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Manufacturing and quality control of 800H alloy seamless butt welded pipe fittings

800H is Incoloy as a branch of 800 series materials, as a high strength austenitic Ni Fe Cr alloy containing C, Si, Mn and limited amount of Al and Ti, its main characteristics are as follows: when the grain size is controlled at grade 2-5 after special solution treatment, it has good creep fracture strength at temperature above 600 ℃, still has good toughness when working under 700 ℃ for a long time, and has good recovery resistance It has high metallurgical stability in long-term high-temperature applications. 800H alloy has been widely used in the fields of nuclear power and chemical industry due to its relatively low price.

Incoloy Alloy 800H Composition (%  by Weight)

Weight% Ni Cr Fe C Al Ti Al+Ti
Alloy 800H 30-35% 19-23% ≧39.5% 0.05-0.10% 0.15-0.60% 0.15-0.60% 0.30-1.20%

Mechanical Characteristics

Nickel Grade

Condition

Available shapes

Ultimate
Tensile
Strength,
ksi (MPa)

Yield
Strength
at 0.2%
offset,
ksi (MPa)

Elongation
in 2″
or 4D, percent

800H

SHT

Sheet, Plate

80 (552)

35 (241)

47

As an important part of 800H pipeline and equipment, the demand for 800H pipe fittings will naturally increase. The research and development of 800H pipe fittings manufacturing, the determination of its manufacturing process and the summary of its quality control points are the basis for pipe fittings manufacturers to provide qualified 800H pipe fittings.

Forming process

It has been proved that the tube and sheet material of 800H alloy has good processability and is suitable for cold or hot working. Because the Ni content is as high as 30.0-35.0%, the transformation from austenite to martensite can be effectively prevented by high Ni content compared with ordinary austenitic stainless steel, which is easy to occur in the forming of ordinary austenitic stainless steel. However, the work hardening phenomenon of 800H material is greater than that of ordinary austenitic stainless steel in the forming process, so the equipment capacity should be considered when cold forming process is adopted. The tube and sheet materials used in forming should be solution heat treated, and intermediate heat treatment may be needed when the cold working deformation is large.
At present, we use 800H (UNS n08810) seamless pipe manufacturing elbow, tee, reducer and other butt welded pipe fittings, the maximum size is dn400, the maximum wall thickness is 16mm. Figure 1 shows the furnace charging picture of dn400 × 16mm elbow after cold push forming.

Fig.1 charging picture of dn400 × 16mm elbow during heat treatment

20200630150728 47106 - Manufacturing and quality control of 800H alloy seamless butt welded pipe fittings

In order to determine whether the tube is cold formed or hot formed, it is necessary to analyze the material properties, process technology, equipment capacity and manufacturing cost. When both hot forming and cold forming can be used, cold forming process should be preferred, because compared with hot forming process, cold forming process usually has the characteristics of one-time forming, good appearance quality and low cost.
According to the specifications of this batch of 800H pipe fittings and the capacity of existing forming equipment, it is determined that all butt welded pipe fittings are formed by cold forming process. The general manufacturing process is as follows:

  • Material re inspection → blanking → forming → heat treatment → mechanical test and grain size detection → sandblasting → end processing → appearance and dimension inspection → ultrasonic and penetration testing → pickling and passivation → marking → packaging

Generally speaking, the forming process of all 800H butt welded pipe fittings is cold forming, but the cold forming process adopted by different varieties is different. The cold forming process of different varieties is summarized as follows:

  • The forming process of elbow is to use seamless pipe with the same diameter as the elbow and cold push forming on a special forming machine. The shape of the elbow is guaranteed by the outer mold and the inner core.
  • The forming process of tee is to use seamless tube with the same diameter as tee, and hydroforming on a special hydraulic press. The shape of the tee is ensured by the inner cavity of the forming die.
  • The forming process of reducer is to use seamless pipe with the same diameter as the big end of reducer, and cold compress the diameter on hydraulic press. The shape of reducer is ensured by the inner cavity of forming die.

These forming processes mentioned above are also the first choice for manufacturing stainless steel butt welded pipe fittings by cold forming method, which have been verified by production practice for many years. Practice has proved that if the equipment capacity is large enough, 800H butt welded pipe can be manufactured by these mature cold forming processes, and the forming process will not produce harmful defects to the products.

800H seamless pipe

Market status

Some industrial developed countries in the world began the market application of Incoloy 800 series alloy materials in the 1950s. Their smelting technology and manufacturing technology of rod, plate and tube have been mature. At present, 800H alloy bars, plates and tubes are mainly supplied by Hastelloy, SMC, ThyssenKrupp VDM, Outokumpu and metallurgy. In China, some scientific research institutions and enterprises have also carried out research on Incoloy 800 [3-5]. So far, our country has carried out research on Incoloy 800 The smelting technology of 800 materials and the manufacturing process of rods, plates and tubes are still in the stage of research and development. Compared with these foreign companies, some Chinese enterprises still need to improve their technology mastery, experience accumulation, brand creation, including scale capacity, especially in the manufacturing of Incoloy 800 seamless pipes.
The maximum size of Incoloy 800 seamless pipe listed in ASTM standard is DN 150 [6]. The supply specifications of seamless pipe from domestic and foreign enterprises are generally as large as DN200 and DN250, and those larger than this specification are provided in the form of welded pipe.

Procurement requirements

As we all know, the raw materials used by pipe manufacturers to produce pipe fittings are usually purchased from abroad, so the quality of raw materials purchased is the primary condition to determine the quality of pipe fittings. However, in terms of raw materials, domestic and foreign suppliers can not provide 800H seamless pipes with the maximum size of dn400, which brings great difficulties to the material procurement. In this case, after communicating with the domestic enterprises with smelting and pipe making experience in manufacturing such materials, it was decided that the smelting and pipe making of this batch of 800H alloy seamless pipes should be carried out by relevant domestic enterprises.
In order to improve the quality of 800H butt welded pipe fittings, ensure the reliability of its use and meet the processing technology of material deformation, the following additional requirements are put forward for the batch of seamless alloy pipe on the basis of conforming to the alloy pipe standard ASTM B407:

  • (1) The content of Al + Ti in the chemical composition should not be more than 0.7%.
  • (2) The grain size is 2-5 grade.
  • (3) The negative deviation of wall thickness shall not exceed 10% of the nominal wall thickness.
  • (4) On this basis, eddy current testing is carried out for alloy pipes below DN 100 and ultrasonic testing is conducted for alloy pipes above DN 100; Penetrant testing is required for both ends of each alloy pipe; endoscopy is used for the inner surface of each alloy pipe (enlarged by 16 times).

Through the joint efforts of raw material smelting enterprises and pipe manufacturing enterprises, the seamless 800H alloy pipe with the maximum nominal size of dn400 and the maximum wall thickness of 16mm, which meets the requirements of material standards and additional requirements of procurement, is finally provided. While accumulating experience, these enterprises have also made contributions to the localization of such materials.

Heat treatment

Role and purpose

Special heat treatment is another important link in quality control of 800H alloy pipe fittings. Its function and purpose are mainly manifested in the following two aspects:

  • (1) It can eliminate the work hardening phenomenon in the process of cold deformation and restore the mechanical properties of the material.
  • (2) The microstructure of the material can be recovered and adjusted to achieve the required grain size and meet the creep resistance requirements of the material at high temperature.

Specification requirements

In the 2010 edition of ASTM B366 for nickel alloy pipe fittings, heat treatment is required for all pipe fittings. The recommended heat treatment temperature for UNS N08810 pipe fittings is 1147 ~ 1177 ℃ (there is a note in the specification that the final heat treatment temperature should be consulted with the material manufacturer), and the cooling method is rapid air cooling or water cooling.
It is found that the heat treatment requirements of UNS n08810 in the previous 2004 edition of ASTM b366 are the same as those in 2010 edition. However, in the earlier version of 1997A, heat treatment was not a mandatory requirement, and the recommended heat treatment temperature was not given. It only indicated that if heat treatment is required, it should be determined by the purchaser and the manufacturer through consultation. According to the changes of heat treatment regulations in ASTM specification, people’s understanding of the importance of heat treatment to nickel base alloy materials is also gradually improved.

Heat treatment process

The main basis for formulating the heat treatment process of 800H alloy pipe fittings are: the heat treatment requirements recommended by ASTM B366, the solution heat treatment temperature of the alloy pipe manufacturer and the heat treatment process test during the trial production of the pipe fittings. The final solution heat treatment process for UNS N08810 seamless butt welded pipe is shown in Table 1.

Table 1 solution heat treatment process of UNS N08810 seamless butt welded pipe fittings

Charging temperature

Holding temperature

Holding time

Cooling mode

8001000

1162

1.5 min / mm, but not less than 15 min

Water cooling out of furnace

Considering the uniformity of heating temperature in the heat treatment furnace, the heat treatment equipment is electric furnace.

The test results of each batch of products after solution heat treatment show that the mechanical properties and grain size of the products meet the expected requirements, which proves that the special solution heat treatment process is correct. Table 2 shows the mechanical property test and grain size test results of the two specifications.

Table 2 mechanical test and grain size test results of some batches

Name, specification

D×T, mm

Tensile

MPa

Yield

MPa

Elongation

%

To attack
(1 set of 3 samples)

J

Grain size

Elbow,
406×16

525

245

54.5

237, 232, 241

3.5

Tee,
325×13

540

215

56.5

206, 198, 184

3.5

Table 2 shows the metallographic pictures of grain size detection of two specifications of products, as shown in Fig. 2 (100 times magnification).

20200630151500 51884 - Manufacturing and quality control of 800H alloy seamless butt welded pipe fittings 20200630151624 31057 - Manufacturing and quality control of 800H alloy seamless butt welded pipe fittings
a) Elbow, 406 × 16 b) Tee, 325 × 13

Fig. 2 metallographic pictures of grain size detection of two specifications of products

Conclusion

The practice of manufacturing butt welded pipe fittings by using multi batch 800H alloy seamless pipe shows that: 1

  • (1) It is technically feasible to produce 800H alloy butt welded pipe by using mature cold forming process of stainless steel butt welded pipe.
  • (2) The primary condition to ensure the quality of 800H alloy butt welded pipe is whether the quality of pipe meets the deformation requirements of forming process without defects. For raw material enterprises, it is a development direction to carry out the research on smelting, pipe making and related processes of 800H pipe, master the key technology, and provide the market with high-quality 800H pipe to meet the growing domestic demand.
  • (3) Heat treatment is another important aspect to control the quality of 800H butt welded pipe. Through special solution heat treatment, the mechanical properties and grain size of the products can reach the expected values, so as to meet the performance requirements of welded pipe fittings under service conditions.

Source: China 800H Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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Reference:

  • [1]  ThyssenKrupp VDM. Nicrofer 3220H/3220HP – alloys 800H/ 800HP[EB/OL]
  • [2]  Special Metals Corporation. INCOLOY alloy 800[EB/OL]
  • [3] Tang Wenxin, Ning Tianxin, et al. Development and application of Incoloy 800H alloy plate, bar and forging 〔 a 〕. Proceedings of the second petroleum and Petrochemical Industry Materials Symposium 〔 C 〕. Chengdu: China Society of metals, 2001.23-25
  • [4] Yang Jinming. Application and performance analysis of Incoloy 800H alloy 〔 J 〕. Refinery design, 2002, 32 (6): 18-20
  • [5] Sun Chaoyang, Liu Jinrong, et al. High temperature deformation flow stress prediction model of Incoloy 800H [J]. Acta metallurica Sinica, 2011, 47 (2): 191-196
  • [6]  ASTM B407-08, Standard Specification for Nickel-Iron-Chromium Alloy Seamless Pipe and Tube[S].
  • [7]  ASTM B366-10, Standard Specification for Factory-Made Wrought Nickel and Nickel Alloy Fittings[S].
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