US2370396A - Powder metallurgy - Google Patents

Description

Patented Fe. 27, E945 UNHTE rare e an POWDER IVE'EAILLUEGY Joseph J. illordiano, Brooklyn, N. Y., assignor to Hardy Metallurgical (Company, New York, N. 82.. a corporation of Delaware No Drawing. Application an 16, 1943,

Serial No. career 7 claims. (Ci. 75-22) ferrous alloy powder, which by itself does not form strong compacts.

As the result of my investigations, I have de There are a number of important advantages in fact that excellent permanent molds and dies for casting can be made from the steel powder of my invention.

ferrous objects. The screened powder is compressed in a die of appropriate shape and the re-' sulting green compact is sintered in a protective atmosphere to produce a steel object, which as indicated above will have higher hardness, higher tensile strength and better ductility and resistance to heat shock than an equivalent object made of the same raw materials by exactly the same procedure except that the fritting and screening steps of the invention are omitted.

Ezvample I To take a specific example, 2 parts by weight of a soft iron powder all,minus 325 mesh is mixed for 2 hours in a drum type mixer with 1 part of crushed white cast iron powder all minus 325 mesh and containing 33% carbon, substantially all of it in a combined state, i. e., as iron carbide. The powder mixture is then heated in a non-carburizing atmosphere, for example an atmosphere of hydrogen, for a time sufiicient to bring about aslight fritting together of the particles. Thus the powder mixture may be introduced into a mufiie held at 800 C. The introduction of the cold powder mixture will drop the temperature to about 700-C. but in a short time (10' minutes) the temperature will rise to 800 C. The mixture ismaintained at this temperature for 5 minutes more and then cooled while it is protected from oxidation.

As indicated above, the atmosphere should be non-carburizing so that the iron powder will I not pick up carbon andbecome hard. An at- In accordance with my invention, I employ fine iron powder and fine ferrous alloy powder.

Preferably, both powders ,are all minus 325 mesh. The powders (for example 2 parts of fine soft iron powder and 1 part of fine crushed white cast iron containing, say, 3% of combined carbon) are thoroughly mixed and then heat treated in a. non-carburizing atmosphere for a short period, say, 5 to 15 minutes at a temperature (say 700 to 800 C.) suificient to bring about a fritting together of the fine particles. Thereafter, the fritted agglomerate is subjected to mild comminuation, for example by screening on a 100 mesh screen. The screened product is much coarser than the original powders and the majority of the particles in the product consist of sintered agglomerate's of finesoft iron particles bon.

The steel powder produced as described above may be treated in the customary way to form the iron powder will be reduced and at the same mosphere of commercial hydrogen is very satisfactory It is reducing and slightly decarburizing in its action so that any oxides present in time there will be a slight decarburization of the cast iron powder, which aids in producing a.

compressible powder.

The fritted powder is permitted to cool to room temperature in a non-oxidizing atmosphere and is screened through mesh to produce a product of which 25% is minus 100 mesh, 35% is minus 200 mesh and the balance all through 325 mesh. Examination of this powder under the microscope shows that the majority of the particles consist of bonded agglomerates of finer particles of both iron and the ferrous alloy, say white cast iron.

The screened powder may be employed to form steel objects in the customary manner by compressing and sintering. Thus the screened powder may be compressed in a die at 50 tons per quare inch to form a green compact which is removed from the die and clntered tor 1 hour at 1200" C. in commercial w en. The slim tered compact is then allowed to cool at room temperature and hardened by being heated to 1 l20 F. in a neutral salt bothered quenched in water.

Steel objects mode in accordance with the foregoing procedure have u hardness or etc Brlnell as compared with e hardness of 335 Brinell for an equivalent piece mode in exactly the some way except thet the powder mixture was hot subjected to the preliminary flitting cued screening steps to egglomemte the soft lrou particles and the cast iron. The procedure is employed to make dies for zinc die costings.

durability.

It is not essential to employ powders all of which. are minus 325 mesh, since excellent results have been obtained with minus cos mesh powders.

eecm ze r1 Approximstely 1.5% of crushed white cost irou total time of 2'? minutes including hosting-uptime.

The diesere subjected to severe heut shocks hut have high Gil e ssueeo ice about 3% carbon. the balance oi the mixture heme carbon-tree iron powder. 7

It electrolytic iron powder is; employed-in the practice of the invention, it is (loch-able to mix the lira-annealed electrolytic powder with the termus alloy powder end to conduct the trltting op eretion on this mixture. In this way, the electrolytlc iron powder is deoxidlsecl and nncaled electrolytic iron particles with finer cast iron par- The resulting fritted powder mixture was el- I pieces were allowed to cool to room temperature.

The coolin was conducted in the furnace. There was no subsequent hardening procedure.

The sintered pieces. after cooling to room temperoture, were subjected to tensile testing. They had tensile strengths of @1300 to 43,600 lbs. per square inch with an elongation in 1 inch of 27%. These values contrast with tensile strengths of 39,290 to 39,900 lbs. per square inch and elongetions of 11 to 13% in 1 inch ohtelued'from the some powders in an identical operation, except that the fritting and screening omratione were omitted.

As indicated above, the ferrous alloy p wder containing the combined carbon need not be white cast iron powder. It may be crushed cast steel powder or for that matter any ferrous alloy containing combined carbon. Thus high carbon ticles containing combined cerhou.

3. In the manufacture of steel. objects by compressing and slnteriug a. mixture or soft iron powder with harder ferrous alloy powder containing combined carbon, the improvement which comprises subjecting the powder mixture to a preliminary frittipg to ogglomerate thc iron particles prises subjecting the powder mixture to a prellminery fritting to agglomerate the iron particles and the ferrous alloy particles, and subjecting the-resulting fritted mass to. mild comminution prior to compressing the powder mixture to form the object, the commlnution'being s0 conducted that the fritted and oommimlted material is subture.

stentielly coarser than the orizinal powder mix- 5. A process for producing a. steel powder suitable for powder metallurgy which comprises fritting a mixture of fine soft iron particles with fine ferrous alloy particles containing substantial proportions of combined carbon and then subferrosilicon, ferromangauese or i'errochrome powders may be used. G 06 results may be obtained by employing a. small proportion, say 2% of high carbon ferrochrome powder containing 67% chromium and 6% carbon and 2. larger percenta e. so M lt of crushed white cast iron containiectine the irittedmixture to mild comminution to produce apowder that is substantially coarser than the original mixture.

6. A process for producing a. steel powder suitoblefor powder metallurgy which comprises fritting a mixture of line soft iron particles with fine ferrous alloy particles containing substantial proportions of combined carbon at a temperature ranging irom 700 to 800 C. and then subjecting the tritted mixture to mild comminution to produce a powder that is substantially coarser than the original mixture. I

'7. A steel powder in which the majority of the particles are fritted agglomerates formed by heat 4 treatment of w loose powder mass or finer soft iron particles with finer lemma alloy particles containing iron carbide.

- JOSEPH J. CORDIANO.