Stainless steel reducers are divided into concentric reducers, eccentric reducers and reducing elbows. The latter two types of stainless steel reducers are non-axisymmetric structural pipe fittings and have to adopt some unique manufacturing processes. Generally speaking, The current manufacturing process method guarantees the quality of stainless steel reducers.
Process method for manufacturing stainless steel reducer
1.1 Manufacturing process of seamed pipe fittings
Both concentric reducers and eccentric reducers with larger diameters can be rolled from sheet metal on a plate rolling machine. This kind of stainless steel reducer has only one radial longitudinal weld. Concentric reducers and eccentric reducers with smaller diameters, as well as reducing elbows of any size, can only be pressed into half shapes through molds, and then the two halves are assembled and welded into a whole pipe fitting. This kind of Stainless steel reducers have two radial longitudinal welds. And the two halves of the eccentric reducer and reducing elbow require two sets of molds respectively.
When sheet metal (roll) is pressed to form stainless steel reducers, cold pressing is generally used. Hot pressing is only used when the capacity of the press is limited. The mold sizes for cold pressing and hot pressing will be slightly different.
1.2 Process methods for manufacturing seamless pipe fittings
The manufacturing process of concentric reducers is the simplest. Use a thicker-walled tube as the blank to enlarge the diameter of one port, or use a tube of equal wall thickness as the blank to reduce the diameter of one port. Regardless of whether it is expanded or reduced, it is usually pressed with a mold. At present, eccentric molds are commonly used in China to press eccentric reducers step by step. The eccentric reducers of level 3 or above produced by this process will have serious gaps at the eccentric end of the small head, inner wall tumors, uneven wall thickness, ellipse, etc. Phenomenon. When the eccentric reducer is replaced by a structure obtained by truncating an obliquely cut concentric reducer, because the oblique section is not a circular surface but an elliptical surface, there will be a problem that the end face and the straight pipe are misaligned and cannot be completely connected. If a new pressing process is adopted, that is, a concentric reducer is first pressed out with a concentric mold, and then the pressed concentric reducer is put into an eccentric mold to be pressed and shaped into the required eccentric reducer, then the inner and outer The surface is smooth, the small head is flat, the wall thickness is uniform, the roundness is qualified, and less raw materials are used.
Reducing-diameter elbows can be formed by step-by-step shrinkage of the mold on the basis of equal-diameter elbows. In order to facilitate shrinkage, an extra piece of elbow wall is cut off on the outer arch of the elbow in an area convenient for manual welding. After the shrinkage, the cut is butt welded there. Of course, the weld must be strictly inspected. In this way, the reducing elbow becomes a half-section pipe fitting with a weld, but the wall thickness is relatively uniform.
When the pipe blank is pressed to form a stainless steel reducer, large-diameter pipes are generally formed by hot-pressing and closing, and it is not suitable to use small-diameter pipes by cold-pressing and expanding.
2 Product testing of stainless steel reducers
The test object, the geometric shape of the eccentric reducer deviates slightly from the standard. The orientation of the surface strain measuring point of a certain eccentric reducer sample is shown in Figures 1 and 2. Ultrasonic non-destructive thickness measurement and thickness measurement with vernier caliper after physical dissection. The comparison of the results of the two methods is shown in Table 2, and other data is shown in Table 3. The measured wall thickness distribution of the eccentric reducer is shown in Figure 3.
There are two wall thickness values in one grid of the eccentric reducer. The upper wall thickness value and the lower wall thickness value are the values measured when the ultrasonic thickness measuring probe transmits and receives the sound wave separation surface parallel and perpendicular to the elbow axis. The eccentricity The error in the average value of the two wall thicknesses of the reducer is 2.16%. Other pipe fittings were tested with the same method and content. The measured ovality of the stainless steel reducer was less than 2%, and the bending radius error of the reducing elbow was also small.
3 Geometric Dimensional Analysis of Stainless Steel Reducers
The wall thickness distribution trends of concentric reducers of large and small sizes are exactly the same. From the big end to the section close to the small end, the wall thickness is from thin to thick, and from the transition section to the small end, the wall thickness is from floating to thin. This is because the inner hole of the small end has been turned after being formed. Some of the wall thickness has been removed. However, the wall thickness of the small end face is thinner than that of the large end face, which is exactly the opposite of the situation of the eccentric reducer. This is caused by the manufacturing process. And when the wall thickness changes along the axial section, the changes between the meridians have obvious regularity, but there is also a certain degree of dispersion.
4 Strength performance analysis
The surface hardness distribution trends of large and small eccentric reducers are roughly the same, but not completely consistent. The main difference is in the hardness of the small end. The hardness of the small end of the small eccentric reducer is higher, while the hardness of the large eccentric reducer is smaller. The hardness of the end has dropped somewhat.
During the manufacturing process of pipe fittings, extrusion processing and geometric shape repair will produce cold work hardening and uneven heat treatment. The actual plastic deformation produced by each part of the pipe fittings is different. Although the product is finally normalized, its mechanical properties will still show a certain isotropic behavior. The opposite sex will have certain adverse effects on the stress distribution. Some scholars’ research on elbows and tees shows that considering the dispersion of test data, the anisotropy of pipe fitting materials is not obvious, and the change in yield strength does not exceed ±5%. The degree of processing deformation of the wooden eccentric reducer is greater than that of the tee, so the change in the yield strength of the pipe fittings may exceed this value.
5 Conclusion
1) The geometric dimensions of the large and small ports of the stainless steel reducer are relatively accurate, but the wall thickness is very uneven. For stainless steel reducers with straight sections, the wall thickness of the small end face of the eccentric reducer is thicker than the wall thickness of the large end face, while the wall thickness of the small end face of the concentric reducer is thicker than the wall thickness of the large end face. To be thin, the wall thickness of the reducing elbow should be relatively uniform. Therefore, during inspection, the geometric dimensions of both the large and small ports must be measured.
2) The wall thickness of the stainless steel reducers tested is all extremely thick. It is recommended that comprehensive wall thickness testing and records be carried out before use. This will provide a basis for online thickness measurement to determine the thinning star, so that the corrosion rate can be accurately reflected and the safe operation of the pipeline can be ensured.
3) The results of the ultrasonic thickness measurement method are slightly larger than those of the caliper thickness measurement method, which makes the results biased towards danger. When measuring the maximum wall thickness, the actual measured value when the probe separation surface is parallel to the axis of the pipe is slightly larger than the actual measured value when the separation surface is perpendicular to the axis of the pipe. Although it is less than 0.5%, it also makes the results biased towards danger.
4) The ovality of the stainless steel reducer is less than 2%, and the bending radius error of the reducing elbow is also small and can be ignored.
5) The surface hardness of both ends of the stainless steel reducer is about 35% lower on average than the surface hardness of the middle section. When the tensile strength is calculated based on the empirical formula 3.5378HB (MPa) in GB 1172-74, the result is still obviously conservative at 6.1%.
6) After final normalizing treatment, the yield strength and tensile strength of the stainless steel reducer products formed by hot pressing of large-diameter pipe blanks are significantly improved.
Author: Maria Yang
Post time: Oct-27-2023