304 stainless steel is the most widely used a chromium – nickel stainless steel, thin – walled seamless steel pipe has good corrosion resistance, heat resistance, low temperature strength and mechanical properties, but also has good processing performance and solderability. It is widely used in the production of equipment and mechanical parts with good comprehensive performance (corrosion resistance and moldability). It is widely used in mechanical, aerospace and other fields. It is mainly made of oil and gas pipes and pressure vessels. Special working environment. Oil and gas pipes, pressure vessels, etc. are limited by the number of cycles that are experienced throughout the life. For example, the pressure vessel is sometimes subjected to only a few load cycles per day. In the practical application of the project its static strength will generally meet the requirements, so its low cycle fatigue fracture characteristics of practical significance. 304 stainless steel components fatigue fracture events occur, resulting in serious consequences.
The tensile fatigue fatigue test of 304 stainless steel samples under stress control was carried out to study the low cycle fatigue fracture characteristics. The experimental results show that the low-cycle SN curve of 304 stainless steel under stress control and the expression of low-period stress life are obtained. The fatigue limit of 304 stainless steel is calculated by the expression. The relationship between strain and life under different stress is analyzed And the relationship between the number of loading and the plastic strain energy; the fracture is observed. The larger the load, the more fatigue source, the smaller the spacing, the faster the fracture of the specimen, the shorter life, the more serious fracture area. This is because when the load is large, the surface of the specimen is easy to form a crack source, the more the number of cracks, the easier the specimen is broken, the fracture surface friction is less, it is more rough; the smaller the load, The longer the time, then the frictional surface friction is more serious, relatively speaking, the more flat.
Shiyan thick-walled seamless steel pipe crack extension area is the most important feature of the fracture surface fatigue stripes, which is to determine whether the basis of fatigue fracture. There are many factors that affect the form and appearance of fatigue stripes. Through the fatigue fracture on the display of fatigue stripes of the material statistics that: under normal circumstances the higher the tensile strength of the material, the more prone to fatigue stripes; toughness of the material is easy to produce fatigue stripes.
Through the study of fracture surface morphology of 304 stainless steel low cycle fatigue fracture specimen, the following results are obtained:
(1) The measured fatigue limit σ-1 (N = 105) of 304 stainless steel is 236.5MPa. The life of the specimen increases with the decrease of the stress amplitude, and the velocity of the specimen decreases with the stress amplitude Increase and decrease.
(2) When the stress amplitude is constant, the strain increases gradually in the process of circulation, and then reaches a certain value and no longer increases and remains stable until the final fault. Under the stress control, the total strain of the specimen at break Increase and increase.
(3) When the microcracks appear on the surface of the specimen, the plastic strain energy reaches the maximum value, and the number of cycles is 1/3 of the total life, and the crack source is formed.
(4) Through the analysis of fracture observation can be: low fatigue fatigue has a number of fatigue sources, fracture morphology and stress level, the greater the stress, the more the crack source, the greater the spacing of fatigue stripes, the greater the area , The more rough.