Research Of Detecting The Quality Of Annealed Centrifugal Ductile Iron Pipe Using Non-destructive Electromagnetic Testing

A large number of centrifugal cast pipes have been used as water supply pipes and drainage channels, they are also been used in manufacturing and agriculture because of the excellent mechanical properties and corrosion resistance. There are more advantages such as light weight, thin wall, well-developed production technology of high efficiency and production, the demand and production of ductile iron pipes is increasing. During casting, the wall of centrifugal ductile iron pipes is so thin that the cooling speed is high, thus the white structure is formed, and it make the toughness of pipes is very low.So the pipes have to been annealed to eliminate the white structure and get more ferrite to improve toughness. The quality of annealing determines the mechanical properties of centrifugal cast pipes. Traditionally, metallographic observation and hardness testing is used to test the quality of annealing, but both the two method will destruct the pipes and time-consuming, and as the samples are randomly selected so that some pipes are undetected.Pearlite is a mechanical mixture of ferrite and cementite, the cementite’s ferromagnetism is weak, so the ferromagnetism of pearlite is weaker than ferrite. During the anneal process, the content of ferrite increases, so the ferromagnetism of material is becoming larger. The non-destructive electromagnetic testing method can test the property of materials according to the magnetic permeability and conductivity of materials without damaging the pipes and coupling. The non-destructive electromagnetic testing method can test can test the ferromagnetism of pipes which has concern with the content of ferrite, so the the quality of Annealing can be evaluated.In this article, samples are annealed after a series period of time, after annealing the content of ferrite is different, then use oscillating magnetometer, electromagnetic pulse detection to detect these samples, and try to find a magnetic factor which is relate to the content of ferrite, and a electromagnetic system can be designed base on this magnetic factor in the future.The oscillating magnetometer testing experiment obtains that when the samples are magnetized saturated, the magnetization is proportional to the content of ferrite. The experiment of measure the initial permeability shows that when the magnetic field intensity is30A/m or so, The initial permeability is proportional to ferrite content.Pulsed electromagnetic experiment results that when testing surface is rough the center frequency of the the induced voltage signal is higher, And the magnetic field would affect the result, but there is no correlation between the center frequency and ferrite content. The experiment of detect the inductance and impedance values obtains that when roughness is higher,the inductance and impedance of the coil is larger, but there is no correlation between the inductance or impedance and ferrite content;...