US2450395A - Malleable cast iron - Google Patents

Description

Sept. 28, 1948.

' fraz/MUM TRE/4 TED /VQ 4 -4 TELL (JR/UM wa BORON TREA TE D H; A. IECKMAN ETAL MALLEABI-*IE CAST v:[RON

Filed Aug. 2., 1944 Patented Sept. 28, 1948 MALLEABLE CAST IRON Harry A. Eckman and Henry W. Maack, Chicago,

Ill., assignors to Crane Co., Chicago, Ill., a corporation of Illinois Application August 2, 1944, Serial No. 547,754

(Cl. 'Z5- 123) 5 Claims.

This invention relates to a novel iron for malleable iron castings and the manufacture thereof. More specifically, it relates to a method for improving certain metallurgical characteristi'cs of blackheart malleable iron, through the use of addition agents or" comparatively recent practical application, to extend its range of usefulness, making the iron suitable for castings havingthicker sections than it would be suitable for without the additions.

In the art of making malleable iron castings, it is generally understood, and in standard specifications usually required, that the castings be free from so-called primary graphite. The latter is free graphite, present in the iron castings before annealing. It is especially detrimental to the strength and toughness of the castings after annealing. It limits the thickness of section of the castings which can be made from such iron and still realize the potential physical or mechanical properties of the iron. One object of our invention is to improve the iron in this respect by adding a suitable modifier.

Generally considered, malleable iron having relatively high carbon content is not as well suited for making thick section castings as iron of lower carbon content. The benets obtainable from the practice of our invention are not confined to high carbon iron however although composition and section thickness are quite closely related. Other elements than carbon in an iron composition have an influence on its characteristics as a malleable iron casting material, also limiting the range of use of sulch iron for thick castings. Silicon is the principal controlling element in this respect. Besides influencing the casting thickness range of the iron, it governs the annealing or malleableizing of the castings. High silicon promotes malleableizing; low silicon retards it. Therefore, the silicon and carbon content in malleable cast iron are required to be carefully adjusted and controlled for any range of section thickness of the castings and length of malleableizing cycle, although these characteristics can now be altered to some extent by the use of suitable modifier additions.

A'modiiier which has the property of preventing the formation of graphite when an Iiron-carbon alloy solidifies and cools is quite generally -known as a carbide stabilizer, so called because it obstructs the breakdown of carbides into iron and graphite or free carbon and sometimes also combined carbon in one or more compounds hav- ,ing appropriate metallographic names and distinctive appearances when examined` through a microscope. So-called pearlite is the principal form of combined carbon in iron-carbon alloys. Numerous metallic and some non-metallic elements act as carbide stabilizers in iron-carbon alloys. Some form more permanent or refractory :compounds than others, resisting breakdownor divorce to different degrees during annealing. The elements chromium, manganese, vanadium, sulfur, selenium, tellurium and boron after the rst 0.10 per cent or so, molybdenum in appreciable amounts, antimony and tin and other metallic and non-metallic elements are retarders of graphitization. Not al1 of these elements, but some others, are known as carbide formers or carbide stabilizers. The' elements tungsten and columbium are recognized as stabilizers. The distinction between carbide stabilizers and annealing retarders in malleable iron is not clear cut, the effect depending on the amount of the element present.

The characteristics most desired in a stabilizer for malleable iron castings are efficiency in pre.- venting the formation of primary graphite in the white or unannealed iron, and the minimum retarding effect on graphitization during annealfor the malleableizing cycle, or both. As an example, `chromium is a stabilizer commonly guarded against in malleable iron foundries because of rits widespread use in low alloy and stainless steels and the likelihood of its presence in steel scrap, a regular ingredient in malleable iron melting furnace charges.

The element tellurium has a very strong stabilizing effect on the carbides in malleable iron. Adding a relatively minute amount has the effect of stabilizing the carbides to such extent that the range of usefulness of the iron with respect to thick sections is considerably increased. As little as a few thousandths of'one per cent of this very potent element has this effect. The benefits of tellurium are not obtainable without drawbacks, however, because it causes coarsening of the temper carbon distribution in the iron and makes annealing more diiiicult, often causing crystalline rims, evident when the castings are fractured. 'Ihe first detriment mentioned, more pronounced `in high carbon than in low'icarbon malleable iron, can be overcome, among other Ways, by the addition of copperin the amount of about one-half of one per cent. An addition of silicon is usually made to overcome the retarding effect of the tellurium when the castings are annealed.

The element boron in minute amounts has a marked effect in accelerating the breakdown or divorcing of the lcarbides during the annealing of malleable iron. In other words, boron, like silicon, acts as an accelerator of malleableizing. But, as with tellurium, the benefits realized are not obtained without sacrifice in part of some desirable properties. Boron in excess of the minute amount required to facilitate annealing coarsens or elongates the temper carbon globules which are precipitated during annealing, therebyv detrimentally aifecting the mechanical properties of the iron. For this reason the consensus of current opinion seems to be that the use of boron Y is undesirable unless it is required to overcome the stabilizing effect of an element which retards graphitization, also present in the iron. As small an amount as .001 per cent of boron is effective in accelerating graphitization.

Our invention comprises adding a stabilizer, preferably tellurium, to the molten iron f or pouring thick section malleable iron castings, the purpose of the stabilizer being to prevent the formation of primary graphite in the castings before annealing, commonly called white iron castings. To counteract the effect of the tellurium in retarding the graphitization of the iron when the white iron castings are annealed, it has been found desirable to add an accelerator of graphitization or malleableizing preferably the element boron in the form of ferroboron. The tellurium and boron act together to make the iron more suitable for thick section blackheart malleable iron castings.

The relative benefits to be gained from applying our invention may be seen more clearly by referring to the accompanying Figures 1, 2 and 3, which are self-explanatory. The facsimiles of the foregoing figures, made'by outlining on a viewing screen the magnified projection of the temper carbon particles in samples of the relatively high carbon content malleable iron described in the above text, illustrate the effect of our invention on the microstructure. Note that the temper carbon particles in untreated iron (Fig. 1) are relatively large and widely separated, those in tellurium treated iron (Fig. 2) larger, and those in iron treated with both 4tellurium and boron (Fig. 3) are smaller and more uniformly distributed.

' The range of analysis (in percent)` of our improved malleable iron is:

Carbon Silicon Manganese Tellurium Boron .(30 to 1.60 .30 t0 .70 .0001 t0 .2 .U01 t0 .05

while a typical analysis has been found to be (in percent) Carbon Silicon Manganese Tellurium Boron 2.25 t0 3.25 .00 1 to .01

| .este 1.20 l .antojo .ooi to .015

complished by adding a carbide stabilizer, namely tellurium, to prevent the formation of primary graphite. It has been discovered that the undesirable effects of the tellurium on the annealing characteristics and mechanical properties are counteracted by an addition of boron, an accelerator of graphitization. The amount of tellurium required to be added depends on the iron to be treated and the improvement to be accomplished, that is, the thickness of the section which is to be cast. The addition of boron is varied roughly in proportion to the amount of tellurium. In iron with about 3 per cent total carbon, when the -tellurium addition is small, say below about .003 per cent, the boron addition is likewise of small amount, .001 or .002 per cent. With a larger addition of tellurium, for example from .004 to .008 per cent, an appropriate amount of boron may be .003 or .004 percent. As a rule it is desirable to keep the amount of the boron addition less than that of the tellurium addition because too much boron tends to cause temper carbon precipitation in a dendritic arrangement detrimental to the mechanical properties of the iron.

We claim:

l. An annealable white iron casting comprising carbon 2.25 to 3.25%, silicon .60 to 1.60%, manganese .30 to .70%, the remainder being substantially iron and small but effective amounts of the carbide forming elements of tellurium and boron to prevent the formation of primary graphite during solidication, the boron accelerating the decomposition of iron carbide during annealing to thereby offset the retarding effect of tellurium in the graphitization of malleable iron.

2. A white iron casting of relatively thick section, substantially free from primary graphite and readily annealable to produce malleable iron, containing approximately 2.25 to 3.25% carbon, .60 to 1.20% silicon, .30 to .70% manganese and a small but effective amount of tellurium upto .02% and up to .015% boron to prevent the formation of primary .graphite during solidification, the boron facilitating the decomposition of carbides during annealing, to thereby offset the retarding effect of tellurium in the graphitization of malleable iron.

3. An annealable white iron casting of relatively heavy section and substantially free from primary graphite, vcontaining at least 2.25% carbon, at least .60% silicon, at least 30% manganese and small amounts of carbide forming elements tellurium and boron to prevent separation of free graphite during solidication, said amounts being .001 to .02% tellurium and .001 .to 015% boron, and the relative amounts of tellurium and boron proportioned so that .the accelerating leffect of .the boron in decomposition .of carbides during annealing offsets the retarding effect yof tellurium in the graphitization of malleable iron.

4. A white iron casting of relatively heavy section and substantially free from primary graphite, composed of the following essential constituents: `Carbon 2.25 to 3.25%, silicon .60 to 1.20%, manganese .30 to .70%, the remainder being substantially iron and small but effective amounts of tellurium from .001 to .015% and boron from .001 to .01% of the mixture, the latter combined amounts being sufcient to obstruct the formation of carbon in free form during solidication and so proportioned that the accelerating keffect of the boron in the-decomposition of carbides oisets the retarding eiect of tellurium in the annealing of the iron to produce'malleable iron.

5. The process of making malleable cast ironY articles of relatively heavy sections comprising casting a mixture containing 2.25 to 3.25% carbon, .60 to 1.60% silicon, .30 to .70% manganese and the balance substantially iron, in the presence of small but effective amounts of tellurium from .01 to .015% and vboron from .001 to .01% ofthe total, suicient to prevent the formation of carbon in free form during solidication, to thereby produce a white cast iron article and then annealing the article under conditions to cause separation of substantially all of the carbon as temper carbon.

HARRY A. ECKMAN.

HENRY W. MAACK.

6 REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,250,488 Lorig July 29, 1941 2,253,502 Boegehold Aug. 2 6, 1941 2,281,460 Smalley Apr. 28, 1942 10 2,331,886 Boegehold oct. 19, 1943 2,364,922 Smalley Dec. 12, 1944 OTHER REFERENCES The Foundry, April 1944, pages 129 and 182. Chemical Abstracts, vol. 34, pages 3223 and 3224.

Certificate of Correction Patent N0.'"2,450,395. September 2s, 194s.

HARRY A. EOKMAN ET AL.

It is hereby certiied that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 7, claim 5, for from .01.. read from .001 l and that the Said Letters Patent should bev read with this correction therein that th same may conform to the record of the ease inthe Patent Oce. M

Signed and sealed this 14th dayof December, A. D. 1948.

[IML] THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Images