EP0956371A1

EP0956371A1 - Austenitic nickel-chromium-molybdenum-silicon alloy with high corrosion resistance to hot chloride-containing gases and chloride

Info

Publication number: EP0956371A1
Application number: EP97951980A
Authority: EP
Inventors: Jutta KLÖWER; Ulrich Heubner
Original assignee: Krupp VDM GmbH
Current assignee: VDM Metals GmbH
Priority date: 1997-01-29
Filing date: 1997-11-26
Publication date: 1999-11-17
Also published as: DE19703035A1; WO1998032887A1; JP2001509210A; CA2279294A1; DE19703035C2

Abstract

The invention relates to an austenitic nickel-chromium-molybdenum-silicon alloy with a markedly improved resistance to the type of hot chlorine-containing gases and chlorides found, for example, in chemical plants, thermal waste-disposal facilities, the exhaust systems of cars and in diesel engines, consisting of: Cr 18-22 %; Mo 6-10 %; Si 0.6-1.7 %; C 0.002-0.05 %; Fe 1-5 %; Mn 0.05-0.5 %; Al 0.1-0.5 %; Ti 0.1-0.5 %; Mg 0.005-0.05 %; Ca 0.001-0.01 %; V max. 0.5; P max. 0.02 %; S max. 0.01 %; B 0.001-0.01; Cu max. 0.5; Co max. 1 %; Hf, Y, Zr and/or rare earth elements 0.02-0.5 %; and the remainder nickel and impurities caused by the melting process.

Description

Austenitic nickel-chromium-molybdenum-silicon alloy with high corrosion resistance against hot chlorine-containing gases and chlorides

The invention relates to an enitic nickel-chromium-molybdenum alloy with additions of silicon.

In plants and units in which hot chlorine-containing gases and chloride-containing deposits occur (plants in the chemical industry, plants for thermal waste disposal, in particular when recycling special waste, plants for recycling biomass, large diesel engines, exhaust systems of automobiles), temperatures of up to 400 ° C ferritic boiler steels are used. At higher temperatures, Nikkei-chromium-molybdenum alloys with 21.5% chromium, 9% molybdenum, 3.7% niobium, 2.5% iron, the rest nickel and inevitable impurities (German material number 2.4856) are used (steel key 1995) .

However, the alloy with the material number 2.4856 is difficult to process. In addition, this alloy suffers a considerable loss of ductility at temperatures above 500 ° C., which can lead to crack formation in components that are pressurized and / or subject to high mechanical stress. To a certain extent, the start of excretion of the ductility-reducing excretions can be delayed by lowering the iron content.

Measures which lead to a significant increase in ductility are described in international patent application WO 95/31579 in which a new alloy is described based on the alloy according to material number 2.4856, which is characterized by increased cold and hot formability and higher ductility.

The new alloy described in this document also has disadvantages. The ductility-increasing measures listed reduce the corrosion resistance against gases containing high chlorine and chloride-containing deposits compared to the alloy with material number 2.4856. Even with this alloy, high corrosion rates occur due to the constantly increasing process and exhaust gas temperatures due to the increase in efficiency. Alloys of the type 2.4856 are also susceptible to hot corrosion due to sulphate-containing deposits, so that there is a considerable need for a different type of alloy with improved resistance to high-temperature corrosion.

The invention has for its object to develop an alloy with a significantly improved resistance to chlorine gas corrosion and chloride-containing deposits compared to the prior art with simultaneous increased resistance to sulphate corrosion and with high ductility in the entire temperature range up to 1000 ° C. The task is solved by a silicon-containing nickel-chromium-molybdenum alloy, which (in mass%) consists of the following components:

Cr 18-22

Mon 6-10

Si 0.6 1.7

C 0.002 - 0.05

Fe 1-5

Mn 0.05-0.5

AI o, 1 0.5

Ti o, 1 0.5

Mg 005-0.05

Ca 001-0.01

V max. 0.5

P max. 0.02

S max. 0.01

Cu max. 0.5

Co max. 1

Hf and / or Y and / or Zr and / or rare earths

0.02 - 0.5% rest of nickel and melting-related impurities.

The alloy according to the invention is distinguished by a significantly increased high-temperature corrosion resistance against chlorine-containing gases, chloride-containing ashes, deposits and salt compounds with simultaneous corrosion resistance against sulphate corrosion and wet corrosion as well as high ductility.

Advantageous further developments of the subject matter of the invention can be found in the subclaims. A preferred alloy is characterized by the following alloy components (in% by mass):

Cr 18 - 20%

Mon 8 - 9.0 o

Si 0.7 - 1.1 o

C 0.02 - 0.015 o,

O

Fe 2.5 - 3.5

Mn 0.05 - 0.1 o

AI 0.1 - 0.3%

Ti 0.1-1.4 o

Mg 0.005-0.015 o

Ca 0.001-0.005 Q, o

V max. 0.01 o

P max. 0.002 g, o

S max. . 0.001 o

B 0.001-0.001 g, o

Cu max. , 0.5%

Hf and / or Y and / or Zr and / or rare earths

0.03 - 0.06 o

Remainder nickel and melting impurities.

The alloy is advantageously suitable, on the one hand, for the production of pipes, in particular composite tubes, sheets, strip material, foils, wires and objects made from these semi-finished products, and on the other hand, corrosion protection applied as cladding or plating.

The advantageous properties of the alloy according to the invention are evident from the following exemplary embodiments. Table 1 shows, by way of example, analyzes of batches from the alloy (AF) according to the invention and the comparison alloys (G, H) lying outside the composition according to the invention. Alloy 2.4856 was used for comparison. All alloy variants were made from cast blocks by hot rolling followed by cold rolling at room temperature. The resistance of the alloy according to the invention to chloride corrosion can be seen from Figures 1 and 2. For the experiments, ground and cleaned test coupons of various test alloys were immersed in an aqueous solution of 1 mol / 1 NaCl, 0.1 mol / 1 CaCl ₂ and 0.25 mol / 1 NaHC0 ₃ , dried at 60 ° C and then at 750 ° C exposed to air over 240 hours. This test simulates the stresses that occur, for example, in exhaust systems of automotive engines (on bellows for decoupling the catalytic converter and engine). Figure 1 shows the metal removal, Figure 2 shows the metallographically determined corrosion attack at the end of the test. In the course of the investigations, it has surprisingly been found that the resistance to chloride corrosion, for example in relation to the alloy 2.4856, could be considerably improved by adding silicon in amounts between 0.6 and 1.7%.

The advantageous influence of silicon can also be seen in Figure 3, which shows the metallographically determined corrosion attack of samples which are in a complex medium (chlorine-containing synthetic waste combustion gas (2.5 g / m ³ HC1, 1.3 g / m ³ S0 ₂ / 9% 0 ₂ , remainder N ₂ ) with simultaneous exposure to boiler ash containing chloride) were stored at 600 ° C for 1000 hours. Compared to the low-silicon batch (Example G), the silicon-containing alloy according to the invention shows a significantly reduced corrosion attack.

Figure 4 shows corrosion attack after 1008 hours of cyclical aging of samples which were coated with a coating of Na ₂ S0 ₄ / KCl before being aged at 750 ° C in an atmosphere containing chlorine and sulfur dioxide. This test is used to test the resistance to sulphate corrosion. How as can be seen from the figure, the alloy according to the invention also shows significantly lower corrosion rates than the alloy 2.4856 currently used under such corrosion conditions with this corrosion stress.

The excellent properties of the alloy according to the invention can be attributed to the silicon additives and the coordination of the alloy elements molybdenum, chromium and iron. The silicon content of the alloy according to the invention should be between 0.6% and 1.7%, since at lower silicon contents the corrosion-inhibiting effect of the silicon no longer occurs and at higher silicon contents with the appearance of embrittling suicides and significant loss of ductility, particularly in the case of medium-sized ones Temperatures (500-800 ° C), can be expected. With silicon contents between 0.5 and 1.7%, the notched impact strength, measured on ISO V notched impact samples, does not drop below 100 J / cm, even after aging for 100 hours at 600 ° C, as shown in Figure 5.

The molybdenum content of the alloy according to the invention is limited to 10%, since, as can be seen from FIG. 4, the susceptibility to sulphate corrosion increases with higher molybdenum contents. A minimum molybdenum content is required to avoid wet corrosion in the event of a drop below the dew point.

The chromium content of the alloy according to the invention should be between 18% and 22% in order to ensure adequate corrosion resistance. Higher chromium contents make it difficult to process nickel-chromium-molybdenum alloys. The alloy should also contain hafnium and / or rare earths and / or zirconium and / or yttrium, if an improved adhesion of protective oxide layers is required for specific applications, for example in automotive exhaust systems at high temperatures and / or with rapid temperature changes. However, the sum of these reactive elements should not exceed 0.5%.

The iron content of the alloy according to the invention is limited to a maximum of 5%, since at higher iron contents in chloride-containing media there is a risk of the formation of volatile iron chlorides. However, a minimum iron content of 1% is required to ensure the processability of the alloy.

The carbon content of the alloy according to the invention is limited to a maximum of 0.05% since there is a risk of intergranular corrosion at higher carbon contents.

The titanium and aluminum contents are each reduced to a maximum of 0.5%, the actually undesirable niobium content to a maximum of. 0.5% is limited, since these elements can lead to a loss of ductility due to the formation of intermetallic phases at medium temperatures. The total addition of niobium, aluminum and titanium should not exceed 1%. However, a minimum content of the oxygen-affine elements aluminum, titanium, magnesium and calcium is required to ensure good oxidation resistance. For processing reasons, the manganese content should be at least 0.05%, but not exceed 0.5%, since higher manganese contents have an unfavorable effect on the resistance to oxidation. 0.001-0.01% boron is also added to improve processability. The levels of phosphorus and sulfur should be kept as low as possible since these surface-active elements reduce both the high-temperature corrosion resistance and the ductility of the alloy.

The alloy according to the invention can be used for strips, foils, sheets, tubes (seamless or welded), wires, as cladding, as cladding or as a composite tube.

The alloy according to the invention can be produced either by block casting or by continuous casting after melting in a vacuum induction furnace or after open melting. The alloy can be remelted, but is not absolutely necessary. The hot shaping is carried out by forging, hot rolling or extrusion, the cold shaping by cold rolling, wire drawing or pilgrims. The production of composite materials, for example plating on carbon steels, can be carried out by one of the customary build-up welding processes, by cold or hot rolling of sheets and strips, by explosive plating or by one of the customary processes for producing bimetallic tubes.

Because of its excellent resistance to chlorination, the alloy is particularly suitable as a strip and sheet, pipe or cladding material for use in hot chlorine-containing gases or in the presence of chloride-containing deposits, such as in plants in the chemical industry, in plants for the thermal treatment of chlorine-containing chemical waste and contaminated soils as well as in automotive exhaust systems (bellows for decoupling the catalytic converter and engine). The excellent resistance of the alloy to complex corrosive salt deposits (Boiler ash) makes the alloy also suitable for use as a cladding and construction material in plants for thermal waste disposal, in large diesel engines, in plants for energy generation from biomass and in plants of the pulp industry.

Claims

claims

1. Austenitic nickel-chromium-molybdenum alloy with addition of silicon characterized by the alloy components (in mass%)

Cr 18 22 o

Mo 6 10 g,

0

Si 0.6 1.7 g, o

C 0.002 - 0.05 g,

0

Fe 1 5 g, "S

Mn 0.05 - 0.5%

AI 0.1 0.5 g,

0

Ti 0.1 0.5%

Mg 0.005-0.05 g, o

Ca 0.001-0.01 g. O

V max. , 0.5 o

P max. 0.02 g,

0

S max. . 0.01 g, o

B 0.001 - 0.01 g, o

Cu max. 0.5 g

0

Co max. . 1 g, o

Hf and / or Y and / or Zr and / or rare earths

0.02 - 0.5 g, o

Remainder nickel and melting-related impurities,

2. Alloy according to claim 1, characterized by the alloy components (in% by mass)

Cr 18 - 20 g, o

Mo 8 - 9.0%

Si 0.7 - 1.1 g,

0

C 0.02 - 0.015%

Fe 2.5 - 3.5 g, o

Mn 0.05-0.1 g, o

AI 0.1 - 0.3 g. ^* o

Ti 0.1 - 0.4 o

Mg 0.005 - 0.015 g "o

Approx. 0.001 - 0.005

V max. 0.01 g, o

P max. 0.002 g. O

S max. 0.001%

B 0.001-0.001 g, o

Cu ma. 0.5 g, o

Hf and / or Y and / or Zr and / or rare earths

0.03 - 0.06%

Remainder nickel and melting impurities

3. Alloy according to claim 1, characterized by a molybdenum content between 6.5 and 9.5%.

4. Alloy according to claim 1, characterized by a silicon content between 0.6 and 1.3%.

5. Use of the alloys according to one of claims 1 to 4 for the production of tubes, sheets, strip material, foils, wires and objects made from these semi-finished products.

6. Use of the alloy according to one of claims 1 to 4 for the production of composite pipes.

7. Use of the alloy according to one of claims 1 to 4 as corrosion protection applied by cladding or plating.