NITROGEN ASSISTED HARDENING OF AN ARTICLE

Description

PRIORITY APPLICATION

This application claims priority from Indian non-provisional application no. 202421045054 filed on 11 Jun. 2024, which is pending and which is hereby incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a method of heat treating an article. More specifically, the invention relates to a method of heat treating stainless steel articles such as double ferrule compression type fittings.

BACKGROUND OF THE INVENTION

Various methods of heat treating stainless steel articles are known in the art. European patent number EP2462253 describes a low temperature carburization of stainless steel using acetylene as the carburizing species, carried out under soft vacuum conditions in the presence of hydrogen or other companion gas. U.S. Pat. No. 11,066,735 describes a process for carburization of an article made of steel. The process involves forming a concentrated solid solution of interstitial carbon throughout the austenitic steel and without any underlying substrate layer so as to improve observed tensile strength, yield strength, and Young's modulus of the treated austenitic steel. U.S. Pat. No. 8,845,823 describes a method of activating an article of passive ferrous or non-ferrous metal by heating at least one compound containing nitrogen and carbon, wherein the article is treated with gaseous species derived from the compound. U.S. patent application No. 20150107723 refers to a partially carbonitriding heat treated stainless steel ferrule, having a first region with a first hardness and a second region with a second hardness, wherein the first region includes a nitrogen layer having a nitrogen concentration higher than a carbon concentration, and a carbon layer formed under the nitrogen layer and having a carbon concentration higher than a nitrogen concentration, so that the first hardness is greater than the second hardness.

However, these methods often need special heat treatment furnaces. Also, the thermal processing of stainless steel often results in a blackening or colouring of the stainless steel surface, and the shiny metallic appearance of the steel is lost.

Therefore, there is a need in the art for a process for heat treating a stainless steel article at low temperatures to impart a balance of good surface hardness and good corrosion resistance while maintaining its original colour and finish, which can be processed in commonly available furnaces.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a method of heat treating an article. The method includes depassivating (102) the article at a temperature in a range from about 200 degrees Centigrade to about 300 degrees Centigrade, at a pressure in a range from about 0.1 millibar to about 10 millibar in an atmosphere comprising hydrogen and an inert gas, to produce a depassivated article, heating (104) the depassivated article to a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade, at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon to produce a preliminary heat-treated article, and maintaining (106) the preliminary heat-treated article at a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon, for a time period in a range from about 10 hours to about 30 hours, to produce the heat-treated article, wherein the heat-treated article retains the appearance of the article prior to heat-treatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a method of heat treating an article in accordance to an embodiment of the invention.

DETAILED DESCRIPTION

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.”

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.

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.

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 qualification of steps as “first” and “second” is for sake of convenience. Unless specified, use of those terms should not be construed as excluding other steps. The use of such terms should not be construed as suggesting that any particular sequence of processing steps need be employed, unless specified.

As used herein, the term “bite type fitting” refers to an article that is used for joining two tubes. The bite type fitting is composed of an outer compression nut and an inner compression ferrule. When the nut is tightened, the ferrule is compressed between the nut and the body of the fitting, thus forming a tight, leak-proof joint.

As used herein, the term “double ferrule” refers to fitting that is composed of an outer compression nut, a front ferrule that compresses on to the body of the fitting, and a back ferrule.

As used herein the term “standard cubic centimetres per minute” or “sccm” is defined as a gas flow rate corresponding to a cubic centimeter of gas flowing in one minute.

As used herein, the term “case depth” refers to the thickness of the hardened layer on an article, after a hardening surface treatment.

One embodiment of the present invention is a method of heat treating an article. The method includes depassivating (102) the article at a temperature in a range from about 200 degrees Centigrade to about 300 degrees centigrade, at a pressure in a range from about 0.1 millibar to about 10 millibar in an atmosphere comprising hydrogen and an inert gas, to produce a depassivated article, heating (104) the depassivated article to a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade, at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon to produce a preliminary heat-treated article, and maintaining (106) the preliminary heat-treated article at a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon, for a time period in a range from about 10 hours to about 30 hours, to produce the heat-treated article, wherein the heat-treated article retains the appearance of the article prior to heat-treatment.

As depicted in FIG. 1 according to an embodiment of the present invention, the method 100 consists of the following steps: depassivating (102) the article at a temperature in a range from about 200 degrees Centigrade to about 300 degrees centigrade, at a pressure in a range from about 0.1 millibar to about 10 millibar in an atmosphere comprising hydrogen and an inert gas, to produce a depassivated article, heating (104) the depassivated article to a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade, at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon to produce a preliminary heat-treated article, and maintaining (106) the preliminary heat-treated article at a temperature in a range from about 300 degrees centigrade to about 400 degrees centigrade at a pressure in a range from about 1 millibar to about 10 millibar in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon, for a time period in a range from about 10 hours to about 30 hours, to produce the heat-treated article, wherein the heat-treated article retains the appearance of the article prior to heat-treatment.

In an embodiment of the present invention, the depassivation step may be a reduction process. In another embodiment of the present invention, the depassivation step removes chromium oxide from the surface of the stainless steel.

In an embodiment of the present invention, the article is subjected to a cleaning step before being heat treated. The cleaning step may include but not limited to a water washing step, an acetone washing step, a degreasing step, a chemical cleaning step, a solvent cleaning step, an ultrasonic cleaning step, a plasma cleaning step, an electrochemical cleaning step, or any other such cleaning step commonly known to one skilled in the art. In an embodiment of the present invention, the article is subjected to an acetone washing step.

In an embodiment of the present invention, the temperature in the depassivation step may be within a range of about 200 degrees Centigrade to about 300 degrees Centigrade. In another embodiment of the present invention, the temperature in the depassivation step may be within a range of about 200 degrees Centigrade to about 250 degrees Centigrade.

In an embodiment of the present invention, the pressure in the depassivation step may be in a range from about 0.1 millibar to about 10 millibar. In an embodiment of the present invention, the pressure in the depassivation step may be in a range from about 1 millibar to about 2 millibar.

In an embodiment of the present invention, the depassivation step is carried out in an atmosphere comprising hydrogen and an inert gas. In another embodiment of the present invention, the inert gas may be argon. In an embodiment of the present invention, the depassivation step may be carried out on a hydrogen flow from about 100 standard cubic centimetres per minute (sccm) to about 200 standard cubic centimetres per minute (sccm), and an argon flow from about 100 standard cubic centimetres per minute (sccm) to about 200 standard cubic centimetres per minute (sccm).

In an embodiment of the present invention, the depassivation step may be carried out in a period from about 30 minutes to about 10 hours. In an embodiment of the present invention, the depassivation step may be carried out in a period from about 30 minutes to about 3 hours. In another embodiment of the present invention, the depassivation step may be carried out for about 1 hour.

In an embodiment of the present invention, the temperature in the heating step may be within a range of about 300 degrees Centigrade to about 400 degrees centigrade. In another embodiment of the present invention, the temperature in the heating step may be within a range of about 350 degrees Centigrade to about 400 degrees centigrade. In another embodiment of the present invention, the temperature in the heating step may be in a range from about 370 degrees centigrade to about 390 degrees centigrade. In another embodiment of the present invention, the temperature in the heating step may be about 380 degrees centigrade.

In an embodiment of the present invention, the pressure in the heating step may be in a range from about 1 millibar to about 10 millibar. In an embodiment of the present invention, the pressure in the heating step may be in a range from about 5 millibar to about 10 millibar.

In an embodiment of the present invention, the heating step is carried out in an atmosphere comprising hydrogen, inert gas, nitrogen and a hydrocarbon. In an embodiment of the present invention, the inert gas may be argon. In an embodiment of the present invention the hydrocarbon may be acetylene.

In an embodiment of the present invention, the heating step may be carried out in a hydrogen flow from about 75 standard cubic centimetres per minute (sccm) to about 100 standard cubic centimetres per minute (sccm), an argon flow from about 35 standard cubic centimetres per minute (sccm) to about 50 standard cubic centimetres per minute (sccm), a nitrogen flow from about 180 standard cubic centimetres per minute (sccm) to about 250 standard cubic centimetres per minute (sccm), and a hydrocarbon flow from about 10 standard cubic centimetres per minute (sccm) to about 20 standard cubic centimetres per minute (sccm).

In an embodiment of the present invention, the maintaining step may be a soaking step. In an embodiment of the present invention, the preliminary heat treated article may be “soaked” at a certain temperature, i.e. the preliminary heat-treated article may be maintained at a certain temperature, under certain conditions of pressure and gas atmosphere.

In an embodiment of the present invention, the temperature in the maintaining step may be within a range of about 300 degrees Centigrade to about 400 degrees centigrade. In another embodiment of the present invention, the temperature in the maintaining step may be within a range of about 350 degrees Centigrade to about 400 degrees centigrade. In another embodiment of the present invention, the temperature in the maintaining step may be in a range from about 370 degrees centigrade to about 390 degrees centigrade. In yet another embodiment of the present invention, the temperature in the maintaining step may be about 380 degrees centigrade.

In an embodiment of the present invention, the pressure in the maintaining step may be in a range from about 1 millibar to about 10 millibar. In an embodiment of the present invention, the pressure in the maintaining step may be in a range from about 5 millibar to about 10 millibar.

In an embodiment of the present invention, the maintaining step may be carried out in a hydrogen flow from about 75 standard cubic centimetres per minute (sccm) to about 100 standard cubic centimetres per minute (sccm), an argon flow from about 45 standard cubic centimetres per minute (sccm) to about 75 standard cubic centimetres per minute (sccm), a nitrogen flow from about 180 standard cubic centimetres per minute (sccm) to about 250 standard cubic centimetres per minute (sccm), and a hydrocarbon flow from about 10 standard cubic centimetres per minute (sccm) to about 20 standard cubic centimetres per minute (sccm).

In an embodiment of the present invention, the maintaining step may be carried out for a time period from about 5 hours to about 50 hours. In another embodiment of the present invention, the maintaining step may be carried out for a time period from about 10 hours to about 30 hours. In an embodiment of the present invention, the third step may be carried out for a time period pf 19 hours.

In an embodiment of the present invention, after the maintaining step, the article may be cooled to room temperature in an atmosphere comprising nitrogen.

In an embodiment of the present invention, the hydrocarbon may be acetylene.

In an embodiment of the present invention, the article may be an austenitic stainless steel article. In an embodiment of the present invention, the article may be composed of “SS 316” grade stainless steel or “SS 304” grade stainless steel or “SS 316Ti” grade stainless steel.

In an embodiment of the present invention, the article may be a double ferrule compression type fitting. In an embodiment of the present invention the article may be a back ferrule of a double ferrule compression type fitting. In yet another embodiment of the present invention, the article may be a gear, a sprocket, a screw, a ball bearing, a roller bearing, a piston pin, a firearm, a chain, a lock shackle, a watch case, a cam shaft, a crankshaft, and the like.

In an embodiment of the present invention, the heat treated article has a Vickers hardness from about 1000 units to about 1100 units.

In an embodiment of the present invention, the heat treated article has a case depth from about 25 microns to about 40 microns. In an embodiment of the present invention, the heat treated article has a case depth from about 27 microns to about 35 microns.

In an embodiment of the present invention, the heat treated article has a corrosion resistance for a time from about 120 hours to about 250 hours in a salt spray corrosion test, as described in the ASTM B117 standard.

In an embodiment of the present invention, the heat treated article retains the colour and surface finish of stainless steel. In an embodiment of the present invention, the heat treated article does not show a blackened surface after the method of heat treating the article is carried out.

EXAMPLES

Example 1: Commercially available ferrules, such as those manufactured by Fluid Controls Pvt. Ltd., Pune, India, were obtained for heat treatment. The ferrule to be heat treated was cleaned in an acetone bath. The ferrule was placed in a furnace and heated to 250 degrees centigrade at a pressure of about 1 millibar in an atmosphere comprising a flow of about 100 standard cubic centimetres per minute (sccm) of hydrogen and an argon flow of about 100 standard cubic centimetres per minute (sccm). The ferrule was held at 250 degrees centigrade for 1 hour at a pressure of about 1 millibar in an atmosphere comprising a flow of about 100 standard cubic centimetres per minute (sccm) of hydrogen and an argon flow of about 100 standard cubic centimetres per minute (sccm), so as to depassivate the surface of the ferrule. The ferrule was then heated to 380 degrees centigrade at a pressure of about 5 millibar in an atmosphere comprising a nitrogen flow of 180 standard cubic centimetres per minute (sccm), an acetylene flow of about 10 standard cubic centimetres per minute (sccm), hydrogen flow of about 75 standard cubic centimetres per minute (sccm) and argon flow of about 35 standard cubic centimetres per minute (sccm). The ferrule was then held at 380 degrees centigrade at a pressure of about 5 millibar in an atmosphere comprising a nitrogen flow of 180 standard cubic centimetres per minute (sccm), an acetylene flow of about 10 standard cubic centimetres per minute (sccm), hydrogen flow of about 75 standard cubic centimetres per minute (sccm) and argon flow of about 45 standard cubic centimetres per minute (sccm), for a time period of about 19 hours. The ferrule was subsequently allowed to cool to room temperature in an atmosphere comprising a nitrogen flow of about 180 standard cubic centimetres per minute (sccm).

The summary of the heat treatment process is described in Table 1 below:

The heat treated ferrule was visually inspected. It was observed that the heat treated ferrule retained the colour of stainless steel. The heat treated ferrule retained its surface finish and lustre. No blackening of the surface was observed.

The hardness of the heat treated ferrule was tested using a Vickers micro-hardness tester. It was observed that the hardness of the heat treated ferrule was about 1000 Hv. The depth from the surface, also referred to as case depth, was found to be about 27 microns.

Further, the ferrules were tested for corrosion resistance in a salt spray chamber. The heat treated ferrule showed corrosion resistance for a time of at least about 120 hours in the salt spray chamber, as described in the ASTM B117 standard.

Heat treated ferrules of various diameters ranging from 6 millimetres to about 42 millimetres outer diameter were tested for leaks according to ISO 19879 standard. Typically, the ferrule was fitted on to a stainless steel tube and tested at 6.3 MPa for a duration of at least three minutes. No leaks were detected in the heat treated ferrules. The heated treated ferrules were tested for leakages due to shock and vibration according to BS EN 61373(2010) standard. The heat treated ferrules were found to meet the specifications laid down in BS 61373(2010) standard. The heat treated ferrules fitted on to stainless steel tubes were also tested for leaks due to misalignment. The ferrules were fitted on to steel tubes and clamped. The tubes were then misaligned from the clamped position up to 45 mm, and the ferrule was crimped. The heat treated ferrules were observed to pass the leak tests as per ISO 19879 standard, in spite of the misalignment.

Comparative Example 1: Non-heat-treated ferrules, such as those manufactured by Fluid Controls Pvt. Ltd., Pune, India, were obtained for comparison. The non-heat treated ferrules were subjected to the same tests as for the heat-treated ferrules, and the results were compared.

It was observed that the non-heat treated ferrules had a lower Vickers micro hardness of about 300 Hv. It was observed that the corrosion resistance of the ferrule, according to ASTM B117 was about 12 hours.

Non-heat treated ferrules were tested for leaks according to ISO 19879 standard. The non-heat treated ferrules employed above were various diameters ranging from 6 millimetres to about 42 millimetres outer diameter as mentioned above in the case of the heat treated ferrules. No leaks were detected in the non-heat treated ferrules. The non-heat treated ferrules fitted on to stainless steel tubes were also tested for leaks due to misalignment. The non-heat treated ferrules were observed to fail various tests described in ASTM F1387 as per ISO 19879 standard.

The comparison of properties of the ferrules described in Example 1 and Comparative Examples 1-2 are summarized in Table 2 below:

ADVANTAGES

The technical advantages brought in by the present invention are as follows:

• • 1. The heat treated articles have a higher surface Vickers hardness as compared to non-heat treated articles.
  • 2. The heat treated articles have a good surface finish, and retain the colour of stainless steel without any observable blackening.
  • 3. The heat treated articles have a high corrosion resistance, as compared to non-heat treated articles.

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.