Treatment of sintered alloys

Abstract: A method for treating sintered alloy is disclosed of standing a portion of sintered alloy at a temperature ranging from about 8⊘⊘ °C to about 13⊘⊘ °C under an atmosphere that contains an amount of water vapor corresponding to dew points ranging from about 5 °C to about 6⊘ °C. The method according to the present invention gives sintered alloy with a satisfactory protective layer that excels in smoothness and uniformity, and that prevents abnormal oxidation. Moreover, the method is especially useful to sintered alloy with a complex structure and/or thin walls, such as a honeycomb structure.

Description: This invention relates to methods for treating sintered alloy to form a protective layer on the surface. This method is especially applicable to sintered alloy articles with protrusions or depressions of their surface, and sintered alloy with a complex structure and/or thin walls, such as a honeycomb structure. To enhance corrosion resistance and lubrication ability, a part made of iron has been known to undergo a water vapor treatment in which they are stood in pressurized steam at a temperature between about 500 °C and about 600 °C to form a coating of Fe₃O₄ on its surfaces. However, this coating does not function as a protective layer against oxidation in higher temperatures. Methods for forming heat-resistant coating have been disclosed before. US-A-4915751 disclosed a two-step method of treating a stainless foil at a temperature ranging from 900 °C to 960 °C and at a temperature ranging from 960 °C to 1000 °C to give an alumina whisker. JP-B-3/1279 disclosed treating a stainless steel foil containing Mg at a temperature ranging from 1000°C to 1150°C in vacuum or under a hydrogen atmosphere; and treating the resultant foil under a carbon dioxide atmosphere. However, the method disclosed in US-A-4915751 requires two steps of heat treatment that make temperature control difficult and increase operational cost. The method disclosed in JP-B-3/1279 is applicable only to stainless steel containing magnesium, and takes time in the surface treatment process. Both these methods are applied to poreless stainless steel manufactured by melting and subsequent rolling. EP-A-390321 (and equivalent JP-A-2/270904) disclosed a method of treating porous sintered Al or Al-containing structures at a temperature ranging from 950°C to 1350°C in an oxidative atmosphere such as air, oxygen, carbon dioxide, or a mixture of hydrogen and water vapour. Specific conditions of the H₂/H₂O surface treatment were not disclosed. Moreover, the coating obtained was said to have only modest durability. DE-A-3419638 describes creating oxidic layers on metal surfaces generally, the processing being characterised by a controlled variation of the oxidation potential of the oxidising treatment atmosphere over a wide range during the formation of the layer. Suggested treatment atmospheres include steam, H₂/H₂O, H₂/CO₂ and inert gas/H₂O. The problem addressed herein is to provide a new method of treating a sintered alloy. According to the present invention, there is provided a method for treating sintered Al-containing alloy, which comprises standing a portion of sintered Al-containing alloy at a temperature of from 800 °C to less than 1300 °C under an atmosphere that contains an amount of water vapor corresponding to a dew point of from 5°C to 60 °C. The present inventors have studied the surface treatment of sintered alloy having protrusions and depressions in its surfaces. Sintered alloy with a metal oxide coating formed under a dry atmosphere, we have found, is prone to undergo abnormal local oxidation. In contrast, sintered alloy with a metal oxide coating formed under an atmosphere with water vapor, is not prone to undergo such abnormal oxidation. Therefore, according to the method in the present invention sintered alloy is treated in a specific temperature range under an atmosphere with water vapor to form a metal oxide on its surfaces, which we find can produce good oxidation resistance of the sintered alloy. The method according to the present invention, involving a chemical reaction between gas and surface, is particularly useful to sintered alloy having protrusions and depressions on its surfaces, including sintered alloy having a complex structure and/or thin walls, such as a honeycomb structure. According to the method in the present invention, sintered alloy to be treated contains Al and have a melting point equal to or higher than a surface treatment temperature. Other elements in the sintered alloy are not particularly restricted, and at least one element selected from Fe, Cr, B, Si, La, Ce, Cu, Sn, Y, Ti, Co, Ni, Ca, alkaline earth metals, lanthanides, Hf, and Zr may be present. The temperature range for surface treatment of sintered alloy in the present invention is from about 800 °C to less than 1300 °C, particularly from about 1000 °C to about 1200 °C. When sintered alloy is treated in temperatures lower than 800 °C, an alumina protective layer formed contains so much iron that its ability for oxidation resistance deteriorates. On the other hand when sintered alloy is treated at temperatures higher than 1300 °C, the fast oxidation on its surfaces during the surface treatment tends to cause a non-uniform protective layer, resulting in a cause of abnormal oxidation and in deterioration of mechanical strength due to grain growth. An amount of water vapor in an atmosphere which sintered alloy is treated under, preferably corresponds to dew points equal to or lower than 60 °C; too much water vapor makes sintered alloy under treatment prone to corrosion during the treatment, and results in deterioration in oxidation resistance and corrosion resistance of the treated sintered alloy. On the other hand too small amount of water vapor makes sintered alloy more difficult to form a uniform coating on the sintered alloy under treatment to result in local oxidation, and oxidation resistance and corrosion resistance in the treated sintered alloy deteriorate; thus an amount of water vapor in an atmosphere which sintered alloy is treated under preferably corresponds to dew points equal to or higher than 5 °C, particularly equal to or higher than 15 °C. Considering the cost of equipment, an amount of water vapor in an atmosphere preferably corresponds to dew points equal to or lower than 40 °C. Favorably an amount of water vapor in an atmosphere is equal to or less than the amount of saturated water vapor around the equipment at a temperature in the surrounding. When an atmosphere for surface treatment of sintered alloy essentially consists of a mixture of hydrogen and oxygen or of a mixture of oxygen and nitrogen, an amount of water vapor preferably corresponds to dew points equal to or higher than 30 °C. An atmosphere for surface treatment of sintered alloy is not particularly restricted, and hydrogen, inert gas, air, oxygen and so on are used. Hydrogen or inert gas is a preferable atmosphere. One possible explanation for this preference is that the absolute amount of oxygen contained in such an atmosphere is smaller than the other atmospheres, and oxidation due to water vapor is presumed to become a dominant oxidation process. Surface treatment time of sintered alloy is preferably equal to or longer than 30 minutes, particularly equal to or longer than one hour because too short a time results in deterioration of protective ability of the protective layer thus formed due to destabilization at the interface between the coating and matrix. Due to a cost factor, time for surface treatment is preferably equal to or less than 10 hours, particularly equal to or less than five hours. As disclosed above, temperature and an amount of water vapor in an atmosphere for surface treatment of sintered alloy considerably affect coating on its surfaces, and other conditions such as an atmosphere and surface treatment time also affect coating. Though how water vapor in an atmosphere for surface treatment plays a role for the formation of a protective layer is not clear, some form of hydrogen that may be produced by oxidation of aluminum by water is presumed somehow to help form uniform coating. As disclosed above, with these methods it is possible to obtain sintered alloy with a satisfactory protective layer of good smoothness and uniformity, and that prevents abnormal oxidation. As a result the present teachings enable the provision of metallic materials with good oxidation resistance at high temperatures, and corrosion resistance. Since it is quite feasible to control an amount of water in an atmosphere corresponding to dew points equal to or higher than 5°C, the method is simple in its industrial application. Examples Embodiments are now described by way of example. (Example 1) Sintered alloy having a composition of Fe-20Cr-5Al (% by weight) with a porosity of 26 % is prepared from Fe powders, Fe-50Al powders, and Fe-60Cr powders as starting materials, and fired at 1320 °C. Sintered alloy thus prepared was used as samples for surface treatment under various conditions to form coating, as tabulated in Table 1. Each of the samples of coated sintered alloy underwent an oxidation resistance test. An amount of total oxidation of each sample after the test was measured, and presence or absence of abnormal oxidation was observed. These results are also tabulated in Table 1. In the oxidation resistance test a sample was stood at 980 °C for 700 hours in an electric furnace, and then weight increase and change in dimension were measured to evaluate the oxidation resistance of the sample. The amount of "total oxidation" of a sample in the results refers to the sum of a weight increase during the surface treatment of the sample and a weight increase during the oxidation resistance test of the sample. (Example 2) Sintered alloy having a composition of Fe-26Al (% by weight) with a porosity of 35 % is prepared from Fe powders and Fe-50Al powders as starting materials, and fired at 1250 °C. Sintered alloy thus prepared was used as samples for surface treatment under various conditions to form coating, as tabulated in Table 2. Each of the samples of coated sintered alloy underwent an oxidation resistance test, as in Example 1. An amount of total oxidation of each sample after the test was measured, and presence or absence of abnormal oxidation was observed, as Example 1. These results are also tabulated in Table 2. (Example 3) Sintered alloy having a composition of Fe-20Cr-5Al-3Si-0.05B (% by weight) with a porosity of 5 % is prepared from Fe powders, Fe-50Al powders, Fe-20B powders, Cr powders, and Fe-75Si powders as starting materials, and fired at 1300°C. Sintered alloy thus prepared was used as samples for surface treatment under various conditions to form coating, as tabulated in Table 3. Each of the samples of coated sintered alloy underwent an oxidation resistance test, as in Example 1. An amount of total oxidation of each sample after the test was measured, and presence or absence of abnormal oxidation was observed, as Example 1. These results are also tabulated in Table 3. As seen from the results in Tables 1, 2, and 3, when a sample of sintered alloy had surface treatment in which the sample was stood in a temperature ranging from about 800 °C to about 1300 °C under an atmosphere that contains an amount of water vapor corresponding to dew points ranging from about 5 to about 60 °C, the sample had good oxidation resistance and did not undergo abnormal oxidation.

Claims: A method of treating sintered Al-containing alloy, comprising: exposing the sintered alloy, at a temperature of at least 800°C but less than 1300°C, to an atmosphere containing an amount of water vapor corresponding to a dew point of from 5°C to 60°C. A method according to claim 1, wherein said atmosphere essentially consists of said water vapor and hydrogen, or of said water vapor and inert gas. A method according to claim 1, wherein said atmosphere essentially consists of said water vapor and oxygen, or of said water vapor and a mixture of oxygen and nitrogen. A method according to any one of the preceding claims in which the alloy is exposed to said atmosphere for from 30 minutes to 5 hours. A method according to any one of the preceding claims in which the sintered alloy constitutes an article having an uneven exposed surface. A method according to any one of the preceding claims, in which said atmosphere contains an amount of water vapour corresponding to a dew point range from 30 to 60°C. A method according to claim 6 in which said atmosphere essentially consists of said water vapour and hydrogen. A method according to any one of claims 1 to 5 in which said atmosphere essentially consists of said water vapour and inert gas, and said atmosphere contains an amount of water vapour corresponding to a dew point ranging from 15 to 60°C.

Inventors: HARADA TAKASHI, OHHASHI TSUNEAKI, TSUNO NOBOU, HARADA, TAKASHI, OHHASHI, TSUNEAKI, TSUNO, NOBOU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1992
Main classification: C23C8
IPC codes: C23C8, C22C1