Fracture Failure Analysis And Fatigue Life Estimation Of The Reciprocating Compressor Piston Rod

The large reciprocating compressor is the indispensable key equipment in chemical production, and its operation reliability directly affects the chemical production. The early fatigue fracture of the piston rod is the main reason of the accident in the compressor. The main objective in this paper is to analyze the reason of the fatigue fracture, and give the method of the piston rod fatigue life estimation.This topic is replied on the agreement among Shandong Hualu Hengsheng Group Co. LTD, Sinochem Pingyuan Chemical Co. LTD, Yangmei group Zibo Qilu First Chemical Fertilizer Co. LTD, Yankuang Group Yishan Chemical Co. LTD and Shandong University. The tests in this paper are mainly to the three broken piston rod of6M32compressor provided by Hualu Hengsheng group Co. LTD. This paper analyzes the reason of piston rod fatigue fracture failure by macroscopic observation, scanning electron microscopy observation, materials tensile test, impact test, hardness test, chemical test, dimensional measurement and surface roughness measurement. And we calculate the stress of the piston rod and the stress in fracture section, according to the structure and operation parameters of6M32compressor. Considering the influence of piston rod working condition, material, etc, we choose the life prediction model based on small damage or nondestructive detection to the piston rod, and compares the results of various methods with the actual life.The analysis shows that, the piston rod fatigue crack initiation in the second phase organization or inclusion position. Nominal stress method is suitable for the piston rod fatigue life estimation. Many factors influence the fatigue life of piston rod, such as load type, the average stress, material performance, microstructure, material internal defects, gap, size effect, heat treatment and surface condition. Miner theory and the revision of Miner theory applies to the piston rod fatigue life estimation.The test results show the main cause of the piston rod fracture failure was that, the mechanical properties of materials below the standard rating, highly stress concentration of dangerous parts reduce the fatigue life, and the fatigue damage occurred under the coactions of the stress concentration and cycle load, gradually fatigue crack extend to fatigue broken. The piston rod surface hardness and surface roughness affect its fatigue life. The surface hardening can improve the fatigue life, and when the surface roughness gets smaller, its fatigue performance will be better.42CrMo and38CrMoAl are the commonly used materials to the piston rod, moreover the ratio of the tensile strength and Brinell hardness to the materials is3.4. And that ratio can be used to estimate the piston rod tensile strength when hardness is known. Analysis shows that in the different fatigue life estimation methods, the way of dealing with average stress Re, stress amplitude AR/2, coefficient of stress concentration Kt, size coefficient ε and surface coefficient β are various. The comparison of the estimate life by lots of chosen estimation models to the actual operation life shows the estimate conclusion by revised Basquin model on Fatigue Strength Exponential Function when the coefficient determining by four point method is most close to the actual life.