US2778757A - Carburized tungsten alloy article - Google Patents

Description

United States Patent CARBURIZED TUNGSTEN ALLOY ARTICLE William W. Wellborn, Pittsburgh, Pa., assignor to Firth Sterling, Inc., Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application August 28, 1953, Serial No. 377,250

1 Claim. (Cl. 148-32) This invention relates to composite carburized articles and process of making the same. The invention is useful in the production of articles such as drawing and extrusion dies, rolls for rolling rod and wire and for parts in general which require abrasion resistance and resistance to impact.

Sintered carbide compositions comprising tungsten carbide or other hard metal carbide bonded by binding metal such as nickel, cobalt or iron, are known to be useful as cutting tools, dies and abrasion resisting articles due to their high abrasive quality. However, such hard metal carbide compositions do not have sufiicient impact resistance for use under certain conditions. In accordance with my invention, I provide composite carburized bodies or articles which comprise an integrally bonded core and a carburized case. The core comprises by weight at least 80% tungsten and the case contains tungsten carbide formed in situ. The core imparts excellent impact resist ance to the article and the case excellent abrasive qualities.

The process of making the composite article involves providing a heavy metal body containing by weight at least 80% tungsten, and thereafter heating the body at a carburizing temperature in contact with a carburizing agent which is a solid at the carburizing temperature employed until a carburized case of desired thickness is formed on the body.

While the heavy metal body, before carburizing, may be constituted entirely of tungsten, it is preferred that it contain both tungsten and a metal binder, the tungsten constituting from 80 to 97% by weight of the body and the binder from 3 to 20%. All proportions given herein are by weight. As binder metals for the tungsten, I may employ nickel, cobalt or iron, or a mixture of any of them. I may use copper or silver or both as binder metals but only in combination with nickel or cobalt or iron or all of them and where copper or silver are used, .they do not constitute over /6 of the total weight of binder metals.

A preferred heavy metal composition contains tungsten, nickel and copper, the preferred proportions being:

Heavy metal bodies which are to be carburized can be made in various ways. The powdered metals can be intimately mixed as by ball milling and then hot pressed into the desired shape. Instead of hot pressing, the mixed powders can be cold pressed and thereafter sintered. The hot pressing, cold pressing and sintering are well known 2,778,757 Patented Jan. 22, 1957 "ice steps in the production of bodies from powdered metal and therefore need not be described in further detail.

Another method of producing the heavy metal body which is to be carburized sebsequently is to produce a green compact of powdered tungsten, place the compact at the bottom of a crucible, then place powdered nickel or a mixture of nickel and copper on top of the compact and heat the crucible until the nickel and copper have melted and flowed into the interstices of the green compact. Another method is to pour nickel or an alloy of copper and nickel on a green compact of tungsten powder so as to cause the compact to become impregnated with the binder metal.

However the heavy metal body is formed, it is then carburized to form a carburized case integrally bonded to the core. For example, if an extrusion die is to be made according to the present invention, a heavy metal body of suitable shape and size for the extrusion die is made in any of the ways which have been described. Thereafter, the die is machined to suitable shape and size, which can be readily done since the heavy metal body does not contain any carbides but merely the metals themselves.

The die is packed in graphite, coke, charcoal or other suitable carburizing agent which is a solid at the carburizing temperature to be employed. Powdered graphite is a preferred carburizing agent and preferably is 100 mesh or finer in size. It should be of high purity since if it contains sulphur it is likely to attack the surface of the heavy metal body. The die is packed in graphite in such manner that all surfaces which are to be carburized are in intimate contact with the graphite and the die so packed in powdered graphite in a graphite boat is placed in a furnace such as a hydrogen atmosphere furnace or other furnace having provision for maintaining a neutral or reducing atmosphere in the furnace. The die is heated at a carburizing temperature until a carburized case of desired thickness is formed on the body. The carburizing temperature may be from 1800 to 2700 F., a preferred range being between 2000 and 2400 F. The time of carburizing will vary according to the thickness of the carburized case desired and according to the particular temperature and particular carburizing agent. Using powdered graphite as the carburizing agent and a temperature of about 2200 F., it has been found that a die whose largest dimension is 1 inch can be carburized to produce a carburized case of 0.030 inch in approximately 4 hours.

The invention has particular utility in the production of dies for extruding aluminum. Aluminum oxide, which is very abrasive, forms on aluminum billets when they are heated for the purpose of extruding them. A die made according to my invention resists the abrasive action of these oxides and, in addition, due to the core of the die which is not carburized, resists the impact involved in carrying out the extrusion. It will be evident that my composite article having an integrally bonded core and a carburized case provides greater impact strength than a similar article made throughout its thickness of tungsten carbide bonded by the same binder metal. On the other hand, my composite article, because of its carburized case, which contains tungsten carbide, provides much greater wear resistance than an article made throughout its thickness of tungsten metal bonded by the same binder metal.

I have found that in order to obtain the desired results in carrying out the carburizing step, the carburizing agent must be one which is a solid at the carburizing temperature employed. It is not satisfactory to employ simply a carburizing gas. For example, I have employed methane gas as the carburizing agent but have found it to be unsatisfactory. It results in blistering of the heavy metal body. It is believed that this is due to premature melting ofthe nickel or other binder m'ctalwhere a binder metal is employed. Where a gas isernployed as the carburizing agent, this premature melting occurs before the carburizing agenthas had an opportunity to produce a carburized case of the desired thickness. Itis' therefore critical in carrying out my method that the carburizing agent be one which is a solid at the carburizing temperature, as contrasted with the use of a gas as the carburizing agent.

The invention is not limited to the preferred embodiment but may be otherwise embodiedor practiced Within the scope of the following claim,

mam:

A composite articlecomprising an integrally bonded core and a carbnrized case, said core comprising by weightab'out 80'to' 97% tungsten'and' 3 to "20%"'binder metal, said binder metal consisting substantially entirely of at least one member of the group consisting of nickel, cobalt and iron and at least one member of the group consisting of copper and silver, the copper and silver being from zero percent to a percent not over /2 the percent of saidbinder metal, saidfcase containing tungsten carbide formed in situ and bonded by said binder metal.

2,242,254 Man fie May 20, 1941