DE3837456C1 - Use of a fully austenitic steel for components which are severely stressed corrosion-chemically and mechanically - Google Patents

Abstract

The invention relates to the use of a fully austenitic steel with at most 0.04% of C, 0-0.69% of Si, 5.4-8.9% of Mn, at most 0.01% of S, 15-30% of Cr, 10.1-24.9% of Ni, 2.01-7% of Mo and 0.31-0.8% of N, the remainder being Fe including usual impurities, with a 0.1 yield stress of at least 350 N/mm<2>, as a material for components, which are severely stressed corrosion-chemically and mechanically, of flue gas purification plants.

Description

The invention relates to a fully austenitic steel as a material for parts of flue gas cleaning systems subject to high levels of corrosion and mechanical stress, Wastewater treatment plants and facilities for Conveying, transporting and storing contaminated sludge. Under "sewage Processing plants "are also understood to include those in which sludge be processed.

In addition to high mechanical loads, parts of such systems and Means very often different acids or mixtures of different Exposed to acids. It is therefore very important that one for such parts of suitable material against the widest possible spectrum is resistant to a wide variety of media. Mechanical and Materials that are highly resistant to corrosion chemicals can also be processed easily be d. that is, they must be easily deformable and weldable with high ultimate strength and finally be inexpensive.

So far come as materials for mechanical and corrosion chemical strong stressed parts of the plant construction once austenitic and ferritic austenitic steels and other nickel-based alloys. The mechanical strength of austenitic steels is different for a large number of Applications insufficient. A disadvantage of ferritic-austenitic Steel is its unfavorable processing behavior, especially when it comes to Hot forming and welding, and their sometimes insufficient corrosion resistance. Nickel based alloys are too expensive for many applications.

The invention is based on the object, one for the above The purpose of finding suitable material that has a high 0.2 proof stress at room temperature has toughness over a wide temperature range remains and which combines high durability with one  mechanical and corrosion-chemical stress shows. So the material should resistant to a variety of different corrosive media. He should easy to weld, good hot formable and inexpensive. In technology For example, steels or alloys are required that are not only corrosion-resistant, but because of the high mechanical stress must also be high-strength and in particular also wear-resistant. The Resistance to wear can be achieved on the one hand through higher strength increase, on the other hand, it is also due to the structure and Alloy composition influenced.

According to the invention, a fully austenitic steel is used to achieve this object
Max. 0.04% C
0 to 0.69% Si
5.4 to 8.9% Mn
Max. 0.01% S
15 to 30% Cr
10.1 to 24.9% Ni
2.01 to 7% Mo
0.31 to 0.8% N
Balance Fe including usual impurities
proposed for parts of flue gas cleaning systems, sewage and sludge treatment systems and devices for conveying, transporting and storing contaminated sludge which are subject to high levels of corrosion and mechanical stress. Such a steel has not only proven to be extremely suitable in a corrosive manner, but also has very good mechanical strength with a 0.2 proof stress of at least 350 N / mm².

Such a composition is already from the US-PS 38 54 937 known.

Preferred compositions of the steel within the above range are specified in the subclaims.

In an austenitic material, the manganese content increases Stack error energy lowered, as a result of which, more stack errors formed, and a strong occurs under mechanical stress  Consolidation in the highly stressed zone. The steel protects itself itself, so to speak. The greater the stress, the stronger it becomes Material solidified in the stressed zone. Enter at the same time mechanical a corrosive stress occurs as a damage mechanism often the so-called erosion corrosion. On this mechanism is based on the wear resistance of the material to be used according to the invention Steel.

To improve the resistance to sulfuric acid, the invention can steel to be used 0.2 to 1.0% Cu are added.

With a steel to be used according to the invention with
0.025% C
0.14% Si
6.90% Mn
0.007% P
0.005% S
23.7% Cr
4.47% Mo
17.61% Ni
0.63% Cu
0.46% N
Rest of Fe

and a 0.2 proof stress of 395 N / mm² was under simulated flue gas conditions found that this steel is better by a factor of 2 Corrosion behavior showed as well-known high-alloyed austenitic steels, like the X1 NiCrMoCu 25 20 5, material no. 1.4539 and the X1 NiCrMoCu 31 27 4; Material number. 1.4563. These materials are used alongside others in flue gas desulfurization plants, see VDI reports No. 674, 1988, Pages 41 to 63. Erosion corrosion occurs mainly in flowing corrosive Media, especially if they contain solids.  

Should the 0.2-proof limit of the steel to be used according to the invention at least 450 N / mm² are raised, the addition of 0.01 to 1.0% Nb and / or V proposed.

Compliance with the additional condition in claim 9 regarding the ratio of chromium to nickel equivalent ensures a significant improvement in weldability. In this regard, reference is made to FIG. 1, which shows the critical pitting temperature as a function of the effective sum% Cr + 3 (% Mo) +15 (% N) of austenitic stainless chromium-nickel-molybdenum steels with and without melting using a TIG welding torch in FIG. 10 % FeCl₃ solution according to ASTM G 48 shows. On the one hand, it can be seen from FIG. 1 that the critical pitting temperature is increased as the effective sum increases. In addition, the course of the lower curve shows that the critical pitting temperature in conventional steels drops significantly with increasing active sum after melting, z. B. at 40 of approx. 70 ° C to 35 ° C. In contrast, steels to be used according to the invention - three samples of alloy No. 14 listed in Table 1 were used - show a significantly smaller drop in the critical pitting temperature during melting. e.g. B. from 90 ° C to approx. 80 ° C. This is an indication of the particularly good weldability of the steels to be used according to the invention.

Table 1 contains the composition of 11 comparative alloys, namely alloys No. 1 to 11, and of nine alloys to be used according to the invention, namely alloys 12 to 20. Samples of alloys No. 1 to 20 were subjected to various corrosion tests. The results are shown graphically in FIGS. 2 to 5.

Fig. 2 shows the limit temperatures for the pitting resistance of various steels in three chloride-containing media, namely a 3% NaCl solution at 950 mV _H , in 10% FeCl₃ · 6 H₂O and finally in synthetic flue gas condensate (7 vol .-% H₂SO₄, 3 vol .-% HCl) after a test period of 24 h. Can be clearly seen from Fig. 2, the superiority of the present invention to be used in alloys Nos. 12, 14, 15, 17 and 18, in which the limit temperature to all three media containing chloride over 60 ° C was. Only the comparative steel 7 could roughly keep up here. The limit temperature for all other comparative alloys is 55 ° C and below.

Fig. 3 shows the limit temperatures for the crevice corrosion resistance of various steels when tested in 10% FeCl₃ · 6 H₂O (cleavage conditions according to the MTI regulation). The test duration was again 24 hours. From Fig. 3 is the superiority of the present invention to be used steels 13, 17 and 18 clearly visible. The comparative steel 7 and the comparative nickel alloy 11 behaved equally well.

Fig. 4 documented for alloy no. 15, the corrosion behavior in the Huey-test (65% nitric acid). Even after 15 cookings, there was no increase in the mass loss of the sample.

Figure 5 contains the results of testing samples in sulfuric acid. After a test period of 24 h, the mass loss rate was determined in g / h · m². The alloys to be used according to the invention were among the best.

The limits for carbon, chromium, nickel and molybdenum were first Line designed to ensure corrosion resistance. For fixing The limit values of manganese were the stabilization of the austenitic Structure and increasing the solubility for nitrogen in the foreground. A minimum quantity of 5.4% was required for this. Higher levels than 8.9% proved to be unfavorable in terms of processing technology. Nickel also increases the resistance to within the specified limits Stress corrosion cracking. Copper ensures corrosion resistance in the specified range in media containing sulfuric acid. Nitrogen in the specified amount increases the strength, austenite stability and corrosion resistance. A low silicon content leads to an increase in Structural stability and good resistance to nitric acid Media.  

By maintaining low sulfur levels, combined with a targeted Reshaping the remaining sulfur increases the corrosion resistance. Only through the sum of these alloying measures has it been possible to the corrosion behavior of the steel to be used according to the invention a wide range of corrosive media cheap. Adding niobium and / or vanadium in the specified amount increases the Strength of the steels. The addition of these two elements can result Precipitation hardening raised the yield strength by at least 100 N / mm² will. This increase in strength is, however, due to a loss in toughness and bought some deterioration in corrosion resistance. At Applications with heavy mechanical loads, such as in flue gas cleaning systems, or when erosion corrosion occurs, such as B. at the treatment of waste water or sludge, the addition of niobium and / or Vanadium lead to a considerable increase in tool life.  

Plate 1

Composition of the investigated materials

Claims (10)

1.Use a fully austenitic steel with
Max. 0.04% C
0 to 0.69% Si
5.4 to 8.9% Mn
Max. 0.01% S
15 to 30% Cr
10.1 to 24.9% Ni
2.01 to 7% Mo
0.31 to 0.8% N
Balance Fe including usual impurities
With a 0.2 proof stress of at least 350 N / mm² as a material for parts of flue gas cleaning systems subject to high levels of corrosion and mechanical stress. 2.Use a fully austenitic steel with
Max. 0.04% C
0 to 0.69% Si
5.4 to 8.9% Mn
Max. 0.01% S
15 to 30% Cr
10.1 to 24.9% Ni
2.01 to 7% Mo
0.31 to 0.8% N
Balance Fe including usual impurities
with a 0.2 proof strength of at least 350 N / mm² as a material for parts of wastewater treatment plants that are subject to high chemical and mechanical stress. 3.Use a fully austenitic steel with
Max. 0.04% C
0 to 0.69% Si
5.4 to 8.9% Mn
Max. 0.01% S
15 to 30% Cr
10.1 to 24.9% Ni
2.01 to 7% Mo
0.31 to 0.8% N
Balance Fe including usual impurities
With a 0.2 proof stress of at least 350 N / mm² as material for parts subject to high chemical and mechanical stresses for facilities for conveying, transporting and storing contaminated sludge. 4. Use of a fully austenitic steel of the composition according to claim 1, but with a carbon content of max. 0.03% for the purpose of claims 1 to 3. 5. Using a fully austenitic steel of the composition according to one of claims 1 to 3, but with a silicon content by Max. 0.2% for the purpose of claims 1 to 3. 6.Using a steel with
Max. 0.03% C
Max. 0.01% S
0 to 0.2% Si
5.4 to 8.9% Mn
20.1 to 26.9% Cr
16 to 20% Ni
3.1 to 4.9% Mo
0.31 to 0.6% N
Balance Fe including usual impurities
for the purpose of claims 1 to 3. 7. Use of a steel of the composition according to one of the claims 1 to 6, but with a chromium content of 23 to 25%, 16 to 18.5% Nickel and 2.8 to 4.6% molybdenum for the purpose of claims 1 to 3. 8. Use of a steel of the composition according to one of the claims 1 to 7, the additional still contains 0.2 to 1% Cu, for the purpose of claim 1 to 3. 9. Use of a steel of the composition according to one of the claims 1 to 8 with a minimum 0.2 proof stress increased to 450 N / mm², which also contains 0.01 to 1.0% Nb and / or V, for which Purpose according to claims 1 to 3. 10. Use of a steel of the composition according to any one of claims 1 to 9, the condition with Cr _equi =% Cr + 1.37 (% Mo) +1.5 (% Si) +2 (% Nb) +3 (% Ti)
and Ni _equi =% Ni + 0.31 (% Mn) +22 (% C) +14.2 (% N) + Cu
fulfilled, for the purpose of claims 1 to 3.