Hot press forming process of reducer and its quality inspection

The hot press forming process of seamed or seamless concentric reducers, eccentric reducers and reducer elbows was summarized. 38 pieces of reducers were tested for shape and wall thickness geometry and surface hardness, and the strength properties of eccentric reducers were also sampled and tested.

1. Manufacturing process of reducer

Reducers have concentric reducer, eccentric reducer and reducer elbow, of which the latter two reducers are non-axisymmetric structure pipe fittings, had to take some unique manufacturing process, on the whole, the current manufacturing process methods for the quality of the reducer is guaranteed.

1.1 Manufacturing process method of seamed pipe fittings

The larger diameter of the concentric reducer and eccentric reducer can be rolled by the plate in the roll forming machine, this reducer only a radial longitudinal welding seam. Smaller diameter concentric reducer and eccentric reducer and any small or large diameter reducer elbow, only through the mold will be pressed into the shape of half of the plate, and then the two halves of the group welded into a whole pipe, this reducer are two radial longitudinal welds. And the eccentric reducer and reducer elbow of the two halves are required respectively two pairs of molds.
Sheet (roll) press forming reducers, generally take cold pressing. Only when the capacity of the press is limited to the use of hot pressing, cold pressing and hot pressing die size will be slightly different.

1.2 Manufacturing process method of seamless pipe fittings

Concentric reducer manufacturing process is the simplest, with a thicker wall thickness of the tube as a blank, expand the caliber of one end or use the same wall thickness of the tube as a blank, reduce the caliber of one end can be. Regardless of the flare or shrinkage, is generally used to press the mold. At present, the domestic common use of eccentric mold pressed eccentric reducer by level, with this process to produce more than three levels of eccentric reducer, in the small head eccentric end will produce serious gap, the inner wall tumor, wall thickness is not uniform and oval and other phenomena. According to the oblique cut concentric reducer cut head structure instead of eccentric reducer, because the oblique section is not a round surface but an elliptical surface, there will be a wrong side between the end face and the straight tube, can not be completely connected to the problem. If the new pressing process, that is, the first concentric die pressed concentric reducer, and then the pressed concentric reducer into the eccentric mold pressed rectification for the required eccentric reducer, the inner and outer surface is smooth, small head flat, uniform wall thickness, roundness qualified, less raw materials used.
Reducing elbow can be made on the basis of the equal diameter elbow by the mold level by level shrinkage. In order to facilitate the shrinkage, the elbow on the outer arch of the convenient hand-welded area to cut a piece of excess elbow wall, and then butt weld the cut after the shrinkage, of course, the weld must be strictly inspected. Thus, the reducer elbow becomes half of the pipe with welded seams, but the wall thickness is more uniform.
Pipe billet press forming reducer, generally using large diameter pipe hot pressed closed-end forming, and should not be used for small diameter pipe cold pressed flare forming.

2. Product testing of the reducer

The geometric shape of the eccentric reducer deviates slightly from the standard, the orientation of the strain measurement point on the surface of an eccentric reducer specimen is shown in Figure 1 and Figure 2, the results of ultrasonic nondestructive thickness measurement and physical dissection after vernier caliper thickness measurement of the two methods are shown in Table 2, and other data are shown in Table 3. The measured wall thickness distribution of the eccentric reducer is shown in Figure 3.
Table.1 Theoretical object table for testing

Item	Outer diameter of large and small ends/mm	Height/mm	Wall thickness/mm	Number of concentric reducers (piece)	Number of eccentric reducing pipes (piece)	Number of reducing elbows (pieces) R=1.5D1
1	168/89	160	5.5	4	4	3
2			9.5	4	4	3
3	219/140	250	6.5	4	4	/
4			9.5	4	4	/

Figure.1 Geometric deviation of eccentric reducer part

Figure.2 Cross-section of eccentric reducer and its equidistant points

Fig.3 Measured wall thickness distribution curve of eccentric reducer

Figure.4 Eccentric reducer warp line measured surface hardness distribution
Table 2 in the eccentric reducer in each cell there are two wall thickness values, the upper wall thickness value and the following wall thickness value are measured when the ultrasonic thickness probe transmits and receives sound wave separation surface parallel and perpendicular to the axis of the elbow, the eccentric reducer two wall thickness average error of 2.16%. Other pipe fittings for the same method and content of the test, the measured ellipticity of the reducer is less than 2%, reducer elbow radius error is also small.
1) Surface hardness.
In order to evaluate the strength of the material and the uniformity of performance, hardness measurement is a simple and easy to use means, measured without destroying the pipe being tested. Measurement methods refer to the “ultra-high-pressure vessel safety monitoring procedures (for trial implementation)” Article 33: “should be in the outer wall of the barrel evenly scribed 5 perpendicular to the barrel of the ring line, in each ring line evenly distributed to take 4 points to do hardness check, hardness value should be in line with the design drawings or the provisions of the standard, the highest value of hardness between the points of the ring line and the lowest value difference should not be greater than 40, the same ring line on each point not Greater than 20”. Test with a hardness tester, respectively, located in the eccentric ramp at the center of the meridian C and its relative circumference of the meridian G on the measurement point, the results of the meridian G see Figure 4. other pipe fittings results are shown in Table 4.
2) Strength performance.
According to GB 1172-74 “ferrous metal hardness and strength conversion values” in the empirical formula σb ≈ 3.5378HB (MPa) will be converted to Brinell hardness for tensile strength, the results are shown in Table 4. 1 eccentric reducer fittings in Table 1 to test the strength properties of the sample 1 in the new Sanshi SHT50005105 microcomputer-controlled electro-hydraulic servo universal The load displacement curve of specimen 1 in the new Sanshi SHT50005105 microcomputer-controlled electro-hydraulic servo universal tester is shown in Figure 5.
Table.2 Comparison of measured wall thickness of eccentric reducer

Section	8 Equidistant point A				8 Equidistant point E
	Ultrasonic thickness measurement/mm	Caliper thickness measurement/mm	Absolute error/mm	Relative error/(%)	Ultrasonic thickness measurement/mm	Caliper thickness measurement/mm	Absolute error/mm	Relative error/(%)
	Parallel/Vertical	H side/B side	Parallel/Vertical	Parallel/Vertical	Parallel/Vertical	H side/B side	Parallel/Vertical	Parallel/Vertical
1	6.5/6.4	5.0/4.8	1.6/1.5	32.7/30.6	5.9/5.9	6.8/5.8	0.4/0.4	6.8/6.8
2	6.9/6.9	5.9/5.7	1.1/1.1	19.0/19.0	6.0/5.9	6.2/6.5	0.35/0.45	5.5/7.1
3	7.6/7.6	6.7/6.3	1.1/1.1	16.9/16.9	7.0/6.8	7.6/7.9	0.75/0.95	9.7/12.3
4	8.8/8.6	7.8/7.8	1.0/0.8	12.8/10.3	8.4/8.3	8.6/8.5	0.15/0.25	1.8/2.9
5	9.5/9.7	9.1/8.8	0.55/0.75	6.1/8.4	9.6/9.0	8.9/9.3	-5	-5
6	6.8/6.8	5.2/5.6	1.4/1.4	25.9/25.9	6.5/6.8	5.9/5.5	0.727272727	0.725388601
7	5.8/5.7	5.5/5.4	0.35/0.25	6.4/4.6	5.9/5.8	5.6/5.6	1.5	1.5
Mean value	7.41/7.39	6.4	1.01/0.99	15.8/15.5	7.04/6.93	7.05	0.14/1.7	/

Table.3 No. 1 eccentric reducer of each cross-sectional circumferential arc length measurement value (mm)

Section	AB	BC	CD	DE	EF	FG	GH	HA	Mean	Perimeter	Diameter
1	66.25	66.25	66.25	66.25	66.25	66.25	66.25	66.25	66.25	530	168.7
2	66.25	66.25	66.25	66.25	66.25	66.25	66.25	66.25	66.25	530	168.7
3	57	54	56	54	58	60	60	60	57.4	459	146.1
4	45	46	50	44	44	49	50	48	47	376	119.7
5	39	40	42	38	38	37	37	39	38.8	310	98.7
6	35.6	35.6	35.6	35.6	35.6	35.6	35.6	35.6	35.6	284.8	90.7
7	35.6	35.6	35.6	35.6	35.6	35.6	35.6	35.6	35.6	284.8	90.7
Geometric dimensions	a1	0	/	c1	0	/	e1	0	/	g1	0
	a2	33	/	c2	0	/	e2	47	/	g2	15
	a3	11	/	c3	0.5	/	e3	11	/	g3	3
	a4	25	/	c4	15	/	e4	15	/	g4	15

Table.4 Average results of hardness testing and extrapolated strength values of reducers

Item	Specification/mm	Wall thickness/mm	Structural style	Hardness value of each part number/HB					Calculated strength/Mpa
				1	2	3	4	Average
1	ϕ168/ϕ89	5.5	concentric	166.8	148.1	155.4	157.1	156.9	Mean 554.90
			eccentric	129.3	130.1	126.3	129.4	128.8	The average value is 455.58, the calculated value of part number 2 is 460.27, and the measured values are 490 and 522
			elbow	112.2	117.5	109.2	/	113	Average 399.77
2		9.5	elbow	103.8	103.4	117.2	/	108.1	Average 382.44
3	ϕ219/ϕ140	6.5	concentric	160.3	156.4	156.5	147.8	155.3	Mean 549.42
			eccentric	140.8	139.2	135.1	127.9	135.8	The average value is 480.26, and the calculated value of part number 2 is 492.46

Table.5 Performance of 20 steel pipes ϕ168×6.0 and their reducers

Mechanical properties at room temperature	σs/MPa	σb/MPa	δ5/(%)	Conclusion
Original data provided by the steel pipe factory	335	475	36	Comply with GB/T8163-1999
Data of steel pipe sampling inspection by the manufacturer	345	480	34
Sample 1 for eccentric reducer	390	490	37.42
Sample 2 for eccentric reducer	436	522	35.3

3. Geometric analysis of the reducer

Figure.5 Load displacement curve of the specimen
According to Table 2 and Figure 3, Figure 4, from the large end section 1 to near the small end section 5, wall thickness are from thin to thick, and from section 5 to the small end section 7, wall thickness from thick to thin, because the small end of the bore after forming the turning process to remove part of the wall thickness, section 5 is the small end of the straight section and the oblique section of the smooth transition zone. But the wall thickness of the small end face are thicker than the wall thickness of the large end face, because the wall thickness of the rough straight tube of the pipe is more uniform. And along the axial section wall thickness changes, the changes between the warp is not uniform, but there is a clear regularity: warp B, C, D composed of straight surface area wall thickness along the axial change is not large, because the processing deformation of the area is small; warp F, G, H composed of oblique surface area wall thickness along the axial change, because the processing deformation of the area is larger.
In addition, the wall thickness distribution trend of the concentric reducers of large and small sizes is identical. From the large end face to the section near the small end, the wall thickness is from thin to thick, while from the transition section to the small end face, the wall thickness is from thick to thin, which is because the small end bore is turned after forming to remove part of the wall thickness. However, the wall thickness of the small end face is thinner than the wall thickness of the large end face, the opposite of the eccentric reducer, which is caused by the manufacturing process. And along the axial section wall thickness changes, the change between the warp has obvious regularity, but also has a certain dispersion.

4. Strength performance analysis

The trend of surface hardness distribution of large and small eccentric reducers is more or less the same, but not exactly the same, the main difference is in the hardness of the small end, the small end of the small eccentric reducer is higher, while the hardness of the small end of the large eccentric reducer has fallen back.
In Table 4, the tensile strengths of the specimens were 6.1% and 11% higher than the strength projections of the empirical formula, respectively. From Table 5, the yield strength and tensile strength of specimen 1 increased by 9.0% and 2.0%, respectively, and the yield strength and tensile strength of specimen 2 increased by 26.4% and 8.8%, respectively, compared with those before the production of the manufacturing plant.
The manufacturing process of extrusion, geometric shape repair, cold hardening, uneven heat treatment, the actual pipe parts produce different plastic deformation, although the product is finally normalized, its mechanical properties will still show a certain degree of anisotropy, which has a certain adverse effect on the stress distribution. The study of elbow and tee shows that, considering the dispersion of test data, the anisotropy of pipe material is not obvious, and the change of yield strength does not exceed ±5%. In this paper, the processing and forming deformation of the eccentric reducer is larger than that of the tee, so the variation of the yield strength of its pipe fittings may exceed this value.

5. Conclusion

• 1) The geometry of the large and small ports of the reducer is accurate, but the wall thickness is very uneven. For the reducer with straight section, the wall thickness of the small end of the eccentric reducer is thicker than the wall thickness of the large end, while the wall thickness of the small end of the concentric reducer is thinner than the wall thickness of the large end, and the wall thickness of the reducer elbow is more uniform. Therefore, when testing, the geometric dimensions of both large and small ports are measured.
• 2) The wall thickness of the tested reducers are ultra-thick, it is recommended that the use of a comprehensive wall thickness test records, online thickness measurement to determine the amount of thinning to provide the basis for accurate reflection of the corrosion rate, to ensure the safe operation of the pipeline.
• 3) Ultrasonic thickness measurement method than caliper thickness measurement method results are slightly larger, thus making the results on the dangerous side. Measurement of wall thickness, probe separation surface and pipe axis parallel to the measured value of the measured value of the separation surface perpendicular to the axis of the pipe compared to slightly larger, although less than 0.5%, but also make the results on the dangerous side.
• 4) The ellipticity of the reducer is less than 2%, the elbowing radius error of the reducer elbow is also small, can be ignored.
• 5) The surface hardness of the two ends of the reducer is about 35% lower than the average surface hardness of the middle section. According to GB 1172-74 empirical formula σb ≈ 3.5378HB (MPa) extrapolated to tensile strength, the results are still significantly conservative 6.1%.
• 6) Using large diameter pipe billet hot press forming reducer products after the final normalizing treatment, yield strength and tensile strength are significantly improved.

Author: Chen Sunyi