EP0455625A1 - High strength corrosion-resistant duplex alloy - Google Patents

Abstract

The invention relates to a readily weldable duplex alloy having excellent corrosion resistance and a high level of mechanical properties, coupled with good cutting properties. After forced cooling from a temperature between 1020 DEG C and 1150 DEG C, the alloy according to the invention contains ferrite and austenite in a ratio of 40 to 60 % and contains, in % by weight, essentially 0.15 - 0.55 Si, 2.0 - 2.9 Mn, 23.0 - 27.0 Cr, 3.0 - 5.0 Mo, 5.6 - 8.0 Ni, 0.5 - 1.0 W, 0.2 - 0.35 N and 0.04 - 0.25 V, the remainder being iron, the ratio value G of the nickel content to the manganese content being greater than 2.0 and smaller than 4.0, and the structure phase factor P, formed from [2.9 x (% Cr) + 2.9 x (% Mo) + 1.4 x (% W) + 4.4 x (% Si) - 2.1 x (% Ni) - 1.0 x (% Mn) - 62.5 (% N)] having a value greater than 40 and smaller than 65.

Description

The invention relates to a high-strength, easily weldable, essentially containing the alloy components C, Si, Mn, Cr, Mo, Ni, W, N and V duplex alloy with excellent resistance to corrosion, in particular general or surface-removing corrosion, pitting and Crevice corrosion as well as stress and vibration crack corrosion, in chloride-containing and phosphoric acid-containing media and in the heat-treated state with a material strength RM of at least 750 MPa, a 0.2 proof stress RP _0.2 of at least 550 MPa and a Charpy-V toughness of at least 100 joules with good Machining properties.

Alloys of this type are required for mechanically highly stressed system parts in corrosive media in the chemical industry and especially in OFF-SHORE TECHNOLOGY when searching for or extracting and distributing oil and natural gas. It is necessary that these materials are easy to weld and easy to work with, can withstand high mechanical loads and have above-average toughness properties even at temperatures below 0 ° C.

Furthermore, the materials must have excellent resistance to all types of corrosion, because chloride-containing and phosphoric acid-containing media in plant parts, pipes and the like, which may be exposed to mechanical direct voltages or mechanical alternating voltages, usually in addition to general surface-removing corrosion, rapidly progressing pitting and intercrystalline crevice corrosion and cause stress and vibration crack corrosion.

Attempts have been made to improve the strength and corrosion resistance of the alloy by increasing the contents of chromium, molybdenum and silicon. Because these elements are ferrite formers, an increase in the ferrite content in the structure is associated with this and a significant deterioration in the hot deformability, the toughness and the weldability as well as embrittlement of the material would be brought about.

In order to avoid these disadvantages, attempts were further made to increase the content of austenite-forming elements and in particular to increase the nitrogen content, because nitrogen is a strong austenite former on the one hand and has a favorable influence on the corrosion resistance on the other hand. However, high nitrogen concentrations are not possible when using a conventional manufacturing technology for such alloys, because due to the jump in solubility during the solidification, gaseous nitrogen is formed, which causes imperfections, for example bubbles and pores, in the casting. It has been shown that the complex requirements regarding the mechanical properties and the corrosion resistance of an alloy can best be met if they have a ferrite and austenite ratio of 1: 1 and in particular the contents of the elements that improve the corrosion resistance are matched in this way are that essentially sufficient workability, Machinability and weldability are present.

For the alloy-technical setting of a ratio of ferrite to austenite in the structure, information is given in the literature on the basis of the Cr equivalent and Ni equivalent, which applies to welding or cooling from the liquid phase. However, parts made from corrosion-resistant duplex alloys are subjected to an annealing treatment in which the proportions of ferrite and austenite are formed depending on the temperature and the alloy composition. Both the influence of temperature and the influence of the concentrations of the individual alloy elements are very large in the known material compositions, so that the corresponding properties cannot always be achieved with certainty and a high, complex property level cannot be set in a targeted manner.

A known nitrogen-containing duplex alloy with high corrosion resistance (EP-A2-0220141) is composed in such a way that it has a ferrite content of 30 to 55%, the corrosion resistance being increased by Cr, Mo and N and reduced by manganese and sulfur. The contents of the elements tungsten and in particular manganese are capped at 0.5% by weight and 1.2% by weight with regard to the resistance in chloride-containing media. During production, however, the most precise and complex technical measures have to be taken to maintain the corresponding characteristics of the material, and it is usually not possible to achieve a desired thermal structural stability in the solution annealing treatment.

EP-A1-0107489 discloses a corrosion-resistant duplex alloy which, in order to improve the mechanical properties and the resistance to pitting corrosion and stress corrosion cracking in sea water, has in particular high manganese contents of 3.5 to 5.0% in order to increase the amount of nitrogen in solution hold and raise the tensile strength and yield strength. The disadvantage here is that at slower cooling speeds of such materials Sigma phase is formed and the toughness is significantly deteriorated.

Mn contents of 5 to 7% by weight, nitrogen contents of up to 0.4% by weight and, to improve the corrosion resistance, copper contents of 1.1 to 3.0% by weight are used in a duplex alloy containing chromium plus 3x molybdenum of greater than 32% by weight according to WO 85/05129. Although such materials already have improved corrosion resistance in the as-cast state, the processability and the mechanical values, in particular the toughness properties, are usually not sufficiently high. Furthermore, the welding behavior also mostly does not meet the requirements with regard to the filler metal.

Furthermore, EP-A1-0320548 discloses a duplex alloy with improved mechanical properties with better toughness values than FERRALIUM alloy 255 or SAF 2205. Characteristic of this alloy is the setting of a ratio of Cr equivalent and Ni equivalent within narrow limits, high nickel contents of 8.0 to 11.0% by weight being maintained due to manganese concentrations of up to 2.0. In the case of a melt-metallurgical production of this material, however, especially at higher nitrogen contents, pore formation can occur in the casting and it can also be difficult to carry out the hot deformation, so that it is preferably used to manufacture forged or, in particular, cast parts.

Attempts have also been made (EP-A3-0151487) to increase the corrosion resistance by means of additional copper contents and to increase the yield strength of the material by means of cobalt concentrations of 0.2 to 4.0% by weight.

DE-B2-26 16 599 discloses an alloy with a composition within wide limits for the production of pipes and pipe connections, which parts are used for the transport and further processing of acid gas. To increase the yield strength, these parts must be subjected to cold working after solution annealing, which causes considerable disadvantages in production, particularly in the case of complicated shapes.

Rest im wesentlichen Eisen.The invention is based on a typical duplex alloy, the composition of which lies essentially within the concentration limits given below in% by weight of the elements.

Rest essentially iron.

The object of the invention is to create a duplex alloy, in particular for the OFF-SHORE application in the petroleum and natural gas sector and for the chemical industry, which can be produced and processed economically and with a high level of production reliability and has good thermoforming properties, whereby the parts made from it are easy to weld and process or machinable and the material has high mechanical properties excellent resistance to all types of corrosion.
In addition to the improved producibility, it is therefore essential that the combination of properties of high material strength, high yield strength, good toughness and resistance to surface-removing corrosion, pitting and crevice corrosion as well as stress and vibration crack corrosion is optimized.

Rest Eisen und herstellungsbedingte Verunreinigungen enthält mit der Maßgabe, daß bei einem Verhältniswert G des Nickelgehaltes in Gew.-% zum Mangangehalt in Gew.-% von größer als 2,0 jedoch kleiner als 4,0 der Gefügephasenfaktor P gebildet aus [2,9x(%Cr)+2,9x(%Mo)+1,4x(%W)+4,4x(%Si)-2,1(%Ni)-1,0x(%n)-62,5x(%N)] einen Wert von größer als 40 jedoch kleiner als 65 aufweist.It has now been shown completely surprisingly that, according to the present invention, this complex object is achieved in that, after a heat treatment by forced cooling from a temperature between 1020 ° C. and 1150 ° C., ferrite and austenite with a ratio of 40 to 60% Alloy in% by weight

The rest contains iron and production-related impurities with the proviso that at a ratio G of the nickel content in% by weight to the manganese content in% by weight of greater than 2.0 but less than 4.0, the structural phase factor P is formed [2.9x (% Cr) + 2.9x (% Mo) + 1.4x (% W) + 4.4x (% Si) -2.1 (% Ni) -1.0x (% n) -62 , 5x (% N)] has a value greater than 40 but less than 65.

This desired level of properties, which takes the complex requirements into account, can obviously only be achieved by synergistic effects of essentially all elements with certain concentration ratios within narrow limits.

The invention is explained in more detail below with the mechanisms of action of the structural parts and the alloy elements.

Duplex alloys are cooled from a solution annealing temperature at which the proportion of austenite and ferrite is set depending on the temperature. Increasing proportions of ferrite increase the strength of the material, but the toughness and corrosion resistance are adversely affected. With a share of 40 to 60% of ferrite in the structure, sufficiently high toughness values are achieved with high strength of the material. It is important for the setting of the microstructure that the solution annealing temperature is from 1020 ° to 1150 ° C., preferably from 1050 ° C. to 1100 ° C.

Carbon is a strong austenite former, but reacts with carbide-forming elements to form carbides that degrade toughness and especially corrosion resistance. Maximum carbon contents of 0.04% by weight, preferably 0.03% by weight, have no adverse effect on the material properties.

Silicon is a strong ferrite former and is required by melt metallurgy to deoxidize the liquid steel. High levels of silicon deteriorate machinability and promote Sigma phase formation, which reduces toughness. Silicon contents of 0.15 to 0.55, preferably 0.2 to 0.5, are therefore essential.

Manganese is an austenite former and increases the nitrogen solubility of the melt, but favors the toughness-reducing excretion of sigma phase in higher concentrations. Low manganese contents cause problems in the melt metallurgy and casting technology with reduced nitrogen solubility and possibly deterioration of the hot formability. Furthermore, manganese binds the sulfur with the formation of sulfide, which sulfide inclusions particularly favor pitting and crevice corrosion. It is therefore essential to the invention that the manganese content of the alloy is present within narrow limits, specifically with a concentration of 2.0 to 2.9, preferably 2.1 to 2.7,% by weight and that the sulfur content is less than 0.005% by weight. -% is.

Chromium is important for establishing a passive state in relation to a corrosion medium and has a ferrite-forming effect. Good corrosion resistance is brought about in the range from 23 to 27, preferably 24 to 26,% by weight of chromium of the alloy.

Molybdenum is particularly effective against pitting and crevice corrosion, especially in chloride-containing media. However, high molybdenum contents can form molybdenum-rich phases, which adversely affect the corrosion resistance of the material. A molybdenum content of the alloy of 3.0 to 5.0, preferably 3.5 to 4.5,% by weight is also important for achieving good weldability.

Nickel is an essential austenite former and is required to adjust the duplex structure with concentrations of 5.6 to 8.0, preferably 6.2 to 7.4,% by weight.

Tungsten in the content limits of 0.5 to 2.0, preferably 0.55 to 0.9,% by weight improves the hot-formability of the alloy decisively and is also essential for increasing the resistance of the alloy to pitting and crevice corrosion.

phases containing copper deteriorate the pitting and crevice corrosion resistance in chloride-containing media, so that the copper content is at most 0.5, preferably at most 0.35,% by weight.

Nitrogen is an extremely important alloying element because nitrogen, as an austenite former, solidifies the austenite phase without loss of toughness. Nitrogen also inhibits the precipitation of intermetallic phases and promotes the chromium distribution between austenite and ferrite. Nitrogen levels of 0.2 to 0.35% by weight are particularly effective.

Vanadium forms fine vanadium carbides, in particular vanadium carbonitrides, in a concentration range from 0.04 to 0.25, preferably from 0.05 to 0.15,% by weight, as a result of which grain refinement and solidification of the material result in improved weldability and thermal structural stability becomes. Lower vanadium contents can lead to coarse grain formation, higher concentrations can lead to a coagulation of the carbonitrides and nitrides which is disadvantageous for the material properties.

An aluminum content of the alloy cannot be absolutely prevented in most of the melt-metallurgical processes, but must be limited to a maximum of 0.06, preferably to a maximum of 0.04,% by weight because of the aluminum nitride formation, which significantly reduces the toughness.

Niobium / tantalum enhances the beneficial effect of vanadium and can contain up to 0.02, preferably up to 0.01,% by weight in the alloy be provided.

Calcium is a particularly effective deoxidation element and a strong sulfide former and improves the purity and the properties, in particular the machinability, of the alloy at concentrations of up to 0.04% by weight. Calcium with a content of 0.001 to 0.015% by weight largely prevents the formation of harmful manganese sulfides, with calcium inclusions having a positive effect on machining.

Magnesium contents of up to 0.02% by weight favor the thermoforming properties and can increase the degree of purity of the material.

Essential to the invention for a high property level of the mechanical and corrosion-chemical-metallurgical characteristics of the material corresponding to the complex requirements of the alloy is a ratio value G of the nickel content to the manganese content with a structure factor P, which reflects the different and differently strong effects of the individual elements on the phase distribution heat treatment.

Completely surprisingly, it has been shown that in a very narrow range of the nickel content to the manganese content of the alloy, that is to say at a ratio G of 2.0 to 4.0, preferably from 2.3 to 3.5, in a structure phase factor P of 40 to 60, in particular from 45 to 59, defined, narrow composition range with high production reliability, significantly improved mechanical properties and outstanding resistance to all types of corrosion can be achieved.

With a narrow selection of a duplex alloy with narrow concentration ranges of the alloy components, determined by the ratio value G and the structural phase factor P, this is a good one Hot deformability given with fine microstructure. Strength values RM of over 750 MPa with an elastic limit RP _0.2 of greater than 550 MPa and a Charpy V toughness of greater than 100 joules of the material can be set, with excellent resistance to corrosion, in particular pitting and crevice corrosion and stress corrosion cracking , given is. Furthermore, a high structural stability and temperature stability of the ferrite / austenite alloy is achieved. The material has good weldability, even in the heat-affected zones of the base material, the properties are not impaired. The machinability, in particular the machinability, of the material is also significantly improved, as a result of which the tool costs are reduced if the material is shaped accordingly.

The invention is further explained on the basis of diagrams with examination results.

• Fig. 1 Versprödungsverhalten in Abhängigkeit vom Ni/Mn-Verhältniswert G
• Fig. 2 Warmstauchversuche in Abhängigkeit vom Ni/Mn-Verhältniswert G
• Fig. 3 Lochkorrosionspotentiale in synth. Meerwasser
• Fig. 4 korrosionsverhalten in Phosphorsäure
• Fig. 5 Zerspanungsverhalten in Abhängigkeit vom Ni/Mn-Verhältniswert G

von Duplex- Legierungen.Show it

• Fig. 1 embrittlement behavior depending on the Ni / Mn ratio value G
• Fig. 2 hot upsetting tests depending on the Ni / Mn ratio value G
• Fig. 3 pitting corrosion potential in synthetic sea water
• Fig. 4 corrosion behavior in phosphoric acid
• Fig. 5 cutting behavior depending on the Ni / Mn ratio value G

of duplex alloys.

1 shows the notched impact strength after a 475 ° C. embrittlement treatment measured in the Charpy V test in relation to the ratio G of nickel content to manganese content, where the maximum achievable toughness is assigned the value 100%. It can be seen that alloys according to the invention in the narrow range between 2.0 and 4.0 of the ratio G have no significant tendency to become brittle, whereas in comparison alloys there is a significant drop in the impact strength due to annealing at 475 ° C. for one hour.

In FIG. 2, the hot formability determined by duplex alloys is compared with the respective nickel to manganese ratio value G by compression tests. In a hot upsetting test, cylindrical samples with a diameter of 12 mm and a height of 18 mm at a temperature of 1150 C were compressed in a press or percussion mechanism to a third of the initial height and the side surface formed by the free spreading was examined for cracks. In these experiments, mesh-like cracks usually develop on poorly deformable materials.

As can be seen from the diagram in FIG. 2, the hot-formability of the material drops sharply from a ratio G of 4, that is to say above the range according to the invention.

Fig. 3 shows the influence of the sulfur and manganese content of duplex alloys on the pitting corrosion potential in synthetic sea water, aerated at a temperature of 80 ° C. The diagram shows that, according to the invention, sulfur contents of less than 0.005% by weight are required to achieve a high resistance of the material to pitting corrosion, the manganese content essentially increasing the nitrogen solubility of the melt or the nitrogen content of the alloy, which improves the corrosion resistance is effected.

The diagram in FIG. 4 shows the corrosion behavior of duplex alloys according to the invention in phosphoric acid as a function of the temperature on the basis of isocorrosion lines a, b and c.

The cutting behavior of ferrite-austenite materials is shown in the diagram in FIG. 5. Cutting behavior is checked by drilling when measuring the total drilling depth and / or by turning when determining the machined volume. Alloys according to the invention with a ratio G (nickel content to manganese content) of 2 to 4 and a calcium content of approx. 0.006% by weight have good machining behavior, whereas comparison alloys, in particular those with low manganese concentrations or high G values, are difficult to machine.

Claims (7)

High-strength, easy-to-weld duplex alloy essentially containing the alloy components C, Si, Mn, Cr, Mo, Ni, W, N and V with excellent resistance to corrosion, in particular general or surface-removing corrosion, pitting and crevice corrosion as well as stress and vibration crack corrosion , in media containing chloride and phosphoric acid and in the heat-treated state with a material strength RM of at least 750 MPa, a 0.2 proof stress RP _0.2 of at least 550 MPa and a Charpy-V toughness of at least 100 joules with good cutting properties, characterized in that that after a heat treatment by forced cooling from a temperature between 1020'C and 1150 ° C ferrite and austenite with a ratio of 40 to 60% in% by weight

The rest contains iron and production-related impurities with the proviso that at a ratio G of the nickel content in% by weight to the manganese content in% by weight of greater than 2.0 but less than 4.0, the structural phase factor P is formed

[2.9 x (% Cr) + 2.9 x (% Mo) + 1.4 x (% W) +4.4 x (% Si) - 2-1 (% Ni) - 1.0 x ( % Mn) - 62.5 x (% N)] has a value greater than 40 but less than 65. Duplex alloy according to Claim 1, characterized in that it is in% by weight

contains Duplex alloy according to Claim 1 or 2, characterized in that it is in% by weight
Calcium 0.001 to 0.015
contains. Duplex alloy according to one of Claims 1 to 3, characterized in that it is in% by weight
Aluminum max 0.025
contains. Duplex alloy according to one of Claims 1 to 4, characterized in that the ratio G is greater than 2.2 but less than 3.5. Duplex alloy according to one of Claims 1 to 5, characterized in that the structural phase factor P has a value of greater than 45 but less than 59. Duplex alloy according to one of Claims 1 to 6, characterized in that it is heat-treated by cooling from a temperature between 1050 ° C and 1100 ° C using water or an inert gas, e.g. hydrogen.

Images