Hot pressing process and testing standard for manufacturing stainless steel reducer

The stainless steel reducer is one of the chemical stainless steel pipe fittings and is used for the connection of two different pipe diameters. It is divided into stainless steel concentric reducer and stainless steel eccentric reducer.

1 Process for manufacturing stainless steel reducer

    The stainless steel reducer has a Concentric Reducer, an Eccentric Reducer and a Reducing El-bow. The latter two stainless steel reducers are non-axisymmetric tubulars. , have to take some unique manufacturing process, in general, the current manufacturing process is guaranteed for the quality of stainless steel reducer.

1.1 Manufacturing method of seamed pipe fittings

    The concentric reducer and the eccentric reducer with larger calibers can be formed by rolling on the buckling machine. The stainless steel reducer has only one radial longitudinal weld. Concentric reducers and eccentric reducers with small calibers and reducers of any size and diameter can only be pressed into a half shape by a die, and then the two halves are welded into one integral pipe. Stainless steel reducers have two radial longitudinal welds. And the two halves of the eccentric reducer and the different diameter elbow respectively require two pairs of molds.

    When the sheet (rolling) is pressed and formed into a stainless steel reducer, cold pressing is generally employed. Hot pressing is only used when the capacity of the press is limited. The size of the mold for cold pressing and hot pressing will be slightly different.

1.2 Manufacturing method of seamless pipe fittings

    The manufacturing process of the concentric reducer is the simplest. The thicker tube is used as the blank, and the one-port diameter or the tube with the same thickness is used as the gross damage, and the diameter of one port can be reduced. Regardless of the flaring or shrinkage, it is generally pressed with a mold. At present, the eccentric reducer is widely used in China to suppress the eccentric reducer. The eccentric reducer of the third grade or above produced by this process will produce serious gaps at the eccentric end of the small head, tumors on the inner wall, uneven wall thickness and ellipse. phenomenon. When the eccentric reducer is replaced by the structure obtained by the truncated concentric reducer, since the oblique section is not a round surface but an elliptical surface, there is a problem that the end surface and the straight tube are misaligned and cannot be completely connected. If a new pressing process is adopted, the concentric reducer is first pressed with a concentric mold, and the pressed concentric reducer is placed in an eccentric die to press the shape into a desired eccentric reducer, then the inside and outside. The surface is smooth, the small head is flat, the wall thickness is uniform, the roundness is qualified, and the raw materials used are less.

    The different diameter elbow can be formed by the die shrinking step by step on the basis of the equal diameter elbow. In order to facilitate the shrinkage, an extra elbow wall is cut off in the area of the elbow that is convenient for manual welding, and the mouth is welded after the shrinkage, and the weld must be strictly inspected. Thus, the reducer elbow becomes a half-welded pipe, but the wall thickness is relatively uniform.

    When the tube blank is pressed and formed into a stainless steel reducer, the large-diameter tube is generally used for hot-pressing and forming, and it is not suitable to use a small-diameter tube for cold-rolling and flaring.

2 Product inspection of stainless steel reducer

    The test objects are shown in Table 1. The geometric shape of the eccentric reducer is slightly deviated from the standard. The orientation of the surface strain measurement point of a eccentric reducer specimen is shown in Figure 1 and Figure 2. The two methods of ultrasonic non-destructive thickness measurement and physical anatomy after vernier caliper thickness measurement The results are compared as 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 table of detection

Fig.1 Geometric deviation of part of eccentric reducer 

Figure 2 eccentric reducer cross section and its equal points

Fig. 3 Measured wall thickness distribution curve of eccentric reducer

Fig. 4 Surface hardness distribution measured by eccentric reducer warp 

In Table 2, there are two wall thickness values in the eccentric reducer. The upper wall thickness value and the lower wall thickness value are measured when the ultrasonic thickness measuring probe transmits and receives the acoustic wave separation surface parallel to the elbow axis and the vertical axis. The value of the average value of the two wall thicknesses of the eccentric reducer is 2.16%. The same method and content were tested on other pipe fittings. The measured ellipticity of the stainless steel reducer was less than 2%, and the error of the radius bend of the reducer was also small.

1. Surface hardness. In order to evaluate the uniformity of the strength and performance of the material, the hardness measurement is a simple and easy means, and the tube to be tested is not damaged during the measurement. The measurement method is stipulated in Article 33 of the "Safety Supervision Procedure for Ultra High Pressure Vessels (Trial)": "There shall be 5 circular lines perpendicular to the cylinder on the outer wall of the cylinder, and 4 points shall be drawn on each loop line. For hardness inspection, the hardness value should conform to the design drawings or standards. The difference between the highest and lowest values of the hardness between the loops should be no more than 40, and the points on the same loop should be no more than 20′′. The test hardness tester is located at the warp line C at the center of the eccentric slope and the measurement point on the warp G at the opposite circumference. The result of the warp G is shown in Fig. 4. The results of other fittings are shown in Table 4.
2. Strength performance. According to the empirical formula in GB1172-74 "Black metal hardness and strength conversion value" ≈ 3. 5378HB (MPa) converts Brinell hardness into tensile strength.

Table 2 Comparison of wall thickness of eccentric reducer

Table 3 No. 1 easurement of the length of the circumferential arc of each cross section of the eccentric reducer

Table 4 Average results of different diameter tube hardness test and estimated intensity value 

Table 5 Performance of 20 steel pipe 168 × 6.0 and its reducer 

3 Gometrical analysis of stainless steel reducers

  According to Table 2 and Figures 3 and 4, from the end face 1 of the large end face to the section 5 close to the small end, the wall thickness is from thin to thick, and from the section 5 to the end face 7 of the small end face, the wall thickness is thick to thin. Because the inner hole of the small end is turned after forming, part of the wall thickness is removed, and the section 5 is a smooth transition zone of the straight end and the inclined section of the small end. However, the wall thickness of the end face of the small end is thicker than the wall thickness of the end face of the large end, because the wall thickness of the straight pipe of the pipe is relatively uniform. And when the wall thickness changes along the axial section, the variation between the warp threads is uneven, but there is obvious regularity: the wall thickness of the straight surface zone composed of warp beams B, C, and D does not change much along the axial direction because the zone The degree of processing deformation is small; the wall thickness of the slope area composed of the warp beams F, G, and H varies in the axial direction because the degree of processing deformation in the area is large. In addition, the wall thickness distribution trends of the concentric and different diameter pipes of the two sizes are completely the same. From the end face of the big end to the section close to the small end, the wall thickness is from thin to thick, and from the transition cross section to the end face of the small end, the wall thickness is from floating to thin, because the small end inner hole is turned after forming. Part of the wall thickness was removed. However, the wall thickness of the end face of the small end is thinner than the wall thickness of the end face of the large end, which is exactly the opposite of the case of the eccentric reducer, which is caused by the manufacturing process. And when the wall thickness changes along the axial section, the change between the warp threads has obvious regularity, but also has a certain dispersion.

Figure 5 Load displacement curve of the sample

4 Strength performance analysis

    The surface hardness distribution trends of the large and small eccentric reducers are about the same, but they are not completely consistent. The main difference is the hardness of the small end, the hardness of the small end of the small eccentric reducer is higher, and the large eccentric reducer is small. The hardness of the end has dropped.

    In Table 4, the tensile strength of the samples was 6.1% and 11% higher than the strength calculations of the empirical formula, respectively. From Table 5, the yield strength and tensile strength of Sample 1 were increased by 9.0% and 2.0%, respectively, before the start of production, and the yield strength and tensile strength of Sample 2 were increased by 26.4%, respectively, before the start of production. And 8.8%.

    Extrusion processing in the manufacturing process of pipe fittings, geometric shape repair, cold work hardening, uneven heat treatment, plastic deformation of different parts of the actual pipe parts, although the product is finally normalized, its mechanical properties will still be a certain direction The opposite sex has a certain adverse effect on the stress distribution. Some scholars have studied the elbow and the tee. The anisotropy of the pipe material is not obvious considering the dispersion of the test data, and the variation of the yield strength does not exceed ± 5%. The degree of deformation of the wooden eccentric reducer is larger than that of the three-way, so the change in the yield strength of the pipe may exceed this value.

5 Conclusion

1. The geometrical dimensions of the large and small stainless steel reducers are more accurate, but the wall thickness is very uneven. For a stainless steel reducer with a straight section, the wall thickness of the end face of the eccentric reducer is thicker than the wall thickness of the end face of the large end, and the wall thickness of the end face of the concentric reducer is larger than the wall thickness of the end face of the big end. To be thin, the wall thickness of the different diameter elbow is relatively uniform. Therefore, when testing, the geometry of both the large and small ports should be measured the most.
2. The thickness of the stainless steel reducer tube is too thick. It is recommended to carry out a comprehensive wall thickness test record before use. It provides a basis for online thickness measurement to judge the thinning star, in order to accurately reflect the corrosion speed and ensure the safe operation of the pipeline.
3. The ultrasonic thickness measurement method is slightly larger than the caliper thickness measurement method, thereby making the result dangerous. When measuring the maximum wall thickness, the measured value when the probe separation surface is parallel to the axis of the tube is slightly larger than the measured value of the partition surface perpendicular to the axis of the tube. Although less than 0.5%, the result is also dangerous.
4. The ellipticity of the stainless steel reducer is less than 2%, and the error of the bend radius of the reducer is also small and can be ignored.
5. The surface hardness of both ends of the stainless steel reducer is about 35% lower than the surface hardness of the middle section. When the tensile strength is estimated according to the empirical formula of 3.5378HB (MPa) in GB 1172-74, the result is still conservatively 6.1%.
6. After the final normalizing treatment, the stainless steel reducer formed by the hot-pressing of the large-diameter tube blank has significantly improved the yield strength and tensile strength.

Source: China Stainless Steel Reducer Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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