US2901384A - Method for treating cast iron castings - Google Patents

Description

United States Patent 2,901,384 METHOD FOR TREATING CAST IRON CASTINGS Lon Saives, Billancourt, France, assiguor to Regie Nationale des Usines Renault, Billancourt, France Application February 10, 1953, Serial No. 336,187 Claims priority, application France February 20, 1952 9 Claims. (Cl. 148-3) The invention relates to a method of treating white cast iron with a view to increasing its toughness.

It is known that copper and nickel facilitate the graphitisation of castings both during the solidification of grey cast iron and during the annealing of white cast iron which can be rendered malleable. It is likewise accepted that nickel is more effective than copper and it is current practice to put up to nickel in high quality grey cast iron but it is not customary to introduce copper or nickel into white cast iron which can be rendered malleable with a black heart, the eifect of these constituents not being sufiicient to compensate for the increase in price.

This invention is characterised by the fact that in white cast iron which can be rendered malleable, the copper, which is present in a concentration of 0.7% to 3%, may have an effectiveness which is very great and quite unexpected, if the graphitisation treatment is carried out in three successive phases:

(l) Austenisation above the temperature for the end of the eutectoid transformation followed by chilling in stages;

(2) Tempering of and formation of nuclei in the graphite by sudden re-heating orbetter still by progressive heating to a precise suitable temperature T, of the order of 450 C., viz. lying between 400 C. and 500 C., and lasting a suflicient length of time, e.g. 24 to 100 hours, more particularly for a period of the order of 36 to 48 hours. In metal chill molds, tempering is suitably carried out for 1 to 24 hours.

(3) Graphitisation of the primary cementite by prolonged maintenance at a temperature considerably higher than the temperature for the end of the eutectoid transformation, e.g. 875 C. to 900 C. for a period of time sufiicient for the total disappearance of the cementite, more particularly 12 hours at 895 C. or 3 hours at 900 C. followed, for example, by cooling in still air. Graphitization may also be carried out, for example, at 875 to 895 C. for one-half to two hours.

In order to demonstrate this new property of copper, examples are given to show that the action of copper is more effective that that of an equal quantity of nickel; bearing in mind the fact that nickel is about five times more expensive than copper, the effectiveness for the same cost of addition is of a much higher order and offers a considerable industrial advantage.

It is possible to ascertain the comparative action of th constituents copper and nickel by working on eutectoid steels with a composition similar to that of white cast iron, that is to say, the composition of Which is obtained by deduction from the primary cementite or proeutectoid.

.Four-rsteels were cast, then austenised at 810 C. 30 minutes, oil-hardened, reheated for 48 hours at T, cooled in still air, reheated to 700 C. for 150 hours and then cooled in still air.; The average number N .of

granules of. graphite; formed per mm. section is given below, beside the chemical compositions, and for two cooling in still air. the primary cementite is complete. The chemical com- Patented Aug. 25, 1959 values of T. For the optimum value of T=450 C., nickel does not bring any improvement in comparison with other cast steels of the same composition, whereas copper, on the other hand, increases the number of nuclei per unit of section by 5,000.

Castings O Si Mn Ni Cu T=450 O. T=500 C.

N/mmfl N/mm."

Passing now to the comparison of white cast iron which can be rendered malleable, it will be seen that the castings appearing in the following table were cast in sand in bars of 15 x 30 mm. and produced a white structure. They were then subjected to the following series of treatments: austenisation at 810 C. lasting 30 minutes, quenching in a salt bath at C. for 1 minute, cooling in still air, reheating to T=450 C. for 48 hours, cooling in still air, reheating to 885 C. for 10 hours and In each case, the graphitisation of positions and the average number N of grains of graphite (fine nodules or ultra fine spherules as the case may be) per mm. are given in the following table:

It will be seen that with an equal concentration, copper produces more than 3.7 to 4 times the number of nodules of graphite. Assuming that nickel is 5 times more expensive than copper, the efiectiveness of copper is about twenty times greater than that of nickel for the same price of addition.

if a comparison is now made with castings devoid of nickel and copper, cast under the same conditions as above and with the same preliminary hardening and nuclear formation treatment as above, but the graphitisation being for 14 hours at 895 C., the slight coalescence of the nodules of graphites has slightly diminished their number. The number of nodules N per sectional unit and the percentage of residual cementite Fe C found are indicated in the following table:

Castings O Si Mn Cu Ni S P N F0 0 21454.--- 2. 32 1. 46 0. 67 0 0. ll 0. 095 2, 500 1' 2.145-11.-- 2. 22 1. 42 0. 66 0 l. 83 0. 10 0. 095 2, 770 0. 5 2.147 2. 33 1. 53 0.71 2. 2 0.1 0 0.12 6, 550 0 0 Si Mn Cu Ni S P r N 2136 1. 15 0. 36 0 0 ll. 12 0. 10 l, 000 2. 39 1. 08 0.38 r 0 2. 13 0. 094 0.097 1, 350 2. 28 1. 43 0.66 2.00 0 0. 10 0. 09 6, 900

The increase in the number of nodules, given the same concentration of addition is "18 times greater for copper than for nickel. Price for .price, the effectiveness of the copper is 90 times greater than that of nickel.

It is known that in malleable castings without special additions, the preliminary hardening must of necessity be marte'nsitic, which is what has been done in the examples given above; but'another remarkable property of copper has been discovered, namely that it enables a large number of fine nodules to be obtained by means of a preliminary bath-hardening. After the preliminary batmhardening carried out, the nuclear formation of the graphite is obtained by annealing for 48 hours at 450 C. and the graphitisation is then obtained by reheating to 895 C. and maintaining at this temperature for 14 hours, followed by cooling in still air. Between the cycles in this treatment, there is a return to room temperature, but it .goes without saying that the passage from 400 C. to 450 C. then to 895 '0. could be made withoutany intermediate cooling and with the same reproducing very fine and very diffuse graphite, which, however, because of its stability, developts at the expense of the primary cementite in the subsequent high temperature operation. Nickel, on the other hand, because of its high solubility in ferrite, has an average composition throughout and its effectiveness remains low.

In these treatments of casting containing copper, an essential characteristic is to observe, the succession of the three operations: preliminary hardening, unclear formation towards 450 0., subsequent graphitisation a'ta high temperature.

An example is given by comparing the correct procedure and two other procedures in which either the preliminary hardening or the tempering for nuclear "formation has been omitted. In this experiment, the control casting without copper is the casting 1995 already mentioned; the analysis of casting 1996 is C:2.45%;

The results are given in the following'table:

Series of treatments Residual, Fe 0% N.

Preliminary hardening Primary graphitisation Tempering Nuclear formation Control, 0u=2% Control, 0u=2% 1995 1996 1995 1996 Nil 810 0. min. Salt, 180 1 minute. 810 0., 30 min. Salt,

180 0., 1 minute.

sult. At the same time, it was desired to demonstrate this remarkable property of copper in causing the pre liminary hardening at the actual temperature of the nuclear formation of the graphite: for this purpose, the castings were austenised at 810 C. for 30minutes, hardened at 450 C. and maintained for 48 hours at this temperature, then heated to 895 C. .for 14 hours and then cooled in still air.

For these two series of treatments, and "for castings of the compositions already given (always cast in sand in 'bars 15 x 3 0 mm., the structure of the casting being perfectly white), the following table gives the results in relation to the percentage of residual primary cementite Fe C and the number N of nodules or spherules of graphite per rn'm. "of section.

.It will be seen that only the triple treatment and-the addition of copper make it possible to reabsorb the entire quantity of primary cemeutite and to have a large number of nodules or spherules of graphite. f

In the accompanying drawings, Figures '1 and 2 show the relationship between the number of nodules or sphertiles of graphite per mm. and the nuclear formation temperature'for periods of maintenance of this temperature of 48 and 96 hours respectively. Figure 3 shows the relationship. between the number of nodules of graphite per mm. and the period of maintaining various nuclear formation temperatures. The curves relate to the same malleable casting with the composition C:2.22%; Si 1.34%; Mn 0.65%; Cu 2%; S 0.12%; P=0.l0%; Al=0.03%.; Ti:0.0 6%. The advantage of F850, Percent N Number of casting 1995 2145-1 2145-11 2041 1995 2145-1 2145-11 2041 type of composition control control Ni= 1.83 0u= 200 control control Ni= 1.83 0u= 2.00

Hardening, 450 0.; Kept 48 hours M450 0.;

Reheatiug 895 0., 14 hourS l2 -5 1 0 250 Preliminary hardening, 400 0.; 3 hours Nuclear formation 450, 0.; 48 hours Graphitisation, 895 0., 14 hours 6 4 0 25 g '50 400 It will be seen that copper is more effective thannickel and that the latter treatment is better than the former. The increase in the number of nuclei given an equal addition is 3.8 times greater for copper than for nickel with the first treatment and 12.3 times greater with the second treatment. Compared on a basis of the same cost of addition, the copper would be 60 times more effective than nickel.

It is believed that this action of copper can be explained by the fact that it is in the neighbourhood of the ternary eutectic content Fea--Fe C-Cu, so that itparticipates completely in the'preliminary hardening while remaining in supersaturation in the ferrite. In the course of the nuclear formation treatment at 450 C., it under goesa centripetal migration which precedes its precipitation. This leads 'to'very high local in'creasesin the copper concentration so that in'these regions the 'instaobserving a period sufficient to have a large numberof nuclei of graphite and the economic disadvantage of prolonging this time-excessively will be seen. Although the most satisfactory result is obtained at the end of 192 hours at 450 C., a period of 48 hours gives an almost identical result at a lower cost. On the other hand periods less than 30 hours sacrifice quality to no purpose.

With these castings containing 2% copper and cast in sand, and with correct treatment, the number of nodules or spherules of graphite per mm. is generally between 3,000 and 15,000 according to the region and'thickness, the average diameter of the spherules of graphite gen; erally being between '1 and 10 microns, usually 4microns. Y The malleable castings containing copper, of the type forming the objectof the invention, and the new precise series of heat treatment, have another remarkable new property in that they can extend the use of white iron castings which can be rendered malleable to greater thicknesses of casting than by the conventional methods while combining white structure at the time of easing with suitability for graphitisation, which are generally contradictory properties. According to the invention, these properties are reconciled by the fact that on solidification, the graphitising action of 2% copper is compensated for bythe whitening action of a corresponding increase in the manganese or by the introduction of molybdenum, or chromium, or the correlative reduction in silicon, whereas when the nuclei of graphite are formed at 450 C., this compensation no longer takes place, by reason of the centripetal migration of the copper which precedes its precipitation and the very high local increases in concentration which result therefrom.

A remarkable consequence of these new properties is the possibility of obtaining, by cupola smelting, by re fining in a reverberatory furnace and by sandcasting, mouldings with a white structure which, after having been subjected to the triple treatment of preliminary hardening, nuclear formation tempering and primary graphitisation annealing, have remarkable mechanical characteristics never before obtained in the industrial casting of white iron castings which can be rendered malleable, and this with a great regularity of production resultingboth from the effect of the copper and from the precision in the heat treatment, without any risk of shrinkage cracks, due to the interrupted hardening, and without deformaand tempering at 650 C., the elastic limitcan be raised to give:

Vickers hardness, kgjmm. 350 Elastic limit, kg./mm. 98 Breaking .load, kg./mm. 102 Elongation, percent 2.4

C, Si, Mn, S, P, Percent Percent Percent Percent Percent The conditions of treating each of the two castings after preliminary hardening and the results obtainednumber of nodules of graphite formed in the nuclear formation, and mechanical properties-are shown below:

Copper intro- Nuclear formation Number Graphitisation Elastic Breaking Elongaduced into the time and temof nuclei time and temlimit, kg./ load, kg./ tion, periron perature per mm! perature mm. mm." cent 15 hours at 500... 18, 000 2 hours at 875 60 85 4 15 hours at 450... 38, 000 1 hour at 875. 67 t 105 6.1

tion because of the limitation of the highest temperature reached in the course of treatment.

The favourable effect of copper can be combined with that of additions of aluminum, titanium and zirconium, these three constituents having an equally favourable action for obtaining a large number of nuclei when the rational procedure of graphitisation with controlled nuclear formation is used. The following example relates to,three castings of white iron cast in sand and having been subjected to the following treatment:

Heating to 810 C. for minutes, quenching in salt at 180 C. for 1 minute, cooling in still air;

Reheating to 450 C. and maintaining for 48 hours, cooling in still air;

Reheating to 895 C, and maintaining for 14 hours, cooling in still air.

The following table gives the residual primary cementite and the number of nodules of graphite:

If, as will be seen, the nuclear formation temperature is higher and the graphitisation time longer for the iron without copper than for that which contains it, it is because it was necessary to adopt these conditions in order to extract the best results from the iron without copper. For the latter, the results would have been considerably poorer than those given in the table if the nuclear formation had been carried out at a temperature as low as 450 C. and if thegraphitisation period had been as short as one hour. Compared with sand-casting, chilled casting has the advantage of making it possible to reduce considerably the graphitisation period at a given temperature. Accordingto the example given above, the disappearance of the primary cementite is complete at 875 in one hour.

Very satisfactory results can be obtained with chill-cast iron casting containing copper with shorter nuclear formation periods than that ,given in the example, for exam- Oastings O Si Mn S P Cu Al Ti Zr Fe O N/mmfl 1 s 1, 200 o s, 500

The mechanical properties which can easily be obtained with these castings" are generally at least equal to the following values for the test-pieces machined into bars and cooled in still air after graphitisation for 14 hours at 895 C., so that the matrix is lamellar pearlite:

But after the triple graphitisation treatment, these castings may also be subjected to a hardening and tempering process; thus by heating to 825 C., oil-hardening ple 3 to 6 hours maintenance at the temperature may be suitable.

White iron castings according to this process may also be produced in moulds the impression of which is constituted by relatively thin parts obtained by the agglomeration of sand or other refractory material, for example, by means of an organic thermosetting plastic material.

I claim:

1. A method of treating cast iron which comprises casting said iron in the form of a white iron casting having a thickness up to about 30 mm. and containing copper in a concentration of 0.7% to 3%, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat at a temperature above the temperature at the end of the eutectoid transformation, followed by step quenching, to a temperature below the M, whereby to form martensite, tempering at a temperature between 400 and 500 C. for 24 to '100 hours to effect the formation of nuclei in the graphite, and subjecting the casting to graphitization of the primary cementite by maintaining the casting at a temperature of about 875 C. to 900 C. for a period of time sufficient for the total disappearance of the cementite.

2. A method of treating cast iron which comprises casting said iron in the form of a white iron casting having a thickness up to about 30 mm. and containing copper in a concentration of 0.7% to 3%, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat at a temperature above thetemperature at the end of the eutectoid transformation, followed by step quenching including homogenization at a temperature slightly greater than the temperature at which martensitic transformation begins, followed by cooling in still air, f

tempering at a temperature between 400 and 500 C. for 24 to 100 hours to effect the formation of nuclei in the graphite, and subjecting the casting to graphitization of the primary cementite by maintaining the casting at a temperature of about 875 C. to 900 C. for a period of time sufiicient for the total disappearance of the cementite.

3. A method of treating cast iron which comprises castingsaid iron in the form of a white iron casting'having a thickness up to about 30 mm. and containing copper in a concentration of 0.7% to 3%, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat at a temperature above the temperature at the end of the eutectoid transformation, followed by step quenching at the nuclear formation temperature to form martensite, tempering at a temperature between 400 and 500 C. for 24 to 100 hours to effect the formation of nuclei in the graphite, and subjecting the casting to graphitization of the primary cementite 'by maintaining the casting at a temperature of about 875 C. to 900 C. fora period of time sufficient for the total disappearance of the cementite.

4. A'method of treating cast iron which comprises casting said iron in the form of a white iron casting having a thickness up to about mm. and containing copper in a concentration of 0.7% to 3%,- subjecting the casting of said white cast iron thus obtained to hardening by ,austenitization with heat at a temperature above the temperature at the end of the eutectoid transformation, followed by step quenching including homogenization at a temperature slightly greater than 'thetemperature at which martensitie transformationbegins, followed by cooling in still air, tempering at a temperature'of about 450 C. for 36 to '48 hours to effect the formation-of nuclei in the graphite, and subjecting the casting to graphitizationof the primary cementite by maintaining the casting at a temperature of about 875 C. to 900 C. for a period of time sufficient for the total disappearance of the cementite and cooling in still air. a

5. A method of treating cast iron which comprises casting said iron in the form of a white iron casting having a1] thickness up to about 30 mm. and containing copper in a concentration of 0.7% to 3% in a metal chill mold, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat at a tempera ture above the temperature at the end of the eutectoid ran formation, followed bystep quenching o a temperature below the M whereby to form martensite, tempering at a temperature between '400and 500 'C..[for 1 to 24 hours to effect the formation of nuclei in the graphite, and subjecting the casting tographitization of the primary cementite by maintaining the casting ata temperature of about 875 C. to 900 C. for .a period of time sufficient for the total disappearance of the cementite.

6. A method of treating cast iron which comprises casting said iron in the form of a white iron casting having a thickness up to about 30 mm. and containing copper in a concentration of 0.7% to 3% .in a metal chill mold, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat at a temperature above the 'temperatureat the end of the eutectoid transformation,followed by step quenching, to a temperature below the M whereby to form martensite, temperingat a temperature between 40.0 and 500 .C. for 1 to 24 hours to effect the formation of nuclei inthe graphite, and subjecting the casting to graphitization of the primary cementite by maintaining the castingat a temperature of about 875 .C. to 895 C. for one-half to'2-hours.

7. .A method as defined in claim 1, wherein the copper concentration is about 2%.

8. A method of treating cast iron which comprises casting saidiron in the form of a white iron casting having a thickness up to about 30mm. and containing copper in a Concentration of 0.7% .to 3%, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat ata temperature above the temperature at the end of the euectoid transformation, followed by step quenching, to a temperature below the M whereby to form martensite, tempering at a temperature between 400 and 500 C, for 24 to hours to effect the formation of nuclei in the graphite, and subjecting the casting to graphitization of the primary cementite by maintaining the casting at a temperature of about 875 C. to 895 C. for 10 to 14 hours.

9. A method of treating cast iron which comprises casting said iron in the form of a white iron casting having a thickness up to about 300 mmrand containing copper in a concentration of 0.7% to 3%, subjecting the casting of said white cast iron thus obtained to hardening by austenitization with heat .at a temperature vabove the temperature at the end of the eutectoid transformation, followed by step quenching, to a temperature below the M whereby to form martensite, tempering at a temperature between 400 and 500 C. for 24 to lOOhours to effect the formation of nuclei in the graphite, and subjecting the casting to graphitization of the primary cementite by maintaining the casting at a temperature of about 900 C. for 3 hours.

References Cited in the file'o'f this patent UNITED STATES PATENTS 1,489,128 Sowers June17, 1924 2,185,894 Hultgren 'Jan. 2, 1940 2,331,886 Boegehold .....V...... Q. Oct. 19, 1943 OTHER REFERENCES

Claims (1)

1. A METHOD OF TREATING CAST IRON WHICH COMPRISES CASTING SAID IRON IN THE FORM OF A WHITE IRON CASTING HAVING A THICKNESS UP TO ABOUT 30 MM. AND CONTAINING COPPER IN A CONCENTRATION OF 0.7% TO 3%, SUBJECTING THE CASTING OF SAID WHITE CAST IRON THUS OBTAINED TO HARDENING BY AUSTEMITIZATION WITH HEAT AT A TEMPERATURE ABOVE THE TEMPERATURE AT TE END OF THE ENTERCOID TRANSFORMATION, FOLLOWED BY STEP QUENCHING TO A TEMPERATURE BELOW THE M3 WHEREBY TO FORM MACTENSITE TEMPERING AT A TEMPERATURE BETWEEN 400 AND 500*C. FOR 24 TO 100 HOURS TO EFFECT THE FORMATION OF NUCLEI IN THE GRAPHITE, AND SUBJECTING THE CASTING TO GRAPHITIZATION OF THE PRIMARY CEMENTITE BY MAINTAINING THE CASTING AT ATEMPERATURE OF ABOUT 875*C. TO 900*C. FOR A PERIOD OF TIME SUFFICIENT FOR THE TOTAL DISAPPEARANCE OF THE CEMENTITE.

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