DE976034C - Procedure to prevent the creeping of long-term steel alloys with long-term stresses over 10,000 hours between 400 and 500 ° C - Google Patents

Description

Steam turbines, steam boilers and high-pressure systems are through after a long period of operation Warm embrittlement extensive damage occurred. About the probable cause is in the »notices of the Association of Large Boiler Owners «has been reported in detail (cf. 1937, No. 63, Pp. 235 to 237, and No. 65, pp. 385 to 396; 1941, No. 85, pp. 171 to 173; 1942, No. 86/87, pp. 14/15; 1943, No. 90/91, pp. 12 to 14).

After the initial failures, it was generally believed that the embrittlement was a result of the Nickel content. Therefore, nickel-free steels were developed and used.

The second measure was to increase the DVM fatigue strength in order to achieve this a higher safety factor is required. However, both measures do not have a remedy the hot embrittlement led.

In the case of hot embrittlement, it is not a question of tempering brittleness, not even hot embrittlement without load, but around the hot embrittlement with load:

Namely, tempering brittleness is usually understood a reduction in the notched impact strength, which occurs with slow furnace cooling of the tempering temperature versus a notched impact strength, which at a faster Cooling of the material is obtained (see Material Handbook Steel and Iron, 1937, Sheet Y / 20).

"Hot embrittlement without load" means the reduction in notched impact strength, which arises when specimens without load are in the temperature range of the embrittlement area for a long time annealed (see report Maurer, Wilms and Kießler in »Stahl und Eisen«, 62, 1942, P. 83, left column, first paragraph).

The temper brittleness does not play a role in the practice of creep-resistant steels, since it is fundamental There is a requirement that steels for heat-resistant objects must be free from temper brittleness. The reduction in notched impact strength in hot embrittlement without load is also for the practice is irrelevant as it does not allow any conclusions to be drawn about the behavior of the material in question under load. For this reason, reference is made to the "Archives for the iron and steel industry", 15, 1941/42, P. 40, right column, last paragraph.

For the present invention, the hot embrittlement with load is decisive; it is namely the damage phenomenon that over 400 ⁰ C, the technical usefulness of the concerned steel products for long term stress at temperatures in question and can lead to unforeseeable by deformation without breaks in its consequences damage. It is tested by examining both smooth and notched bars of the steel in question, which is in a certain tempered state.

On the basis of tests ("Archiv für das Eisenhüttenwesen", 1948, p. 46) in which only notched samples were tested, the assumption was made that a higher tempering temperature would reduce the creep strength, ie the load capacity in kg / mm ² up to Break, improve. The question of embrittlement was not dealt with.

While in the aforementioned investigation only notched samples because of the lack of comparison with corresponding smooth samples a statement about the embrittlement behavior of steel is impossible, other researchers ("Archiv für Metallkunde", 1949, p. 163) believe It was found that the creep rupture strength when loaded up to 5000 hours by a previous long-term starting would be reduced so much that there is no practical benefit from it could pull. These researchers therefore made the proposal to use components that were already loaded in practice Repeated tempering from time to time in order to regenerate the steel.

The invention is directed to the fact that the creep rupture strength is over 10,000 hours The higher load is independent of the hardening and tempering resistance, from the previous idea experts from the fact that one by a higher rated hardening resistance as a result of the application lower tempering temperatures have an advantage in terms of creep rupture strength would.

It is also based on the finding that the tendency to embrittlement with the generally common Steels containing Mo, CrMo and CrMoV, which are additionally can still contain up to 2% Ni, due to an excessively high hardening and tempering resistance is, and that the embrittlement is prevented if the steels after hardening at certain tempered at higher temperatures and thus determined the maximum tensile strengths Do not exceed maximum values.

According to the invention, in order to prevent embrittlement under long-term ^{stresses of over 10,000 hours at temperatures between 400 and 550 0} C, long-lasting steels which have the usual molybdenum content (at least about 0.25% Mo) and optionally chromium and vanadium in a total content of contain three elements up to about 3% and carbon from 0.20 to 0.45%, proceed in such a way that the steels after hardening from temperatures between about 900 and 950 ⁰ C at temperatures between about 700 and 750 ° C depending on the Steel alloy are tempered in such a way that the tensile strength at room temperature is reduced to previously unconventional low values, with the proviso that the level of the respective tempering temperature is selected according to the composition of the treated steel according to the following information:

Steels with about

0.20 to 0.28% carbon,

0.40 to 0.70% manganese,

0.15 to 0.35% silicon,

0.25 to 0.35% molybdenum

should be tempered in such a way that the strength (at room temperature) in the quenched and tempered condition is below 60 kg / mm ² ;

Steels with about

0.20 to 0.28% carbon, 0.40 to 0.70% manganese, 0.20 to 0.40% silicon, 0.80 to 1.10% chromium, 0.40 to 0.50% molybdenum

should be tempered in such a way that the strength (at room temperature) in the quenched and tempered condition is below 70 kg / mm ² ;

Steels with about

0.30 to 0.38 ¹ Vo carbon, 0.40 to 0.70 ⁰ zo manganese, 0.20 to 0.40% silicon, 0.80 to 1.10% chromium, 0.40 to 0.50% molybdenum

should be tempered in such a way that the strength (at room temperature) in the quenched and tempered condition is below 75 kg / mm ² ;

Steels with about

0.20 to 0.28% carbon, 0.30 to 0.80% manganese, 0.15 to 0.35% silicon, 1.20 to 1.40% chromium, 0.50 to 0.60% molybdenum, 0.15 to 0.25% vanadium

should be tempered in such a way that the strength (at room temperature) in the quenched and tempered condition is below 80 kg / mm ² , and

Steels with about

0.30 to 0.38% carbon,

0.30 to 0.80% manganese,

0.15 to 0.35% silicon,

1.20 to 1.40% chromium,

0.50 to 0.60% molybdenum,

0.15 to 0.25% vanadium

should be tempered in such a way that the strength (at room temperature) in the quenched and tempered condition is below 90 kg / mm ² .

The steels can also contain up to 2% nickel
contain.

The results of comparative tests are shown below.

The tests determined:

1. Composition of steels
(Table 1),

2. Creep strength according to DVM
DINA 117/18 and creep rupture strength
(Table 2),

3. Embrittlement with long-term use
(Table 3).

The following materials were tested:

Tabel

stole C. Si Mn P. S. Cr Mon V I. 0.22 0.38 0.68 0.027 0.023 0.13 0.30 _ 2 0.21 0.37 0.60 0.015 0.015 0.76 0.56 0.16 3 0.17 0.37 0.26 0.014 0.012 1.09 0.54 0.19 4th 0.25 0.41 o, 34 0.010 0.010 1.05 0.52 0.20 5 0.29 0.43 o, 34 0.010 0.013 1.04 0.60 0.19 6th 0.20 0.29 0.82 0.013 0.015 1.40 0.56 0.21

To 2. The fatigue strength was determined according to the guidelines of the preliminary standard A 117/18. For the following values were determined for the individual qualities:

Table 2
DVM creep strength

stole Stretch limit tensile strenght DVM creep strength 500 ° C 550 ° C Creep strength
(at 500 ° C) 100,000 hours kg / mm ² kg / mm ² 450 ° C 13.5 5.5 10000 hours 7.5 I. 40.0 55.4 17.5 19.5 8, o 14th 8th - 65.0 31.Ο 30.0 20.0 l6.5 14.5 2 64.4 75.2 41.0 38.0 21.0 21 14.5 77.0 86.0 50.0 32.0 21.5 23.5 15th 3 84.1 91.7 49.Ο 32.0 15.0 28 16 - 4th 73.8 82.8 44.0 36.Ο 22.0 27 15th 5 80.3 89.2 48.0 26.5 18.0 27 15.5 6th 76.1 85.0 34.O 24

To 3. The embrittlement, as it occurs during operation, ie the longer periods of use, was determined by long-term tests. Both smooth and notched specimens were used to determine this creep rupture strength. If a material does not become brittle after prolonged use, the smooth specimen breaks before the notched one. This is because the smooth specimen gradually receives higher stresses than the notched specimen as a result of the continuous reduction in cross-section under tensile load as a result of the flow. This, on the other hand, does not change its cross-section due to the flow restriction caused by the notch. If, on the other hand, a material becomes brittle, the smooth sample does not expand. It does not change its cross-section and maintains the same tension. The notched sample, on the other hand, has a stress peak with the same cross-section as a result of the notch effect, which is significantly higher than the
Stress on the smooth specimen is.

Table 3

Heat treatment C oil 720 ° C. strength Ver stole 900 ° C oil 68o ° C. at
2O ⁰ C
kg / mm ² brittleness
after hours I. 9OO ⁰ C oil 720 ° C. 55 OO I. 95O ° C oil 680 ° C. 65 3OOOO 2 950 ° C oil 660 ° C. 75 OO 2 950 ° C oil 700 ° C. 86 3000 3 95O ° COl 700 ° C. 92 200 4th 950 ° C oil 700 ° C. 83 100 OOO 5 950 ° 89 2OOOO 6th 85 7OOOO

The results are shown in Table 3. In these tests steel 1 and 2 are in the higher strength level becomes more brittle with increasing loading time. With low cold strength on the other hand, they do not become brittle.

The tendency to embrittlement is increased by the low carbon content and high quenched and tempered resistance, as the steels harden poorly and cannot be tempered to such a high degree. So has z. B. Steel 3 with the lowest C content of 0.17% has the highest hardening and tempering resistance (Table 3) and is accordingly very embrittled. From Table 3 it can be seen that is very strongly suppressed at annealing temperatures of 700 ⁰ C from the embrittlement and is completely suppressed at 720 ⁰ C.

• If higher cold strength values are required, the tempering temperature cannot be reduced, rather, more alloying elements have to be added in order to increase the basic strength.

A simple and cheap means of doing this is the C content.

The investigations show that melts of the same composition give the same creep strength over longer test times, even if, for. B. by increasing the hardening temperature, the initial fatigue strength is increased. The increase in the cold strength also has no effect on the creep rupture strength. This results from the results of melts 1 and 2. The increase in cold strength by 10 kg / mm ² resulted in an increase in DVM creep strength of 6 and 8 kg / mm ^2, respectively. With 10,000 hours of testing time, however, the gain in creep rupture strength is only 2.5 kg / mm ² . For a test time of 100,000 hours, however, there is no longer any gain. Accordingly, components with a long service life are independent of the cold strength in terms of their creep behavior.

Claims (2)

Patent claims: i. Process to prevent embrittlement in the event of long-term ^{stress of over 10,000 hours at temperatures between 400 and 550 0} C, fatigue-resistant steels which have the usual molybdenum content (at least about 0.25% Mo) and, if necessary, chromium and vanadium in a total content of the three elements up to about 3% and carbon from 0.20 to 0.45%, characterized in that the steels after hardening from temperatures between about 900 and 950 ⁰ C at temperatures between about 700 and 750 ⁰ C depending on the steel alloy so that the Tear strength at room temperature reduced to previously unused low values is, with the proviso that the tensile strength for steels with about 0.20 to 0.28 ¹¹ zo carbon, 0.40 to 0.70 ⁰ zo manganese, 0.15 to 0.35 ⁰ Zo silicon, 0.25 to 0.35 ⁰ Zo molybdenum, Remainder of technical iron, under 60 kgZmm ² , for steels with approx 0.20 to 0.28 ⁰ Zo carbon, 0.40 to 0.70 ⁰ Zo manganese, 0.20 to 0.40 ⁰ Zo silicon, 0.80 to 1.10 ⁰ Zo chromium, 0.40 to 0 , 50 ⁰ Zo molybdenum, Remainder of technical iron, 75 under 70 kgZmm ² , for steels with approx 0.30 to 0.38 ⁰ Zo carbon, 0.40 to 0.70 ' ⁰ Zo manganese, 0.20 to 0.40 ⁰ Zo silicon, 0.80 to i, io ^fl Zo chromium, 0.40 to ο , 5ο ° Ζο molybdenum, the remainder technical iron, under 75 kgZmm ² , for steels with approx 0.20 to 0.28 ⁰ zo carbon, 0.30 to 0.80 ⁰ zo manganese, 0.15 to 0.35 ⁰ zo silicon, 1.20 to 1.40 ⁰ zo chromium, 0.50 to 0, 60 ⁰ Zo molybdenum, 0.15 to 0.25 ⁰ Zo vanadium, the remainder technical iron, under So kgZmm ² , for steels with about 0.30 to 0.38 ¹⁰ zo carbon, 0.30 to 0.80 ⁰ zo manganese, 0.15 to 0.35 ⁰ Zo silicon, 1.20 to 1.40 ¹⁰ zo chromium, 0.50 to 0.60 ⁰ zo molybdenum, 0.15 to 0.25 ¹⁰ zo vanadium, Remainder of technical iron, ' is below 90 kgZmm ² . 2. Application of the method according to claim ι those steels mentioned therein which still ^{contain up to 2 0} zo nickel. Publications considered: Archives for the Ironworks, Vol. 19, p. to 48; Vol. 20, pp. 229 to 242; "Technische Mitteilungen Krupp", research reports, March 1943, pp. 143 to 174; Material sheets of the Association of German Ironworkers, 630-51; 630-55. © 209 '762/7 1.