DE1232759B - Martensite-hardenable chrome-nickel steel - Google Patents

Description

GERMAN PATENT OFFICE

German KL: 40 b-39/20

EDITORIAL

Number: 1 232 759

File number: J 29037 VI a / 40 b

1 232 759 Filing date: September 22, 1965

Opened on: January 19, 1967

The invention relates to a martensitic hardenable stainless steel having strength and toughness, for example as a material for pressure vessels, aircraft parts or the like makes suitable.

The invention relates primarily to stainless steels, both after cooling to room temperature after the solution heat treatment as well as after the cooling that follows hardening are martensitic, including those steels after the solution heat treatment Cooling to room temperature are essentially austenitic, but by freezing or Cold working can be converted into the martensitic state.

A disadvantage of the known age-hardenable stainless steels is that they have a high yield strength can not be achieved without loss of toughness. In addition, these steels are often extensive and expensive heat treatments are required.

Few of the known age-hardenable stainless steels have a yield strength of 140 kg / mm ² or more, but their toughness is extremely poor. Overaging these steels has improved toughness at the expense of strength. The term toughness is to be understood as meaning, above all, the high notched impact strength with a high ratio of notched tensile strength to tensile strength. Elongation and constriction values that arise in tests with unnotched specimens are not a reliable benchmark because these steels have internal or external notches when used for structural parts.

The notch sensitivity is a measure of the deformation behavior of a metal under a high level local exposure. Cracks, nicks, or bubbles are known to be the cause and starting point are for spreading bugs. It is well known that local stress concentrations occur at such fault locations on. Metals that are resistant to spreading due to their plastic fluidity of cracks are called notch-tough. Metals that do not have this property, are sensitive to notches and tend to break brittle. Despite satisfactory yield strength, elongation and Constriction values of unnotched specimens, this brittle fracture phenomenon can occur. the Crack propagation can occur e.g. B. caused by the heat treatment and the strength. It is known that the minimum size of a notch is the cause of the brittle fracture with decreasing stretch martensite hardenable Chrome-nickel steel

Applicant:

International Nickel Limited, London
Representative:

Dr.-Ing. G. Eichenberg

and Dipl.-Ing. H. Sauerland, patent attorneys,

Düsseldorf, Cecilienallee 76

Named as inventor:

Clarence George Bieber, Ramsey, Ν. J. (V. St. A.)

Claimed priority:
V. St. v. America September 23, 1964
(398 771)

limit and tensile strength of the metal increases. As a result, avoiding brittle fracture in metals with yield strengths of, for example, 70 to 105 kg / mm ^{2 is} not as difficult as in metals with yield strengths of 140 kg / mm ² and more. In connection with the present invention, the steel with a yield strength of 140 kg / mm ² and more should have a ratio of the notched tensile strength to its tensile strength of at least one with a notch factor of K _t of 10 or more in order to be considered to be notch-resistant. The ratio should preferably be at least 1.2: 1.

Steels with a yield strength of 98 to 140 kg / mm ² should have a high toughness, which is expressed in values for the elongation of at least 10%, preferably 12%, for the necking of at least 40%, preferably of at least 50%, and for the notched impact strength of at least 7 kgm. Likewise, steels with a ^{yield strength exceeding 140 kg / mm 2 should have} an elongation of at least 10%, a necking of at least 40%, a high notch tensile strength and a ratio of

609 758/257

Have notched tensile strength to tensile strength of at least 1 and preferably 1.1.

The object on which the invention is based is now to produce steels with the above to create resulting properties without the expense of heat treatment measures for development having to accept their technological properties.

This object is achieved by a steel that consists of 11 to 13% chromium, 9 to 11% nickel, 1.5 to ro 3% molybdenum, the sum of 0.8 times the chromium content, the nickel and molybdenum content 20 to 23%, 0.1 to 0.5% titanium and / or 0.05 to 1% niobium, 0.5 to 1.6% aluminum, where the total aluminum and titanium content does not exceed 1.9% and the ratio of the nickel content to the total content of aluminum and titanium is at least 5: 1, up to 0.03% carbon, 0 to 0.2% manganese and 0 to 0.2% silicon, the remainder iron with impurities from the melting process consists.

The aluminum is particularly important for hardening. Low aluminum steels have a good toughness, in particular a low notch sensitivity, while steels with higher Aluminum contents have a higher yield point with relatively good notch toughness.

Steels with a yield strength of 140 kg / mm ² and more should contain 1 to 1.6% aluminum, preferably 1.1 to 1.5%. The total aluminum and titanium content preferably does not exceed 1.8%. If the aluminum content significantly exceeds 1.6%, the notch toughness of the steel is adversely affected. On the other hand, at least 1% aluminum is required to achieve the highest possible yield strength.

If yield strengths between 105 and 140 kg / mm ^{2 are} sufficient, but the steels are to have a maximum possible notched impact strength of at least 7 kgm, the aluminum content should be 0.5 to 1%. If the aluminum content is below 0.5%, the yield point is too low. To ensure the best notched impact strengths, the aluminum content must be 0.9%.

The chromium content of the steel must not be below 11% and is preferably at least 11.5%, around the steel, regardless of the beneficial effect of molybdenum on certain corrosive ones Media that give the desired resistance to corrosion. On the other hand, · chromium levels impair of over 13% the technological properties of the steels are negative and also make the Use of extensive heat treatments, such as intermediate annealing, is required.

At least 9% nickel must be given for a good combination of strength and toughness to ensure. Excessive nickel contents stabilize the austenite with aging and limit the The hardening temperature range without the risk of residual austenite remaining and over-hardening can be applied. For this reason the nickel content does not exceed 11%. To continue To ensure good toughness, the ratio of the nickel content to the total content is of aluminum and titanium at least 5: 1.

Molybdenum also promotes corrosion resistance to certain atmospheres, in particular chlorine-containing atmospheres and acids such as

Sulfuric acid and phosphoric acid, so that at least 1.5% molybdenum is required. When the molybdenum content however, if it exceeds 3%, the austenite tends to be durable. Advantageously exceeds the molybdenum content is therefore not 2.5%.

The steels according to the invention are intended to prevent harmful grain boundary precipitation during hardening contain at least one of the elements titanium and niobium. Although the niobium content can be up to 1%, it preferably exceeds it 0.5% not. The titanium content should not exceed 0.5%, but preferably not exceed 0.35%. Too high Titanium content creates precipitates and other difficulties in the manufacture of the steel.

The carbon content of the steel should be kept as low as possible and not exceed 0.03%, because otherwise not only intergranular corrosion occurs, but also the temperature range the martensite transformation is reduced and the toughness of the steel is impaired. Of the Carbon content should therefore and so that it does not act as a hardener, as low as possible and preferably Do not exceed 0.02%.

The same applies to the contents of silicon and manganese, the total content of which is not more than 0.25% and the individual content advantageously not more than OJ ⁰ D.

In order to achieve the best properties, for example a yield strength of over 150 kg / mm ² , an elongation of at least 10%, a necking of over 45% and a ratio of notch tensile strength to tensile strength of at least 1.1, the steels should be 11.5 to Contain 12.75% chromium and 9 to 10.75% nickel, 1.75 to 2.5% molybdenum, the sum of 0.8 times the chromium content and the nickel and molybdenum content being 20.5 to 22%, 0.2 up to 0.35% titanium and / or 0.2 to 0.5% niobium, 1.1 to 1.5% aluminum with a total aluminum and titanium content of at most 1.8%, up to 0.03% carbon, 0 to 0, 15% manganese and 0 to 0.15% silicon with a total manganese and silicon content of at most 0.25%.

Accompanying elements caused by melting, such as calcium, cerium, up to 0.5% vanadium, up to 1% tantalum, up to 0.5% copper, up to 0.1% beryllium, up to 0.01% boron and up to 0.05% zircon can also be used to be available. The levels of impurities such as sulfur, phosphorus, hydrogen, oxygen and nitrogen, should be kept as low as possible. The steels according to the invention are cobalt-free, with the exception of cobalt contents in the order of magnitude of impurities.

The steels according to the invention can be melted in air and the homogeneously solidified blocks can be processed hot or cold by forging, pressing, rolling, etc. Multiple annealing and hot forming results in a continuous homogenization of the cast structure. The deformation is advantageously carried out at 980 to 1095 C and at a temperature on the finishing stand of 870 to 815 C. Following the deformation, the steels are ^{solution annealed for several hours at temperatures of 870 to 980:} C and then cooled, for example in air. When cooling after the solution heat treatment, the structure is essentially converted into martensite, but it can also be converted into the martensitic state by deep cooling and / or cold deformation.

A special advantage of steels with a total content of chromium and nickel of no more than However, 23% is that freezing or cold working is not required.

To explain the heat treatment and the technological properties of the steels according to the In the following, four of these steels (No. 1 to 4) with four others not belonging to the invention are considered to be the invention Steels (A to D) compared. The composition of these steels results from the Table I. Starting materials did not add more than 0.03% carbon to the steels registered. They all contained less than 0.15% manganese and less than 0.15% silicon. With everyone The rest of the steel consisted of iron including impurities.

Table I.

stole Cr Ni Mon Al Ti Nb No. ° / o 'Vo % % % % 1 12th 11 2 1.3 0.3 2 12th 10 2 1.3 0.3 3 12th 9 2 1.3 0.5 4th 12th 9 2 1.3 0.3 A. 15th 10 2 1.3 0.3 B. 14th 10 2 1.5 0.3 C. 14th 8th 2 1.5 0.3 D. 11 12th 2 1.3 0.3

15th

Samples were taken from these steels for the tensile test and subsequent heat treatment subject to:

Heat treatment A:

1. Solution heat treatment for 1 hour at 870 ° C and subsequent cooling in air.

2. Freeze for 16 hours at -75 ° C (dry ice).

3. Cure at 480 "C for 1 hour.

Heat treatment B:

1. Solution heat treatment for 1 hour at 980 ° C and subsequent cooling in air.

2. Cure for 4 hours at 480 ³ C

Heat treatment C:

1. Solution heat treatment for 1 hour at 980 "C and subsequent cooling in air.

2. Freeze for 16 hours at -75 ^; C (dry ice).

3. 4 hours curing at 480 ° C.

The Rockwell hardness of the samples in the different stages of the heat treatment, for example after cooling after the solution heat treatment, the low temperature treatment and after cooling following curing were determined. The values determined in this way result from Table II.

Table II

25th

30th

Rockwell hardness, RC stole
No. warmth
treatment Solution
glow Freezing Harden 870 = C 980 C 1 C. 28 32 50 2 C. 34 35 50 3 B. 30th 48 4th B. 29 48 A. C. 6th 32 43 B. C. 32 31 48 C. A. 32 31 47 D. C. 21 30th 47

The resulting properties of the steels after the end of the heat treatment is illustrated Table III. None of the steels examined was cold worked before or after hardening.

Table III

stole Stretch limit tensile strenght strain Constriction Notched tensile strength Notch tensile strength / No. kg / mm kg / mm ² % ° / o kg / mm ² tensile strenght 1 144.3 157.0 13th 50 230.5 1.51 2 162.3 171.8 12th 58.5 239.3 1.39 3 158.6 169.1 12th 51 177.8 1.12 4th 165.5 173.3 13th 56.5 218.8 1.25 A. 53.5 102.8 33 71 145.4 1.42 B. 127.8 145.5 18th 57 197.2 1.40 C. 155.4 160.2 10 42 99.2 0.612 D. 132.0 148.7 16 60.5 222.0 1.49

It can be seen from Table III that the steels according to the invention have markedly improved technological properties compared to the conventional steels. With the exception of steel C, the steels not covered by the invention have a yield strength of less than 140 kg / mm ² and significantly less than the preferred 150 kg / mm ² . Although steel C had a good yield point, the ratio of notch tensile strength to tensile strength, which gives a measure of the toughness of the steel, was far below 1. It can be stated that the chromium contents of steels A and B were too high and the sum 0.8 times the chromium content as well as the nickel and molybdenum content of these two steels exceeded the required maximum of 23%. Steel A was in

Claims (5)

Following the cooling that took place after the solution heat treatment, it was completely austenitic. This results from the very low Rockwell hardness of 6 (see Table II). In addition, this steel contained a large amount of retained austenite after hardening and cooling. Steel C contained too much chromium and too little nickel. Finally, the nickel content in steel D was too high. Steels 1 to 4, on the other hand, had very good technological properties. Steels 3 and 4, whose total chromium, nickel and molybdenum content was 23%, show that optimum properties can also be achieved without low-temperature treatment or cold deformation. This is a significant advantage when the steels are used for the manufacture of large boilers. Such boilers are welded, which is why solution heat treatment and hardening are required after the welding in order to produce the properties of the base metal in the weld zone. However, it would be extremely difficult and cumbersome to subject the kettle to a low-temperature treatment before hardening. A comparison of steel 1 with steels 2, 3, 4 makes clear the advantages associated with the fact that the sum of 0.8 times the chromium content and the nickel and molybdenum content is not more than 22%. As mentioned earlier, there are numerous uses that require steels with yield strengths from 105 to 150 kg / mm2. In these cases, steels according to the invention with an aluminum content of 0.5 to 1% can be used. Such steels have a remarkable ability to absorb extremely high impact energies. The heat treatment and the properties of three steels (No. 5, 6 and 7) with 0.5 to 1% aluminum are explained as examples. Their composition is shown in Table IV. The steels contained less than 0.03% carbon, less than 0.15% manganese and less than 0.15% silicon. In each case the remainder consisted of iron including impurities. Table IV SteelCrNiMoA! TiNbNo.% "/ O ° / o% ° h% 5121020,80,2612920,80,2712920,80,5 Steels Nos. 5, 6 and 7 were neither subjected to cryogenic treatment nor cold worked. Samples of these steels were subjected to heat treatment B or the similar heat treatment D, during which they were cured for 2 hours at 540: C. After the heat treatment, the values listed in Table V resulted: Table V Steel No. Heat treatment Yield limit kg / mma Tensile strength kg / mm'2 elongation »/» constriction% Kerhcrfiliiix- toughness kgm5B135,2142,9166710,0D126,5135,2186519,06B137,1144,11564,510,4D128,8136,916,566,516,47B141,2147,515657,7D131, 1142,016,563,515.1 The table shows that steels No. 5 and 7 have an exceptionally good notched impact strength, which is well over 7 kgm, which can be increased by heat treatment to over 18 kgm and with a remarkably high yield point of 125 kg / mm² and more Neten steels B and D have a yield strength comparable to steels No. 5, 6, 7, but not as good notch impact strength. Patent claims: 1.Martensite hardenable chrome-nickel steel, consisting of 11 to 13% chrome, 9 to 11% Nickel, 1.5 to 3% molybdenum, the sum of the nickel and molybdenum content as well 0.8 times the chromium content is 20 to 23%, 0.1 to 0.5% titanium and / or 0.05 to 1% Niobium, 0.5 to 1.6% aluminum, the total content of aluminum and titanium being 1.9% does not exceed and the ratio of the nickel content to the total content of aluminum and Titanium is at least 5: 1, up to 0.03% carbon, 0 to 0.2% manganese and 0 to 0.2% silicon, The remainder is iron with impurities from the melting process. 2. Steel according to claim 1, but wherein the total content of chromium, nickel and molybdenum Does not exceed 23%. 3. Steel according to claims 1 and 2, characterized in that the aluminum content is at least 1%. 4. Steel according to claims 1 to 3, consisting of 11.5 to 12.75% chromium, 9 to 10.75% Nickel, 1.75 to 2.5% molybdenum, with the sum of 0.8 times the chromium content and the Nickel and molybdenum content is 20.5 to 22%, 0.2 to 0.35% titanium and / or 0.2 to 0.5% niobium, 1.1 to 1.5% aluminum, the total content of aluminum and titanium being 1.8% does not exceed, up to 0.03% carbon, in each case not more than 0.15% manganese and silicon, whereby the total content of manganese and silicon does not exceed 0.25%, the remainder being iron with melting-related Impurities. 5. Steel according to claims 1 to 4, but wherein the carbon content is 0.02%, the manganese content 0.1% and the silicon content does not exceed 0.1%.