Description:
1. Title of the Invention: Method of treating a surface

2. Technical Field:
The present invention generally relates to a method of treating a surface of a metallic article. More particularly, the invention describes a method of treating a surface of a metallic article by a friction stir processing process.
3. BACKGROUND
Various methods of treating metallic surfaces are known in the art. Chinese patent CN110565025B describes a method of friction stir processing a surface of comprising a high carbon steel, followed by tempering at 300 degrees centigrade for 20 hours, to produce high strength steels. US patent US7416102B1 describes a method of friction stir welding using a tool with a frustoconical tip. While US patent US7225969B2 describes a method of friction stir processing a surface to reduce surface porosity or change its mechanical properties. Korean patent application KR20160041485A describes a method of modifying a metal structure by friction stir processing. However, in all the above mentioned processes the friction stir processed surface is susceptible to failure due to fatigue and wear, especially in high-load applications such as railway wheels, etc. Further, the friction stir processing method has a high process variation, and the stir processed surface often has high roughness, and a large number of surface defects.

Therefore, there is a need for a method of treating a surface of a metallic article, which obviates the aforesaid drawbacks.
4. OBJECTS OF THE INVENTION / SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of processing a region on a metallic surface. The method includes the steps of selecting the region to be processed on the metallic surface, friction stir processing the region to be processed on the metallic surface to obtain a friction stir processed region, heat treating the friction stir processed region to obtain a heat treated region. The heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region on a metallic surface prior to the method of processing.

Another object of the present invention is to provide a method of processing a region on a metallic surface. The method includes the steps of selecting the region to be processed on the metallic surface, friction stir processing the region to be processed on the metallic surface to obtain a friction stir processed region, heat treating the friction stir processed region to obtain a heat treated region. The method includes the step of tempering the heat treated region to obtain a tempered heat treated region, and quenching the tempered heat treated region. The heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region prior to the method of processing.

Yet another object of the present invention is to provide a method of processing a region on a metallic surface. The method includes the steps of selecting the region to be processed on the metallic surface, friction stir processing the region to be processed on the metallic surface by applying a pinless and shoulderless friction stir tool with a downward force in a range of about 250 Newton to about 350 Newton on the region to be processed, rotating the pinless and shoulderless friction stir tool at a rate in a range of about 1500 revolutions per minute to about 2500 revolutions per minute. The method includes the step of scanning the region to be processed by the rotating tool, to obtain a friction stir processed region, and, heat treating the friction stir processed region to obtain a heat treated region. The heat treated region has an austenitic content that is about 10 percent greater compared to the austenitic content of the region prior to the method of processing.

These and other objects of the invention herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however that the following descriptions, while indicating embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing the spirit thereof, and the embodiments herein include all such modifications.

5. Brief Description of Drawings
The embodiments of the invention are illustrated in the accompanying drawings, throughout which the reference letters indicate corresponding part in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:
FIG. 1 illustrates a method of processing a region on a metallic surface in accordance with an embodiment of the invention.
FIG. 2 illustrates a method of processing a region on a metallic surface in accordance with an embodiment of the invention.
FIG. 3 illustrates a method of processing a region on a metallic surface in accordance with an embodiment of the invention.
FIG. 4 is an electron micrograph showing a cross section of the friction stir processed region, according to an embodiment of the invention.
FIG. 5 is a graph showing the hardness as a function of depth for a region on a metallic surface processed according to an embodiment of the invention.
FIG. 6 is an electron micrograph showing the microstructure of the heat treated region according to an embodiment of the present invention.
FIG. 7 is an electron micrograph showing the scar profile of a Stribeck test on a heat treated region according to an embodiment of the present invention.
FIG. 8 is a graph showing the coefficient of friction as a function of log of velocity as measured during a Stribeck test on a heat treated region according to an embodiment of the present invention.

6. DETAILED DESCRIPTION OF THE INVENTION
While various embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions may occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed.
In the specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. “Substantially” means a range of values that is known in the art to refer to a range of values that are close to, but not necessarily equal to a certain value.

Other than in the examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, and the like, used in the specification and claims are to be understood as modified in all instances by the term “about.” In some aspects of the current disclosure, the terms “about” or “approximately” are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the terms are defined to be within 10 percent, alternatively within 5 percent, alternatively within 1percent, or alternatively within 0.5 percent.
As used herein, the term “substantially” and its variations are defined as being largely but not necessarily wholly what is specified as understood by one of ordinary skill in the art, and in one non-limiting aspect substantially refers to ranges within 10 percent, within 5 percent, within 1 percent, or within 0.5 percent.
Various numerical ranges are disclosed herein. Because these ranges are continuous, they include every value between the minimum and maximum values. The endpoints of all ranges reciting the same characteristic or component are independently combinable and inclusive of the recited endpoint. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations. The endpoints of all ranges directed to the same component or property are inclusive of the endpoint and independently combinable. The term “from more than 0 to an amount” means that the named component is present in some amount more than 0, and up to and including the higher named amount.
As used herein, “combinations thereof” is inclusive of one or more of the recited elements, optionally together with a like element not recited, e.g., inclusive of a combination of one or more of the named components, optionally with one or more other components not specifically named that have essentially the same function. As used herein, the term “combination” is inclusive of blends, mixtures, alloys, reaction products, and the like.

As used herein, the term “friction stir processing” refers to a method of changing the properties of a surface through plastic deformation. This is done by forcibly inserting a tool into the surface to be processed, and rotating the tool in a stirring motion. The rotating tool is moved across the surface whose properties are to be modified, to produce a friction stir processed region.
As used herein, the term “plastic deformation” refers to a permanent deformation that occurs when a material is subjected to a stress higher than its yield strength.
As used herein, the term “tempering” refers to a process of heat treating a metal to increase its toughness.
As used herein, the term “quenching” refers to the rapid cooling of a hot article in water, oil, air, or a molten salt, to achieve certain material properties.
One embodiment of the present invention refers to a method of processing a region on a metallic surface. The method includes the steps of selecting the region to be processed on the metallic surface, friction stir processing the region to be processed on the metallic surface to obtain a friction stir processed region, heat treating the friction stir processed region to obtain a heat treated region. The heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region on a metallic surface prior to the method of processing.
In an embodiment of the present invention, the friction stir processed region has a hardness from about 855 Vickers Hardness (HV) to about 861 Vickers Hardness (HV).
In an embodiment of the present invention, the heat treated region has a hardness of about 10 percent lower than the hardness of the friction stir processed region. In another embodiment of the present invention, the heat treated region has a hardness of about at least 750 Vickers Hardness (HV). In an embodiment of the present invention, the heat treated region has a hardness in a range from about 825 Vickers Hardness (HV) to about 875 Vickers Hardness (HV).
In an embodiment of the present invention, the region to be processed has an austenitic content in a range from about 10 percent to about 20 percent of the austenitic content of the region on a metallic surface prior to the method of processing. In an embodiment of the present invention, the heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region on a metallic surface prior to the method of processing. In an embodiment of the present invention, the heat treated region has an austenitic content in a range from about 20 percent to about 30 percent of the austenitic content of the region on a metallic surface prior to the method of processing.
In an embodiment of the present invention, the metallic surface comprises a hardenable steel. In an embodiment of the present invention, the metallic surface comprises a steel that can undergo a martensitic transition. In another embodiment of the present invention, the steel comprises a SAE2100, AISI 4140, AISI 8620 or 440c grade steel or combinations thereof.
In an embodiment of the present invention, the method of processing a region on a metallic surface further includes the step of tempering the metallic surface at a temperature from about 180 degrees centigrade to about 250 degrees centigrade for a period in a range from about 55 minutes to about 195 minutes to obtain a tempered metallic surface.
In an embodiment of the present invention, the method of processing a region on a metallic surface further includes the step of quenching the tempered metallic surface. In an embodiment of the present invention, the step of quenching is carried out in a salt bath, or an oil bath. In an embodiment of the present invention, the quenching step is carried out in a salt bath. In an embodiment of the present invention, the salt bath is maintained at a temperature in a range from about 150 degrees centigrade to about 200 degrees centigrade. In another embodiment of the present invention, the salt bath is maintained at a temperature of 200 degrees centigrade. In another embodiment of the present invention, the quenching step further includes the step of air cooling and cooling in a cold water bath after quenching in the salt bath.
In an embodiment of the present invention, the heat treating is carried out in a carbon-rich atmosphere at a temperature in a range of at least about 840 degrees centigrade to about 950 degrees centigrade for a time period in a range from at least about 40 minutes to about 80 minutes.
In an embodiment of the present invention, the carbon-rich atmosphere that may include carbon monoxide, hydrogen, a hydrocarbon and combinations thereof. In an embodiment of the present invention, the carbon-rich atmosphere comprises from about 10 percent to about 30 percent of carbon monoxide, from about 30 percent to about 50 percent hydrogen, and from about 1 percent to about 10 percent of a hydrocarbon. In an embodiment of the present invention, the carbon-rich atmosphere comprises about 20 percent of carbon monoxide, about 40 percent of hydrogen and about 5 percent of a hydrocarbon. In an embodiment of the present invention, the hydrocarbon is an alkane, or mixtures of alkanes. In another embodiment of the present invention, the hydrocarbon comprises from about 1 carbon atom, to about 10 carbon atoms, and mixtures thereof. In another embodiment of the present invention, the hydrocarbon is a mixture of propane and butane. In another embodiment of the present invention, the hydrocarbon is a liquefied petroleum gas (LPG).

Another embodiment of the present invention refers to a method of processing a region on a metallic surface. The method includes the steps of selecting the region to be processed on the metallic surface, friction stir processing the region to be processed on the metallic surface to obtain a friction stir processed region, heat treating the friction stir processed region to obtain a heat treated region. The method further includes the step of tempering the heat treated region to obtain a tempered heat treated region, and quenching the tempered heat treated region. The heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region prior to the method of processing.
Yet another embodiment of the present invention is to a method of processing a region on a metallic surface. The method includes the steps of selecting the region to be processed on the metallic surface. The method includes the step of friction stir processing the region to be processed on the metallic surface by applying a pinless and shoulderless friction stir tool with a downward force in a range of about 250 Newton to about 350 Newton on the region to be processed. The method further includes the step of rotating the pinless and shoulderless friction stir tool at a rate in a range of about 1500 revolutions per minute to about 2500 revolutions per minute, scanning the region to be processed by the rotating tool, to obtain a friction stir processed region, and heat treating the friction stir processed region to obtain a heat treated region. The heat treated region has an austenitic content that is about 10 percent greater compared to the austenitic content of the region prior to the method of processing.
A method of processing a region on a metallic surface in accordance with one embodiment of the present invention will now be described with reference to the drawings.
The flowchart depicted in FIG.1 is an example embodiment of the method 100 of processing a region on a metallic surface. The method 100 includes the steps of: selecting 102 the region to be processed on the metallic surface, friction stir processing 104 the region to be processed on the metallic surface to obtain a friction stir processed region, and heat treating 106 the friction stir processed region to obtain a heat treated region. The heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region on a metallic surface prior to the method of processing.
FIG 2, a flowchart depicting a method 200 of processing a region on a metallic surface according to an embodiment of the present invention is shown. The method 200 includes the steps of: selecting 202 the region to be processed on the metallic surface, friction stir processing 204 the region to be processed on the metallic surface to obtain a friction stir processed region, heat treating 206 the friction stir processed region to obtain a heat treated region. The method further includes the step of tempering 208 the heat treated region to obtain a tempered heat treated region, and quenching 210 the tempered heat treated region. The heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region prior to the method of processing.
A flowchart shown in FIG. 3, is a process 300 of processing a region on a metallic surface. The method 300 includes the steps of: selecting 302 the region to be processed on the metallic surface, friction stir processing 304 the region to be processed on the metallic surface by applying a pinless and shoulderless friction stir tool with a downward force in a range of about 250 Newton to about 350 Newton on the region to be processed. The method includes the step of rotating 306 the pinless and shoulderless friction stir tool at a rate in a range of about 1500 revolutions per minute to about 2500 revolutions per minute, scanning 308 the region to be processed by the rotating tool, to obtain a friction stir processed region. The method includes the step of heat treating 310 the friction stir processed region to obtain a heat treated region. The heat treated region has an austenitic content that is about 10 percent greater compared to the austenitic content of the region prior to the method of processing.

Examples:

Example 1:
A soft steel of grade SAE52100 was obtained. A region to be hardened was selected on the surface of the steel. The selected region was friction stir processed using a rotating shoulderless pinless tool. The tool tip had a width of about 4 mm. A plunging force of about 500 Newtons was applied between the tool and the surface. The tool was rotated at a speed of 2000 revolutions per minute, and was contacted with the surface for 15 Seconds. The selected region was scanned using the rotating tool to ensure friction stir processing of the entire selected region, to obtain a friction stir processed region. The surface was then heat treated at 855 degrees centigrade for about 45 minutes in a carbon rich atmosphere. The carbon rich atmosphere includes about 20 percent of carbon monoxide, about 40 percent of hydrogen and about 5 percent of liquefied petroleum gas (LPG). The heat treated surface was then tempered at a temperature of 200 degrees centigrade for a time of 145 minutes. The tempered surface was then quenched in a salt bath.
The amount of retained austenite was measured according to ASTM standard E375 by 4-peak method using X-ray diffraction. The amount of residual austenite was found to be about 25 plus or minus5 percent.
Comparative Example 1:

A soft steel of grade SAE52100 was obtained. A region to be hardened was selected on the surface of the steel. The selected region was friction stir processed using a rotating shoulderless pinless tool. The tool tip had a width of about 4 mm. A plunging force of about 500 Newtons was applied between the tool and the surface. The tool was rotated at a speed of 2000 revolutions per minute, and was contacted with the surface for 15 seconds. The selected region was scanned using the rotating tool to ensure friction stir processing of the entire selected region, to obtain a friction stir processed region. The friction stir processed region had the hardness of about 861 Vickers Hardness(HV) with grain refinement.
The amount of residual austenite was found to be about 25 plus or minus5 percent for friction stir processed and heat treated surface wherein heat treated non-processed region was found to be about 15 plus or minus5 percent.
The hardness of processed region was 861 Vickers Hardness(HV) which was reduced to 805 Vickers Hardness(HV) after hardening treatment.
The hardened region had high grain refinement and small carbide size.
Comparative Example 2:

A soft steel of grade SAE52100 was obtained. The steel was heat treated at 855 degrees centigrade for about 45 minutes in a carbon rich atmosphere that includes about 20 percent of carbon monoxide, about 40 percent of hydrogen and about 5 percent of liquefied petroleum gas (LPG). The heat treated steel was then tempered at a temperature of 200 degrees centigrade for a time of about 145 minutes. The tempered steel was then quenched in a salt bath.
FIG. 4 shows an electron micrograph of a cross section of a friction stir processed sample, prepared by a method that is described in Comparative Example 1. The micrograph shows the presence of a friction stir processed zone 402, and a heat affected zone 404 adjacent to the friction stir processed zone. The heat affected zone has a thickness of about 180 microns. Grain refinement is observed in the friction stir processed zone. This grain refinement results in an improvement in wear and hardness properties of the surface. A high hardness of about 855 Vickers Hardness(HV) to about 861 Vickers Hardness(HV) was achieved up to a depth of about 300 microns from the surface. The micrograph 400b shows the cross section of a sample prepared by a method as described in Example 1. The micrograph shows the presence of a friction stir processed zone 402a. A heat affected zone is not observed here.
FIG. 5 is a graph showing the variation of hardness as a function of depth below the surface in accordance to an embodiment of the invention. It can be seen that for a friction stir processed sample prepared according to the method described in Comparative Example 1, the hardness begins to drop above a depth of about 250 microns, and drops to about 20 percent of the hardness of the surface as depicted by curve 502. However, in case of a heat treated article prepared according to an embodiment of the resent invention, the hardness was almost invariant with depth, as seen in curve 504. The hardness value was almost similar to the hardness profile of a through hardened sample prepared by a process as described in Comparative Example 2, for which the hardness profile is shown by curve 506.
FIG. 6 shows electron micrographs of the grain structure of the region to be processed, and a heat treated region prepared according to an embodiment of the present invention. Micrographs 602a and 602b show the microstructure of the region to be processed, after treatment with picric acid etchant and nitric acid etchant respectively, while 604a and 604b show the microstructure of the heat treated region, after treatment with picric acid etchant and nitric acid etchant, respectively. Picric acid and nitric acid were used as etchants while preparing specimens for electron microscopy, to selectively etch the grain boundaries, and thus allow the grain structure to be seen clearly. The image 606 shows the refined grains in the heat treated region, vis-à-vis the original grains in the non-heat treated region. It can be observed from these images, that grain refinement is observed in the heat treated region.
FIG. 7 shows the electron micrographs of the scar profile of a Stribeck test. The micrographs 702 and 704 refer to the micrographs of the surface after pin on disk testing. It was observed that there was a 25 percent reduction in scar depth in the heat treated region seen in micrograph 704 as compared to the micrograph 702 for the sample prepared according to a method described in Comparative Example 2. Micrograph 706 and 708 show the wear on a surface prepared according to a method described in Comparative Example 2, and Example 1, respectively. A reduction of about 6 percent in the scar width was observed in the heat treated sample. A reduction in the wear rate implies an increase in the life of the surface, and is especially important where sliding wear is seen, for example in sliding contacts.
FIG. 8 is a graph showing the variation of coefficient of friction as a function of log of the velocity during a Stribeck test according to an embodiment of the invention. It is observed that the coefficient of friction of the heat treated sample is lower than the coefficient of friction of the region to be processed, the friction stir processed region as described in Comparative Example 1, a through hardened sample as described in Comparative Example 2, and a heat treated region according to an embodiment of the present invention. It is observed that the slope of the Stribeck curve increases as seen in curve 802, as compared to the curve 804 for a through hardened sample as described in Comparative example 2, and the heat treated region possesses a low coefficient of friction at high revolutions per minute. A low coefficient of friction reduces energy loss and heat generation due to friction, making the system more efficient.

Advantages
The technical advantages brought in by the present invention are as follows;
1. The treated surface has a high hardness.
2. The heat treated region does not show a heat affected zone adjacent to the treated surface, thus, preventing a deterioration of properties at the heat affected zone.
While considerable emphasis has been placed herein on the components and component parts of the various embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the embodiments without departing from the scope and spirit of the invention. These and other changes in the various embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Claims:

We Claim:
1. A method of processing a region on a metallic surface, the method comprising the steps of:
selecting the region to be processed on the metallic surface;
friction stir processing the region to be processed on the metallic surface to obtain a friction stir processed region;
heat treating the friction stir processed region to obtain a heat treated region; and
wherein the heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region on a metallic surface prior to the method of processing.

2. The method of claim 1, wherein the heat treated region has a hardness of about 10 percent lower than the hardness of the friction stir processed region.

3. The method of claim 1, wherein the heat treated region has an austenitic content in a range from about 20 percent to about 30 percent.

4. The method of claim 1, wherein the metallic surface comprises of a hardenable steel.

5. The method of claim 1, further comprising the step of tempering the metallic surface at a temperature from about 180 degrees centigrade to about 250 degrees centigrade for about 55 minutes to about 195 minutes to obtain a tempered metallic surface.

6. The method of claim 5, further comprising the step of quenching the tempered metallic surface.

7. A method of processing a region on a metallic surface, the method comprising the steps of:
selecting the region to be processed on the metallic surface;
friction stir processing the region to be processed on the metallic surface to obtain a friction stir processed region;
heat treating the friction stir processed region to obtain a heat treated region;
tempering the heat treated region to obtain a tempered heat treated region;
quenching the tempered heat treated region;
wherein the heat treated region has an austenitic content that is at least about 10 percent greater as compared to the austenitic content of the region prior to the method of processing.

8. The method of claim 7, wherein the heat treating is carried out in a carbon-rich atmosphere at a temperature of at least about 840 degrees centigrade to about 950 degrees centigrade for a time of at least about 40 minutes to about 80 minutes.

9. A method of processing a region on a metallic surface, the method comprising the steps of:
selecting the region to be processed on the metallic surface;
friction stir processing the region to be processed on the metallic surface by applying a pinless and shoulder less friction stir tool with a downward force in a range of about 250 Newton to about 350 Newton on the region to be processed;
rotating the pinless and shoulder less friction stir tool at a rate in a range of about 1500 revolutions per minute to about 2500 revolutions per minute;
scanning 306 the region to be processed by the rotating tool, to obtain a friction stir processed region,
heat treating 308 the friction stir processed region to obtain a heat treated region; and
wherein the heat treated region has an austenitic content that is about 10 percent greater compared to the austenitic content of the region prior to the method of processing.

10. The method of claim 9, wherein the heat treated region has a hardness of about 90 percent of the hardness of the friction stir processed region.