JPS648045B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of finely hardening the structure of only the surface of a lid made of ductile cast iron to impart wear resistance. Iron covers for manholes, etc., have changed from those that are placed on shelves on the inner wall of the cover frame to ones with steeply sloping sides that fit into the inner side of the cover frame, which increases the weight of passing vehicles and makes them difficult to use in winter. The surface hardness is high due to harsher service conditions such as the use of spiked tires.
Moreover, there has been a demand for high toughness as a whole. In general, methods for hardening the surface of cast products include flame hardening, induction hardening, and casting using a chilled metal. Re-heating increases the work process and increases energy consumption, and
The method using a cold metal requires complicated work, and both methods are unsuitable for manufacturing iron lids due to operational and equipment considerations. The purpose of the present invention is to form a fine pearlite structure on the surface without reheating after casting, and to obtain an iron lid with increased wear resistance by hardening only the surface without impairing toughness. do. The ductile casting referred to in the present invention is TC: 3.4~
3.8%, Si: 2.6-3.5%, Mn<0.4%, P<0.1%,
S<0.02%, the balance essentially consisting of Fe, and having a pearlite matrix. The temperature for removal from the mold is in the range of 800 to 1000°C, and rapid cooling is performed from this high temperature range to a temperature of approximately 400 to 500°C. If the product is cooled too quickly to a low temperature, the stress relaxation of the product will be insufficient, and problems will remain regarding the strength of the iron lid, which is subjected to repeated loads from passing vehicles. The method of the present invention uses a mold in order to take out a cast product in a high temperature range. In addition, water is usually used as the cooling medium for rapid cooling, and a method is adopted in which water is sprinkled on the surface of the iron lid. By rapidly cooling the surface of the ductile cast iron lid from 800 to 1000°C as described above, the pearlite structure on the surface becomes finer, the hardness increases without losing toughness, and the wear resistance improves. Once rapid cooling is stopped, heat is absorbed from inside, the temperature rises again, and residual stress relief annealing is performed on its own.
Residual stress in the product is alleviated to the same extent as with normal casting methods. Examples are shown below. In this example, C: 3.6%, Si: 2.9%, Mn:
The test was carried out on a manhole cover made of ductile cast iron with a composition of 0.3%, P: 0.03%, S: 0.015%, and the balance essentially consisting of Fe, and the manhole cover had a shape as shown in Figures 1 and 2. The lid is cast in a mold at 1400°C, removed from the mold at 950°C, and immediately cooled only on the surface of the iron lid to approximately 450°C using a water spray method using a device as shown in Figure 3. did. In Fig. 3, 1 is the iron lid, 2 is the surface plate, and 3 is the water sprinkler unit. The microscopic structure (×400) of the surface of the casting thus obtained is shown in Figure 6a, and the hardness test results, abrasion test results, and load test results are shown in Tables 1 and 2, respectively.
Table 3 shows the results. As a comparative example, a ductile cast iron lid of the same shape was cast in a sand mold under the same conditions and the same test was conducted. The results are shown in Figure 6b, Tables 1 and 2, respectively.
Also listed in Table 3.

【table】

【table】

[Table] This product load test is a test to determine the amount of deflection and breaking load when a load W of 25 tons is applied using the equipment shown in Figures 4 and 5. In Figures 4 and 5, 1 is the iron lid, 4 is the receiving frame, and 5
1 is a cast iron stand, 6 is a dial gauge support point, 7 is a rubber seal, 8 is a loading plate, 9 is a dial gauge mounting bar, and 10 is a dial gauge. As is clear from the above examples, according to the method of the present invention, there is no loss in load strength compared to comparative products made by sand casting, and the surface hardness of the ductile cast iron lid is on average H _B 300 or more, and after cooling. , it is possible to obtain an iron lid having high abrasion resistance equivalent to that subjected to a surface hardening method such as a flame hardening method. Moreover, in the surface hardening method of iron covers such as manholes according to the present invention, the entire matrix is a uniform pearlite structure, and only the extremely thin surface layer is a fine pearlite structure that is continuous with the matrix. It has both toughness to withstand the passage of heavy vehicles and abrasion resistance to the passage of highly abrasive objects such as spiked tires, and also has the effect that the hardened surface layer does not peel off.

[Brief explanation of the drawing]

FIGS. 1 and 2 are explanatory diagrams showing the shape and size of the manhole manufactured in the example of the present application, respectively, and FIG.
The figure is an explanatory diagram showing the same rapid cooling, Figures 4 and 5 are explanatory diagrams each showing the product load test method, and Figure 6 is a 400x microscopic photograph showing the structure of the iron lid.
FIG. 6a shows the structure 1 mm below the surface of the iron lid treated according to the present invention, and FIG. 6b shows the microscopic structure at the same position of the iron lid obtained by the conventional method using a sand mold.

Claims (1)

[Claims] 1 T.C3.4-3.8% by weight, Si2.6-3.5% by weight, Mn
<0.4% by weight, P<0.1% by weight, S<0.02% by weight,
Molten cast iron, the remainder of which is essentially Fe, is poured into an iron lid mold, and after solidification, the entire surface of the iron lid taken out from the mold is rapidly cooled to 400 to 500°C, and then air cooled. A surface hardening method for ductile cast iron lids.