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- EXTRA EDITION
In the first part of this three-part series, we talked about the effect cold shortness has on the use of cast iron in refrigeration equipment. Now we will look at porosity and abrasion resistance which must be, after cold shortness, the “condition number two” in appreciating the qualities of a cast iron used in refrigeration systems.
Porosity and abrasion resistance are discussed together, since they are close to one another by the ratio Si/Ni or Si/Ni+Cr, and also because generally a piece exposed to wear must be gas-proof, too. We must not think only of a cylinder liner or a piston, but also a valve body, pump casing, trunk of ejector jet pumps, etc., which are exposed to wear and tear owing to the erosion of the metal under the action of the shocks due to the flow of the liquid or the gas.
If the cast iron is not gas-proof, it is due structurally to long-shaped graphite separations. In order to ensure the cast iron is gas-proof, the graphite must be in the form of short, compact (nearly round) flakes and as uniformly distributed as possible. The refinement of graphite can be made by a proper heat treatment or by alloying with nickel. Nickel leads to the formation of suitable graphite especially in gray cast iron proper in silicon. If it is well correlated with the silicon content, nickel lessens the effect of hardening, which leads to the appearance of the hard points of the casting and increases the density of the latter. Meanwhile the porosity formation tendency is reduced. An important field of applying cast iron alloyed with nickel is that of the crankcases of refrigeration compressors. For the execution of arming, used in the refrigeration industry, gas-proof cast iron is also recommended.
The interchanging ratio between silicon and nickel is chosen accordingly to working conditions. It is of 1/2 to l/4 and increases for silicon-short cast irons up to 1/6. Silicon is advantageous for wear resistance, due to the formation of silico-ferrites, more wear resistant than ferrites. Nickel also acts in the same direction. The optimal ratio between silicon and nickel must be established after suitable tests in the case of pieces from gas-proof and wear resistant cast iron. One cannot diminish the nickel content, because there would appear porosities, but one cannot diminish either the silicon content, because the wear resistance will be lessened.
This ratio may be determined as a function of the thickness desired of the cast-iron piece (Fig. 1 and 2), for different meltings (2.75 percent C; 3.l to 3.5 percent C and 3.4 to 3.6 percent C).
A further improvement of the basic metal structure and of wear resistance can be made also by alloying with chrome (up to 0.5 percent). Good results have been obtained in this instance by the Factory of Refrigerating Machines in Odessa, which used cast iron on the cylinders of refrigeration compressors running on CFC-12 and HCFC-22. The cast iron used has a pearlitic base with graphite in the form of small flakes. Cast irons 3 and 4 alloyed with copper have been used in order to compare characteristics (Fig. 3).
If the cast iron is not gas-proof, it may be due to fine fissures, which appear as a result of heat treatment. Blows caused by gases coming out from the solution, or by gases of the reaction, as well as shrink holes produced by normal contractions are seldom the reason for the leakage of cast irons. In the case of cast irons rich in phosphorous, a large contraction may lead to the appearance of so-called drops of transpiration. These drops are the segregations of some residual eutectics with low melting point, which are pressed out from the piece by the pressure of the contraction. By establishing the content of carbon, silicon, and phosphorous, this unpleasant property is much diminished. It has been observed, too, that porosities disappear by diminishing simultaneously the percentage of carbon and phosphorous.
Generally, wear appears owing to mechanical removal of some small particles from the material, surface deformation or cold hardening of the material, and oxidation phenomena between the metals in contact.
There is an issue to condition the degree of wear resistance by surface hardness. Is hardness a criterion in determining wear? Is wear inversely proportional to hardness? Hardness of a material does not allow a sure conclusion upon its behavior in conditions promoting wear. Experiments have shown that in the case of high quality cast irons, sliding-friction wear increases with the hardness difference of the parts working together. Wear of the moving part is always larger than that of the resting one; that is why the first needs a relatively larger hardness for an equal wear time of both parts. Hardness will have been used only as a means of expression of a practically ascertained condition.
It has been observed that uniformly pearlitic cast iron with small shaped graphite and sufficient lubrication endures specific pressures of over 20 daN/cm2 without any mark of gripping. Wear is also very much influenced by the form of the graphite, as well as by its quantity.
Fine graphite has the best behavior. The amount of graphite gives the material certain lubricating properties, making loose its structure at the same time.
Silicon diminishes the carbide content; therefore hardness is diminished and made uniform as a result of increasing hardness of the silicon-ferrite. At the same time, at high silicon contents, the graphite reduces its shape. Optimum silicon contents at sliding wear (removable liner) are situated between l.75 to 2 percent.
Nickel also reduces the carbide content, but increases hardness in parallel with refinement of graphite grains. Nickel also reduces oxidation, which can be considered as the main cause of wear of the plunger case assembly in the presence of lubricating oil.
Chromium up to 0.75 percent has the same influence as nickel. At higher percentages it acts unfavorably in the sense of diminishing machinability and resistance to bending.
Copper does not reduce hardness, but increases it a little.
Sulphur, especially in the form of iron-sulphide, promotes cementitical solidification, which leads to the formation of hard points (in cast iron) and to large inner stresses, which promote fissures; thus, mechanical characteristics are directly influenced. In the case of removable liners, the cementitic structure leads to a rapid wear and tear of the piston and of piston rings. The practical conclusion of a great number of research works is that in cast iron, the sulphur content must be diminished as much as possible. A sulphur content not exceeding 0.08 to 0.l0 percent is considered generally as satisfactory.
Then there is the influence of different alloying elements upon wear behavior of cast iron (Fig. 4). Compressor cylinders without lubrication must be formed of cast iron with high antifrictional properties and with an improved structure as compared to that used for cylinders with wet working. Casting of such cylinders must be made centrifugally, increasing density, resistance, and wear resistance. For compressors with a high number of revolutions and high pressure, for the execution of cast iron removable liners, the following composition are used successfully: 2.6 to 3.0 percent C; 1.4 percent Si; 0.8 to 1.3 percent Mn; 1.8 to 2.2 percent Cr; 16 to 17 percent Ni; 5 to 7 percent Cu. In the mass of the cylinder of compressors graphite must be small and medium grained and of linear or spherical form. Eutectic graphite and ferrite are allowed only in a proportion of maximum 5 percent of the ground-slide surface of each extract.
In order to increase the wear resistance, porous chromium plating is used on the inner surface of the shirt of the cylinder. Thus, a hardness of 800 to 1000 HB can be obtained and a wear resistance of four to five times larger than that of a liner without chromium plating. It is important that chromium plating of liners reduces one-and-a-half to two times the wear resistance of segments and diminishes friction since oil is retained in the pores of the deposited chromium layer. Thickness of the chromium layer may be of 0.05 to 0.25 mm. However, it is recommended to renounce the impregnating of the walls with different tightening solutions and to the chromium plating of a surface exposed to wear, since an alloyed cast iron, founded in technologically suitable conditions, can assure tightness and a wear resistance necessary for removable liners used in refrigeration compressors.
In the construction of pistons for slow refrigeration compressors, functioning with ammonia and halogenated refrigerants, cast iron alloyed with nickel is used successfully. But there is a tendency to replace it with aluminum alloys, replacement imposed by diminishing of the piston mass, especially for high speed compressors. Graphitized aluminum is used successfully, too.
Resistance and Hardness at Low Temperatures
Low temperatures do not have a very great influence upon resistance and hardness of cast-iron pieces. According to some research, stretching resistance and Brinell hardness remain unchanged down to -150°C, or increases insignificantly.
Generally, one can observe that also at low temperatures, not only at temperatures above 0°C, the variation of resistance with dilatation of length is not linear. The non-linear relationship between tension and deformations appears because of the effect of concentration of the efforts, owing to the granules of graphite, especially in the case of cast irons with flaked graphite.
At design, the cast iron quality used for manufacturing a piece — ensuring that its mechanical properties cover the working strains of the piece, respectively — must have a safety coefficient. The value of these coefficients is chosen as a function of the nature of the strain and in function of the application of the piece. Thus, it is not recommended for the designer to use too high of a coefficient for the design of a crankcase for compressors in transport refrigeration because the heavy construction would result in a product that was uncompetitive in the market.
At design, the exact consideration of working strains is sometimes impossible, especially in the case of fatigue strain, where it is very difficult to determine the loading cycle. A sure coincidence between the strains considered in design and working strain can be obtained only by studying the presumed manner of rupture of the piece. Resistance can be used only in the case of a piece exposed to traction in a static regime. But these strains are very seldom. In most cases (over 90 percent), by making a detailed analysis of the sort of strain, we can observe that it is a matter of fatigue strain, where ultimate tensile strength cannot be used. The resistance values for different qualities of cast iron, at different fatigue strains are intuitive and can be used successfully in the activity of designing cast-iron pieces, being the result of a wide practical experience.
In the case of pieces working at low temperatures, one can observe from the preceding section, that irrespective to the structure of the basic metallic mass and to the form of the graphite, resistance increases with decreasing temperature. This means that resistances corresponding to a temperature of +20°C are covering in the case of exploitation of the piece at low temperatures.
While the notching effect of the inserted graphite is the reason for the resistance decrease, as compared to a basic mass without graphite, the contraction effect of graphite and its effect to deviate the course of stresses determine the elastic behavior to deviate from Hook’s law. As shown earlier, this deviation does not depend on the basic mass, but on the heterogeneous mixture: basic mass - graphite. Cast iron, in contrast with steel, does not have an accurately defined flowing limit. Elongations increase faster than loadings until finally, after surpassing the separation resistance, the break appears and it has practically no elongation. Owing to the absence of this well defined (natural) limit, the resistance corresponding to a remnant elongation of 0.2 percent is taken as flowing limit. (Similarly, the traction resistance is established conventionally, too, and it corresponds to an elongation of 0.03 percent). For pieces of high precision owing to the very little deformations (under 1 percent) for flow resistance and traction resistance, in calculations we may take the flow resistance as design resistance. Generally, for cast irons except cast irons with nodular graphite, the ratio of flow resistance to traction resistance is equal to unity, thus, one more reason to take the flow resistance for design resistance.
Publication date: 10/22/2012