Modified "NiResist", "Nomag" Cast Irons and Technological Processes of their Manufacture
 
Sheyko A., Bondarevskyy V., Sluchovskyy O., Zeleny B.
Physique-Technological Institute Metals and Alloys of
National Academy of Science of Ukraine. Kiev.
(Note:  Highlighted words are in place of Greek symbols)
 
"NiResist" and other austenitic class cast-irons allow us to obtain cast-iron castings with the complex of high physical, mechanical and working properties (high corrosion and erosion properties, heat resistance, cold resistance, non-magnetic, etc.)
 
In Physico-Technological Institute of Metals and Alloys in Kiev in Ukraine since 1960 we had been occupied for a long time with problems of ductile cast irons of an austenitic class. Moreover, we have been investigating not only "NiResist" and "Nomag" cast irons but also austenitic cast irons with lower Ni content including Ni-free and manganese-copper austenitic cast irons.
 
Here we shall report some aspects of obtaining and properties of "NiResist" and "Nomag" cast irons that, as we consider, have theoretical and practical interest.
 
In our investigations of "NiResist" ductile cast-iron the content of Ni in it ranged from 12 to 20%, Mn content from 0.5 to 4%, Cu content up to 6%, Cr content from 0 to 2%.
 
Cast irons of pre-eutectic, eutectic and post-eutectic compositions were investigated.
 
From this cast iron the thick-walled castings were manufactured. For example, centrifugal castings, weighing from 8 to 25 tons with the thickness of the wall up to 120mm for the parts of pulp and paper equipment. The thin-walled castings weighting from 0.5 to 4 kg were also manufactured for oil pumps.
 
Cast iron was modified by Ni-Mg alloying composition or by alloying composition containing Mg, Ca, REM, Si and Fe.
 
In modification of "NiResist" and more rarely "Nomag" cast irons we could often observe occurrence of their demodification. This is the occurrence when together with globular graphite in separate areas of the casting the graphite of eutectic-like shape takes place. Hence all negative consequences for the properties of cast iron. This occurrence is specific for high-alloyed cast irons. In modification of austenitic cast-irons by the modifier of Ni-Mg alloying composition type (or complex Fe-Si-Mg-Ca-4REM modifier containing Mg, Ca, Ce) the instability of modification process was considered to be one of different reasons independent on the melting conditions. The modification instability showed itself in the fact that in austenitic cast iron structure together with globular graphite the numerous colonies of so-called frame-type eutectic graphite have formed, that on 1/3 reduced strength properties and on 2/3 reduced plastic properties of modified cast iron.
 
We connect this occurrence with peculiarities of microinhomogeneous structure of the melt of cast iron before modification and crystallization and, first of all, with the character of micro-distribution of C atoms in the melt. Structural state of the melt, in its turn, to a great extent is determined by the technological parameters of metallurgical process (by the state of initial charge materials and sequence of their melting, heat and time conditions of the manufacture).
 
Our conducted diffractional investigations liquid cast iron structures obtained in different technological conditions of melting allow us to make this conclusion. The investigations were conducted on X-ray v-v diffractometer in neutral atmosphere of helium. The measuring of intensity of monochromatized Mo-K_alpha emission, scattered by free surface of the melt, was conducted in the interval of magnitude of dispersion vector S from 1.0 to 12.0 Å [ 4pi/lambda sinv, lambda-length of wafe of Mo-K_alpha 2v-angle of dispersion]. The interfering functions and functions of radial distribution of atoms
[4pir² lambda(r)] (integral Furies-analysis) were calculated from the obtained diffraction patterns (pictures). Beforehand the investigations of alloy (when Fe content 75.4%, Ni content 21.4%, C content 3.35%) was conducted. Obtained results show that the process of micro-distribution of graphite depends on sequence of charge components and heat and time conditions of the melting. Graphite dissolution simultaneously with carbide formation and its dissolution. This process proceeds at about liquidus temperature for continuous time [from 6 to 8 hours at a temperature of 30-50^o above liquidus line].
 
At a certain sequence of injection of C and Ni into the melt the process of graphite dissolution in this melt can be accompanied by the formation of intermediate phase - carbide with equal atom composition with MeC type hexagonal lattice (the analogue of WC or MoC carbides). We suppose that Ni carbide formation takes place.
 
The metastable double-phased state (carbide + fluide) on balanced liquidus line stays preserved for continuous time (8 hours and more). Under crystallization of this double-phased system the decomposition of MeC carbide with precipitation of graphite occurs.
 
The increase in a temperature on 200-300^oC (higher than T_lig) quickly brings the system to homogeneous liquid state. At the same time microhomogeneous structure of the melt is characterized by essentially inhomogeneous distribution of C atoms in microvolumes of the melt. All the above-mentioned is illustrated by Figure 1. On this figure some of sequentially-obtained diffractograms of the triple alloy at a temperature of 1260^oC are represented. We found out that in time the intensity of graphite lines reduces (at S=18.5 mm^-1, and 37 mm^-1), and intensity of reflections from MeC carbide lattice increases.
 
At continuous enough isothermal holding the X-ray pattern becomes typical for the liquid state. The character of microinhomogeneity of liquid alloy in essentially caused by uneven atom distribution on the melt microvolumes.
 
Microareas, strongly enriched by C, have carbide-like type of package (atom distribution in this sort of microareas is like their distribution in hexagonal lattice of the MeC carbide). Microareas, consisting mainly of metal atoms (with small C content) have BCC or FCC-like type of atom package (austenite or cementite-like type of package). The results of modeling of the melt structure within the framework of quasi-poly-crystal model.
 
Thus, if at a high temperature the complete melting of the Ni carbide formed, didn't occurred, and we cool the melt, so, in the area of liquid-solid state complete decomposition of this melt will occur. In a solid of cast iron this carbide isn't observed. We suppose that this occurrence affects the process of cast iron demodification. This process can't be eliminated by the increased content of Mg and other modifying elements in cast iron.
 
For the investigation of Fe-Ni- Mn-C alloy, two samples of metal of the same composition Fe=70.4%; Ni=9.8%; Mn=5.9%; C=3.8% were smelted. Main differences in the melting were in the sequence of charge components, injected into the melt. During the melting of sample 1 mn charge component was injected into the melt the last, and during the melting of sample 2 Ni was injected the last.
 
X-ray structure investigations were conducted about liquidus line. First diffractograms were measured after 20 minutes isothermal holding, second ones - after 2 hours.
 
For conduction of the integral analysis (the calculation of the functions of atom distribution) the second curves of typical liquid sort (without additional sharp side tenons) first diffractograms of sample I and 2 (curve 1 and 2) and one of diffractograms of above-mentioned Fe-Ni-C alloy are presented on Figure 1.
 
The comparison of diffractograms shows, that the most expressed diffraction effects (sample 2) [at 1.6; 2.2; 4.0Å .], observed for the investigated alloy correspond to the most intensive lines of refletion of MeC carbide hexagonal lattice in liquid-solid triple alloy (curve 3). Unlike triple alloy, in this case we can observe only separate diffraction effects. These effects are marked feebly, and in sample 1 they are completely absent. Nevertheless by them we can estimate parameter "a" hexagonal lattice of MeC carbide which small fraction is typical for the melt of sample 2 of investigated alloy. Parameter "a" for Fe-Ni-Mn-C alloy is equal 0.3-031 mm. For triple alloy parameters "a" and "c" were equal to (0.315 ± 0.004 mm) and (0.38 ± 0.005 mm) accordingly.
 
It this lattice the shortest is the distance between atoms of metal and carbon and it is equal to 0.263 mm. The distance between metal atoms [6 nearest neighbors] is equal to parameter "a" of the lattice (0.3-315 mm).
 
In liquid state, owing to the weak despersive ability C atoms, the coordination of MeC makes just a small contribution to the curve of radial atom distribution. [This is a peculiarity of X-ray structure analysis.] . Therefore the curve of atom distribution 4pir² S(r) (Figure 2) reflects on the whole the distribution of metal atoms. [In this very figure i(s) of the second diffractograms of samples 1 and 2 are also represented].
 
It can be seen from the 4pir² S(r) that for second sample the distance (when r =0.3 mm) which is correspondent to the distance between Me-Me atoms in the hexagonal lattice of MeC carbide.
 
We suppose, that a melt of sample 2 contains a considerable part (share) of carbide-like-typed microclusterings and microclusterings consisting mainly of metal atoms with lambda-solution type package (GammaLIK-like type) [maximum at r=0.26 mm].
 
The melt of sample 1 consist mainly of the micro-clusterings of second type lambda-solution type, more saturated with C. Longer the most probably distance between atoms -r₁, for the first sample in comparison with the second one (0.265 mm in comparison 0.260 mm). [Here is different 2% when error is 0.5%].
 
Hence, the method of melting of Fe-Ni-Mn-C alloy influence considerably micro-distribution of C atoms in the melt. For Fe-Ni-Mn-C alloy (70.4%Fe, 9.8%Ni, 5.9%Mn, 3.8%C) the order of input of charge materials in furnace clucible essentially influences the micro decomposition of C atoms in this alloy. In a liquid state the part of microareas strongly enriched (saturated) with carbon atoms is much bigger in case, when the last from charge materials Ni is injected into liquid melt. When injecting Mn as a last from charge materials, the microdistribution of C atoms is more homogenous.
 
From Nomag ductile cast iron the large-sized castings of pressure (forcing) rings for turbogenerator stators, castings for electrotechnical industry and ets were manufactured.
 
We investigated the influence of Ni (from 6 to 12%), Ni-Mg alloying composition and Fe-Si-Ca-Mg-2REM alloying composition on the proportion of structural components at cooling rate of from 0.41 to 4.1 K/s and mechanical properties of these cast-iron when Cu content in them was 3%. On increasing the rate of austenitic ductile cast iron by Ni, the total amount of carbide component is decreased. The same happens on decreasing the speed of crystallization. The increase in Si content decreases the carbide component in cast irons, but it increases the quantity of graphite inclusions that especially shows itself at high cooling speeds. With reduction in cooling speed the quantity of graphite inclusions in the volume decreases, which is explained by the increase in carbon solubility in austenite.
 
Austenitic cast irons contain both the elements promoting graphitization (Ni, Cu, Si) and those preventing it (carbide-forming element Mn).
 
Taking into account complex influence of the above-mentioned elements on graphitization processes and cast iron structure formation, first of all it was necessary to define an optimal content of Si and Mn and their influence on the stability of an austenitic base.
 
As a result of conducted investigations we found out that destabilizing influence of Si shows itself in the interval of cooling speeds from 3 to 25 K/min. Upon increasing cooling speed the concentration-kinetic threshold of Si destabilizing influence moves in the direction of higher Si concentrations (Figure 3.)
 
The study of Mn influence on obtaining austenitic structure showed that under the conditions when the cooling speeds ranged from 12 to 0.5 K/s, the minimum amount of decomposition products of austenite could be recorded when Mn content was 6-6.5%. (Figure 4.)
 
Lower content of Mn leads to destabilization of austenitic matrix, while its higher content leads to a considerable increase in the amount of carbide-austenite component and a pearlite-like component, impoverished austenite of which is instable. (Figure 5)
 
It is well known that cast irons are the alloys with developed segregation processes on the grain level. The investigation showed that the microdistribution of alloying elements in grains of austenitic cast irons is uneven. Segregation of Ni and Cu is inverse and reaches 2% for each element. Mn segregation is direct and reaches 4-4.5%. (Figure 6.)
 
We discovered that the distribution of C and Si indicates the direct segregation of these elements. The obtained data allows us to explain the existence of a pearlite-like component on grain periphery. The consideration of the combined influence of a cooling speed, the alloying by Ni and the content of Si allowed us to discover some objective laws in correlation of structure components in lean-alloyed austenitic cast irons:

The investigation of mechanical properties of austenite cast iron in cast and heat-treated conditions was conducted under their modification NiMgREM and FeSiMgCa-2REM alloying composition with Ni content from 6 to 12%, Si content from 2 to 3% and constant Mn content.
 
Stable and plastic properties of lean-alloyed austenitic cast iron showed their evident dependence on above-mentioned factors. (Figure 8.)

The tendency of alloys with austenitic structure to distorting transformations (any kind of mechanical tests and mechanical treatment are considered to be deformation) and also to plasticity caused by transformation and plasticity caused by twinning takes place in austenitic cast irons.
 
We also investigated mechanical properties of "Nomag" cast iron under the conditions of compound stressed state at a complex force-temperature influence. On figure 9, you can see general result at a temperature a 173 K, 293 K, 473 K. Rather high isotropy of cast iron mechanical properties attracts the attention at the positive temperatures. The proximity of experimental points and rated value sigma_b (by Colon's criterion) and sigma₀₂(By Myzes criterion) testify to it.
 
The investigations were performed on tubular samples under 7 variants of state of major stresses. It was achieved by the combination of using axis load and its direction (compression or tension) and tangential load (internal pressure) and their relation.
 
We should emphasize the correlation K= -I (simultaneous axis contraction and tangential tension). Under this correlation of major stresses the level of values of sigma₀₂ is equal to 50-60% from the level of sigma₀₂ at mono-axis tension K=0. Using this technique as a macromodel of the stressed state at the grain level at the interaction of II type with the applied load one can suggest the possibility of deformational transformations at lower stresses. (Figure 9.)
 
The expediency of using austenitic cast irons for castings can be defined not only by high mechanical properties but also by opportunity to obtain the complex of special properties (cyclic ductility, low magnetic penetrability, hydroabrading and corrosion resistance).
 
We found out certain dependencies of magnetic permeability bath on the degree of alloying by Ni and content of Si and on cooling speed in the interval from 0.4 to 4.1 k/s. These dependencies belong to cast state. The increase in cooling speed and increase in Ni content reduce magnetic permeability. With the rise of Si content in cast-iron the magnetic permeability will increase. (Figure 10.)
 
Heat treatment allows us to improve magnetic permeability of austenitic cast- iron of alloying degree. This permeability doesn't exceed µ=1.3 * 10^-6 H/m.
 
Increased demands of hydroabrading resistance of lean-alloying austenitic cast iron are made on materials of different function pumps. The influence austenitic cast-iron structure and angle of attack of abrading particles on hydroabrading was investigated. The structure with 10-15% of isolated carbide inclusions in austenitic matrix was found as optimal one.
 
The comparative tests, conducted in laboratory and on special stand, showed 1.3-1.6 times higher hydroabrading resistance of developed austenitic cast iron in comparison with alloyed steels. (Figure 11.)
 
"Nomag" cast iron has a high corrosion resistance in a number of corrosive mediums (iodine-bromide water, layer liquid, ammonia water, etc.).
 
Corrosion resistance in seawater under continuous nature tests ranges from 0.045-0.055 mm/year.
 
We have developed and mastered technological processes of manufacture the casting of high technological complication from ductile austenitic cast iron for pumps, compressors, energetic machines, etc.

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