphite and enable the formation of dendritic graphite
5.23 Eliminate the "heritability" of cast iron
5.24 improve the uniformity of the organization of the casting in section
5.25 Facilitate the shrinkage of the casting. Similarly, the ferrofluid insulation also has a similar effect of ferrofluid superheating.
5.3 on the influence of ferrofluid breeding treatment
Fertilization treatment is in the molten iron into the cavity before the casting, the fertilizer attached to the molten iron to change the metallurgical state of the molten iron, so as to improve the microstructure and properties of cast iron. For gray cast iron, the inoculation treatment is to obtain A-type graphite, pearlite matrix, fine eutectic group organization, as well as to reduce the tendency of white mouth at the thin wall or corner of the casting and sensitivity to the wall thickness of the casting; for malleable cast iron, in order to shorten the short annealing cycle, increase the allowable wall thickness of the casting and improve the structure of the organization; for ductile iron, in order to reduce the tendency of white mouth of the casting, increase the spheroidization rate and improve the roundness of graphite.
5.4 Influence on the characteristics of the furnace charge
In actual production, it is often found that changing the metal charge (for example, using pig iron of different origin or changing the ratio of the charge, etc.) and the chemical composition seems to have no change in the case of cast iron with different organization and performance, which indicates that the nature of the raw material directly affects the nature of the cast iron melted with it, called cast iron: "heredity" For this reason, the The use of increased iron temperature and the use of a variety of iron ingredients can eliminate this "heredity" and improve the organization and properties of cast iron.
Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.