| To improve the oil extraction in the oilfield, the perforating bullet is widely used to perforate downhole. Bullet body, a key component of perforating bullet, can enhance the lateral perforating bomb wave impulse, improve the kinetic energy of the metal jet, delay and reduce the interference of rarefaction wave on metal jet, and ensure the perforation depth. The results showed that the higher the strength of bullet body is, the greater the value of the perforating depth is. So, increasing the strength of bullet body is one of effective ways to improve the perforating depth. Because of the complex structure and high machining cost, the bullet body is made by the cold extrusion technology which can improve the production efficiency and decrease the production costs while the choice of body materials is limited into20#steel. However, the explosion fragments of20#steel are very large and prone to form the lock down-hole accident, which will append the salvage. In recent years, new technical requirements are proposed with the development of the exploitation technologies of tilted and horizontal wells, and full-bore perforating technique. So, developing new bullet body materials and machining technology are currently an urgent research project. In this work, the microstructure, properties and forming process are studied, and the feasibility of preparing the new bullet body by powder metalluary materials and craft method is explored. This can provide a theoretical basis and technical support for industrialization, and meet the oil industry demand for new perforating bullet.Based on thermodynamic principles and alloying powder metallurgy sintering mechanism, the Fe-Cu-C series, Fe-Ni-C series and Fe-Ni-Cu-C series alloys were optimized and designed. The effect of Ni, Cu and C alloying elements on microstructure and properties of bullet body were investigated. The strengthening mechanism of alloying elements was analyzed. The results showed that the strengthening effect of Cu on microstructure and properties is very obvious. Cu could promote the formation of the pearlite structure and reduce the pearlite lamellar spacing. The tensile strength, hardness and pore rate in the matrix increase while the elongation decreases with the increase of Cu content. Cu was melted and uniformly distributed in the pores matrix. With Cu is more than2%and cools slowly, the dissociative Cu and Cu-rich phase appeared. Ni Ni also promoted the formation of the pearlite. The ferrite content decreased and pearlite content significantly increased with increasing the Ni content. The tensile strength and hardness of the alloy tended to increase. At first the elongation of the alloy increased and then decreased with the increase of Ni content when the same C content was kept. The elongation of the alloy reached the highest value and then decreased at3%Ni content. The Ni-rich austenite layer in the grain boundaries appeared at2%Ni content. The Ni-rich austenite layer tended to thicken with the increase of Ni content. Mechanical test results showed that, for iron-based powder metallurgy materials, the comprehensive strengthening effect of Cu and Ni is much larger than the single element and they play a complementary role. As a comparison, the mechanical properties of Fe-Ni-Cu-C alloy are superior to Fe-Ni-C and Fe-Cu-C alloy.The effect of compaction-forming method, initial pressure, repeated pressure and holding time on microstructure and density distribution were systematically studied. The press forming method of bullet body was optimized by the theoretical analysis and process experimetns. The structure of bullet body, initial and repeated pressure forming mold was optimized. The results showed that the initial pressure and repeated pressure played an important role on the initial pressure density and repeated pressure density. When the initial pressure density was close to7.1g/cm3and repeated pressure density reached to7.4g/cm, the density did not increase when continuing to increase the pressure. Namely, the repeated pressure density did not exceed95%of the theoretical density. It had the adverse effects if the initial pressure density was too high or low. The microstructure and density distribution was better at the initial pressure of600MPa and repeated pressure of800MPa. The conventional multi-pressure method of bullet body is difficult to obtain a higher density and uniform density distribution. We used the compensatio extrusion and enhanced compaction method to obtain the high density powder metallurgy sintering bullet body with certain density gradient. The average density of the bullet body was7.3g/cm3. The technology had an important role in obtaining high density powder metallurgy products. Mechanical experimental results showed that the density had a great influence on the mechanical properties. When the sintering density of the billet was above6.9g/cm3, the tensile strength and elongation increase significantly while the hardness slightly changed. When the density of bullet body was7.3g/cm3, the tensile strength and elongation of Fe-Ni-Cu-C alloy could reach647MPa and3.78%, respectively.Sintering process of bullet body was systematically studied. The strengthening mechanism of sintering and sintering kinetics was analyzed and discussed. Sintering kinetic analysis shows that the sintering rate depends primarily on the sintering temperature. Sintering temperature had a great influence on microstructure, density and properties of bullet body. At first the density of bullet body alloy decreased and then increased with increasing the sintering temperature. The density tended to a stabilized value when the temperature reaches1200℃. The pore volume decreased with increasing the sintering temperature. The shape of the pore became irregular oval or spherical and distributed independently. The tensile strength, hardness and elongation of bullet body increased with increasing sintering temperature. The results show that Cu and Ni had an enhanced sintering effect on the sintering process. Cu could produce liquid phase sintering and play a strengthening effect on the diffusion of the alloy elements when the sintering temperature is higher than1083℃. Cu was melted and easy to form pores. It caused the volume expansion, the decreasing of the density and anti-densification. Ni-rich austenite grain layer formed by Ni and Fe grains provided a fast diffusion path for Fe atoms and alloy elements. The sintering volume shrinkage of the bullet body appeared and resulted in sintering densification. This could compensate for the anti-densification of Cu. Cu had an enhanced sintering effect. On the other hand, a number of molten Cu pores formed. The explosion of bullet body was easy to form small pieces by adjusting the sintering parameters to control the porous shape. So the bullet body presented excellent low debris. The results show that the sintering technology is the best with the beginning of sintering temperature of850℃, holding time of1hour and the repeated sintering temperature of1100℃-1150℃, holding time of1.5hours.In this paper, the effect of process parameters on plastic deformation, flow behavior and stress was analyzed using finite element simulation of bullet body by adopting cold extrusion of powder billet. The results show that the peak of the axial tensile stress was significantly higher than other technology in a cylindrical shape. With a frustoconical black in the process of the molding, the whole deformation was easy to flow uniformly. The required extrusion pressure and the minimum value of the effective stress were the lowest. The formation could be realized after simple extrusion. The simulation provided the basis of reasonable process design and a new pathway preparation of fabricating the bullet body.As a comparision of different burst tests, the results show that the critical target penetration depth and pore size of the bullet body by powder metallurgy exceeded the standard value. Penetration depth of the bullet body was higher than that of20#steel bullet body. The bullet body presented excellent low debris and could completely replace20#steel bullet body. |
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