US2153906A - Method of heat treating chromiumcontaining corrosion and/or heat resisting steels - Google Patents

Description

A ril 11, 1939. """'2,153,906

R. H. ABORN ET AL METHOD OF HEAT TREATING CHROMIUM'CONTAINING CORROSION AND/OR HEAT RESISTING STEELS Filed Oct. 22, 1952 2 Sheets-Sheet 2 CUPPOS/O/V ATMC/( l 1 l 400 600 6'00 1000 00 m0 f INVENTOR @BE/PT/i /4BOEN.

ATTORNEYS Patented r. H, 1939. i I

METHOD CONT nnsrsc s'rnans DE AT TEE-AG CQRBOSION to York.

N. Y a corporation billion 3? Application October 22, 1932, Se No. 639%73 6 Claims.

This invention relates to the art of metallurgy and more particularly to the art of heat treating metals to produce and/or to develop therein specific desired physical and/ or chemical properties. More particularly this invention relates to the heat treatment of corrosion resisting alloys containing as a corrosion resisting element the metal chromium.

Heretofore in the art many alloy compositions have been proposed for corrosion resisting purposes in which the chromium content thereof has been in part or in whole relied upon to impart thereto chemical or heat resistant or both chemical and heat resistant properties. In such type alloys it has heretofore been noted that the heat and/ or corrosion resisting properties of the alloys are in marked degree destroyed or lowered through the presence in the alloy of the element carbon. The deleterious effect of carbon is attributed generally to the fact that chromium reacts with carbon in the proportion of four (4) atoms of chromium to one (1) atom of carbon to form chromium carbide compounds thereby removing the chromium thus combined from the t alloy composition.

readily done as the chromium carbide content of the alloy hardens the alloy, and moreover when the alloy is to be used as a corrosion resistingalloy the presence of this chromium carbide distributed about the grain boundaries weakens the metal for use under some conditions and in relatively small cross-sections may seriously lower the resistance of the metal to deterioration when exposed to attack by corroding solutions or gases. Accordingly, it has heretofore been recognized that in corrosion resisting chromium-containing alloys it is preferable to maintain as low a carbon content as is economically practical.

It has been found, however, that under the most careful of metallurgical methods heretofore proposed for the economical production of such alloys a carbon content under .1 per cent is diificult to economically attain and even this low percent- (Cl. lid-@LE) age of carbon has sometimes been found detrimental. .It has therefore been proposed to add to the alloy a so-called special carbide forming or fixingelement in such an amount as will eflicientl and p eferentially combine with the carban to formmetalcarbide compounds which are substantially sle under the proposed service conditions of. the alloy. Such special carbide forming elements comprise one or more of the elements, titanium, zirconium, tantalum, columbium, vanadium, tungsten, molybdenum, uranium and the like.

It may be appreciated that when the carbon content of any. given chromium containing alloy is low it is exceedingly cult to introduce into the alloy any one of these special carbide forming elements ln eaactly the equivalent combining proportions. This dimcultyis due in part to the fact that these elements also serve primarily as degasifying elements in the molten metal bath and the gas content of any given molten alloy varies widely from melt to melt. The dimculty is also due in part to the fact that in low carbon molten metal baths which are substantially free from these special carbide forming elements the carbon is present therein as metal carbide compounds and in relatively extreme dilution. It-is, therefore, necessary to add to the metal bath an excess of the special carbide forming element, the precise amount of the excess being variable from melt to melt with any given alloy depending upon these variable factors.

It is also to be appreciated that any appreciable encess'of any one of these special carbide forming elements above that necessary to react mth contained gases and carbon materially ef ieots the resultant alloy product. In some alloys desired for special-service conditions this excess might be beneficial; in other alloys this excess is often 'detrimentaL- it is cult to maintain this excess of special carbide forming constituents to a desired 10w cical methods or manufacture.

It is one of the objects oi the present inventlon tov provide a method whereby a desired minimum or s desired maximum of special carbide forming constituent may be consistently and emciently maintained in such chromium containlug-heat and/or'corrosion resisting alloys.

Another object of this invention is to provide a method of heat treating chromium containing heatand/or corrosion resisting alloys containing a desired minimum or maximum of added special carbide forming constituent to insure the comunder present metallurplete utilization of the carbide forming constituent in its contemplated capacity.

Another object of this invention is to improve the method of manufacture and the heat and/or corrosion resisting properties of chromium containing alloys.

Other objects and advantages will become apparent as the invention is morefully disclosed.

In accordance with the objects of the present invention we have discovered that by proper heat treatment subsequent to forming the alloy by any of the heretofore proposed methods of manufacture, and either before or after fabrication into desired articles of manufacture, that we may insure the complete or substantially complete combining of the carbon content of the alloy with the special carbide forming constituent and that 7 this desired result can be obtained irrespective of the total amount of the special carbide forming constituent in the alloy above that amount approximating but above the theoretical molecular combining weight of the constituent. We can, therefore, add to the alloy during forming a desired minimum or maximum of the special carbideforming element or any desired excess of the same over that theoretically necessary to combine with the carbon content of the alloy and thereafter through the special heat treatment process hereinafter disclosed insure that the primary purpose of the added constituent in combining with the carbon is fulfilled and the remainingexcess (if any) is available to perform its desired function in degasifying or in modifying the crystal structure or physical characteristics of the alloy.

Briefly stated, the present invention resides in the discovery that the'soluhilities of various metal carbide compounds in any given metal or alloy composition varies with the metal or alloy temperature and that each metal carbide compound has a different solubility curve in any given metal or alloy. It is, therefore, possible by proper heat treatment to preferentially dissolve or to preferentially precipitate metal. carbide compounds within metal or alloy structures.

As a specific embodiment of the present invention and not in any sense as a limitation of the nature and scope thereof, we will describe the present invention as it has been applied to the beneficiating of nickel-chromium steels of the so-called stainless type comprising chromium approximately 18 per cent; nickel approximateLv 8 per cent, carbon approximately 0.1 per cent. To this steel during manufacture has been added a proportion of a special carbide forming constituent, specifically titanium, in an amount in' substantially slight excess of that theoretically necessary to empirically combine with the estimated total carbon content of the alloy.

Before further disclosing the nature and scope of the present invention reference should be made to the accompanying drawings wherein- Fig. 1 illustrates the typical solubility curves for titanium carbide, tungsten carbide and chromium carbide compounds in an alloy known in the art as nickel-chromium stainless steel .of the so-called 18-8 type.

Fig. 2 illustrates the beneficial effects produced on a nickel-chromium stainless steel alloy containing a proportion of special carbide forming constituent, when treated in accordance with the present invention.

- Fig. 3 illustrates the beneficial effects obtained arsaooo annealed state and-in the so-called cold rolled state.

Referring to Fig. 1 we have determined that the solubility of chromium carbide (Curve I) in stainless steels of the 18-8 type remains low and substantially constant with increased temperatures up to about 700 C. At temperatures above about 700 C. the solubility and the rate of solu' tion rapidly increases until at or about 1100 C. the solubility approximates .25 per cent carbon, which is above the estimated carbon content of the alloy composition herein specifically described.

With the same steel alloy base we have discovered that the solubility of titanium carbide (Curve 2) remains low (and lower than chromium carbide) and substantially constant with increased temperatures up to about 900 0., and at temperatures above 900 C. while the solubility increases and the rate of solution increases the total carbon dissolved in the alloy base at these higher temperatures is less than that brought in solution as chromium carbide. At 1100 C. as indicated on the chart .25 per cent carbon is dissolved as chromium .carbide while only .03 to .04 per cent carbon is in solution as titanium carbide at the same temperature.

It is, therefore, possible to heat treat such an 1&8 stainless steel alloy containing carbon combined as chromium carbide and a proportion of titanium substantially equivalent to that necessary to empirically combine-.with the carbon to form titanium carbide compounds, to temperatures above about 700 C. for a requisite time interval necessary to effect solution of the chromium carbide compound and subsequent precipitation of the carbon content thereof as titanium carbide compounds.

It is to be appreciated that preferably the specific heat treating temperature employed should be chosen with respect to the carbon content of the alloy and the relative rates of solution of the chromium and titaniumcarbides so as to provide as wide a differential between chromium carbide solution and titanium carbide precipitation as is convenient and to thereby facilitate the process. And it is also to be appreciated that the specific heat treating temperature applied is in part to be chosen with respect to the desired or the'desired minimum dissolved carbon content of the alloy, as where two metal carbide compounds such as titanium and chromium carbides are present in any given metal base, and have differing solubilities at any given temperature, the amounts of each carbide compound retained in solution-in the metal base at that temperature will depend upon well recognized principles of solution and precipitation, the preponderance of one metal constituent of the metal carbide compounds over the other, the time, the relative rates of solubilities, solution pressures and the like factors.

In general it is recognized that under any given set of heat treating conditions and with any given alloy composition a certain equilibrium of carbide solution and carbide precipitation will be reached and the alloy thus heat treated will retain in solid solution (if quenched from this heat treating temperature) this equilibrium percentage of carbon.

It is apparent, therefore, that to obtain any desired minimum of residual dissolved carbon in accordance with the present invention it is only necessary to experimentally determine the extent and effective scope of these factors with any given alloy and with any given special carbide forming constituent.

solid solution (if in sufficient excess) would react directly with the chromium carbide in accord- -ance with Equation (0) to thereby form titanium carbide as a precipitate, the chromium content of the carbide .then passing into solid solution. In each of the hypothetical alternatives (d) or (e') a residual carbon content in accordance with the solubility chart of Fig. 1 also would be retained in solid solution.

When the quenched alloy or any one of the heretofore enumerated heat treated alloys are heat treated to temperatures between about 1750 F. to about 2500 F. (955 C. to 1370 C.) the titanium, carbon and chromium are all again taken up into solid solution in which state they may be retained by suitable quenching to temperatures below about 900 F. (480 0.). This is indicated by Equation (I).

When the alloy is quenched from a temperature range 1300-F. to 1700 F (705 C. to 925 C.) irrespective of whether the alloy has previously been heated to temperatures above this range or not provided that the time interval within this range has been sufllcient to permit the obtainance of substantial equilibrium between chromium', titanium and carbon content, and is thereafter exposed to temperatures below about 900 F. (480 C.) a totally different combination of these elements is perceived than that obtained in Equation (0) Under these new conditions it is to be noted that the titanium has combined with the carbon (in part or in whole depending upon the time interval involved relative excess of Ti essential to such combination, solution pressures, and the equilibrium residue of TiC-CnC of the quench temperature, etc.) and has precipitated as TiC whereas the chromium remains in solid solution together with residual and excess titanium and carbon.

The same'situation is found even where the alloy is reheated to temperatures between 900 F. (480 C.) to 1350 F. (735 C.) or to temperatures between 1350 F. (735 C.) to 1750 F. (955 0.). the previous heat treatment has been insuflicient to precipitate all except the equilibrium residue of carbon then more-titanium carbides will precipitate out on the reheating.

When the temperature is raised to above 1750 F. (955 C.) the titanium carbide is redissolved and all of the elements chromium, titanium and carbon return to solid solution substantially as indicated in Equation (9').

In applying the teachings of the chart of Fig. 1 and the teachings of the above table to the heat treatment of titanium containing nickel chromium steels of the 18-8 type of the specific embodiment herein disclosed, we have found that these alloys may be materially improved by heat treatment to temperatures ranging from about 1300 F. (705 C.) to about 1800 F. (980 0.). Following heat treating, the alloy may be either quenched to room temperatures or may be slow cooled to room temperatures as desired. More-J over, we have found that this beneflciating heat treatment may be applied to such alloys in the so-called annealed" state, or before or after so- "called cold rolling, fabricating, etc. without detrimental result. We have found that heat treating of the alloy within this temperature range serves primarily to stabilize the carbon content thereof through the converting oi the same into stable titanium carbide compounds, which compounds thereafter are not susceptible to change In the latter range if the time interval ofdecreases with increase in temperature and that substantially the same results may be obtained at a lower temperature in a longer time interval In general it may be stated that the reas may be obtained at a higher temperature in a shorter interval. The precise time interval required at any given temperature with any given alloy may be readily ascertained by experiment.

To illustrate the beneficial results obtainable by the practice of the present invention we have prepared the charts of Figs. 2 and 3.

In Fig. 2 we have illustrated the eflect of heat treatment at various temperatures on the corrosion-resisting properties of a so-called annealed' 18-8 alloycontaining titanium. The specimen, annealed at 2100 F., was subjected to a thermal gradient of the range indicated for a period of 1 hour; following this treatment the whole specimen was exposed at 1100 F. for 100 hours in order to detect latent susceptibility to intergranular attack. In the chart the solid line denotes the corrosion resisting properties measured in any arbitrary unit by the special electrical resistance method above described. It may be noted that up to'a gradient temperature of about 1000 F. there was a uniform degree of attack. At temperatures above about 1000 F. the corrosion resistance rapidly decreased; however at gradient temperatures between 1400 F. to-1600 F. corrosion resistance rapidly returned to a maximum approximating 100 per cent. .Thereafter this maximum corrosion resistance was rapidly lost by heat treatment at successively higher heat treating temperatures.

To indicate the eflect of prolonged periods of heat treatment the experiments resulting in the production of the solid line were repeated using 4 hour periods of heat treatment instead of 1 hour. The dotted line indicates the displacement obtained in that portion of the curve X-X of the solid line. This shows that 4 hours heat treatment at about 1400 F. is substantially equivalent to a 1 hour treatment at about 1600 F.

To illustrate the benefit obtainable upon heat treating the same material at a determined temperature preceded by various standard mill processing steps, the chart of Fig. 3 has been prepared. In the chart Curve I indicates the results obtained on the so-called annealed alloy which has been heat treated to temperatures approximating 2150 F. and quenched. When samples of this alloy are subsequently heat treated for one hundred hours at the temperatures indicated on the bottom horizontal line of the chart, it is found that at temperatures between about 1000 F. and a about 1200" F. the resistance to intercrystalline peratures the susceptibility to corrosion is again evidenced at treating temperatures between about 1000 F. and 1200 F. as indicated in Curve 3. The extent of this susceptibility however is markedly less than with annealed metal which has not previously been given the heat treatment in the temperature range 1300 F. to 1800 F. This result may be due for example to the fact as indicated in Fig. 1 that the titanium carbide formed during the 1600 F. heat treatment is less readily soluble in the austenite at 2150 F. than is chromium carbide or free carbon which normally is present in the annealed" alloy at 2150 F. or that the period of time of heat treatment at 1606" F. was too short to obtain completeprecipitation of the carbon content of the alloy as titanium carbide.

When the heat treated, cold rolled and annealed metal tested in Curve 3 is re-treated within the preferred heat treatment range (1300 F. to 1800 F.) it is found as indicated in Curve 3 that the susceptibility. to inter-crystalline corrosion is'again reduced within the range 1000 F. to 1200 F. to a relatively low minimum over that noted with respect to the original annealed and not so heat treated metal of curve i. The result indicated in Curve 3 indicates that either the time interval of heat treatment at 1600" F. is too short to obtain substantially complete precipitation of the carbon as titanium carbide or that the residual or equilibrium amount of titanium and chromium carbides retained in solution at.

that treating temperature is sufiicient to subsequently deleteriously effect the corrosion resistance of the alloy at these elevated temperatures. A longer time of heat treatment at 1600 F. or a longer time of heat treatment at a lower treating temperature within the range of i300 F. to 1800 F. would serve to still further reduce this susceptibility to corrosion.

Having broadly and specifically defined the present invention it is apparent that many modifications and departures may be made therein without departing essentially from the nature and scope thereof. Instead of applying the broad inventive idea to the specific 18-8 nickel chromium alloy, we may also apply it to any other chromium containing austenitic or non-austenitic alloy wherein the chromium content is in major part relied upon to impart the desired corrosion and/or heat resisting qualities.

Instead of utilizing titanium as a special carbon forming or carbon fixing agent we may use any one or more or the herein above enumerated special carbon forming elements. When any one or more of these other special carbon forming elements such as tungsten are used it is to be understood that the heat treatment range herein specified with respect to titanium need not necessarily apply and in our broadest con cept we contemplate heat treating in a temperature range which provides the optimum opportunity for the special carbide forming agent that is added, to combine with the carbon content of the alloy and to precipitate out as a subsantially stable carbide compound leaving the chromium substantially free to perform its desired corrosion resisting and/or heat resisting function. This temperature range for the purposes of the present invention will be defined as the optimum special carbide precipitation temperature'range.

Nor is the present invention limited in its scope to a minimum or maximum addition of the special carbide forming element-to the alloy as obviously where it is desired to remove only a part of the carbon or where it is desired to utilize an excess amount of the carbide forming element to modify the alloy structure or the physical properties of the alloy, the herein described heat treatment within the optimum special carbide precipitation temperature range may be applied.

The advantages of heat treating these chromium containing alloys containing the special carbide forming element within this optimum temperature range prior to the use and application of the alloy in the arts or in service are many. Besides those herein above enumerated it permits the use, for example, of a minimum amount of a special addition element other than thosehereinbefore enumerated, such as aluminum, silicon, magnesium and the like metals, whose presence in excess may have an unfavorable infiuence on the non-metallic content of the steel or those elements which in excess promote the development of ferrite thereby altering the desirable characteristics of the so-called austenitic type steels or alloys. By this special heat treatment method defined herein, these elements may now be used in their minimum theoretical and desired amounts without the heretofore ex-. perienced detrimental features.

What we claim is:

l. The method of preventing loss of corrosion resistance in austenitic iron and steel alloys containing chromium, carbon and at least one of the strong carbide forming elements at temperatures below about 800 C., which comprises a stabilizing heat treating step applied to the alloy prior to use at temperatures within said damaging range, the temperature of said heat treatment being between about 730 C. and about 950 6., and the time interval of said heat treating being at least sumcient to obtain a substantial conversion of the carbon content of the saidalloy into carbide compounds of the said strong carbide forming element.

2. The method of treating austenite iron and steel alloys containing chromium, carbon and titanium to stabilize said alloy against subsequent deterioration through chromium carbide precipitation when the said alloy in service is subjected to elevated temperatures ranging up to 800 C. which comprises heat treating said alloy prior to use in said range of elevated temperatures at a temperature above about 950 C. for a time interval necessary to put all of the carbon content thereof in solid solution, cooling said alloy to a temperature within the range of about 730 C. to about 959 and maintaining the alloy at said temperature for a time interval sufdcient to obtain substantial precipitation of the carbon content or the alloy as titanium carbide com pounds.

3. The method or treating austenite iron and steel alloys containing chromium, carbon and titanium to stabilize said alloy against subsequent deterioration through chromium carbide precipitation when the said alloy in service is subjected to elevated temperatures ranging up to about 800 C. which comprises heat treating said alloy all cipitation oi the carbon content thereof as titanium carbide compounds.

4. The method oi! preventing loss 01' corrosion resistance in austenitic iron and steel alloys containing chromium, carbon and at least one of the strong carbide forming elements at temperatures within the carbide precipitation range. which comprises a stabilizing heat treating step applied to the alloy prior to use at temperatures within said damaging range, temperature of said heat treatment being between about 1400 I". and about 1600" F., and the time interval of said heat treating being at least sumcient to obtain a substantial conversion of the carbon content of the said alloy into carbide compounds of the said strong carbide forming element.

5. The method of preventing loss of corrosion resistance in austenitic chromium-iron; alloys containing carbon and a strong carbide-forming constituent at temperatures within the carbide precipitation range. which comprises a stabilizing heat treatment applied to the alloy as a final step prior to use at temperatures within such antaaoe range, the temperature of said'heat treatment including temperatures between about 1400 F. and 1600 F. and the time interval of said heat treatment being at least suflicient to form noncorrodible carbide compounds between the carbon and the strong carbide-forming constituent.

6. The method of preventing loss of corromon resistance in austenitic iron and steel alloys containing chromium, carbon and sumcient titanium to at least empirically combine with the said carbon to form titanium carbide compounds when the alloys in service are to be subjected to temperatures ranging from above atmospheric temperatures to about 800 0., which comprises heat treating the alloy prior to such service at a temperature within the range 130 C. to about 950 C. for a suflicient time interval eflective .to obtain a conversion of substantially all of the said carbon into said titanium carbide compounds.

ROBERT H. ABORN.

JOHN J. BRU'IHERFORD.

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