JP2683666B2 - Method for producing low thermal expansion cast iron with excellent wear resistance - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing low thermal expansion cast iron, particularly low thermal expansion cast iron having excellent wear resistance. [Prior Art] With the recent progress in industrial technology, the need for ultra-precision machining has increased, and along with this, prevention of thermal deformation of processing machines has become an important issue in maintaining accuracy, and various measures have been taken. It became so. Application of a low thermal expansion material is one of the means. As the low thermal expansion material, Fe-Ni binary alloy or Fe-Ni
-Intern of a ternary alloy of Co is known, but a drawback is that workability and castability are extremely poor. In order to solve such a problem, for example, Japanese Patent Publication No. 60-51547.
As described in Japanese Patent Laid-Open No. 62-63648 and Japanese Patent Laid-Open No. 62-63648, low thermal expansion cast iron has been proposed. The structure of this low thermal expansion cast iron is shown in FIG. [Problems to be Solved by the Invention] The conventional low thermal expansion cast iron has good workability and castability because graphite crystallizes in the matrix as shown in FIG. 4, but the matrix is austenite. Therefore, it is soft and has a problem in wear resistance when applied to a precision machine part having a sliding surface. An object of the present invention is to provide a low thermal expansion cast iron excellent in wear resistance while maintaining workability and castability, and a method for producing the same. Before going into the description of the present invention for achieving the above object, the process of completing the present invention will be described. The present inventors have noted that it is effective to precipitate carbide in the matrix in order to improve wear resistance. Therefore, Cr having a large forming ability was selected as a carbide forming element, and low thermal expansion cast iron containing Cr was examined. An example of the result will be described. Table 1 shows the composition (% by weight) of the sample. Table 2 shows the average coefficient of thermal expansion of each sample. Sample No. A is a conventional low thermal expansion cast iron, and Samples No. B and No. C are comparative examples. Sample No. D is described later. As is clear from Table 2, the average coefficient of thermal expansion increases with the increase of Cr content. It was also found that the addition of Cr deteriorates the graphite shape, adversely affects the mechanical properties, and deteriorates the workability. Therefore, it was found that the addition of Cr was not good as a means for improving wear resistance. The V, Nb, and W carbides were also examined, but the same results were obtained. Based on the above results, the inventors of the present invention increase the coefficient of thermal expansion and improve the wear resistance without deteriorating the mechanical properties due to the deterioration of the graphite shape, in order to cementite the graphite in cast iron. The idea that it is effective to do so was investigated for making graphite into cermetite. In Table 2
The No. D sample is a sample in which cast iron having the same composition as that of the conventional material of No. A was heated and maintained within the eutectic start and end temperature sections, and then cooled with oil to make the graphite cementite. The coefficient of thermal expansion is slightly higher than that of the conventional material of No. A due to the precipitation of cementite, but it is smaller than that of the comparative materials of No. B and No. C containing Cr. [Means for Solving Problems] The present invention has been made on the basis of the findings of the present inventors described above. The first invention provides Fe and Ni, or Fe and Ni and Co. After melting the surface layer of low thermal expansion cast iron as the main component, to form a structure in which graphite and cementite are dispersed in the matrix of the surface layer by rapid solidification, from room temperature to 50 ° C. of the layer having the graphite and cementite. A second invention is characterized by a method for producing a low thermal expansion cast iron having an average thermal expansion coefficient of 2.70 × 10 ⁻⁶ / ° C. or less and excellent in wear resistance. Fe and Ni,
Alternatively, after melting the surface layer of the low thermal expansion cast iron containing Fe, Ni and Co as the main components, it is rapidly solidified to form a structure in which graphite and cementite are dispersed in the matrix of the surface layer, and the graphite and cementite are included. The average coefficient of thermal expansion of the layer from room temperature to 50 ° C is 2.70 × 10 ^-6 /
It is characterized by a method for producing a low thermal expansion cast iron which is not more than 0 ° C and has excellent wear resistance. [Operation] In the low thermal expansion cast iron of the present invention described above, the improvement of wear resistance is achieved by the presence of high-grade cementite precipitated in the matrix. Ni is an element that promotes graphitization by reducing the solubility of carbon in Fe. In the manufacturing method of the present invention, after melting the surface layer of the cast iron for cementitizing the graphite in the cast iron, it is rapidly solidified, or the cast iron is heated and maintained at a temperature between the eutectic start temperature and the end temperature to obtain the resonance graphite. Is melted and then rapidly solidified to suppress the formation of graphite, thereby forming cementite as a non-equilibrium phase. This gives a structure in which graphite and cementite are dispersed in the matrix. The amount of cementite is adjusted by adjusting the amount of C in cast iron or by performing graphitization at an appropriate temperature. [Example] Example 1 A low thermal expansion cast iron material composed of Fe-1.7% C-2% Si-1% Mn-39% Ni was heated and held for 2 hours in an Ar gas atmosphere electric furnace at 1225 ° C, and then cooled with oil. . The composition represents% by weight. The same applies to the second and subsequent embodiments. The microstructure is shown in FIG. It can be seen that the structure is composed of spheroidal graphite and network-like cementite dispersed in the matrix. Next, the abrasion resistance was investigated by a polishing type abrasion test. The test was carried out by fixing # 80 emery paper on a rotary table and reciprocating the test piece on the rotary table for 3 minutes with a load of 800 g, and the difference in weight before and after the test was taken as the amount of wear. The test results are shown in FIG. In FIG. 2, a comparative material a has the same composition as the material of the example of the present invention, but has cast iron having a structure in which only graphite is dispersed in the matrix, and a comparative material b has Cr added by 2.5%. The samples according to the examples of the present invention have a smaller amount of wear than the comparative material a having the same composition and a structure in which only graphite is dispersed in the matrix and the comparative material b containing 2.5% of Cr, and the abrasion resistance is greatly improved. You can see that
Further, since graphite was present, no problem of workability occurred. Although the heating and holding temperature was 1225 ° C. in this example,
The heating and holding temperature is arbitrarily selected in the range between the eutectic start temperature and the end temperature of the applied material. Further, water cooling or air blast cooling may be adopted as the cooling means, and the heating and holding time is determined according to the size of the applied material. Example 2 The surface of a low thermal expansion cast iron material composed of Fe-1.2% C-2% Si-1% Mn-39% Ni was remelted at a temperature above the liquidus temperature using a TIG welding machine and rapidly solidified. . As is clear from the microstructure shown in FIG. 3, it can be seen that the remelted and rapidly solidified layer has a structure in which fine spheroidal graphite and cementite (worm-like) are dispersed. As a result of the abrasion test, it was confirmed that the abrasion resistance was very excellent. The present embodiment is effective for parts in which wear resistance is required on the surface portion. Example 3 A surface of a low thermal expansion cast iron material made of Fe-1.7% C-2% Si-1% Mn-32% Ni-8% Co was remelted at a temperature above the liquidus temperature using a TIG welding machine, It was solidified rapidly. Example 2
Similar structure and wear test results were obtained as described above. (Effects of the Invention) As described above, according to the present invention, the structure of low thermal expansion cast iron containing Fe and Ni, or Fe, Ni and Co as the main components, has graphite and cementite dispersed in a matrix. As a result, it is possible to improve wear resistance without impairing low thermal expansion, workability and castability. Therefore, precision machine parts that dislike thermal deformation and have a sliding surface to which a high load is applied, such as a plunger. It can be widely applied to cams, gears, mechanical seals, machine tools and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a micrograph showing the metallographic structure of cast iron of Example 1 of the present invention, FIG. 2 is a characteristic diagram showing the results of wear test in Example 1, and FIG. 3 is of the present invention. FIG. 4 is a photomicrograph showing the metallographic structure of the cast iron of Example 2, and FIG. 4 is a photomicrograph showing the metallographic structure of the conventional low thermal expansion cast iron.

Claims (1)

(57) [Claims] After melting the surface layer of low thermal expansion cast iron mainly composed of Fe and Ni, or Fe and Ni and Co, to form a structure in which graphite and cementite are dispersed in the matrix of the surface layer by rapid solidification, the graphite And a method for producing low thermal expansion cast iron with excellent wear resistance, characterized in that the layer having cementite has an average thermal expansion coefficient of 2.70 × 10 ^-6 / ° C or less from room temperature to 50 ° C. 2. The low thermal expansion cast iron containing Fe and Ni or Fe and Ni and Co as main components is heated and held at a temperature between the eutectic start temperature and the end temperature to melt the eutectic graphite and then rapidly cool it to form a matrix. A method for producing a low thermal expansion cast iron having excellent wear resistance, which is characterized by forming a structure in which graphite and cementite are dispersed and having an average thermal expansion coefficient of 2.70 × 10 ^-6 / ° C or less from room temperature to 50 ° C.