Microstructure of the Surface Layer during Diffusion Chromizing of Armco Iron and Steel 35
Abstract Generated abstract
This study examines the microstructure and properties of surface layers formed by diffusion chromizing of Armco iron and grade 35 steel. Specimens were treated for 10 hours at 1000, 1100, and 1200°C in a ferrochrome, alumina, and ammonium chloride powder mixture, followed by metallographic examination, hardness measurements, and corrosion tests. In Armco iron, chromizing produced a chromium solid solution layer whose depth increased with temperature and which showed good resistance in fresh water but not in salt solution. In steel 35, chromium diffusion was more limited and produced a thin, complex carbide-containing layer with very high surface hardness, while corrosion resistance in water remained poor.
Full Text
Microstructure of the Surface Layer during Diffusion Chromizing of Armco Iron and Steel 35
I. V. Slosman
Introduction
Diffusion chromizing of steel products (cementation with chromium) is a comparatively new method of chemical-thermal treatment of steel, and one that has not yet been sufficiently studied. However, the data from already published investigations make it possible to consider that such treatment provides the possibility of substantially changing certain properties of steel products. When products made of steels with medium and high carbon content are cemented with chromium, the surface layer of these products acquires a very high hardness, unattainable by any other method of thermal or chemical-thermal treatment, and the ability to retain this hardness when heated to high temperatures. When mild steel is cemented with chromium, the surface layer of the cemented products acquires high resistance to corrosion in various media: in air, in fresh water, in nitric acid, and in other media.
The beneficial effect of cementation with chromium on the properties of steel products explains the interest that this new type of steel treatment has aroused among metallurgical researchers. In this connection it should be noted that the principal regularities occurring during cementation with chromium were studied in the works of our Soviet investigators, of which the most important are the works of V. I. Nikolaev [1], N. A. Izgaryshev and E. S. Sarkisov [2], S. S. Kanfor [3], V. I. Arkharov [4], A. P. Gulyaev and G. N. Dubinin [5], E. M. Morozova [6], and other authors.
Aim of the Work and Method of Investigation
The aim of the present work was to study the microstructure of the surface layer obtained during cementation with chromium of iron and steel, and to establish the relationship between the character of the microstructure and the properties of this layer.
The experiments were carried out in a cementing compound that consisted of a mixture of powders of ferrochrome, alumina, and ammonium chloride. Ferrochrome—the principal component of the cementing compound—is the source by means of which the surface of the cemented specimens is enriched with chromium. Alumina is introduced to prevent the ferrochrome particles from sintering and from adhering to the surface of the cemented specimens at high temperatures. This component does not take direct part in the course of the reactions occurring during cementation. Ammonium chloride is an active participant in the process. It is introduced into the mixture in order to create, in the cementation space, an atmosphere of chromous chloride—a gaseous medium without which the rate of diffusion of chromium is very small. In addition, ammonium chloride
creates a positive pressure in the cementation space, excluding the possibility of oxidation of the specimens and ferrochrome during cementation.
The components of the cementing mixture were taken in the following quantitative ratio (by weight):
| Component | Content |
|---|---|
| Ferrochrome | 65% |
| Aluminum oxide | 30% |
| Ammonium chloride | 5% |
After calcination to remove moisture and volatile components, the cementing-mixture components were thoroughly mixed in an electric mixer.
Cementation was carried out in an iron cup having a height of 120 mm, an internal diameter of 60 mm, and a wall thickness of 3 mm. The arrangement of the specimens and cementing mixture in the cup is shown in Fig. 1. After packing, a lid was welded tightly to the cup; a small hole (1 mm in diameter) was drilled in the lid for the escape of gases. This method of packing is more labor-intensive than the methods used by other investigators (for example, coating with clay instead of welding on the lid), but it gives greater assurance against oxidation of the specimens and cementing mixture at high temperatures. Both in our experiments and in the experiments of other investigators performed by this procedure [3], no cases of oxidation of the specimens or cementing mixture were observed.
Fig. 1. Packing arrangement of the specimens.
Cementation was carried out in an electric furnace with silite heaters. The cups were loaded into a cold furnace and heated together with the furnace to the cementation temperature; in one cup, specimens of Armco iron and grade 35 steel were cemented. Heating of the furnace to the cementation temperature lasted 2–3 hours, and holding at the cementation temperature lasted 10 hours. Cementation was carried out at three temperatures: 1000, 1100, and 1200°. After the ten-hour holding period, the furnace was switched off and the cups with the specimens were cooled together with the furnace.
After the cup had cooled, the lid welded to it was cut off and the specimens were freed from the cementing mixture. To study the microstructure of the ce-
a metallized layer, a polished section was prepared from each specimen. The face intended for preparation of the polished section was first ground down to a depth of 1–2 mm, which was known to exceed the depth of the metallized layer.
Mechanism of Cementation
When the cup with the cementing agent and the specimens is heated, the ammonium chloride included in the cementing agent dissociates according to the reaction:
\[ \mathrm{NH_4Cl = NH_3 + HCl}. \]
Ammonia leaves the furnace, having time partly to decompose into nitrogen and hydrogen,
\[ 2\mathrm{NH_3 = N_2 + 3H_2} \]
as is evidenced by combustion of the gas issuing from the opening of the cementation cup, while hydrogen chloride reacts with the ferrochrome powder, forming chlorides of iron and chromium:
\[ \mathrm{Fe + 2HCl = FeCl_2 + H_2}, \]
\[ \mathrm{Cr + 2HCl = CrCl_2 + H_2}. \]
At the cementation temperatures chromium chloride is in the vapor state (boiling point of \(\mathrm{CrCl_2} = 840^\circ\)) and reacts with the iron of the specimen being cemented according to the following exchange reaction:
\[ \mathrm{Fe + CrCl_2 = FeCl_2 + Cr}. \]
Iron chloride, which is a gas at an even lower temperature \((650)^\circ\), is removed from the furnace together with the other gases, while chromium atoms replace the positions of the departed iron atoms and diffuse into the specimens being cemented to a depth determined by the temperature and duration of the cementation process.
On the other hand, owing to the concentration difference of iron at the surface and in the deeper layers of the specimens being cemented, diffusion of iron atoms to the surface of the specimens takes place, and the conditions for the exchange reaction are re-created.
Macrostructure and Properties of the Cemented Layer
The microstructure and properties of the cemented layer are determined mainly by the composition of the steel subjected to cementation, since with different steel compositions the nature of the cemented layer is also different.
In the present work, specimens made of “Armco” iron and grade 35 steel were subjected to cementation.
The microstructure of cemented specimens of Armco iron is shown in Figs. 2 and 3.\(^1\) Figure 2 shows the microstructure of a specimen that underwent cementation at a temperature of \(1100^\circ\), and Fig. 3 at a temperature of \(1200^\circ\). Etching of the polished sections was carried out with a 4-percent solution of nitric acid.
From these figures it is evident that, in the microstructure of Armco iron cemented with chromium, at a certain distance from the surface of the specimen there is a sharp line separating a light layer, not etched by nitric acid, from the main mass of the specimen. The presence of this separation line is well explained by the mechanism of cementation analyzed above.
\(^1\) The photographs of the microstructure were obtained in the diploma work of engineer E. V. Bilchugova, carried out in 1951 under the author’s supervision.
In fact, at all carburizing temperatures used, the iron is in the gamma state, and in the first period of the process the diffusing chromium forms a gamma solid solution with iron. The solubility of chromium in gamma iron is limited to 13% (Fig. 4). Therefore, when the chromium concentration as a result of the diffusion process becomes higher than this limit, a rearrangement of the crystal lattice of gamma iron into the lattice of alpha iron occurs,
Fig. 2. Magnification 100. Armco iron. Chromized at 1100°, 10 hours.
Fig. 3. Magnification 100. Armco iron. Chromized at 1200°, 10 hours.
alpha, in which the chromium concentration can reach any value. This rearrangement naturally begins at the surface of the specimen, where the chromium concentration reaches the limit of its solubility in gamma iron earlier than in other places, and spreads from the surface inward. And since the solid solution of chromium in iron, in which the chromium concentration is more than 13%, possesses, in comparison with iron and less concentrated solid solutions, a considerably higher chemical resistance to nitric acid, then under ordinary etching of transverse sections of iron chromized with chromium, a line should appear separating the chromized layer with a chromium concentration of more than 13% from the rest of the specimen.
The depth of the light, unetched layer depends on the chromizing temperature. A ten-hour hold at a temperature of 1000° gives a very small layer with a depth of 0.03–0.04 mm. At 1100° the layer depth reaches 0.15 mm (Fig. 2), and at 1200°—0.4 mm (Fig. 3).
The chromized Armco-iron specimens were subjected to corrosion testing in the following media: in fresh water—cold and hot (at a temperature of 80°), with alternating exposure in hot water and in air (8 hours in water at 80°—16 hours in air), and in a 3-percent solution of common salt. These tests gave the following results.
In testing in cold fresh water, all chromized specimens showed high resistance to corrosion. After three months’ exposure in water, no signs of corrosion were observed on the specimens, and the water remained as clear as before the beginning of the test.
Tests in fresh water at a temperature of 80° were carried out in a thermostat. These tests showed that specimens chromized at a temperature of 1000° proved insufficiently resistant to corrosion in hot—
hot water. In some of these specimens corrosion began in the first days of testing and had a local, pitting character. The same specimens that were cemented at higher temperatures (1100° and 1200°) possessed high resistance. Twenty-five days after the start of the test, no signs of corrosion were observed on them.
The specimens that possessed high resistance to corrosion in hot water also proved resistant under alternating exposure to hot water and air. After 10 cycles of the hot-water–air test (16 hours in air—8 hours in water), no signs of corrosion were observed on these specimens.
Fig. 4. Phase diagram of the iron–chromium system.
Corrosion tests in a 3-percent solution of sodium chloride showed that, in most specimens, corrosion began on the first or second day after the start of the test, and only individual specimens had higher resistance. In all cases the corrosion was local and pitting in character. Therefore it should be assumed that chromizing by the method used in the present work cannot provide a sufficient guarantee against corrosion in salt water.
The microstructure of the cemented specimens of steel 35 is shown in Figs. 5, 6, and 7.
Figure 5 shows a photograph of the microstructure of a specimen that was cemented at a temperature of 1000°. Cementation of steel 35 at this temperature does not lead to the formation of a shiny outer layer that is not etched by nitric acid, i.e., a layer containing more than 13% chromium. This, however, does not mean that, at such a comparatively moderate temperature, diffusion of chromium into steel does not occur at all. Diffusion of chromium also takes place at this temperature, but the rate of diffusion of chromium into steel proves to be considerably lower than the rate of diffusion into iron, since the carbon dissolved in gamma iron apparently impedes the diffusion of chromium.
Fig. 5. Magn. 100. Steel 35. Chromized, \(1000^\circ\). 10 h.
Fig. 6. Magn. 100. Steel 35. Chromized, \(1200^\circ\). 10 h.
It is seen from Fig. 5 that the grain size of steel carburized at a temperature of 1000° for 10 hours is substantially different at the surface of the specimen and in the deeper zones: at the surface of the specimen the grain size is considerably smaller. This is explained by the fact that the presence in the surface layer of the carburized specimen of even comparatively small amounts of chromium hinders the growth of the austenite grain during prolonged holding in the range of high temperatures.
The microstructure of a specimen of steel 35, which was carburized at a temperature of 1200°, is shown in Fig. 6. As is seen from this figure, a light zone, not susceptible to etching by nitric acid, formed at the surface of the specimen. This zone is, by its nature, a zone of chromium carbides.
Fig. 7. Magnification 500. Steel 35. Cemented with chromium, 1200°, 10 h.
The chromium atoms, diffusing into the steel at high temperatures, enter into chemical interaction with the carbon dissolved in gamma iron, forming chromium carbides that are stable at high temperatures.
A distinctive feature of the carburized zone obtained in steel at a high carburizing temperature is the division of this zone into two layers. The outer layer, adjacent to the very surface of the specimen, is very bright and shiny. The non-etching layer located deeper is paler. Between these two non-etching layers there passes a fairly distinct line of separation.
Such “double” non-etching layers were obtained only when steel specimens were carburized and were never obtained when Armco iron specimens were carburized. From this it is clear that the phenomenon of splitting of the non-etching zone is connected with the carbon present in the steel. It may be explained as follows.
In chromium carburizing of Armco iron, the boundary of the non-etching layer is the place in the specimen up to which, at the carburizing temperature, gamma → alpha recrystallization has occurred, caused by the diffusing chromium when its concentration in the gamma solid solution reaches the limiting value. This limit (Fig. 4) is approximately 13%, i.e., it practically coincides with the chromium concentration at which chromium solutions in iron are not etched by nitric acid.
Under the influence of carbon, the chromium concentration at which the gamma region closes increases. From Fig. 8 it is seen that this increase is...
quite substantial: at a carbon content equal to 0.4%, the chromium concentration at which the gamma region closes reaches 27–28%.
On this basis, it may be considered that the line separating the outer non-etching layer from the inner non-etching layer is the boundary up to which, at the cementation temperature, the gamma → alpha recrystallization process has occurred in the steel. The chromium concentration at this boundary depends on the carbon content in the steel, is in agreement with the diagram shown in Fig. 8, and is always considerably greater than 13%. The boundary between the inner non-etching layer and the rest of the specimen is, as in the case of Armco iron, the boundary separating non-etching regions, in which there is no less than 13% chromium, from etching regions with a lower chromium concentration.
If the considerations stated are consistent with reality, then the hardness in different regions of the cemented zone should be different. It should be greatest in the outer non-etching layer. In this layer, as a result of diffusion processes, not only the greatest chromium concentration is produced, but also the greatest carbon concentration—owing to diffusion of carbon from deeper layers, as noted by S. S. Kanfor [3], E. M. Morozova [6], and other authors. In the inner non-etching layer, in which the chromium and carbon concentrations are lower, one should naturally expect a lower hardness. Finally, the etching layer, in which the chromium concentration does not reach 13%, should have the lowest hardness.
Fig. 8. Influence of carbon on the position of the gamma region in iron–chromium alloys.
Measurement of the hardness of various regions of the cemented zone was carried out on a PMT-3 instrument under a load of 50 g. The results of this test are clearly visible in the photograph of Fig. 7. They fully confirm the assumptions stated.
Tests of cemented specimens of steel 35 for resistance to corrosion in water gave negative results: corrosion began within the first day and in all cases had a local, pitting character. This may be explained both by insufficient depth of the cemented layer and by its insufficient continuity.
Conclusions
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In chromizing Armco iron, the case-hardened layer is a layer of a solid solution of chromium in iron. Case hardening at \(1200^\circ\) for 10 hours makes it possible to obtain a layer \(0.4\ \mathrm{mm}\) deep, in which the chromium concentration is greater than \(13\%\). This layer has high resistance to corrosion in cold and hot fresh water, but is insufficiently resistant to corrosion in a 3-percent aqueous solution of table salt.
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In chromizing steel 35, a case-hardened layer of considerably smaller dimensions (less than \(0.1\ \mathrm{mm}\)) and of more complex structure is obtained. The outer part of this layer consists of chromium carbides or complex chromium–iron carbides. The case-hardened surface in this case has very high hardness. Converted to Rockwell hardness numbers (scale “C”), this hardness exceeds 72 units.
References
- Nikolaev V. I. Corrosion of Metals. Publishing House of the Institute of Applied Mineralogy, 1931.
- Izgaryshev N. A. and Sarkisov E. S. Doklady of the Academy of Sciences of the USSR, 7, 1938.
- Kanfor S. S. Bulletin of NII-13, 1–2, 1944.
- Arkharov V. I. Transactions of the Institute of Metal Physics and Metallurgy. Ural Branch of the Academy of Sciences, 4, 1944.
- Gulyaev A. P. and Dubinin G. N. Bulletin of Machine Building, 5, 1945.
- Morozova E. M. Surface Hardening of Steel by Methods of Chemical-Thermal Treatment, Mashgiz, 1949.