Method for Activating Surface of Metal Member

Abstract

A passivated film on a surface of a high-alloy steel member makes it difficult to apply diffusion treatment, such as gas nitriding or gas carburizing, that forms a nitrided layer, carburized layer or carbonitrided layer on the surface of the steel member. An activating treatment method is provided for the surface of the metal member. This method is not accompanied by problems of conventional activation treatment with a halide, such as furnace deposits, furnace wall erosion and effluent gas detoxification treatment, and is useful as pretreatment for diffusion treatment. According to this method, the passivated surface of the high-alloy steel member can be activated by using a gas commonly employed in gas heat treatment, and forming HCN gas in a heating furnace while making use of catalytic action of the steel member or a surface of the furnace.

Description

TECHNICAL FIELD [0001] The invention of the present application relates to a method for the pretreatment of a metal member to activate a surface of the metal member before applying diffusion treatment such as nitriding or carburizing to the metal member. BACKGROUND ART [0002] To improve mechanical properties such as abrasion resistance and fatigue strength, gas nitriding or gas carburizing that forms a nitrided layer or carburized layer in a surface of a metal member is widely applied primarily to members made of iron-based material. [0003] Upon applying such treatment to a surface of a member made of alloy steel, especially high-alloy steel, the penetration and diffusion of nitrogen or carbon into the surface of the metal member is prevented by a passivated film (an oxide or the like) which exists on the surface of the member, thereby possibly resulting in the occurrence of poor treatment or uneven treatment of the member as a problem. Before such diffusion treatment, activation tr...

AI Technical Summary

Benefits of technology

[0025] A passivated film on a surface of a high-alloy steel member makes it difficult to apply diffusion treatment, such as gas nitriding or gas carburizing, that forms a nitrided layer, carburized layer or carbonitrided layer on the surface of the steel member. According to the present invention, an activating treatment method is provided for the surface of the metal member. This method is not accompanied by problems of conventional activation treatment with a halide, such as furnace deposits, furnace wall erosion and effluent gas detoxification treatment, and is useful as pretreatment for diffusion treatment. According to this method, the passivated surface of the high-alloy steel member can be activated by using a gas commonly employed in gas heat treatment, and forming HCN gas in a heating furnace while making use of catalytic action of the steel member or a surface of the furnace. BEST MODES FOR CARRYING OUT THE INVENTION

[0026] The present invention will next be described in more detail based on best modes for carrying out the invention.

[0027] According to Patent Document 2 referred to in the above, CH3. (methyl radicals) formed by the pyrolysis of acetone in the formula (1) reduce an oxide film on a surface of a metal member. The CO formed in the above-described formula (1) and (2) reacts with ammonia as atmosphere gas on the metal surface to form HCN. HCN acts on the metal oxide film in accordance with the above-described formula (5).

[0028] From a comparison between the formula (2) and the formula (5), the CH3. formed by the pyrolysis of acetone and HCN (the reaction product of CO, the other pyrolyzate, with ammonia as atmosphere gas) are similar to each other in their action on the passivated film. The present inventors, therefore, presumed that the existence of both CH3. and HCN would be a sufficient condition for the activation of the surface of a high-chromium alloy steel member but would not absolutely be a necessary condition. Paying attention to HCN, the present inventors, therefore, endeavored to develop a method for the formation of HCN on a metal surface and also to ascertain effects of HCN for the activation of the surface of a metal member.

[0029] An investigation was conducted on the formation of HCN by introducing a nitriding atmosphere gas (NH3:N2=1:1 by molar ratio) together with gases selected from various carbon-containing compounds, which are gaseous at normal temperature and pressure, respectively into a Muffle furnace made of SUS310S and heating them to 550° C. As a result, it has been clearly ascertained that carbon monoxide, carbon dioxide, acetylene, ethylene, propane and butane each forms HCN when combined with ammonia.

[0030] An experiment was then conducted in a similar manner as described above except that the inner wall of the Muffle furnace was replaced by bricks, and an analysis was performed for the amount of HCN formed. In each case, HCN was not detected. From those results, it has become evident that the catalytic action of a metal surface is an essential condition for the HCN-forming reactions between ammonia and these gases.

Problems solved by technology

Upon applying such treatment to a surface of a member made of alloy steel, especially high-alloy steel, the penetration and diffusion of nitrogen or carbon into the surface of the metal member is prevented by a passivated film (an oxide or the like) which exists on the surface of the member, thereby possibly resulting in the occurrence of poor treatment or uneven treatment of the member as a problem.

However, the surface activation of a metal member by such a chloride results in the erosion of a furnace wall made of bricks or a metal by HCl formed through decomposition, and in gas nitriding or gas softnitriding, HCl so formed reacts with ammonia as atmosphere gas to form ammonium chloride, which not only deposits in the furnace or an exhaust system to cause troubles but also remains on the surface of the metal member (work) to induce reductions in the corrosion resistance and fatigue strength of the member.

The activation method of the surface of the metal member with the fluorine compound (NF3), however, requires sophisticated treatment for the detoxification of NF3 and HF contained in effluent gas, which prevents the wide-spread adoption of the method.

The above-described activation methods for the surfaces of metal members, which make use of halides, respectively, involves problems such as troublesome furnace deposits, furnace wall erosion and the need for detoxification treatment facilities for effluent gas.

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