RU91613U1 - CENTRIFUGAL CAST PIPE FROM HIGH-STRENGTH IRON IRON WITH SPHERICAL GRAPHITE - Google Patents

Abstract

Annealed centrifugal cast tube of high-strength nodular cast iron, annealed, characterized in that it is made of cast iron containing 3.0-4.0% carbon, 3.0-3.5% silicon, 0.40- 0.60% manganese, 0.030-0.045% magnesium, 0.03-0.10% vanadium, annealed at a temperature of 890-950 ° C and subsequent tempering at a temperature of 670-700 ° C.

Description

The utility model relates to pipes intended for transportation of aggressive media, and can be used in the manufacture of corrosion-resistant pipes for oil and gas pipelines.

Known cast iron hot-deformed seamless pipe made of high-strength cast iron containing, along with other elements by weight of 2.1-3.8% carbon; 1.0-3.5% silicon; 0.05-0.80% manganese (RF patent 2243046, IPC F16L 9/02). However, cast iron hot-deformed pipes, as shown by the practice of their use, do not possess the necessary corrosion resistance in oilfield environments.

The closest set of essential features to the proposed utility model is a centrifugal cast pipe made of high-strength cast iron with spherical graphite, annealed at a temperature of 840-870 ° C and subsequent regulated cooling (RF patent 2175986, IPC C21D 9/08). Such pipes have high strength and plastic properties, but do not possess corrosion resistance and impact toughness for oil and gas pipelines.

The problem to which the claimed utility model is directed is to improve the quality of centrifugal cast iron pipes by increasing their corrosion resistance in various aggressive environments and cold resistance, as well as expanding the existing arsenal of pipes for the construction of oil and gas pipelines.

The problem is solved by the fact that the centrifugal cast tube of high-strength cast iron with spherical graphite, annealed, in contrast to the prototype, is made of cast iron containing 3.0-4.0% carbon, 3.0-3.5% by weight silicon, 0.40-0.60% manganese, 0.030-0.045% magnesium, 0.03-0.10% vanadium, annealed at a temperature of 890-950 ° C and subsequent tempering at a temperature of 670-700 ° C.

The technical result provided by the implementation of this technical solution is as follows.

The implementation of the proposed pipe made of cast iron of the specified chemical composition allows to increase its corrosion resistance in various aggressive environments. Moreover, the silicon content in the range of 3.0-3.5 wt.% Contributes to the saturation (up to 16% Si) of the surface layer at the metal-corrosive environment, i.e. the effect of natural silicification is observed, which prevents further corrosion deep into the metal. When the silicon content is in amounts of less than 3.0 wt.%, The necessary level of corrosion resistance is not provided, and when the silicon content is more than 3.5 wt.%, Pig iron embrittlement and loss of plastic properties occur. The limitation of the manganese content to 0.60 wt.% Is explained by the fact that a larger amount of manganese contributes to the formation of a pearlite base and bleaching of cast iron. Studies have also shown that the microalloying of cast iron with magnesium together with vanadium in the indicated amounts leads to an improvement in the visco-plastic properties. Magnesium deoxidizes and modifies cast iron, providing graphular globularization. However, a magnesium content of less than 0.03 wt.% Is not sufficient for the formation of compact spherical graphite and high viscous-plastic properties, and an increase in the amount of magnesium in excess of 0.045 wt.% Leads to a decrease in ductility of cast iron. In the production of cast iron in the melt, the presence of dissolved gases is inevitable: oxygen, hydrogen, nitrogen. Atomic hydrogen is absorbed by graphite, oxygen deposits on graphite inclusions in the form of oxides, nitrogen remains in solid solution. Since vanadium is a strong nitride-forming element, its introduction in the presence of nitrogen dissolved in the melt of cast iron leads to the formation of finely divided nitride inclusions, which increases strength and, in addition, helps to increase the visco-plastic properties and cold resistance. Moreover, vanadium less than 0.03 wt.% Is not enough to bind nitrogen to nitrides, and its content in an amount of more than 0.10 wt.% Contributes to the bleaching of cast iron with the formation of ledeburite in the cementite structure.

The combination of necessary mechanical properties and corrosion resistance of the described pipe is also ensured by the fact that the pipe is subjected to the specified heat treatment. Centrifugally cast spheroidal cast iron pipes have a non-uniform microstructure and uneven graphitization, which is explained by different cooling rates: in the surface layers, where the cooling rate is higher, more cementite (ledeburite) and fewer graphite inclusions are formed than in the middle part of the pipe wall. During annealing in the proposed temperature range, the residual (eutectic) cementite decomposes with the formation of a ferrite-pearlite structure with a uniform degree of graphitization throughout the volume of cast iron. In the case of heating below the indicated temperature of 890 ° C, complete decomposition of ledeburite does not occur, which leads to a deterioration in ductility and toughness. An increase in the annealing temperature above 950 ° C leads to an increase in austenitic grain and subsequently to a decrease in strength and visco-plastic characteristics. Tempering at a temperature of 670-700 ° C ensures the decomposition of perlite cementite into ferrite and graphite and allows the formation of a ferrite structure of cast iron with a uniform distribution of graphite inclusions, which leads to a significant improvement in the visco-plastic properties. Moreover, tempering at temperatures below 670 ° C does not lead to the complete decomposition of cementite, and overheating above 700 ° C helps to reduce ductility and viscosity.

The essence of the claimed utility model and the achieved technical result are illustrated by an example of a specific implementation and data from studies. Centrifugally cast pipes with a diameter of 159 mm and a wall thickness of 10 mm were made of high-strength cast iron with spherical graphite, the chemical composition of which is shown in Table 1. The pipes were annealed at a temperature of 950 ° C, held at this temperature for 2 hours, and cooled in air and subsequent vacation at a temperature of 680 ° C for 2 hours with cooling in air. The test results of these pipes are shown in table 2 and indicate both sufficient mechanical strength of the proposed pipes and their corrosion resistance in aggressive environments containing hydrogen sulfide and carbon dioxide. In addition, these pipes, in contrast to the known analogues, have high cold resistance, which makes it possible to use them in the construction of oil and gas pipelines in areas with different climatic conditions.

Table 1 No. p / p Mass fraction of elements,% FROM Si Mn Mg V Fe and inevitable. impurities one 4.0 3,5 0.60 0,035 0,03 Rest 2 3.3 3.2 0.55 0,040 0.06 3 3.0 3.0 0.40 0,030 0,03 four 3.4 3.2 0.45 0,030 0.08 5 3,5 3.4 0.50 0,045 0.10 table 2 No. p / p Mechanical properties Corrosion properties (according to NACE TM0177 Corrosion rate in H ₂ S-content medium, mm / year Corrosion rate in CO ₂ -containing. medium, mm / year Resistance to SKRN (method A) σ _in , MPa σ _t MPa δ,% KCV ^{+ 20C} , kgcm / cm ² KCV ^-60C , kgcm / cm ² Destruction time, hour. Thresholds, load from min. guarantee. yield strength, σ _th ,% one 605 545 7 2.0 0.5 0.1 150 70 2 513 427 21 5.5 4.0 0.3 720 70 3 475 365 13 2,3 1,0 1,0 320 70 four 520 400 10 1,2 0.7 0.25 400 70 5 540 490 fifteen 2.0 1,0 0.3 110 70

Claims (1)

Annealed centrifugal cast tube of ductile cast iron with spherical graphite, characterized in that it is made of cast iron, containing 3.0-4.0% carbon, 3.0-3.5% silicon, 0.40- by mass 0.60% manganese, 0.030-0.045% magnesium, 0.03-0.10% vanadium, annealed at a temperature of 890-950 ° C and subsequent tempering at a temperature of 670-700 ° C.