RU2807775C1 - Chloride corrosion-resistant steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to austenitic steel used for the manufacture of parts for the chemical and oil industries, energy and nuclear production. Steel contains components in the following ratio, wt.%: carbon no more than 0.2, manganese from 5 to 15, silicon from 0.6 to 3, chromium from 11.0 to 20.0, nickel from 12.0 to 14.0, molybdenum from 2.0 to 6.0, nitrogen not less than 0.005 and not more than 0.4, copper not more than 2.0, boron from 0.2 to 0.6, rhenium from 0.1 to 4.5, tungsten from 1 to 6, phosphorus no more than 0.003, arsenic no more than 0.04, antimony no more than 0.09, bismuth no more than 0.03, iron and inevitable impurities - the rest.

EFFECT: steel is resistant to chloride corrosion cracking in chloride water vapour (dry, overheated, saturated), in aggressive boiling concentrated solutions of any chlorides, hydrochloric acid and wet organochlorine compounds.

1 cl, 1 dwg, 5 tbl

Description

The invention relates to the chemical, oil industry, energy and nuclear production, namely to the production of austenitic steel resistant to chloride corrosion cracking for the manufacture of pipelines, tanks, parts of oil equipment, ship hulls, offshore structures, heat exchanger pipes, pumping pipelines, walls of welded tanks , tanks, columns, coils, heating chambers, evaporators, water-cooled reactors, once-through steam generators, double-circuit reactors and pipelines, the main circulation circuit in a boiling-water reactor, shut-off valve parts operating in hot aqueous and aqueous-organic media containing chloride (halide) , thiocyanate, organochlorine, sulfide, alkaline compounds.

Corrosion cracking occurs when steel is exposed to periodic or permanent tensile stresses and an aggressive corrosive environment. In this case, on the surface of the metal, even if it is not affected by general corrosion, branched or unbranched microcracks appear, turning into cracks developing inside or between crystals.

The initiation of a crack at low stresses that do not cause rupture of the oxide film locally disrupts the integrity of the passive layer with the participation of an adsorbed activator ion, leading to the formation of corrosion. Thermodynamic and kinetic factors lead to easier local activation of the stressed metal, accelerating the formation of soluble metal-chloride compounds due to an increase in defects and diffusion permeability of the protective film. The regulator of the relationship between the aggressiveness of the fracture environment and the localization of dissolution is the hydrogen ion. A decrease in pH to a low value leads to activation of the side walls of the crack and an increase in corrosion.

When atomic hydrogen is released in the corrosion process, it, adsorbed on the surface, diffuses into the region of the complex-stressed state of the metal at the edge of the crack with diffusion and the formation of an interstitial solid solution, martensitic phases, and pores with high pressure of molecular hydrogen. A crack develops along a hydrogenated region due to brittle fracture or accelerated dissolution of metal in steels. In austenitic steels, there is increased solubility and a low rate of diffusion of hydrogen in austenite. Embrittlement by introduced hydrogen occurs in structurally and deformation-unstable steels (12Х18Н10 Russia). Stable austenitic steels containing 15–25% Ni do not become embrittled at high hydrogen contents of 50–100 cm ³ /100 g, and there are no changes in the phase composition of the crack. Corrosion cracking of steel X17N15M3 (Russia) occurs with an increase in the corrosion activity of the hydrogen-saturated metal at the crack and its rapid dissolution in a hot (90° C) chloride solution of H ₂ SO ₄ + SeO ₂ . After keeping steel in solution for a long time, excess hydrogen leaves, martensitic phases and surface microcracks remain.

Inhibition of corrosion cracking at very high chloride concentrations is possible due to delocalization of corrosion or a decrease in the oxygen concentration in the solution. Testing of steels 12Х18Н9 (Russia), AISI 304 (USA) in water at 200 - 260 ° C under a load of 320 MPa at a concentration of 33% NaCl , the time to corrosion cracking was 0.5 hours, at 50% NaCl - 125 hours.

Under production conditions, when the solution level changes, the concentration of water impurities during evaporation, boiling in cracks, gaps, a low chloride content in water does not guarantee the absence of corrosion cracking of stressed steel, because the resulting real concentration of chloride on the metal surface is greater than the initial value.

Chloride corrosion cracking of austenitic steels occurs in water vapor (dry, overheated, saturated) with the transfer of chlorides dropwise with the volatility of HCl , in solutions of any chlorides, hydrochloric acid, wet organochlorine compounds CCl ₄ , CHCl ₃ , C ₂ H ₅ Cl , aggressive boiling concentrated solutions - MgCl ₂ , CaCl ₂ , ZnCl ₂ , LiCl .

With increasing tensile stresses, chloride corrosion cracking in steels increases, and the time of resistance to corrosion cracking decreases. In the low-stress region, the nature of chloride corrosion cracking is similar, but the time to failure increases. It has been experimentally established that heat-treated steels 12Х18Н9, 12Х18Н10Т, 08Х18Н10, operating in an aggressive environment with a high concentration of chlorides and in the presence of residual stresses of 200 - 210 MPa, are capable of operating until failure for 1200 hours; when the load is reduced to 100 - 120 MPa, the time to destruction increases by 30 – 40%. Residual tensile stresses after cold deformation of austenitic steels can cause corrosion cracking, while the structure of austenitic steels changes and martensitic phases appear. For all steels under study, with an increase in the degree of cold deformation of 5–30%, the resistance to corrosion cracking decreases. Mechanical processing, during which compressive residual stresses remain in the part, increases resistance to corrosion cracking by 20 - 30%. Removing the hardened layer by etching or polishing also increases resistance to chloride corrosion cracking by up to 15%.

Corrosion-resistant heat-resistant steel is known, intended for use as a material for power engineering in the production of various heat exchange equipment in thermal and nuclear power engineering, operating for a long time at temperatures of 500-600°C (patent No. 2543583 [1]). Steel contains components in the following ratio, wt.%: carbon 0.06-0.10, silicon 0.02-0.3, manganese 0.3-0.8, chromium 11.5-13.0, nickel 0, 8-1.2, molybdenum from 0.8 to less than 1.0, vanadium 0.15-0.30, niobium 0.05-0.15, nitrogen from more than 0.04 to 0.07, sulfur 0.001-0.010 , phosphorus 0.001-0.015, copper 0.01-0.10, calcium from more than 0.005 to 0.015, cerium from more than 0.01 to 0.05, boron 0.001-0.005, aluminum 0.05-0.15, iron - the rest . The total content of carbon and nitrogen does not exceed 0.16%. An improved complex of mechanical properties of heat-resistant, corrosion-resistant welded steel and higher values of short-term and long-term strength, resistance to pitting corrosion are achieved due to the fact that carbon, manganese, silicon, chromium, nickel, molybdenum, vanadium, niobium, nitrogen, phosphorus were introduced into the steel , sulfur, copper, iron, calcium, cerium, aluminum and boron. The ratio of the specified alloying elements and the accepted restrictions are chosen in such a way that the steel provides the required level of mechanical, corrosion and technological properties.

This steel cannot be used in aggressive environments. In oxidizing and strongly acidic environments during long-term operation, intergranular corrosion occurs along the grain boundaries of steel, which is associated with the diffusion mobility of carbon, depending on boron, phosphorus, silicon and the passive state of molybdenum.

Well-known complex alloyed two-layer corrosion-resistant steel (patent No. 2206632 [2]), used in nuclear energy in the manufacture of heat exchange equipment. Corrosion resistance, increased strength against corrosion cracking in a steam-water environment at 500 °C of this steel is provided by two layers: the main layer, containing carbon, silicon, manganese, chromium, nickel, molybdenum and iron, and the cladding layer, consisting of carbon, silicon, manganese, chromium , nickel, molybdenum, titanium, copper and iron. In this case (Nb+2Ti)/C≥35 and the thickness of the cladding layer should be 0.2-0.5 of the total thickness.

Manufacturing this steel in layers requires a special manufacturing technology, for example, using a vacuum chamber with layers of steel placed in it, and one of the layers must be in the form of a powder, or manufacturing from powders in layers, followed by compaction with pressing and cold or hot plastic deformation, or forging combined with rolling. Therefore, the mass introduction of this steel in the chemical and oil industries is not advisable. In addition, there are no studies on the resistance of layers to corrosion cracking in chloride-containing environments.

The invention is known: austenitic corrosion-resistant steel for chloride-containing environments and a product made from it (patent No. 2413031 [3] prototype). Steel contains carbon, silicon, chromium, nickel, manganese, nitrogen, copper, boron, molybdenum, hafnium, iron and inevitable impurities in the following ratio of components, wt.%: carbon ≤0.02, manganese 1.0-2.0, silicon ≤0.8, chromium 16.0-18.0, nickel 8.0-9.5, molybdenum 2.5-4.0, nitrogen 0.10-0.20, copper 0.3-0.9 , boron 0.001-0.005, hafnium 0.001-0.01, iron and inevitable impurities the rest. The content of molybdenum, boron and hafnium is related by the dependence (B+Hf)·(5Mo)=0.035-0.25. Hot-rolled sheets with a thickness of 3-10 mm and cold-rolled sheets with a thickness of 0.8-3.0 mm are made from steel. The duration of high-quality operation of welded products is increased due to high corrosion resistance against pitting corrosion and stress corrosion in chlorine-containing environments and at elevated temperatures, combined with increased strength and sufficient manufacturability during hot and cold forming.

The disadvantage of this invention is that it has insufficient resistance to cracking in concentrated chloride solutions. In addition, the steel contains hafnium. As a chemical element, it is refractory and inextricably linked with zirconium; their chemical bond is very strong; hafnium and zirconium can be separated at great cost. Hafnium is almost never found in its pure form.

The problem to be solved by the invention is to create a chloride-corrosion-resistant steel against cracking in concentrated chloride solutions and in high-temperature aqueous environments, operating under constant or variable loads.

The technical result is ensured by the selected ratio of individual chemical components in steel and is achieved by the fact that the creation of chloride-corrosion-resistant steel will increase the time to corrosion cracking by 80 - 90%, equipment productivity by 80 - 90%, and increase the throughput of installations by 70 - 80% .

The objective is achieved by the use of austenitic steel containing carbon, silicon, chromium, nickel, manganese, nitrogen, copper, boron, molybdenum, iron and inevitable impurities, resistant to chloride corrosion cracking, characterized in that it additionally contains rhenium, tungsten , phosphorus, arsenic, antimony and bismuth in the following ratio of components, wt.%: carbon no more than 0.2, manganese from 5 to 15, silicon from 0.6 to 3, chromium from 11.0 to 20.0, nickel from 12.0 to 14.0, molybdenum from 2.0 to 6.0, nitrogen not less than 0.005 and not more than 0.4, copper not more than 2.0, boron from 0.2 to 0.6, rhenium from 0. 1 to 4.5, tungsten from 1 to 6, phosphorus no more than 0.003, arsenic no more than 0.04, antimony no more than 0.09, bismuth no more than 0.03, iron and inevitable impurities - the rest.

The essence of the invention lies in the fact that the found ratio of the main alloying components allows the formation of an austenite structure that increases resistance to chloride corrosion cracking.

The results of experimental studies of the influence of alloying components on the resistance of steel to chloride corrosion cracking are presented in Table. 1.

Table 1

Influence of the content of alloying components in austenitic steel on stress-corrosion cracking

Nickel content, % Chromium content, % Solution Load under tension, MPa Conditional yield strength σ _0.2 , MPa Time to corrosion cracking, h 15 - 20 11 - 16 MgCl ₂ , 40% concentration 210 220-290 1500 40 - 45 11 - 16 MgCl ₂ , 40% concentration 250 320 - 350 1300 40 - 45 18 - 20 chloride containing 1 g/l Cl ^- water vapor at 200 – 350° C 180 157 - 196 900 25 – 46 20 - 37 MgCl ₂ , 40% concentration 180 200 700 until 3 20 - 37 Chloride containing 1 g/l Cl ^- water vapor at 300 °C 150 216 1000 9-11 18 – 21 boiling NaCl, 25% concentration 300 500 up to 800 9-11 18 – 21 boiling NaCl, 10% concentration 300 500 1100 10 18 Chloride containing 5 g/l Cl ^- water vapor at 270 °C + NaOH, pH 8 - 10 250 276 1500 9 18 10% FeCl ₃ , 3% NaCl + H ₂ S at 20°C 70 220 170 10 18 Boiling 60% CaCl ₂ 290 230 100 18 18 Boiling 60% CaCl ₂ 350 230 1000 10 18 Boiling 44% MgCl ₂ 290 230 40 20 18 Boiling 44% MgCl ₂ 290 230 1000 10 18 Boiling 44% MgCl ₂ 350 230 100 50 18 Boiling 44% MgCl ₂ 350 230 1000

With increasing chromium content, resistance to corrosion cracking decreases (Table 1), which is explained by the increased content of harmful impurities in austenitic steels. Resistance to chloride corrosion cracking when alloyed with titanium and niobium is reduced by 20%. Alloying with molybdenum up to 2% increases the corrosion resistance of austenitic steel, but if the nickel content is less than 12%. An increase to 6% molybdenum and a decrease in nickel content to 12% to increase resistance to chloride corrosion cracking is possible when working in a high-temperature aqueous environment and a boiling 40 - 44% MgCl ₂ solution. An increase in the manganese content to 20% in austenitic steels leads to a decrease in the resistance to stress-corrosion cracking, which is associated with the interaction of sulfur with manganese and the formation of non-metallic active corrosion inclusions. Up to 6% tungsten and up to 40% nickel reduce resistance to corrosion cracking. The recommended ratio is 1 – 6% tungsten and up to 14% nickel.

As a result of experiments, it was revealed that with partial replacement of nickel 3 - 6% with nitrogen up to 0.4% and manganese from 5 to 15%, such steel is susceptible to chloride corrosion cracking in any aggressive solutions.

When conducting bending experiments, it was revealed that with an increase in silicon content from 2 to 6%, the resistance of steels to chloride corrosion cracking in solutions of sodium chloride and magnesium chloride increases due to the creation of the protective properties of silicon dioxide and complex oxides. The use of a sodium chloride solution with saturated steam at 260 °C with a silicon content of 0.2 to 4.5% showed the same cracking time.

Copper content up to 2% slows down chloride corrosion cracking. Increasing the copper content to 4% did not reveal an increase in crack resistance. An increase in boron content from 0.2 to 0.6% made it possible to increase the time before the onset of corrosion cracking in an aggressive solution of magnesium chloride by 15–25%. Lower boron content did not reveal changes in the time to chloride corrosion cracking.

Exceeding the carbon content of more than 0.2% in the solid solution reduces the resistance of austenitic steel to corrosion cracking due to the formation of cracks. The absence of intergranular corrosion is possible when the carbon content is below the limit of its solubility in steel 0.009 - 0.03%, but these values reduce the structural strength of the steel. Therefore, if carbon (0.01 - 0.2%) is in solid solution and does not change the phase composition of steel when heating an aggressive environment, then intracrystalline cracking does not occur. But if the formation of chromium carbides occurs, the resistance of steel against corrosion cracking decreases and intergranular cracks appear, the faster the higher the carbon content. The carbon content depends on nickel and silicon, which increase the activity of carbon.

With a decrease in nitrogen content from 0.1 - 0.3% to 0.005%, phosphorus to 0.003%, arsenic to 0.04%, antimony to 0.09%, bismuth to 0.03%, the time to corrosion cracking increases by 50 - 60% in any aggressive environment.

The introduction of rhenium into steel makes it possible to increase heat resistance due to its refractoriness, corrosion resistance due to resistance to oxidation and the inability to dissolve in hydrochloric, hydrofluoric and sulfuric acids, and the absence of reaction with nitrogen and hydrogen. Rhenium simultaneously increases the strength and ductility of molybdenum and tungsten, withstands repeated cooling and heating, high shock loads, vibrations, contact with aggressive substances at a temperature of 1000°C, is technologically feasible, and weldable. It is economically feasible to use in chloride-corrosive highly concentrated solutions of chlorides and in high-temperature aqueous environments operating under constant load the mass ratio of rhenium at the upper limit while reducing tungsten and molybdenum to the lower limit, and vice versa, in weakly concentrated solutions of chlorides and in medium-temperature aqueous environments, working under variable load, the mass ratio of rhenium is at the lower limit while increasing to the upper limit of tungsten and molybdenum, which will increase the time to corrosion cracking by 80 - 90%.

In experimental studies, we used the proposed composition, manufactured by powder metallurgy using nanotechnology to obtain a nanocluster fullerent equiaxed structure with non-identical grain boundaries (Fig. 1). The resulting semi-finished product was subjected to heat treatment: quenching temperature 1150 - 1200°C, cooling medium - air.

The results of the mechanical properties (Table 3) of the steel of the proposed composition (Table 2) were carried out on a 150x50x4 mm plate under a load of 210 MPa. It can be argued that alloying with rhenium leads to a stable increase in the values of the proof strength σ _0.2 and the ultimate strength _σ in the temperature range from 20 to 1200°C. Strain before failure increases to 62%.

Table 3. Mechanical properties of test steel in the hardened state

Experience nominal yield strength σ _0.2, MPa tensile strength σ _in , MPa Strain, ε, % Steel 1 850 1000 60 Steel 2 950 1100 62 Prototype 342 N/mm ² 685 N/mm ² 56

Tests for resistance to chloride corrosion cracking under the influence of tensile stresses were carried out on a 150x50x4 mm plate in a 44% MgCl ₂ solution, the results are shown in Table 4, and pipes with a diameter of 150 mm, a length of 200 mm, a wall thickness of 5 mm in a chloride-containing 5 g/ l Cl ^- water vapor at 300°C + NaOH at variable load (Table 5).

Table 4. Corrosion resistance properties of test steel in a 44% MgCl ₂ solution

Experience Tension, MPa Time to destruction, hours Steel 1 595 1500 Steel 2 665 1700

Table 5. Properties of test steel for corrosion resistance in chloride-containing 5 g/l Cl ^- water vapor at 300°C + NaOH

Experience Corrosion rate cracking g/m ² hour Steel 1 0.007 Steel 2 0.006

Analysis of the results shows that the proof strength and tensile strength of the proposed steel are 35 - 45% higher compared to existing steels containing the main components nickel, chromium, tungsten, vanadium, molybdenum.

The resistance of the proposed steel against cracking in concentrated chloride solutions and in high-temperature aqueous environments operating under constant or variable loads was shown by a decrease in the rate of corrosion cracking and an increase in time to failure by 80 - 90%.

To increase resistance to chloride corrosion cracking when working with variable loads and in a high-temperature chlorine-containing aqueous environment, steel, in addition to the specified components, contains tungsten in the amount of 6.0, molybdenum no more than 6.0, nickel no more than 12.0, and rhenium no more 0.1 wt.%.

To operate in highly concentrated chloride-corrosive aggressive environments under constant load, steel, in addition to the indicated components, contains nitrogen no more than 0.005, tungsten no more than 1.0, molybdenum no more than 2.0, rhenium up to 4.5 wt.%.

It is economically feasible to use in chloride-corrosive highly concentrated solutions of chlorides and, operating under constant load, the mass ratio of rhenium at the upper limit while reducing tungsten and molybdenum to the lower limit, and vice versa, in weakly concentrated solutions of chlorides and in medium-temperature aqueous environments, operating under variable load the mass ratio of rhenium at the lower limit while increasing to the upper limit of tungsten and molybdenum, which will increase the time to corrosion cracking by 80 - 90%. The recommended maximum temperature for using the developed steel composition for a long time (up to 10,000 hours) is 1050 – 1100ºС, operating life is under load from 1000 to 12,000 hours.

The use of expensive rhenium in the steel composition and variation with other components will increase the productivity of equipment in the chemical, oil industry, energy and nuclear production by 80 - 90%, and increase the throughput of installations by 70 - 80%.

Literature.

1. Patent No. 2543583 C2 Russian Federation, IPC C22C 38/54. Heat-resistant corrosion-resistant steel: No. 2013127543/02: application. 06/17/2013: publ. 03/10/2015 / A. S. Oryshchenko, G. P. Karzov, A. S. Kudryavtsev [etc.] ; applicant Federal State Unitary Enterprise "Central Research Institute of Construction Materials "PROMETEUS" (FSUE "CNII KM "PROMETEUS").

2. Patent No. 2206632 C2 Russian Federation, IPC C22C 38/50, B32B 15/18, C22C 38/58. Double-layer corrosion-resistant steel: No. 2001121204/02: application. 07/27/2001: publ. 06/20/2003 / G. P. Karzov, V. G. Markov, V. A. Yakovlev [etc.] ; applicant Federal State Unitary Enterprise "Central Research Institute of Structural Materials "Prometey", State Enterprise "Experimental Design Bureau "Gidropress".

3. Patent No. 2413031 C1 Russian Federation, IPC C22C 38/58, C22C 38/54. Austenitic corrosion-resistant steel for chloride-containing environments and a product made from it: No. 2009137647/02: application. 10/13/2009: publ. 02/27/2011 / E. Kh. Shakhpazov, A. P. Shlyamnev, G. A. Filippov [etc.]; applicant Federal State Unitary Enterprise "Central Research Institute of Ferrous Metallurgy named after I.P. Bardin" (FSUE "TsNIIchermet named after I.P. Bardin").

Claims (1)

Austenitic chloride corrosion cracking resistant steel containing carbon, silicon, chromium, nickel, manganese, nitrogen, copper, boron, molybdenum, iron and inevitable impurities, characterized in that it additionally contains rhenium, tungsten, phosphorus, arsenic, antimony and bismuth with the following ratio of components, wt.%: carbon no more than 0.2, manganese from 5 to 15, silicon from 0.6 to 3, chromium from 11.0 to 20.0, nickel from 12.0 to 14.0, molybdenum from 2.0 to 6.0, nitrogen not less than 0.005 and not more than 0.4, copper not more than 2.0, boron from 0.2 to 0.6, rhenium from 0.1 to 4.5, tungsten from 1 to 6, phosphorus no more than 0.003, arsenic no more than 0.04, antimony no more than 0.09, bismuth no more than 0.03, iron and inevitable impurities - the rest.