High toughness ductile iron is a kind of gray cast iron with excellent mechanical properties. The molten iron is solidified to form spherical graphite. The strength and toughness of high-toughness ductile iron are higher than other cast irons. Generally, before pouring, a small amount of spheroidizer (usually magnesium, rare earth magnesium alloy or rare earth alloy containing cerium) and inoculant) are added to the molten iron, usually ferrosilicon), sometimes replacing cast steel and malleable cast iron in the machinery manufacturing industry. So, let's understand the difficulties and control of high toughness ductile iron production together!

Difficulties in the production of high toughness ductile iron

Due to the slow cooling of the thick section of this casting, the solidification time of the metal liquid is long, and the internal porosity of the casting is easy to occur.

In the manufacture of high toughness ductile iron, the heat treatment temperature according to the existence of free cementite or pearlite in the cast tissue and the use of high-temperature heat treatment of 900 to 950. However, in order to obtain high tensile strength, yield strength, elongation, the previous heat treatment of ferrite, to the production tissue and delivery time has brought great difficulties, due to high production costs, complex process, long production cycle, the need to obtain the matrix ferrite in the casting state. Therefore, the difficulties in producing this material are mainly as follows

A, casting for the specified area of the radiographic inspection, how to solve the internal shrinkage of the casting;

B. The method of ensuring more than 90% of the ferrite matrix in the slab state;

c. The method of making the material have sufficient tensile strength and yield strength

d. approach to obtain an adequate growth rate (

18%; after alloying treatment, the specified elongation is obtained;

c. Use a good alloying treatment process.

Dissolution control

1. Selection of raw materials

In the production of high toughness ductile iron, the content of some alloying elements such as Cu, Cr and Mo should be strictly controlled. Because many trace elements are sensitive to the deterioration of the ball, such as tungsten, antimony, tin, titanium, vanadium and so on. It is very necessary to choose high purity raw materials. The contents of Si, Mn, S and P in raw materials are less (Si<1.0, Mn<0.3, S<0.03, P<0.03). Titanium has great influence on spheroidization and should be controlled. However, high titanium is the characteristic of pig iron in China, which is mainly related to the metallurgical process of pig iron.

2. Desulfurization

The sulfur content of the original liquid iron determines the addition amount of the spheroidizing agent, the S content in the original liquid iron before the spheroidizing treatment is restrained below 0.02%. The higher the sulfur content in the original liquid iron, the more the amount of the spheroidizing agent is added, otherwise the castings with good spheroidizing are not obtained. When the sulfur content of the raw liquid iron before the spheroidizing treatment is high, it is necessary to carry out the desulfurization treatment.

3.Mo alloy treatment:

Mo alloying treatment adopts the eddy current process, in order to ensure the effective absorption of Mo, the addition amount is controlled at 0.5-1.0%, which is adjusted according to the Mo content. Strict requirements should be imposed on the grain size of the alloy.

4. spheroidizing agent and spheroidizing treatment

In the case of manufacturing a thick spherical iron member having a large cross section, a heavy rare earth element having a high damping resistance such as yttrium can be increased by adding a heavy rare earth element to a spheroidizing agent at a certain ratio in order to improve the damping resistance, while ensuring the content of Mg having a spherical action. The test and production practice of many domestic factories show that the spheroidizing agent has also achieved good results in the actual production and application of our company. Relevant data show that the use of Re-Mg and yttrium-based heavy rare earth composite nodulizer as a thick-walled large cross-section ball iron production of nodulizer is very ideal, in the case of high carbon inoculation good, carburizing body will not appear. In addition, yttrium and phosphorus can form high melting point inclusions, yttrium spheroidizing ability is second only to magnesium, but its anti-recession ability is much stronger than magnesium, will not return sulfur, so yttrium can be added in excess, reduce and disperse phosphorus eutectic, further improve the ductile iron elongation. A spheroidizing process was used. The control of the spheroidizing treatment mainly controls the reaction speed, and in the spheroidizing treatment, the spheroidizing reaction time is controlled to about 2 minutes in order to increase the absorption rate of magnesium, control the reaction speed, and improve the spheroidizing effect.

In contrast, in the case of a low-medium Mg, Re spheroidizing agent and a complex spheroidizing agent of yttrium-based heavy rare earths, the addition amount of the spheroidizing agent is determined by the amount of residual Mg.

Preventing the decline of spheroidization: on the one hand, the reason for the decline of spheroidization is that Mg and RE elements escape from the molten iron, and on the other hand, it is also related to the gradual decline of the culture effect. In order to prevent spheroidization recession, a, maintain sufficient spheroidization element content in molten iron; C, reduce the sulfur content of the original molten iron to prevent oxidation of the molten iron; C, shorten the residence time after spheroidization treatment of molten iron; D, spheroidization treatment is carried out on the molten iron. After scraping slag, in order to prevent Mg and RE elements from falling out, the surface of the molten iron is tightly covered with covering agent to isolate air, reduce the element out.

The above is the high toughness ductile iron production difficulties and control, if you need to know more, can contact us at any time!