Characteristics Of Metal Magnetic Memory Signals Of The34CrMo4Steel Of High Pressure Cylinders Under Tensile And Fatigue Testing Load

The high pressure cylinder is a kind of pressure vessel with large usage and strong mobility under cyclic loading. During the usage of high pressure cylinder, stress-concentration zones will be accelerated to mechanical fatigue damage and corrosion.Based on magnetic memory effect of ferromagnetic structures in geomagnetic field, metal magnetic memory testing technology (MMMT) is used to collect self magnetic leakage field in stress-concentration areas under service load. In this paper, tensile testing and fatigue testing were taken on the34CrMo4plate specimen, which is made from a newly prepared high pressure cylinder. The variation of magnetic memory signals of34CrMo4steel with tensile stress and fatigue cycles were studied. The hardness tests were carried out to measure the surface hardness of specimen which has been experienced different tests. The main results are listed as followed:1The tensile stress has great influence on magnetic memory signals. It was found that both magnetic field intensity Hp(y) and|K|firstly increased with the increase of tensile stress σ in the elastic range; then they reached maximum value under yield stress. However, Hp(y) and|K|decreased quickly in the plastic range.2Hardness testing of the specimens, which is closed to yielding or in the necking region under tensile testing, showed that the hardness of the stress concentration area is lower than that nearby areas.3The fracture location of the tensile specimen was not right in the zero-cross point of Hp(y), which usually has a distance about0～15mm. 4Hp(y) varied regularly with the increase of fatigue cycles N. the Hp(y) firstly changed quickly at the first stage, then Hp(y) comes into a stage range, which varied slightly with the increase of fatigue cycles, finally, the distribution range of Hp(y) and the value of K near fracture positions increased with the increase of N.5Compared to the other regions, the hardness of the stress concentration area are higher than that of the other areas of the fatigue specimen.