EP0006953A4

EP0006953A4 - Unmagnetizable cast steel alloy, use and making thereof.

Info

Publication number: EP0006953A4
Application number: EP19780900291
Authority: EP
Inventors: Walter Gysel; Adolf Trautwein
Original assignee: Georg Fischer AG
Current assignee: Georg Fischer AG
Priority date: 1977-11-30
Filing date: 1979-06-21
Publication date: 1980-01-09
Also published as: IT1108126B; US4285725A; IT7868152A0; WO1979000328A1; EP0006953A1

Abstract

An unmagnetizable cast steel alloy having the composition C max. 0.30%, Si max. 2.00%, Mn 4.00-20.00%, Cr 10.00-20.00%, Ni 4.00-12.00%, Mo max. 3.00%, N<u2>u O.02-0.20% the remainder being iron, with a magnetic permeability //c </= 1.20 and a Cr NiMn equivalence factor f = 6.5% Cr -0.4.% Ni + 0.1% Mn + 0.075.% Cr .% Ni + 0.013.% Cr .% Mn 0.02.% Ni.% Mn wherein -6 </= f </= + 2 fulfills in an optimum way the whole of the following properties: low permeability; homogeneous resistance and hardness; stable flexibility at low temperatures; homogeneous magnetic permeability in big solidified sections; high capability to form chips and good weldability without microfissure and a sufficient limit to elongation. Other points of the invention relate to the use of these alloys in presence of intense magnetic fields and also to a process according to which after welding the alloy is heat-treated

Description

Non-magnetizable cast steel alloy, its use and manufacturing process

The invention relates to a non-magnetizable cast steel alloy, the use thereof and a method for producing the alloy.

Hitherto, cast components for fixing magnetic coils but also highly stressed parts of electrical machines (for example single-phase alternators) which must not cause any disturbances or restrictions in the magnetic flux flow, have preferably been used as austenitic spheroidal graphite cast iron of the GGG-NiMn 13 7 or GGG type -NiMn 23 4 or austenitic cast steel according to steel sheet 390 or ASTM A-296 CF 20 is used.

These alloys all have the power part that none of these cast iron-carbon alloys can fulfill the entirety of the property package required for such parts, namely:

low permeability n homogeneous strength and toughness values up to large wall thicknesses,

Structural stability at low temperatures down to -196 ° C and temperature changes, homogeneous magnetic permeability with large solidification cross sections in the range of 100 - 500 mm also in the residual solidification zone, - machinability and weldability, which is at least as good as for the standardized stainless steel cast alloys stanchions, e.g. Material number. 4308 or 4408 (DIN 17 445), yield strength or 0.2 proof strength of at least 250 N / mm. 2, weldable without micro-cracks.

Depending on the composition, high-alloy CrNiMn steel castings can be fully austenitic or, with a corresponding increase in the Cr content or lowering of the Ni and / or Mn content, can also contain more or less large amounts of ferrite in the austenitic basic structure. The austenitic phase is non-magnetic with a very low magnetic permeability (λi 4- 1.001), while the ferritic phase is ferro-magnetic with correspondingly high permeability values. For this reason, in two-phase austenitic ferritic alloys, the magnetic permeability increases very strongly with the ferrite content (FIG. 5). For so-called non-magnetic alloys with very low permeability, therefore, only fully austenitic alloys are used, provided that these steels can be used as wrought alloys. In the case of cast steel, that is to say the use of the alloys in the cast, not subsequently deformed state, this route is not feasible. Fully austenitic CrNiMo cast steel alloys are practically not crack-proof weldable due to their great susceptibility to hot cracks during welding. This problem does not occur with wrought alloys (forged and rolled steels) in this sharpness because these steels

OM

^• are considerably more resistant to hot cracks during welding due to the reshaping and subsequent subsequent graining of the structure by recrystallization with the aid of heat treatment.

The weldability of cast CrNiMn steel alloys is known to improve by leaps and bounds if these alloys have certain ferrite contents. It is not important how much ferrite these alloys are in use e.g. at room temperature, but what ferrite levels occur in the welded state. When welding, i.e. in the equilibrium state near the melting point, the ferrite content should be approximately 5% ferrite. Our investigations show that these ferrite contents are already reached when welding alloys if they only contain about 2% ferrite in the as-cast state at room temperature.

Well weldable CrNiMn cast steel alloys with ferrite components ^> 2% are known, but not for the intended use as ui-non-magnetic, stainless steel cast parts, since the permeability is too high due to the ferrite content,

The object of the invention is to avoid the above disadvantages and to fulfill the property package listed above. In particular, a non-magnetizable and at the same time crack-proof weldable cast steel alloy is to be created.

The task is solved by the characterizing features of the main claim.

The alloy according to the invention can advantageously be used for plant parts in nuclear fusion reactor plants where field strengths H of more than 10 ³ Oersted prevail, but is also suitable for uses at temperatures below -150 ° C. It was found that the reason for the remarkably favorable behavior during welding of ^alloys' in An¬ essentially standardized on certain of ferrite in erstarrungsmorpholo¬ cal peculiarities of the alloy system FeCrNiMn should be sought. In the alloy range up to 20% Cr, up to 15% Ni and up to 20% Mn, a peritectic melting surface separates the austenitic primary solidification from the ferritic primary solidification. Starting from the partially known three-substance systems FeCrNi and FeCrMn and supported by alloy tests, the following relationship was found for the peritectic melting surface :

f = 6.5 -% Cr - _0/4 •% Ni + 0.1% Mn + 0.075 • •% Cr - 0.013% Ni +% Cr • •% Mn - 0.02% Ni • •% Mn

Quaternary FeCrNiMn alloys with a CrNiMn equivalence factor f> 2 solidify primarily austenitic and are therefore fully austenitic at room temperature. Alloys with f <ζ 2 solidify primarily ferritic. The ferritic solidification is replaced by a binary peritectic reaction if the f values are not too low. If f < _. 0, the solidification of the alloys ends with the peritectic reaction. For alloys with 0. ^ F ^ T 2 there is an austenitic residual solidification after the ferritic primary solidification and the binary peritectic reaction, which likewise leads to full austenites. The important peritectic reaction causes austenite to be formed by dissolving primarily formed ferrite, in contrast to the primary austenitic or. austenitic residual solidification, where austenite is formed from the melt without the participation of ferrite.

When welding, which is a re-melting, the phase reactions occur in reverse order. With full

OiV.PI Austenites melt in each of the two cases, i.e. for f 2 the grain boundaries primarily formed by residual solidification. In the area of the melting line of the austenitic welding, this leads to dreaded hot cracks, which practically preclude absolutely safe weldability. In cases f ^ 2, where solidification ends with the peritectic reaction, the ferrite / austenite phase boundaries melt. From a morphological point of view, this is a completely different starting point, which means that hot cracks do not occur during welding. Knowledge of this state of affairs now allows the selection of such alloys which have just as much ferrite in terms of their weldability as possible and as little as possible with regard to the lowest possible magnetic permeability. For this purpose, the alloy elements Cr, Ni and Mn that are decisive for the structure must be brought into the context defined above, which results in a strict selection of alloys.

As the CrNiMn cast steel alloys tend to be out of equilibrium when they cool down after casting and welding, subsequent heat treatment may be necessary to adjust the system-inherent equilibria. The effect of such heat treatments with regard to a reduction in the ferrite content is shown in FIG. 4, which is explained in more detail below.

Due to the combined increase in both the nitrogen and the manganese content in the inventive _. , Stahlgusslegie¬ tion succeeds in raising the yield point, compared to conventional pure austenitic chromium-nickel steels, without accepting porosities due to nitrogen excretions, which can occur in the case of strong segregation in large casting cross-sections due to insufficient nitrogen solubility.

The same effect could also be achieved by other elements, for example by carbon or phosphorus. However, it has been shown that increasing carbon contents as a result of increasing hardness of the martensite which arises during the machining, lead to a deterioration in the machinability and that, on the other hand, increasing phosphorus contents also have a severe impact on the toughness values even in the solution-annealed state.

The cast steel alloy according to the invention is preferably used with a carbon content of C 0.06% according to claim 2, to limit the carbide deposits and to avoid embrittlement, in particular during stress relief annealing. Further advantages of the low carbon content are the better machinability and the Protection against intergranular corrosion. The decrease in the yield strength associated with a lower carbon content is compensated for by a correspondingly increased nitrogen content. The crack resistance during welding is significantly increased if an S content is selected according to claim 4.

The chromium and nickel content in the cast alloy according to the invention depends on the operating temperature of the system parts. At low operating temperatures, high chromium / nickel contents should be selected to ensure austenite stability.

An exemplary embodiment of the invention is explained below, the compositions always being given in percentages by weight.

example

A cast steel alloy according to the invention of the composition

C Mn Si PS Cr Ni Mo N ₂

0, 044 11, 6 0, 74 0, 028 0, 010 15, 7 8, 72 0, 05 0, 155%

_/ y QU RE _ OMP The rest of the iron with the usual accompanying elements and impurities gave the following mechanical properties in cast component-like samples with a wall thickness of 200 mm:

Yield strength Rp 0, _r 2 281 N / mm ² max. Tensile strength R 466 N / mm ²

Elongation at break ^A 5 39%

Constriction Z 47%

Impact energy ^K CV 178 joules

Brinell hardness HB _. 159 where the equivalence factor was f = - 0.916.

A conventional austenitic cast steel according to the steel iron sheet ASTM A-296 CF 20 of the composition

C Mn Si PS Cr Ni Mo N ₂ 0.17 1.16 0.82 0.009 0.010 19.7 8.62 0.04 0.060%

The rest of iron, accompanying elements and impurities, on the other hand, result in a lower yield strength and impact energy. The machinability of the steel casting alloy according to the invention is also superior to that of CF 20 or other comparable iron-carbon steel casting alloys. CF 20 is also not weldable without micro-cracks.

Reference is made to FIGS. 1 to 4. 1 shows the graphical comparisons between the alloy A according to the invention (filled circles) and other alloys (filled triangles and quadrilaterals) when turning. The cutting speed is V (m / min.) On the abscissa and the tool life on the ordinate T VB 0.4 (min) discontinued.

Fig. 2 relates to milling. The cutting material here is Widia TT 40. The board dimension is TNAF 2504 ZZR. The feed s _z = 0.11 mm / tooth and it was not cooled. The cutting speed V (m / min.) Is plotted on the abscissa and the tool life L (mm / tooth) is plotted on the ordinate.

_O PI Fig. 3 refers to the comparisons when drilling. The material is CF 20. The tool is an HSS twist drill with a diameter of 5 mm. The feed s = 0.06 mm / rev. It was cooled with an oil emulsion. The cutting speed V (m / min.) Is plotted on the abscissa and the tool life L (mm) is plotted on the ordinate.

FIG. 4 shows the relationship between the CrNiMn equivalence factor f of the alloys according to the invention on the abscissa and the ferrite content (Fer) in% on the ordinate, where

curve I the state after casting,

II T1 II "" "" welding,

II III "" "" annealing at 1100 ° C

II II -T-T7 II II II I «II II 8 Ω ^ C

V "" "" "" represents 650 ° C

To the left of the vertical axis through f = 2 is the ferritic and to the right of it the austenitic primary solidification area.

5 shows the relationship between the permeability eabI, which is plotted on the abscissa, and the ferrite content Fer in%, which is plotted on the ordinate.

A preferred embodiment of the alloy is evident from claim 3. Factor f = -2 was chosen. After solidification, point 1 on curve I of FIG. 4, the ferrite content is 3%. When welding, point 2 on curve II, the ferrite content increases to 6%. The ferrite content can be reduced to 0.1% by an annealing treatment, point 5 on the intersection of the abscissa and curve V, which corresponds to a / U value of 1.02. If necessary, this / u-Vfert can be reduced further by appropriate heat treatment.

The favorable magnetic permeability of the cast steel alloy according to the invention also remains in components of large cross section in the range from 100 to 500 mm, preferably from 200 - 300 mm, also obtained in the residual solidification zone by a high austenite stability and a large one. Homogeneity of properties is achieved even with a modest amount of alloying. Particularly in the case of extremely strong magnetic fields of, for example, 10 ^** Oersted field strength, as are required in fusion reactors for shaping the plasma, the alloy according to the invention has clear advantages over conventional alloys.

The measurement of the magnetic permeability of the steel casting alloy, according to the example, was carried out with the aid of the type 1.067 magnetoscope (Dr. Förster Institute) and gave the following values for a test specimen of dimensions 200 x 200 x 300 mm (approx. 250 Λ in the roughness depth) the cross section of 300 mm:

For example, particularly advantageous cast steel alloys have the composition: in% by weight

C Mn Si Cr Ni N max. 0.06 9-11 max. 1.0 14-16 7.0-8.0 0.10-0.15

Balance iron, possibly accompanying elements and impurities and the composition: in% by weight - ^' •

C Mn Si Cr Ni N max. 0.06 10-12 max. ^" 1.0 18-20 8.0-9.5 0.1-0.2

Remainder iron and possibly accompanying elements and impurities. The equivalence factors are f = - 1.61 and f = - 1.635.

Claims

Patent claims

1. Non-magnetizable cast steel alloy, characterized by the composition:

C max. 0.30%

Si max. 2.00%

Mn 4.00 - 20.00%

Cr 10.00 - 20.00%

Ni 4.00 - 12.00%

Mo max. 3.00%

N2 0.02 - 0.20%

Balance iron, by a magnetic permeability ΛJ. ^ 1.20 and by a CrNiMn equivalent factor: f = 6.5 -% Cr - 0.4 •% Ni + 0.1 ^• % Mn + 0.075 •% Cr •% Ni + 0.013 •% Cr •% Mn - 0.02 •% Ni. % Mn, where - 6 ^ f ^ + 2.

2. Alloy according to claim 1, characterized by the composition:

C max. 0.06%

Si max. 1.00%

Mn 9.00 - 12.00%

Cr 14.00 - 20.00%

O Ni 7, 00 - 10, 00% N ₂ 0, 1 - 0, 2% Mo max. 1.50%

3. Alloy according to claim 2, characterized by an f-range between -2.5 and -1.5 and a permeability

4. Alloy according to one of claims 1 to 3, characterized ge indicates that the sulfur content max. 0.02% and preferably max. Is 0.005%.

5. Alloy according to one of claims 1 to 4, characterized by a magnetic permeability ΛI ^ .1.05 with large solidification cross sections in the range of 100 - 500 mm, preferably from 200 - 300 mm, also in the residual solidification zone.

6. Use of the cast steel alloy according to one of claims 1 to 5 for parts of the system which are exposed to electromagnetic fields of a field strength H of approximately ιo3 Oersted.

7. Use according to claim 6, characterized in that the plant parts are used in nuclear reactor plants.

8. Use according to one of claims 1 to 8, at an operating temperature of the system parts below -150 ° C.

9. A method for producing an alloy according to one of claims 1 to 5, characterized in that after welding is annealed at a temperature between 650 ° C and 1'100 ° C.