AU2002363303A1 - Method for forming pearlite in an iron based article. - Google Patents

Description

REACTIVE HEAT TREATMENT TO FORM PEARLITE FROM AN IRON CONTAINING ARTICLE

FIELD OF THE INVENTION

[0001] The instant invention is directed to a method for producing pearlite from an iron containing article by reactive heat treatment.

BACKGROUND OF THE INVENTION

[0002] Because it is relatively inexpensive, carbon steel is the workhorse of the petrochemical industry. Chromium alloying is known to improve the corrosion resistance of carbon steel, but chiomium is an expensive element. Thus, approaches whereby corrosion resistance can be achieved without expensive alloying are desirable.

[0003] Peaiiite is a microstructural constituent of steels which is made up of alternating layers of feπite (body centered cubic iron) and cementite (Fe₃C). The pearlite micro structure is particularly resistant to certain forms of acid corrosion such as, for example, corrosion by organic acids. Thus, pearlite could be a ready substitute for expensive chiomium alloying, however, the strength characteristics of peaiiite limit its use as a bulk structural material for many applications since peaiiite is produced from carbon steels containing at least 0.77% carbon.

[0004] Thus, what is needed in the ait is a process for producing peaiiite from an iron containing article which process preserves the mechanical properties of the article. BRIEF DESCRIPTION OF THE FIGURES

[0005] FIG. 1 depicts scanning election micrographs showing (a) surface pearlitic structure on pure iron after reactive heat treatment at 775°C for 1 hour in 50% CO:50% H₂ environment and (b) enlarged area on surface revealing the ferrite (Fe) and cementite (Fe₃C) forming as roughly parallel lamellae, or platelets, to produce a composite lamellar two-phase structure. In this scanning electron micrograph the cementite lamellae appear light and the ferrite appeals recessed, because it has etched more deeply than the cementite. These figures show the final product having the peaiiite surface, which is produced in accordance with this invention.

[0006] FIG. 2 depicts the thickness vaiiation of surface peaiiite formed by the method of this invention as a function of reaction time at 775°C in 50% CO:50% H₂ as well as 97.5% CO:2.5% H₂ environments.

[0007] FIG. 3 depicts the thickness vaiiation of surface peaiiite formed by the method of this invention as a function of H₂ content in CO at 775°C for 1 hour.

[0008] FIG. 4 depicts the thickness vaiiation of surface peaiiite formed by the method of this invention as a function of temperature in 50% CO:50% H₂ environment for 1 hour.

SUMMARY OF THE INVENTION

[0009] The present invention is directed to a process for producing peaiiite from an iron containing article comprising the steps of, (a) heating an iron containing article comprising at least 50 wt % iron for a time and at a temperature sufficient to convert at least a portion of said iron from a feπϊtic structure to an austenitic structure, (b) exposing said austenitic structure, for a time sufficient and at a temperature of about 727 to about 900°C, to a carbon supersaturated environment to diffuse carbon into said austenitic structure and (c) cooling said iron containing article to form a continuous peaiiite structure.

[0010] A carbon supersaturated environment is herein defined as an environment in which the thermo dynamic activity of carbon is greater than unity. It is known that CO is the most potent carbon transferring molecule and the presence of hydrogen in carbon monoxide tends to facilitate carbon transfer. The following reactions can lead to the transfer of carbon to the metal surface from carbonaceous environments.

CO + H₂ = C + H₂0 [1]

2CO = C + C0₂ [2]

CFL_t = C + 2 H₂ [3]

[0011] Reaction [1] has the fastest kinetics: therefore CO-H₂ gas mixtures are the preferred gas mixtures to be used as the carbon supersaturated environments. Typical hydrogen contents in carbon monoxide can range from about 2.5 vol % to about 90 vol %, preferably about 10 vol % to about 60 vol %.

[0012] The iron articles utilized in the instant invention need not contain any carbon. It is sufficient for the carbon which forms the pearlite structure to come from the environment to which the iron article is exposed.

[0013] According to the instant invention, austenite is converted to a continuous peaiiite layer. As shown in figure 4, the preferred temperature range for the conversion is about 727 to about 900°C. Above this temperatuie, the pearlite phase will lose its continuity and fail to provide corrosion protection. [0014] Times and temperatures for conversion of fenϊtic iron to austenitic iron are well known in the art.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The instant invention involves exposing an iron containing article, where the iron has been converted to the austenitic state, to a carbon supersaturated gaseous environment and then cooling the article to obtain a continuous layer of peaiiite. The prefened temperatuie range for accomplishing the conversion of austenite to pearlite is shown in Figure 4. The prefened composition of the carbon supersaturated environment coπesponds to the plateau region in Figure 3. In this range, the reaction times are shorter to obtain a specific thickness of pearlite and therefore, gas compositions in this range are economically more attiactive. The reaction times to achieve vaiious thicknesses of continuous peaiiite can be deteixrrined by reference to Figure 2.

[0016] The process can be used to obtain any thickness of continuous peaiiite. It can also be used to completely convert the iron-containing article to pearlite. Thus, the production of peaiiite in the instant invention can be easily controlled to prepare a continuous layer of pearlite, or to convert all of the iron contained in the article to a continuous peaiiite structure. Hence a peaiiite structure can be a continuous layer of peaiiite on the surface of the iron article being acted upon, or a completely converted peaiiite article. The thickness of peailitic layers can be controlled by the carbon supersaturated environment, the temperatuie and the exposure time. Such exposure times are readily determin- able by the skilled artisan, as depicted in Figure 2. [0017] Shown in figure 3 are results for the thickness vaiiation of surface pearlite formed on pure iron after reactive heat treatment at 775°C for 1 hour as a function of the composition of carbon supersaturated gas mixtures. Maximum thickness of surface pearlite was obtained in a specific range of CO-H₂ gas composition. Typical hydrogen contents in carbon monoxide can range from about 2.5 vol % to about 90 vol %, preferably about 10 vol % to about 60 vol %.

[0018] The thickness of the peaiiite layer can be any thickness desired. All that is necessary is to alter the exposure time to the carbon supersaturated gaseous envkonment at the noted temperatures. For thinner layers, the exposure time will be less, and for thicker layers the exposure time will be greater. Typical exposure times can range fiom about 1 minute to about 50 hours, preferably fiom about 5 minutes to about 25 hours, and most preferably from about 10 minutes to about 10 hours. Thus, the exposure time and temperatuie will be those necessary to form a desired thickness of peaiiite following step (c). It is important to note that the entire iron containing article can be converted to pearlite if desired in which case the thickness of the aiticle will be the desired thickness.

[0019] Typical layer or structuie thickness will thus range from at least about 10 microns up to the thickness of the iron aiticle being acted on, preferably from about 10 to about 1000 microns, more preferably from about 10 to about 500 microns.

[0020] When converting the iron containing aiticle from the fenϊtic crystal structure to the austenitic crystal structure, all that is necessary is for the aiticle to be heated. One skilled in the art can easily determine the time and temperature necessary to accomplish such ciystal structure conversion by reference to any published Fe-C phase diagram (See for example: ASM Specialty Handbook, Carbon and Alloy Steels, Ed., by J. R. Davis, p.366 (1996) ASM International).

[0021] The cooling step (c) will deteπnine the lamellar spacing of the peaiiite formed. The cooling rate for a desired coarseness, or lamellar spacing, of the peaiiite is easily determined by the skilled aitisan taking into account the pearlite formation temperature, cooling rate and iron containing aiticle composition.

[0022] The iron containing aiticle to be acted upon will contain at least about 50 wt % iron. The aiticle can be composed entirely of iron. The amount of carbon contained in the aiticle can range from less than 0.77 wt % down to 0 wt % carbon. Thus, the instant invention allows the skilled aitisan to prepare peaiiite fiom an iron containing aiticle with much better mechanical properties than carbon steels containing 0.77 wt % or more carbon. The iron containing aiticle may further comprise other components including, but not limited to chromium, silicon and manganese. All that is necessary for the instant invention is that the article being acted upon contains at least about 50 wt % iron.

[0023] Additionally, an aiticle already having an amount of peaiiite in combination with feπϊte, can be subjected to the instant invention to convert the feπϊte to pearlite.

[0024] The carbon supersaturated environment to which the iron containing article is exposed is any carbon supersaturated environment. The thermo- dynamic carbon activity in the supersaturated environment is greater than 1. Examples of suitable environments include, but are not limited to CO, CH₄, or other hydrocarbon gases, such as propane (C₃H₈) and mixtures thereof with H₂, 0₂, N₂, C0₂, and H₂0.

[0025] The instant invention allows the skilled aitisan to produce steels having both coι osion resistance and mechanical properties far superior to those of carbon steels containing 0.77 wt % or more caibon. This is because the steel's mechanical properties improve as the caibon content decreases. In the instant invention, the amount of carbon diffused into the iron containing article from the carbon supersaturated environment is utilized to produce peaiiite. The portion of the iron containing article not converted to peaiiite, is unchanged and maintains the mechanical properties it possessed prior to treatment in accordance with the instant invention. Thus, for example, the amount of caibon necessary to form a pearlite layer of desired thickness can be diffused into the iron containing article thus foiTning peaiiite. The mechanical properties of the remaining non-peailitic portion of the article will be unchanged.

[0026] The following examples are illustrative and are not meant to be limiting in any way.

Example 1 :

[0027] Iron of 99.99% purity is heated to a temperatuie of 775°C in a hydrogen environment in a vertical quartz reactor tube and held at that temperatuie for ~5 minutes. Thereupon, the environment is changed to 50% CO-50% H₂. After 1 hour of exposure, the metal sample is cooled by lowering the furnace surrounding the quartz reactor. After the sample has attained room temperatuie, the surface microstructure is examined by scanning electron microscopy. Figure la reveals that a peaiiite surface layer of 100 micron thickness has formed on the iron surface. A magnified view of the peaiiite microstructure, showing alternating layers of fenite and cementite, is depicted in figure lb. By changing the duration of exposure to the caibon supersaturated gaseous environment, the thickness of the peaiiite layer can be changed. This is shown by the graph in Figure 2.

Claims (8)

CLAIMS:

1. A process for producing peaiiite fiom an iron containing article comprising the steps of, (a) heating an iron containing aiticle comprising at least 50 wt % iron for a time and at a temperature sufficient to convert at least a portion of said aiticle from a feπitic structure to an austenitic structure, (b) exposing said austenitic structure, for a time sufficient and at a temperature of about 727 to about 900°C, to a caibon supersaturated environment to diffuse carbon into said austenitic structure and (c) cooling said iron containing aiticle to form a continuous peaiiite structure.

2. The process of claim 1 wherein said iron containing aiticle further comprises silicon, manganese, and mixtures thereof.

3. The process of claim 2 wherein said caibon supersaturated environment is selected from the group of gases consisting of CO, CFL,, hydrocarbon gases, C₃H₈ and mixtures thereof with hydrogen, oxygen, nitrogen, carbon monoxide, and water.

4. The process of claim 3 wherein said carbon supersaturated environment is a CO/H₂ gaseous environment.

5. The process of claim 4 wherein when said CO/H₂ gaseous envkonment is selected as said caibon supersaturated envnonment, the hydrogen contents in carbon monoxide ranges from about 2.5 vol % to about 90 vol %.

6. The process of claim 1 wherein said time sufficient to diffuse carbon into the austenitic structure ranges from about 1 minute to about 50 hours.

7. The process of claim 6 wherein said thickness of peaiiite is at least about 10 microns.

8. The process of claim 5 wherein the hydiogen content in carbon monoxide ranges fiom about 10 vol % to about 60 vol %.