GB1563726A - Method of making sintered metal articles - Google Patents

Description

(54) METHOD OF MAKING SINTERED METAL ARTICLES (71) We, FEDERAL-MOGUL CORPORATION, a corporation organised under the laws of'the State of Michigan, United States of America, of P.O. Box 1966, Detroit, Michigan 48235, United States of America, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to a method of making sintered metal articles.

Relatively small forged iron base metal articles can be produced from wrought stock or powdered metal, the latter being a relatively new technique.

To produce such an article of high Rc hardness, generally about Rc 60, which means substantially high martensitic content, coupled with good surface finish and dimensional stability requires subjecting the wrought article after forging to at least the additional steps of cooling, machining, and rehardening. Each subsequent step is not only expensive but introduces different metal conditions which must be accounted for in the next process step.

The forging of powdered metal preforms has consequently come into acceptance as a metal working process because of its chief advantage in eliminating the machining operations necessary with wrought material. In such process pre-alloyed metal powders are admixed with graphite and lubricant, compacted into briquettes, sintered, and forged.

While desirable, the full benefits of this relatively new procedure have not been fully realized.

For example, in the manufacture of powdered iron base metal articles having hardness at the working surface of Rc 60 or thereabouts it has heretofore always been thought necessary to either a) use a carbon content of 0.2 - 0.3 percent by weight and then carburize and subsequently harden the article, or b) use a carbon content of 0.5 - 0.7 percent and then austenize the article and quench it. While there are different specific ways to accomplish each processing method (a and (b), these are well known in the art and it is sufficient for purposes of illustrating our invention to explain that while each achieves the surface hardness and surface finish desied, each such method causes undesired dimensional changes in the article resulting from the heat treatments carried out in the process.

The present invention aims to overcome the above stated problems heretofore associated with powder metallurgy forging and permit the one-step forging of articles having the desired density, hardness, and closer dimensional tolerances, as well as articles which are free from surface and internal defects. In addition it is an aim of the invention to provide a process capable of producing an article of substantially uniform high Rc hardness throughout, rather than mere surface hardness as obtained with carburizing.

According to the present invention there is provided a method of making a through hardened scale-free forged sintered metal article having a density of at least 99.5% of theoretical and a surface hardness of at least Rc 40, comprising compressing an iron alloy powder as herein defined into a briquette, sintering the briquette into a sintered preform and subsequently adjusting the temperature of the preform so as to put the preform in an austenitic condition, forging the preform in a one step operation while it is in the austenitic condition so as to produce a forged article quenching the forged article substantially immediately after forging and while it is still in the austenitic condition, and stress-relieving the quenched article.

As used herein the term "iron alloy powder" covers a powder of iron alloy as well as a powder comprising iron and one or more other metal powders which together form the desired alloy when the process of the invention is carried out.

The invention will be further described by way of example only with reference to the accompanying drawing which shows a flow diagram illustrating the main procedural steps employed in carrying out the method of the present invention.

Referring to the drawing, there is shown a flow diagram of the method, generally designated 10, in which the initial step is preparation of the pre-alloyed metal powder 12.

The formulation of the powder will vary widely dependent upon the characteristics desired in the final article. Iron, of course is the major constituent with varying amounts of such metals as manganese molybdenum, and nickel being added. Carbon, in the form of graphite, is added prior to briquetting. In actual operation, it is preferred to first alloy the iron and other metals in the usual manner and to atomize the alloy to produce an iron alloy powder. The iron alloy powder is then admixed with the graphite and with a lubricant. Such lubricants are conventionally waxy or fatty materials which will be burned off in the sintering furnace as hereinafter described. The methods used to alloy and to atomize are those conventional to this art and form no part of the present invention.

Typically, the iron alloy powder can have the following composition: % by Weight Manganese 0.25 - 0.5 Molybdenum 0.25 - 0.5 Nickel 0.25 - 2.25 Carbon 0.2 - 0.9 Iron Remainder A preferred carbon content for use in the above shown range is 0.5 to 0.9% The powder is then formed into a briquette 14 or green compact by pressure compression in a die. This briquetting is used to control densification, shape, and general dimensional tolerances. Commonly, briquetting can accomplish densification up to about 85% of theoretical. Here, again, the type of compaction that is used can be varied: such as uniaxial or isostatic.

After compression, the briquette or preform is sintered 16. This is accomplished by placing the preform into a sintering furnace. These conventionally contain a preheating zone for lubricant burn-off, a hot zone for sintering, and cooling zone. Normally, sintering of ferrous materials, is carried out at a temperature of 2050 to 2100"F. Sintering is carried out in a protective atmosphere, i.e., one free of oxygen, in order to pevent internal oxidation and formation of oxidative scale on the surface of the preform. The furnace atmosphere in which the preforms are sintered contains the carbon potential to maintain the desired combined carbon level of the material. Any of the protective atmospheres conventionally used for this purpose are suitable. Surface enrichment to increase the carbon content and thus provide a carburized condition on the surface of the preform can also be provided by using a gaseous atmosphere of high carbon potential. It has been found in practice that such a step not only helps maintain hardness but also eliminates general cracks resulting from the subsequent quenching step.

An essential feature of the present invention is the heating of the preform 18 after sintering to a temperature such that the preform is in an austenitic condition prior to forging. In conventional sintering furnaces, the preform is cooled to ambient temperature and consequently, the preform must then be subsequently reheated to place it in an austenitic condition. The particular temperature to which the preform is heated above the minimum temperature (austenising temperature) required to place it in an austenitic condition is chosen is varied depending mainly on the shape and cross-sectional thickness of the preform. Thus, for example, a preform having an austenizing temperature of about 15500F can be adjusted to a preforging temperature of 1600 to 1900"F. Preferably the heating is conducted immediately after sintering so that as little heat as possible is lost and consequently less heat is required to heat the preform to an austenitic condition.

Immediately after being brought to the proper temperature. the preform is subjected to the forging operation 20 in a protective atmosphere wherein a single blow (one-step forging) of approximately 60 to 80 tons per square inch is used to shape the preform into the desired finished article.

The forged article is then preferably permitted to attain a uniform temperature throughout, whilst still in the austenitic condition, and is then promptly quenched in a conventional quenching bath 22.

It is preferred to use a conventional quenching oil bath although other quencing baths, such as water containing ethylene glycol, can be used.

It has been found that the careful regulation of the temperature prior to forging and quenching eliminates the need for any further heat treatment or working of the article to attain the desired tolerances and hardness throughout the article. Moreover, by operating in a protective atmosphere during sintering and forging, formation of any oxidative scale is avoided. The article 26 is in a finished condition after removal 24 from the quenching bath.

With respect to hardness, the present process obtains hardness that are 40-62 Rc or higher. It will be evident that, if a lower hardness is desired, the finished article can be further heat treated. Similarly, the product can be selective or surface hardened.

The invention will be further described in connection with the following examples which are set forth for purposes of illustration only.

EXAMPLE 1 An iron alloy powder was prepared having the following formula: % by Weight Manganese 0.25 Molybdenum 0.5 Nickel 1.8 Carbon .6 Iron 96.85 100.00 The iron, manganese, molybdenum and nickel were alloyed in the usual manner and the alloy atomized to produce iron alloy powder. This powder admixed with graphite to supply the carbon and with a lubricant (Acrawax--a hard, white, synthetic wax having a melting point of 94-97"C; Acrawax is a Registered Trade Mark). was placed into the die cavity of a standard briquetting press where pressure was applied and the powder formed into a briquette (green compact) of sufficient strength to permit further handling.

The briquette was then sintered in a sintering furnace at approximately 2050"F. for about 30 minutes in a protective atmosphere. The furnace was of the available type containing a zone for lubricant burn-off, a hot zone for sintering, and a cooling zone. The sintered preform was at a temperature of about 200"F. when removed from the sintering furnace.

The sintered preform was then heated to a temperature of approximately 16500F., in a protective atmosphere in an electric heating core. This temperature was above the temperature of 15500F. required to put the material in an austenitic condition.

The heated preform was then promptly transferred to a forging press and formed by a stroke of approximately 60 tons per square inch pressure into a forged stator clutch race having a smooth O.D. and a splined I.D. After forging. the forged article was allowed to stabilize (i.e. the temperature throughout the article became uniform) and cool to its austenizing temperature of about 15500F. and promptly quenched in a conventional oil bath. The article was then stress relieved.

The article was free of surface oxidative scale and was tested and found to have a density of 7.82 grams/cc (min.) or a density 99.6% that of theoretical; 7.87 grams/cc being the accepted theoretical maximum density. Most important, the product had a hardness, throughout the part of Rc 59-62. Equally importantly, the article had the desired shape and dimensions, thus eliminating the need for further processing to obtain the desired hardness of the product.

EXAMPLE 2 An iron alloy powder is prepared in Example 1 but having the following composition: % by Weight Nickel 0.5 Manganese 0.3 Molybdenum 0.5 Carbon 0.65 Iron Remainder 100.0 The graphite and lubricant are added as in Example 1 and the mixture deposited in the die cavity of the die set of a conventional briquetting press and compressed into a briquette.

This briquette, then sintered, is heated, forged, quenched and stress relieved as described in Example 1. The forged article thus produced was also found upon inspection to possess an excellent surface quality free from oxidation scale and other surface imperfections and immediately salable without further treatment. Its hardness if found by test to extend entirely through the article, with similar properties extending in all directions.

EXAMPLE 3 A nickel-content iron alloy powder is prepared as in Example 1 having the following formula: % by Weight Nickel 2.0 Manganese 0.25 Molybdenum 0.3 Carbon 0.5 Iron Remainder 100.0 The processing steps of Example 1 are followed to form a finished forged article. The forged article thus produced is also found upon inspection to possess an excellent surface quality free from oxidation scale and other surface imperfections so as to be immediately saleable without further surface treatment. The hardness of the article, as in Example 1 and 2 above, is found by test to extend entirely through the article, with similar properties extending in all directions, and in this instance was Rc 55-57.

Experience in carrying out the process of the present invention has shown that such an excellent surface quality is obtained that the product can be sold and shipped as it was forged, without the usual machining to remove 20 to 30 thousandths of an inch in order to eliminate the surface deterioration which has hitherto occurred. Furthermore, the obtaining, by this method, of the ultimate product immediately after forging. causes the tolerances required when, as previously, the dimensions of the article changed when the article was carburized after forging. Thus, the quenching immediately after forging, according to the present method, eliminates the extra tolerance spread acquired by variations in dimensions occurring during the subsequent reheating required by previous procedures, such as carburization.

The high hardnesses achieved by the forging and quenching method of the present invention are ideally suited for mechanical parts which are required to sustain heavy running loads during operation, especially where the hardnesses are so high that the material cannot be machined but can only be ground, namely above 35 Rockwell "C".

Anti-friction bearing races, for example, will wear rapidly and will not survive unless they have hardnesses in the neighborhood of 57 or 58 Rockwell"C".

WHAT WE CLAIM IS: 1. A method of making a through hardened scale-free forged sintered metal article having a density of at least 99.5% of theoretical and a surface hardness of at least Rc 40, comprising compressing an iron alloy powder (as herein defined) into a briquette, sintering the briquette into a sintered preform and subsequently adjusting the temperature of the preform so as to put the preform in an austenitic condition, forging the preform in a one step operation while it is in the austenitic condition so as to produce a forged article quenching the forged article substantially immediately after forging and while it is still in the austenitic conditon, and stress-relieving the quenched article.

2. A method as claimed in claim 1 wherein the powdered alloy composition comprises, for each 100% by weight, 0.25 to 0.5% manganese, 0.25 to 0.5% molybdenum, 0.25 to 2.25% nickel, 0.2 to 0.9% carbon and the remainder iron.

3. A method as claimed in claim 1 wherein the carbon content of the powdered alloy composition is 0.5 to 0.9%.

4. A method as claimed in any one of claims 1 to 3 wherein the sintering of the preform is conducted in an atmosphere of high carbon potentional to produce surface carburisation of the preform.

5. A method as claimed in any one of claims 1 to 4 wherein said adjustment of the preform temperature prior to forging is carried out immediately after sintering.

6. A method as claimed in any one of claims 1 to 5 wherein after the forging step and prior to the quenching step the temperature of the forged article is allowed to become

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

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. This briquette, then sintered, is heated, forged, quenched and stress relieved as described in Example 1. The forged article thus produced was also found upon inspection to possess an excellent surface quality free from oxidation scale and other surface imperfections and immediately salable without further treatment. Its hardness if found by test to extend entirely through the article, with similar properties extending in all directions. EXAMPLE 3 A nickel-content iron alloy powder is prepared as in Example 1 having the following formula: % by Weight Nickel 2.0 Manganese 0.25 Molybdenum 0.3 Carbon 0.5 Iron Remainder 100.0 The processing steps of Example 1 are followed to form a finished forged article. The forged article thus produced is also found upon inspection to possess an excellent surface quality free from oxidation scale and other surface imperfections so as to be immediately saleable without further surface treatment. The hardness of the article, as in Example 1 and 2 above, is found by test to extend entirely through the article, with similar properties extending in all directions, and in this instance was Rc 55-57. Experience in carrying out the process of the present invention has shown that such an excellent surface quality is obtained that the product can be sold and shipped as it was forged, without the usual machining to remove 20 to 30 thousandths of an inch in order to eliminate the surface deterioration which has hitherto occurred. Furthermore, the obtaining, by this method, of the ultimate product immediately after forging. causes the tolerances required when, as previously, the dimensions of the article changed when the article was carburized after forging. Thus, the quenching immediately after forging, according to the present method, eliminates the extra tolerance spread acquired by variations in dimensions occurring during the subsequent reheating required by previous procedures, such as carburization. The high hardnesses achieved by the forging and quenching method of the present invention are ideally suited for mechanical parts which are required to sustain heavy running loads during operation, especially where the hardnesses are so high that the material cannot be machined but can only be ground, namely above 35 Rockwell "C". Anti-friction bearing races, for example, will wear rapidly and will not survive unless they have hardnesses in the neighborhood of 57 or 58 Rockwell"C". WHAT WE CLAIM IS:

1. A method of making a through hardened scale-free forged sintered metal article having a density of at least 99.5% of theoretical and a surface hardness of at least Rc 40, comprising compressing an iron alloy powder (as herein defined) into a briquette, sintering the briquette into a sintered preform and subsequently adjusting the temperature of the preform so as to put the preform in an austenitic condition, forging the preform in a one step operation while it is in the austenitic condition so as to produce a forged article quenching the forged article substantially immediately after forging and while it is still in the austenitic conditon, and stress-relieving the quenched article.

2. A method as claimed in claim 1 wherein the powdered alloy composition comprises, for each 100% by weight, 0.25 to 0.5% manganese, 0.25 to 0.5% molybdenum, 0.25 to 2.25% nickel, 0.2 to 0.9% carbon and the remainder iron.

3. A method as claimed in claim 1 wherein the carbon content of the powdered alloy composition is 0.5 to 0.9%.

4. A method as claimed in any one of claims 1 to 3 wherein the sintering of the preform is conducted in an atmosphere of high carbon potentional to produce surface carburisation of the preform.

5. A method as claimed in any one of claims 1 to 4 wherein said adjustment of the preform temperature prior to forging is carried out immediately after sintering.

6. A method as claimed in any one of claims 1 to 5 wherein after the forging step and prior to the quenching step the temperature of the forged article is allowed to become

substantially uniform throughout the article.

7. A method of making a through hardened scale-free forged sintered metal article substantially as hereinbefore described in any one of the Examples.

8. A through hardened scale-free forged sintered metal article when made by the process of any one of claims 1 to 7.