SE529428C2 - Austenitic stainless steel alloy component, e.g. tube, for use in supercritical water oxidation plants comprises predetermined amounts of chromium and nickel - Google Patents

Abstract

An austenitic stainless steel alloy component for use in supercritical water oxidation (SCWO) plants comprises 15-30 wt.% chromium and 20-35 wt.% nickel. The component is intended to be in direct contact with supercritical or near supercritical solution. It may further comprise 0.5-6.0 wt.% copper, 0.01-0.10 wt.% carbon, not >4.0 wt.% molybdenum, 0.2-0.6 wt.% niobium, 0.4-4.0 wt.% tungsten, 0.10-0.30 wt.% nitrogen, 0.5-3 wt.% cobalt, and 0.02-0.10 wt.% titanium. An independent claim is included for use of an austenitic stainless steel alloy comprising 15-30% chromium (Cr) and 20-35% nickel (Ni) in supercritical water oxidation plants.

Description

529 428 a This process takes place even if the content of organic material in the water is low (3 to 6%), which means that the excess heat is always available for heating the waste water that passes through the preheater towards the reactor. In addition, the plant includes apparatus for treating the treated aqueous phase leaving the reactor, such as a steam boiler, a cooler, pressure reducing devices, a gas-liquid separator, and so on. All these components are connected through different pipelines and are regulated by other components, such as valves, accumulators, pressure reducing devices, liquid oscillators, injectors, nozzles, filters, separators, etc.

In practice, the SCWO process can be used to neutralize an almost infinite number of waste products. Thus, municipal water sludge can be treated during complete destruction of the organic substances therein. Inorganic material in the wastewater can then be deposited as a non-hazardous material or used as a raw material for recycling purposes. Another application is waste products that contain valuable inorganic materials. The organic pollutants are completely destroyed, leaving behind a purely inorganic phase, which can be recycled. Examples of this application are the decolorization of pulp during the recycling of paper fillers, and the treatment of spent catalysts during the recycling of precious metals. Furthermore, active pharmaceutical ingredients can be eliminated from waste water, and halogenated waste destroyed without the formation of hazardous by-products, such as dioxins. The SCWO process is particularly interesting as an alternative to incineration, as waste products containing nitrogen can be destroyed without forming NOX.

The environment in a SCWO facility is generally very difficult. In particular, the environment inside the reactor vessel and the pipelines connected to it can be corrosive, with a number of substances being very aggressive relative to the materials in the various components. For example. there are acids, such as nitric acid, sulfuric acid and hydrochloric acid, which are highly corrosive in the range 270 to 380 ° C. A surface in contact with the aggressive and corrosive substances therefore runs the risk of corroding or otherwise deteriorating in a short time. 529 428 3 In order to overcome the problems caused by the aggressive conditions in SCWO plants, the choice of material for the various components in the plant is difficult, but crucial. Today, a common construction material is Alloy 625, which is used in plant equipment, such as the heater, preheater, reactor, boiler and evaporator, as well as in simple components such as pipes and plates. The main reason why Alloy 625 is used is that it withstands high pressures (250 bar) and high temperatures (600 ° C) and is quite resistant to the corrosivity of the process liquid, e.g. in the temperature range 270 to 380 ° C, which means that the appliances and components have an acceptable service life. However, a serious disadvantage of high-alloy nickel-based varieties, such as Alloy 625, is that they are very expensive due to the high nickel and molybdenum content, which results in heavy investment costs for the construction of the plants. Another austenitic stainless steel alloy, similar to Alloy 625 in the high nickel content, is C 276. Both of these grades contain eo% nkel or more; In order to reduce the cost of the construction material, attempts have also been made, in SCWO plants, to use low-alloy varieties, such as 304 L and 316 L, which require rather moderate amounts of nickel (8 to 15%) and chromium (18 to 20%). and therefore are inexpensive relative to the high alloy varieties. However, it has been found that 304 L and 316 L do not withstand the high pressure (221 bar) or the high corrosivity of the liquid in the area immediately below the supercritical temperature point (374 ° C). The patent literature describes the use of nickel-based and high-alloy stainless steel alloys in SCWO plants in e.g. US 6958122 (lnconel and Hastelloy) and US 5804066 (lnconel), while the use of the low alloy 316 L is mentioned in US 5823220.

For the sake of completeness, it may also be mentioned that the use of ceramics and / or cermets, mainly as claddings and coatings on structural components in SCWO plants, has been proposed by US 5358645, US 5461648 and US 5545337. Although ceramics are quite resistant to corrosion, they do limit 529 428 4 the freedom of construction to an unreasonable extent, which means that the various constructions on the plant can not be carried out in the best way, with regard to e.g. mechanical strength, weldability, function, etcetera.

The above-mentioned shortcomings in the known construction materials hinder the development and commercialization of the SCWO technology to an attractive alternative to the traditional methods of waste destruction, such as incineration, despite the fact that supercritical water oxidation is superior in your respects, e.g. environmentally, and with regard to the ability to dispose of hazardous waste products in a safe manner. Therefore, there is still a need for a construction material that is reasonably inexpensive and nevertheless suitable for its purpose in terms of corrosion resistance, mechanical strength, temperature strength, weldability and machinability.

Summary of the Invention The present invention provides an austenitic stainless steel which meets the above needs, namely in the form of a variety called SANICRO®25 as described, e.g. in EP 1194606 B1. Thus, it has been found that an alloy made mainly in the manner specified in said patent documents not only successfully withstands high mechanical loads, but also provides acceptable corrosion protection in SCWO environments, despite the fact that SANlCRO®25 is based on fairly moderate amounts of expensive constituents (especially nickel) and therefore less expensive to manufacture than Alloy 625 and Alloy C 276.

As will be seen from the following report of a corrosion test, SANlCRO®25 is at least as good as, and in some cases even better than, the high alloy grade A 625, in terms of corrosion resistance and service life.

An austenitic stainless steel alloy of the present invention comprises (by weight) 20 to% nickel (Ni) and 15 to 30% chromium (Cr). In a more preferred embodiment, the alloy comprises 20 to 35% nickel (Ni), 15 to% chromium (Cr) and 0.5 to 6.0% copper (Cu). In practice, the alloy of the invention may advantageously comprise (by weight) 20 to 35% nickel (Ni), 15 to 30% chromium (Cr), 0.5 to 6.0% copper (Cu), 0.01 to 0, 10% carbon (C), 0.20 to 0.60% niobium (Nb), 0.4 to 4.0% tungsten (W), 0.10 to 0.30% nitrogen (N), 0.5 to 3.0% cobalt (Co), 0.02 to 0.10% titanium (Ti), not more than 4.0% molybdenum (Mo), not more than 0.4% silicon (Si) and not more than 0 , 6% manganese (Mn), balanced with iron and normal steelmaking contaminants.

Brief Description of the Drawings Figure 1 illustrates the weight change of two alloys, according to the invention and Alloy 625, when exposed to a simulated SCWO environment at 350 ° C for 125 hours.

Figure 2 illustrates the weight change of two alloys, according to the invention and Alloy 625, when exposed to a simulated SCWO environment at 600 ° C for 125 hours. Detailed Description of the Invention The components of the alloy used according to a preferred embodiment of the invention are discussed below (all percentages are by weight).

Nickel Nickel is an essential component for the purpose of ensuring a stable, austenitic structure. The structural stability depends on the relative amounts of the ferritic stabilizers such as chromium, silicon, tungsten, titanium and niobium, as well as austenitic stabilizers such as nickel, carbon and nitrogen. To attenuate the formation of sigma phases after a long time at elevated temperatures, especially when a high content of chromium, tungsten and niobium, needed to ensure high temperature corrosion resistance and high creep rupture strength, is used, the nickel content should be at least 20% and preferably at least 22%. , 5%. It can also be 25% or higher. At a specific chromium level, an increased nickel content dampens the growth rate of the oxide in 529 428 6 and improves the tendency to form a continuous chromium oxide layer. However, in order to keep the material cost at a reasonable level, the nickel content should not exceed 35% and preferably not 32%. With the above factors in mind, the nickel content of the alloy is limited to the range of 20 to 35%.

Chromium Chromium works to improve the general corrosion resistance and oxidation resistance. In order to achieve a sufficient durability in these respects, a chromium content of at least 15% is prescribed. Preferably 20% chromium or more can be added. However, if the chromium content were to exceed 27% and approach 30%, the nickel content would have to be further increased to give a stable austenitic structure. A chromium content exceeding 30% would make it necessary to increase the nickel content to a level that is too high (above 35%) to ensure a cost-effective composition. For these reasons, the chromium content is limited to between 15 and 30%, preferably between 20 and 27%.

Copper Copper is added to give a copper-enriched, and in the matrix och distributed and uniformly precipitated phase, which contributes to an improvement of the creep rupture strength. Such an effect requires an amount of at least 0.5% copper, whereby a marked improvement is achieved by around 2%. Copper is also added to improve the overall corrosion resistance_ to sulfuric acid. An excessive amount of copper (6% or more) would lead to a reduced processability. Also for economic reasons, the Cu content should be kept moderate, e.g. at 3.5%. With these considerations in mind, the copper content is limited to the range of 0.5 to 6%, preferably 2 to 3.5%.

Carbon Carbon is a component that has the effect of providing the appropriate tensile strength and creep rupture strength required for high temperature steels. However, if too much carbon is added, the toughness of the alloy decreases and the weldability may deteriorate. In addition, an excessively high carbon content would reduce corrosion resistance in SCWO environments. For these 529 428 'ß reasons, the carbon content is limited to a maximum of 0,1 ° />. Preferably it can amount to at least 0.04% and at most 0.08%.

Niobium Niobium is generally recognized for contributing to the improvement of creep rupture strength by precipitation of carbonitrides and nitrides. However, an excessive amount of niobium can reduce weldability and machinability. With these considerations in mind, the niobium content is limited to the range of 0.20 to 60%. Preferably, the niob content should be at least 0.33% and at most 0.50%.

Tungsten and Molybdenum Tungsten is added to improve the high temperature strength, mainly through solution curing, and a minimum of 0.4% is needed to achieve this effect. Tungsten and molybdenum also contribute to the general corrosion resistance in SCWO environments.

However, both molybdenum and tungsten favor the formation of the sigma phase. Tungsten is considered to be more effective than molybdenum in improving strength. For this reason and for economic reasons, the molybdenum content is kept low, not higher than 0.5%, preferably lower than 0.02%. In order to maintain sufficient workability, the tungsten content should not exceed 4% and therefore the tungsten content is limited to the range 0.4 to 4%, preferably 1.8 to 3.5%.

Nitrogen Nitrogen, like carbon, is known to improve strength at elevated temperatures, e.g. above 500 ° C, and the creep rupture strength partly stabilize the austenitic phase. However, if nitrogen is added in excess, the toughness and ductility of the alloy decrease. For these reasons, the nitrogen content is limited to the range 0.1 to 0.30%, preferably 0.20 to 0.25%.

Cobalt Cobalt is an austenite stabilizing element. The addition of cobalt can improve the high temperature strength by solidification enhancement and suppression of sigma phase formation after long exposure times at elevated temperatures. However, in order to maintain production costs at a reasonable level at 529 428 X, the cobalt content should be in the range of 0.5 to 3.0%, if added.

Titanium Titanium can be added in order to improve the creep rupture strength by precipitation of carbon hydrides, carbides and nitrides. However, an excessive amount of titanium can reduce weldability and machinability. For these reasons, the titanium content is limited to the range of 0.02 to 0.10%, if added.

As examples of components or structural elements made of the steel alloy according to the invention, the following can be mentioned: pipes, sheet metal, rods, rods, strips, foils, claddings, blocks, sleeves, wires, beams, supports, columns and profiles. All of these components can in turn be used (individually or in combination) to design the various appliances and devices included in a complete SCWO plant, such as a reactor, an oxygen tank, a water sludge tank, an evaporator, a preheater, a steamer, a radiator, a gas / liquid separator, as well as various valves, accumulators, pressure reducing devices, liquid oscillators, injectors, nozzles, filters and separators. Pipes and sheets are easy to manufacture from the steel alloy described above.

By using components of the present invention, i.e. components consisting of a steel alloy of the type specified above, it is expected that the material costs associated with the construction of SCWO facilities will decrease by approximately 25 to 40% compared to the costs of high alloy varieties, such as Alloy 625, for the important upstream equipment and downstream of the plant's reactor. Thus, the invention will contribute positively to the future development and use of the SCWO technology, as a method of disposing of organic waste products in a manner that is harmless to the environment. 529 428 “in Test Report Three different superalloys were tested to determine the corrosion resistance in near-critical and supercritical water conditions. The superalloys were subjected to a pressure of 29 MPa and temperatures of 350 ° C and 600 ° C, respectively, for 125 hours. To simulate a difficult SCWO environment, the solvent contained chloride ions and oxygen.

The composition of the tested alloys is described in Table 1 and the experimental conditions are summarized in Table 2. The two coils differ only in the temperatures used and thus in the density of the liquid.

Table1 Alloy Al Si Ti Cr Mn Fe Co Ni Cu Nb Mo W 1 - - - 23.6 0.7 42.5 1.9 24.1 2.7 0.4 - 4.1 2 0.2 0.2 - 28.2 5.0 27.6 - 32.1 - 0.7 5.8 - A | loy625 0.3 - 0.3 23.1 - 2.1 - 60.5 - 4.5 9.1 - Table | 2 Test no. Time Temp. Pressure Density Mass fl fate Solvent [h] [° C] [MPa] [g / cma] [m / min] composition [mol / I] Run 125 350 29 0.64 1.0 1.0 H 2 O + 0.05 HCl 1 +0.5 02 Run 125 600 29 0.087 1.0 1.0 H 2 O + 0.05 HCl 2 +0.5 02 Rectangular specimens were cut from the alloys and ground (80 to 1000 mesh) and polished (9 to 0.25 μm diamond). The specimens were weighed before and after being subjected to the experimental conditions identified above. The surface layers formed during the experiments were examined by field emission scanning electron microscopy (SEM) and X-ray microanalysis (EDX). Further examination of the specimens was done by optical microscopy. Corrosion attacks were evaluated by microscopic observations. 529 428 / Û Results for 350 ° C The test pieces from the test were cleaned with distilled water and acetone and finally blow-dried for weighing and to carry out fl your examination steps. The test resulted in a significant material loss, demonstrated by the weight change. Weight and dimension before exposure and the amount of weight change after the test run at 350 ° C are given in Table 3.

The mean values are presented in Figure 1.

Table 3 Alloy XYZ Weight before Weight after A [g] A [° / °] [mm] [mm] [mm] [9] [91 1 «20.0 9.68 1.95 3.005 2.365 -0.640 -21, 3 2 20.7 9.89 1.9 3.033 2.120 -0.913 -30.1 Alloy _ 20.5 10.0 1.7 2.803 2.120 -0.683 -24.4 625 Table 4 EDX results of the scale layers formed on the test pieces during exposure at 350 ° C [wt%] Alloy 1 2 Alloy 625 O 29.9 36.9 - 36.3 Cl 2.0 0.3 1.7 AI 0.3 0.8 0.9 Si - - 0.1 Ti - - 0.5 Cr 30.8 23.4 18.9 Mn 0.6 1.8 0.7 Fe 18.6 23.2 13.1 Co 1.3 - - Ni 9.8 5 .8 14 -529 428 Il Cu 2 - - Nb 0.6 2 5.1 Mo - 3.6 7.1 W 4.3 2 1.4 The specimens were imaged by scanning electron microscopy (SEM) and then the elemental composition of the surface layers was analyzed by energy dispersive. X-ray spectroscopy (EDX). The microanalytical results of the surfaces revealed a composition of the scale of mainly oxides and small amounts of chlorine, the results are listed in Table 4.

Results for 600 ° C No significant corrosion attacks were observed on the samples after exposure.

Only a thin oxide layer remained on the sample surface, indicated by a slight weight gain. Weight and dimension before exposure and the weight change after the test run at 600 ° C, are given in Table 5. The mean values are presented in Figure 2.

The specimens were reproduced by scanning electron microscopy (SEM) and then the elemental composition of the surface layers was analyzed by energy dispersive X-ray spectroscopy (EDX). The EDX analysis confirmed an oxidic composition of the thin layers formed. A summary of the EDX results is listed in Table 6.

Table 5 Alloy X [mm] Y [mm] Z [mm] Weight Weight A [g] A [%] before [g] after [g] 1 20.0 9.60 1.6 2.729 2.745 0.016 0.06 2 20 .3 9.75 1.8 2.872 2.889 0.017 0.06 A || 0y 625 20.6 10.1 1.7 2.839 2.848 0.009 0.03 '529 428/2 Table 6 EDX results of the scale layers formed on the test pieces during exposure at 600 ° C [wt%] Alloy 1 2 Alloy 625 O 30.3 30.9 27.3 Cl - - - Al - - 0.5 Si - - - Ti 1.8 1 0.9 Cr 3 9.9 6.5 Mn 1.6 2.7 1.5 Fe 41 31.9 4.9 Co Ni 21.1 22.5 53.3 Cu - - -Q Nb <0.1 - 4, 8 Mo - 1.1 <0.3 W _ _ _

Claims (11)

10 15 20 25 30 1. KRAV 529 428/3 A component for supercritical water oxidation plants made of an austenitic stainless steel alloy comprising, in% by weight: 15 to 30% chromium (Cr), 20 to 35% nickel (Ni), the component being in direct contact with a near. critical or a supercritical aqueous solution A component according to claim 1, wherein the alloy comprises, in% by weight: 15 to 30% chromium (Cr), 20 to 35% nickel (Ni), 0.5 to 6.0% copper (Cu). A component according to claim 1, wherein the alloy further comprises 0.01 to 0.10% carbon (C). A component according to claim 1, wherein the alloy further comprises not more than 4.0% molybdenum (Mo). A component according to claim 1, wherein the alloy further comprises 0.2 to 0.6% niobium (Nb). A component according to claim 1, wherein the alloy comprises 0.4 to 4.0% tungsten (W). A component according to claim 1, wherein the alloy comprises 0.10 to 0.30% nitrogen (N). A component according to claim 1, wherein the alloy further comprises 0.5 to 3% cobalt (Co). 10 10. A component according to claim 1, wherein the alloy comprises 0.02 to 0.10% titanium (Ti). A component according to any one of the preceding claims, which is in the form of a plate, a tube, a rod, a rod, a strip, a foil, a cladding, a sleeve, a block, a wire, a beam, a carrier, a pillars or a profile. Use of an austenitic stainless steel alloy comprising 15-30% Cr and 20-35% Ni in supercritical water oxidation plants, the component being in direct contact with a near-critical or a supercritical aqueous solution.