RU2693718C2 - Duplex stainless steel for production of shutoff and control valves - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to duplex stainless steel used for production of shutoff and control valves of corrosion-active gas media with high content of hydrogen sulphide. Steel contains components at the following ratio, wt. %: carbon 0.01–0.04, silicon 0.3–0.5, manganese 0.9–1.2, chromium 22.5–24.0, nickel 5.8 –7.0, molybdenum 3.5–4.8, nitrogen 0.16–0.25, copper 3.0–3.3, niobium 0.27–0.37, cerium and/or lanthanum 0.001–0.004, sulfur ≤0.004, phosphorus ≤0.004, aluminum 0.01–0.02, calcium 0.001–0.004, yttrium ≤0.005, iron is the rest. After electroslag remelting, it has structure containing 50–60 vol. % of ferrite, inside which ferritic grains of which there are nitrides and nitrite carbonitrides of size ≤300 nm.

EFFECT: higher visco-plastic characteristics of steel in combination with high corrosion resistance and resistance to corrosion cracking in aggressive gas mediums of hydrogen sulphide.

1 cl, 2 tbl, 2 dwg

Description

The invention relates to the field of metallurgy and relates to duplex stainless steel for the production of valves and regulating valves of corrosive gas environments with a high content of hydrogen sulfide.

Known duplex stainless steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, boron, sulfur, cobalt, tungsten, copper, ruthenium, aluminum, calcium, iron and inevitable impurities in the following ratio of components, wt. %: carbon ≤ 0.03, silicon ≤ 0.5, manganese ≤ 3.0, chromium 24-30, nickel 4.9-10, molybdenum 3.0-5.0, nitrogen 0.28-0.5, boron ≤ 0.003, sulfur ≤ 0.01, cobalt ≤ 3.5, tungsten ≤ 3.0, copper ≤ 2.0, ruthenium ≤ 0.3, aluminum ≤ 0.03, calcium ≤ 0.01, iron and inevitable impurities rest.

(EA 009108, C22C 38/44; C22C 38/52; C22C 38/54, published 10/26/2007)

Known duplex stainless steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, boron, sulfur, cobalt, tungsten, copper, aluminum, calcium, phosphorus, titanium, cerium and / or lanthanum, vanadium, niobium, magnesium, tin and iron in the following ratio of components, wt. %: carbon ≤ 0.03; silicon 0.005-1.0; manganese 0.1-7.0, chromium 18.0-25.0; nickel 0.5-5.0; molybdenum ≤ 1,5; nitrogen 0.1-0.3; boron ≤ 0,005; sulfur ≤ 0.0001-0.001; cobalt ≤ 2.0; tungsten ≤ 1.0; copper ≤ 2.0; aluminum ≤ 0.05; calcium 0.001-0.004; phosphorus ≤ 0.05; titanium 0.003-0.05; cerium and / or lanthanum 0.005-0.05; vanadium 0.05-0.5; niobium 0.01-0.15; magnesium ≤ 0.003; tin 0.01-0.2; iron else.

(EP 2770076, C22C 38/00, C22C 38/58, published 08/27/2014)

Known duplex steels have a sufficiently high corrosion resistance in liquid chloride-containing environments in combination with improved mechanical properties and processability. However, in gaseous media containing hydrogen sulphide up to –25 vol. %, known steel prone to stress corrosion cracking, which limits their use for the manufacture of valves and control valves.

The closest in technical essence is duplex stainless steel containing carbon, silicon, manganese, chromium, nickel, molybdenum, tungsten, nitrogen, cobalt, copper, niobium, cerium and / or lanthanum and iron, in the following ratio of components, wt. %: carbon ≤ 0.12; silicon ≤1,0; manganese ≤ 2.0; chromium 20.0-35.0; Nickel 3.0-12.0; molybdenum 0.5-10.0; tungsten 2.0-8.0; nitrogen 0.05-0.5; cobalt 0.01-2.0; copper 0.1-5.0; niobium ≤0.2; cerium / or lanthanum ≤ 0.2%; iron else.

(JPH 09209087, C22C 38/00; C22C 38/58, published 08/12/1997)

Known steel has high strength and corrosion resistance in aqueous chloride-containing environments in the presence of hydrogen sulfide. However, in gaseous media containing hydrogen sulfide up to 25 vol. %, known steel is also prone to stress corrosion cracking, which makes it impossible to use it for the production of stop and control valves.

The task and the technical result of the invention is to increase the visco-plastic characteristics of duplex steel in combination with increased corrosion resistance and resistance to stress corrosion cracking in aggressive gaseous environments of hydrogen sulfide and carbon dioxide.

The technical result is achieved by the fact that duplex stainless steel for the production of stop and control valves contains carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, copper, niobium, cerium and / or lanthanum, sulfur, phosphorus, aluminum, calcium, yttrium and iron in the following ratio, wt. %: carbon 0.01-0.04, silicon 0.3-0.5, manganese 0.9-1.2, chromium 22.5-24.0, nickel 5.8-7.0, molybdenum 3, 5-4.8, nitrogen 0.16-0.25, copper 3.0-3.3, niobium 0.27-0.37, cerium and / or lanthanum 0.001-0.004, sulfur ≤0.004, phosphorus ≤0.004, aluminum 0.01-0.02, calcium 0.001-0.004, yttrium ≤0.005%, the rest is iron, while it has a structure containing 50-60 vol. % ferrite, and after electroslag remelting inside the ferritic grains are located nitrides and carbonitrides of niobium ≤300 nm.

Calcium and yttrium optimize the chemical composition of non-metallic inclusions. Calcium aluminates of complex composition, being sufficiently resistant in environments containing gaseous hydrogen sulfide, reduce the tendency of steel to local forms of corrosion.

Yttrium at concentrations of less than 0.005 wt. % in electroslag remelting forms refractory crystalline compounds, which are forced crystallization centers. In addition, yttrium in a concentration range of ≤0,005 wt. % grinds the dendritic grain, which contributes to the improvement of the viscoplastic properties of duplex steel. This has a positive effect on the results of stress corrosion tests. The content of phosphorus should be limited to 0.004 wt. % The nonmetallic inclusions formed in the indicated concentration range are not collectors for corrosive components of the gaseous medium.

The aluminum content in steel is 0.01-0.02 wt. % in combination with calcium content of 0.001-0.004 wt. % provides aluminates with spherical shape and small size. At low concentrations of sulfur in the metal ≤0,004 wt. % not observed the formation of sulfide shells on the surface of aluminates, which increase their melting point.

The carbon content in steel is 0.01-0.04 wt. % in combination with a nitrogen content in the range of 0.16-0.22 wt. %, provides minimal formation and as a result of the low content of large carbides of type Me ₂₃ C ₆ , located mainly on the grain boundaries of austenite and ferrite, causing brittle fracture under load.

It is at the stated ratio of carbon and nitrogen that nitride hardening acts, providing an increased complex of mechanical properties of steel.

Manganese in concentrations of 0.9-1.2 wt. % not able to cause the formation of σ-phase, which actively degrades the plastic properties of steel and reduces its corrosion resistance.

The silicon content of 0.3-0.5 wt. % due to the presence in the steel according to the invention of aluminum, calcium and rare earth metals, cerium and / or lanthanum (0.001-0.004 wt.%).

Nickel at concentrations of 5.8-7.0 wt. % stabilizes the γ-region, and also increases the corrosion resistance of steel, in particular, reduces the tendency to transcrystalline corrosion cracking.

The chromium content of 22.5-24.0 wt. % in combination with the optimal nitrogen content of 0.16-0.22 wt. % allows you to prevent the formation of undesirable large chromium nitrides of the type Cr ₂ N along the grain boundaries.

The claimed nitrogen content provides preferential bonding of niobium to resistant nitrides and carbonitrides and at the same time eliminates the possibility of the formation of gas porosity in the ingot of electroslag remelting.

The molybdenum content of 3.5-4.8 wt. % in combination with the optimal chromium content helps to reduce the number of complex compounds of the excess phase (intermetallic compounds) enriched with iron, chromium, nickel, molybdenum and copper.

- фазы, концентрирующейся преимущественно в теле зерна и отвечающей за повышение прочности материала.The copper content of 3.0-3.3 wt. % allows maximum corrosion resistance to steel cracking under stress. Being a surface-active element, copper is concentrated on the surface of the grains, has an inhibitory effect on the rate of reactions that occur on the surface of the product, especially in the zone of formation and development of cracks. Copper additives weaken corrosion processes on the steel surface, forming a surface copper-containing layer, preventing the penetration of corrosive components of the gaseous medium into the metal. In addition, the positive effect of copper is associated with the formation of fine excess

- phase, concentrating mainly in the body of the grain and responsible for increasing the strength of the material.

The invention can be illustrated by the following example.

Duplex stainless corrosion-resistant steel for the production of valves and control valves, stable in the environment of hydrogen sulfide was obtained according to the following technological scheme:

- smelting of consumable electrodes in an open induction furnace by fusing pure charge materials with protection of the metal bath from excessive saturation with atmospheric nitrogen by supplying argon to its surface;

electroslag remelting of electrodes with diffusion deoxidation of a slag bath with aluminum mixed with silicocalcium, when yttrium is reduced to metal from yttrium oxide previously introduced into the slag;

- forging an ESR ingot with an optimized degree of annealing and obtaining a semi-finished billet for the manufacture of valves;

- heat treatment of the semifinished product, including heating the semifinished product of the workpiece, holding it in the temperature range 1050-1070 ° C and subsequent quenching in water;

- mechanical processing of parts and production of samples for testing.

From the data presented in Table 1 of the mechanical tests, it follows that the duplex steel according to the invention possesses enhanced viscoplastic characteristics, as compared with the known steel.

Studies of the microstructure (Fig. 1 and 2) of duplex steel samples showed the presence of an excess phase in the structure of particles (niobium nitrides and carbonitrides) characterized by sizes ≤ 300 nm and preferential location of ferritic grains in the body.

FIG. 1. The microstructure of steel with a ferrite content of 57%.

2. Niobium nitride and carbonitride (light globules) in ferritic grains of duplex steel containing 57% ferrite (dark phase) and 43% austenite (light phase)

From the results presented in Figures 1 and 2, it follows that the steel according to the invention has a high complex of strength, plastic and corrosion properties precisely due to the formation and stabilization of the duplex structure (Figure 1) and the location of small particles of the excess phase (niobium nitrides and carbonitrides) inside the ferritic grains (Figure 2).

Tests for stress corrosion cracking were carried out in the test center, in accordance with the requirements of the standard NACE TM0177, type A: the samples were kept in a solution of 5.0 mass. % NaCl and 0.5 wt. % CH ₃ COOH saturated with H ₂ S to 2530 ppm for 720 hours with a constantly increasing load on the H50KT testing machine.

As a result of the test, for 720 hours. The samples had no cracks or damage. An assessment of the tendency of duplex steel according to the invention for stress corrosion cracking has shown its stability in a gaseous medium containing up to 25 vol. % hydrogen sulfide (table 2).

The data presented showed that the duplex hoist according to the invention ensures the achievement of the technical result achieved: an increase in the visco-plastic characteristics of duplex steel in combination with enhanced corrosion resistance and resistance to stress corrosion cracking in aggressive hydrogen sulfide gas environments.

Claims (1)

Duplex stainless steel for the production of stop and control valves containing carbon, silicon, manganese, chromium, nickel, molybdenum, nitrogen, copper, niobium, cerium and / or lanthanum, sulfur, phosphorus, aluminum, calcium, yttrium and iron, characterized by that it contains components in the following ratio, wt.%: carbon 0.01-0.04, silicon 0.3-0.5, manganese 0.9-1.2, chromium 22.5-24.0, nickel 5 , 8-7.0, molybdenum 3.5-4.8, nitrogen 0.16-0.25, copper 3.0-3.3, niobium 0.27-0.37, cerium and / or lanthanum 0.001- 0.004, sulfur ≤0,004, phosphorus ≤0.004, aluminum 0.01-0.02, calcium 0.001-0.004, yttrium ≤0.005, iron - the rest, while electroslag remelting it has a structure containing 50-60 vol. % ferrite, inside the ferritic grains of which are located nitrides and carbonitrides of niobium with a size of ≤300 nm.

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