Continuous Cooling Microstructure Transformation Behavior And Mechanical Properties Of Steels Used For Oil Fracturing Pump Hydraulic End

With the gradual development of oil exploitation to low-permeability and ultra-deep oil fields,the exploitation environment has become more and more severe which puts forward higher demands on the performance of the hydraulic end steel of the fracturing pump.The hydraulic end of the fracturing pump is generally dominated by large forgings and the weight of the single unit can reach 6t.During the process of production and manufacturing,the forgings are easy to be affected by the cooling rate,resulting in uneven microstructure and residual stress,which affects its performance.In order to study the microstructure transformation and evolution of hydraulic end steel under continuous cooling conditions,4330M steel and 17-4PH stainless steel were chosen as the research objects and thermal simulation method was used to determine the behavior of continuous cooling microstructure transformation.SEM and TEM were used to characterize the microstructure of 4330M and 17-4PH steels and the relationship between cooling rate,microstructure and property was established.The tensile and impact tests were done to measure mechanical properties of the two steels and the fracture mechanism was explored through fracture analysis.The results were included as following:The influence of multiple alloying elements of 4330M steel prolonged the incubation period of pearlite transformation and reduced the transformation rate so that there was no pearlite transformation at the experimental cooling rate range of0.08?10?/s.During the process of continuous cooling,the transformations of F+GB,GB+LB+M and M occurred successively while the critical cooling rate of martensite transformation was about 3?/s.Within 0.08?0.158?/s cooling range,the granular bainite was the dominate microstructure and the average size of M/A island in granular bainite was reduced from 0.49?m to 0.37?m,the content increased from 22.15%to 25.77%and then decreased to 24.52%to strengthen the matrix as the cooling rate increases.The hardness increased rapidly from 330 HBW to 375 HBW.Combined with the Vickers hardness of individual constituents and quantitative analysis of color metallography,hardness-volume model fraction is established as follow:HBW=-0.07-4.69·f_F+4.02·f_GB+4.63·f_LB+4.82·f_M(R²=0.991),which indicates that the model is suitable to predict the HBW of the 4330M steel accurately.Because of the high content of alloying elements Cr,Ni,Cu and other alloying elements,17-4PH stainless steel has excellent hardenability and the lath martensite microstructure is obtained at the cooling rate range of 0.05?5?/s.As the cooling rate increasing,the width of the lath progressively refined.At the cooling rate range of0.05?0.3?/s,austenite undergone thermal stabilization,martensite stopped transformation at some temperature ranges and“platform-like”phenomenon appeared on the dilation curves.As the cooling rate increased,the degree of austenite stabilization tended to be flat and this phenomenon disappeared completely when the cooling rate reached 0.5?/s.During austenitizing treatment at 1040?,the free energy of precipitation transformation of Nb C is-13.67 k J/mol<0 which promoted Nb C precipitation from supersaturated austenite and the free energy was further reduced with the decrease of temperature to boost precipitation.At low cooling rate of0.08?/s,copper-rich phase formed by equilibrium precipitation from supersaturated solid solution.During the continuous cooling process,the variation of martensite transformation fraction with temperature can be predicted by the following K-M equation:f_M=1–exp[–0.06076(116.11–T)].After heat treatment,the microstructure of 4330M steel was tempered sorbite formed by mechanically mixing recrystallized ferrite and precipitated cementite while the microstructure of 17-4PH were lath martensite and dispersed?-Cu phase which kept an orientation relationship of(11?0)_M?(11?1?)_Cu[002?]_M?[022?]_Cu.Recrystallized ferrite had good ductility,while lath-shaped tempered martensite had excellent strength and toughness.Coupled with the strengthening effect of the precipitated phases,both steels can reach the requirements of 1000MPa level.From the fracture analysis,it was found that the fractures of the two steels were dimples and a small amount of tearing ridges,which presented ductile fracture.The amount and depth of dimples of 4330M steel were larger and deeper than that of 17-4PH stainless steel,so the impact energy was higher.