GB1600876A

GB1600876A - Method and apparatus for prediciting metallographic structure

Info

Publication number: GB1600876A
Application number: GB20199/78A
Authority: GB
Original assignee: ELECTRO NITE; Electro Nite International NV
Current assignee: ELECTRO NITE; Heraeus Electro Nite International NV
Priority date: 1977-05-18
Filing date: 1978-05-17
Publication date: 1981-10-21
Also published as: SE7805633L; PL206866A1; IT1095991B; DE2821352C2; DE2821352A1; SE447027B; CH626727A5; IT7823443A0; PL118845B1

Abstract

According to the method, the procedure is such that a first part (14) of a sample solidifies at a first speed of solidification, and such that a second part (15) of the said sample solidifies at a second, lower, speed of solidification. The temperature of the first part (14) of the sample is measured, and after solidification of the first part (14) of the sample, this measurement of the temperature is used in order to determine the thermal conductivity of the sample. A thermal conductivity parameter is then used in order to predict, by means of an empirically fixed relationship, the metallographic structure. <IMAGE>

Description

(54) METHOD AND APPARATUS FOR PREDICTING METALLOGRAPHIC STRUCTURE (71) We, ELECTRO-NITE N.V., a body corporate organized and existing under the laws of Belgium of Grote Baan 27a, 3530 Houthalen (Belgium), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates generally to a process for rapidly predicting a metallographic structure, such as the degree of nodularity, of castings to be made from molten metal before casting and apparatus for performing the same.

It is known that the composition of molten metal may be estimated by recording the liquidus and eutectic temperatures using known cooling curves. Such method provides information such as a change in phase, carbon equivalent, etc. It is also known to determine the thermal conductivity of solid bodies by measuring the heat flow through such body when subjected to a thermal gradient.

It has been observed that in grey cast iron in the solid state, the thermal conductivity is substantially influenced by the graphitic structure and that a nodular cast iron, for instance, has a lower conductivity than a lameller or equivalent composition.

When pouring cast iron, it is important that analysis be carried out very quickly and before casting. If one waits too long before completing a pour of a nodular cast iron, a large percentage of the magnesium which is present in the molten metal will dissipate or will otherwise be lost and there is a likelihood of making an unsatisfactory cast.

The industry has long sought a simple, rapid and effective way of accurately predicting the degree of nodularity of a cast iron before casting of the same. One method proposed heretofore is disclosed in U.S. Patent No. 3.670,558 wherein the properties of a nodular cast iron are evaluated by way of a comparative study of conventional cooling curves and a set or family of curve segments. The method proposed by said patent has not proven to be acceptable.

Thus, at the present time, the degree of nodularity of a cast iron can only be determined with precision if the sample is cooled and evaluated by ultrasonic or metallographic analysis, and the like.

According to the present invention, there is provided a process for predicting the metallographic structure of a casting, wherein a mass of molten metal is placed in a receptacle and allowed to solidify whilst the temperature in a point of the mass is measured during cooling and plotted in function of time, wherein said point lies within a first part of said mass which is caused to cool at a faster rate than the remaining part thereof, so that said remaining part causes a deviation, by thermal reflection, of the plotted curve after solidification of said first part, said deviation serving to predict the metallographic structure of the casting.

Also according to the present invention, there is provided apparatus for carrying out the process and comprising a crucible having means for ensuring a faster cooling of a part of the metal poured into said crucible than the remaining part, as well as a temperature sensor, wherein said means consists in providing said crucible with a cavity having two contiguous portions with a smaller transverse dimension for said first part so that the ratio of surface area to volume of the second cavity portion is smaller than that of the other portion, said temperature sensing means being located within the cavity for said first part.

Further according to the present invention, there is provided apparatus for carrying out the process and comprising a crucible having means for ensuring a faster cooling of a part of the metal poured into said crucible than the remaining part, as well as a temperature sensor, wherein said means consists in a heat insulating jacket located around a part of said crucible, thereby retarding the cooling of the metal located in said part with respect to the remaining part.

For the purpose of illustrating the invention, there is shown in the accompanying drawings embodiments thereof which are presently preferred. In the drawings: Figure 1 is a sectional view through a first type of crucible usable in practicing the method of the present invention.

Figure 2 is a sectional view through another type of crucible usable when practicing the method of the present invention.

Figure 3 is a graph showing a cooling curve wherein temperature is plotted against time using a single thermocouple.

Figure 4 is a sectional view through another crucible.

Referring to the drawings in detail, wherein like numerals indicate like elements, there is shown in Figure 1 a first form of apparatus for use in practicing the method of the present invention. Thus, there is shown a crucible designated generally as 10, the diameter of the upper part 12 being much greater than the diameter of the lower part 13. As used herein, the word crucible is intended to include sample cups, molds used for IN-MOLD processes. and the like. The crucible 10 constructed in this manner makes it possible to cool a sample of molten metal at two different rates. Thus, the lower portion 14 of a sample in the crucible 10 will cool more rapidly than the upper portion 15 of the sample in crucible 10 because the ratio of surface area to volume of the upper portion is smaller than that of the lower portion.

in Figure 2. there is illustrated a second form of apparatus for use in practicing the method of the present invention. In Figure 2, there is shown a crucible 10' of constant diameter. The lower portion 13' of the crucible 10' is placed in a jacket 18 of heat insulating material. As a result of the jacket 18, the portion of the sample in the upper portion 19 of the crucible 10' will solidify at a faster rate as compared with the rate of solidification of the sample in the lower portion of the crucible 10'.

Although the crucibles 10 and 10' are slightly different in construction as to their contour. the principle utilized in the method of the present invention is the same. In order to prevent the formation of shrinkage cavities in the sample, it is preferable to design the crucibles in such a way that the portion having the fastest rate of solidification is in the lower portion of the crucible.

The temperature of the sample in portion 14 or 19 is sensed by a sensing element such as a thermocouple 22. The thermocouple 22 is preferably generally perpendicular to the centre line or axis of the crucible 10.

Another practical embodiment of this invention is shown in Figure 4 i.e. a crucible 30. In Figure 3, there is shown a cooling curve of time versus temperature obtained from a sample of cast iron cooled in crucible 30. In Figure 3, the designation "X" indicates the time to cool through the 100"C range from 1160"C to 10600C.

Crucible 30 is similar to crucible 10 but only contains a single alumel-chromel thermocouple 32 within lower portion 34. Crucible 30 has a larger portion 36 coextensive with portion 34. Typical dimensions for portion 34 are diameter of 18.5 mm; height 29 mm; and thermocouple 32 spaced from the bottom of portion 34 bv 6 mm. Typical dimensions for portion 36 are diameter of 50 mm and a height of 46 mm. For the reasons described above. that part of the sample in portion 34 will cool at a faster rate than the part of the sample in portion 36. A crucible as disclosed in US patent 4 056 407 can be modified to have the features of crucible 30.

When analysing a specimen of cast iron, it is recommended that the cooling time starting with the casting of the sample and ending with solidification of portions 14 or 34 should not be less than 1-1/2 minute in order to prevent the formation of carbides.

Furthermore, the total duration of analysis should be no longer than 5 minutes in order for the method to produce economic advantages. With these goals in mind, the dimensions of the crucible 10 are preferably chosen to meet the goals. Thus, the crucible 10 is preferably made from foundry sand with a resinous material binder, and with the inner diameter of the crucible lower part 13 being about 18.5 mm and the inner diameter of part 12 is about 50 mm.

With a single temperature sensing element used in that portion of the crucible which cools the fastest, AT is obtained from a graph as shown in Figure 3.

Thus, the present invention comprehends obtaining a parameter of heat conductivity such as a AT after pouring a sample into a crucible and thereafter using the parameter to predict whether or not the nodularity of castings to be made from the entire ladle will be satisfactory whereby pouring can commence. If the nodularity is unsatisfactory, corrections may be made before the molten metal is poured or the entire ladle is pigged, thereby saving the molds and other operating costs. Nodularity can change with time. Hence it may be desirable to use the present invention to check nodularity after all castings have been poured to confirm the quality of the last poured castings.

When the present invention is used as part of an IN-MOLD process, a variety of variations are possible. For example, the main mold cavity could correspond to portion 36 of crucible 30 and portion 34 of the crucible 30 could be a small auxiliary cavity and arranged to cool at a faster rate. The casting resulting from the auxiliary cavity would be removed along with the sprue.

An electronic microprocessor or desk calculator may be interfaced with a digital pyrometer for use as a computer analysis control equipment with red, yellow and green lights. The present invention will drastically reduce quality control problems and eliminate the cost of casting of molten metal with insufficient nodularity.

WHAT WE CLAIM IS: 1. A process for predicting the metallographic structure of a casting, wherein a mass of molten metal is placed in a receptacle and allowed to solidify whilst the temperature in a point of the mass is measured during cooling and plotted in function of time, wherein said point lies within a first part of said mass which is caused to cool at a faster rate than the remaining part thereof, so that said remaining part causes a deviation, by thermal reflection, of the plotted curve after solidification of said first part, said deviation serving to predict the metallographic structure of the casting.

2. A process according to claim 1, wherein said curve is plotted over a chosen period of time.

3. A process according to claim 1, wherein said curve is plotted over a chosen temperature range.

4. A process according to claim 3, wherein the molten metal is cast iron and the metallographic structure is nodularity, wherein said temperature range has an upper limit of about 1160"C and a lower limit of about 1060cm.

5. A process according to claim 1, wherein said receptacle is a mold having a molding cavity and a second cavity communicating therewith, said second cavity being intended to contain said first part of the mass of molten metal and the molding cavity, said remaining part.

6. An apparatus for carrying out a process according to any one of claims 1-4, comprising a crucible having means for ensuring a faster cooling of a part of the metal poured into said crucible than the remaining part, as well as a temperature sensor, wherein aid means consists in providing said crucible with a cavity having two contiguous portions with a smaller transverse dimensions for said first part so that the ratio of surface area to volume of the second cavity portion is smaller than that of the other portion, said temperature sensing means being located within the cavity for said first part.

7. An apparatus for carrying out a process according to any one of claims 1-4, comprising a crucible having means for ensuring a faster cooling of a part of the metal poured into said crucible than the remaining part, as well as a temperature sensor, wherein said means consists in a heat insulating jacket located around a part of said crucible, thereby retarding the cooling of the metal located in said part with respect to the remaining part.

8. An apparatus according to claim 6, wherein said crucible is a mold, said first cavity part being an auxiliary cavity to the main mold cavity which is said second part.

9. A method for predicting the metallographic structure of a casting substantially as described hereinbefore with reference to the accompanying drawings.

10. Apparatus for carrying out a method for predicting the metallographic structure of a casting constructed and arranged substantially as hereinbefore described and shown in Figure 1, 2, or 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. When the present invention is used as part of an IN-MOLD process, a variety of variations are possible. For example, the main mold cavity could correspond to portion 36 of crucible 30 and portion 34 of the crucible 30 could be a small auxiliary cavity and arranged to cool at a faster rate. The casting resulting from the auxiliary cavity would be removed along with the sprue. An electronic microprocessor or desk calculator may be interfaced with a digital pyrometer for use as a computer analysis control equipment with red, yellow and green lights. The present invention will drastically reduce quality control problems and eliminate the cost of casting of molten metal with insufficient nodularity. WHAT WE CLAIM IS:

1. A process for predicting the metallographic structure of a casting, wherein a mass of molten metal is placed in a receptacle and allowed to solidify whilst the temperature in a point of the mass is measured during cooling and plotted in function of time, wherein said point lies within a first part of said mass which is caused to cool at a faster rate than the remaining part thereof, so that said remaining part causes a deviation, by thermal reflection, of the plotted curve after solidification of said first part, said deviation serving to predict the metallographic structure of the casting.