US2435946A - Process for decarburizing austenitic manganese cast iron - Google Patents

Description

Patented Feb. 10, 1948 PROCESS FOR DECARBURIZIN G AUSTEN- ITIG MANGANESE CAST IRON Alfred Gordon Evans Robiette and Peter Francis Hancock, Erdington, Birmingham, signors to Birlec Limited, Birmingham,

land

No Drawing. Application In Great England, as- Eng- June 30, 1943, Serial Britain February 27,

4 Claims. (01. 148-16) The present inventionrelatesto improved iron alloys, and to a process for treating castings of austenitic white cast iron alloys, in which the principal alloying agent is manganese.

We have discovered that castings of austenitlc white cast iron alloys in which the principal alloying agent is manganese may be subjected to a decarburising treatment whereby the average carbon content of such castings is decreased and a product is obtained havingphysical properties similar to those. associated with austenitic mangan'ese. steel and/or whereby a graded structure with regard to the carbon content. is developed and a product is obtained which for certain pur poses may be equivalent or superior to a similar alloy of homogeneous carbon content such as austenitic manganese steel.

It is an object of the inventionto provide an improved iron alloy in which the'principal alloying ingredient is manganese.

It is another object of the invention to provide an improved iron alloy having a special combination of properties. T

It is a further object of the invention to provide an improved iron alloy productshaving. a wear-or abrasion resistant surface and'integral therewith a tough underlayer. i i

It is also an object of theinvention -,to provide an iron alloy product having a surface with the wear and abrasion properties of manganese steel and having a core with the properties of man'- ganese cast iron. r i

It is still another object of the invention to provide an integral iron alloy product having a hard martensitic' surface supported by a tough austenitic underlayer land a core of austenitic manganese cast iron.

The invention also contemplates a process of treating castings of austenitic manganese white cast iron whereby the aforesaid iron alloys and iron alloy products are produced.

The invention further contemplates a process of treating castings of austcnitic manganese white cast iron to impart to the surface thereof properties similar to those of Wear or abrasion resisting manganese steel.

It is also withinthe contemplation of the invention to provide castings of an iron-manganese alloy having an increasing carbon content from the surface to the core and having a special combination of properties. V Other objects and advantages of the invention will become apparent to those skilled in the art from the following description.

In accordance with the present invention, castings of austenitic manganese white cast irons are decarburised by an agent which is relatively nonoxidising to the manganiferous austenite, but is decarburising to the carbon content. The decarburising agent may be gaseous, liquid or solid.

In carrying the invention into practice, the decarburising agent preferably consists of a gas or gaseous mixture maintained at substantially atmospheric pressure, that is to say, at a pressure of not less than half an atmosphere and not exceeding two atmospheres. The gaseous decarburising agent may be passed into the treatment chamber of a furnace to form an atmosphere which, if desired, may be recirculated by a fan, The re-circulation may be effected within the furnace itself by means of a fan or through a system which comprises the furnace and an external conduit. The re-circulation system may include means for regenerating the atmosphere by adding a controlled supply of air or other oxygen-containing gas whereby the carbon monoxide and hydrogen formed during the decarburising reaction are re-oxidised to carbon di: oxide and water vapour. The system may also include a vent to prevent the setting up of excess internal pressure. Such an atmosphere should contain one or more of the constituents hydro: gen, water vapour and carbon dioxide as decarburising agents; if carbon dioxide is present there should also be. enough carbon monoxide present to reduce oxidation of the austenite to such a degree that the desired dimensions of-the finished castings will not be detrimentally affected. Similarly the hydrogen-watervapour ratio should be such that the gas mixture is non-oxidising to the same extent. The mixture may be diluted with inert constituents such as nitrogen.

Alternatively, in a solid or liquid the gaseous atmosphere to the treatment chamber, the said atmosphere may be generated within the furnace itself. Thus, for example, the castings may be treated in contact with molten salts containing oxidising the decarburising agent ma be agents or giving rise to the same under the conditions of the reaction, for example by using molten barium chloride, with or without barium oxides or a mixture of sodium chloride and manganese dioxide. When treating the castings with a solid decarburising agent they may be heated in the present of iron ore with or without the addition of small amounts ofcarhon.

form; also, instead of feeding The cast alloy may be heated, in the presence of any of the above decarburising agents, to a temperature between about 700 C. and the temperature of incipient fusion of the alloy under treatment, the temperature and duration of treatment depending upon the thickness of the section of the casting and the properties desired.

When the castings are of relatively thick section, and a long decarburising treatment isnecessary to produce the required average carbon content, it may be that there is: an excessively deep zone of low carbon at the surface. In this case the castings may be submittedto a recarburising treatment, in a carburising atmosphere by which the carbon content? of the surfacezone is increased. If a thin surface zone of low: carbon content is desired, a further short decarburising treatment may be given after the recarburising step.

The composition of the alloy as cast should preferably be in the range of about 9% to 25% man anese, about 2% to 6% of carbomnot more than about 2.0% of silicon, and not more than about 0.15% of phosphorus. Part of the manganese may be'replaced'by nickel and/or copper to such an extentthat the-alloy may contain up to a total of aboutf24% clinickeland copper. The nickel will substitute "an: approximately equal weight of manganese-whilethe-copper will substitute approximately half its weight of manganese. It will beun derstood 'that the expression faustenitic manganese cast irons as used herein includes-those alloys-in which part of the manganese is substituted by nickel and/or-copper'but in which the manganese is not less than "5%. Other alloying elements such as chromium, moiybdenum, titanium, tungsten and vanadium may alsobe present in quantities not exceeding about 5% of any one element and'not-exceeding about 12% m an, and it-is notintended to exclude the presence of these elements in -such amounts.

In a preferredapplication of the'inventiomthe cast alloy is decarburised bya heatitreatment in an atmosphere obtained-by partiallyburning fuel gases such as coal gas, coke oven gas, producer gas, butane, propane or other hydrocarbons. Thus for example when usingan atmosphere produced from coal gas, having-Ian approximate composition comprisingabout 45% to 48% of hydrogen, about'1'i% to 19% of methane, about 18% to'22i% of carbon monoxide,-about 2%:to 4% of carbon dioxide, about 0.5% to 1% of oxygen, about 6% to 9% of nitrogen, and about'.1.-5% to 2.5% of illuminants, the gas is mi-xed with-about 2%. 'to 3 parts ofair by volume and the mixture urnt in a closed combustion chamber with or without a, catalyst andthe resulting products of qqmbustion, aftercooling to remove excess water vapour, have an approximate composition comprising about 14.% to-8% of hydrogemabout 1 1% to 6% or carbonmonoxide,:about:6% to 9% of carbon dioxide, about ,1 to.2 of water vapour, the balance being nitrogen. This gas will vary between these limits depending .upon .the exact gas, to air ratioand upon the-temperature ofthe combustion chamber and in. regard to the water vapour content upon. the temperature to which theme is co led, Alternatively, other methods removin water vapour an'be employed;

, Th f llowi g s a or; an atmosphere using charcoal producer gals mixed with air. Charcoal producer gas has an approximat composition comprising about 30% 'to'-32% of carbon monoxide, ,about'1% to 3% of carbon dioxide, about '2%' to 4% of hydrogen,

example of th production of carbon monoxide, about 7.5

about 1% of water vapour, the balance being nitrogen. This ga is mixed in the ratio of 4 parts of gas with one part of air and is burnt either inan externally-heated chamber or in the furnace chamberitself togive'a gas having an approximate composition comprising about 20% of carbon dioxide,

about 2% of hydrogen, and about 1% of water -vaD01 the balance being nitrogen.

The process of decarburisation apparently operates in the following manner. The gaseous, liquid or solid decarburising medium is out of chemical equilibrium with the carbon content of the cast iron with the result that it absorb car- .bon readily, from the surface of the castings. If gaseousmedium is'used containing hydrogen,

water vapour orcarbon dioxide, the reactions in- 'volved apparently are:

The carbon in the'above reactions is represented for simplicity as .elemental carbon, but in fact is mainly in the'form of metallic carbides or the solution of carbon in iron known as austenite. The surface of theparts being treated rapidly tends to reach equilibrium with the gas, and this equilibrium carbon content depends upon the ratio of carburising and decarburising constituents of the gaseous mixture at the temperature of treatment. It is evident, therefore, that the final surface carbon contentv can be largely controlled by the gas composition; Preferably the gas composition is such that it is in equilibrium with a carbon content of between zero and 0.8% so as to get rapid carbon removal. It should be stated that the equilibria alter with the temperature in accordance with well-known chemical laws and the necessary 'gas composition may, therefore, vary with the temperature of treatment. Furthermore, the rateof diffusion of carbon to the surface is governed by'the temperature and this is usually the limiting factor.

During the decarburising treatment, carbon is continuously removed from the surface of the casting by reaction with the decarburising agent, and simultaneously carbon diffuses from the centre, where the carbon content is high, to the surface, where it is low. As the treatment progresses, the depth of the decarburised zone increases, and if carried far enough will reach the centre of the section. Continuation of the treatment will cause a lowering of the carbon content at the centre, and a consequent flattening ofv the carbon content gradient from the centre of the surface; Theoretically, in the limit, a uniform carbon content would be obtained, of a value in equilibrium with the decarburising medium. In practice, the time required to approximate this result would be very long except in the case. of thin sections. The characteristics of the carbon gradient are continually varying throughout the treatment, and these characteristicsfor a given thickness of section are influenced by the time and temperature of treatment, by the composition of the decarburising medium, bythe original carbon-content of the a1loy,'an'd by thediifusicn constants of carbon in the austenite.

By suitable control of these factors, a carbon content gradient of any desired characteristics, within certain limits, may be produced. Preferably, in practice, the decarburised castings have a graded carbon content increasing from a valuein the range of 0.05% to 0.9% at the surface, to a value in the range of 0.9% to 6.0% at the core.

The metallurgical structure and the physical properties of an alloy containing manganese in the contents specified, together with the other elements, will depend upon the carbon content. It isevident, therefore, that a range of metallurgical structures can be produced from the surface to the core of the section thus treated. For example, after treatment and quenching of a casting from a temperature of approximately 1000" to 1100 C., the core may consist of a structure substantially similar to that in the original alloy, namely, carbide set in a matrix of austenite. This core will merge into an intermediate zone of substantially pure austenite, the depth or thickness of this zone depending upon the content of austenite forming elements such as manganese, nickel or copper and the-time of treatment. Then, approaching the surface there will be a zone of martensite plus austenite, and with certain compositions a zone of pure martensite on the surface.

In putting the process into commercial production, normally it is found that there is formed a very thin surface skin of ferrite and/or oxide generally of a few thousandths of an inch in thickness. This, for commercial purposes can be disregarded and is incidental to the process and the product. It should, therefore, be understood that, when we refer to surface zones, we mean zones immediately underlying said thin skin of ferrite and/or oxide.

The physical characteristics of such a section would be somewhat as follows: The core would be of moderate strength and would possess low ductility, but would be supported by a tough ductile band of austenite of considerable thickness. The majority of this band would be of relatively stable austenite if quenched from a high temperature of the order of about 1000 to 1100 C. This austenite would become more unstable towards the surface where it merges intothe hard martensite, This martensitic surface is brittle, but is backed by the said tough austenitic band. The resulting articleas a whole will have a hard wear-resisting surface with a high rate of work hardening, a tough, strong body or intermediate band and acore which is moderately strong, but not ductile. The relative thickness of these bands or 'zones can be adjusted by the time and temperature of treatment and by control of the atmosphere during the various stages of treatment and they will also vary with the thickness of section. In thin sections the original core structure may be completely removed and the tough 'austenite structure can extend right to the centre.

The complex structure resulting from treatment in accordance with the present invention may have a possible application in the manufac' ture of armour plating, whilst for other purposes it would give rise to ideal wear and abrasion resisting properties combined with adequate strength and ductility. The resulting product is moreover cheaply and simply produced from Zan alloy cast iron which can be readily melted and cast by any of, the well-known methods at present in use for cast and malleable irons.

The process of the invention may be applied to any type of castings. Such castings may include those made by a centrifugal process which are usually of hollow cylindrical shape; after and elongation figures are a measure of strength about 6% t0 9% tion,;sucli castings could be slitlongitudinally" and rolled, forged, or otherwise manipulated.

If it is desirable that the surface layer of any part ,or parts of a casting should retain its high: carbon content, such part or parts may be protected from the action of the decarburising agent by any known process for protecting surfaces,

such as by applying an impermeable coating of enamel or metallic deposit.

The process of the present invention enables products to be produced having a wear resistant surface zone containing at least one of the constituents austenite and martensite and a tough underlying zone or layer containing at least one of the constituents austenite and carbide. The products preferably have ultimate tensile strengths lying between 18 and 45 tons/sq. in. elongations on two inches between 8 and 35 and surface hardnesses between 200 and 400 V. P. N. with a high rate of work hardening. The tensile and ductility of the material as a whole, and the capacity for work hardening and surface hardness are a measure of the wear resistance- I of the material.

Material required to have primarily high wear resistance would have analysis and treatment to give the highest obtainable surface hardness andcapacity for work hardening, whereas for toughness, the analysis and treatment would aim at the optimum tensile and elongation figures.

Although the present invention has been de-' scribed in conjunction with preferred embodi ments, it is to be understood that modifications and variations may be resorted to without de-- parting from the spirit and scope of the inven-" tion, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview'and scope of the inventionand appended claims.

Having described our invention what we claim 1 and desire to secure by Letters Patent is:

1. A process of treating a casting of austeniticcarbidie white cast iron having a structure com-- prised of austenite and free massive carbides and containing over 2% and up to about,6% carbon, about 9% to 25% manganese, and the balance essentially iron which comprises decarburizing said casting in a solid state at substantially atmospheric pressure and above about 700 C. with a gaseous decarburizing medium which is substantially non-oxidizing to the manganifer-ous austenite in said casting but is decarburizing to the carbon therein and which contains about 8% to 14% hydrogen, about 1% to 2% water vapor, carbon dioxide, about 6% to 11% carbon monoxide and the balance nitrogen, continuing said decarburizing treatment for a, period of time sufficient to reduce thecarbon content throughout the entire thickness of the casting and to produce a graded carbon content increasing toward the core and comprised when quenched of a work hardenable surface zone having a carbon content of less than 0.9% and a structure containing martensite on an underlayer comprised of unstable austenite merging into a deeper tough core of more stable austenite from which all free massive carbides have been removed, and quenching said casting having said graded carbon content from approximately 1000 to 1100" C.

2. A process of treating a casting of austeniticcarbidic white cast iron having a structure comprised of austenite and free massive carbides and.

treatment in accordance with the present inven- 5 t ining ov r 2% and up to about 6% carbon,

about1'9 to.-- 25%; mang nese=-andthe e-balance essentially iron which comprises =-decarburizing said c'astingin-a solid state at substantially atmospheric pressureand above-about 700 0-.- with a gaseous: decarburizing medium which is .substantially, non-oxidizing tothe manganiferous austenite in said castingbut-is decarburizing to the carbon therein -oontinuin 'said .decarburizing treatment for a period of--.time-suflicient to reduce the carbon :contentf-throughout the entire thickness of thecasting and to=producein the casting a graded carbon contentincreasing tow-ard the core andncomprisedw-hen quenched of a worle hardenable surface zone havingp a carbon content -of--1ess than-0.9 and-a, structure. conj tainingmartensite on-:an underlayer comprised of unstableaustenite-merging into a deeper tough core :of more stableyaustenite from which all free f massive-J carbides: have I been removed,

and quenching said casting #having said. graded, carbonvcontent fromapproximately-1000 to-1100-C. 1 3r A-processof treating .a; casting. of austeniticcarbidic white castironhaving astructure comprised :ofaustenite and freemassive carbides and containing over 2% and up to about-6%- carbon,

24 rotat -least oneof: the 1 elements selected from the -g-roupnconsisting of nickel and. copper, the

amount ofm-anganese plus nickel .plus' one-half thecopper; being; 9%..to 25%of. the composition,

and the balance' essentia-lly ironwhich comprises] decarburizing {said castingin a solidrstate tat; substantially atmospheric pressure" and above about '7-00:'- C. w-ith a gaseous decarburizing medium which is substantially non -exidizing to; the Iman-, ganiferous'austenite iii-said casting; but is decarburizing to the carbon therein, continuing, said decarburizing treatment for aperiod-of. time.

sufficient: to reduce the carbon content throughout the entire thickness of-thecastingand to produce in the casting a gradedcarbon content increasingwtoward thecore and. comprisedwhen quenched of a work hardenable surface zone having -a carbon content-of less than 0.9% and a structure containing martensiteon. an underlayercomprised of" unstable austenite] merging intoa deeper tough core of more-stable austenite' from which all free! massive carbides have been removed,,and quenching said .casting having said graded carbon contentfrom approximately 1000 to 1-100 C,

42v A-process of treating a castingof austeniticcarbidic white cast iron havin'gfa. structure comprised-.ofaustenite and free massive carbides and containingz over 2% ,and toabout 6%, carbon,-

about 9%. to-'25%.; manganese, and the=balance essentially iron which comprises decarburizing. said' casting in-a, solid state at substantiallyjatmosphericpressure andabove about 700C. with.

a gaseous decarburizing medium which, issubstantially non-oxidizing to the manganiferous. austenite in said casting but is decarburizing to:

the carbon therein and which contains about 2%- hydrogen, about 1% water vapor, about carbon dioxide, about 20% carbon monoxideand the balance nitrogen, continuing said decarburizing treatment for a period of time sufficient to reduce the.carbon content throughout the entire thickness vo1 the casting'andto produce a graded.

carbon content increasing toward the core and comprised when quenched of a work hardenable surface. zone having a carbon content offless than-0.9 and a structure containing martensite;

on an underlayer comprised ofiunstab'le austenite.

merginginto a. deeper toughcore of more stable, austeniteirom which all free massive carbides, have. been removed, and. quenching. said casting.

having said graded carbon content from approximately 10003 to. 1100.? C..

4 ALFRED GORDON Efwn'ss ROBIEIITE;

PETERIRANCIS HANCOCK.

. REEERENCES CITED:

The following references are of record in the file of this "patent? I I UNITED STATES PATENTS OTHER REFERENCES f"A11oy"Cast Irons, 1938, page 16. "Revue de Meta1lurgie,. July 1936;pp. 466-472;

Am. Foundrymen.s Assoc.,