Acta Metall. Sin.(Engl. Lett.)Vol.23 No.1 pp72-80 February 2010
Forming process and numerical simulation of making
upset on oil drill pipe
Huaping TANG 1,2)∗, Changqian HAO 1), Yongzheng JIANG 1) and Lei DU 1)
1) School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China 2) School of Mechanical Engineering, Nantong University, Nantong 226007, China
Manuscript received 12 August 2009; in revised form 6 December 2009
The flow stress behavior of grade G-105 steel (API standard) for oil drill pipe in hot compressive deformation was studied by isothermal compression tests on Gleeble-1500 thermal-mechanical machine. Referring to established empirical formulas, the constitutive equation of the flow stress for 105 steel was obtained. For the grade G-105 oil drill pipe (φ114.3×10.92 mm), through proper design of the die and applying suitable process parameters, and using coupled rigid viscoplastic thermo-mechanical finite element method, the forming process of making upset on oil drill pipe was nu-merically simulated. The simulation results show that the internal transition section of the oil drill pipe is not only continuous and smooth but also long (up to about 220–250 mm).
KEY WORDS Oil drill pipe; Constitutive equation; End upsetting; Numerical simulation
1 Introduction
Drill pipe which is responsible for transfer torque, makes the bit to drill down and deepen oil well, and is indispensable in the field of oil exploitation. Moreover, it bears the tensile stress arising from the weight of its own. At the same time, the internal wall of the oil drill pipe is eroded by slurry at high pressure which should be cleaning and discharging clastic rock from oil well. Thus, drill pipe worked is subjected to the combined effects of bending, twisting and pulling. The working condition of the oil drill pipe is extraordinarily complex. In view of this, usually the end of the oil drill pipe is required to upset for the purpose of enhancing the connection strength between drill pipes[1].
The process of end upsetting on oil drill pipe is achieved by firstly heating the end of drill pipe locally, and then changing its inner diameter and outer diameter to some extent so as to increase the wall thickness (Fig.1). In accordance with the change of the wall thickness, the upsetting process can be divided into three types[2]: external upset, internal upset and internal-external upset, of which internal-external upset is the most common type.
End upsetting on oil drill pipe is a highly difficult forming process. Few industrial companies can manufacture oil drill pipe maturely and therefore monopolize the market,
∗Corresponding author. Professor, PhD; Tel.: +86 731 88879351.
and there has been little reported in this field so far.
Currently, the upsetting portion, especially the internal transition section of oil drill pipe is apt to appear defects like concave, groove, fold, short length and small radius (Fig.2) as a result of improper design of the dies and unreasonable parameters used in the forming process. Stress concentration usually occurs in the transition section producing high stress concentration. The unstable quality of the drill pipe product will produce serious safety menace[3−5]. The internal taper length (M iu) and the radius of curvature (R) of the drill pipe have been regarded as the most important geometry parameters which affect the fatigue strength to a great extent. Therefore, efforts have been made by researchers to make a long tapered transition to minimize stress concentration.
Traditionally, the design of the dies and the determination of technologies used in metal forming process mainly rely on the previous design experience. Manufacturers expect to eliminate defects through a commonly way of trial-and-error. However, the end upsetting on the oil drill pipe is rather complex, and the factors affecting the quality of metal form-ing, such as the upsetting ratio, shape of dies, heating temperature, heating length and lubrication are complicated also. Undoubtedly, the traditional manufacturing method of a product will be at high cost, low productivity and long development cycle. Nowadays, finite element method has become an effective tool in analyzing and solving these prob-lems in metal forming. The application of the finite element numerical simulation method in metal forming process can provide detailed and reliable information, by which we can successfully optimize the forming parameters and the design of dies[6].
In this work, based on experimental study of the thermoplastic deformation charac-teristics, a constitutive equation of G-105 steel (API standard) was established, and the forming process of making an internal-external upset on oil drill pipe (φ114.3×10.92 mm) was simulated.
Fig.1 Schematic drawing of making upset on oil drill pipe (1-Oil drill pipe; 2-Clamping machine; 3-Die; 4-Punch).
2 Establishment of Constitutive Equation[7−9]
In order to simulate the metal behavior accurately under forming temperature, it is necessary to investigate the flow stress of the oil drill pipe metal at first.
The steel used is hot-rolled and belongs to grade 105. The compressive deformation of the steel was investigated at temperatures ranging from 750◦C to 1100◦C and strain rates ranging from 0.01 s−1 to 10 s−1. The cylindrical specimens were firstly heated to 1200◦C at speed of 10◦C/s; then preserved about 300 s; finally, cooled to the test temperature at a speed of 2.5◦C/s and preserved 30 s. The data of the true stress-true strain relationship (Fig.3) of G-105 steel was obtained under different conditions.
Based on a comprehensive analysis and evaluation on the various mathematical models of metal, XU et al.[10] proposed a mathematical model of the true stress-true strain and its form is clear and simple.
σ=Aεaε˙bexp[−(cT+ dε)] (1)
where σ is true stress, MPa; εtrue strain; ˙εstrain rate, s−1;T temperature, ◦C; A,a,b,
c,dparameters related with material.
Based on the material data obtained from experiment, the parameters of the constitu-tive Eq.(1) were regressed: A=5104.4,a=0.314,b=0.112,c=0.003 andd=0.7114.
The correlation coefficient is 0.9272, which indicates the significance of the regression. Consequently, model (1) is precise enough to describe the deformation behavior at high
0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 S t r e s s / M P a Strain 750 o C 800 o C 900 o C 1000 o C 1100 o C (a) 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 S t r e s s / M P a Strain 750 o C 800 o C 900 o C 1000 o C 1100 o C (b) 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 300 S t r e s s / M P a Strain 750 o C 800 o C 900 o C 1000 o C 1100 o C (c) 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 300 350 S t r e s s / M P a Strain 750 o C 800 o C 900 o C 1000 o C 1100 o C (d)
Fig.3 True stress-strain curves of the oil drill pipe: (a) ˙ε=0.01 s−1; (b) ˙ε=0.1 s−1; (c) ˙ε=1 s−1;
temperature of the grade 105 oil drill pipe.
σ = 5104.4ε0.314ε˙0.112exp[−(0.003T+ 0.7114ε)] (2) 3 Numerical Simulation
During the process of end upsetting on oil drill pipe, the metal transfer is obvious in volume. Namely, the movement volume of metal in plastic deformation is larger than in elastic deformation. Considering that the end upsetting process is carried out under a certain temperature, a coupled rigid viscoplastic thermo-mechanical finite element method was adopted. It should be noted that the final process and parameters adopted is based on actual situation, extensive finite element analysis and comparison. The process design of end upsetting on oil drill pipe consists in an appropriate combination of a number of parameters. Therefore, this paper will also reveal the typical influence of the die and temperature on forming process, which can verify the advisability of the process designed and the rationality of the finite element model.
3.1 End upsetting process
When developing end upsetting process, the number of steps is mainly determined by the ratio of the final wall thickness to its original.
The upsetting ratio tou/t is restricted to less than 1.5. If it0s more than that, a new step should be added. touandtstand for the final wall thickness and its original thickness, respectively.
According to the actual manufacturing capacity in factory, the G-105 oil drill pipe (φ114.3×10.92 mm) with an outer diameter of 125 mm and an inner diameter of 70 mm was finally designed, which complies with the API standard. So,tou=27.5 mm, tou/t=2.518.
The forming process was divided into three steps, containing three heating steps and three end upsetting. Internal-external upset was adopted in each step. The upset ratios were in Table 1.
Table 1 Allocation of three upset ratios
Upsetting step Outer diameter of drill pipe/mm Inner diameter of drill pipe/mm Upsetting ratio
1 120.4 88 1.484
2 122.5 78 1.373
3 125 70 1.236
Heating of the oil drill pipe can be divided into three zones: full heating, room tem-perature and gradient heating between them. The fully heated part of the three upsetting steps was 1180–1250◦C. The fully heated length l
1 in the first step was 600–650 mm. The fully heated length in the second step was designed to be l2=l1 −∆l1+ 50, where l1 is the first fully heated length; ∆l1 is the shortened length in the first step. Similarly, the fully heated lengthl3 wasl3=l2−∆l2+ 50. In the gradient heating zone, the oil drill pipe presented transition temperature distribution from fully heated part to room temperature part and the length was 50 mm. Electrical induction heating method was used in all steps. In the process of end upsetting on the oil drill pipe, the shape of the die at the transition part plays an important role in determining the quality of the drill pipe. As illustrated in
Fig.4, we believe the importance of the die can be mainly embodied by the parameter
n, which decides the metal resistance there and thus controls the volume of metal flow in all directions. The appropriate value of
ncan ensure excellent quality at the internal upset portion. In this process, the reasonable value of nis about 50 mm.
Fig.4 Schematic drawing of die.
3.2 Establishment of finite element model
Because the structure and boundary conditions of the oil drill pipe, die and punch are all in line with the characteristic of axial symmetry, axial symmetric model was used to establish the finite element model in Msc.marc software (Fig.5).
3.3 Material properties
The flow behavior of grade 105 oil drill pipe steel under high temperature was simulated by writing the thermoplastic constitutive Eq.(2) in Msc.marc software. Considering the influence of temperature, the material model of the oil drill pipe in finite analysis was defined as rigid viscoplastic model. Because of relatively small deformation of the die and punch in all upsetting steps, they were defined as hot-rigid body in analysis, which merely considered the effect of hot exchange but deformation. The material of the die and punch is 4Cr5MoSiV1 and 38Cr2Mo2VA, respectively. Material properties are listed in Table 2.
3.4 Boundary conditions
All freedoms of the nodes along the outer bound of the die were constrained, and the part of the pipe which extended out the die (the right part of the pipe in Fig.6) was glued
Fig.5 Finite element model (1-punch; 2-die; 3-drill pipe). Table 2 Physical parameters of the materials[13]
Material property Drill pipe Die Punch
Density/(kg/m3) 7474 7800 7600
Emissivity 0.7 0.7 0.7
Thermal expansion coefficient/(◦C−1) 14.5×10−6 13.2×10−6 13.84×10−6
Specific heat/(J/kg·K) 460 571 393.5
Conductivity/(W/(m·K)) 29 28.4 40.23
Contact heat transfer coefficient 10000 W/(m2·◦C)
Heat transfer coefficient 200 W/(m2·◦C)
Fig.6 Boundary conditions (1-punch; 2-axial symmetry centerline; 3-die; 4-drill pipe; 5-clamping line).
to the fixed borderline numbered 5 in Fig.6 to simulate clamping. Besides, 0.9 was defined as the plastic heat generation coefficient. The three upsetting steps were all at a speed of 250 mm/s.
3.5 Friction
Because of too large normal friction between oil drill pipe and the die during end upsetting, shear friction model, which based on shear stress, was adopted. The friction coefficient between drill pipe and the die was 0.25, and the friction coefficient was 0.15 between the drill pipe and the punch[14].
4 Simulation Results
Numerous parameters were involved in end upsetting process, here we mainly focus on the forming results (Fig.7) of the drill pipe after the three steps. Since the upsetting process contained three heating and three upsetting steps, calculation of genetic algorithm in pass was used in this finite element analysis. Namely, the latter procedure was simulated on the basis of the previous one.
Referring to Fig.7a, the process of metal flow was observed: the outer wall of the drill pipe firstly contacted with the inner wall of the die under the force of punch; then, the inter wall of the pipe began to form along the surface of the punch because of metal resistance coming from the transition section of die.
As shown in Fig.8, there was a temperature drop at the outer and inter wall of the oil drill pipe, while there was a temperature increase on the middle part of the pipe wall as a result of plastic heat generation.
The node coordinates at the internal transition section of the oil drill pipe after the third step were got and plotted in Fig.9 (original curve). In order to make a quantitative analysis, the original curve was fitted. The maximum standard residual error was 0.52 mm, and the correlation coefficient was 0.99348. From the fitted curve, we can see that the upsetting parameters designed could manufacture an oil drill pipe withM iulength of 230 mm, which is much better than the level of 140 mm currently available in market. Obviously, what we have done in this paper can effectively make the internal transition of the oil drill pipe smooth and its performance better.
5 Discussion
In this investigation, the productive use of the finite element technique disclosed the influence of the die and temperature on making upset on oil drill pipe, which helps us to understand some typical problems in forming process. In order to demonstrate the superiority of the process designed, some typical problems will be discussed as follows:
Fig.7 Simulated forming process of the drill pipe: (a) first upsetting when t=1.84 s; (b) first finished upsetting; (c) second finished upsetting; (d) third finished upsetting (full model); (e) third finished upsetting (transition section).
(1) The shape of the die at the tran-sition part plays an important role in the end upsetting. In order to compare and find out the optimal value of n, respectively, we tookn=20 mm, 30 mm, 40 mm, 50 mm and the end upsetting process was simulated in different cases. The simulation results were shown in Fig.10. In the case of n=20 mm, it is rather steep at the internal transition section of the drill pipe and the value of the M iuis only about 60 mm, which is less than the minimum requirement (76.20 mm) of the API standard. When n=30 mm and 40 mm, theM iuis 155 mm and 160 mm, re-spectively, which were approximately equal. When n=60 mm, it can be seen from the simulation that the end upsetting couldn0t be made because of too small resistance arising from the transition location of the die. So it can be concluded that different values of n will result in the metal mov-ing at different proportion in each direc-tion. A too short transition part of die will cause a too steep slope at the internal tran-sition part of drill pipe, weakening the
mech-Fig.8 Temperature on the end of the drill pipe after the third upsetting.
0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.034 0.036 0.038 0.040 0.042 0.044 0.046 R a d i a l l o c a t i o n / m Axial location / m 1 2 Equation y=Intercept+B1*x 1 +B2*x 2 W eight No weighting Residual sum 1.64196E-5 of squares
Adj. R-square 0.99348
Value Standard error Intercept-0.04859 0.00114 B
B1 0.27125 0.00393 B2 -0.19677 0.00336
Fig.9 Fit of nodes location at internal tran-sition section after final upsetting (1-fitted curve; 2-original curve).
anical properties of it[15].
(2) Concerning the design of the heating lengthlion the oil drill pipe, in some practices
liwas just defined as the former fully heated lengthli−1subtracted by the shortened length ∆li−1 in forming process. Namely, the location of the transition temperature part on the oil drill pipe is unchanged in each upsetting step. As shown in Fig.11, such process will make obvious bulges at the junction of fully heated part and transition temperature part, affecting the forming quality. However, methods described in this paper can effectively avoid the defect. It should be noted that bulges can not be eliminated at all, but adopting the methods can control it to an acceptable range.
Fig.11 Bulge defect after the second upsetting (a) and the third upsetting (b).
(3) There should be a clear borderline between the fully heated part and the room temperature part on the oil drill pipe. The length of the transition temperature part is generally in the range of 30–50 mm. A too long length will result in reduced clamping stiffness, while a too short will aggravate additional stress and make bulge as shown in Fig.11.
6 Conclusions
(1) The flow behavior of Grade 105(API standard) oil drill pipe is studied on Gleeb-1500 thermal-mechanical machine and a constitutive equation of the G-105 steel is obtained by means of regression analysis: σ=5104.4ε0.314ε˙0.112exp[−(0.003T+ 0.7114ε)].
(2) According to the law of plastic flow, through designing dies properly and controlling important process parameters, the forming process of making an internal-external upset on the drill pipe (φ114.3×10.92 mm) is simulated. The results show that the length of the internal transition section (M iu) can reach 230 mm and the surface there is smooth.
(3) This work discloses the law of metal flow during the end upsetting process and validates the feasibility and superiority of the designed forming process.
REFERENCES
[1] B.M. Wang,The Process of Oil Drill Pipe in Foreign(Metallurgical Industry Press, Beijing, 1983) p.3 (in Chinese).
[2] API SPECIFICATION 5D. Specification for Drill Pipe [S] p.12. [3] W.G. Chunn and W. R.T. Moore,US US5184495(9 February 1993). [4] T.E. Winship,US US5517843(21 May 1996).
[5] E. Takeuchi and H. Miyoshi,US US4845972(11 July 1989).
[6] S.S. Xie and L. Li,Finite Element Simulation Technology and Its Application in Metal Forming(Science press, Beijing, 2008) p.1 (in Chinese).
[7] H.G. Zhang, Y. He, X.F. Liu and J.X. Xie,Acta Metall Sin43(2007) 930 (in Chinese). [8] J.M. Zhou, L.H. Qi and G.D. Chen,Mech Sci Technol14 (2005) 212 (in Chinese). [9] S.L. Wang, X.Y. Ruan and X.L. Yu,J Shanghai Jiaotong Univ30(1996) 20 (in Chinese). [10] Y.S. Xu, J.T. Liu and M. Nie,J Appl Sci15 (1997) 379 (in Chinese).
[11] P. Zhao and J.W. Zhang,CN CN200510027402(3 January 2007). [12] J.W. Zhang and W. Wang,CN CN200610028551(2 January 2008).
[13] Z.Z. Chen and D.N. Lan, Die Steel Manual (Metallurgical Industry Press, Beijing, 2002) p.370 (in Chinese).
[14] H.X. Li,Friction and Lubrication in Pressure Processing(Metallurgical Industry Press, Beijing, 1993) p.132 (in Chinese).
[15] D.N. Wang,Principal of Metal Plastic Forming (China Machine Press, Beijing, 1982) p.109 (in Chi-nese).
