Analysis of gray cast iron processing performance

1 Analysis of cast iron processing performance

The evaluation of the machining performance of cast iron is the cutting speed at a certain tool life under the same conditions. Usually use a bow saw machine, choose the same new saw blade, cut the test bar under the same conditions, compare the cutting speed.

In metal processing, the first is the plastic deformation of the thin layer of chips to release heat, commonly known as the effect of cutting temperature on tool life, the second is the abrasive wear during the cutting process. Generally, the chips of high-strength gray iron are slightly curled, indicating that there is plastic deformation accompanying heat precipitation. This is related to the shape amount of graphite in the cast iron, the size of the different structures amounts in the matrix. The abrasive wear is also related to the cast iron. Hard-phase particles, such as carbides, nitrides, phosphides, etc. are related to hardening cast iron poor processing performance. To improve processing performance, it is necessary to start with low cutting temperature reduce abrasive wear.

2 The influence of molten iron temperature

2.1 Self-deoxidation phenomenon

The cupola melts the cast iron, all the droplets have gone through the high temperature zone. The oxidation zone is generally greater than 1800°C. At this time, almost all metal elements some carbon nitride can be melted, so that the original internal structure is more completely disrupted , then deoxygenate sometimes through the reduction zone. The above-mentioned conditions do exist in the induction furnace to melt cast iron. Instead, the molten iron is polluted by the atmosphere. It is remelted three times at 1454℃, the number of white holes increases successively, which means that every time the melting is performed, there is more oxidation the graphite crystalline core decreases. retain the original performance of some charge. During the melting of molten iron, many people have found that the molten iron looks particularly "bright" when the temperature of the molten iron is above 1480℃, the oxidized inclusions on the surface of the molten iron suddenly decrease. When the temperature of the molten iron is heated to 1538℃, the molten iron will feel red hot. , Very clean, this phenomenon shows that the molten iron has a "critical point", beyond which the molten iron has self-deoxidation phenomenon, the oxygen carbon are combined to form CO gas run out of the molten iron surface.

2.2 Common molten iron temperature regulations

According to the information of many foundries in advanced industrial countries the company’s visits consultations for several meetings, most foundries have iron tapping temperatures ranging 1500-1550°C to 1420 to 1430°C, casting temperatures greater than 1400. British experts also insisted that the melting temperature of cast iron should be around 1540℃ when they came to the factory for guidance. The melting temperature is too high, especially for long-term high-temperature melting, too little oxygen in the molten iron, which is conducive to inoculation, the chilling tendency is serious. The melting temperature is too low, various casting defects are easily produced. Dongfeng Company uses induction furnace for melting. In the past, the melting temperature was between 1390 1420°C, the pouring temperature was often between 1280 1320°C, which is about ℃ lower than that of foreign countries. Low temperature can easily cause uneven internal organization.

3 The impact of gestation

The purpose of cast iron inoculation is to eliminate white mouths, improve processing performance, refine eutectic clusters, obtain graphite, make graphite uniformly distributed, improve structure, improve mechanical properties, reduce section sensitivity. Inoculation is the most economical method. It only accounts for the cost of castings, but it doubles the value of castings.

The inoculation treatment can control the subcooling of the molten iron within 10℃, double the number of eutectic clusters, promote the precipitation of graphite in type A, reduce the length of graphite to within 100um. However, the inoculant is good, the temperature is low, the amount is large, the particles are large, the molten iron cannot be melted, resulting in hard spots inside the casting.

3.1 The temperature of molten iron during incubation

In order to have a good inoculation effect, the molten iron must have supersaturated oxygen, that is, it can form a "hatred" crystal nucleus oxygen with the silicon element in the inoculant. When the temperature of the molten iron is low, the inoculation becomes deoxidation, the casting is prone to pores needles. Defects such as holes, slag inclusions, mixed graphite, etc., the temperature of the molten iron is high the oxygen content is too low, which is prone to white mouth decarburization. In addition, it is required that the holding time of the molten iron above the equilibrium temperature should be long. If it is too long, it will cause ①to volatilize too much cause lean carbon ②cause hypoxia, so that "'the crystal surface is destroyed, the nucleation effect is achieved. ③Many high-concentration micro regions disappear Severely, too much supercooled graphite independent phases that should appear are generated.

To control the overheating temperature of molten iron in inoculation of molten iron, it is actually to control the oxygen content of molten iron, so that oxygen silicon will generate "enemy" nucleus particles instead of causing oxide slag. Some reports believe that gray iron breeds the best oxygen content in some cases. In production, it is generally believed that the silicon content is at one time, the oxygen content is maintained at the same temperature at one ℃, which is the best time to incubate the molten iron. In the past, Dongfeng Company's processing temperature was low, the inoculation effect was good, the temperature was too low, the inoculant could be completely melted, resulting in the phenomenon of "silicon-rich phase" hard spots.

3.2 Inoculation method

Our factory's gray iron inoculation stipulates in the operating procedures that "the iron must be discharged to the bag to cover the bottom of the bag before it starts to be evenly scattered into the inoculant with the iron flow. The time for adding the inoculant accounts for about the iron time, so accurate Control the amount of iron produced the amount of inoculants added.” However, in the implementation, I saw that the weighed inoculant was poured into the bottom of the bag before the molten iron was poured. If there is still molten iron at the bottom of the bag, the inoculant was poured early, sometimes it would take a while to produce iron. This kind of inoculation method, coupled with inoculant, only does give birth well, but also has the problem of early decline.

Nowadays, there have been multiple inoculation methods such as re-inoculation when the second inoculation is inverted, instantaneous inoculation, follow-up inoculation, intra-inoculation. In 1976, experiments were carried out on the cylinder.

The low hardness of the inoculation in front of the furnace indicates that a large amount of ferrite is produced. The inoculation with the flow can increase the amount of pearlite more obviously, the hardness uniformity is good. The above shows that to obtain castings with excellent processing properties, it is also important to adopt a good inoculation method.

4 Conclusion

In order to improve the processing performance of iron castings, the following links must be grasped the perspective of melting

①Choose appropriate ingredients ② high-quality charge ③Fast melting to shorten high temperature time ④Appropriate molten iron treatment process ⑤Control the crystallization process.