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Manufacturing and Quality Control for Seamless Buttweld Pipe Fittings Made of Alloy 800H

Manufacturing and Quality Control for Seamless Buttweld Pipe Fittings Made of Alloy 800H

800H is a branch of Incoloy 800 series materials. As a high-strength austenitic Ni-Fe-Cr alloy containing C, Si, Mn and limited Al and Ti, its main characteristics are: after special solution treatment When the grain size is controlled at grade 2 to 5, it has good creep rupture strength when used at temperatures above 600 °C, and still has good toughness under 700 °C for a long time; it has good resistance to reduction, Oxidation, nitriding medium corrosion and resistance to oxidation-reduction alternating medium corrosion performance; high metallurgical stability in long-term high temperature applications [1]. Due to this excellent characteristic and relatively low material price of 800H alloy, it has been widely used in the fields of petroleum, chemical industry and nuclear power.

As an important part of 800H pipes and equipment, the demand for 800H pipe fittings naturally increases. Carrying out research and development on the manufacture of 800H pipe fittings, determining its manufacturing process, and summarizing its quality control points are the basis for pipe fitting manufacturers to ensure that they provide qualified 800H pipe fittings.

1 Forming process

Practice has proved that 800H alloy tube and plate materials have good processability and are suitable for cold working or hot working. Since its Ni content is as high as 30.0-35.0%, compared with ordinary austenitic stainless steel, the high Ni content can effectively prevent the transformation from austenite to martensite during the forming process. It is easy to occur in the forming of stainless steel. However, the 800H material exhibits a larger work hardening phenomenon than ordinary austenitic stainless steel during the forming process, so the equipment capacity should be considered when using the cold forming process. The tube and plate materials used for forming should be in the state of solution heat treatment, and intermediate heat treatment may be required when the amount of cold working deformation is large.
At present, we use 800H (brand UNS N08810) seamless pipe to manufacture elbows, tees, reducers and other butt welded pipe fittings with a maximum size of DN400 and a maximum wall thickness of 16mm. Figure 1 is a picture of the furnace installation of the DN400×16mm elbow after cold pushing and heat treatment.
To determine whether the pipe fittings are cold-formed or hot-formed, it is first necessary to analyze the material properties, process technology, equipment capabilities and manufacturing costs. Cold forming should be the first choice when either hot forming or cold forming can be used, because cold forming usually has the characteristics of one-shot forming, good appearance quality and lower cost compared with hot forming.
According to the specifications of this batch of 800H pipe fittings and the capacity of the existing forming equipment, it is determined that the forming method of all butt welded pipe fittings adopts the cold forming process. The overall manufacturing process is as follows:
Material re-inspection→cutting→forming→heat treatment→mechanical test and grain size inspection→sand blasting→end processing→appearance and size inspection→ultrasonic and penetrant inspection→pickling and passivation→marking→packaging
In general, the forming process of all 800H butt welded pipe fittings is cold forming, but the cold forming process methods used by different varieties are different. The cold forming process methods of different varieties are briefly described as follows:
---The elbow forming process is to use the seamless pipe with the same diameter as the elbow, and cold push it on a special forming machine. The shape of the elbow is guaranteed by the outer mold and the inner core.
---The three-way forming process is to use the seamless pipe with the same diameter as the three-way, and hydraulically expand it on a special hydraulic press to form. The shape of the tee is guaranteed by the inner cavity of the forming die.
---The forming process of the reducer is to use the seamless pipe with the same diameter as the big end of the reducer, and cold-compress the diameter to form on the hydraulic press. The shape of the reducer is guaranteed by the inner cavity of the forming die.
The above forming processes are also the first choice for manufacturing stainless steel butt welded pipe fittings by cold forming, which has been verified by many years of production practice. Practice has proved that if the equipment capacity is large enough, 800H butt welded pipe fittings can be manufactured by these mature cold forming processes, and the forming process will not cause harmful defects to the product.

2 800H seamless pipe

2.1 Market Status

    Some industrialized countries in the world started the market application of Incoloy 800 series alloy materials in the 1950s [2], and their smelting technology and manufacturing technology of rods, plates and tubes have matured. At present, the 800H alloy rods, plates, tubes and other products in the domestic market are mainly provided by American Hastelloy, American SMC, German ThyssenKrupp VDM, Sweden OUTOKUMPU, Japan Metallurgy and other companies. In my country, some scientific research units and enterprises have also carried out research on Incoloy 800 [3-5]. So far, the smelting technology of Incoloy 800 material and the manufacturing process of rod, plate and tube are still in the research and development stage. Compared with these foreign companies, some Chinese enterprises still need to improve in terms of technology mastery, experience accumulation, brand creation, including scale capability, especially in the manufacture of Incoloy 800 seamless pipes.
The maximum size of Incoloy 800 seamless pipe listed in the ASTM standard is DN150 [6]. The supply specifications of seamless pipes by domestic and foreign enterprises are generally as large as DN200, and the largest is DN250. Larger than this size, they are provided in the form of welded pipes.

2.2 Purchasing Requirements

As we all know, the raw materials used by pipe fittings manufacturers to produce pipe fittings are usually purchased from external sources, so the quality of the purchased raw materials is the primary condition for determining the quality of pipe fittings. The design requirements of this batch of 800H butt welded pipe fittings are all seamless pipe fittings, but in terms of raw materials, domestic and foreign suppliers cannot provide 800H seamless pipes with a maximum size of DN400, which brings great difficulties to material procurement. In this case, after communicating with domestic enterprises that have experience in smelting and pipe making of such materials, it was decided that the smelting and pipe making of this batch of seamless 800H alloy 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 their use and meet the processability of material deformation processing, the following additional requirements are put forward for this batch of seamless alloy pipes on the basis of conforming to the provisions of the alloy pipe standard ASTM B407:
(1) The content of Al+Ti in the chemical composition shall not exceed 0.7%.
(2) The grain size is 2 to 5 grades.
(3) The negative deviation of wall thickness shall not exceed 10% of the nominal wall thickness.
(4) Hydraulic test is carried out for each alloy tube; on this basis, eddy current testing is carried out for alloy tubes below DN100, and ultrasonic testing is carried out for alloy tubes with DN100 and above; both ends of each alloy tube shall be subjected to penetration testing; The inner surface of the alloy tube was examined endoscopically (16x magnification).
Through the joint efforts of raw material smelting enterprises and pipe-making enterprises, we finally provided seamless 800H alloy pipes with the largest nominal size of DN400 and the largest wall thickness of 16mm that meet the requirements of material standards and additional procurement requirements. While accumulating experience, these companies have also contributed to the localization of such materials.

3 Heat treatment

3.1 Function and purpose

The use of special heat treatment is another important part of the quality control of 800H alloy pipe fittings. Its function and purpose are mainly manifested in the following two aspects:
(1) Eliminate the work hardening phenomenon of the material during the cold deformation process of the product, and restore the mechanical properties of the material.
(2) Recover and adjust the structure of the material to achieve the required grain size and meet the creep resistance requirements of the material under high temperature conditions.

3.2 Specification Requirements

In the 2010 edition of the specification for nickel alloy pipe fittings ASTM B366 [7], all pipe fittings are required to be heat treated. The recommended heat treatment temperature for UNS N08810 fittings is 1147 to 1177°C (there is a note in the specification stating that the final heat treatment temperature should be consulted with the material manufacturer), and the cooling method is rapid air cooling or water cooling.
Upon review, the heat treatment requirements for UNS N08810 in the previous 2004 edition of ASTM B366 are the same as those in the 2010 edition. However, in the earlier version 1997a, heat treatment was not a mandatory requirement, and no recommended heat treatment temperature was given, only a reminder that if heat treatment was required, it would be determined by the purchaser in consultation with the manufacturer. Judging from the changes in ASTM specifications for heat treatment, people's awareness of the importance of heat treatment to nickel-based alloy materials is also gradually improving.

3.3 Heat treatment process

    The main basis for formulating the heat treatment process of 800H alloy pipe fittings is: the heat treatment requirements recommended by ASTM B366, the solution heat treatment temperature of the alloy pipe manufacturing enterprises 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 fittings is shown in Table 1.

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

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

4 Conclusion

The practice of manufacturing butt welded pipe fittings using multiple batches of 800H alloy seamless pipes shows that:

(1) It is technically feasible to manufacture 800H alloy butt welded pipe fittings by the mature cold forming process of stainless steel butt welded pipe fittings.

(2) The primary condition for ensuring the quality of 800H alloy butt welded pipe fittings is whether the quality of the pipe meets the deformation requirements of the forming process without defects. For raw material enterprises, it should be a development direction to carry out in-depth research on the smelting, pipe making and related processes of 800H pipes, master key technologies, and provide the market with high-quality 800H pipes to meet the growing domestic demand.

(3) Heat treatment is another important aspect to control the quality of 800H butt welded pipe fittings. Through special solution heat treatment, the mechanical properties and grain size index of the product can reach the expected value, so as to meet the performance requirements of butt welded pipe fittings under service conditions.

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