Selected problems of risers machining

There are many research projects around the world regarding various aspects of the deep hole machining process, i.e.:  impact of the tool construction on the dimensional accuracy of the drilled holes [1] and the integrity of the surface after drilling and boring of deep holes [2],  thermal deformation of the workpiece during drilling deep holes [3],  impact of the tool geometry on chip evacuation [4] and the dynamics of the coolant flow to improve the cooling effect when drilling deep holes and flushing the chips [5],  optimization of the design of the deep hole drilling tool [6]. The article describes in a simplified way the problems that occurred during technological trials of processing risers. The tests were carried out at CELSA Huta Ostrowiec in Ostrowiec Świętokrzyski. The semi-finished entry angle for the machining process was free rod with diameter ⌀ ~ 370 mm, length 16 140 mm and weight 10 328 kg. The main problem associated with the length of the produced risers is the drilling of long holes and the uneven heating of the workpiece during the turning process, which is the reason for the pipe to bend when removing subsequent layers of machining allowance.

There are many research projects around the world regarding various aspects of the deep hole machining process, i.e.:  impact of the tool construction on the dimensional accuracy of the drilled holes [1] and the integrity of the surface after drilling and boring of deep holes [2],  thermal deformation of the workpiece during drilling deep holes [3],  impact of the tool geometry on chip evacuation [4] and the dynamics of the coolant flow to improve the cooling effect when drilling deep holes and flushing the chips [5],  optimization of the design of the deep hole drilling tool [6].
The article describes in a simplified way the problems that occurred during technological trials of processing risers.The tests were carried out at CELSA Huta Ostrowiec in Ostrowiec Świętokrzyski.The semi-finished entry angle for the machining process was free rod with diameter ⌀ ~ 370 mm, length 16 140 mm and weight 10 328 kg.
The main problem associated with the length of the produced risers is the drilling of long holes and the uneven heating of the workpiece during the turning process, which is the reason for the pipe to bend when removing subsequent layers of machining allowance.

Production process of risers
The first stage of production of risers was the execution in CELSA Hutta Ostrowiec using the forging method in the F22 grade, weight ~10 328 kg, in the form of a bar with a diameter of ⌀ ~370 mm and length 16 140 mm.Overall dimensions forgings have machining allowances on the diameter of 25.6 mm to 110 mm and length -1270 mm (material for destructive testing after heat treatment: ~400 mm, material for thermal barriers and technological surplus: 300 mm, material for fastening -in a special holder in a vertical furnace during heat treatment: 570 mm).
After the forging and the initial heat treatment, the riser was pre-machined and bored, and the axial hole was bored.Then a qualitative heat treatment was performed in a vertical gas furnace.The heat-treated riser fabric had to meet very high strength requirements: tensile strength Rm ≥ 670 MPa, yield strength Re ≥ 550 MPa, sample elongation A4 ≥ 40%, tapering rate ≥ 40%, impact strength KV ≥ 100 J at temperature -40 °C.
In addition, the pipe material was subjected to nondestructive tests: ultrasonic (UT) and magnetic (MT) to confirm the high requirements of steel cleanliness specified in the material specifications.After receiving positive results of destructive and non-destructive tests, it was possible to proceed to the final machining (turning outer diameters and opening the hole) and to carry out a measurement report on geometrical quantities of the product.

Turning raw forging
The turning of the raw tube forging took place on a heavy TCH160 type lathe.The rolling operations of the nipple (circumferential strips on the outer diameter of ~120 mm, used to support the forgings of the holders) were made using a centering sleeve, allowing for precise axial alignment of the workpiece and support with roller supports.
The made tube should have a roundness deviation not exceeding 0.04 mm, radial run-out maximum 0.02 mm and a surface roughness Ra < 1.25 μm.During turning, the forging was supported by three roller rests.The turning parameters, depending on the cross-section of the cutting layer, were within the following limits: cutting speed vc = 40÷60 m/min, feed f = 0.4÷1.0mm/rev.

Drilling hole Ø 150 mm and boring at Ø 175 mm
Drilling of the hole ⌀ 150 and successive boring was performed on a deep hole drilling machine type KŻ 1920, using special drilling and boring heads of their own design, mounted on a drilling rod (STS system, fig.2), working at the cutting speed vc = 60 m/min and feedrate vf = 9÷12 mm/min.The changes in pressure on the oil supply to the head were in the range of 0.1÷6.0bar and were dependent on the head's construction, hole length, bar and head diameter, cutting data and the type of chips.The oil flow -200, 400, 600 l/min -was adjustable.Before each boring process, pilot holes were made, the shape and dimensions of which are shown in fig. 3 and in the table.AND.During drilling of the holes there were problems with the removal of chips through the holes of the drill rods (Fig. 4).The difference in wall thickness in individual cross-sections of risers did not exceed 3 mm over the entire length of the hole -after drilling operations.

Thermal treatment
In order to minimize the error in the straightness of the workpiece, the heat treatment of the risers was already carried out in the vertical furnace at the stage of thermal improvement, where a universal disc sling was used to suspend the pipes.It allowed to hook two or four pieces at once, for example: riders, shafts or rods.
Two opposite holes in the plate were used -one to suspend the riser, and the other -to the counterweight.It was impossible to suspend one pipe due to the lack of a suitable handle.
After the heat treatment of the pipe, its elongation was increased by ~15 mm on a length of 10 746 mm and a straightness error of ~15 mm / 15,000 mm.
The pipe was subjected to a straightening process and thermal treatment was repeated.The rectilinearity of the pipe was again measured -the straightness error on the entire length of the pipe was 7 mm.
After heat treatment, the hardness of the tube was measured with a EQOTIP 550 hardness tester using the Leeba dynamic method at points as shown in the diagram in fig. 5.The results of the hardness measurements are presented in tab.II.
Based on the analysis of the measurement results, it was found that the difference in the hardness of the pipe along the angular positions G1, G2, G3, G4 and distance was within 47÷54 HB, and in particular cross-sections contained in the range of 7÷13 HB.
Then, the next stage of machining was started.The machining was started by calibrating the "straightening out" of the outer surface of the pipe by means of a connection with a suitably adjusted axis (fig.1).The distance between the axes and a part of the shaft length is achieved due to the eccentric fixing of the shaft in the centering sleeve with the help of adjustment bolts.Then, from the moved axis, a "tube" (fig.3, tab.I, tab.II) was made to insert the boring head and the boring was started at Ø 190 mm, with the cutting parameters: vf = 8÷12 mm/min and vc = 60 m/min.
Drilling and hole boring is carried out using the STS method, i.e. one-shot method, in which the flushing oil is fed through a gap between the machined hole and the surface of the drilling rod.The chips are discharged through the hole of the rod.During the boring, the pipe was supported by five steadies.
The problem of difficult chip breaking from the material from which the pipe was made, was solved by reducing the feed.In this way, relatively regular and easy-to-rinse chips were obtained.The use of a higher feed resulted in the formation of hard-breakable shavings, which blocked the drill rod and prevented further processing.
Obtained dimensions of the hole diameter Ø 190 mm at the entrance are Ø 190.08 mm, and at the exit -Ø 190.14 mm, the diameter of the mole was Ø 190 + 0.05 / + 0.03.
Then the turning operations were prepared, preparing the boring pipe at Ø 210 mm and Ø 219.4 mm (tab.I), and a pipe meeting the requirements shown in fig. 5 was obtained.Measurements of the diameters at the beginning and end of the hole to be removed from Ø 190 mm to Ø 210 mm showed that the entry hole diameter was 0.04 mm higher than at the output (input Ø 210.08 mm, output Ø 210.24 mm).During mechanical machining, the thin walls of the pipe, communes = 19.56mm, were warming up, which is why it was necessary to use very abundant cooling.

Description of the method of wall thickness measurement
Measurements of wall thickness of risers after each turning, drilling and boring operation were carried out using ultrasonic thickness gauges, with a measuring accuracy of 0.1 mm.The wall thickness measuring points were prepared by turning the outside diameter of the 30 mm wide strips, maintaining the transverse runout maximum 0.05 mm and the surface roughness Ra = 1.25 μm.Preservation of the same position of the measuring points on subsequent measurements allowed to follow changes in the rectilinearity of the hole.

Conclusions
 Differences in pipe surface hardness indicate the place and direction of the pipe's curvature when removing the subsequent surplus layers.In place of areas with higher hardness, the curvature of the pipe was convex (beating outwards).One should strive to achieve the same hardness on the entire surface of the pipe. The pipe straightness error after heat treatment should not exceed 4 mm.Larger curvature of pipes considerably extends the machining time, and in exceptional cases there may be a lack of machining allowance. It should be noted that the pipe is significantly elongated after the heat treatment and that a sufficiently large portion of material is placed on the pipe end faces. The problem of poor drilling performance and hole boring in materials that give breakable chips are still to be solved.The use of low drilling and boring parameters allowed the chips to flow out with the oil, but at the cost of extending the processing time. Large influence on the straightness of the pipe after machining has the number of pipe supports used during machining.When processing pipes with a length of ~16 m, it is advisable to use five steadies.A smaller number of steadies contributes to a greater cross-section of the pipes, and a larger number of stops keeps them very difficult during machining due to the necessity of frequent relocation.

Fig. 1 .
Fig. 1.Method of supporting the riser during the turning process

Fig. 4 .
Fig. 4. Form of chips making their removal difficult