| Oil industry is a pillar of national energy sources. With the development of economy,the domestic demand for oil is becoming huger. Therefore, to increase deep oilexploitation in domestic oilfields is becoming more and more urgent. Fracturing pump isthe key equipment in the process of oil production, which is currently widely used in deepoil recovery operation. However, sudden destruction often happened to the fracturingpump during actual using process what seriously affected the normal production, resultingin huge economic losses. So, it is imperative to find the reason why fracturing pumpfailure occurs.25Cr2Ni4MoV is a kind of fracturing pump material, and this kind of alloy steel hasvery excellent comprehensive mechanical properties. Research shows that fracturing pumpis influenced by both concentrated hydrochloric acid solution and tensile stress. Stresscorrosion cracking may be one of the reasons which lead to sudden failure to fracturingpump. Four point bend specimens of25Cr2Ni4MoV steel are used to investigate the stresscorrosion cracking (SCC) in ultra-high concentration hydrochloric acid (20wt.%). As aresult of four point bend experiments, curves of relationship between deflection and timeare found out. The results show that the stress corrosion cracking process of25Cr2Ni4MoV steel under the condition of ultra-high concentration hydrochloric acid isdivided into three typical stages: crack initiation, crack propagation and instant fracture.At the same time, SCC lifetime decreases along with the applied stress, showing a typicalstress corrosion cracking characteristics.After SCC is determined to be one of the main causes of fracturing pump failure, thispaper also explores the influence of alloy elements and material microstructure to25Cr2Ni4MoV steel’s stress corrosion performance, and then analyzes the mechanismhow SCC occurred.25Cr2Ni4MoV and30CrNi2MoV steel’s SCC contrast experiment results showedthat the SCC lifetime of30CrNi2MoV steel is shorter for lower Cr/C ratio and it indicatesthat increasing Cr/C ratio can improve SCC performance. Through the control of heat treatment process parameters, such as quenchingtemperature, holding time and tempering temperature,25Cr2Ni4MoV steel’smicrostructure is changed. The experimental results show that the smaller originalaustenite average grain size and the higher tempering temperature can bring the longerSCC lifetime.Scanning electron microscopy (SEM) is used to observe the microscopiccharacteristics of crack initiation period and extended period of25Cr2Ni4MoV steel’sSCC. The results showed that crack initiation mainly emerges in original austenite grainboundaries, and crack expansion has a relationship with the crack tip stress concentration.Generally, the expanding crack seems to be a "Z" glyph or approximate straight.Transmission electron microscopy (TEM) is used to observe the original austenitegrain boundaries and found that there are many discrete stripe precipitates (second phaseparticles) along with the grain boundaries. Energy diffraction spectrum (EDS) is used toanalyze the elements in grain boundaries and found that Cr element content in grainboundaries is1/2or lower than that of the matrix content. There existing precipitates alongwith the original austenite grain boundaries and at the same time, where Cr content islower than the matrix, both of which make the grain boundaries is easy to become anodeand be corroded preferentially in ultra-high concentration hydrochloric acid solution, andthen micro-cracks are formed along the grain boundaries as corrosion going on. Themicro-crack which is perpendicular to the applied stress direction keeps growing, thenreaches critical crack length, and finally becomes SCC’s main crack. |
PDF Full Text Request