JPS6120602B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a forged powder metal article with high Rc hardness that does not require further machining, surface treatment and heat treatment after quenching.

Relatively small wrought iron-based metal articles can be manufactured from wrought stock or powdered metal;
Manufacturing from powdered metals is a relatively new technology for compacting metal powders.

To manufacture such metal articles having a high Rc hardness, typically around Rc60 (which means a substantially high martensite content) combined with good surface finish and dimensional stability, the workpiece after forging is It is necessary to subject it to at least the additional steps of cooling, machining and rehardening. However, each subsequent step is not only costly, but also introduces different metal conditions that must be taken into account in the next processing step.

Forging of powder metal preforms has consequently become accepted as a metal processing method due to its cost advantages in eliminating the machining operations required when using forged continuous stock. In such processing methods, prealloyed metal powders are mixed with graphite and a lubricant, compressed into briquettes, sintered, and then forged. Although this relatively new method is desirable, its benefits have not yet been fully realized.

For example, when producing a powdered iron-based metal article with a hardness of about Re60 at the worked surface, a carbon content of 0.2 to 0.3% by weight is used to carburize and then the workpiece is hardened, or b It has always been considered necessary to austenitize the article using a carbon content of 0.5 to 0.7% by weight, followed by rapid cooling. There are various ways to carry out each of processing methods a and b and are well known in the art. For purposes of describing the invention, it will suffice to point out that each method achieves the desired surface hardness and surface finish, but has the drawback of causing undesirable dimensional changes in the article itself due to thermal effects.

The method of the present invention overcomes the aforementioned problems associated with powder metallurgy forging and provides articles with the desired density and tighter dimensional tolerances and free of surface and internal defects in a single step forging. be. Furthermore, the method of the present invention produces articles having substantially uniformly high Rc hardness throughout, rather than just surface hardness as obtained by carburization.

Accordingly, the present invention comprises: compressing an iron alloy powder having a carbon content of 0.2 to 0.9% by weight into a pricket; sintering the pricket into a sintered preform; adjusting the temperature of the article to a temperature at which it remains in the austenitic state; forging the thus heated preform in one step into a forging while retaining it in the austenitic state; retaining the forging in the austenitic state; The forged product is forged in one step to produce a forged product; the forged product is kept in an austenitic state and quenched in an oil quenching bath immediately after the above forging process; the thus quenched forged product is taken out of the quenching bath and the forged product is relieving stress from the article to produce a forged powder metal article having a surface hardness of at least Rc40 and a surface finish and dimensions that require no further machining other than surface polishing; having a density of at least about 99.5% of the value. The present invention provides a method for manufacturing a wholly hardened scale-free forged powder metal article.

The method of the present invention is generally carried out according to the flowchart outlined at 10 in the accompanying drawings. In this flowchart, the first step is the preforming of an iron alloy powder, the composition of which will vary widely depending on the properties desired in the final article. Of course, iron is the main component, to which metals such as manganese, molybdenum, and nickel are added in various amounts. Carbon is added in the form of graphite before precketing. In practice, iron and other metals are first alloyed by conventional methods and the resulting alloy is pulverized to form an iron alloy powder. This iron alloy powder is then mixed with graphite and a lubricant. Such lubricants are commonly used as waxy or fatty substances which are burned in a sintering furnace as described below. Alloying and micronization methods are conventional in the art.

As a typical example, the iron alloy powder may have the following composition: wt% manganese 0.25-0.5 molybdenum 0.25-0.5 nickel 0.25-0.25 carbon 0.2-0.9 iron balance initial carbon in the raw mixed powder of the present invention The reason why the content range is 0.2 to 0.9% by weight is determined with the expectation that this carbon content will be reduced when the preform is sintered in a sintering furnace. This is because if the initial carbon content is not within the above range, the final product will not have a hardness of at least Rc40.

The resulting powder is then formed into prickets 14 by pressure in a mold. This pricketting is done to control density, shape, and general dimensional tolerances. Normally, pre-ketization is about 85% of the theoretical value.
It is possible to achieve a density of up to The type of compression used here can also vary, such as general compression or isostaie compression.

After compression, the preform is sintered at 16. This is done by introducing the preform into a sintering furnace. Sintering furnaces are conventionally provided with a preliminary zone for lubricant combustion, a hot zone for sintering, and a cooling zone. Sintering of ferrous metal materials usually takes about
It is carried out at a temperature of 1121-1149℃ (2050-2100〓). Sintering is carried out in a protective atmosphere, ie, an oxygen-free atmosphere, to prevent internal oxidation and the formation of oxidative scales on the surface of the preform. The atmosphere within the sintering furnace has a carbon potential that maintains the desired bonded carbon level in the preform. Any protective atmosphere customary for this purpose is suitable. A surface enrichment effect may also be provided which increases the carbon content of the preform and thus brings about a carburized condition on its surface. In fact, this process not only retains hardness; It has been found to eliminate common cracking that can occur during subsequent quenching steps.

Prior to forging, the sintered preform is heated in step 18 to a temperature that maintains it in its austenitic state. As the preform is cooled to room temperature in a conventional sintering furnace, the preform must then be reheated above its austenitic temperature; the particular temperature above the selected austenitic temperature is primarily the temperature of the preform. Varies depending on shape and cross-sectional thickness. For example, a preform having an austenite temperature of about 850°C can be adjusted to a preforging temperature of about 870-1040°C.

Immediately after heating to the appropriate temperature, the preform is subjected to a forging operation 20 in a protective atmosphere, in which the preform is formed into the desired shape by a single forging of approximately 60-80 tons/6452 ^cm2 . Formed into a finished product.

The forging is then allowed to reach a uniform temperature around its austenite temperature and subjected to a conventional quenching bath 2.
Quench quickly in 2. Although it is preferred to use a conventional oil quenching bath, other quenching baths such as water containing ethylene glycol can also be used.

Careful control of the temperature prior to forging and quenching has been found to avoid the need for subsequent heat treatment or processing of the article to achieve the desired tolerances and hardness throughout the molded article. Furthermore, by performing the sintering and forging in a protective atmosphere, the formation of oxidizing scales is avoided. After removal 24 from the quench bath, the article 26 is placed in a finished condition.

Regarding hardness, the method of the invention gives a hardness of Re40-62 or higher. It will be clear that the finished product can be further heat treated if a lower hardness is desired. Similarly, the finished product can be surface hardened if desired.

Next, the present invention will be further explained by implementation.

Example 1 An iron alloy powder having the following composition was prepared; wt% manganese 0.25 molybdenum 0.5 nickel 1.8 carbon 0.6 iron 96.85 100.00 Iron, manganese molybdenum and nickel were alloyed by a conventional method, and the resulting alloy was pulverized to produce iron. An alloy powder was obtained. This powder is mixed with graphite to provide carbon and a lubricant (Acrawax, a hard white synthetic wax with a melting point of 94-97°C), and the resulting powder is placed into the mold cap of a standard preket forming press. Pressure was applied to form the powder into prickets with sufficient strength for handling in subsequent steps.

The prickets were then sintered in a sintering furnace at about 1120% in a protective atmosphere for about 30 minutes. The sintering furnace used was of a known type equipped with a lubricant sintering zone, a hot sintering zone and a cooling zone. The sintered preform was at a temperature of approximately 94°C when removed from the sintering furnace. This preform was then heated in a protective atmosphere in an electric heating core to a temperature of approximately 900°C. This temperature is higher than the austenite temperature (843°C) of the molding material.

The heated preform was quickly transferred to a forging press and formed into a forged stator clutch race with smooth O, D and splines I, D, by a blow of about 60 tons/6.452 cm ² . After forging, the forging was stabilized, cooled to an austenite temperature of approximately 843°C, and quenched in a conventional oil quench.

The article, removed from the oil bath and released from stress, has no oxidative scale on the surface and has a tested density of 7.82 g/cc, or 99.% of the theoretical value (7.87 g/cc).
g/cc is the theoretical maximum density allowed). Most importantly, the product had an overall hardness of Rc 59-62. Also importantly, the article has the desired shape and dimensions;
Therefore, no further processing was required to obtain the desired hardness of the product.

Example 2 An iron alloy powder having the following composition was prepared in the same manner as in Example 1: % by weight Nickel 0.5 Manganese 0.3 Molybdenum 0.5 Carbon 0.65 Iron balance 100.00 Graphite and lubricant were added as in Example 1 and the mixture was prepared in a conventional manner. The material was put into the mold cavity of the die set of a preket molding press and compressed into a pricket.
This pricket was then sintered, heated, forged, hardened, and stress relieved in the same manner as in Example 1. The forgings thus produced were found by testing to have excellent surface quality, free of oxide scale and other surface defects, and could be sold as is without further treatment. The hardness was good throughout the article, with similar properties in all directions.

Example 3 Nickel-containing iron alloy powder having the following composition was prepared in the same manner as in Example 1.

Weight% Nickel 2.0 Manganese 0.25 Molybdenum 0.3 Carbon 0.5 Iron balance 100.00 A finished forged product was manufactured according to the processing process of Example 1. The resulting forgings were tested and found to have excellent surface quality, free of oxide scale and other surface defects, and can be sold as is without further surface treatment. Similar to Examples 1 and 2, the hardness of the article was good throughout the article (Rc55-57) and had similar properties in all directions.

From the practical tests of the method of the present invention, the products obtained were:
It has been found that it is possible to sell and transport forged products without the need for the usual machining process that removes 1/20,000 to 1/30,000 x 2.54 cm to remove surface deterioration that occurs in the case of conventional methods. Furthermore, obtaining a final product immediately after forging according to the present invention introduces tolerances that are required if the dimensions of the article change when the article is carburized after forging, as in conventional methods. Therefore, quenching immediately after forging according to the method of the present invention eliminates the extra tolerance spread resulting from dimensional changes that occur during reheating, such as subsequent carburizing, required in conventional methods.

The high hardness achieved by the forge-quenching process according to the invention makes it difficult to support heavy operating loads during operation, especially when the material is so hard that it cannot be machined and can only be crushed, i.e. Re35 or higher. It is a practical match for the required mechanical parts.
For example, anti-friction bearing races must have a hardness around Re57-58 or they will wear quickly and have a short lifespan.

Although the preferred embodiments of the present invention have been described above, this is not intended to limit the present invention to the specific embodiments described above, and any other methods, modifications and equivalents that may be included within the essence and scope of the present invention are It is intended to protect property.

[Brief explanation of the drawing]

The drawing is a flowchart showing the main steps taken in carrying out the method of the invention.

Claims (1)

[Claims] 1. Compressing an iron alloy powder with a carbon content of 0.2 to 0.9% by weight to form a pricket; sintering the pricket to form a sintered preform; adjusting the temperature to maintain the austenitic state; forging the thus heated preform in one step to form a forged product while maintaining the austenitic state; carrying out the above forging process while maintaining the forged product in the austenitic state; Immediately thereafter quenching in an oil quenching bath; the thus quenched forging is removed from the quenching bath, stress is removed from the forging and the forging has a surface hardness of at least Rc40 and requires no further machining other than surface polishing. producing a forged powder metal article having a surface finish and dimensions that are not equal to and having a density of at least about 9.95% of theoretical; A method of manufacturing a wholly hardened scale-free forged powder metal article. 2 Powdered iron alloy contains 0.25 to 0.5 manganese depending on weight.
%, 0.25-0.5% nickel, 0.2-0.9% carbon, and the balance iron. 3. The forged powder metal article according to claim 1 or 2, wherein the preket is sintered in a gaseous atmosphere with a high carbon potential, and this sintering imparts a carburizing effect to the surface of the preform. manufacturing method. 4. A method for manufacturing a forged powder metal article according to claim 2 or 3, wherein the powdered iron alloy contains 0.5 to 0.9% carbon. 5. Temperature control of the preform before forging is carried out immediately after sintering, so that the residual heat of the sintered preform is utilized for the purpose of reaching a temperature range at which the preform remains in the austenitic state; 2. A method of making a wrought powder metal article according to claim 1, which conserves thermal energy to achieve a desired hardness by eliminating the need for expended heat in subsequent carburization. 6. The method for producing a forged powder metal article according to claim 1, wherein the temperature of the forged product is substantially stabilized after the forging process and before the quenching process.