Ferritic ductile iron

Ductile iron is a high-strength material developed in the mid-20th century. By spheroidizing the carbon in pig iron, this new type of material, which is inexpensive and has comprehensive properties close to steel, was obtained. Its excellent cost performance has been successfully used to cast some parts with complex stress and high requirements for strength, toughness, and wear resistance.

Ductile iron is a high-strength material developed in the mid-20th century. By spheroidizing the carbon in pig iron, this new type of material with low price and comprehensive performance close to steel is obtained. Its excellent cost performance has been successfully used to cast some parts with complex stress and high requirements for strength, toughness, and wear resistance.

The matrix of ferritic ductile iron is ferrite, which can be obtained from the as-cast state or after annealing. Its mechanical properties, especially plasticity and toughness, have been greatly improved, and its strength is even higher than that of carbon steel. It has rapidly developed into a new casting material second only to gray iron. The so-called 'replacing steel with iron' mainly refers to this material.

The carbon contained in pig iron exists in the form of graphite, and its morphology and metal matrix structure have a great influence on its toughness. Flake graphite seriously cuts the metal matrix, and stress is concentrated at its sharp corners, so the casting is brittle, the impact value is very low, and the strength is greatly weakened.

As long as the matrix structure is qualified, the higher the spheroidization rate of ductile iron, the better the plasticity. The matrix structure of ferritic ductile iron is mainly ferrite, and the higher the content, the better the plasticity. If the contained pearlite can be controlled below 10%, it will not have a great impact on the performance because it is dispersed.

Under the premise of proper inoculation technology, increasing the carbon equivalent will increase the ferrite content. However, if the carbon equivalent exceeds a certain threshold, it is easy to cause the graphite spheres to float. In addition, the thickness and cooling speed of the casting will also cause the graphite spheres to float. Generally, low carbon is suitable for thick and large parts, and high carbon is suitable for thin and small parts. Strengthening the inoculation process should also reduce the carbon equivalent.

The role and influence of each element in ductile iron: Appropriate carbon equivalent is beneficial to graphitization and spheroidization. Silicon is an element that strongly promotes spheroidization, and its inoculation effect can refine eutectic clusters and disperse phosphorus eutectic. Manganese will hinder the decomposition of cementite and pearlite, so try to control the existence of manganese.

The phosphorus element will form a brittle phase, especially the ternary phosphorus eutectic or composite phosphorus eutectic will make the finished product brittle, and appropriate measures need to be taken. If the sulfur content is too high, the spheroidization will be unstable, and too many sulfide inclusions will be generated, which will seriously affect the toughness.

Only primary iron with very little silicon, manganese, phosphorus and impurities can produce high-quality ductile iron products. Many countries use high-purity pig iron with good results. However, the impurity content of Chinese pig iron is high, which makes the performance of the as-cast state unstable. Annealing can be used to improve castings with high performance requirements.

According to the quality requirements of castings, it can be divided into two annealing methods: high and low temperature stages and low temperature. The room temperature impact values of the two are relatively close and stable, and the low temperature impact values are slightly different, while the product obtained by the two-stage heat treatment method has a higher low temperature impact value and a smaller amplitude. Because two-stage annealing makes the structure uniform and reduces the segregation of carbides and grain boundary elements.

Ductile iron

What is ductile iron?

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