RU2506336C1 - Titanium-based alloy - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention refers to metallurgy, and namely to titanium-based alloys, and can be used in equipment components of chemical productions, in welded connections of shipbuilding industry. Titanium-based alloy contains the following, wt %: aluminium 4.3-6.3, molybdenum 1.5-2.5, carbon 0.05-0.14, zirconium 0.2-1.0, oxygen 0.06-0.14, silicon 0.02-0.12, iron 0.05-0.25, niobium 0.3-1.20, ruthenium 0.05-0.14, and titanium is the rest. Total content of silicon and iron shall not exceed 0.30 wt %.

EFFECT: alloy has increased stability to crevice and pitting corrosion in corrosive media with increased salt content and at the temperature of up to 250 °C.

1 tbl, 1 ex

Description

The invention relates to non-ferrous metallurgy, in particular to the creation of high-strength titanium-based alloys with increased resistance against crevice and pitting corrosion in aggressive environments with high salinity (up to 3.5% NaCl), pH 2.5-4.0 and temperatures up to 250 ° C.

The alloy is intended for use in equipment elements: chemical industries, offshore equipment and shipbuilding, including welded structures of marine equipment.

Titanium-based alloys are known for use in aggressive environments (Grade 13 and Grade 15, Grade 26 and Grade 27, Grade 28 and Grade 29 no ASTM B 265-98, RU 2439188 C22C 14/00, RU 2426808 C22C 14/00 ) These alloys, having good corrosion resistance, nevertheless have certain disadvantages that limit their use in environments with high salinity (up to 3.5% NaCl), pH 2.5-4.0, and temperatures up to 250 ° C.

The disadvantages of these alloys are for some - a low level of strength, for others - low values of ductility. In addition, despite the corrosion resistance, the listed alloys are used to a limited extent in critical loaded nodes of marine equipment.

Known alloy based on titanium with ruthenium and palladium (RU 2203974 C22C 14/00 05/07/2001). This alloy contains a high content of β-stabilizing elements, including iron, which leads to structural heterogeneity and a decrease in resistance to crevice and pitting corrosion in environments with high salinity (3.5% NaCl), pH 2.5-4.0, and temperature up to 250 ° С.

The closest in content of the ingredients is an alloy based on titanium containing wt. %: aluminum 0.25-7.5, molybdenum 0.1-3 0.0, carbon up to 0.3, zirconium 0.1-10.0, oxygen up to 0.3, silicon 0.1-1.0 , iron 0.1-2.0, niobium 0.1-10.0, titanium the rest (GB 785293, C22C 14/00).

Of the alternative compositions of the specified alloy (GB 785293, C22C 14/00), the composition of the alloy was selected as a prototype, the qualitative and quantitative composition of which corresponds to the qualitative and quantitative composition of the claimed alloy.

This alloy, which has sufficiently high strength and ductility characteristics, can be used in welded structures. The disadvantage of the prototype alloy is reduced resistance to crevice and pitting corrosion in aggressive environments with high salinity (up to 3.5% NaCl), pH 2.5-4.0 and temperatures up to 250 ° C.

The technical result of the claimed invention is the creation of an alloy with higher resistance to crevice and pitting corrosion in aggressive environments with high salt content (3.5% NaCl), pH 2.5-4.0 and a temperature of up to 250 ° C compared with the alloy prototype.

The technical result is achieved due to the fact that the composition of the known alloy containing aluminum, molybdenum, carbon, zirconium, oxygen, silicon, iron, niobium, titanium, the rest, additionally introduced ruthenium in the following ratio of components (wt.%):

Aluminum 4.3-6.3;

Molybdenum 1.5-2.5;

Carbon 0.05 ÷ 0.14;

Zirconium 0.2-1.0;

Oxygen 0.06 ÷ 0.14;

Silicon 0.02 ÷ 0.12;

Iron 0.05 ÷ 0.25;

Niobium 0.3 ÷ 1.20;

Ruthenium 0.05-0.14;

Titanium rest

In this case, the total content of iron and silicon should be less than or equal to

[Si] + [Fe] ≤0.30

In the claimed invention, alloying and impurity elements are in a ratio that provides high resistance to α - crevice and pitting corrosion in aggressive environments with high salinity (3.5% NaCl), pH 2.5-4.0 and temperatures up to 250 ° C.

Silicon and iron in the claimed invention are involved in complex microalloying. Fulfillment of the claimed ratio [Si] + [Fe] ≤0.30 eliminates the formation of segregation of these elements along the boundaries of structural blocks and grains and provides a uniform regulated structural state, which increases the electrochemical uniformity of the alloy, which is necessary to increase resistance to crevice and pitting corrosion.

Exceeding the declared total content of [Si] + [Fe] reduces the resistance to crevice corrosion of the inventive alloy, especially welded joints, since the remelted metal of the heat-affected zone (HAZ) and the weld loses the ability to intragranular deformation. In this case, the deformation is localized along the boundaries of large grains characteristic of the molten metal and HST, forming microcracks, which contributes to the development of crevice corrosion.

Aluminum in the claimed range of 4.3-6.3% intensively increases the strength characteristics of the alloy, eliminates the formation of the α ₂ phase and provides good deformation characteristics in the production of semi-finished products. An increase in aluminum beyond the limits stated in the alloy reduces the corrosion resistance and increases the tendency to corrosion cracking due to the occurrence of ordering processes in the α phase.

The molybdenum content is limited to 1.5-2.5%, because when the molybdenum content is more than 2.5% in the welded joint, a low-plastic α-phase is formed, deformation ability decreases, which contributes to the formation of microcracks and reduces the resistance to pitting corrosion.

Molybdenum increases the strength characteristics of the alloy, blocks the process of ordering and the formation of the α ₂ phase and, expanding the two-phase (α + β) region, provides an increase in the technological properties of the alloy.

The carbon content in the inventive alloy is limited by the selected limits, since when the content is more than 0.14%, carbon is released in the form of rounded carbide inclusions along the grain boundaries, which reduce corrosion resistance. When the carbon content is less than 0.05%, the strength of the alloy decreases.

Neutral β-stabilizer - zirconium in the range of 0.2-1.0% in combination with aluminum provides a uniform distribution of alloying components in the α-phase, reduces intracrystalline segregation, which increases the resistance to crevice and pitting corrosion.

With a content of more than 1.0% zirconium reduces the toughness and increases the tendency to corrosion cracking, which contributes to a decrease in resistance to pitting corrosion.

Isomorphic β-stabilizer - niobium within the specified limits of 0.3-1.20% is included in the composition of the alloy as a technological additive that provides the introduction of the required amount of carbon and its uniform distribution in the alloy structure without the formation of carbides, therefore, within the specified limits it increases the structural uniformity of the alloy and corrosion resistance to crevice and pitting corrosion.

Ruthenium in the amount of 0.05-0.14% in the inventive alloy is a microalloying and cathode-modifying additive, promotes passivation by reducing the overstrain of the hydrogen evolution reaction and increases the corrosion resistance to crevice and pitting corrosion.

Microalloying the alloy with ruthenium shifts the electrochemical potential of the alloy to a region of stable passivity, which excludes pitting formation.

With a ruthenium content of less than 0.05% in these aggressive media, passivation does not occur. Complete passivation in chloride solutions with a ruthenium content of up to 0.14% is due to the facilitation of the cathodic hydrogen reduction reaction (H ⁺ + e → H). The content of ruthenium beyond the specified limit is inefficient and inappropriate.

Execution example.

Ingots with the chemical composition of the inventive alloy and prototype alloy were smelted (table 1).

The ingots were forged into billets and rolled into 4.0 mm thick sheets, from which 4.0 × 35 × 35 mm samples were then made for corrosion testing for crevice and pitting corrosion.

Corrosion tests were carried out on samples - plates measuring 35 × 35 × 4 mm. The test procedure met the requirements of GOST9.912-89 (ST SEV 64461-88).

In order to accelerate, corrosion tests were carried out in an autoclave in a 20% NaCl solution at a temperature of 250 ° C for 200 hours. The pressure was 40 ata. The test results are shown in table 2.

The tendency to crevice corrosion was assessed based on the results of the study of mass loss in the dimension of 10 ^-3 g / (dm ² hours) and the presence of local damage.

An assessment of the propensity for pitting was performed visually when examining the surface of the samples using optical equipment with an increase of x12. Pitings with a diameter of more than 0.1 mm were detected.

Pitting surface lesions larger than 4.0 mm were detected on a prototype alloy sample. On the sample of the inventive alloy, no surface lesions were found, the surface of the samples was shiny. The presented results show that the inventive alloy in terms of resistance to crevice and pitting corrosion exceeds the similar characteristics of the known prototype alloy. This allows you to increase the resource of various items of equipment when operating in aqueous solutions with a high chloride content at elevated temperatures up to 250 ° C and a pH of 2.5 to 2-3 times.

Table 1 The chemical composition of the inventive alloy and prototype alloy based on titanium Alloy Composition number Al Mo C Zr O ₂ Si Fe Nb Ru Si + Fe≤0.30 Ti The claimed one 4.3 1,5 0.05 0.2 0.14 0.02 0.25 0.3 0.14 0.27 rest 2 5,0 2,5 0.10 0.7 0.08 0.05 0.20 0.5 0.10 0.25 rest 3 6.3 2.0 0.14 1,0 0.06 0.12 0.05 1.20 0.05 0.17 rest Prototype GB785293 6.0 2.0 0.15 1,5 0.14 0.15 0.25 1,5 - 0.40 rest

table 2 Corrosion resistance of the inventive alloy and prototype alloy Alloy Composition number Environmental characteristic Corrosion test results, test duration 200 hours Crevice corrosion Pitting corrosion Degree of lesions Surface condition The claimed one 20% NaCl solution aerated, saturated CO ₂ , pH = 3.5, temperature 250 ° С No defeat Glossy surface No defeat The surface of the samples is shiny, no damage 2 No defeat Glossy surface No defeat The surface of the samples is shiny, no damage 3 No defeat Glossy surface No defeat The surface of the samples is shiny, no damage Prototype GB785293 Local defeat Gray film on the surface Surface ulcerated Ulcers with a diameter of more than 4.0 mm

Claims (1)

A titanium-based alloy containing aluminum, molybdenum, carbon, zirconium, oxygen, silicon, iron, niobium and titanium, characterized in that it additionally contains ruthenium in the following ratio, wt.%:
Aluminum 4.3 ÷ 6.3 Molybdenum 1,5 ÷ 2,5 Carbon 0.05 ÷ 0.14 Zirconium 0.2 ÷ 1.0 Oxygen 0.06 ÷ 0.14 Silicon 0.02 ÷ 0.12 Iron 0.05 ÷ 0.25 Niobium 0.3 ÷ 1.20 Ruthenium 0.05 ÷ 0.14 Titanium rest
the following relation is fulfilled:
[Si] + [Fe] ≤0.30.