Development of Large Diameter 800H Alloy Seamless Butt Welding Pipe Fittings

Because 800H alloy has good high-temperature mechanical properties, high-temperature creep resistance, oxidation resistance and corrosion resistance, it has been more and more widely used in industrial pipelines, especially in petroleum, chemical, nuclear power and other fields. In this case, the demand for 800H alloy pipes, plates, rods and other profiles is also increasing. Regarding the supply of Incoloy 800 series alloy seamless pipes, the specifications currently available in the domestic and foreign markets are usually DN200, up to DN250, and the larger pipes are welded pipes. In terms of Incoloy 800 series alloy seamless pipe standards, the maximum seamless pipe specification listed in the ASTM specification is DN150.
In the 800H (UNS N08810) alloy material purchase order of a domestic project, the maximum specification of 800H alloy pipe and 800H butt-welded pipe fittings is DN400, and the design stipulates that the pipes and fittings are seamless structures. This design requirement brings raw material purchases It is very difficult. Related companies at home and abroad cannot provide such large-size 800H alloy seamless pipes.

When pipe fitting manufacturers encounter unusually used raw material grades, raw materials that are not available in the market, and the number of ordered products is small, the usual solution is to use forgings as blanks to complete butt-welded pipes by machining. Manufacturing. In the case that only small-sized pipes can be purchased and the required butt-welded pipe fittings are too large, the solution is to use the method of first expanding the pipe and then manufacturing or adopting the process of expanding the diameter. However, none of these methods are applicable to this case.

The reasons are as follows:

• 1. The large number of butt-welded pipe fittings in the order is large, and the cost of using forgings is huge;
• 2. The purchase contract will package seamless pipes and pipe fittings for purchase, so even if forgings are used as the high-cost plan of raw materials or the use of pipe expansion/diameter expansion to complete the manufacture of large-size butt-welded pipe fittings, the problem of large-size seamless pipes in the purchase contract cannot be solved.

In order to solve the problem of large-caliber seamless pipes required by the procurement contract and the manufacture of pipe fittings, pipe fitting manufacturers communicated with domestic metallurgical and pipe-making enterprises with experience in manufacturing 800H alloy through market research, and finally determined that this batch of 800H alloy seamless pipes will be produced by domestic relevant enterprises cooperated to tackle key problems, and completed all the work from smelting to tube making.

800H alloy seamless pipe

Additional requirements

According to the designer’s opinions, relevant information, and considering the processability of pipe deformation processing, the 800H alloy seamless pipe meets the requirements of the standard ASTM B407, and puts forward additional contract requirements. Table.1 lists the corresponding provisions of the standard and the additional requirements of the contract.

Table.1 Corresponding regulations of the standard and additional requirements of the contract

	Al+Ti/%	Grain size	Negative deviation of wall thickness/%	Non-destructive testing methods
ASTM B407 Stipulate	0.85-1.20	Grade 5 or coarser	≤12.5	Water pressure/vortex
Additional requirements	≤0.7	Level 2-5	≤10	Water pressure + eddy current / ultrasonic + endoscope

The quality of raw materials is the primary condition that determines the quality of pipe fittings. The above additional requirements for 800H alloy seamless pipes are mainly for the needs of material service and manufacturing process of pipe fitting.

• (1) According to relevant research data, the increase of Al and Ti in the 800H alloy is beneficial to the creep strength, but is detrimental to the toughness [2]. In order to make the material have good creep rupture strength above 600 ℃, and still have good toughness when working for a long time below 700 ℃, the content of Al + Ti should be controlled not more than 0.7% [3].
• (2) One of the main properties of 800H alloy is resistance to high temperature creep. When the grain size is controlled at 2-5, the creep rupture strength of the material will be significantly improved [2-3].
• (3) It is required that the negative deviation of the pipe wall thickness does not exceed the nominal wall thickness in %. This is to consider the possible wall thickness loss when the butt welded pipe fitting is deformed to ensure that the wall thickness of the butt welded pipe fitting meets the requirements.
• (4) The additional non-destructive testing requirements are that each alloy pipe is subjected to a hydraulic test; on this basis, eddy current testing is performed on alloy pipes below DN 100, and ultrasonic testing is performed on alloy pipes of DN 100 and above; both ends of each alloy pipe must be tested for penetration, and the inner surface of each alloy tube is inspected with an endoscope (16 times magnification). All these requirements are mainly to ensure the quality of 800H alloy seamless pipes when they leave the factory.

The quality of 800H alloy seamless pipe

After the trial production of the first batch of large-diameter 800H alloy seamless pipes was completed, the pipe fitting manufacturer carried out trial production of butt welded pipe fittings. In the Penetration Testing (PT) of the formed and heat-treated elbows and pipes, it was found that the inner and outer surfaces of the pipe fittings partially appeared point-shaped defects with a diameter of not more than 1.5mm, which were irregularly distributed, and the defects were polished. eliminate. This defect in alloy pipes can no longer meet the quality requirements of pipe fittings.
The subsequent technical analysis believes that defects are likely to occur in the initial stage of material manufacturing, and the cause should be found from the smelting of the material including the blooming stage. In view of the reasons for the defects, the metallurgical enterprises conducted analysis and research and further improved the process technology. With the joint efforts of related enterprises, we finally provided the 800H alloy seamless pipe (maximum nominal size DN400, maximum wall thickness 16mm) that meets the material standard regulations, purchase additional requirements and the quality requirements of the pipe fitting manufacturing process for the alloy pipe. Subsequent trial production results of pipe fittings show that the large-diameter 800H alloy seamless pipe provided after the improved process technology no longer has the above defects, which meets the quality requirements of the pipe fitting manufacturing process for alloy pipes.
Through the process of producing large-size 800H alloy seamless pipes, relevant companies have accumulated technical experience in manufacturing large-diameter 800H alloy seamless pipes, which has opened up ways for the subsequent market applications of such products.

Forming process of 800H butt welded pipe fittings

800H alloy material has good deformation processing performance and is suitable for cold processing or hot processing. It should be noted that the work hardening phenomenon of 800H alloy during the material forming process is greater than that of ordinary austenitic stainless steel. Therefore, the equipment capacity should be considered when the cold forming process is adopted. The pipe used should be in a solution heat treatment state. When the cold working deformation is relatively large In large cases, intermediate heat treatment may be required to eliminate work hardening to avoid cracking of the pipe during the forming process and resulting in scrap.
According to the butt-welded pipe fitting specifications in the 800H alloy material purchase order and the company’s existing forming equipment capabilities, the pipe fitting manufacturer considers the cold forming process compared to the hot forming process that usually has the characteristics of one-time forming, good appearance quality and lower cost , It is determined that all 800H alloy butt-welded pipe fittings will be manufactured by cold forming process. The overall manufacturing process flow is: material reinspection → blanking → forming → heat treatment → mechanical test and product particle size inspection → sandblasting → end processing → appearance and size inspection → ultrasonic and penetrating inspection → pickling and passivation → marking → package.
The varieties of 800H alloy seamless butt-welded pipe fittings include elbows, tees and reducers. The cold forming process methods of different varieties are shown in Table.2.
The process method shown in Table.2 is an advanced forming process for manufacturing stainless steel butt welded pipe fittings that has been verified by many years of production practice. It can be used for the manufacture of 800H alloy butt welded pipe fittings.
Table.2 Cold forming process methods of different types of butt welded pipe fittings

Variety	Process name	Method description
Elbow	Cold push	The seamless pipe with the same diameter as the elbow is used for cold push forming on a dedicated forming machine. The shape of the elbow is guaranteed by the outer mold and the inner core.
Tee	Hydraulic bulging	The seamless pipe with the same diameter as the tee is used, and it is formed by hydraulic expansion on a special hydraulic press. The shape of the tee is guaranteed by the cavity of the forming die.
Reducer	Reduced diameter	A seamless pipe with the same diameter as the large end of the reducer is used, and the diameter is cold compressed on a hydraulic press to form. The shape of the reducer is ensured by the inner cavity of the forming die.

At present, the pipe fitting manufacturers adopt the cold forming process to manufacture 800H alloy elbows, tees, reducers and other seamless butt-welded pipe fittings with a maximum size of DN400 and a maximum wall thickness of 16rnmo. .

Fig.1 800H alloy seamless butt welding pipe fittings

Heat treatment of 800H butt welded pipe fittings

The role of heat treatment

Heat treatment is another important part of 800H alloy quality control. Through the use of specific co-solvent heat treatment for 800H alloy pipe fittings:

• ① It can eliminate the hardening of the material during the cold deformation of the product and restore the mechanical properties of the material;
• ② Restore and adjust the structure of the material to achieve the required grain size. Meet the requirements of creep resistance of materials under high temperature conditions [3-4].

Heat treatment temperature specified in the specification

If the product specification stipulates a certain parameter, the manufacturer should usually manufacture according to the stipulated parameter, unless the technical index stipulated by the specification cannot be met after the manufacture according to this parameter.
In the nickel and nickel alloy pipe fittings specification ASTM B366 [5], the recommended heat treatment temperature for UNS N08810 pipe fittings is 1147～1177 ℃, and the specification has a note on this, stating that the final heat treatment temperature should be consulted with the material manufacturer.
In the specification for nickel-iron-chromium alloy seamless pipe ASTM B407, the minimum final heat treatment temperature given to US N08810 seamless pipe is 1121 ℃, and there is no comment on this.
The same is the ASTM material specification, the same material used in UNS N08810. The minimum heat treatment temperature given by the seamless pipe specification and the pipe fitting specification is 26 ℃.
After consulting, the lowest temperature of the final heat treatment of UNS N08810 alloy seamless pipe by the pipe manufacturer is 1130 ℃.

Heat treatment process of UNS N08810 butt welded pipe fittings

When formulating the heat treatment process for 800H alloy butt welded pipe fittings, the heat treatment requirements of ASTM B366 were referred to, the solution heat treatment process of alloy pipe manufacturing enterprises was analyzed, and the heat treatment process test of UNS N08810 alloy butt welded pipe fittings was carried out. According to the test results, a solution heat treatment process was developed. The main content of the solution heat treatment process is shown in Table 3.
Table.3 The main content of the solution heat treatment process of UNS N08810 pipe fittings

Furnace temperature/℃	Holding temperature/℃	Holding time	Cooling method
800-1000	1162±5	1.5min/mm, but not less than 15min	Water cooling

The heat treatment equipment adopted is a box-type electric furnace. The furnace loading temperature is determined at 800-1000 ℃, the purpose is to make the heated workpiece reach the holding temperature as quickly as possible, shorten the residence time of the austenite material in the sensitive temperature zone, so as to reduce the precipitation of carbides between the products and improve the corrosion resistance of the material [6-7].

According to the heat treatment process test results, the holding temperature is determined to be 1162 ℃. This temperature is also the intermediate value of 1147 and 1177 ℃ given in ASTM B366. The holding temperature of solid solution heat treatment performed by alloy pipe manufacturers is 1130 ℃, although this temperature is higher than the minimum heat treatment temperature given by ASTM B407. But it is lower than the minimum heat treatment temperature given by ASTM B366. From the comparative results of the grain size test. The grain size of the material heat-treated at 1130 ℃ is significantly smaller than that of the product heat-treated at 1162 ℃.
According to the results of heat treatment experiments. Control the holding time of the workpiece to 1.5min/mm but not less than 15min of the wall thickness. This is to obtain the expected value of the material grain size. Experiments have proved that too long holding time will cause the grain size of the material to grow too much.

Test results

The test results of the grain size and mechanical properties of the 800H alloy seamless surfacing pipe fittings show that after a specific solution heat treatment. The grain size and mechanical properties of each batch of the product have reached the target requirements and product specifications.

Figure 2-Figure 5 are the grain size re-inspection of 800H alloy pipes of 4 specifications and the grain size inspection pictures of butt welded pipes (magnification 100 times).

Table 4 shows the mechanical properties of the 800H alloy pipes of 4 specifications and the mechanical properties of the butt-welded pipe fittings.
Table.4 Mechanical properties of 4 specifications of alloy pipes and butt-welded pipe fittings

Specification/mm	Name	Tensile strength/Mpa	Yield strength/Mpa	Elongation/%	Hardness/HB
DN400 × 16	Pipes	530	275	57.5	121-138
	Pipe fittings	525	245	54.5	126-130
DN350 × 13	Pipes	515	210	55	125-133
	Pipe fittings	540	215	56.5	123-139
DN250 × 9.5	Pipes	555	310	49	90-110
	Pipe fittings	525	282	56.5	98-136
DN200 x 8.2	Pipes	580	395	48	90-110
	Pipe fittings	555	325	56	88-136

Conclusion

• 1) Through the development of large-diameter 800H alloy butt-welded pipe fittings, the production of large-size 800H seamless pipes has been promoted. It provides useful experience for the future market application of large-diameter 800H alloy seamless pipes and seamless butt-welded pipe fittings.
• 2) The quality of the 800H combined pipe is the primary condition for controlling the quality of butt welded pipe fittings. Relevant metallurgical and pipe-making enterprises should further strengthen the research and development of the 800H smelting, pipe-making and supporting process technology to provide the market with high-quality 800H alloy materials to meet the growing market demand.
• 3) Practice has proved that if the equipment capacity meets the estimated tonnage required for the forming of 800H alloy butt welded pipe fittings. The cold forming process of stainless steel butt welded pipe fittings can be used in the manufacture of 800H alloy butt welded pipe fittings.
• 4) The use of a specific heat treatment process is another important condition for controlling the quality of 800H alloy butt-welded pipe fittings. Through specific solution heat treatment, the mechanical properties and product particle size indexes can reach the expected value to meet the requirements of 800H alloy butt-welded pipe fittings in use.

Author: Guo Shun significant, Yang Zhenfei

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.)

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References:

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