DE1558635B2 - High-strength, stable austenitic corrosion-resistant steel for the production of evaporator tubes and superheater tubes - Google Patents

Description

The invention relates to a stable austenitic corrosion-resistant steel which is used as a material for Manufacture of evaporator tubes that are used at high temperatures and pressures, suitable is, especially for superheater tubes for evaporators with supercritical pressure.

The following requirements are generally made of materials that are used to manufacture such pipes to deliver:

1. High creep rupture strength;

2. high structural stability even with long-term stress;

3. good weldability;

4. good corrosion and oxidation resistance;

5. good deformability.

Stable austenitic corrosion resistant steels according to the present invention contain:

0.01 until 0.20% Carbon, ο, ι until 1.0% Silicon, ο, ι until 4.0% Manganese, 0.05 until 0.15% Phosphorus, 12.0 until 18.0% Chrome, 12.0 until 20.0% Nickel, 0.5 until 2.5% Molybdenum, 0.1 until 2.0% Niobium, 0.001 until 0.05% Boron,

Remainder iron with melting-related
impurities other than nitrogen.

VerDie known austenitic heat-resistant heat-resistant materials that were previously used for the manufacture of evaporators Steels include the following alloy groups:

1. 18-8 Cr-Ni steels;

2. steels the Group "X Carbon, 0.12% the following guidelines following Ί : 8 Cr Ni Mo V Nb 16 13 Silicon, 0.75% until Composition: Manganese, 0.50% Carbon, 0.30 to 0.10% Chrome, 0.02% Manganese, 1.0 to 0.60% Nickel, 0.015% Silicon, 15.5 to 1.5% Molybdenum, 16.0% Phosphorus, 12.5 to 17.5% Vanadium, 14.0% Sulfur, 1.1 to 14.5% Nitrogen, 2.5% Chrome, 0.60 to 1.5% )% C, but less than 10% C + 0.4, 3.0% Nickel, 0.85% , 2% niobium 0.45% Molybdenum, more than IC 0.10% 3. 17-14 Cu-Mo steels 0.25% Copper, maximum 1 setting: rest Niobium, Titanium, Iron.

maximum until 0.75% Silicon, until 0.15% Carbon, 14.75 until 16.50% Chrome, until 1.0% Silicon, 13.50 until 16.50% Nickel, until 7.0% Manganese, 1.25 until 1.85% Molybdenum, until 11.0% Nickel, 1.00 until 1.85% Tungsten, until 16.0% Chrome, 0.80 maximum 1.30% Niobium + tantalum, until 1.2% Molybdenum, rest 0.15% Nitrogen, until 0.40% Vanadium, Iron. until 1.25% Niobium, Cr-Ni-Mn steels as follows 0.009% ι boron, maximum Iron. 0.2 5.5 9.0 14.0 0.8 0.15 0.75 0.003 rest

4. 15-15 N steels of the following composition:

maximum 0.15% carbon,

maximum 2.0% manganese,

maximum 0.030% sulfur,

maximum 0.040% phosphorus,

. Other steels were also considered useful for the stated purpose. The known steels however, all of them have lower creep resistance at high temperatures than those composites of the present invention Steels. Other known special steels achieve high heat strengths, contained however, expensive alloying elements such as cobalt, etc. in relatively large quantities, thereby reducing the Costs will be increased sharply.

In another austenitic steel known for the production of pipes, the high temperature strength is increased by precipitation hardening with the addition of phosphorus, and the toughness and creep strength (Creep breaking strength) at high temperatures is further improved by adding boron. Such steels contain considerable amounts of precipitates, such as carbides and nitrides, as the Precipitation hardening is achieved through additives, in particular nitrogen, to the steel. It is undesirable To use such steels as a material for evaporator tubes because of their corrosion resistance is significantly reduced. In addition, such steels have poorer machinability during manufacture of pipes and because of their thickness have poor bending and welding properties, so that they even if they have desirable properties in view of the use of some pipes should not be used for the production of evaporator tubes. It therefore arises the task of specifying a steel for the manufacture of evaporator tubes that meets the numerous above specified requirements and which also has an improved creep resistance.

In many years of study the inventors

found that it is possible to use an alloy steel with a purely austenitic stable structure and lower Tendency to produce precipitates even with long-term exposure to high temperatures, the high creep resistance and low price combined.

The steel according to the invention is made of a heat-resistant austenitic steel, which the elements Contains chromium, nickel and molybdenum, as well as some other additives that are listed below, obtained by adding an amount of niobium tailored to the carbon content, which

Prevents the separation of chromium carbide, which makes the base steel mainly in its resistance against intergranular corrosion and its heat resistance is improved. Another sizable one The high-temperature strength of the steel alloyed with niobium is increased by adding very small amounts of boron Amounts achieved along with such amounts of phosphorus that increase the strength of the matrix cause.

From the above it can be seen that the steel according to the invention is distinguished by that it receives its high heat resistance without the precipitates that are required for this purpose in the known steels must be effected.

F i g. 1 of the drawing shows resistance curves of steels of the invention and of steels known Composition;

F i g. Fig. 2 shows the influence of each alloying element, which together with boron make up the starting steel was added on the creep rupture strength;

F i g. 3 shows the structure of steels assembled according to the invention in comparison with known ones and "similarly composed" steels magnified 500 times.

The steels according to the invention can be based on the tests described below and the the results obtained can be better understood.

The steels A and B according to the invention listed in Table 1 and the known steels C, D, E, F and G were tested at 700 ^° C. in order to determine the relationship between the applied load and the time that elapsed until breakage. The results are shown in FIG. 1 shown graphically. In addition, the breaking strength values of all steels listed in Table 1 are given in Table 2 after they have been annealed ^{at 700 ° C. for 10,000 hours.}

Tabel

stole Ele
ment C. Si Mn P. Cr Ni Mon Nb B. V Zr W. Co Remarks Invention A. 0.02 0.59 2.80 0.092 13.89 13.72 1.56 0.45 0.008 appropriate
Steels B. 0.08 0.50 2.72 0.093 15.74 14.02 1.62 1.17 0.008 1 0.12 0.51 1.46 0.144 13.63 16.47 1.47 0.94 0.008 / 0.008 to 0.012% B.
\ 0.05 to 0.15% P 2 0.08 0.55 1.43 0.091 13.50 16.21 1.38 0.98 0.015 / More than 0.012% B
\ 0.05 to 0.15% P 3 0.09 0.52 1.45 0.132 13.48 16.35 1.42 1.03 0.014 / More than 0.012% B
\ 0.12 to 0.15% P 4th 0.12 0.53 1.41 0.081 13.34 16.10 1.35 1.61 0.006 0.10 to 0.20% C 5 0.10 0.55 1.45 0.094 13.80 16.50 2.10 0.87 0.008 2.0 to 2.5% Mo 6th 0.12 0.50 1.47 0.088 13.75 16.62 1.35 1.10 0.005 “More like that
Stole" C. 0.08 0.74 2.87 0.010 19.67 14.89 1.53 0.95 0.014 Acquaintance
Steels D. 0.10 0.50 6.00 - 15.0 10.0 1.0 1.0 0.007
0.009 0.25 E. 0.06 0.71 1.74 0.027 17.35 13.26 2.41 - - - - 0.03 F. 0.12 0.47 1.45 0.014 20.50 17.16 2.70 1.05 - - - 0.12 2.88 19.12 G 0.13 0.48 1.21 0.148 18.05 12.03 2.05 - 0.048 - - 0.06

Table 2

QtQ St. Breaking strength in kg / mm ² OLaXIl after 10,000 hours at 700 ° C A. 16.2 B. 16.0 1 16.0 2 15.8 3 16.3 4th 14.8 5 15.8 6th 15.0 D. 10.2 E. 6.5 F. 12.1

From Fig. 1 and Table 2 it can be seen that the steels composed according to the invention have a significantly higher creep strength of at least 14.8 kg / mm ² after treatment for 10,000 hours at 700 ° C. than the known steels.
In addition to the notch impact values for the steels A and B composed according to the invention and for the known steels C and D, as listed in Table 1 and which have been obtained after simple solution annealing, also the values for the same steels after this following the solution heat treatment, they were heated ^{to 750 ° C. for 3000 hours and then quenched in water.} The notch impact values listed in Table 3 represent a criterion for the structural stability that can be expected with the later long-term loading of the steel. They indicate that the invention. Chairs have superior stability properties in terms of both temperature and exposure time.

Table 3

stole

A.
B.
C.
D.

Impact value (kg-m / cm ² )

solution annealed

24.9
19.2
20.8
15.2

solution annealed
+ 3000 hours 700 ° C / water

10.1
8.1
0.4
6.0

The following are the reasons why the amounts of in the production of the Additional alloy components added to steels according to the invention to those specified in each case Limits need to be constrained.

F i g. 2 shows the influence of the elements Zr, V, Mn, P or N, when these are added together with boron to the Cr-Ni-Mo-Nb steel used as the starting steel, on the creep rupture strength of the starting steel containing boron. The abscissa of this figure shows the contents of the elements added to the starting steel together with boron, and the ordinate shows the ratio of the time it takes to break at 700 ° C (load 18 kg / mm ² ) for the boron-containing steel Base steel with one of the added elements applied at the break time for the boron-containing base steel without further additives.

From Fig. Figure 2 shows that phosphorus is the only alloying element that has creep rupture strength when it is added to the base steel together with boron. The influence of the simultaneously im Steel elements present phosphorus and boron is great. The remaining elements examined show the opposite effect. They lower the creep strength of the steel. This is especially true with nitrogen the case. The steels according to the invention are therefore essentially characterized in that they have none Contain nitrogen.

Boron has an especially increasing effect on the creep strength of the starting steel when it is very is present in small quantities. However, if the amounts fall below 0.001%, the effect diminishes. Larger amounts than 0.05% deteriorate the weldability. This means that the addition of Boron in the last-mentioned amounts is not desirable. It should also be noted that the addition of boron in amounts smaller than 0.005% have little effect the increase in high-temperature toughness and that already amounts to more than 0.012% improve the weldability worsen slightly.

Phosphorus is mainly contained in the matrix in the form of a solid solution, whereby it is the Strength of the base mass increased. The effectiveness of the phosphorus is, however, less remarkable if the content is below 0.05%. Phosphorus contents above 0.15% increase the tendency of the chromium carbide to precipitate, destroy the plastic deformability, increase the difficulties in pipe production and deteriorate the flexibility of the steel. Even set contents of more than 0.12% phosphorus already reduces the steel's resistance to stress corrosion.

Carbon influences the steel properties through the formation of carbides. Steels with high carbon contents are at the place where high short-term tensile strengths are required. Also, carbon increases that Carbide precipitates, which embrittles the steel and lowers the corrosion resistance, if the C content is higher than 0.20%. A C low-carbon steel is therefore desirable, if a highly stable steel with a low tendency to precipitate carbide is required over a long period of time, The above mentioned low-carbon steels, the C-contents of which are below 0.01%, are only unsatisfactory

ίο have strength properties and are difficult to manufacture. Carbon additions of more than 0.1% result in steels that tend to precipitate carbides and have an unstable structure.
Niobium not only has a preventive effect with regard to the precipitation of chromium carbide, but it also greatly increases the high temperature strength (high temperature strength) of the steel. The contents to be added depend on the carbon content of the base material, and they should be approximately equal to 10 times the carbon content, i.e. 0.1 to 2.0% for the steel according to the invention. It should be noted, however, that niobium additions of more than 1.5% cause the formation of intermetallic compounds, which makes the structure unstable.

Molybdenum increases the strength as an atom dissolved in the matrix. However, it has less of an effect Measure if it is less than 0.5%, and it promotes ferrite formation and forms intermetallic compounds with the other elements, which the steel with long-term heating become brittle at higher temperatures, provided that the element is present in a content of more than 2.5%. Such Embrittlement is of course undesirable. It should also be noted that even with Mo levels over 2.0% promoted the formation of intermetallic compounds and the development of an unstable structure is favored.

An addition of more than 16% nickel does not significantly increase the heat resistance of the steel, at least not to the extent that would be desirable in view of the increased costs caused by the addition. Contents of less than 0.01% Zr and less than 0.1% V are in view of increasing the high temperature strength and toughness only ineffective. In view of the increasing production costs nothing is gained by adding more than 0.05% Zr and more than 0.5% V either.

In Fig. 3 are photomicrographs (magnification 500facli) of the microstructure of the steels A and B and of similar composite steel C, as listed in Table 1, reproduced, namely according to the following treatment: solution treatment 1 hour 1 200 ⁰ C, then 3000 hours 750 ° C and quenching in water. In this figure, numerals (1), (2) and (3) represent steels A, B and C.

As already emphasized, it is necessary that steels for the manufacture of evaporator tubes are stable Have a structure whose toughness is not even with long-term use at high temperatures sinks. As will be shown below, the steel according to the invention fully meets this requirement.

From the images (1) and (2) of FIG. 3 shows that the structure of the steel according to the invention after long-term annealing hardly differs from that before long-term annealing. This proves that the structure is largely stable. The one shown in Figure (1) of FIG. 3 shown steel A shows with its C content of 0.02% has very little precipitations, and it shows a satisfactory toughness, like the notch

impact value of 8 to 10 kg-m / cm ² in Table 3 shows. On the other hand, the similarly composed steel C - shown in Figure (3) of FIG. 3 - a structure in which a large amount of intermetallic compounds are precipitated at the grain boundaries and within the grain. This steel shows a remarkable drop in toughness, which is expressed in an impact value of less than 1 kg-m / cm ² , as can be seen from Table 3. It is obvious that steel C, which had a purely austenitic structure in the solution-annealed state, was almost completely converted into ferrite due to the precipitation of intermetallic compounds such as the sigma phase during long-term annealing because of its content of 20% Cr and 14% Ni .

The steel according to the invention should therefore chromium in amounts of 12 to 18% and nickel in amounts of 12 to 20% to ensure that it has adequate corrosion resistance, oxidation resistance (scale resistance) and unchanged Has toughness even after long-term use. The steel should also contain silicon as a deoxidizer in amounts of 0.1 to 1.0%. These Quantities can be added without loss of strength.

The steel can also contain manganese in amounts of 0.1 to 4.0%. Manganese works like silicon as a deoxidizer and improves the machinability of the steel. In this context, should It is mentioned that manganese contents greater than 4.0% should be avoided as they reduce the strength of the Noticeably lower Stahls.

As already emphasized in the introduction, the inventive Steel that is not only used as a material for the manufacture of evaporator tubes can, but generally as a material with high creep resistance and high corrosion resistance at elevated temperatures, without the use of expensive alloying elements with relatively can be generated at low cost. The use of this steel makes it possible to use evaporator tubes smaller wall thickness and therefore lower weight and lower price, and there can be evaporators that work at higher temperatures and higher pressures. This becomes essential contribute to the spread of evaporators with supercritical pressure.

Claims (9)

Patent claims: 1. Stable austenitic corrosion-resistant steel with high heat resistance and high fatigue strength at high temperatures, especially for the production of evaporator and superheater tubes, characterized by the following composition: 0.001 to 0.1 to 0.1 to 0.05 to 0.001 to
12.0 to 18.0%
12.0 to 20.0% 0.5 to 2.5% 0.1 to 2.0% 0.20% carbon,
1.0% silicon,
4.0% manganese,
0.15% phosphorus,
0.05% boron, Chrome, Nickel, Molybdenum, Niobium, The remainder is iron with impurities from the melting process. 2. Steel according to claim 1, characterized in that the boron content is 0.005 to 0.012% amounts to. 3. Steel according to claim 1, characterized in that the phosphorus content is 0.05 to 0.12% amounts to. 4. Steel according to claim 1, characterized in that the niobium content is 0.1 to 1.5%. 5. Steel according to claim 1, characterized in that the phosphorus content is 0.05 to 0.12%, the boron content is 0.005 to 0.012% and the niobium content is 0.1 to 1.5%. 6. Steel according to at least one of the preceding claims, characterized in that the carbon content is 0.01 to 0.1%. 7. Steel according to claim 5, characterized in that the carbon content is 0.01 to 0.1%, the phosphorus content 0.05 to 0.12%, the nickel content 12.0 to 16.0% and the molybdenum content Is 0.5 to 2.5%. 8. Steel according to at least one of the preceding claims, characterized in that that in addition 0.01 to 0.05% zirconium is present. 9. Steel according to at least one of the preceding claims, characterized in that that an additional 0.1 to 0.5% vanadium is present. 1 sheet of drawings 009 525/194