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Method for oxygen free carburization in atmospheric pressure furnaces

a technology of oxygen-free carburization and atmospheric pressure furnace, which is applied in the direction of solid-state diffusion coating, metal material coating process, coating, etc., can solve the problems of additional safety risks, difficult to generate and precisely control endo-gas atmospheres, and high cost of operation

Inactive Publication Date: 2008-06-26
PRAXAIR TECH INC
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0008]The present invention may be characterized as a method of treating a metal part in an atmospheric pressure furnace comprising the steps of: (i) heating the metal part into the atmospheric pressure furnace having a gas atmosphere substantially free of oxygen, the gas atmosphere comprising a carbon containing gas, a reducing gas, and an inert gas; (ii) controlling the gas concentrations of the carbon containing gas, reducing gas, and inert gas to inhibit formation of metal-oxides on the metal part and to maintain a carbon saturation factor of between about 0.75 and 1.10 with negligible cementite or other metallic carbide formation; and (iii) removing the treated metal part from the atmospheric pressure furnace.

Problems solved by technology

Unfortunately, endo-gas atmospheres are somewhat difficult to generate and precisely control, are typically more costly to operate, and involve additional safety risks due to the gas atmosphere flammability and toxicity.
In addition, because the endo-gas atmosphere includes oxygen-laden gases, the resulting formation of inter-granular oxidation (IGO) within the metal part cannot be avoided.
Unfortunately, the current methods of controlling endo gas type atmospheres during the carburization process in an atmospheric pressure furnace have often resulted in a trade-off or balancing of the undesired effects in the carburized metal part and are not able to avoid IGO.
Such vacuum carburization process allows for the use of less complex gas atmospheres when compared to endo-gas atmosphere systems and can produce parts with low inter-granular oxidation, but require expensive and complex vacuum furnaces.
In addition, such vacuum carburization process typically operate in a very narrow range of low pressure, as too low of a pressure yields inadequate or inefficient carburization rates, and too high a pressure (above about 0.1 atmosphere) produces undesirable sooting of the metal parts.
Also, the low atmosphere density within a vacuum carburization process and system makes it difficult to carburize blind holes and recesses.

Method used

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  • Method for oxygen free carburization in atmospheric pressure furnaces
  • Method for oxygen free carburization in atmospheric pressure furnaces
  • Method for oxygen free carburization in atmospheric pressure furnaces

Examples

Experimental program
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example 1

[0065]This example involved the carburization of a piece of ¾″×2″ SAE 9310 steel alloy rod to a desired effective case depth of about 0.050 inches using the oxygen-free carburization model. A nitrogen atmosphere containing 0.6% propylene and 5.0% hydrogen was equilibrated in a laboratory tube furnace atmosphere at about 1700 degrees F. until the O2 concentration stabilized. The carbon uptake in the tube furnace was empirically determined to be about 1.0% based on a SAE 1010 shim sample (0.001″ thick) placed in the furnace and contacted with the oxygen-free gas atmosphere for about 15 minutes.

[0066]The relevant process inputs were entered into the oxygen-free carburization model and a desired hydrocarbon gas concentration profile as a function of time was established to achieve the desired effective case depth. The hydrocarbon gas concentration profile 98 used is depicted in FIG. 4. Hydrogen gas concentration during the carburization process was maintained at about 5.0%. The sample w...

example 2

[0068]This example involved the carburization of a SAE 4140 steel alloy sample to a desired effective case depth of about 0.047 inches in an operating atmosphere pressure carburization furnace using the oxygen-free carburization model. A nitrogen, propylene and hydrogen gas atmosphere was equilibrated in an operating atmosphere pressure carburization furnace at about 1700 degrees F. until the O2 concentration stabilized.

[0069]The relevant process inputs were entered into the oxygen-free carburization model and a desired hydrocarbon gas concentration profile as a function of time was established to achieve the desired effective case depth. The upper hydrocarbon limit was constrained by the hydrocarbon injection equipment. The hydrocarbon gas concentration profile 99 used is depicted in FIG. 5. Hydrogen gas concentration during the carburization process was maintained between about 1.19% and about 2.99%. The sample was placed in a loading vestibule and purged with nitrogen to remove r...

examples 3 and 4

[0071]These examples involved the carburization of a SAE 9310 steel alloy sample (Example 3) and a SAE 4820 steel alloy sample (Example 4) to desired effective case depths of about 0.034 inches in an operating atmosphere pressure carburization furnace using the oxygen-free carburization environment but keeping the hydrocarbon gas profile constant as a function of time. A nitrogen atmosphere containing about 1.7% propylene and 5.0% hydrogen was equilibrated in an operating atmosphere pressure carburization furnace at about 1700 degrees F. until the O2 concentration stabilized.

[0072]As with the earlier described examples, the samples were placed in a loading vestibule and purged with nitrogen to remove residual oxygen. The samples were then introduced to the furnace atmosphere at about 1700 degrees F. and allowed to come to temperature. The propylene gas flow and hydrogen gas flows were then introduced into the furnace. The hydrocarbon gas concentration profile was kept constant at ab...

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Abstract

A method of oxygen-free heat treatment of a steel part in an atmospheric pressure furnace is disclosed. The present batch treatment process employs an oxygen-free controlled gas atmosphere including hydrogen gas in concentrations between about 1.0 percent to 10.0 percent, a hydrocarbon gas, such as propylene, in concentrations of between about 0.1 percent and 10.0 percent that varies as a function of time, with the balance of the gas atmosphere being nitrogen. The present continuous furnace treatment process employs a plurality of zones with each zone including an oxygen-free controlled gas atmosphere including hydrogen gas in concentrations between about 1.0 percent to 10.0 percent that varies across the different zones, a hydrocarbon gas, such as propylene, in concentrations of between about 0.1 percent and 10.0 percent that varies across the different zones, with the balance of the gas atmosphere within each zone being nitrogen. The presently disclosed oxygen-free heat treatment processes, preferably carburization processes, use a precisely controlled atmosphere to minimize inter-granular oxidation, eliminate the formation of soot and cementite or other metallic carbides, and avoid hydrogen embrittlement.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 11 / 645,447 filed Dec. 26, 2006, the disclosure of which is incorporated by reference herein.FIELD OF THE INVENTION[0002]The present invention relates to methods of carburizing metal parts in an oxygen-free gas atmosphere, and more particularly to a method of varying in a controlled way the hydrocarbon and hydrogen gas concentrations in the furnace atmosphere to carburize metal parts with very low levels of inter-granular oxidation, and without formation of cementite or other metallic carbides in and / or soot on the carburized metal part.BACKGROUND[0003]Carburizing is a well-known process by which carbon atoms are adsorbed on the surface of a metal part and are introduced into the metal part via diffusion and is intended to make the surface of the metal part harder and more abrasion resistant. Carburization of steel generally involves a heat treatment ...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): C23C8/20
CPCC23C8/20C21D1/74C21D1/76
Inventor CONNERY, KAREN ANNENOVAK, RICHARD A.HO, SIMONGUPTA, AMITABHDE WILDE, ROBERT MARCEL
Owner PRAXAIR TECH INC
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