Analysis on the bending process of TP347H stainless steel pipe

This paper briefly analyzes the material characteristics of TP347H, the problems easily occurred in the process of bending and extrusion, and the corresponding process measures taken, so as to avoid the hot crack of stainless steel and obtain qualified products.

With the expansion of urban scale and the acceleration of urbanization in China, the production and accumulation of urban garbage are increasing year by year. All kinds of industrial garbage and urban domestic garbage which are difficult to be treated in time have posed a huge threat to people’s living environment. Using municipal solid waste for heating is one of the ideal methods to solve the problem of municipal solid waste. The purpose of waste incineration is to burn waste as much as possible, make the burned materials harmless and fully reduce the volume, reduce the generation of new pollutants and avoid secondary pollution. The biggest advantage of incineration treatment is good reduction effect, which reduces the volume and weight of incineration waste by more than 90%. The heat energy generated by waste incineration is used for heat supply to make urban waste become new energy, which is not only beneficial to environmental protection, but also can obtain obvious economic and social benefits. Due to the complex composition of garbage and the strong corrosiveness of flue gas after combustion, the flue gas temperature of boiler superheater is higher, so TP347 is often used as the material of high temperature superheater. Due to the space layout, the steel pipe adopts small R bend radius, and the steel pipe is easy to use φ51 × 5. The bend radius is R34, which is difficult to make. Combined with the company’s experience in trial production and post production, the TP347H steel pipe is introduced φ51 × 5. The manufacturing process of bend radius R34 is introduced.

Austenitic stainless steel background

The domestic brand of TP347H is 1cr19ni11nb, which has good cold workability and weldability, and does not need heat treatment after welding. Table 1 shows the chemical composition of TP347H, and table 2 shows the mechanical properties of TP347H
Table 1 Chemical Composition% of TP347H steel

Element Mass percent content Element Mass percent content
C 0.04 0.10 Nb 0.75
Mn 2.00 Ni 9.00 13.0
Si 0.75 P 0.045
Cr 0.75 S 0.03

Table 2 room temperature mechanical properties of TP347H steel

Performance index TP347H
yield strength δ s/Mpa 205
Tensile strength δ b/Mpa 515
Elongation δ/% 35

Function of various elements in stainless steel

  • Cr: the main alloy element for stainless steel to obtain corrosion resistance. Due to the protection of dense and stable Cr2O3 film formed by chromium, the penetration corrosion of medium to metal matrix is prevented.
  • Ni: the element that forms and stabilizes austenite. When the nickel content continues to increase to about 8%, in general, the single-phase austenite structure can be obtained, that is, 18-8 austenitic stainless steel, which is widely used, has better corrosion resistance than ferritic stainless steel and martensitic stainless steel with the same chromium content, and has better processability, weldability, plasticity and impact toughness at low temperature.
  • C: Carbon plays a beneficial and harmful role in stainless steel. Carbon can improve the thermal strength of austenitic steel, but the affinity between C and Cr is very large, and it is easy to combine with (FeCr) 23c6 in stainless steel. This kind of carbide precipitates along the grain boundary, which will cause CR poor zone and intergranular corrosion.
  • NB, Ti: titanium and niobium are easier to form carbides than chromium. The presence of titanium and niobium in stainless steel will make the carbon in the steel combine with titanium or niobium as much as possible. In this way, chromium in the steel can exist in the solid solution as stably as possible, so that there is enough chromium content in the solid solution to ensure corrosion resistance, so as to ensure that chromium does not precipitate along the grain boundary, and that there is no chromium poor zone at the grain boundary, It can effectively prevent intergranular corrosion of stainless steel.

Intergranular corrosion

The corrosion extending along the boundary between metal grains. At room temperature, the solubility of carbon in austenite is very small, about 0.02% ~ 0.03%, but the carbon content in general austenitic stainless steel exceeds this value, so the excess carbon will continue to diffuse to the austenite grain boundary, and combine with the nearby Cr element to form chromium carbide carbide between grains, such as (FeCr) 23c6. When the mass fraction of Cr in the grain boundary is less than 12%, a “chromium poor region” will be formed. Under the action of corrosive medium, this region is easy to be corroded, so it is called “intergranular corrosion”, as shown in Figure 1.
20210721012806 52828 - Analysis on the bending process of TP347H stainless steel pipe
Fig.1 intergranular corrosion

Austenitic sensitization range

If the residence time of austenitic stainless steel is too long in the range of 420 ~ 850 ℃, the C element in austenite will be fully analyzed to the austenite grain boundary, and the C element and Cr element will fully combine to form chromium carbide, thus accelerating the intergranular corrosion.

Solution treatment

The purpose of solid solution heat treatment of austenitic stainless steel is to dissolve the alloy carbides, such as (FeCr) 23c6, produced or precipitated in the previous processing procedures into austenite again to obtain a single austenite structure, so as to ensure the material has good mechanical properties and corrosion resistance, and fully eliminate the stress and cold work hardening phenomenon. The solid solution treatment process is shown in Figure 2.

Process test

According to the different process positions of pipe bending, four groups of process tests are made, as shown in Figure 3. The relevant mechanical properties and metallographic structure are compared, and the most suitable group is selected as the manufacturing process.
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Fig.2 solution treatment process
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Fig.3 four groups of process tests
Group 1:

  • Cold bending R75;
  • The second group: cold bending R75 + hot extrusion R34;
  • The third group: cold bending R75 + hot extrusion R34 + solution treatment;
  • The fourth group: cold bending R75 + solution treatment + hot extrusion R34;
  • Each group of pipe bend shall be tested according to the test name and number in Table 3.

Table 3 name and quantity of bend test for each group

Test name Microscopic metallographic test Hardness test Tensile test Bending test Intergranular corrosion
Number of tests
  • Two temperature gradient zones
  • 2 pieces bends on the outermost side
  • 5 points at bend
  • 5 o’clock at straight section
2
  • 2 face bends
  • Back bend 2
2

Sample location:
Microscopic metallography:
a. The outermost part of the bend has the largest tensile deformation;
b. The temperature gradient section of the straight section;
Hardness test:

a.

  • Marking on bend micro metallographic specimen;

b.

  • Five points were randomly selected at the straight section;
  • Tensile test: straight section;
  • Bending test: straight section;
  • Intergranular corrosion: straight section, temperature gradient section;

20210721013738 84254 - Analysis on the bending process of TP347H stainless steel pipe
Figure 4

Mechanical test

(1) The mechanical properties of each group of test specimens on the straight section are shown in Table 4.
Table 4 mechanical properties of straight section
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(2) The mechanical properties of each group of test specimens on the bend are shown in Table 5.
Table 5 mechanical properties of bend
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Hardness test

See Table 6 for hardness test results of straight section and bend of each test specimen.
Table 6 hardness test results of straight section and bend (Vickers HV)
20210721014736 53274 - Analysis on the bending process of TP347H stainless steel pipe
Hardness result analysis: after cold bending, work hardening occurs at the bend, and the hardness value increases. Hot extrusion has no effect on work hardening. The solution treatment can obviously improve the cold work hardening, but the hardness of the temperature gradient section increases.

Microscopic metallography

  • (1) The metallographic structure of bend of four groups of specimens is shown in Figure 5.
  • From the analysis of the metallographic structure of the bend, it can be concluded that cold bending causes dislocation, deformation and distortion of the outermost lattice of the bend, and there are many black spots. It is speculated that the black spots are carbides.
  • (2) The metallographic structure of the straight section of the four groups of specimens is shown in Fig.6.

20210721015150 87929 - Analysis on the bending process of TP347H stainless steel pipe
Fig. 5 metallographic structure of bend of 4 groups of specimens
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Fig. 6 metallographic structure of straight section of group specimens
From the metallographic analysis of the straight section:
The first group is microscopic metallography of raw materials. Compared with the first group, the second group has no obvious difference in microstructure and grain size. The grains of the third and fourth groups are finer than those of the first and second groups, and the grain sizes of the third and fourth groups are almost the same.
Outermost side of bend:

  • Grain size: group 1 > group 2 > group 3 > group 4

There were a lot of black tissues in the first and second groups. The black tissues in the third and fourth groups were less than those in the first and second groups, but still more than the raw materials.

Scanning electron microscope

Scanning electron microscope (see Figure 7) and energy spectrum analysis (see Figure 8) were used to analyze the particles on the grain boundary before and after solution.
The results of energy spectrum analysis show that the granular structure is carbide, but mainly niobium carbide. The mass fraction of Nb is 79.38%, while that of Cr is 1.90%. Only a small part of Cr is combined with C. NB is preferentially combined with C to ensure the mass fraction of Cr in stainless steel, prevent the phenomenon of “poor chromium” and ensure the corrosion resistance of stainless steel.

Test summary

After cold bending of TP347H pipe to R75, the bend deformation produces work hardening, the hardness value increases from 240 to 307, the material becomes hard and brittle, there is basically no ductility, grain dislocation and deformation. When the bend of R75 is hot extruded to R34, the microstructure has no obvious change, and the hardness value still reaches 301. Due to the short time of hot extrusion, the microstructure has not been changed. Therefore, the effect of hot extrusion on the microstructure is not obvious. But the material is easy to crack when it is extruded under the condition of brittleness. The third and fourth groups added a solution process to the bend, the third group was solution treatment after hot extrusion R34, and the fourth group was solution treatment after cold bending R75, and then hot extrusion. The grain size of the third and fourth group is smaller than that of the first and second groups, and the fourth group is the smallest. However, the hardness values of the third and fourth groups increase in the temperature gradient section, and there are more black particles in the microscopic metallographic phase. The black structure should be mainly niobium carbide, with a small amount of chromium carbide, iron carbide, silicon carbide and so on. NB element plays a very important stabilizing role in the black structure. When the TP347H bend is extruded, the bend hardening causes the pipe cracking, and the work hardening is reduced by solid solution treatment before extrusion to restore the material properties. The metallographic and mechanical properties of the extruded bend were tested to meet the requirements of material strength.
20210721015256 22326 - Analysis on the bending process of TP347H stainless steel pipe
Fig.7 Comparison of scanning electron microscopy before and after solid solution
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Figure.8 energy spectrum analysis of particles on grain boundary

Develop P347 steel pipe φ51 × 5, manufacturing process of bend radius R34

According to the process test, at last we chose to bend R75 first, then solid solution treatment, and finally extrusion to R34. After trial production of several products, we tested all the pipe heads by Pt without any quality problems, which ensured the product quality and production progress, and completed the solidification of the production process.

Conclusion

Manufacturing process of small R TP347H pipe bend has certain difficulty, easy to cause material cracking, pipe head scrap, through the test process comparison, a good solution to the relevant problems, achieved good production efficiency, to ensure that the manufactured products meet the relevant technical requirements and specifications.
Authors: Gong Deping, Wei Xiaoyan

Source: Network Arrangement – China Pipe Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference:

  • [1] ASME II materials section a iron based materials 2015 edition [S]
  • [2] Cai Jian. Analysis on manufacturing process of TP347H stainless steel tube panel 〔 J 〕. Shanghai: industrial boiler, 2017 (3): P54 ~ 56
  • [3] Wang Xiaomin. Engineering materials [M]. Harbin: Harbin Institute of technology, 2002
  • [4] Xia Lifang. Metal heat treatment technology [M]. Hal: Harbin Institute of Technology Press, 1998
  • [5] Cui Zhongqi. Metallurgy and heat treatment M. Beijing: China Machine Press, 2000
  • [6] KIO Sr, Pickering FB. Niobium in stainless steel. Compilation of literature on the application of niobium in stainless steel production, 2003:74

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