Article by Mr. Hiroyuki Fujii, Nas Toa Co. Ltd., Tokyo, Japan.

A member of the Nippon Yakin Kogyo Group and headquartered in Tokyo, Japan, Nas Toa Co. Ltd. is a manufacturer of welded stainless steel and high-performance alloyed pipes and tubes. The company has a branch in Osaka and two factories in Japan – the Chigasaki and Kawasaki plants, with a further plant in Samutprakarn in Thailand (see Header/Fig. 1).

It produces a wide range of welded pipes and tubes made from austenitic stainless steel, super austenitic stainless steel, Duplex, IncoloyTM, InconelTM, HastelloyTM, MonelTM, and pure nickel, which are sold throughout the Asia–Oceania region. These are available in the size range: 6 to 1676.4 mm OD, with thicknesses of 0.7 to 40 mm, and a maximum length of 12 m (these figures are based on 304 grade and may vary somewhat with the material). They comply to various standards, including ASTM, ASME, ISO, and JIS. The company has a number of strategic products, which are listed in Fig. 2.

Fig 2. Nas Toa’s pipes and tubes showing the variety of materials available.

Fig 2. Nas Toa’s pipes and tubes showing the variety of materials available.

1. Manufacturing technology Over the years Nas Toa have built up much knowledge and experience about the market situation in Asia–Oceania and also in the production of the specialty, high-grade products with which they service this region.

1.1. Super duplex stainless steel (SDSS)

In recent years there has been a considerable rise in demand for SDSS pipes and tubes against the backdrop of deep sea development and the growth in demand for desalination plants in Asia– Oceania. Special characteristics of SDSS pipes and tubes are their excellent corrosion resistance and high strength. Nevertheless, the level of technical difficulty involved in producing pipes and tubes made of SDSS is considerable.

There are many points that a manufacturer has to pay particular attention to such as the α/γ ratio control in the weld metal, the prevention of denitrogenation from the weld metal during the welding process in order to maintain corrosion resistance, the prevention of toughness deterioration due to σ phase precipitation influenced by the welding heat, and finally the prevention of pipe and tube deformation during the annealing process. Through experience, Nas Toa has overcome these difficulties with regard to its SDSS pipes and tubes, in welded and annealed conditions.

Fig 3. Microstructure of SDSS at weld metal.
Fig 3. Microstructure of SDSS at weld metal.

1.1.1. α/γ ratio control in weld metal (see Fig. 3)

Where welding takes place without filler metal, the α ratio in the weld metal becomes too high, so Nas Toa has learnt to controls the α/γ ratio through heat treatment. On the other hand, when welding takes place with filler metal, a filler metal with a relatively high nickel content (higher than that of the mother metal) is used to obtain a suitable α/γ ratio in the welded condition.
1.1.2. Prevention of denitrogenation from the weld metal during the welding process to maintain corrosion resistance

Nitrogen raises the γ ratio and improves the corrosion resistance in duplex stainless steels. However, nitrogen easily escapes from the molten metal. A suitable shield gas therefore has to be used to avoid denitrogenation.

1.1.3. Prevention of toughness deterioration due to σ phase precipitation influenced by welding heat
In the case of welding pipes and tubes in a welded condition, welding with proper heat input management is important. In contrast, when welding pipes and tubes in an annealed condition, there is no need to manage the heat input so strictly because even though the σ phase precipitation occurs during welding, the σ phase disappears after the annealing process. Of course, rapid quenching after the annealing is necessary to achieve this.

1.1.4. Prevention of the deformation of pipes and tubes during the annealing process
Duplex stainless steels have a poor strength under elevated temperatures. If duplex stainless steels are annealed in the same way as austenitic stainless steels, the duplex stainless steel pipes and tubes become deformed by their own weight, and shape correction becomes difficult. Therefore, the company uses a jig to prevent deformation during the annealing process. There are many ways in which a manufacturer should manage SDSS pipe and tube production.

Heat treatment after welding is sometimes difficult in the case of on-site girth welding by fabricators. In such cases, proper heat input management should be carried out. Interpass temperature management and heat input control are important, and not only issuing WPS/PQR but also following it without fail is also very important. Furthermore, gas seal techniques to avoid oxidized scale formation during the welding operation or descaling after welding is necessary for maintaining corrosion resistance.

2. Super austenitic stainless steels

Super austenitic stainless steels are used in the area of environment related equipment such as FGD equipment, chemical plants, and offshore equipment. The σ phase can easily precipitate in super austenitic stainless steels when compared with grade 304, due to its high chromium and molybdenum content. Nevertheless, the degree of σ phase precipitation in super austenitic stainless steel is better than SDSS.

3. High nickel alloys

For environment preservation or cost reduction, the application in chemical plants and the equipment used under elevated temperatures become severe and the range of use for high nickel alloys becomes increasingly greater. For welded pipes and tubes made of high nickel alloys, prevention of hot cracking and oxidized scale is important.

3.1. Prevention of hot cracking and blowholes

High nickel alloys are more likely to incur hot cracking and blowholes than grade 304, therefore welding current control, interpass temperature control, the selection of the applicable welding material, and the conditions of use and the cleanliness of the groove are important.

3.2. Prevention of oxidized scale

The oxidized scale after welding reduces corrosion resistance, so oxidized scale formation can be prevented by suitable gas shielding. In cases where oxidized scale forms after welding, this should be removed by pickling, grinding or sandblasting.

4. Square pipes

Some applications for the use of square pipes require a tight tolerance on the internal radius at the four corners. Grade 304 is temperature dependent due to its mechanical properties, so NAS TOA preheats material in winter to ensure a tight tolerance.

5. Some applications

5.1. High-performance alloys

Welded pipes and tubes made of high-performance alloys such as IncoloyTM, InconelTM, HastelloyTM, MonellTM and pure nickel are used for chemical plants, heat exchangers, and equipment used under elevated temperatures.

5.2. Special alloys

SDSS (UNS S32750) welded pipes and tubes are used for seawater desalination plants, FPSOs (Floating Production, Storage and Offloading), etc. 6Mo grade super austenitic stainless steel welded pipes and tubes are used for environmental related facilities and NAS354NTM (UNS N08354, ASME Code Case 2543, 23Cr-35Ni-7.5Mo-0.2N) with a corrosion resistance level between 6Mo grade super austenitic stainless steel and HastelloyTM is used for waste disposal facilities and salt production facilities.

5.3. Special specification

Pipes and tubes used in a nation’s infrastructure, such as electric power plants and gas facilities are required to meet strict specifications, even when the material may be a common grade such as 304, 304L, 316, 316L. NAS TOA also supplies pipes for the Large-scale Cryogenic Gravitational Wave Telescope (LCGT) as a national level project in Japan.

Fig 4. Storage racks for nuclear fuel.
Fig 4. Storage racks for nuclear fuel.

5.4. Square pipe

Square pipes at Nas Toa are usually made of 304 grade and are used and for the structure of food machinery and semiconductor manufacturing equipment. On the other hand, duplex square pipes which have a high corrosion resistance and strength are used as racking for vessels. Boron-containing stainless steel (18%Cr-8%Ni-1%B), which can absorb neutrons, is further used in storage racking for used nuclear fuel (see Fig. 4).

About the Author:

Hiroyuki Fujii

Hiroyuki Fujii is currently a general manager of quality assurance department of NAS TOA Co., Ltd., Quality control manager and Senior welding engineer. In 1987, after graduated materials science and engineering, faculty of engineering, Tohoku university, received a job with a stainless steel company as a researcher. In 2011, transferred to NAS TOA as a deputy general manager of quality assurance department.

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