Non-magnetizable steel casting alloy, its use and process of manufacture

PCT No. PCT/CH78/00040 Sec. 371 Date July 30, 1979 Sec. 102(e) Date July 23, 1979 PCT Filed Nov. 24, 1978 PCT Pub. No. WO79/00328 PCT Pub. Date June 14, 1979 A non-magnetizable steel casting alloy with 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.sub.2 0.02-0.20% ______________________________________ the remainder being iron, and with a magnetic permeability .mu..ltoreq.1.20 and with a CrNiMn 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.ltoreq.f.ltoreq.+2, meets in the best manner the total set of the following properties: deep permeability, homogeneous strength and toughness values, structural stability at low temperatures, homogeneous magnetic permeability in large solidification cross sections, good machinability and weldability without micro-cracks, and a sufficient yield point. Furthermore, uses in high field intensities and low temperatures and a process in which a heat treatment takes place after welding are subject matter of the invention.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a non-magnetizable steel casting alloy, the use 
thereof and a process for manufacturing the alloy. 
2. Description of the Prior Art 
Heretofore, for cast components for fixing magnet coils and also for highly 
stressed parts of electrical machines (e.g. single-phase rotary current 
generators) which may not cause any problems or constrictions of the 
magnetic flux, preferably austenitic spheroidal cast iron of the type 
GGG-NiMn 13 7 or GGG-NiMn 23 4 or austenitic cast steel according to basic 
pig iron specification 390 or ASTM A-296 CF 20 are used. 
These alloys all have the disadvantage that the total set of properties 
required of such parts can not be met by any of these cast iron-carbon 
alloys, namely, 
deep permeability .mu. 
homogeneous strength and toughness values up to large wall thicknesses, 
structural stability at low temperatures to -196.degree. C. and during 
changes in temperature, 
homogenous magnetic permeability in large solidification cross sections in 
the range of 100-500 mm even in the residual solidification zone, 
machinability and weldability which is at least as satisfactory as for the 
standard rustproof steel casting alloys, e.g. Material No. 4308 or 4408 
(DIN 17 445), 
yield point or proof limit of at least 250 N/mm.sup.2, 
weldable without micro-cracks. 
Depending on the composition, high-alloy CrNiMn cast steel may be fully 
austenitic or, with an appropriate increase of the Cr content or reduction 
of the Ni and/or Mn content, may additionally contain more or less high 
portions of ferrite in the austenitic basic structure. The austenitic 
phase is non-magnetic, with a very low magnetic permeability 
(.mu..ltoreq.1.001), while the ferritic phase is ferromagnetic with 
correspondingly high permeability values. For this reason, in two-phase 
austenitic-ferritic alloy, the magnetic permeability strongly increases 
together with the ferrite content (FIG. 5). Therefore, for so-called 
non-magnetic alloys with very low permeability, exclusively fully 
austenitic alloys are used if these steels are to be used as wrought 
alloys. This course can not be taken in the case of cast steel, since the 
alloys cannot subsequently deformed. Due to their high susceptibility to 
heat cracks during welding, fully austenitic CrNiMo steel casting alloys 
are practically not weldable without cracking. This problem does not arise 
in the wrought alloys (forging steels and rolled steels) to such an extent 
since these steels are much stabler in respect to the susceptibility to 
heat cracks during welding as a result of the deformation and the 
consequently possible subsequent change in granulation of the structure 
through recrystallization by means of a heat treatment. 
As is well known, the weldability of the cast CrNiMn steel alloys is 
improved significantly, when these alloys have certain ferrite contents. 
In this regard, it is not important how much ferrite these alloys have in 
the state of use, for example, at room temperature, but what ferrite 
contents they have during the welding state. During welding, i.e. in the 
state of equilibrium in the vicinity of the melting point, the ferrite 
content should be about 5%. Our tests have shown that these ferrite 
contents are already achieved during the welding of alloys when they 
contain only about 2% ferrite in the cast state at room temperature. 
It is true that CrNiMn steel casting alloys with ferrite contents of more 
than 2% are known; however, they can not be used as non-magnetic, 
rustproof steel castings since the permeability is too high due to the 
ferrite content. 
SUMMARY OF THE INVENTION 
The invention is based on the object to avoid the above-mentioned 
disadvantages and to meet the above-recited set of properties. More 
particularly, a non-magnetizable steel casting alloy is to be provided 
which simultaneously is immune to weld cracking. 
This object is met by the characterizing features of the main claim. 
The alloy according to the invention can be advantageously used in 
components of nuclear fusion reactor plants where field intensities H of 
more than 10.sup.3 Oersted prevail, however, they can also be used at 
temperatures below -150.degree. C. 
It has been found that the reason for the strikingly more favorable 
behavior during the welding of the alloys in the presence of certain 
ferrite portions is to be found in the morphological peculiarities in 
respect to solidification of the alloying system FeCrNiMn. In the alloying 
range of up to 20% Cr, up to 15% Ni and up to 20% Mn, a peritectic melting 
interface separates the austenitic primary solidification from the 
ferritic primary solidification. Starting from the partially known ternary 
systems FeCrNi and FeCrMn and supported by alloying tests, the following 
relationship was found for the peritectic melting interface: 
f=6.5-%Cr-0.4.multidot.%Ni+0.1.multidot.%Mn+0.075.multidot.%Cr.multidot.% 
Ni+0.013.multidot.%Cr.multidot.%Mn-0.02.%Ni.multidot.%Mn

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Quaternary FeCrNiMn alloys with a CrNiMn equivalence factor f&gt;2 solidify 
primarily austenitically and, therefore, are fully austenitic at room 
temperature. Alloys with f&lt;2 solidify primarily ferritically. When the f 
values are not too low, the ferritic primary solidification is followed by 
a binary peritectic reaction. In the case of f&lt;0, the solidification of 
the alloys terminates with the peritectic reaction. In the case of alloys 
with 0.ltoreq.f.ltoreq.2, an austenitic residual solidification, which 
also leads to fully austenitic products, takes place after the ferritic 
primary solidification and the binary peritectic reaction. The important 
peritectic reaction has the result that austenite is formed by dissolving 
the primary-formed ferrite, contrary to the primary austenitic or 
austenitic residual solidification, in each of these cases the austenite 
being formed from the melt without the participation of ferrite. 
During welding which constitutes a remelting, the phase reactions take 
place in the reverse order. In fully austenitic products, in each of the 
two cases, i.e. for f&gt;2, the grain boundaries formed by primary residual 
solidification are melted. In the range of the line of melting of the 
austenitic welding, this results in the dangerous heat cracks which 
practically exclude an absolute safe weldability. In the cases of 
f.ltoreq.2, in which the solidification concludes with the peritectic 
reaction, the phase boundaries ferrite/austenite will melt. From a 
morphological viewpoint, this is a completely different situation which 
has the result that heat cracks do not occur during welding. The knowledge 
of this situation makes possible the selection of those alloys which have 
just as much ferrite as necessary in respect to their weldability and as 
little as possible in respect to a low magnetic permeability. For this 
purpose, the alloying elements Cr, Ni and Mn which are important for the 
structure must be brought into the relationship defined above, resulting 
in a strict selection of the alloy. Since the CrNiMn steel casting alloys 
have the tendency to reach a state of imbalance during the cooling after 
the casting and welding, a subsequent heat treatment may become necessary 
in order to adjust the equilibrium which is imminent to the system. The 
effect of such a heat treatment in respect to a reduction of the ferrite 
content is shown in FIG. 4, which shall be explained in more detail 
hereinbelow. 
The combined increase of the nitrogen and the manganese contents in the 
steel casting alloys according to the invention makes it possible to 
significantly raise the yield point, compared to conventional purely 
austenitic chrome nickel steels, without creating porosities due to 
nitrogen precipitation as they may occur in the case of a strong 
segregation in large casting cross sections due to insufficient nitrogen 
solubility. 
The same effect could be achieved also by other elements, for example, by 
carbon or phosphorus. 
However, it has been found that increasing carbon contents result in a 
deterioration of the machinability due to the increasing hardness of the 
martensite created by the cold forming during machining and that, on the 
other hand, increasing phosphorus contents negatively effect to a 
significant degree the toughness values even in the solution-treated 
state. The steel casting alloy according to the invention is preferably 
used with a carbon content of C 0.06% according to claim 2 in order to 
limit the carbide precipitations and to avoid an embrittlement, 
particularly during stress annealing. Additional advantages of the low 
carbon content reside in the better machinability and the protection 
against intercrystalline corrosion. The reduction in yield point caused by 
the lower carbon content is compensated by an increased nitrogen content. 
The immunity to cracks during welding is significantly increased if a S 
content according to claim 4 is chosen. 
The contents of chromium and nickel in the steel casting alloy according to 
the invention depends on the operating temperature of the plant 
components. In the case of low operating temperatures, high 
chromium/nickel contents should be chosen in order to insure the austenite 
stability. 
In the following, an embodiment of the invention is explained wherein the 
compositions are always given in percent by weight. 
EXAMPLE 
A steel casting alloy according to the invention with the composition 
______________________________________ 
C Mn Si P S Cr Ni Mo N.sub.2 
______________________________________ 
0.044 11.6 0.74 0.028 
0.010 15.7 8.72 0.05 0.155% 
______________________________________ 
the remainder being iron with the usual accompanying elements and 
impurities resulting in cast structural componentlike samples with a wall 
thickness of 200 mm, in the following mechanical properties: 
______________________________________ 
Yield point Rp 0.2 281 N/mm.sup.2 
Max. tensile strength 
Rm 466 N/mm.sup.2 
Elongation at 
rupture A.sub.5 39% 
Necking Z 47% 
Notched bar impact 
work K.sub.CV 178 Joule 
Brinell hardness HB 159 
______________________________________ 
wherein the equivalence factor f was -0.916. 
On the otherhand, a conventional austenitic cast steel pursuant to ASTM 
A-296 CF 20 of the composition 
______________________________________ 
C Mn Si P S Cr Ni Mo N.sub.2 
______________________________________ 
0.17 1.16 0.82 0.009 
0.010 19.7 8.62 0.04 0.060% 
______________________________________ 
the remainder being iron, accompanying elements and impurities, resulted in 
a lower yield point and notched bar impact work. 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. Moreover, CF 
20 is not weldable without micro-cracks. 
It is referred to FIGS. 1 to 4. FIG. 1 illustrates the graphical comparison 
between the alloy A according to the invention (solid circles) and the 
other alloys (solid triangles and squares) in the case of turning. The 
cutting speed V (m/min.) is plotted on the abscissa and the total life T 
VB 0.4 (min) is plotted on the ordinate. 
FIG. 2 relates to milling cutting. The cutting material is Widia TT 40. The 
plate dimension is TNAF 2504 ZZR. The feed is s.sub.z =0.11 mm/tooth and 
no cooling was performed. The cutting speed V (m/min) is plotted on the 
abscissa and the endurance L (mm/tooth) is plotted on the ordinate. 
FIG. 3 relates to the comparisons in the case of drilling. The work 
material is CF 20. The tool is a HSS twist drill with a diameter of 5 mm. 
The feed is s=0.06 mm/rot. Cooling was performed by means of an oil 
emulsion. The cutting speed V (m/min) is plotted on the abscissa and the 
endurance 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, wherein 
______________________________________ 
the curve 
I shows the condition after 
casting, 
the curve 
II shows the condition after 
welding, 
the curve 
III shows the condition after 
annealing at 1100.degree. C. 
the curve 
IV shows the condition after 
annealing at 850.degree. C. 
the curve 
V shows the condition after 
annealing at 650.degree. C. 
______________________________________ 
The region of the ferritic primary solidification is to the left of the 
vertical axis through f=2 and the region of the austenitic primary 
solidification is to the right of this axis. 
FIG. 5 shows the relationship between the permeability .mu. which is 
plotted on the abszissa and the ferrite content Fer in % which is plotted 
on the ordinate. 
A preferred embodiment of the alloy results from Claim 3. In this claim, 
the factor f=-2 was chosen. After solidification, point 1 on curve I of 
FIG. 4, the ferrite content is 3%. During welding, point 2 on curve II, 
the ferrite content increases to 6%. By an annealing treatment, point 5 on 
the point of intersection of the abscissa and the curve V, the ferrite 
content can be reduced to 0.1% which corresponds to a .mu. value of 1.02. 
If necessary, this .mu. value can be further reduced by an appropriate 
heat treatment. 
The favorable magnetic permeability of the steel casting alloy according to 
the invention is maintained also in structural components of large cross 
sections in the range of 100-500 mm, preferably 200-300 mm, even in the 
residual solidification zone, because a high austenitic stability and a 
high homogeneity of the properties is achieved even with modest alloying 
expenses. Especially in the case of extremely strong magnetic fields of, 
for example, 10.sup.3 Oersted field intensity, as they are required in 
fusion reactors for the shaping of the plasma, the alloy according to the 
invention provides significant advantages over the conventional alloys. 
The measurement of the magnetic permeability of the steel casting alloy 
according to the example was performed by means of the magnetoscope of the 
Type 1.067 (Institute Dr. Forster) and resulted in the following values 
for a test specimen of the dimensions 200.times.200.times.300 mm (about 
250 .mu.m peak-to-valley height) over the cross section of 300 mm: 
______________________________________ 
.mu.(G/Oe) 
Treatment 10 50 100 150 200 240 mm 
______________________________________ 
1100.degree. C./10h/H.sub.2 O 
1.011 1.017 1.013 
1.011 
1.019 
1.013 
______________________________________ 
Especially advantageous steel casting alloys, for example, have the 
composition in percent 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 
______________________________________ 
the remainder being iron and possibly accompanying elements and impurities, 
and the composition in percent 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 
______________________________________ 
the remainder being iron and possibly accompanying elements and impurities. 
The equivalence factors are f=-1.61 and f=-1.635, respectively.