GB2055404A

GB2055404A - Gas nitriding steel

Info

Publication number: GB2055404A
Application number: GB8020504A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-06-26
Filing date: 1980-06-23
Publication date: 1981-03-04
Also published as: GB2055404B

Abstract

A metal treatment (nitriding) process for producing a hard surface layer on an alloy steel component comprises maintaining the component for a period of up to four hours at a temperature between 480 DEG - 600 DEG C in an atmosphere of nitrogen and ammonia and then maintaining the temperature for a further period of up to one hour in an inert atmosphere following which the component is allowed to cool in an inert atmosphere, e.g. nitrogen. During the nitriding treatment the ammonia content of the exhaust gas from the retort may be maintained at 28-32% by volume.

Description

SPECIFICATION Metal treatment process This invention relates to a nitriding process for producing a hard surface layer on an alloy steel component, i.e. a component formed from steel containing alloying metals such for example as chromium and molybdenum.

Two nitriding processes are known for producing a hard surface layer on such a component, these being the cyanide bath process often termed the liquid nitriding process and the other being the ammonia process often termed the gas nitriding process.

In the cyanide bath process the component is immersed in a molten cyanide salt for a period of time after which it must be cooled and washed to remove all traces of the cyanide. The process is dirty and hazardous and the quality of the end product is not consistant. Moreover, there is a problem in getting rid of the spent cyanide salt fom the bath and the problem of disposing of the liquid be it water our a special sovent, which is used to wash the cyanide from the component. The process also demands a considerable amount of energy most of this being required for the purpose of maintaining the cyanide salt in a liquid state.

In the ammonia process the component is maintained at a high temperature in a retort whilst ammonia is passed through the retort. The time during which the component is maintained in the retort may be as long as 40 hours. After the component has been removed from the retort and allowed to cool the working surfaces must be machined to at least partly remove an excessively thick surface layer of Fe N. This process also demands a large amount of energy to heat up the retort and maintain it at the desired temperature.

The object of the invention is to provide a nitriding process for producing a hard surface layer on an alloy steel component in a simple and convenient form.

According to the invention a nitriding process for producing a hard surface layer on an alloy steel component comprises maintaining the component in an atmosphere of nitrogen and ammonia at a temperature of between 480"C-600"C for a period of time up to four hours, maintaining the temperature for a further period of time up to one hour in an inert atmosphere, and allowing the component to cool in an inert atmosphere.

In carrying out the process in accordance with the invention the alloy steel components are loaded into a retort which is then purged of air using an inert gas, e.g. nitrogen. The component is brought up to temperature in the inert gas and when the required temperature has been attained, i.e. between 480"C and 600"C preferably 560"C + 10 , the first part of the process is carried out and this can be regarded as the activating stage.

A mixture of nitrogen and ammonia is passed through the retort and the atmosphere in the retort is maintained so that it comprises between 17 and 18 parts by volume of ammonia and between 28 and 29 parts by volume of nitrogen the remainder being hydrogen. The time during which the flow of the nitrogen/ammonia mixture is maintained is between 1 and 4 hours. In order to provide accurate control of the atmosphere in the retort of the exhaust gases from the retort are analysed and the ammonia content kept art between 28-32% by volume. It is important to maintain the accurate control through the activating stage. During this stage chemical reactions take place and nitrides are produced. The ammonia breaks down to produce active monatomic nitrogen and hydrogen. The time required depends upon the temperature and the thickness of the layer of nitrides required.

Next follows the diffusion stage during which an inert gas preferably diatomic nitrogen, is supplied to the retort. The temperature is maintained. Conveniently the flow rate of the nitrogen during this stage is arranged to be equal to the combined flow rates of the ammonia and nitrogen during the activating stage. This is to ensure that there is no possibility of air entering the retort.

The diffusion stage lasts for about one hour after which the retort is allowed to cool. During cooling the atmosphere within the retort is maintained inert using nitrogen until the components are cool enough to be unloaded. During the diffusion stage inward diffusion of the nitrides takes place.

The resulting components can be utilised without further machining.

A typical alloy steel capable of being subjected to the process described comprises by weight 6.0% Molybdenum, 6.0% Tungsten, 4.0% Chromium, 2.0% Vanadium, 0.85% Carbon and the remainder Iron.

The surface of the components is whitish grey in colour having uniform texture. An examination of the surface layer reveals that at the surface there is a thin layer of Fe N backed up by layers of Fe2N, Fe3N, and Fe4N in that order, to a total thickness of about 0.075mm. The surface hardness value is between 1000 and 1150 Vickers. The surface has a low coefficient of friction and a marked resistance to scuffing which is due to the Fe N.

In the ammonia process a thick layer of Fe N is produced which whilst being resistant to scuffing, is brittle and hence machining of the component is necessary to redude the thickness of the layer.

1. A nitriding process for producing a hard surface layer on an alloy steel component comprising maintaining the component in an atmosphere of nitrogen and ammonia at a temperature of between 480" - 600 C for a period of time up to four hours, maintaining the temperature for a further period of time up to one hour in an inert atmosphere, and allowing the component to cool in an inert atmosphere.

2. A process according to claim 1 in which the temperature is in the range 550 Cto 5700C.

3. A process according to claim 1 or claim 2 in which the exhaust gases of the retort during the first mentioned period of time contain ammonia in the range 28 - 32% by volume.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Metal treatment process This invention relates to a nitriding process for producing a hard surface layer on an alloy steel component, i.e. a component formed from steel containing alloying metals such for example as chromium and molybdenum. Two nitriding processes are known for producing a hard surface layer on such a component, these being the cyanide bath process often termed the liquid nitriding process and the other being the ammonia process often termed the gas nitriding process. In the cyanide bath process the component is immersed in a molten cyanide salt for a period of time after which it must be cooled and washed to remove all traces of the cyanide. The process is dirty and hazardous and the quality of the end product is not consistant. Moreover, there is a problem in getting rid of the spent cyanide salt fom the bath and the problem of disposing of the liquid be it water our a special sovent, which is used to wash the cyanide from the component. The process also demands a considerable amount of energy most of this being required for the purpose of maintaining the cyanide salt in a liquid state. In the ammonia process the component is maintained at a high temperature in a retort whilst ammonia is passed through the retort. The time during which the component is maintained in the retort may be as long as 40 hours. After the component has been removed from the retort and allowed to cool the working surfaces must be machined to at least partly remove an excessively thick surface layer of Fe N. This process also demands a large amount of energy to heat up the retort and maintain it at the desired temperature. The object of the invention is to provide a nitriding process for producing a hard surface layer on an alloy steel component in a simple and convenient form. According to the invention a nitriding process for producing a hard surface layer on an alloy steel component comprises maintaining the component in an atmosphere of nitrogen and ammonia at a temperature of between 480"C-600"C for a period of time up to four hours, maintaining the temperature for a further period of time up to one hour in an inert atmosphere, and allowing the component to cool in an inert atmosphere. In carrying out the process in accordance with the invention the alloy steel components are loaded into a retort which is then purged of air using an inert gas, e.g. nitrogen. The component is brought up to temperature in the inert gas and when the required temperature has been attained, i.e. between 480"C and 600"C preferably 560"C + 10 , the first part of the process is carried out and this can be regarded as the activating stage. A mixture of nitrogen and ammonia is passed through the retort and the atmosphere in the retort is maintained so that it comprises between 17 and 18 parts by volume of ammonia and between 28 and 29 parts by volume of nitrogen the remainder being hydrogen. The time during which the flow of the nitrogen/ammonia mixture is maintained is between 1 and 4 hours. In order to provide accurate control of the atmosphere in the retort of the exhaust gases from the retort are analysed and the ammonia content kept art between 28-32% by volume. It is important to maintain the accurate control through the activating stage. During this stage chemical reactions take place and nitrides are produced. The ammonia breaks down to produce active monatomic nitrogen and hydrogen. The time required depends upon the temperature and the thickness of the layer of nitrides required. Next follows the diffusion stage during which an inert gas preferably diatomic nitrogen, is supplied to the retort. The temperature is maintained. Conveniently the flow rate of the nitrogen during this stage is arranged to be equal to the combined flow rates of the ammonia and nitrogen during the activating stage. This is to ensure that there is no possibility of air entering the retort. The diffusion stage lasts for about one hour after which the retort is allowed to cool. During cooling the atmosphere within the retort is maintained inert using nitrogen until the components are cool enough to be unloaded. During the diffusion stage inward diffusion of the nitrides takes place. The resulting components can be utilised without further machining. A typical alloy steel capable of being subjected to the process described comprises by weight 6.0% Molybdenum, 6.0% Tungsten, 4.0% Chromium, 2.0% Vanadium, 0.85% Carbon and the remainder Iron. The surface of the components is whitish grey in colour having uniform texture. An examination of the surface layer reveals that at the surface there is a thin layer of Fe N backed up by layers of Fe2N, Fe3N, and Fe4N in that order, to a total thickness of about 0.075mm. The surface hardness value is between 1000 and 1150 Vickers. The surface has a low coefficient of friction and a marked resistance to scuffing which is due to the Fe N. In the ammonia process a thick layer of Fe N is produced which whilst being resistant to scuffing, is brittle and hence machining of the component is necessary to redude the thickness of the layer. CLAIMS

4. A process according to claim 1 comprising the initial step or purging the retort of air using an inert gas and raising the component to the required temperature in the inert gas.

5. A process according to claim 1 in which the inert atmosphere is composed of diatomic nitrogen.

6. A nitriding process for producing a hard surface layer on an alloy steel component substantially as hereinbefore described.