US1803340A - Process of annealing and heat treating cast iron - Google Patents

Description

' Patented May 5, 1931 meant I PAUL s. woven, or rrrrsnnnsn, rnnrasynvenm FIR/0035901 ANNE-MING AND EAT TREATING CAST IRON No Drawing. Application filed November 10, 1925, Serial No. 88,172. Renewed November 18, 1929.-

lhis invention relates to improvements in processes of annealing and heat treating, socalled white cast iron, whereby a new and useful product is obtained.

5 The object of the invention is to provide an alloy having the desirable qualities found in hardened and tempered high carbon steel of high tensile strength, high elastic limit, hardening and tempering properties, and

hardness and toughness of structure of such a character as to be highly resistant to abrasion and wear, furthermore to provide anenumerated above has apparently been reached. 1

In order to clarify the later-exposition of my invention, it will be necessary to give a review of the art of casting and annealing malleable iron, and also of hardening and tempering steel.

Malleable iron before annealing is known technically asflwhite cast iron. This is the state in which it solidifies after casting from the molten product as drawn-fromthe furname or cupola. The alloy known as white. ,cast iron consists of silicon, carbon, varyingsmall amounts of sulphur, manganese, phosphorus and occasionally traces of other elements, with the balance made up of pure iron. The elements other than silicon, carbon and iron are unessential to my invention, provided they are present in such proportions that the resultant product after the standard annealing treatment will be commercial malleable iron. These elements therefore' will hereinafter be disregarded and the alloy spoken of as if it were a simple alloy ofsilicon, carbon and iron.

lieved by heating up to temperatures,

expected from a physical inspection, closely Silicon is only important'in its catalytic action on the carbon, much silicon, as in grey iron, tending to make the carbon precipitate from the solution, as it cools after casting, in

relatively large flakes. These are familiar to the trade as the gray flakes observed in the fracture of sand cast pig iron. The malleable iron industry is founded on the phenomenon that, with relatively rapid cooling as when cast in a sand mold, and with silicon in the proportion of approximately 1.10% to .70% and carbon approximately 2.20% to 3.00%, the carbon will remain in combination with the iron in the casting down to normal temperatures, and the resultant structure will be what is known as white cast iron,

an aggregate of iron carbide and pearlite. 4 However, this entity is in a state of insta bility and if the rigidity of the mass is reusually slightly in excess of the critical tem-- perature of the alloy, and holding at this temperature for a number of hours, the carbon will precipitate from the solid mass in the finely divided state called temper carbon. The resultant product, after this separation is complete and the mass has cooled to atmospheric temperatures, is a fairly homogeneous mixture of temper carbon in .a matrix of pure iron or ferrite. There are some other phenomena occasioned by the annealing process, such as a slight decarbonization of the alloy, which somewhat reduces the amount of total carbon in the commercial product from the amount present in the white cast iron before annealing. This decarbo'nizing takesplace largely in the surface or skin portions of the castings which are thus softened and rendered less resistant to wear and abrasion due to the decarbonizing. The distribution of combined carbon is, after this decarboniz'ing, not uniform throughout the casting and the hardness is variable throughout with the. variation content of combined carbon. The resultant product may now be considered as a very dense sponge of ferrite with the interstices filled with temper'carbon, and, as would be Y approaches low carbon steel in its properties of tensile strength, elastic limit and ductility, with the added property ofgreat resilience under shock due to the temper carbon cushions scattered throughout the mass.

I haye defined above white cast iron as its compositionis understood in malleable iron foundry practice, where it is castand cooled in a sand mold. It is possible, however, to use an iron of considerably higher carbon or silicon and, if cast in a metallic mold or a mold which has a high heat conductivity, still get a product which consists of cementite and pearlite and which has a fracture silvery white in appearance. I wish it to be understood that when the term white cast iron is hereinafter used that I do not wish to limit the term to the composition and proportion of elements which is employed to make the white cast iron of the malleable founder, but also to include iron of other compositions such that when solidified from the molten state, it is essentially an aggregate of cementite and pearlite.

1 When commercial steel with a relatively high carbon content, 25% and over, is heated above its critical temperature range and suddenly cooled, as for example quenching in water, its hardness is materially increased. Steel above the critical range is a solid solution of carbon in iron or perhaps iron carbide in iron and is called gamma iron. Steel below the critical temperature range is called alpha iron. 1 The name austenite has been given to gamma iron. There are several well defined transition stages which the solution gamma iron assumes as it cools to the alpha or atmospheric condition. These different entities, which have clearly developed prop: erties, are called martensite, troostite, sor-' bite and pearlite. To harden carbon steel, it is heated through the critical temperature, when it assumes the austenitic condition. it is then cooled in a suitable medium, as for example quenching in water or other 'quick cooling substance if the properties of martensite are to be retained, or oil quenched or air cooled for sorbite or sorbitic pearlite, and furnace cooled for pearlite, pearlite being the softest and most ductile product of the series. Frequently, for more reliable commercial production, the steel is first hardened by quenching in water and then tempered by heating to temperatures below the critical temperature range. The hardened condition obtained by the above quenching process is of a transitional nature and there is an instability of structure which, when the rigidity of the mass is relieved to some extent by comparatively low heat, the compound tends to stabilize itself as sorbite or pearlite. This treatment does not noticeably decarbonize the surface or skin portions of the steel article and hence these portions are highly resistant to wear and abrasion and the hardness and distribution of combined carbon are substantially uniform throughout the steel article.

Experiments in the annealing of white cast iron indicate that combined carbon cannot be entirely broken up into ferrite and temper carbon above the critical range. On heating white cast iron to a given temperature and holding for a number of hours, the percentage of combined carbon gradually decreases and finally becomes a constant. This constant value depends on the annealing temperature and for any temperature used in practice varies from about .57% to 1.00%, increasing with the temperature. As the metal cools slowly through the critical range and immediately below it, this residual ccmcntite decomposes into temper carbon and ferrite. For this reason. the cooling of malleable iron, .through the critical temperature and for some 200 degrees below this temperature, is very slow, preferably some 8 to 10 degrees per hour, in order to insure a complete elimination of cementite from the annealed product. A test bar taken from the furnace above the critical temperature range and quenched shows what appears to be a martensitic struc-l ture with temper carbon inclusions. With the exception of the temper carbon particles, this is a structure characteristic of hardened high carbon steel, and is susceptible to strengthening and toughening by a simple tempering treatment.

In my invention I heat white cast iron of a composition as outlined heretofore, for a number of hours above its critical temper ature and then quench in any suitable medium and finally temper to get any desirable predetermined quality. This same result may be reached by quenching in a medium with such a specific heat and heat conductivity that the castings will be cooled down more slowly than in the process outlined above andI will obtain the sorbitic condition directly which was obtained above b quenching and tempering. Furthermore, i a greater degree of softness combined with great strength and high elastic limit is desired, I will find for the annealing furnace a combined carbon-temperature curve for uniform cooling through the critical temperature range. By means of this curve I will take the metal from the furnace when it has reached any desired and predetermined combined carbon content. Immediately, after withdrawal from the furnace, the metal will either be quenched and tempered, or cooled through a suitable medium to give the required structure without tempering, Furthermore either the tempering or slow quenching mediums can be so constituted and manipulated as to give in the case of castings a desirable finishing color.

' It appreciated that I have invented a new and useful process of obta' an unusual alloy of iron and carbon, the other elements of a nature and in a proportion regularly present .in commercial malleable iron.

' This alloy has the desirable qualities found in hardened and tempered high carbon steel, of high tensile strength, high elastic limit, hardening and tempering properties, and hardness and toughness of structure of such a characteras to be highly resistant to abra sion and Wear. In addition to the above properties this alloy possesses the qualities of commercial white cast iron, as defined by malleable iron foundry practice, of facility of handling when molten and accuracy to detail when cast in a mold. And again in the finished state it will possess the quality of malleable iron of high resistance to shock, due to the cushioning effect of the temper carbon particles scattered throughoutthe mass.

What I claim and desire to secure by Letters Patent is 1. A process for treating metal which consists in heating white cast iron to a temperature in excess of the critical temperature range, holding it at approximately that temperature for a suflicient length of time to produce a structure containing between about 25% and about 1% of combined carbon, removing it from the furnace while at a temperature in excess of the critical range, quenching or rapidly cooling it in a medium whichwill have such a cooling effect on the metal that the austenite of the solid solution shall be changed and set as any one of the structures called troostito-sorbite, sorbite, sorbitic-pearlite, pearlite or any combination of these structures, while retaining the content -of combined carbon substantially unchanged.

2. A process for treating metal which consists of heating white cast iron to a temperature in excess of the critical temperature range, holding the same. at approximately that temperature for a suflic'ient length of.

time to precipitate some of the combined carbon as temper carbon, removing from the furnace when theresidual carbon in combination with the ,iron has reached the carbon-iron ratio of medium or high carbon steel, quenching it in some medium which will have a rapidly cooling effect of such a 1 rate that the austenite of thesolid solution shall be changed to one of thefstructures called troostito-sorbite, sorbite, sorbiticpearlite or pearlite or any combination of these structures,'while retaining the aforesaid content of combined carbon substantiall unchanged.

3. he process of treating white cast iron which consists in heating the iron at a temperature in excess of the critical temperature range of the iron'for severalQhours, cooling 'the iron from about that temperature in a v being so heated, reheating the product at a constituenbchanging temperature below the critical temperature thereby producing, a product containing the content of. combined carbon substantially unchanged, and softer and more ductile than before the reheating, and cooling the reheated product.

4;. A process of heat treating metal which consists in heating white cast iron above its critical temperature range sufficiently long to precipitate all .but between about 25% and about 1% of the combined carbon therein, quenching the iron when the content of combined carbon remaining is within the aforesaid range and in a manner to produce a martensitic structure containing between about 25% and about 1% of combined carbon, then reheating the iron to below its critical temperature range in a manner to convert the martensitic structure into any one or more of the structures called troostitosorbite, sorbite, sorbitic pearlite or pearlite without substantially decreasing the content of combined carbon.

. 5. The process of treatingan iron casting of approximately the chemical analysis of white cast iron which comprises heating such a casting above its critical temperature range to precipitate all but between about 25% and about 1% of the combined carbon, quenching .the casting in a manner to produce a martensitic structure containing between about 25% and about 1% of combined carbon, then reheating the quenched casting to below the critical temperature range sufliciently long to convert the structure of the quenched casting into a structure containing iron carbide and substantially free from .martensite or troostite without substantially decreasing the content of combined carbon. 6. An iron casting approximating white cast iron in chemical analysis and containing ferrite, uncombined carbon and between about 25% and about 1% of iron carbide substantially free from martensite and troostite, the hardness of the casting being substantially uniform throughout.

7. An iron casting approximating whit-e cast iron in chemical analysis and contain-' ing ferrite, uncombined carbon, and between about 25% and about 1% of iron carbide substantially free from martensite and troostite, the surface portions of the casting having a hardness'and a resistance to'wear and abrasion characteristic of hardened and tempered high carbon steel.

Signed at Bridgeport, in the county of Fairfield and State of Connecticut, this-7th day of November,-A. D. 1925.

" PAUL S. MENOUGH.

manner .to. obtain a product containing be- 1 tween about 25% and about 1% of combined carbon, and being softer than the iron before