Method of producing powder metal parts using induction sintering

Abstract

A method of forming a part from a metallurgical powder includes compressing a metallurgical powder including no more than 0.3 weight percent of lubricant within a die to provide a green compact. The green compact is subsequently sintered by induction heating up to a sintering temperature of the green compact and maintaining the compact at the temperature for a time sufficient to provide a sintered compact. The sintered part may be subsequently hot formed and / or forged to densify and adjust the dimensions of the sintered compact.

Description

[0001] Not applicable.[0002] Not applicable.TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION[0003] The present invention is generally directed to a method of forming parts from metallurgical powders. More particularly, the present invention is directed to a method of forming powder metal parts including the step of sintering a green compact by induction heating. Even more particularly, the present invention is directed to a method of forming powder metal parts including the step of induction sintering a green compact including little or no internal lubricant and, optionally, further including a hot forming or forging step subsequent to the induction sintering step. The method of the present invention obviates the need for sintering a green compact using a conventional sintering furnace, such as a belt, pusher, or batch sintering furnace. The method of the present invention may provide substantial cost and time savings.DESCRIPTION OF THE INVENTION BACKGROUND[0004] The fo...

AI Technical Summary

Benefits of technology

[0008] In conventional powder metallurgy techniques, internal lubricants are employed as a means of facilitating removal of the green compact from the die and to aid in particle rearrangement during compaction. In the absence of an internal lubricant, the powder metal parts may adhere to the surface of the die, which may result in galling of the die surface after only a very small number of compacts have been formed in the die. Consequently, the die wears quickly and has to be replaced on a frequent basis, resulting in substantial expense. Lubricants, while alleviating the problems of adherence and die damage, are problematic in a number of ways. For example, the cost of the lubricants increases the cost of the finished parts. The lubricants also must be substantially removed from compacts during processing, and the evolution of gaseous lubricant decomposition byproducts from compacts during sintering may cause micro-cracks in the part and / or raise environmental concerns. Thus, eliminating internal lubricants would be of great value.

[0009] As just noted, the internal lubricant must be substantially removed from the powder metal compacts during processing. Failure to remove the internal lubricant reduces the maximum part density that may be achieved and otherwise compromises the properties of the finished part. For example, corrosion resistance of stainless powder metals may be degraded if all internal lubricant is not removed.

[0010] Conventional electric or gas-fired belt sintering of iron-base powder metal compacts typically subjects the compacts to high temperatures for about 20 minutes, allowing the internal lubricant to decompose and escape. Electric and gas-fired sintering furnaces are relatively inefficient heating devices and require large energy expenditures. Thus, the length of the sintering step required to process each compact involves substantial energy consumption and cost, and the sintering step constitutes a significant portion of the overall processing time and expense necessary to produce powder metal parts. In addition, belt, pusher, or batch sintering furnaces are large and expensive machines, requiring significant capital expense and a large floor space in the plant. Thus, in addition to the elimination of internal lubricant, it also would be advantageous to form powder metal parts without the need for conventional belt, pusher, or batch sintering.

[0011] Prior efforts to eliminate the use of internal lubricants in the fabrication of powder metal parts have met with only moderate success. One approach, seen in U.S. Pat. No. 5,682,591 issued to Inculet ("the '591 patent"), teaches the use of an electrostatic process to charge lubricant particles which are then sprayed onto the die surface prior to each molding cycle. It is intended that lubricant on the die surface would prevent adhesion of the compact and inhibit die wear. The entire disclosure of the '591 patent is hereby incorporated herein by reference. Success with the process of the '591 patent, however, has been limited because coverage of the die surface by the lubricant spray can be inconsistent if improperly sprayed. At the high molding pressures required to form green compacts, even one mis-spray can produce die damage. Molding dies could better withstand insufficient die coverage resulting from mis-sprays if lower molding pressures could be used. However, lower pressures result in compacts with lower green strength and lower densities. Green strength refers to the ability of a green compact to maintain its size and shape during handling and storage prior to sintering. Low densities may be raised in subsequent working operations, but a green strength less than about 1,000 lbs. / in.sup.2 (psi) is unacceptable because the compacts may easily crumble or fracture when handled.

[0012] Accordingly, it would be advantageous to provide a method of forming powder metal parts wherein little or no internal lubricant must be included in the metallurgical powder. It also would be advantageous to avoid the necessity for the use of conventional belt, pusher, or batch sintering techniques in order to reduce time and expense associated with those processes.

Problems solved by technology

In the absence of an internal lubricant, the powder metal parts may adhere to the surface of the die, which may result in galling of the die surface after only a very small number of compacts have been formed in the die.

Consequently, the die wears quickly and has to be replaced on a frequent basis, resulting in substantial expense.

Lubricants, while alleviating the problems of adherence and die damage, are problematic in a number of ways.

For example, the cost of the lubricants increases the cost of the finished parts.

The lubricants also must be substantially removed from compacts during processing, and the evolution of gaseous lubricant decomposition byproducts from compacts during sintering may cause micro-cracks in the part and / or raise environmental concerns.

Failure to remove the internal lubricant reduces the maximum part density that may be achieved and otherwise compromises the properties of the finished part.

Electric and gas-fired sintering furnaces are relatively inefficient heating devices and require large energy expenditures.

Thus, the length of the sintering step required to process each compact involves substantial energy consumption and cost, and the sintering step constitutes a significant portion of the overall processing time and expense necessary to produce powder metal parts.

In addition, belt, pusher, or batch sintering furnaces are large and expensive machines, requiring significant capital expense and a large floor space in the plant.

Prior efforts to eliminate the use of internal lubricants in the fabrication of powder metal parts have met with only moderate success.

Success with the process of the '591 patent, however, has been limited because coverage of the die surface by the lubricant spray can be inconsistent if improperly sprayed.

At the high molding pressures required to form green compacts, even one mis-spray can produce die damage.

However, lower pressures result in compacts with lower green strength and lower densities.

Low densities may be raised in subsequent working operations, but a green strength less than about 1,000 lbs. / in.sup.2 (psi) is unacceptable because the compacts may easily crumble or fracture when handled.