US2219320A - Heat treating process for white cast iron - Google Patents

Description

Patented Oct. 29,:1940

1 No Drawing Application July 22, 1935,

. SerlalNo.285,955 I I, I}

4011 m s. (onus- 21.7);-

I'his invention relates to a heat-treating process for white cast iron, by which such white cast iron may --be givenimprov'ed Properties; and to, the

product of such process; This'application'is a 5 continuation in part of my co-pending applicationSerial No. 76,294, filed April 24, .1936.

White cast iron has been used as such in a limited way, but its properties are so poor, ex- :ceptin-respectto its extreme hardness and wear m resistance, that; it has comparatively few.,engineering applications. It has low strengthJs brittle, has no. ductility, may notbe-worked, and is machinable only with great difllculty. Its pri- -mary use, other than when it is remelted and 1 convertedinto a product such;as-st eel,.is-;as a

basis for malleablei'ron, which, isthe cheapest form into which white cast iron may be changed.

To producemalleable iron,.the white cast iron is cast into the desired pieces, and such castings are heated to a temperature well above the critical point of themateriaL are ,held at that tem- I ;perature for aperiod of days, and; are then very slowly cooled. The process is long, and requires expensive furnace and heating. equipment, so that 125 itisitself quite expensive. 5

In the ,malleabilization process, the carbon which appears in the originabwhite cast iron in the form of cementite is transformed into --.graphite which-appearsas nodules distributed 30 substantially throughout the finished product.

Thus, malleable iron has a matrix of substantially, carbon-free iron with graphite particles distributed therein. V i

In ,thepresent process, it is, my object to give -the white cast iron difierent propertiesthan are ,given toit by the malleabiliz ation process and r which in many respects are better qualities than those in malleable iron. Itis aiurther object of myinvention to. securethesenimproved Properties by a relatively simple and short process.

Microscopic examinationshowsthe product to have a matrixof iron and cementite with'the cementite: thereof .largely inaspheroidized'; form,

and thatthe additional carbon which is present 45 is in the iormcof substantially disconnected par- -ticles or masses of massive cementite. a

The product hassubstantially improved prop- Its tensilestrengthis above 75,000and. usually above 80,000 pounds per square inch or 50 more. The product is readily machinable, it may ,be forgedat a-temperature of about 1400 to 1460 erties.

' v F. and may be welded with eitherza'n electric arc weld or an, acetylene, weld. It'hasa-high resistance to abrasion, and takes a highpolish. -It t 56 may be given subsequent heat treatments such tween about 1600and17009FJ as those applied, steeL lto cause changes in the properties of the carbon containing matrix simi- I: lar tothose whichoccur in steel.

; In generaLtheprocess is performed upon white iron castingsby heating them in aiurnace to a 5 Y temperature substantially above the AC1-3 point of the material, holding them at'that temperature for a periodof time, the temperature and i the holding time being sufllcient to cause some of the cementite to go into solution in the aus- 10 tenite and tosecureat least partial diffusion, and -,.equalization in the austenite, son that there is .iormed a substantially homogeneous matrix-oi substantially saturated'austenite. The temperaturefand holding time, however, is insumcient to 5 -cause any substantial" formation. of graphite either during such holding orduring subsequent steps in the'process. The castings are then .fur-

nace cooled to ajtemperature between 700" and 800 F., at'which temperature'they, should be held a suflicient time" to equalize the temperature throughout the body or the white cast iron; and

are then quenched, preferably in water. The heatingmay be 'doneatas rapid azrate as convenient, but with? the. usual precautions l ag'ainst warpage. The temperature to which they are heated should be above. the Aci-epoint oi' 4 the white cast iron, .inorder toisecur'e the ;for-

' mation of austenite to dissolve cementite, and

should beconsiderably above such point On-the-ao other hand, the temperature should not be so :high that graphite williorm-or that too great a tendency toward graphitization-will-occur., I

have =foundthat a the tmeperature should be-bef -The time duringwhich the metal is held at this high temperature aiterwthe furnace has; been brought to the high temperature should be in accordance *with the ausual practice:innthe heat -treatment of steel, to permitthe austeniteto dis 40 solvemore cementite andto permit diffusion and equalization ofthe cementite which is dissolved -:in the austenite. That is',--the time during which :the'metalis held at the desired high temperature,

from the'time the furnace reaches such temperature, shouldabe 'for approximately one hour for eachinch of metal, calculated'inaccordance with iusual heat-treating practice.

at the, conclusion ofg the holding time, the metaleis cooled ata relatively slow rate-but, fast enough to avoid .graphitization. Preferably, it

should :be furnace co01ed,, as by-perinitting'the .4 pieces to. remain m ths iurnac and t cool with the iurnace, or by, removing the pieces from r the. high-temperature furnace and immediately charging them in a furnace which is held at the temperature to which they are to be cooled. Ex-

pressed in cooling rates, I have found that the cooling should not be below about 250 F. per hour and may be faster, up to about 1000 F. per hour, these cooling rates being average cooling rates and not necessarily uniform throughout the cooling operation.

The cooling is carried to a temperature of about 700 to 800 F., most desirably 720 F., is held there for any period necessary to equalize the temperature of the piece .at that point, and may be held a longer period at that temperature. If the cooling is in the range of about 250 to 400 F. per hour, and the castings are small, the holding period at the 700-800 F. temperature may be omitted. j

At the conclusion of the cooling and any low-' temperature holding time, the castings are rap idly quenched, preferably in a liquid quenching medium; and I havefound that a water quench gives best results. y

A study of this process, and of.-the product thereof, indicates that metallurgically the process acts as follows: When the white cast iron reaches its Ac1 3 temperature, its pearlitic portions form austenite in which is dissolved the cementite from such portions. As the iron is heated more and held at l600-l700 F., the austenite dissolves cementite from the excess which is present. In the untreated white cast iron, this excess cementite occurs largely in the form of thin layers or stringers, which form a network through the original material, and also in the form of larger and relatively thick masses. At'the high-temperature the austenite becomes substantially saturated, a largepart of the stringers are dissolved,

especially at their thinner portions, and diffusion and equalization of the dissolved cementite Theheat treatment is'not so severe nor long continued that graphite is formed, although I,

believe a tendency toward graphite formation is unavoidably set up during the heating necessary 50 to get a desirableamount of thecementite. in so:

lution and diffused.

stringer form but instead in .a more spheriodized 5 form. At the Ari-a point the austenite is changed to' pearlite, which during the continued; cooling is itself at' least partiallyspheroidized. Thereis thus formed a matrix of pearlite and speroidized cemeptite. which is substantially con 50 tinuous although it'has embedded in it the undissolved masses ofcementite which are now substantially disconnected.

As has been stated, during the treatment of the white iron castings at the high temperature, 65 I believe there is a tendency set up for graphite to be formed. 11 the 'coolingis carried to room temperature at'the slow rate, the graphite actually separates out, and its presence in anysubstantial amount-materially weakensthe resulting 70 product. Quenching-from the desired temperature of approximately 7-20*F., however,substantially preventsthis formationof graphite.

The slow cooling to the quenching temperature produces in the matrix'properties of toughness 75 and ductility, and makes the product readily for about 30 to 45' minutes.

suits.

- I f H Per cent Carbon 2.40 Silicon 1.00 Manganese 0.35 Phosphorous 0.155 Sulphur 0.075

This analysis is believed to be typical of commercial white cast iron, and the test bars upon examination showed structures typical of high grade clean white cast iron.

Several of these test bars were placed in a cold furnace and heated in about 1 hours to a temperature of 1675 F. and held at that temperature The current was then shut off and the furnace allowed to cool to 720. The cooling took about 3 hoursso that the average cooling rate was about 270 per hour..

ples of the treated bars were welded in a forge fire, others were welded by acetylene, and others by the use of an electric arc, with satisfactory re- The metal wasfound capable of taking a high polish upon being machined and ground, and was readily drilledwith a plain carbon steel drill of No. 6 0,size. i

ExamPZe-Z -Test .bars similar tothose of Example l'werefplaced'ina furnace'and heated for about 1% hours at-r. which' time their color indicatedthatftheyhad' attained a temperature of -aboutJ160047001Fiend were held at that temperature for" thirtyminutes- The metal was then permitted to' 'cool in the furnace as rapidly as it l f'wouldQandi whenf'the metal changed color from During slow cooling above theAn-apoint, the] excess cementiteseparates not in its original,"

"heat to black heat'in cooling, the operator started stroking oneof the pieces of metal in the furnace with-a file. When the metal showed a bright mark when stroked with'the file the bars to have substantially the same properties. I

Example 3." Severali variously sized commercial white iron castings .were carefully loaded in a containerin such a way that the weight of the several pieces would not tend to deform them, and the containeriwas placed in afurnace and heated to about 1700 F. The pieces were held I' at this temperature for 'a'time calculated at the rate of one hour for each inch of metal of the thicker portions of the castings." The furnace was then permittedto coolto a temperature of approximately 720 during which time the charge was withdrawn two or three times for shortexamination periods.

Ihe castings were held at720 F. for about the same time as the holding time at the high temperature, and were then quenched in water.

The resulting treated castings were compared in various respects with the known properties'of identical pieces which had been malleabilized and were found'satisfactory for commercial use. The process had been performed under non-production conditions within about eleven hours; while the malleabilizing process appliedto the comparativepieces normally took a period of five days.

I claim as my invention: I Y

1. A process to quick anneal white iron castings with the aid of a water quench in the short cycle of time of hours or less, said process involving heating the white iron casting for a about 1 hours until it attains a temperatureof' approximately 1675 degrees F. thereby to get a more homogeneous condition by eliminating the 1700' F., holding it at that temperature to secure substantial saturation of the austenite, furnace cooling it at a rate of from 250 F. to 400F.

perature.

per hour to between 800 F. and 700 F., and then subjecting it to a rapid quench from that tem- 3. A heat-treating process for whitecast iron, comprising heating the white cast iron to a temperature substantially above its Aer-s temperature and holding it at such temperature to cause austenite to be formed and to dissolve a substantial part of the original stringer formation of massive cementite and to secure a more homog'eneous condition of the matrix but substantially avoiding graphite formation both during such holding and in subsequent process operations, subjecting the cast iron to cooling conditions in ,a temperature not below about, 700 F. until it reaches a uniform temperature of approximately 700" to 800 F., and quenching the cast iron from the temperature to which it is cooled.

4. A heat-treating process for white cast iron, comprising heating a white-iron casting to a temperature-of about 1600" to 1700" F., holding it at that temperature a time sufliclent to cause a considerable portion of the stringer formation of cementite to be dissolved while substantially avoiding graphitization, cooling the cast iron at a rate of between about 250 and 1000 F. per hour to a temperature of about 700 to 800 F., holding it at such temperature any time necessary to secure a uniform temperature in it, and quenching it from that temperature.

COLLET JONES.