Manufacturing process and testing standards for hot pressing of stainless steel reducing pipes

Stainless steel reducers are divided into Concentric Reducer, Eccentric Reducer and Reducing El-bow. The latter two stainless steel reducers are non-axisymmetric structural pipe fittings, and some unique manufacturing processes have to be adopted. Generally speaking, the current manufacturing process is guaranteed for the quality of stainless steel reducers.

1. Process method for manufacturing stainless steel reducer

      Stainless steel reducers are divided into Concentric Reducer, Eccentric Reducer and Reducing El-bow. The latter two stainless steel reducers are non-axisymmetric structural pipe fittings, and some unique manufacturing processes have to be adopted. Generally speaking, the current manufacturing process is guaranteed for the quality of stainless steel reducers.

1.1 Manufacturing process of seamed pipe fittings     Concentric reducers and eccentric reducers with larger diameters can be rolled and formed on the plate rolling machine. This stainless steel reducer has only one radial longitudinal weld. Concentric reducers and eccentric reducers with smaller calibers, as well as reducer elbows of any size and diameter, can only be pressed into a half shape through a mold, and then the two halves are welded into an integral pipe fitting. This stainless steel reducer has two radial longitudinal welds. And the two halves of the eccentric reducer and the reducer elbow require two sets of molds respectively.     When sheet metal (rolling) is pressed to form stainless steel reducers, cold pressing is generally adopted. Hot pressing is only used when the capacity of the press is limited, and the mold size for cold pressing and hot pressing will be slightly different.

1.2 Manufacturing process of seamless pipe fittings     The manufacturing process of the concentric reducer is the simplest. Use a tube with a thicker wall thickness as the blank, expand the diameter of one port or use a tube of equal wall thickness as the cap, and reduce the diameter of one port. Regardless of flaring or shrinking, it is generally pressed with a mold. At present, eccentric molds are generally used in China to suppress eccentric reducers step by step. For eccentric reducers with more than three levels produced by this process, there will be serious gaps, inner wall tumors, uneven wall thickness, and ellipse at the eccentric end of the small head. When the eccentric reducer is replaced by the structure obtained by obliquely cutting the truncation of the concentric reducer, because the oblique section is not a circular surface but an elliptical surface, there will be a problem of misalignment between the end surface and the straight pipe, and the problem that it cannot be completely connected. If a new pressing process is adopted, that is, the concentric reducer is first pressed with a concentric mold, and then the pressed concentric reducer is put into an eccentric mold to be reshaped into the required eccentric reducer, the inner and outer surfaces are 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 gradually shrinking the mold on the basis of the equal-diameter elbow. In order to facilitate the necking, cut off a piece of redundant elbow wall on the outer arch of the elbow where it is convenient for manual welding, and then butt weld the cut at this place after shrinking. Of course, the weld must pass strict inspection. In this way, the reducing elbow becomes a half-section pipe fitting with a weld, but the wall thickness is relatively uniform.     When the tube blank is pressed to form a stainless steel reducer, it is generally used to form a large-diameter tube by hot pressing and closing, and it is not suitable to use a small-diameter tube to form a small-diameter tube by cold pressing and expanding.

2 Product inspection of stainless steel reducer The detection objects are shown in Table 1. The geometric shape of the eccentric reducer deviates slightly from the standard. The orientation of the surface strain measurement point of an eccentric reducer sample is shown in Figure 1 and Figure 2. The results of the ultrasonic nondestructive thickness measurement and the vernier caliper thickness measurement after physical anatomy are compared in Table 2. Other data are shown in Table 3. The measured wall thickness distribution of the eccentric reducer is shown in Figure 3. In Table 2, 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’s sending and receiving sound wave separation surface is parallel to and perpendicular to the axis of the elbow. The error of the average value of the two wall thicknesses of the eccentric reducer is 2.16%. The same method and content were tested for other pipe fittings. The ellipticity of the measured stainless steel reducer is less than 2%, and the error of the bending radius of the reducer is also small.

1) Surface hardness. In order to evaluate the strength and performance uniformity of materials, hardness measurement is a simple and easy method, and the measured pipe will not be damaged during the measurement. The measurement method refers to Article 33 of the "Ultra-high Pressure Vessel Safety Supervision Regulations (Trial)": "Five circular lines perpendicular to the cylinder should be evenly drawn on the outer wall of the cylinder, and 4 points should be evenly distributed on each circular line for hardness inspection. The hardness value should meet the design drawings or standards. The difference between the highest value and the lowest hardness value of each point between the circular lines should not be greater than 40, and each point on the same circular line should not be greater than 20". The hardness tester used for the test is respectively located at the measuring points on the meridian C at the center of the eccentric slope and the meridian G at the opposite circumference. The results of the meridian G are shown in Figure 4. The results for other fittings are shown in Table 4.

2) Strength performance. According to the empirical formula ≈3.5378HB (MPa) in GB1172-74 "Conversion Value of Ferrous Metal Hardness and Strength", the Brinell hardness is converted into tensile strength.

3 Geometric dimension analysis of stainless steel reducer     The wall thickness distribution trends of the large and small concentric reducers are exactly 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 section to the end face of 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 forming to remove part of the wall thickness. However, the wall thickness of the end face of the small end is thinner than that of the end face of the large end, which is just 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 changes between the meridians have obvious regularity, but there is also a certain dispersion.