US1931109A - Heat and shock resisting cast iron - Google Patents

Description

' Patented Oct. 17, 193 3 their or ie- 1,931,1M HEAT AND snoox nnsrsrrno oas 'r men Stanley w. Ewing and ham n. Schwartz,

Cleveland, Ohio,- assignors, by measure assignments, to Link-Belt Company, Chicago, iii, a,

corporation of Illinois No Drawing.

a clai Our inventionrelates to iron castings'which in use are exposed to repeated heating and cooling which set up severe local stresses in the castings and tend tooause breakages or to deform the 5 castings. For uses such as grate and furnace bars or other installations in which the iron is 'subjected to repeated heating and cooling, cast iron or malleable iron castings are largely used.

Cast iron, when it contains large quantities of cementite, is extremely apt to crack under severe I service conditions, or when it contains pearlite and large flakes of graphite is so brittle as likewise to crack, while malleable iron, because of large quantities of ferrite, readily warps and dic torts out of shape. v

v Wehave-found that iron castine of normal compositions used for conversion into malleable iron when properly annealed so as to be substantially free from cementite and containing a relatively high amount of temper carbon and a 'balanced amount of ferrite and .pearlite, are sufiicieritly stiff to resist deformation under extremely arduous conditions of-intense localized heating and frequent cooling, and sufllciently '25 ductile 'to withstand shocks. Such material is more resistant to destruction by abrasive wear than are the cast or malleable iron materials aforesaid.

Wehave found that iron castings containing,

after being cast, from 2% to 3.5% of carbon, and silicon, manganese, phosphorus and sulphur in the percentages usual in iron intended for use in malleable iron castingsand in which the carbon is practicaly all in the form of cementite, give excellent results after being subjected to the followin'g heat treatment.- I

The castings aretreated at an elevated temperature of, for'example, between 1500 and 1700 F but necessarily above the lower critical point 40 commonly called A1, until practically all the cementite is destroyed. Such heat treatment may may be cooled from the high heat at any conveniently rapid rate to" slightly below 1400" F., 55 and maintained at a nearly constant temperature Application January 31, 1925 Serial No. 6,140

(er. rea -21.8)

somewhat under'for example, ,ataiBOO F.-for a period of about ten hours before final cooling.

Obviously the time or rate of cooling will be a function of the composition of the metal.

When cast iron containing 2.75% carbon practically all combined as cementite has been treated as described above, a typical specimen of our resulting product will contain substantially 2.00% of temper or graphitic carbon. and agraphitic, approximately 25% having been lost through oxidation. Results will be impaired if the casting when cooled contains more than the eutectoid composition, which is approximately .83% combined carbon, the excess over would be in the form of free or pro-eutectoid cementite. Normally the combined carbon is be tween .30 and .75 and is usualy about 50%, While the silicon usually ranges-between .60% and .80%. When cooled at a slow rate (but not slower than 10 F. per hour) from about 14200 we have found that in the resulting structure the .carbon is present in graphitic nodules surrounded by areas of ferrite and pearlite.

A characteristic formation of material as shown in micrographs comprises nodules oi carbon surrounded by comparatively large areas of ferrite, whichin turn are surrounded by either continuous or separated areas of pearlite. We have found, however, that iii-cooling from above 1400 F. some ferrite is apt to separate out and that in the resulting product the characteristic formation referred to above does not always appear, although the areas of ferrite and pearlite are sufliciently distributed to produce a satisfactory material.

Additions of small percentages of chromium, tin, selenium, tellurium, antimony or boron, abnormal ratios of manganese and sulphur, in which either is present in an amount substantially more than required for combination with. the other, will retard the formation of graphitic carbon and ferrite for agiven cooling rate, and also serve to stabilize the pearlite, while the presence of high silicon, titanium, aluminum, zirconium or' uranium will increase therate of formation of graphitic carbon and ferrite. Variations in the composition by additions of the elements just referred to will require corresponding changes in the cooling ratel, or' vice versa, to produce satisfactory results.

Our improved material is particularly efiective when used for grate and furnace bars, stolrer links, and-mother installations in which the material is subjected repeatedly to high tempera tures, repeated shocks, or severe abrasion. When Vol - effected at a rate not less than 10 F. per hour to bars are subjected at times to rather intense localized heating due to the sintering material vburning through in spots, which sets up very severe local stresses, grate bars embodying our invention have a life of several times that of either cast iron or malleable iron bars, due to stiffness, which prevents warping, and their ductility, which prevents cracking.

The terms and ,expressions which we have employed are used as terms of description and not of limitation, and we have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and'described, or portions thereof, but recognize that various modifications are possible within the scope of the invention claimed.

. Whatwe claim is:

1. A grate bar comprising-an ironcasting substantially free from cementite, and having the major portion of its carbon in the form of nodules of graphite, the dominant portion of the casting being formed of substantial areas of both ferrite and pearlite, the casting being sufliciently stiff to resist deformation at a moderate heat and being suflicently ductile to prevent crackingunder heating stresses.

2. A grate bar comprising an iron casting substantially free from cementite, having the major portion of its carbon inthe form of nodules of graphite, areas of ferrite surrounding the nodules of graphite, areas of pe'arlite surrounding the areas of ferrite, the casting being sufficiently stiff to resist deformation at a moderate heat and being sufllciently ductile to prevent cracking under heating stresses.

3. A granular ferrous metal including grains of free ferrite, graphitie carbon and iron carbide the latter being distributed between the grains of ferrite and about the graphitic carbon.

4. The process of treating malleableized cast iron which comprises successive heating and cooling thereof to derive therefrom a structure con taining free'carbon ferrite, and iron carbide; the heating being carried above the critical temperature but below the melting point to cause the formation of iron carbide and the cooling being enable the retention of iron carbide together with 7 casting being formed of substantial areas of both ferrite and pearlite, the casting being sufficient ly stiff to resist deformation at a moderate heat and being sufiieiently ductile to prevent cracking under heating stresses.

'I. A furnace element comprising an iron casting substantially free from cementite, having the major portion of its carbon in the form of nodules of graphite, areas of ferrite surrounding the nodules of graphite, areas of pearlite surrounding the areas of ferrite, the casting being sufficiently stiff to resist deformation at a moderate heat and being sufficiently ductile to prevent cracking under heating stresses.

8. A Stoker element comprising an iron casting substantially free from cementite, and having the major portion of its carbon in the formof nodules of graphite, the dominant portion of the casting being formed of substantial areas of both ferrite and p'earlite, the casting being sufficiently stiff to resist deformation at a moderate heat and-being sufliciently ductile to prevent cracking under heating stresses. i r

9. A stoker element comprising an iron casting substantially free from cementite, having the".

major portion of its carbon in the form of nodules of graphite, areas of ferrite surrounding the nodules of graphite, areas of pearlite surrounding the areas of ferrite, the casting being sufficiently stiff to resist deformation at a moderate 'heat and being sufliciently ductile to prevent cracking under heating stresses.

- STANLEY W. EWING.

HARRY A. SCHWARTZ.