WO2000009776A1

WO2000009776A1 - Selective case hardening for metal articles

Info

Publication number: WO2000009776A1
Application number: PCT/US1998/016557
Authority: WO
Inventors: Steven V. Marx; Peter C. Williams
Original assignee: Swagelok Company
Priority date: 1998-08-12
Filing date: 1998-08-12
Publication date: 2000-02-24
Also published as: AU9016698A

Abstract

A selectively case hardened steel article by low temperature carburization includes a body made of a chromium bearing nickel or ferrous based alloy, for example stainless steel, a first portion of the body having a first hardness characteristic, and a second portion of the body having a second hardness characteristic; the second portion being less than the entire body and substantially free of carbides. One method for selective case hardening by low temperature carburization includes the steps of applying a carbon blocking mask over surface areas of the article that are not going to be carburized; activating those surface areas that are to be carburized; diffusing carbon into the activated surface areas at a temperature below which carbides form; and removing the carbon blocking mask.

Description

SELECTIVE CASE HARDENING FOR METAL ARTICLES

Technical Field of the Invention

The present invention relates to processing techniques for articles of stainless steel

and other alloys, such as, for example, tube coupling ferrules. More particularly, the

invention relates to processes for selective case hardening such articles substantially

without the formation of carbides.

Background of the Invention

As is well known, stainless steel is commonly used for many parts and

assemblies. One example is a ferrule used as part of a fluid coupling for joining tube

ends. The degree to which the stainless steel must be used will vary from application to

application. In some high purity systems, for example in the semiconductor and

biotechnology fields, lower carbon stainless steel such as 316L for example, is commonly

used. Many chemistries for stainless steel are used, and other chromium bearing nickel or

ferrous based alloys are known and used other than stainless steel.

One attribute of some stainless steel alloys is that they are relatively less hard than

other steel alloy materials. As a result in some applications, such as ferrules, the stainless

steel article or part is provided with a hardened surface, referred to generally and herein

as case hardening. The concept of case hardening is to transform a relatively thin layer of

material at the surface of the part by enrichment of carbon or other ingredients to make

the surface harder than the base metal alloy. This disclosure is directed to case hardening

of an article by enrichment of carbon. The article thus retains in bulk the desired formability of stainless steel without the softness of the standard chemistry base metal at the article surface.

Stainless steel alloy parts are case hardened by a process generally known as

carburization. Carburization is a process by which carbon atoms are diffused into the

surface of the article. In some applications, such as ferrules, it is desirable to only case

harden certain portions or areas of the ferrule, referred to herein as selective case

hardening. Known selective case hardening processes are performed at high

temperatures. However, case hardening processes performed at high temperatures greater

than about 1000 °F (for stainless steel alloys) promote the formation of carbides in the

hardened surface.

It is desired, therefore, to provide a new article of manufacture that is in the form

of an article made of a chromium bearing nickel or ferrous based alloy and that is

selectively case hardened over a surface area that is less than the entire area of the article

without the formation of carbides. It is a further object of the invention to provide a

carburization process for selective case hardening at low temperature that does not

promote the formation of carbides.

Summary of the Invention

In accordance with one embodiment of the invention, an article of manufacture is

provided that is characterized by a body comprising a chromium bearing nickel or ferrous

based alloy; a first portion of the body having a first hardness characteristic; and a second portion of the body having a second hardness characteristic; the second portion being less

than the entire body.

The present invention also contemplates methods for producing a selectively case

hardened article including in one embodiment the steps of disposing a carbon blocking

mask over a selected surface portion of the article; activating an unmasked surface

portion of the article; and carburizing the unmasked portion of the article. The present

invention further contemplates a product made by such a process.

These and other aspects and advantages of the present invention will be apparent

to those skilled in the art from the following description of the preferred embodiments in

view of the accompanying drawings.

Brief Description of the Drawings

The invention may take physical form in certain parts and arrangements of parts,

preferred embodiments and a method of which will be described in detail in this

specification and illustrated in the accompanying drawings which form a part hereof, and

wherein:

Fig. 1 is an elevation in longitudinal cross-section of a conventional ferrule;

Figs. 2A-2C illustrate diagrammatically a low temperature selective case

hardening process.

Detailed Description of the Preferred Embodiments

With reference to Fig. 1, a conventional ferrule 10 is illustrated. This ferrule 10 is

but one example of countless many articles and parts that can be used with the present invention. While the invention is described herein with reference to a 316 type stainless

steel ferrule, such description is intended to be exemplary in nature and should not be

construed in a limiting sense. The present invention finds application with any part or

article made of a chromium bearing nickel or ferrous based alloy and that is to be case

hardened.

Furthermore, although the preferred embodiments are described herein with

specific reference to articles made of stainless steel alloys, such descriptions are

exemplary in nature and should not be construed in a limiting sense. The present

invention is applicable to many types of chromium bearing ferrous or nickel based alloy

chemistries, including but not limited to alloy 316, alloy 316L, and alloy 304 stainless

steels, alloy 600, alloy C-276 and alloy 20 Cb, to name a few examples.

The ferrule 10 is illustrated in Fig. 1 in partial cross-section only. This particular

ferrule is a rear ferrule that is used as part of a two ferrule system. Such ferrules and

ferrule systems including the ferrule geometry are well known and are fully described in

United States Patent Nos. 4,915,427 and 3,103,373, the entire disclosures of which are

fully incorporated herein by reference.

The ferrule 10 is characterized by a tapered nose portion 12, a central body 14 and

a rear drive surface 16. In a tube coupling, the rear drive surface 16 engages a wall of a

nut that axially drives the nose of the ferrule 10 into a rear camming mouth of a front

ferrule (not shown). This action, among other things, causes the nose portion 12 of the

ferrule 10 to be driven radially inward to grip a tube end. The geometry of the ferrule 10 illustrated in Fig. 1 is exemplary in nature and will vary substantially depending on the

particular ferrule system. The ferrule 10 could also be used in a single ferrule system in

which case the nose portion 12 is driven into a camming mouth of a forward coupling

element.

A common but not exclusive material for the ferrule 10 is 316 stainless steel alloy.

To enable driving the ferrule 10 into an enhanced grip of a tube end, it is desirable in

some applications to case harden the ferrule 10. Additionally, in some applications it is

also desirable to case harden only portions of the ferrule 10. For example, if the nose

portion 12 can be selectively case hardened, overall performance of the ferrule system can

be improved in many cases.

As used herein, case hardening means to provide a relatively thin carburized layer

at the surface of the ferrule 10 to increase the surface hardness as compared to the base

alloy metal used for the ferrule 10. Carburization is a preferred method for case

hardening the ferrule 10, and in accordance with one aspect of the present invention, new

carburization processes are used which permit selective case hardening of the ferrule 10

over a selected area that is less than the entire part. In those cases in which it is desired to

case harden an entire part, however, the carburization processes of the present invention

can still be used.

Carburization in general is a process by which carbon atoms are diffused into the

base alloy in solution. In order to diffuse the carbon atoms into the stainless steel, the

chromium oxide layer must be removed. This step is generally known as activation or de-passivation. The surface must be activated because the oxide layer presents a

substantial barrier to carbon atoms. Once activated, the surface can be carburized by diffusion at an elevated temperature.

The diffusion process can be accelerated by performing the carburization at a high

temperature, for example, greater than 1000 °F. However, such high temperature

diffusion can readily and quickly produce carbides which are carbon/chromium

molecules. Carbides tend to reduce the chromium of the base alloy in some cases.

In order to prevent or substantially eliminate the formation of carbides, the present

invention contemplates a carburization process for selective case hardening that is

performed at a temperature that is below a carbide promoting temperature. For many

chromium bearing alloys, such as 316 stainless steel for example, carbides tend to readily

form at carburization temperatures greater than 1000 °F. Therefore, selective case

hardening processes of the present invention are performed at a temperature less than

about 1000 °F for stainless steel alloys. The time period during which carburization takes

place also affects carbide formation. Even at temperatures below 1000 °F, carbides can

form if the base alloy metal is exposed to the carbon source for a long enough period of

time. In accordance with another aspect of the present invention, carburization is

performed below a carbide promoting temperature and for a time period less than that

which permits carbides to form. Thus, the invention contemplates a time-temperature

profile that substantially prevents the formation of carbides during a selective case

hardening process. As an example of such a time-temperature profile, carbides readily form in 316

stainless steel above 1000 °F, as fast as within an hour. However, below this

temperature, for example in the 800-950 °F range, carbides will not form until about a

week or more, particularly at the lower temperature range. This is but one example, and

the particular time-temperature profile used in any specific carburization process for

preventing carbide formation will depend on a number of factors including but not

necessarily limited to the carburization temperature and the alloy chemistry of the base

metal .

Figs. 2A-2C illustrate in a representative manner (and in partial cross-section) the

various steps of a preferred carburization process in accordance with the invention for

selective case hardening of a stainless steels article, in this example a ferrule. The general

steps of the process are 1 ) applying a carbon blocking mask over surface areas of the

article that are not going to be carburized; 2) activating those surface areas that are to be

carburized; 3) and diffusing carbon into the activated surface areas. After the process is

complete the carbon blocking mask is removed. Step 1) can be performed in at least two

ways, the first being to apply a mask over the entire part and then removing (as by

etching, for example) the portions overlying the surface area to be carburized; and the

second being to only selectively apply the carbon blocking mask.

In Fig. 2A, the article 10 has a carbon blocking mask 20 applied over the entire

surface area thereof. It is important to note that in Figs. 2A-2C relative dimensions such as the respective thickness of the various layers are greatly exaggerated for clarity and ease of illustration and explanation.

In the present embodiment, the carbon blocking mask 20 is formed of copper that

can be applied to the article by any suitable method, in this case by electroplating.

In Fig. 2B the result of the next step is illustrated. Portions of the copper mask 20

have been removed, in this example the nose area 22, such as by chemical etching, to

expose the base metal. The etching process can be achieved in any number of ways. In

the illustrated case, the nose portion 22 is easily accessed and the copper can be etched by

simply dipping the nose portion 22 into a nitric acid bath. The acid removes the copper

without attacking the base metal of the part 10.

As soon as the copper is selectively removed and the article 10 is again placed in

air, the exposed unmasked portion 22 is passivated or de-activated and forms a chromium

oxide layer. This passivation is virtually instantaneous with exposure to air and occurs

with any stainless steel. The passive oxide layer that forms over the unmasked area 22 is

a carbon blocking layer. In order to carburize the unmasked nose portion 22. the

unmasked area 22 needs to be activated.

In order to activate the unmasked portions, the article is exposed to a hydrogen

halide gas mixture such as hydrogen chloride (HCl) and nitrogen at one atmosphere and

at an elevated temperature for a time history that minimizes or acceptably limits the

attack on and migration of the carbon blocking mask. HCl will attack, for example, a

copper mask. Over time the copper will tend to migrate away from the article and be - Si -

carried off by the HCl gas. The activation time however will be short as HCl is a very

good activation gas. So the time period of exposing the article to the HCl gas should be

kept short enough to prevent loss of the copper mask. A thicker copper mask can also be

applied if required to allow sufficient activation time. A suitable temperature-time profile

is 600-650 °F for about one hour. Other activating gases include but are not limited to

hydrogen fluoride (HF), and the activating gas selected will determine in part the time-

temperature profile used.

After activation of the unmasked areas, in one embodiment of the invention, the

carburization process is carried out whereby carbon atoms are diffused into the article 10

by exposing the article 10 to carbon monoxide (CO) gas. The carbon atoms easily diffuse

into the unmasked base metal area 22. The temperature for diffusion is kept below about

1000 °F (for stainless steel alloys) to prevent the formation of carbides. The carbon

atoms diffuse into a solid solution with the base metal. In accordance with this aspect of

the invention, a selective case hardening process is provided that allows for gaseous

carburization. In this embodiment, the carburizing gas mixture includes carbon

monoxide and nitrogen at one atmosphere pressure at a temperature less than 1000 °F, for

example in the range of 800 °F to 980 °F. The diffusion process can take up to two

weeks or so at these lower diffusion temperatures depending on the amount of carbon to

be diffused into the article.

Those skilled in the art will understand that the diffusion time period will

determine the depth of the carbon hardened surface because diffusion rate is temperature dependent. Since time also is related to the temperature related formation of carbides, the

carburization diffusion process should be controlled to achieve the desired case depth

using a time-temperature profile that prevents the formation of carbides for the particular

alloy in use. For example, because carbide formation is a function of time and

temperature, in cases where a deep case is desired it may be necessary to reduce the

temperature during the diffusion process as time goes by to prevent carbide formation.

The lower the temperature of diffusion the longer the diffusion process can last without

carbides forming. The drawback is the added time it may take to reach a desired

diffusion depth. But in many cases, by keeping the carburization temperature below that

temperature at which carbides readily form, for example less than 1000 °F for 316

stainless steel, the article can be case hardened to a sufficient depth without carbides

forming.

The various processes described herein involving exposing the article to gas can

be accomplished with conventional and commonly available equipment such as a pit

furnace as is well known to those skilled in the art.

After the activation step is completed, the activating gas is purged from the gas

chamber but the activated article surface is not exposed to atmosphere, otherwise the

article will immediately become passivated again. The activation gas is replaced in the

chamber with the carburizing gas mixture. In this example, carburization is performed

using a gas mixture of carbon monoxide, hydrogen and nitrogen at one atmosphere. An

exemplary gas mixture is 0.5-60% volume carbon monoxide and 10-50% volume hydrogen, remainder nitrogen, at one atmosphere. Again, the carburization diffusion is

done within a time-temperature profile and particularly at a temperature below that which

would allow or promote the ready formation of carbides in the base metal alloy (e.g. less

than 1000 °F for stainless steel alloys, for example in the range of 750-950 °F) for about

one week (again the time parameter will vary depending on the carburization depth

required).

Fig. 2C shows the end result after carburization. After the carbon atoms are

diffused into the unmasked portion 22, in this case the ferrule nose, a selectively case

hardened portion 30 of the article 10 has been formed that is harder than the remaining

portions of the article. The relative thickness of the hardened portion 30 is exaggerated in

the drawings for clarity, and in practice may only be 0.001 to 0.003 inches thick, for

example. This thickness is but one example. The depth of the diffusion will depend on

the desired mechanical properties and functional requirements of the final article. In Fig.

2C the copper mask 20 has been removed as by etching for example, after the diffusion

process is completed. Removal of the mask layer allows re-passivation of the article 10

to provide a chromium oxide layer, including the surface area of the case hardened nose

12.

Various alternatives to the preferred process will be readily apparent to those

skilled in the art. For example, other metals besides copper may be used for the carbon

mask including silver, again applied by electroplating. Other metals may serve as carbon

blocking masks. The invention has been described with reference to the preferred embodiment.

Obviously, modifications and alterations will occur to others upon a reading and

understanding of this specification. It is intended to include all such modifications and

alterations insofar as they come within the scope of the appended claims or the

equivalents thereof.

Having thus described the invention, it is claimed:

1. An article comprising:

a body comprising a metal alloy;

a first portion of said body having a first hardness characteristic; and

a second portion of said body having a second hardness characteristic;

said second portion being less than the entire body.

2. The article of claim 1 wherein said body comprises a generally

homogenous chromium bearing alloy with said second portion comprising a carburized

surface area substantially free of carbides.

3. The article of claim 1 wherein said second portion is formed as a

carburized area at selected locations of the body.

4. The article of claim 1 wherein said alloy comprises a chromium bearing

nickel or ferrous based alloy.

5. The article of claim 4 wherein said alloy comprises stainless steel.

6. The article of claim 1 wherein said second portion comprises a carbon

impregnated surface over a selected area of said body to produce a harder portion than

said first portion.

7. The article of claim 6 wherein said second portion is substantially free of

carbides.

8. The article of claim 1 wherein said body is a ferrule. 9. An article comprising:

a body comprising a chromium bearing alloy;

a first portion of said body having a first hardness characteristic; and

a second portion of said body less than the entire body with said second portion

being carbon hardened compared to said first portion and being substantially free of

carbides.

10. The article of claim 9 wherein said alloy comprises a chromium bearing

nickel or ferrous based alloy.

11. The article of claim 9 wherein said second portion comprises a carburized

area over a selected portion of the body.

12. A method for selectively carburizing an article, the method comprising:

disposing a carbon blocking mask over a selected surface portion of the article;

activating an unmasked surface portion of the article; and

carburizing said unmasked portion of the article.

13. The method of claim 12 wherein said step of disposing said mask

comprises the steps of:

depositing a carbon blocking material over the entire article; and

selectively removing portions of said blocking material.

14. The method of claim 13 wherein said step of depositing material

comprises applying a copper plate to the surface of the article.

15. The method of claim 14 wherein said copper plate is selectively removed.

Claims

16. The method of claim 12 wherein said activating step comprises exposing

said unmasked portion to a hydrogen halide gas at an elevated temperature.

17. The method of claim 16 wherein said carburizing step comprises exposing

said activated portion to a hydrogen bearing carburizing gas at an elevated temperature

below a temperature at which carbides would readily form.

18. The method of claim 12 wherein said carburizing step comprises exposing

said activated portion to a carburizing gas at an elevated temperature below a temperature

at which carbides would substantially form.

19. The method of claim 12 wherein said mask is applied by electroplating

copper.

20. The method of claim 12 wherein said carburizing step is performed with a

gas mixture of carbon monoxide and nitrogen at about one atmosphere pressure at a

temperature not greater than about 1000 °F for a stainless steel alloy and for a time period

less than that required for the formation of carbides in the selected temperature range.

21. An article made by the following process:

activating an unmasked surface portion of the article;

carburizing said unmasked portion of the article.

22. The process of claim 21 comprising the step of removing said mask after

said carburization step.

23. The process of claim 22 wherein said carburizing step is performed below

a temperature that promotes the formation of carbides.

24. The method of claim 12 comprising the step of removing said mask after

said carburization step.