Study on the characteristics of metal magnetic memory signal of X70 pipeline steel

In view of the zero crossing phenomenon of the normal component of the magnetic memory signal on the surface of the ferromagnetic component in the crack or stress concentration, many scholars have disputes about it. In this paper, through the simulation of the crack by wire cutting, the normal component of the magnetic leakage signal on the surface of the pipeline component is stretched on the tensile testing machine, and the normal component of the magnetic memory signal on the surface of the pipeline component is analyzed. It is found that the zero crossing phenomenon does exist in the normal component of the crack, but the zero crossing occurs with the pulling when the load increases to a certain extent, it will not drift after passing the zero point. The results show that the normal component cannot be used to judge the crack or stress concentration simply.


1.Introduction
Metal magnetic memory (MMM) detection method is a new NDT method proposed by Russian experts represented by Professor dubov in 1997. As soon as this method is put forward, it gets the attention of the international community. Scholars at home and abroad begin to study the theory of MMM method and its application in the crack detection of components [1,2] . At present, in the research of MMM detection of component cracks, the main focus is on the MMM signal characteristics of component cracks. Professor dubov, a Russian scholar, thinks that the normal component of magnetic memory leakage signal on the surface of ferromagnetic component has zero crossing point in judging the crack or stress concentration, but many scholars have doubts about it [3,4,5] . In order to solve this problem, the characteristics of metal magnetic memory signals of ferromagnetic components are studied.

Experimental object
In the test, X70 pipeline steel is taken as the test object, the length of pipe fittings is 200 mm, the inner diameter is 58 mm, and the outer diameter is 60 mm. The cracks with the depth of 0.5mm, 1mm and 1.5mm were processed by wire cutting at the distance of 30mm, 60mm, 90mm, 120mm and 150 mm from the end point to simulate the cracks. Connecting screws are machined at both ends of the pipe for fixing on the tensile testing machine as shown in Fig.1.

Test Method
In order to study the MMM detection signal characteristics of specimens under different tensile forces, this paper has carried out four tests in the elastic deformation stage of stretching, each of which includes two parts, i.e. the test during loading and the test after unloading. The detection direction is shown by the arrow in Fig.1. When the specimen is loaded to a certain test force on the tensile testing machine, we keep the test force unchanged and carry out MMM test on the effective area of the specimen. All MMM tests start from the left end of the tensile specimen in Fig.1 and end at the right end. Detection conditions are showed in Tab.1. has obvious zero crossing phenomenon, but with the increase of the tensile load, the zero crossing has obvious drift, as shown in Fig.2, 3

Relationship between zero crossing position and tensile test force
In this paper, the relationship between zero crossing position and tensile test force in MMM test of tensile test is statistically analyzed. The statistic is the difference between zero crossing position and prefabricated crack position, which is defined as Lx Δ . The same rule is obtained. Take the statistical results of the first crack at the left end of No. 1 test piece as an example, as shown in Fig.6. From Fig.6, it can be seen that at the lower stress level (the first loading stress), no zero crossing occurs. When the stress increases to a certain level, zero appears for Hp . As the stress continues to increase, there is a form of movement that tends to zero for Lx Δ . When the stress exceeds a certain level, the stress continues to increase, and it will no longer change, but remain at zero. From the above analysis, it is easy to draw a conclusion that Hp zero crossing is a kind of detection surface phenomenon. Because of its position drift, Hp zero crossing is not the only reliable detection signal feature used to determine the crack position.

Conclusions
(1) Under the action of tensile load, the detection