Experimental Study On Law Of Change Of Stress Corrosion Threshold Of 4130X Steel

The project is derived from the 13th Five-Year National Key Research and Development Program“Life Prediction and Key Technologies for Life Extension of extended Service Pressure Equipment”?2016YFC0801905?,and focus on typical pressure equipment materials,researching on Life Prediction Method and Safety Assessment of Extended Service Equipment under Unsteady working Conditions.In order to obtain the effect factors of the material stress corrosion threshold K_ISCC,and to establish the mathematical model between K_ISCC and hydrogen sulfide concentration,a series of studies were conducted in this paper.Taking the fracture mechanics as the theoretical basis and the 4130X cylinder steel as the research object,experimental studies were conducted from the two perspectives:the effect of hydrogen sulfide corrosion on the fracture toughness of materials,and the stress corrosion threshold value.Non-steady state conditions?different corrosion environments and different external stress?were considered,the changing law of material stress corrosion thresholds were discussed.And theoretical analysis of the stress field at the crack tip is performed based on the test data,the stress field and the plastic zone size of the crack tip before and after the corrosion were analyzed under the combined effect of stress and corrosion.This study can provide experimental basis for follow-up study of long-term service life prediction.First,this article studied the effect of simple corrosion factors on fracture toughness of materials.Three-point bend specimens were used for COD experiments,and fracture toughness of two states of 4130X steel?before and after heat treatment?were measured,21 groups of P-V curves were obtained by using the original specimens and 1600 ppm hydrogen sulfide pro-corroded specimens.Corresponding to 21 groups of?_R-?a relations?In the air:6 sets of original materials and 6 sets of heat-treated materials;after corrosion:3 sets of original materials and 6 sets of heat-treated materials?.For heat treatment materials,resistance curves were fitted based on the?_R-?a relations,fracture toughness were calculated and the K_I values were converted.For raw materials,because it has no stable crack propagation in the test process,the upper limits of K_I were defined.Secondly,this article studied the law of change of stress corrosion threshold,modified WOL specimens were used and the constant displacement experiments were conducted.Three gradient of H₂S concentrations?200ppm?800ppm?1600ppm?and three gradients of different external loads were considered separately,a total of 18stress corrosion thresholds were obtained?original materials:9 results,heat-treated materials:9 results?.Due to the low yield strength of the original material,the experimental results did not meet the plane strain requirements,so the upper limits of the stress corrosion threshold of the original material were defined as well.The main work and innovation are as follows:?1?The mathematical models between the 4130X material K_ISCC?K_e?and the hydrogen sulfide concentration C_H2S in the stress corrosion environment were established through experiments.In engineering applications,when the hydrogen sulfide concentration is known,the stress corrosion threshold value of the material can be deduced from this model.?2?The effects of two factors,hydrogen sulfide concentration and initial load were considered in the experiment,the results show that hydrogen sulfide concentration is the main influencing factor,the stress corrosion threshold value decreases exponentially with the increase of H₂S concentration.The initial load is a secondary factor,it mainly affects the stress corrosion threshold at low concentrations,as the concentration of hydrogen sulfide increases,the effect gradually weakened.?3?Commonly used high pressure cylinder steel 4130X were evaluated in this paper.A systematic experimental study was carried out and the key parameters of mechanical properties,fracture properties,corrosion properties,stress corrosion properties of 4130X steel of before and after heat treatment were obtained.Quantitative evaluation results were given,which can be used for engineering applications.