Production process of stainless steel pipe
Stainless steel pipe is one of the rapidly developing varieties of steel in China, and its output has increased nearly 15 times in recent 20 years. However, due to the gap between process technology and equipment level, many high-grade varieties can not be produced, especially stainless steel and high nickel alloy steel pipes for chemical and nuclear power industries still basically rely on imports.
On the key devices of some important equipment in chemical industry, electric power (especially nuclear power) and other industrial departments, stainless steel and high nickel alloy long pipes are often used, with an outer diameter of 16-38mm and a length of 15000-36000mm. For example, the stainless steel long tubes used in the stripper, high-pressure condenser and High-Pressure Scrubber of 520000 t urea and 300000 t synthetic ammonia large urea unit are 316L (urea grade) and 2RE69 (Swedish steel grade, i.e. 00Cr25Ni22Mo2N), and their typical specifications are Φ 25mm × 2.5mm ×- 22000mm.
For the long stainless steel tubes in the high-temperature section of superheater and reheater of 300000-600000 kW large power station units, the steel used is 304H austenitic stainless steel, and the typical specification is Φ 16mm × 1.2-3.0mm × 12500-24000mm.
The nuclear power industry is a major user of stainless steel and high nickel alloy tubes. According to statistics, a 1 million KW nuclear power plant needs 1255T of various steel pipes, including 200t of U-shaped heat transfer pipe of steam generator. A 1.3 million KW PWR nuclear power plant requires 2850t of various steel pipes, including 850t of stainless steel and high nickel alloy pipes.
The materials used for U-shaped heat transfer tubes of steam generators in large PWR nuclear power plants are generally Inconel 690, Incoloy 800 and Inconel 600, and their typical specifications are: Φ 19.05mm × 1.09mm ×- 36000mm and Φ 22mm × 1.2mm ×- 28000mm.
The stainless steel tube Market in the United States also has a great demand for long stainless steel tubes and high nickel alloy tubes for boilers. It is understood that the average annual demand for long stainless steel tubes for boilers in the U.S. market is 4000t, the steel grades are 304H, 321h, 316H and 347h, and the standard is astma213; For high nickel alloy, including high nickel alloy long pipe, the annual demand is 12000t, and its standards are astmb163 and B616.
Because these products have the characteristics of “small diameter” and “super length”, they have special requirements in production technology, and generally implement “professional” production.
Plymouth tube company is an 82 year old manufacturer of long stainless steel tubes for power stations. It has two long tube professional production plants in Maryland and Louisiana to provide high-quality, safe and reliable water heater U-tubes for users in the power industry for more than 40 years. At present, the company mainly has the following four categories of stainless steel pressure pipes:
- (1) U-tubes for boilers, condensers, auxiliary heaters and heat exchangers of AUSTENITIC FERRITIC duplex stainless steel and ferritic stainless steel;
- (2) Austenitic stainless steel sanitary and general purpose pressure pipes;
- (3) Seamless steel tubes for nickel and nickel chromium alloy condensers and heat exchangers;
- (4) Nickel chromium iron alloy pressure pipe. The types, uses and specifications of stainless steel long pipes of Plymouth steel pipe company are shown in Table 1.
Table.1 product range of stainless steel long pipes of Plymouth steel pipe company
|Purpose||Type of steel pipe||Outer diameter/mm||Maximum length/m|
|Mechanical tube||Seamless steel pipe or welded pipe||3～51||40|
|Pressure pipe*||Seamless steel pipe or welded pipe||3～51||40|
|Feedwater heater pipe*||Seamless steel pipe or welded pipe||16～25||40|
|Condenser tube*||Welded steel pipe||13～51||40|
|Aviation tube||Seamless steel tube||3～25||15|
|Nuclear driven structural tube||Seamless steel tube||3～25||43|
|High purity stainless steel pipe||Seamless steel pipe||3～25||40|
Note: * is a universal U-bend.
In addition to Plymouth steel pipe company, there are also Salem, Greenville, Teledyne Rodney metals and other long pipe professional production plants in the United States. Most of them use hot extruded wool pipes imported from abroad or cold processed finished pipes as raw materials. In the specialized long pipe workshop, they are transformed into various specifications after 2-3 passes of cold rolling, cold drawing and reprocessing Stainless steel and high nickel alloy special pipes for various purposes for domestic sales and export.
VALLOUREC of France is a famous company in the world that produces stainless steel and high nickel alloy long tubes for nuclear industry. It has a history of producing stainless steel and high nickel alloy long tubes for nearly 40 years. The five categories of “group products” for nuclear industry produced by VALLOUREC specialty tubes division are:
- (1) Fuel cladding tube is used for nuclear reactors such as pressurized water reactor nuclear power plant (PWR), fast increment reactor nuclear power plant (FBR) and light water reactor nuclear power plant (LWR);
- (2) U-shaped heat transfer tube of steam generator (the maximum developed length of U-shaped tube is 36000mm);
- (3) Stainless steel seamless steel pipe for nuclear industry;
- (4) Stainless steel extra long steel pipe for feedwater heater and steam water separation Reheater for nuclear industry, with a length of 50000mm.
- (5) Condenser titanium pipe (for seaside nuclear power plant).
Sandvik, Sweden, is also one of the world’s famous companies producing stainless steel and high nickel alloy long tubes for the nuclear industry. In 1968, the company established the world’s first professional production line for producing U-shaped heat transfer tubes of steam generators for nuclear industry, specializing in the production of stainless steel and high nickel alloy long tubes (Incoloy 800). The outer diameter of the long stainless steel pipe for special purposes is 8-34mm, and the maximum length exceeds 30000mm. The materials of high nickel alloy long pipe mainly include Incoloy 800, Inconel 600 and Inconel 690, with specifications of Φ 16-22mm × 1.0mm up to 28000mm long.
Langenfeld stainless steel pipe factory of Mannesmann company in Germany is also a professional manufacturer of stainless steel and high nickel alloy long pipes. Its production specifications are Φ 4-25mm × 0.4-4.0mm × 20000mm, the main varieties are 300 series stainless steel, nickel and high nickel alloy.
The common features of this kind of stainless steel and high nickel alloy long pipe products are:
- (1) Small caliber and super length. Its slenderness ratio is generally more than 500.
- (2) Important use. It is generally used in the “core” parts of some important devices.
- (3) The requirements for product quality are extremely high, and the whole long pipe is almost required to be “defect free”.
- (4) Most users will require delivery in the state of U-bend.
Due to the special purpose and harsh working conditions of this kind of steel pipe, very strict requirements are put forward for the selection of product materials and quality control. Among them, “U-shaped heat transfer tube of nuclear power steam generator” and “urea grade stainless steel precision tube” can be regarded as the “best” among the “fine products” of stainless steel and high nickel alloy long tubes. Among many foreign stainless steel tube production enterprises, only a few factories can produce them. At present, this kind of steel pipe basically depends on import in China.
Due to the characteristics of “small diameter, super length and high quality”, the production process of stainless steel and high nickel alloy long pipe has different characteristics compared with the general production process of stainless steel pipe.
Fine material policy
The production blank of stainless steel and high nickel alloy long pipe can be hot extruded capillary, or cold drawn cold-rolled intermediate pipe or finished pipe close to the finished size after several cold processes. Before the blank pipe enters the long pipe workshop, it must pass the nondestructive inspection and degrease the internal and external surfaces. For long high nickel alloy tubes used in nuclear industry, the first process after the blank tubes enter the long tube workshop is to polish the outer surface to remove all defects on the surface of the blank tubes. The blank with any defect shall be strictly prohibited from entering the long tube workshop, and the capillary tube produced by the two roll cross rolling piercing process shall be avoided. For the U-shaped heat transfer tube used in the steam generator of nuclear power plant, the user clearly stipulates that only the hot extrusion tube can be used as the blank tube, mainly considering the high production and use cost of stainless steel and high nickel alloy long tubes. Some nuclear industry stainless steel and high nickel alloy long tube production workshops, even after the blank enters the long tube workshop, begin to implement the “closed management” from blank to finished product production, so as to ensure the product quality and economic indicators of stainless steel and high nickel alloy long tube production.
Cold working process of stainless steel and high nickel alloy long pipe
The cold working process of stainless steel and high nickel alloy long pipe can adopt cold rolling process, cold drawing process and cold rolling cold drawing combined process.
1) Cold rolling process
The cold rolling process of steel pipe is characterized by “periodicity of deformation process”. The primary deformation is large, which is 3-4 times that of cold drawing, and the length of rolled pipe is not limited by the length of rolling mill bed, which is particularly important for the cold processing of long stainless steel and high nickel alloy pipes. However, there will be “wavy” defects on the surface of the cold-rolled steel pipe, resulting in the longitudinal periodic uneven wall thickness of the steel pipe. This defect is sometimes unclear by visual inspection, but has a hand feel.
2) Cold drawing process
The cold drawing process of steel pipe includes non mandrel drawing, short mandrel drawing, long mandrel drawing and floating mandrel drawing. The process is characterized by “continuity of deformation process”. After cold drawing, the surface quality and dimensional accuracy of steel pipe are relatively high, but its primary deformation is small, and the drawing length is limited by the length of cold drawing machine body.
3) Cold rolling cold drawing combined process
At present, more foreign stainless steel tube manufacturers still adopt the “cold rolling cold drawing combined process dominated by cold rolling” to produce stainless steel and high nickel alloy long tubes. Some specialized long tube production workshops are often equipped with 1-2 small tonnage extended cold drawing machines to meet the process requirements of drawing with small deformation due to changing the specifications of stainless steel and high nickel alloy long tubes and strengthening the surface of finished products. For example, the long tube workshop for nuclear industry of Sandvik company in Sweden is equipped with a long drawing machine with drawing force of 0.1mn (10t), drawing speed of – 30m/min (adjustable speed) and backstage length of 40m for reference Φ The specification of finished pipe less than 40mm is changed and the process needs of small deformation strengthening on the surface of Incoloy 800 alloy long pipe.
Different stainless steel and high nickel alloy long pipe production plants adopt different long pipe production processes.
Ti steel pipe investment company a + P (aceles + Pollocks) plant and fine tube Ltd in the UK are important manufacturers of stainless steel and high nickel alloy long tubes in the UK, using cold drawing as the main production process.
Ti (a + P) company was founded in 1899 and has a history of 109 years. Its main products are 300 and 400 series stainless steel and Nimonic, Inconel and Incoloy series high nickel alloy steel pipes. The product specifications are Φ 0.5-60mm × 0.02-3.80mm。 The processing equipment used are: 12 0.02-0.3mn (2-30t) cold drawing machines, including 1 0.3mn hydraulic cold drawing machine; One kpw30vmr high-speed long stroke cold rolling mill is only used as the “blooming” of capillary tubes. Taking advantage of its large deformation, it can shorten the production cycle and reduce metal consumption, and the utilization rate is not high. The cold processing of products are finished by cold drawing process, because there are surface wavy defects in the length direction of cold-rolled finished pipes, which is taboo for pipes used in nuclear industry.
Fine tube Ltd was founded in 1942 and has a history of 66 years. Its products are also 300 and 400 series stainless steel and Nimonic, Inconel and Incoloy series high nickel alloy steel pipes. The product specifications are Φ 0.5-40.0mm × 0.05-3.25mm。 The equipment used are: 7 sets of 0.02-0.15mn (2-15t) cold drawing machines (including 1 set of 0.1mn DC drive cold drawing machine and 2 sets of 0.15mn DC drive cold drawing machines); One kpw30vmr high speed pipe mill; 2 ordinary pilger cold rolling mills. The production process is also based on cold drawing. The cold rolling mill is mainly used for “capillary rolling and reducing passes”, and its primary deformation can reach 76%.
In the cold drawing process, the floating mandrel drawing process is usually used in the fixed wall pass or small-size “work in process” pass, while the long mandrel drawing process is generally used in the cold drawing of large-size “work in process”.
When the floating mandrel is used for drawing, because the metal deformation follows the “law of minimum resistance”, when the steel pipe is plastically deformed, the floating mandrel can “find its own center and position automatically” according to the balance condition of deformation force in the deformation area. Therefore, the wall thickness of the steel pipe tends to be uniform after cold drawing, and the internal surface finish of the drawn steel pipe is improved. In addition, the manufacturing of the floating mandrel is relatively simple, The service life is also relatively long.
In the United States, many stainless steel and high nickel alloy long tubes are produced by long mandrel drawing process. The elongation coefficient of long mandrel drawing can reach 1.3-1.5, and the metal deformation process during drawing is relatively stable, the inner wall quality of steel pipe is relatively good, and the steel pipe after drawing is relatively straight. At the same time, due to the use of high-speed rotary hammer machine, the head loss is reduced by 50% and the yield is improved. However, when drawing long mandrel, the length of drawn steel pipe is limited by the length of cold drawing machine body, and the manufacturing requirements of long mandrel are also relatively high.
In Monroe steel pipe plant of Plymouth tube company, stainless steel and high nickel alloy long pipes are drawn with long mandrel on two extended cold drawing machines with drawing force of 0.2-0.3mn (20-30t), and the drawing speed is 30-50m/min. Generally, imported cold-rolled and cold drawn pipes are used, i.e. the specification is Φ 31.75-50.80mm × 2.40-5.16mm × 11000-12000mm finished pipe conforming to astma312 standard is used as raw material. After entering the long pipe workshop, it is drawn to the finished product specification through two times of long mandrel drawing and one time of empty drawing. The total drawing deformation is 75%. After the long mandrel is drawn, the wall thickness of the long pipe is relatively uniform, and the drawn steel pipe is also relatively flat, which is conducive to the subsequent heat treatment and finishing process.
In recent years, German smsmeer company has successively launched kpw-25vmr, skw25vmr, skw25hmrk (skw – twice feeding and twice rolling in one week) and other new high-speed, high-precision and long stroke pilger cold rolling tube mills to meet the needs of small diameter and long tube production, which are deeply favored by users.
Due to the progress in roll pass design technology, torque and inertia force balance technology and structural design, the rolling speed and steel pipe rolling accuracy of the new small diameter, long stroke, high-speed and high-precision pilger cold rolling mill have been further improved. For the steel pipe rolled by kpw25vmr cold rolling mill, the internal and external diameter tolerance shall reach 0.08mm; For the steel pipe rolled by kpw18vmr cold rolling mill, the wall thickness accuracy reaches 0.005mm.
The main foreign professional manufacturers of stainless steel and high nickel alloy long tubes directly use the kpw25vmr cold rolling mill of smsmeer company to roll the finished products of stainless steel and high nickel alloy long tubes for nuclear industry, which has achieved satisfactory results and can meet the requirements of high efficiency and high quality.
Table 2 and table 3 show the technical characteristic parameters and typical compression of Pilger cold rolling mill manufactured by German smeer company.
Table.2 technical characteristic parameters of Pilger cold rolling mill manufactured by German smeer company
|Model of pipe mill||Operation mode||Stroke/(r·min-1)||Maximum blank diameter/mm||Maximum finished pipe diameter/mm||Minimum finished pipe diameter/mm||Ring die diameter/mm||Rolling length/mm||Passing length/mm|
Note: C – continuous feeding; D – discontinuous feeding.
Table.3 typical compression capacity of Pilger cold rolling mill manufactured by German smeer company
|Material||Tube blank size/mm||Finished product size/mm||Feed rate/mm||Surface shrinkage /%||Elongation coefficient||Output/(m.h-1)||Model of pipe mill|
|Φ54×4.2||Φ25 × 2.0||12||78||4.5||400||KPW75VMR|
|Φ102×8.0||Φ70 × 3.5||13.9||69||3.23||255||KPW100YM|
Note: KPW – one round trip is one rolling process and one feeding.
The kpw50vmr pilger pipe mill manufactured by German smsmeer company is adopted by French valorek company for rolling Φ 22.2mm × 1.27mm × 28000mm, Φ 19.05mm × 1.02mm × Inconel 600 and Inconel 690 long tubes of 36000mm specification are used for U-shaped heat transfer tubes of nuclear power steam generators.
Sweden Sandvik company adopts a self-made long stroke, high-speed, high-precision, double line pipe rolling mill with a length of 40m and can be rolled Φ 22mm × 1.2mm × 19000-21000mm high nickel alloy Incoloy 800, Inconel 600 and Inconel 690 long tubes are also used for U-shaped heat transfer tubes of nuclear power steam generators.
Practice has proved that from the requirements of stainless steel and high nickel alloy long pipe products for surface quality and dimensional accuracy, the finished products are directly rolled by modern advanced pilger cold rolling mills skw25hmrk and skw50lc, which can fully meet the technical requirements of “high-quality” and “best”.
Therefore, for the rolling of high nickel alloy long tubes used in the nuclear industry, one or more skw25hmrk and skw50lc cold rolling mills can be used according to the requirements of the output and specifications of the finished products. The process of directly rolling the finished products is the best choice for the main deformation equipment for the cold processing of high nickel alloy long tubes. For the cold working of long stainless steel pipe, the combined process of cold rolling and cold drawing is a more favorable choice.
For a long time, “copper plating + butter lime” has been used as lubricant when rolling stainless steel pipes in China, that is, electroless copper plating is carried out on the surface of stainless steel pipes, and then the colloidal liquid composed of No. 3 industrial grease and lime in a certain proportion is manually applied and dried at 200-300 ℃. Although this lubricant has the advantages of low cost, good lubrication performance and convenient material acquisition, it also has some disadvantages, such as complex production and operation, high labor intensity, low production efficiency, large area occupation, serious environmental pollution and so on. Moreover, if the production is not careful and the coating is not uniform, it will lead to defects such as crater and “roughening” on the surface of the rolled steel pipe, which will affect the qualified rate of the steel pipe.
Chlorinated oils with different additives are widely used as lubricants for cold rolling and cold drawing of stainless steel tubes abroad, such as iloform series lubricants of Castrol company in Britain. Iloformtdn81 rolling oil (a highly chlorinated extreme pressure oil mixed with fatty oil additives) is used for cold rolling of stainless steel pipes; During cold drawing of stainless steel pipe, iloformtdn87 drawing oil (a non emulsified oil with deeply refined mineral oil as base oil, high content of chlorinated additives in the oil and fatty oil substances that can further improve the lubricating performance of the oil) is generally used; When the high-speed rolling mill rolls stainless steel pipes, iloformtdn90 rolling oil (a thick paste lubricant for extreme pressure rolling, a rolling oil containing chloride, fatty compounds and mineral oil) is used for the lubrication of the mandrel and the inner wall of the steel pipe. The three lubricating oils have good compatibility in use.
The cold rolling and cold drawing of stainless steel pipe adopts rolling oil and drawing oil lubricant, which not only simplifies the lubrication process and improves the environmental conditions, but also improves the surface quality of steel pipe and reduces the consumption of tools and dies.
There are many foreign manufacturers of cold rolling and cold drawing lubricants for stainless steel tubes. The varieties of cold rolling and cold drawing lubricants used by various stainless steel and high nickel alloy long tube manufacturers are also different. For example, TI company (a + P), the main manufacturer of steel pipes for aviation and atomic energy industries in the UK, uses tdn81 drawing oil produced by britishpchociem when cold drawing steel pipes; The tdn81 lubricating oil of Castrol company is used by precision pipe company in cold rolling steel pipe.
When drawing stainless steel and high nickel alloy long pipes with long mandrel, Monroe steel pipe plant of Plymouth steel pipe company of the United States uses towerdraw and tc-78 drawing oil produced by tower oiinc to lubricate the internal and external surfaces. When the steel pipe is cold drawn, the circulating pump system and nozzle are used for the lubrication of the outer surface of the steel pipe. The paste lubricant is directly and continuously sprayed onto the outer surface of the steel pipe to enter the die hole, and is brought into the deformation area by the steel pipe being drawn, so as to realize the lubrication of the steel pipe and the die surface; For the lubrication of the inner surface, the ejector is used to inject the lubricating oil into the inner hole of the steel pipe, and when the long mandrel enters the inner hole of the steel pipe, the paste lubricating oil is continuously coated on the long mandrel with a brush, so as to lubricate the inner surface of the long tube and the long mandrel during cold drawing, so that the long tube can be easily and smoothly removed after being drawn on the long mandrel.
At present, “butter lime” lubricant widely used in domestic stainless steel pipe production is not suitable for the production of stainless steel and high nickel alloy long pipes. Because the use of “butter lime” lubricant has neither quality standard nor operation standard, it is generally “fine work and rough work” on site, and there are too many variables in the process of manual production and operation; At the same time, the basic characteristic indexes of “butter lime”, such as friction coefficient, anti biting ability and rolling load, are still different from those of rolling oil, so the defect rate is higher than that of rolling oil under the same conditions. Table 4 lists the comparison of performance indexes and test results of various lubricants.
In order to change the backward situation of lubrication technology in the cold rolling and drawing process of stainless steel tubes in China, improve product quality and improve the workshop production environment, the original No. 5 plant of Shanghai Iron and steel imported 10t tdn81 cold rolling lubricant from Castrol company in Britain in 1983, and properly transformed the domestic lg55 cold rolling tube mill, and then began the process test of rolling stainless steel with tdn81 cold rolling lubricant; At the same time, with the support of the former Ministry of metallurgy and the Ministry of nuclear industry, together with the former China Petrochemical Corporation and the Research Institute of petrochemical industry, the subject of jointly developing domestic cold rolling lubricating oil for stainless steel pipe was put forward. In 1985, a new domestic cold rolling lubricating oil str-1 for stainless steel tubes and the corresponding degreaser cs68 were successfully developed.
The domestic cold rolling lubricating oil str-1 for stainless steel pipe was tried on the lg55 cold rolling mill of the former No. 5 plant of Shanghai Iron and Steel Co., Ltd. and expanded on the LG80 and LG55 pipe mills of Daye Steel Co., Ltd. and the LG55 and LG30 pipe mills of the former Jiangxi Steel Co., Ltd. More than 2 million meters of stainless steel pipes have been rolled, and satisfactory results have been obtained. The rolled products have passed the technical appraisal organized by the former Ministry of metallurgy and China Petrochemical Corporation, which provides China with the “fluidization” and “continuity” of lubricants and lubrication processes for cold-rolled stainless steel and high nickel alloy long pipes in the production of stainless steel and high nickel alloy long pipes “Automation” and “localization” have laid a good foundation.
Table. 4 Comparison of performance indexes and test results of various lubricants
|Lubricant||Oil film strength/MPa||Bite resistance/MPa||Friction factor||Rolling pressure/kN||Rolling defect rate /%|
|Note||4 ball testing machine||Md-320 end face friction tester||TIMKEN ring block friction tester||MD-320 end face friction tester||LG55、 Φ 57mm × 3 mm→ Φ 41mm × 1.6 mm||LG55、Φ 57mmx3 mm→Φ41mmx1.6mm||Flat roll rolling compression rate 20%||Inspection results of 9663 pieces (inner wool, rolling and folding)|
Degreasing process and degreasing agent of stainless steel pipe
1) Degreasing process
The degreasing process in the production of stainless steel pipe is generally carried out before the heat treatment or inspection process to prepare for the heat treatment and inspection process of steel pipe.
Degreasing process is one of the important processes in the production process of stainless steel pipe. During the production of stainless steel pipe, if the degreasing process is not in place, the steel pipe with lubricant and oil on the surface will enter the bright continuous heat treatment furnace for heat treatment, which will lead to the following two results:
(1) After heat treatment, the surface of stainless steel pipe will appear “negative color” (carburization), which reduces its intergranular corrosion resistance, especially ultra-low carbon stainless steel pipe;
(2) It destroys the atmosphere in the bright heat treatment furnace protected by hydrogen, so that the surface of the stainless steel pipe after heat treatment can not meet the requirements of brightness.
At present, HF + HNO3 solution is used as degreasing agent in the degreasing process of most domestic stainless steel pipe manufacturers, and some pickling waste liquid is also used as degreasing solution. However, both from the degreasing principle and practical application, it is proved that this method is not suitable for the production of stainless steel and high nickel alloy long pipes.
The production of stainless steel pipes abroad, especially the production of stainless steel and high nickel alloy long pipes, attaches great importance to the degreasing process, and the degreasing agent and degreasing process are also different. Generally speaking, there are three forms:
- (1) immersion degreasing process, which adopts trough degreasing device, which is generally used for degreasing the internal and external surfaces of ordinary length steel pipes.
- (2) Continuous on-line degreasing process is generally used for degreasing the outer surface of long pipe production.
- (3) Jet off-line degreasing process is generally used for degreasing the inner surface of long pipe production.
- The immersion degreasing device adopted by British precision pipe company is designed and manufactured by Royal chemical company (ICI). The unit is 10m long, 1.2m wide and 3M high. 1.1.1 trichloroethylene organic solvent is used as degreasing agent. In the degreasing device, a 600mm high steel plate is used at the bottom of the tank to separate the degreasing tank into two chambers (“coarse degreasing chamber” and “fine degreasing chamber”) along the length direction. There are steam pipes at the bottom of the tank to heat the degreasing agent. Multiple rows of condensing pipes are set along the middle and upper part of the tank wall to condense the vaporized trichloroethylene into liquid and return to the fine degreasing chamber.
Circulation, filtration Device for purifying degreasing solution (Fig.1).
Fig.1 Schematic diagram of degreasing device designed and manufactured by Royal chemical company
1 – degreasing tank 2 – condensate pipe 3 – coarse degreasing chamber 4 – fine degreasing chamber 5 – purified liquid pipe and valve 6 – purification device 7 – pump 8 – filter screen 9 – blowdown valve 10 – dirt tank 11 – sewage liquid pipe and valve 12 – diaphragm 13 – steam heating pipe 14 – scale removal pump 15 – steam dry chord area
After starting the degreasing device, first turn on the steam and heat the degreasing agent to a slight boiling state. Trichloroethylene vapor gradually rises to the condenser area, and a “dry chord” of trichloroethylene vapor is formed in this area. The degreasing operation starts from placing the greased steel pipe in the special stainless steel pipe spreader and lifting it into the degreasing device. The steel pipe is first placed in the “coarse degreasing chamber” for 4-5min. During this period, tilt it several times, pour out the coarse degreasing solution from the inner hole of the steel pipe, lift it into the “fine degreasing chamber” for 20-30min, tilt it several times, and then lift it into the “dry string area” to receive trichloroethylene steam bath for 2-3min, Then tilt the tube bundle, pour out the degreasing agent, lift out the degreasing device, and dry it with compressed air one by one. The degreasing of a bundle of steel pipes takes 30-40min.
The long pipe workshop of valorek company in France adopts a continuous on-line degreasing process to degrease the outer surface of the long pipe on-line. That is, the long pipes rolled by pilger cold rolling mill pass through a combined device one by one on the conveying roller table. In the device, the long tube is continuously degreased on the outer surface by trichloroethylene degreasing agent degreasing, soft brush scrubbing, foam plastic scrubbing, hot water cleaning, blowing drying and drying 5 (Fig.2).
Fig.2 Schematic diagram of degreasing device for outer surface of continuous long pipe
1 – steel pipe 2 – roller table 3 – degreasing device 4 – brushing device 5 – scrubbing device 6 – jet cleaning device 7 – steel pipe surface drying device
The inner surface of the long pipe is degreased by off-line jet degreasing process. First, clean the wool felt plug with the compressed air to blow through the inner hole of the long tube, clean the oil inside the surface, then insert the foam plastic soaked with trichloroethylene into the inner hole with an air gun to defuse the inner surface of the long tube. Blow repeatedly 3-4 times until the plug does not change color.
The degreasing device in the long pipe workshop of Sweden Sandvik company is composed of a “longitudinal continuous” outer surface degreasing device arranged at the outlet end of the finished pass pipe mill and a “transverse mobile” inner surface degreasing device arranged behind the pipe mill and next to the discharge bench.
The degreasing of the outer surface starts from the head of the rolled steel pipe leaving the pipe mill. Firstly, a cork plug is plugged at the pipe end, and a group of continuous degreasing devices for the outer surface of steel pipe are arranged at the outlet end of the pipe mill. The device is composed of two adjacent iron boxes, one of which is filled with recycled “kerosene degreaser”, the other is “separated by two”, half of which is a steam bath box and the other half is a water washing box. The outer surface degreasing of the long pipe is to promote the steel pipe to degrease through kerosene degreasing agent, steam bath steam washing and clean water washing one by one with the help of rolling force to complete the outer surface degreasing of the long pipe. The “longitudinal continuous” outer surface degreasing device of the long pipe is shown in Figure 3.
After degreasing the outer surface, the long pipe enters the receiving bench. A “transverse mobile” inner surface degreasing device (Fig.4) is set on one side of the material receiving bench. The device is composed of five controllers that can move longitudinally and control the working sequence and cycle of degreasing agent, steam box, water washing box, etc.
Fig.3 Schematic diagram of degreasing device for outer surface of “longitudinal continuous” long pipe
1 – pipe mill 2 – kerosene degreaser 3 – steam bath box 4 – water washing box
Fig.4 Schematic diagram of degreasing device for inner surface of “transverse mobile” long pipe
1 – pipe rolling mill 2 – kerosene degreasing agent 3 – steam bathroom 4 – clean water flushing room 5 – manual controller 6 – degreasing steel pipe 7 – soft facing degreasing material table 8 – horn shaped receiving pipe
The degreasing of the inner surface of the long pipe starts after the long pipe enters the feeding bench. First, install a “double horn” shaped plastic plug at the tail of the pipe end, and then blow out the installed plastic plug together with a cork at the head to wipe off the residual oil on the inner surface of the long pipe. Then, the controller is started, and the kerosene, steam, and clear water are used to continuously wash the inner surface of the steel pipe at a certain pressure. Finally, a plastic foam plug with a white cloth is blown through the inner hole of the steel pipe, and the white cloth does not change color.
The degreasing method produced by the director of Monroe steel pipe plant of Plymouth steel pipe company is as follows: the external surface adopts “continuous on-line degreasing process” and the internal surface adopts “jet off-line degreasing process”. The long stainless steel and high nickel alloy tubes cold drawn by the long mandrel enter the external surface degreasing device online, and then degrease the external surface of the long tube through the steps of coarse degreasing in the degreasing tank, ultrasonic oscillator device and fine degreasing tank (Fig.5).
Fig.5 Schematic diagram of continuous on-line degreasing device for outer surface of director pipe of Monroe steel pipe plant
1 – degreasing long tube or long mandrel 2 – coarse degreasing tank 3 – ultrasonic oscillator device 4 – transmission device 5 – fine degreasing tank
Magnus1007, PPG (poly propylene glycol) and polypropylene glycol aqueous degreaser are used. The working temperature of the degreaser aqueous solution in the degreasing tank is 41.7 ℃ (107 ℉). In order to improve the degreasing effect, the degreasing device is also equipped with an ultrasonic oscillator device. Through ultrasonic oscillation, the residual oil on the outer surface of the long greased pipe is loose and easy to remove.
During the production process, the long mandrel must be degreased every time it is drawn to keep the inner surface of the long tube clean. After degreasing by the above degreasing process, the surface of the long tube and long mandrel is clean and dry.
The degreasing of the inner surface of the long tube after being drawn by the long mandrel and degreasing of the outer surface is carried out on the platforms on both sides of the feeding bench of the bright heat treatment furnace. Use a high-pressure air gun (the pressure of compressed air is 60.48kpa) to pass the soft plastic plug soaked in acetone through the inner hole of the long pipe for three consecutive times to clean the inner surface of the long pipe.
The long tubes degreased on the inner and outer surfaces shall meet the feeding requirements of the bright heat treatment furnace, and the long tubes shall be fed into the furnace according to batches on the feeding bench for bright heat treatment.
2) Degreasing agent
Degreasing agents used in the production of stainless steel and high nickel alloy long tubes abroad are essentially organic solvents, such as trichloroethylene, trichloroethane, carbon tetrachloride, etc. This kind of degreasing agent can achieve the effect of “once cleaning” in the “on-line continuous” degreasing process on the outer surface of long pipe. However, the toxicity of degreasing agent and the environmental pollution caused by the regeneration and treatment of degreasing waste liquid are still a difficult problem.
The degreasing agent used by Monroe steel pipe plant in the United States is non-toxic and has good degreasing effect. It is said to be an alkaline soapy water. If so, there are two possibilities: (1) because the deformation of Monroe steel pipe plant when drawing with long mandrel is relatively small, and the deformation of each pass is not more than 15%, the deformation resistance during cold drawing is relatively small, and the requirements for oil film strength, bite resistance and other indicators of cold drawing lubricant are not high. Its cold drawing lubricant may be “water-soluble” rather than drawing oil（ 2) Its degreasing agent is a special “killer product” for this kind of lubricant. Because if there is a corresponding “killer degreaser” relative to the lubricant, the problem will be much simpler and the effect will be much better. For example, the degreasing agent cs68 developed corresponding to domestic rolling oil str-1 has a good degreasing effect (its price is almost similar to that of rolling oil str-1).
It can be seen that in the production process of stainless steel pipes in China, especially stainless steel and high nickel alloy long pipes, the lubricants and degreasing agents used should be “localized” and “serialized” in production, and there is still a way to go to realize “online”, “continuous” and “once clean” in use.
An important feature of the production process of stainless steel and high nickel alloy long pipe is that the heat treatment process must realize “continuity” and “brightening”, which is a necessary process condition to ensure product quality in the production process of long pipe.
So far, most domestic stainless steel pipe manufacturers use open flame roller hearth furnace without protective gas during steel pipe heat treatment, and a few manufacturers still use electric contact heating heat treatment process. Therefore, the surface oxidation of the steel pipe after heat treatment is serious, the bending degree of the steel pipe is relatively large, and the performance of the product is uneven.
As early as the 1980s-1990s, some domestic steel pipe plants introduced advanced hydrogen protected continuous bright heat treatment furnaces from abroad. For example, the original No. 5 plant of Shanghai Iron and Steel Co., Ltd. introduced a mesh belt bright heat treatment furnace with an hourly output of 300kg from Drever U.K. of Britain, and a four muffle bright heat treatment furnace with an hourly output of 275kg from Great Wall Steel Co., Ltd. In recent years, some stainless steel pipe manufacturers have gradually adopted domestic hydrogen protected continuous bright heat treatment furnace in recent years, and achieved good results.
The continuous bright heat treatment furnace has been widely used in the production process of foreign stainless steel pipe factories, whether it is “in-process” or finished steel pipe, which basically cancels the pickling process, shortens the turnover time of the production process, improves the environmental conditions of stainless steel pipe production, and is conducive to the improvement of product quality.
In order to meet the production demand of long tubes with different varieties, specifications and output, the continuous bright heat treatment furnace has the following three forms:
- (1) Roller hearth continuous bright heat treatment furnace. This is a high productivity heat treatment furnace, with a general hourly output of 1000-2000kg, which is mainly suitable for the heat treatment of large diameter stainless steel pipes.
- (2) Mesh belt continuous bright heat treatment furnace. The hourly output of this heat treatment furnace is generally 200-1000kg, the diameter of the treatable steel pipe is 3-80mm, the wall thickness is 0.2-5.0MM, and the length can exceed 40m.
- (3) Muffle type continuous bright heat treatment furnace. The furnace is mainly used for heat treatment of small diameter and high demand stainless steel pipes. The fuel can be gas, heavy oil or electricity. The furnace has low energy consumption and the shielding gas is only in the muffle tube. It is an economical heat treatment furnace. The hourly output is generally 35-275kg, the number of muffle pipes in the furnace is generally 2-8, the diameter of the treatable steel pipe is 1.5-67.0mm and the wall thickness is 0.18-4.00mm.
- For the shielding gas of bright heat treatment furnace, the general stainless steel pipe can use the gas (75% H2 + 25% N2) decomposed and purified by ammonia (NH3), while for the long high nickel alloy pipe, especially the pipe for nuclear power, high-purity hydrogen must be used. Crude hydrogen (99.56% H2) can be obtained by electrolyzing water, and 99.99% high-purity hydrogen can be obtained after secondary purification.
Therefore, from an economic point of view, it is more advantageous to set up two kinds of gas sources for one continuous bright heat treatment furnace and select different shielding gases according to different products.
The continuous bright heat treatment furnace used in the production of stainless steel and high nickel alloy long tubes abroad is as follows.
- (1) British precision pipe company adopts muffle type continuous bright heat treatment furnace for nuclear power pipes and aviation pipes with a diameter of Λ 38mm. The furnace has 5 muffle tubes, the maximum heating temperature is 1200 ℃, and the furnace length is 41m; The shielding gas is provided with two kinds of gas sources: high-purity hydrogen and ammonia decomposition gas; The fuel is heavy oil, the traction speed is 0.2-2.0mm/min, and the inlet and outlet of both ends of the furnace are sealed with wheat.
- (2) The net belt continuous bright heat treatment furnace is used in the long pipe workshop of valorek company in France. The mesh belt width of the furnace is 610mm and the hourly output is 300kg; The fuel is electricity and three-stage heating mode is adopted; The shielding gas is high-purity hydrogen with purity of 99.99%, hydrogen consumption of 80m3/h and dew point of – 70 ℃.
- (3) Monroe steel pipe plant of Plymouth steel pipe company adopts roller hearth bright heat treatment furnace. The furnace is 94.3m long and 1.4m wide; The length of the furnace heating section is 13.7m, which is divided into three sections for temperature control, and the maximum heating temperature is 1200 ℃; Ammonia decomposition gas is used as shielding gas (75% H2 + 25% N2); The cooling section is 21m, and the steel pipe is cooled by hydrogen convection cooling; The hourly output of the furnace is 1000kg, and the length of the front and rear feed in and feed out roller table and bench are 30m respectively.
- (4) Swedish Sandvik company adopts muffle type bright heat treatment furnace. The furnace has 2 muffle tubes with an inner diameter of 150mm; The fuel is gas; The shielding gas is 99.99% high purity hydrogen. The furnace is composed of preheating section, heating section and cooling section, in which the preheating section is 7m long and the temperature of preheating section is 600-900 ℃; The heating section is 10m long and the heating temperature is 900-1150 ℃; The cooling section is 24.5m long and is cooled indirectly by water jacket.
- (5) Tistainlestubes in the UK adopts roller hearth bright heat treatment furnace. The furnace is 79M long, 1.15m wide and 180mm high; Electric heating, power 800KW; The heating temperature is 750-1170 ℃, and the heating speed is 10-100m/h; Hydrogen purity 99.99%, dew point – 58 ℃; The maximum length of treatable steel pipe is 23m. The biggest feature of the furnace is that it has a hydrogen recovery device, which can be recycled. Therefore, the consumption of hydrogen is only 30-50m3/h.
It can be seen that realizing the “continuity” and “brightening” of long pipe heat treatment process is the key process condition to ensure the quality of long pipe in the production of stainless steel and high nickel alloy long pipe.
Straightening process of stainless steel pipe
The straightening of stainless steel pipe after heat treatment generally adopts roller straightener. For the straightening of stainless steel and high nickel alloy precision long pipes, it is required that there is no straightening trace on the steel pipe surface after straightening, and the bending degree of the steel pipe after straightening is required to reach 0.5mm/m.
There are two straightening processes for long tubes, namely roll straightening and stretch straightening.
The traditional roll straightening process adopts 6-10 roll straightener, and for stainless steel and high nickel alloy precision long tubes, all parts in direct contact with the long tubes (including loading and unloading bench, feeding and export parts, material pulling and receiving devices, conveying and lifting tools, etc.) shall be made of soft materials or coated. During operation, “soft contact” and “light landing” shall be achieved to avoid bumping or scratching the surface of the long pipe, especially on the feeding and discharging roller table. When straightening the long pipe with small diameter, the steel pipe has been rotating at a high speed from the bite of the straightening roller to the throw out at the end of straightening. There is a roller pressing device on the pipe roller table, but the surface of the steel pipe will inevitably be scratched during rotation and swing.
The straightening process of stainless steel and high nickel alloy long tubes can be simplified by using a tensile straightener. Tension straightener was originally mainly used for straightening profile, which is called tension torsion straightener.
The main parameters of tension torsion straightener are tensile force, torsional moment and torsional angle, which mainly depends on the material and section size of straightened profile. One end of the main drive of the tension torsion straightener is fixed, and the other end can move freely according to the length of the profile. During straightening, the two ends of the profile are respectively clamped by the torsion head and the stretching head through the special-shaped insert, and then the twisted profile is twisted by the torsion head at one end of the main drive, and then the profile is stretched by the stretching head at the free end to a certain amount, so that the distortion and bending of the profile can be corrected and straightened at the same time. Generally, the tension torsion straightener adopts hydraulic transmission.
There is no problem of cross-section distortion during long tube tension straightening. Therefore, the general long tube tension straightener does not have the function of torsion. At the same time, because the cold rolling process and long mandrel or floating mandrel drawing process are adopted for the long pipe during cold processing, the deformation of the metal during cold rolling and cold drawing is relatively uniform and stable, and then the heat treatment is carried out through the hydrogen protective heat treatment furnace. Therefore, the long pipe after heat treatment is relatively straight, the bending deformation is relatively small and gentle, which creates favorable conditions for its tensile straightening.
The Monroe steel pipe plant of Plymouth steel pipe company of the United States uses the tensile straightening process to straighten the long stainless steel and high nickel alloy pipes produced by the long mandrel drawing process. According to their experience, the straightening effect can be achieved when the tensile deformation of long pipe reaches 2.54mm (0.1in), and the surface of long pipe straightened by tension will not have defects such as scratch and surface straightening trace when straightened by roller straightener. In addition, the equipment of the stretching straightening process is simple, the floor area is small, and there is basically no noise during straightening.
The stretch straightening of stainless steel and high nickel alloy long tubes is a new application of stretch straightener in the field of steel tube straightening. Further research and development of its stretch straightening equipment and process will certainly make new achievements.
Transfer between processes
More than 80% of the defects produced in the production of cold-rolled and cold drawn stainless steel pipes come from poor surfaces. This is because there are many processes and long cycle during cold processing of stainless steel pipes. Multiple processes, transportation in repeated turnover and taking off, landing and parking in the process will inevitably lead to bumps between steel pipes, between steel pipes and tools and bench, which are easy to lead to surface defects of steel pipes. Especially for the production of long stainless steel and high nickel alloy pipes, because the steel pipe has the characteristics of “long” and “thin”, even in the process of short-distance transmission, it is inevitable to cause bumps on the surface of the steel pipe. Therefore, in the cold working process of stainless steel pipe, the operation, parking mode and device of steel pipe between processes must be highly concerned.
It is generally believed that for the process layout of the specialized production workshop of stainless steel and high nickel alloy long pipes and the connection between various processes, the specialized production should be carried out first according to the characteristics of “long” and “fine” products. The process layout should be “linear”, “clear longitude and latitude” and “no turning back”; In terms of equipment configuration, we should achieve “less but better”, and try to select high-efficiency and high-precision equipment; Simplify the process design. In the specialized production workshop of nuclear long pipe, it is necessary to “produce finished products together” and eliminate the interference of “work in progress” as far as possible. Secondly, the tools and bench in direct contact with the steel pipe shall be made of soft materials or coated with soft materials, and the transfer of stainless steel and high nickel alloy long pipes shall be “soft contact” and “light landing” to avoid scratching the surface.
American special steel company is an enterprise that once provided heat transfer tubes for ineonel600 steam generators for American nuclear submarines. In addition to the cold rolling mill, cold drawing machine, straightener, pipe cutter, traveling crane, vacuum furnace and other equipment in the nuclear pipe workshop of the company, the roller table, bench, material rack, handrail and guardrail on the plane of the workshop are made of hard wood. The scene is like a “fairy tale world”, realizing “soft contact and light landing” in the nuclear pipe production process, The scratch and bruise on the surface of the nuclear tube are effectively prevented.
In the production of stainless steel and high nickel alloy long pipes abroad, due to the adoption of professional production mode, the process layout is vertically and horizontally orderly and the lines are clear. There are basically two modes for the transmission of long pipes between various processes:
- (1) The “continuous mode” is mainly roller conveying and supplemented by small tonnage crane hoisting. Longitudinal roller table transmission and transverse bench transition connection are adopted, and the auxiliary processes (lubrication, degreasing, etc.) are designed and arranged online; For some independent and batch processes (such as vacuum desensitization treatment), small tonnage crane is used to assist transmission. This transition mode is adopted in the long pipe workshop of valorek company in France.
- (2) Small tonnage (5-8T) crane hoisting is mainly used to realize the “frog jumping mode” of long pipe transportation. The characteristic of adopting this mode is to professionally design the lifting tools for each process according to the operation characteristics of each process to ensure absolute safety in use. Swedish Sandvik company adopts this transition mode.
In the long pipe workshop of Plymouth steel pipe company of the United States, when producing stainless steel and high nickel alloy long pipes, the continuous conveying mode dominated by roller table is adopted for the conveying between processes. The lubrication process of long tube during long mandrel cold drawing and the degreasing process of long tube and mandrel after cold drawing are carried out online. The long pipe and long mandrel are transported by roller table at different levels. The conveying roller table in the long pipe workshop is not only distributed at different levels and implements grade separation, but also can be lifted and overturned. In addition to the small tonnage crane used for lifting tools and molds in the workshop, there is basically no crane in production. Only a large “C” shaped hook with a length of about 30m is placed in the workshop for packaging and shipping of finished products. There are few people in the workshop, the production is in good order, and the “steel pipe does not fall”.
As for the transfer mode between processes in the production process of long pipe, it is generally considered that the continuous mode of “mainly roller conveying and supplemented by small tonnage crane hoisting” is more favorable for the product quality assurance, automatic control of production process and improvement of production environment in the long pipe specialized workshop.
Purification of production environment
The production of nuclear stainless steel and high nickel alloy long tubes is required to be carried out in an air-conditioned and purified environment. In particular, the finished product testing (including ultrasonic, eddy current and other nondestructive testing, shape and dimensional tolerance testing of straight pipes and “U” pipes, performance testing, etc.), finished product marking, packaging and other processes must be completed in an air-conditioned and purified environment.
The TGL long pipe workshop of valorek company in France and the 8500m2 (340m×25m) old plant built in 1970 adopt flat roof sealing structure. The 5280m2 (240m×22m) plant newly built in 1978 adopts the daylighting gas building structure, and adopts the thermal insulation structure of lined steel plate and wrapped metal plate. The air in the workshop is filtered and the temperature and humidity are controlled. The temperature is generally maintained at 15-20 ℃.
The 9300m2 (310m×30m) long pipe workshop of Sweden Sandvik company adopts steel structure workshop, 300mm thick brick wall, double-layer glass window, concrete polished and painted floor, air conditioning is adopted in the workshop, and the temperature is maintained at 20 ℃.
The nuclear pipe workshop of British precision pipe company has a plant area of 11214m2, steel structure plant, FRP big wave tile roof, and the indoor temperature is controlled at 15-20 ℃.
In addition to the large long tube workshop using the sealed form of air conditioning purification for the production of nuclear tubes, there are also small nuclear tube plants using the form of “clean room” for the production of nuclear tubes. For example, American special steel company uses a “cleaning room” with an area of about 800m2 to complete the production of some key processes of nuclear and aviation pipes.
Stainless steel pipe is one of the rapidly developing varieties of steel in China. In the past 20 years, the output of stainless steel seamless steel pipe in China has increased nearly 15 times. However, the grade of stainless steel pipe products is not high, and many high-quality varieties can not be produced. In particular, stainless steel and high nickel alloy long pipes for chemical and nuclear power basically rely on imports.
According to the relevant national plans issued by China in 2004, China’s total installed capacity of nuclear power will reach 40 million KW by 2020, that is, 40 nuclear power plants of 1 million KW will be built. By then, China’s nuclear power installed capacity will account for about 9% of the total installed capacity of the power industry. It is predicted that more than 50200t stainless steel tubes, including stainless steel and high nickel alloy long tubes, will be required to build 40 nuclear power plants of 1 million KW.
Therefore, developing the production of high-grade stainless steel pipes, including stainless steel and high nickel alloy long pipes, is not only the need for the development of national industry and cutting-edge technology, but also the need for China to change the long-term dependence on imports of such high-precision products as soon as possible and realize localization. At the same time, it is also the inevitable trend of the production process and technology development of stainless steel pipes and high nickel alloy precision pipes in China.
Authors: Zou Zihe, Fu Fengren
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.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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