Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
1998-08-10
2001-07-10
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S235000, C148S218000, C148S225000, C148S216000, C148S319000
Reexamination Certificate
active
06258179
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to carburized parts, methods for producing same and carburizing systems. The invention deals more particularly with an improved method for carburizing or carbonitriding steel at higher efficiency, and with a method for uniformly diffusing and precipitating fine cementite alone or together with fine nitrides in high percentage on the surface of steel by incorporating the above carburizing and carbonitriding method. The invention further relates to carburized parts such as rolling steel parts and their production method and carburizing system, which utilize the above carburizing and carbonitriding method for promoting the diffusion of cementite and nitrides.
BACKGROUND ART
In a typical known carburizing method, a workpiece is carburized in a carburizing atmosphere with a carbon potential equivalent to Acm transformation temperature or less, after raising temperature to the carburizing temperature range. Then, carbon is further diffused at the same temperature and with a carbon potential of 0.7 to 0.9 wt %. After temperature is lowered to about 850° C., the workpiece is quenched. Alternatively, the workpiece once cooled subsequently to the prior diffusion process is heated again to about 850° C. and then quenched (reheating hardening).
In recent years, high-temperature carburization is attempted, in which RX gas and butane gas are used as carrier gas and enriched gas respectively and such RX gas carburization is performed at a high temperature of 950° C. to 1,000° C. in order to increase the yield of carburized or carbo-nitrided steel. As other known high-temperature carburizing methods, the following carburization techniques are taken at high temperatures: (a) vacuum carburization in which carburization and diffusion are carried out in a reducing atmosphere in which hydrocarbon gas is decomposed at a reduced pressure; (b) N
2
-base carburization in which carburization and diffusion are carried out in an atmosphere in which N
2
gas mixed with hydrocarbon gas is heat-decomposed. The RX gas carburization method and N
2
-base carburization method often use a continuous carburizing furnace to enable mass production. In the above methods, a target value of the carbon content at the surface of the carburized layer after carburization is such a value with which 0.7 to 0.9 wt % an eutectoid constituent can be achieved and, generally, there are precipitated no carbides on the surface of the carburized layer.
A special carburization technique, high-carbon carburization is also known, in which two or more carburizing cycles are repeated, at least one of which is carried out in an atmosphere with a carbon content equivalent to Acm transformation temperature or more so that carbides are dispersed in the surface layer of steel. This technique aims to increase the rolling strength of steel parts and an example of which is disclosed in Japanese Patent Publication (Kokoku) No. 62-24499 (1987). According to one embodiment of this publication in which RX gas carburization is incorporated, a workpiece is pre-carburized for 6 to 12 hours at a temperature ranging from 930 to 980° C. with a carbon potential in the range of from the eutectoid carbon content to the value equivalent to Acm transformation temperature. After cooled by air or quenched once, the workpiece is again heated by raising temperature at a rate of 20° C./min. or less to a re-carburizing temperature of 750 to 950° C. Subsequently, carburization is carried out for 6 hours at 900° C. while the carbon potential (=1.85) equivalent to Acm transformation temperature or more being maintained, whereby 30% by volume or more cementite is precipitated in the carburized surface layer of 0.1 mm. In addition to the description of the above carburizing technique, the publication has reported that the steel having 30% by volume or more cementite precipitates exhibits superior rolling life. In the high-carbon carburization which causes cementite diffusion, the steel workpieces are required to contain 0.5 wt % or more Cr in most cases, as disclosed in Japanese Patent Publication (Kokai) No. 6-17225 (1994).
Since a problem presented by the prior art RX gas carburization method lies in the carburization reaction based on CO—CO
2
gas, there is inevitably created a grain boundary oxidized zone or imperfect hardened zone on the surface layer of steel after carburization, which results in, when taking a gear for example, decreases in the bending strength of the dedendums and in the strength of the tooth flanks. Due to the recent trend towards more compactness and higher load caused by transmitted power in reduction gears, there arise demands for a carburizing method which is able to prevent oxidization reaction under a carburizing atmosphere.
Carburization treatment in which the carbon content of the surface of a workpiece is controlled by controlling the amount of CO
2
in a carburizing atmosphere is usually carried out at a temperature of 900 to 950° C., because it is extremely difficult to control carbon potential by controlling the amount of CO
2
under high-temperature carburizing conditions. This inevitably involves long processing time. For example, it is well known that treatment of large gears takes two or more days and therefore incurs very high treatment cost. In addition, prolonged carburization treatment often causes an increase in the depth of a grain boundary oxidized zone or imperfect hardened zone so that the tooth flanks must be ground in some cases, resulting in a further increase in the cost of manufacture of gears.
When performing the above RX gas carburization at high temperatures for instance, for adjusting carbon potential to 1.5±0.1 during a carburization phase at a temperature of 1,000° C., the amount of CO
2
in the furnace must be controlled within the range of about 0.035 to 0.045%. For adjusting the carbon content of the surface to 0.8±0.1 wt % during a diffusion phase, the amount of CO
2
must be controlled within the range of 0.1±0.02%. For adjusting the carbon content of the surface to 1.5±0.1 wt % during carburization at a temperature of 1,100° C., the amount of CO
2
must be within the range of 0.015 to 0.020%, and for adjusting the carbon content of the surface to 0.8±0.1 wt %, the amount of CO
2
must be within the range of 0.035 to 0.05%. As understood from above, there are many problems in the control of carbon potential.
As attempts to solve the problems suffered by the above RX gas carburization method such as prolonged treatment and the creation of abnormal surface layers (e.g., a grain boundary oxidized zone), there have been proposed the vacuum carburization method and the N
2
-base carburization method, in which carburization is performed in an atmosphere of hydrocarbon gas having a pressure of about 10 torr or less. However, the vacuum carburization method reveals the problem that even if the amount of CH
4
within the furnace is measured and controlled, it cannot be used as an index for carbon potential and therefore CH
4
needs to be added in an amount several times to several tens of times more than a theoretical value during carburization. This method thus fails in practically controlling carbon potential so that precipitation of coarsened cementite on the carburized surface layer cannot be prevented during carburization. Although some measures have been taken to avoid these undesirable results, for example, by stopping a supply of carburizing gas in the course of carburization to effect diffusion treatment for a specified period, these techniques are taken under various operating conditions which are determined according to the depth of carburization, the type of steel used and others, as described in the reference book written by Takeshi Naito. That is, the determination of operating conditions is highly dependent on the know-how, which leads to instability in quality, high maintenance cost for the system, and involvement of vacuum troubles. For adjusting the carbon content of the surface to 0.7 to 0.9 wt %
Hamasaka Naoji
Takayama Takemori
Armstrong Westerman Hattori McLeland & Naughton LLP
Komatsu Ltd.
Yee Deborah
LandOfFree
Carburized parts, method for producing same and carburizing... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Carburized parts, method for producing same and carburizing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Carburized parts, method for producing same and carburizing... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2507225