DE1533158B1 - Use of a rollable and weldable stainless steel for the production of objects which are intended for use under neutron radiation and at temperatures between -200 and +400 ° C, and as welding filler material - Google Patents

Description

The invention relates to the use of a rollable and weldable, neutron radiation-resistant stainless steel with high tensile strength and toughness within a wide temperature range, both in the hardened and in a tempered state, for the manufacture in particular of welded objects, e.g. pressure vessels, where it is desirable ₅ to reduce the material thickness by using materials with high strength, as well as a welding filler material for steels to be welded of the same composition.

Austenitic stainless steels are mainly used for welded equipment made of corrosion-resistant steel. These steels are known to be 18-8- (18% Cr, 8% Ni), 18-10- (18% Cr, 10% Ni), 18-10-Mo- (18% Cr, 10% Ni + about 1 up to 4% Mo), 25-12- (25% Cr, 12% Ni), 25-20- (25% Cr, 20% Ni) etc. steel. They are characterized by very good toughness, good ductility and very good weldability. Compared to a non-alloy or low-alloy steel with a structure consisting of ferrite and pearlite, bainite or tempering martensite, they have a low strength. The 0.2% proof stress is particularly low and is generally below 25 kg / mm ² . Because of these conditions, a part exposed to mechanical loads, for example a pressure vessel made of austenitic, stainless steel, must often have a wall thickness that leads to uneconomically high costs. In order to reduce the wall thickness of the stainless steel part, it is therefore sometimes necessary to use a stainless lining in connection with certain high-strength, unalloyed or low-alloy steels or so-called composite steels, a process which in both cases leads to certain technical difficulties, e.g. welding problems, leads. Despite their low strength, austenitic steels are widely used because of their high toughness, good formability and especially because of their easy weldability.

In those cases where greater strength is required and the requirements for corrosion resistance are only moderate, certain types of stainless steel are still used today, their

ao great strength is achieved through a high carbon content. With this are the so-called martensite steels meant that achieve their high strength through hardening and tempering processes. With these In steels, chromium is the main alloying element. As an example, there are some stainless Swedish ones Standard steels with a chromium content of 13 to 17% called:

Chemical composition or condition according to the standard regulations

7oMn

VoCr

VoNi

State

2303-4
2321-3

0.18-0.25
0.15-0.25

maximum 0.8 maximum 1.0

maximum 1.0 maximum 1.0

12.0-14.0
16.0-18.0

1.25-2.25

Tempered up to 90 to 105 kg / mm ²

tensile strenght

Tempered up to 85 to 105 kg / mm ²

tensile strenght

The disadvantage of these steels is the high 4 ° to preheat during and after the welding process Carbon content by which these types of steel are recommended to be tempered.

be more sensitive to cracking during welding, there are also ferritic-austenitic stainless steel

Because the martensite, which is in the heated zone steels, the comparatively high strength limits forms, is hard and brittle. In order to have cracking, such as the following standard steels: To avoid such steels it is necessary to use them 45

stole % C VoCr VoNi VoMo 0.2% proof stress
kg / mm ² tensile strenght
kg / mm ² 2324 maximum 0.10 24-27 4.5-6.0 1.3-1.8 minimum 42 minimum 60

The weldability of steel grades of this type is, however, less good as they are resistant to grain growth are sensitive. When welding, the material in the heated zone becomes coarser, and thus becomes the steel is brittle.

The steel for the purpose according to the invention contains

0.020 to 0.040%,
preferably 0.030 to 0.040% carbon,

0.2 to 1.0%,
preferably 0.3 to 0.5% silicon,

0.2 to 2.0 0/0,
preferably 0.6 to 1.0% manganese,

15 to 18%,
preferably 16 to 17% chromium,

4 to 6%,
preferably 4.5 to 5.5% nickel,

0 to 2%.
preferably 0.8 to 1.3% molybdenum,

0.02 to 0.08%,
preferably 0.02 to 0.05% nitrogen,

The remainder is iron and production-related impurities, the content of the alloy components being in Dependence on each other is selected so that the sum of chromium equivalent + nickel equivalent between 24.5 and 26.2 and the difference from the

3 4

1.4 times the value of the chromium equivalent and the austenite present in the steel also contributes to the tough nickel equivalent is between 16.5 and 19.0. This contributes to the heat affected zone. The steel is Chromium equivalent is calculated as therefore not sensitive to during the welding process

Cracking and can be welded without preheating

° / o Cr + ° / o Si + ° / o Mo.5 will be. To get maximum toughness properties

The NicVelämiivaient as received, it is expedient after welding

the nickel equivalent as within a temperature range of 500 to

<y _o Ni _ | _ o, 5. % Mn + 30 (° / ₀ C + ° / ₀ N). 630 ⁰ C to be remunerated. This will reduce the sweat

tensions balanced and the martensite in the

After hardening, such a structure contains a tempered hardening zone.

Martensite, ferrite and austenite. Consists of analytical and a characteristic feature of steel

From a structural point of view, this steel can be viewed as an additional benefit in that it has a certain content transition form between an austenitic and ferrite. With hardenable steels with high

ferrite-martensitic steel. The problem was with the carbon content

The phase composition for the analysis according to the invention is that as far as possible no ferrite is present

appropriate alloy should be used. It is well known that ferrite is found in martenrungen can be taken from the drawing. In the static steels with a high carbon content Diagram is the chromium equivalent, i.e. the sum of the toughness reduced so that the content of ferrite

0 / r j-0 / c-io / constituent alloy elements on a low

/ 0 Cr + / o bi + I ₀ Mo, _{20 value} g ^^ _who had to, if a high viscosity

as the abscissa and the nickel equivalent, i.e. the sum was required. On the other hand, it turned out-

0 / vr: 4th λ «. 0 / M _n 1 on. toi η 4- "/ ISH that if there is a low carbon

I ₀ im -f up / 0 Mn + ju Uo ^ t / 0 in.), _{Salary also then be} . _{martensitic tribes}

plotted as the ordinate. The good toughness in question is obtained when ferrite is in the structure Alloys lie within the dashed area 25 occurs.

ABCD in the three-phase surface of austenite + Mar- It is therefore possible to determine the chromium content in one

tensite + ferrite. These alloys contain after as high a value as about 16% to keep and also to ^{add to the rolling and hardening at about 1000 0} C and about 1% molybdenum. This results in tempering at 500 to 63O ⁰ C to 5 to 15% ferrite, considerably improved corrosion resistance in Ver-10 to 40% austenite and the remainder of tempering martensite. 30 is the same as for the known ferrite-free steels

The steel is annealed at about 1000 ⁰ C, obtained in air results achieved whose chromium content is cooled, and is then possible within a temperature limited to about 14% when no molybdenum raturbereichs 500-630 ⁰ C started. In the case of which is present, and may only be about 13%, if a hardening temperature consists of the structure of austenite content of about l ° / ₀ molybdenum is desired. + 5 to 15% ferrite. Since the steel according to the invention is hardenable, sub-temperature or below, no transformation takes place, it separates itself from the ferrite-containing steels, that of ferrite, while austenite is converted into martensite - described in French patent specification 803 361. However, there will be a few and the 0.020 to about 0.300% C, 1.3 to 3.0% percent austenite will not be converted. Mn, 1.5 to 6.5% Ni, 16.0 to 23.0% Cr and given

By heating the hardened steel up to 500 40 if it contains up to about 2.5% Cu or 3% W, up to 630 ⁰ C the martensite is tempered and the hardening tensions are that two phases, ferrite + austenite, are common for tempered steels. In doing so, which is obtained, the aim of using the toughness of the steel is increased. However, this type of steel differs from other steels, 45 intergranular corrosion, a property which, in the tempering process within the specified austenitic steels with a high carbon temperature range, also forms austenite, which is characteristic.

is stable even at low temperatures. The amount The great toughness of steel can also be seen in it

of the resulting stable austenite increases seen with increasing that the curve for the impact stressing participating annealing temperature up to about 630 ⁰ C. 50 at different temperatures without heated is rectilinear one further at higher temperatures, then any appreciable transition from a high austenite formed no longer permanent. It goes to a low value and, in the event of a transition, on cooling to room temperature in Marten temperature, it will run below -196 ° C. Converted as a transitional site. By changing the temperature temperature in impact tests, the and the duration of the tempering can usually be referred to different temperatures at which the curve for the strengths with 0.2% yield strengths between 50 and impact stress versus temperature for 80 kg / mm ^{2 is} obtained will. A longer tempering time, impact test specimen with an area of 10 χ 10 mm ² , acts in the same way as increasing the tempering - a V-notch 2 mm deep and a carrier temperature, a distance of 45 mm, the value 2.8 kgm above.

Despite the low carbon content, the steel has increased 60. For welded systems, especially for great strength. It differs from pressure vessels is the use of material with martensitic steels with a high carbon and a low transition temperature are desirable, content due to its easy weldability. It is often prescribed that the transition temperature

The temperature formed during welding in the heated zone should be below 0 or -2O ⁰ C. The martensite according to the invention is tough because of the low carbon content of steel at -196 ° C. Therefore, in this zone of about 5 kgm, this value is significantly above no embrittlement, as is the case with chromium steels with 2.8 kgm, the value that is the case as a criterion for the excessively high carbon content. Dar im 'gang temperature. is seen.

The steel further shows only a small drop of the 0.2% yield strength up to temperatures of 300 to 400 ⁰ C.

The steel has a good resistance to neutron radiation and has a transition temperature below -80 ° C ^{even after a radiation dose of 3 · 10 ie} neutrons / cm ^{2 (> 1 MeV).}

The steel has, therefore coextensive with unalloyed steel and much lower than austenitic steel has a low coefficient of expansion which is at 20 to 100 ⁰ C 12 * 10 ~. ⁶

The properties of the steel which are important for the intended use according to the invention are explained in the examples below.

example 1

Steel sheet with a thickness of 14 mm was made from a steel of the following composition:

C 0.038%

Si 0.53%

Mn 0.76%

P 0.017%

S 0.005%

Cr 16.0%

Ni 4.7%

Mo 1.02%

N 0.038%

the rest consisted of iron and accidental impurities.

The steel sheet was ^{heated to up to 1000 °} C. and this temperature was maintained for 1 hour. It was then cured in air and tempered at 600 ° C., the latter temperature being maintained for 6 hours. The following strength values were obtained for the steel sheet heat-treated in this way:

0.2% proof stress 60 kg / mm ²

Tensile strength 91 kg / mm ²

Elongation (measuring length 5 d) 20%

Impact work at 20 ° C 7.2 kgm

Brinell hardness 278 kg / mm ²

The following strength values were obtained at higher temperatures obtain:

There were no cracks either at the welds or in the zone exposed to the heat of welding.

The useful application according to the invention thus also extends to the use as a welding filler material for steels of the same composition to be welded.

Example 2

70-mm steel sheet of the same composition as in Example 1, heating was cured on to 1000 ⁰ C in air and annealed for 6 hours at 600 ⁰ C after 2 hours. The following strength values were obtained:

0.2% proof stress 64 kg / mm ²

Tensile strength 92 kg / mm ²

Elongation 18%

Impact work 7.3 kgm

Example 3

A round forged part having a diameter of 170 mm and a length of 250 mm was annealed with 4 hours of heating at 1000 ⁰ C, then cooled in air and annealed for 6 hours at 575 ° C. The steel had the following composition:

Weight percent

30th

40 C.

Si ..
Mn
P ..
S ..
Cr.
Ni.
Mon
N

0.035 0.30 1.00 0.035 0.025 16.5 4.7 1.08 0.030

45

0.2% proof stress,
kg / mm ²

Tensile strenght,
kg / mm ²

Elongation (measuring length
5 <*).%

100 ° C 200 ° C 300 ° C 400 ° C 500 ° C

60
84

60
80

60 79

16

59 73 14

51 63 14 The following strength values were obtained:

0.2% proof stress 76 kg / mm ²

Tensile strength 92 kg / mm ²

Elongation (measuring length 5 d) 20%

Shrinkage 57%

Impact work, at 20 ⁰ C 14 kgm

The following strength values were obtained at higher temperatures obtain:

55 0.2% proof stress, kg / mm ² .

Tensile strength, kg / mm ²

Impact work at different temperatures: elongation (measuring length 5 rf),%

Impact work,
kgm

_6o shrinkage,%

200 ° C 300 ° C 70 68 80 73 18th 17th 70 70

400 ° C

62 72 16 69

Test temperature in ° C -196 I -74 I —20 I 0 I +20 Impact work at different temperatures:

5.0

6.3

6.7

7.1

7.2

The steel sheet was welded with the aid of coated electrodes of similar composition.

Impact energy, kgm

Test temperature in ° C -196 I -74 I -20 0 I +20

6.2

12th

13th

14th

Example 4

Test specimens for the tensile test and the impact test were taken from 70 mm sheet steel from the same batch as in Example 1 and irradiated ^{at 240 to 260 ° C. in a nuclear reactor.}

Strength tests before and after the irradiation showed the following values:

Ultimate tensile tests Irradiating
lung
tempe-
rature
⁰ C + 2O ⁰ C train
firm
speed
kg / mm ^a Deh
tion
° / o Radiation dose 265 0.2 ° / o-
Stretching
border
kg / mm ² 88
102 18th
17th Not irradiated
5 · 10 ¹⁸ neutrons / cm ²
(> IMeV) 66
90

Transition temperatures for impact tests

The test pieces were 3 × 3 mm in size. T ₀₁₁₇ is the temperature at which the impact energy of 0.17 kgm is reached. It corresponds approximately to the temperature 7 " _{2) 8} at which the impact work of 2.8 kgm is reached in normal test bodies.

Irradiating T Vickers lung hardness Radiation dose tempe- rature ⁰ C kg mm ^{2 0} C -192 293 Not irradiated -137 332 5.00 x 10 ¹⁸ neutrons / cm ² . 265 -122 344 1.1 · 10 ¹⁹ neutrons / cm ² .. 265 -80 369 3 · 10 ¹⁹ neutrons / cm ² ... 265

(Neutron energy> 1 MeV.)

Claims (3)

Patent claims: 1. Use of a rollable stainless steel with a structure of martensite with 5 to 15% ferrite and 10 to 40% austenite, consisting of 0.020 to 0.040% carbon, 0.2 to 1.0% silicon, 0.2 to 2.0% manganese, 15 to 18% chromium,
4 to 6% nickel, 0 to 2% molybdenum, 0.020 to 0.080% nitrogen, The remainder is iron and the manufacturing-related impurities, the alloy additives so are agreed that the sum of the as % Cr +% Si +% Mo chromium equivalent calculated as% Ni + 0.5% Mn + 30 (% C +% N) calculated nickel equivalent between 24.5 and 26.2 and the difference between 1.4 times the value of the chromium equivalent and the nickel equivalent between 16.5 and 19.0, for the production of welded objects that must be resistant to neutron radiation and also + 400 ° C, a 0.2% proof stress must have kgm of at least 50 kg / mm ² or at -200 ⁰ C, a notch impact energy of at least 2.8. 2. Use of a steel according to claim 1, consisting of 0.03 to 0.04% carbon, 0.3 to 0.5% silicon, 0.6 to 1.0% manganese, 16 to 17% chromium,
4.5 to 5.5% nickel, 0.8 to 1.3% molybdenum, 0.02 to 0.05% nitrogen, for the purpose of claim 1. 3. Use of a steel according to claim 1 or 2 as a welding filler material for to be welded Steels of the same composition. 1 sheet of drawings 909 581/75