Quality Control of Nickel Alloy Tubing (Tech Center of Shanghai HY Industry Co., Ltd)
At present, about one-third of the world’s oil and gas fields contain H2S gas. Because such oil and gas wells contain a large amount of H2S, CO2, C1- and other corrosive media, ordinary carbon steel, low-alloy steel and even 13Cr steel, 22Cr steel and other materials of oil well pipe It has been unable to meet the requirements of the mining environment. Compared with general stainless steel, high nickel alloy not only has good resistance to general corrosion, local corrosion and stress corrosion, but also has good comprehensive mechanical properties and cold and hot processing properties. In recent years, it has been used in the manufacture of corrosion-resistant oil well pipes. Has been widely used.
In china, the research and development of high nickel alloy oil well pipes started relatively late, but with the increasing R&D strength of domestic steel pipe companies, the production technology of nickel alloy oil pipes has broken the foreign monopoly. At present, a domestic pipe factory has successfully produced the first batch of nickel alloy tubing made of Inconel 600 and HASTELLOY G3 in China. Due to the numerous production and inspection procedures of nickel alloy tubing, the quality control process should grasp key points and be targeted.
Main production process and its quality control:
Production of Inconel 600 and HASTELLOY G3 nickel alloy tubing has mainly gone through the following steps:
smelting of blanks;
forging of blanks;
hot extrusion of blanks;
solution treatment of pierced billet
Cold rolling of pierced billet;
Tube processing of smooth tube;
Marking and packaging of finished tube.
Among them, (1), (3), (4), and (5) are the most critical production processes. The control of these processes will determine the quality of the product, and it needs to be monitored.
1 Smelting of billets
Unlike ordinary tubing and casing steel, the chemical composition of high nickel alloys such as Inconel 600 and HASTELLOY G3 has the following characteristics:
A large amount of alloying elements are added, such as Cr, Ni, Mo, W, etc.;
Content of C is usually It should be controlled below 0.015%;
Content of P and S should usually be controlled below 0.03%.
In view of the higher chemical composition requirements, the smelting of high nickel alloy billets is often carried out in the following way: EAF+AOD (VOD) +ESR or VIM+ESR.
For nickel alloy tubing, the quality of billet smelting quality control will directly affect the implementation of subsequent processing procedures and the performance of the final product. When the billet smelting quality is unqualified, forging cracking (there are inclusions), hot extrusion delamination (high oxygen content, inclusions), cold rolling cracking (the presence of inclusions), insufficient product strength (presence of pores), etc. The quality of the product brings fatal harm. Therefore, in order to ensure the quality of the product and the smooth progress of the subsequent processes, during the supervision of this process, the following inspections should be carried out on the blanks of each electroslag furnace:
Results of chemical composition inspection;
Evaluation results of inclusions;
Whether there are defects such as bubbles and shrinkage holes.
2 Hot extrusion of blanks
Nickel alloy tubing is often formed by hot extrusion, but its hot extrusion is more difficult. In the hot extrusion process, it is necessary to comprehensively consider the extrusion capacity of the equipment, the influence of temperature on the alloy deformation resistance, and the extrusion rate on the alloy deformation. The influence of resistance, etc., can design a reasonable hot extrusion process. The deformation resistance of high nickel alloys is highly sensitive to temperature and strain rate. Generally speaking, its flow stress will increase rapidly as the deformation temperature decreases and the strain rate increases. Tests have shown that high-nickel alloys deform in the temperature range above 1 100°C, which can effectively reduce flow stress and reduce equipment load. Therefore, the hot extrusion temperature of high-nickel alloys is often set in this area. In this process, if the hot extrusion process is not properly controlled, defects such as cracks, folds, cracks, cracks, delamination and scars will usually appear on the inner and outer surfaces of the hot extrusion pierced billet. Therefore, the quality control process should focus on checking the internal and external surfaces of the pierced billet for the above-mentioned defects.
3.Solution treatment of pierced billet
ISO 13680 “Technical Conditions for the Supply of Corrosion-resistant Alloy Tubing and Coupling Seamless Pipes for the Petroleum and Natural Gas Industry” stipulates  that the total precipitated phase of nickel alloy tubing cannot exceed 1%, and the σ precipitated phase cannot exceed 0.5%. Due to the particularity of the composition of the high-nickel alloy tube, after the hot extrusion process, a second phase distributed along the grain boundary will often appear, such as the nitride phase, the carbide phase, and the σ phase. The appearance of these precipitated phases will affect the material. Corrosion resistance, plastic toughness, etc. cause serious hazards, so they must be eliminated by solution treatment. In addition, solution treatment can also play a role in eliminating work hardening, so as to facilitate the second cold rolling process. In the quality control process of this process, it is necessary to check the temperature, holding time and cooling water pressure of the heat treatment furnace.
4,Cold rolling of pierced billet
Generally speaking, the mechanical properties of ordinary oil pipes are guaranteed by hot rolling (heat treatment) process, and nickel alloy oil pipes often appear harmful second phases during the hot working process, so other ways to achieve their mechanical properties are needed. At present, nickel alloy oil pipes such as Inconel 600 and HASTELLOY G3 are usually cold rolled to control the mechanical properties of the tube body. The advantages of cold rolling are roughly as follows: (1) It can avoid the harmful second phase; (2) By setting a reasonable amount of rolling deformation, products with high strength and good impact toughness can be produced; (3) Products with high dimensional accuracy, good surface quality, and high finish.
During the supervision of this process, the following two aspects of inspection are required: (1) The dimensions of the cold-rolled tube, such as outer diameter and wall thickness; (2) The surface quality of the cold-rolled tube, such as: Pits, scratches, surface cracks.
Main inspection procedures and quality control
According to ISO 13680 standard requirements, nickel alloy tubing needs to be inspected in the following aspects: (1) Physical and chemical performance inspection; (2) Non-destructive inspection inspection; (3) Finished pipe surface quality inspection; (4) Pipe straightness test. Based on my own quality control experience, the author summarized the problems that are prone to occur in the above-mentioned inspection items.
1 ,Physical and chemical performance inspection
The physical and chemical properties of nickel alloy tubing are an important basis for judging whether its quality is qualified. Any problem with physical and chemical properties may bring a fatal threat to the safety of the pipe body. Therefore, special attention should be paid to the quality control process. According to the requirements of the ISO 13680 standard, the physical and chemical performance inspection of nickel alloy tubing mainly includes the following seven aspects: chemical composition analysis, tensile test, impact test (or flattening test), hardness test, hydraulic test (optional), Metallographic inspection, CO2, H2S corrosion test . The author found that in the process of tensile test and metallographic inspection, the inspection results often do not meet the ISO 13680 standard. Here is a brief analysis of it.
Supervision of tensile test. For nickel alloy tubing, the strength failure phenomenon is often encountered during the tensile test, and this phenomenon is more common in the inspection process of high steel grade and large wall thickness nickel alloy tubing, and we need to add in the quality control process. Great control. The mechanical properties of nickel alloy tubing are usually achieved by cold rolling deformation. When the cold rolling deformation is insufficient, the strength often fails to meet the specified requirements. For thin-walled and low-grade steel pipes, the deformation is relatively easy to control, and the probability of insufficient strength due to insufficient deformation is small; when the wall thickness of the pipe is thicker and the steel grade is higher, the control of its deformation will change. It is more difficult to get it, and it is very prone to the phenomenon of substandard strength, such as: couplings and other large-wall products.
Supervision of metallographic inspection. According to the requirements of the ISO 13680 standard, the total amount of precipitated phases in the finished high-nickel alloy tube should be less than 1%, and the total amount of σ phases should be less than 0.5%. As mentioned earlier, during the hot extrusion process of nickel alloy tubing, a large number of intermetallic precipitates will appear between the austenite grain boundaries. When the solution treatment process is not appropriate (such as unreasonable temperature setting and too short holding time) , A large number of precipitated phases between grain boundaries cannot be eliminated. Therefore, in the process of quality control, attention should be paid to the identification of metallographic photos to determine whether the precipitated phase exceeds the standard.
2,Non-destructive inspection (ultrasonic)
Microstructure of the nickel alloy tubing delivered in the cold rolled state is usually a coarse austenite structure, and the structure is uneven, and at the same time the acoustic performance is anisotropic, causing the nickel alloy tubing to have messy intergranular and grain boundaries during the ultrasonic inspection process. Grass-like echo and low signal-to-noise ratio bring many difficulties to ultrasonic flaw detection. In the process of flaw detection of nickel alloy tubing, the following three types of defects need to be detected: longitudinal defects, lateral defects, and delamination defects. In the quality control process of this process, the following two aspects should be paid attention to:
Ultrasonic flaw detection sample tubes must be made of the same high-nickel alloy material;
Whether the sensitivity calibration of longitudinal, transverse and delamination defects is qualified.
In addition, due to the large stress inside the nickel alloy tubing delivered in the cold rolled state, delayed cracks may appear after being placed for a period of time, and special attention should be paid to the quality control process.
Inspection of finished pipe surface quality and pipe body straightness
As the hardness of nickel alloy tubing is generally low, its HRC is generally below 35, and defects such as scratches and pits are easy to appear during production and hoisting. In addition, because the delivery state of the nickel alloy tubing is cold-rolled, the product tubing is prone to bending and deformation under the action of stress, causing the straightness of the tubing to exceed the standard. Therefore, the quality inspector should conduct random inspections on the surface quality and straightness of the finished pipes before they are put into storage.
At present, although the domestic production technology of nickel alloy tubing has broken the foreign monopoly, there are many production and inspection processes for this type of product, and the production process is difficult to control. There is unstable product quality during the production process, and more supervision is required. Strengthen quality control to ensure the quality of steel pipes delivered to the oil field.
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