| In recent years,it has become an urgent task for China to vigorously develop its oil and gas industry under the condition of regular oil and gas exploitation,the construction of tight oil and gas resources has become an inevitable choice for the overall development of China’s oil and gas industry.The fluid end is an important part of the equipment required for fracturing and acidizing construction,the core technology of shale oil and gas well development to obtain ideal production,and the valve box is the key part of the fluid end assembly,which has very strict material requirements.The application of 17-4PH stainless steel valve box promotes the development of fracturing equipment to the characteristics of high pressure,high power,large displacement,compact structure and long operation life.The manufacturing process of stainless steel valve box forgings is quite difficult.Due to the narrow deformation temperature range during forging process,the surface stress is different form the stress state of the core,thereis surface tensile stress,the surface cracking problem is the technical bottleneck that needs to be broken through in the mass production of stainless steel valve box forgings in China.In this paper,the surface crack problem of 17-4PH valve box forgings in the process of free forging is studied.In addition,the study on the formation and propagation of cracks caused bu microstructure factors(δ-ferritic phase and NbC phase)was extend.The key factors of surface cracking during valve box forging were investigated,in the meanwhile,the mechanism of cracking was revealed.Base on the finite element calculation,a prediction model of valve box surface cracking initiation was proposed,and the influence of micro-structure factors on cracking tendency was compared quantitatively.Finally,the improvement strategy was put forward to achieve industrial trial production.The rheological behavior of 17-4PH stainless steel was studied in this paper.The flow stress curves of 17-4PH stainless steel ingot material and forging material were measured through thermal compression test by Gleeble-3500 thermal simulation test machine,and then,the differences of peak stress,peak strain and critcal strain under different micro-structure states were compared.By using the forging material hot working diagram,the optimum process window for hot working of 17-4PH stainless steel was obtained.The surface cracking of 17-4PH stainless steel valve box forgings during forging is mainly related to the surface "fold" defect and the second phase of δ-ferrite and NbC in the micro-structure.The uneven surface deformation of the ESR ingot during upsetting leads to different degrees of "folds",which provides the development conditions for the formation of mode-I open crack.The deformation of δ-ferrite and NbC is inconsistent with that of the matrix,resulting in micro-cavity initiation at the interface between the second phase and the matrix.The crack propagation accords with the damage theory of ductile materials,and the crack initiation state can be predicted by the critical cavity expansion ratio parameter.According to the criterion of critical expansion ratio(VGC),the function relation between the critical opening displacement and the cavity expansion ratio parameter of crack front in 17-4PH stainless steel under the elastic-plastic condition was established through finite element simulation,and the critical opening displacement of crack tip in different states were obtained,and the hot forging cracking was studied.A prediction model of 17-4PH stainless steel valve box forging surface crack initiation was established simultaneously.Based on the physical test of hot forging,the prediction model of surface crack initiation for 17-4PH stainless steel valve box forging was further modified and verified,and the effect of process parameters such as strain rate and deformation temperature on surface crack propagation was clarified.The evolution kinetics of δ-ferritic phase and NbC phase in 17-4PH stainless steel at different deformation temperatures were illuminated,and the precipitation of 8-ferritic phase amount control model of separating and NbC phase nucleation rate-temperature curve(NrT curve),as well as the amount of phase shift-temperature-the relative theoretical curve of time(PTT curve)were obtained by combining the surface crack forecasting model.In addition,the tendency of increasing the initiating surface cracks during forging from both aggregated and uniformly distributed NbC phases was discovered,however,the mechanisms are different between the two.The relevant results obtained in this paper are also applicable to the behavior of internal cracks in forgings,but because the parameter conditions of the core usually fail to reach the critical condition of crack behavior,the relevant results are presented on the surface.In order to better reflect the engineering problems,"surface cracks are described".Based on the above research achievements,a process improvement strategy was proposed for the hot crack problem during the free forging process of 17-4PH stainless steel valve box forgings,furthermore,the corresponding industrial trail production of the valve box forgings had been carried out.It was found that the thermal plasticity was improved after the process optimization,and the surface crack propagation was effectively inhibited during the forging process. |
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