US2105114A - Alloy steel tool - Google Patents

Description

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James P. Gill, Latrobe, Pa, assior to w dium-Alloys Steel Company, Latrobe, Wa a corporation of Pennsylva No Drawing. Application November 13, 1937,. Serial No. 174,407

1 Claim.

This invention relates to alloy steels, and more particularly to the typeknown as high speed steels, which may be employed in the making of cutting tools, dies and other tools. I

There are a number' of high speed steels in commercial use today which usually contain as alloying elements, tungsten, chromium and vanadium; or molybdenum, chromium and vanadium; or tungsten, molybdenum, chromium and vanadium. It is unusual for such steels to contain a carbon content in excess of 370%, since it is generally considered that a higher carbon content makes forging difiicult and increases brittleness to such a degree as toymake the steel generally unsatisfactory.

.In investigating different co loying ingredients, particularly num, chromium and vanadium, Ihave discovered a new balanced composition and a new relationship between the carbon and vanadium contents which can be effectively used to increase materially the cutting efficiency of the steel without material decrease in toughness and yet still have a steel which is readily forgeable and can be readily annealed.

I find for example, that when a high speed steel of the so-called molybdenum type contains approximately 9.00% molybdenum and 4.00% chromium, the vanadium content should be approximately three times that of the carbon contentwhen the latter is in excess of..70%, for best cutting efficiency. Increasing the carbon and vanadium contents in this ratio results in a steel of the molybdenum type which has materially better cutting efiiciency, forgeability and annealibility than previously known steels of this type. Maintaining this relationship in the carbon and vanadium content does not increase the brittleness of the steel to the same extent as where the carbon content increases at a lesser ratio.

Increasing the carbon content in this ratio also results in. a steel which can be readilyforged even when the carbon content is substantially in excess of 370%. P

In the practice of my invention, tools may be formed from steels containing from 370% to 1.50% carbon, from 2.00% to 4.50% of vanadium, from 7.00% to 11.00% of molybdenum, and from 3.00% to 6.00% of chromium, with the vanadium content approximately three times that of thecarbon content, but always within the' limits of in excess of two and one-half to one and less within the range of my invention which prove very satisfactory:

In addition .to the above elements, small amounts of manganese, silicon, sulphur and phosphorus are assumed to be present, with the remainder of the alloy substantially all iron.

Notwithstanding the broader ranges set forth above, the compositions of my improved steel will preferably fall within the narrower limits which may be defined as follows:

Limits Carbon .75- 1.20 Silicon .20- .35 Manganese .10- .30 Sulphur Less than .03 Phosphorus Less than .03 Chromium 3.75- 4.75 Molybdenum 53.25-10.00 Vanadium 2.00- 4.25

Additionally, notwithstanding the narrower limitsset forth above. there will be a desired range within the narrower range which may be defined as follows:

Steels made of the above approximate compositions will be found to have a substantial in crease in wearing ability with increase in the carbon content. As the carbon content increases, while maintaining the proper ratio with the content. of vanadium, it will be found that toughness decreases slowly. While forgeability will decrease slightly with the increase in carbon and vanadium content it does so slowly and the steel will remain forgeable with the carbon content substantially in excessof 310% and with the vanadium content in proper proportion thereto.

Additionally, in 'a composition of that kind the material can be freely annealed and has excellent cutting properties.

.amount to approximately 2%.

In high speed steels of compositions similar to those set forth in the tables given above, and having a ratio of carbon and vanadium as stated, the addition of cobalt from small amounts up to. and including as much as 12% may materially increase the cutting ability of the steel. Also, if nickel is introduced into steels of this type the strength is increased without material decrease in the cutting ability. The nickel content which may be used ranges from an effective Additionally, other well-known alloying ingredients may be added to an extent less than one and one-half percent of the composition in the aggregate.

' Since this improved steel reacts to heat treatment more uniformly than other molybdenum high speed steels, and the same hardness results can be consistently obtained using the same quenching and tempering temperatures, it will be desirable to set forth various methods of heat treating this material.

Forging.-The improved steel should be heated slowly and uniformly to a forging temperature of from 1900 to 1950 degreesFahrenheit. It willforge somewhat easier if heated to a temperature of from 1300 to 1400 degrees Fahrenheit and. held at this temperature for several hours before heating to the higher temperature. After forging, it is necessary to cool slowly as the steel is self-hardening like all high speed steels. Slow cooling may be either in a furnace or by burying in'a heat insulating material such as ashes, lime, mica, infusorial earth, etc.

Annealing To properly anneal, it is best to pack in a container using sand or lime to which a very small amount of charcoal may be added. The container in which the material is to be annealed should be of such a size that only a small amount of packing is necessary, otherwise it may insulate the heat from the steel and cause an unnecessary time for heating to the annealing temperature; An annealing temperature of approximately 1550 degrees Fahrenheit is best and the steel should be cooled preferably with the furnace and at a temperature not over about 40 or 50 degrees an hour until a temperature of about 1200 Fahrenheit has been reached when'it may be cooled more rapidly.

HardeningP-The entire process of hardening will depend greatly upon the heat treating equipment available. Controlled atmosphere furnaces or salt baths are preferable for hardening but when these are not available a semi-muflle furnace will serve satisfactorily, in which case it is necessary to cover the material with borax to prevent partial decarburization of the surface. If a semi-muffle furnace is used, the pre-heat should be from about 1300 to 1400 degrees Fahrenheit.

As soon as the steel has reached the temperature of the preheat it should be covered with borax. When semi-muiile furnaces are used, either for the pre-heat or high heat, it is advisable to use a plate in the bottom of the furnace to prevent the borax attacking the furnace bottom. For maximum hardness a temperature of approximately 2225 degrees Fahrenheit is satisfactory for most types of tools. Single point cutting tools may be heated somewhat higher and fine edge tools, such as taps, should be heated to a temperature about 25 degrees lower. As soon as the steel has reached the temperature of the furnace it should be oil quenched, The same hardening temperatures apply for controlled atmosphere furnaces or for salt baths but when these furnaces are available it is desirable to use a pre-heatin-g temperature in the vicinity of about 1500 degrees Fahrenheit. It is unnecessary to .use a borax covering in controlled atmosphere furnaces. It is difilcult to state the lengt-hof time necessary for holding at the highheat as this depends considerably upon the size of the tools being treated and'upon the type of equipment in use. In general, the same comparable time. should be used as with othertypes of high speed steels.

Tempering.-The resistance of a high speed steel to loss of hardness resulting from the tempering temperature will depend somewhat upon the amount of alloys that have been dissolved and the amount of austenite present after hardening. This is indicated by the grain size. If the steel is highly overheated then it will resist tempering to a more noticeable degree than if properly heated or underheated. Any table which gives the hardness resulting from the tempering temperature should be based on about the average grain size to be expected from hardening for the general run of tools such as drills, reamers, milling cutters, etc. It should not be based on the structure that is oftentimes found in single point cutting tools which may be overheated to produce a comparatively coarse grain and, therefore, have considerable austenite resulting from the higher hardening temperature. The following table of drawing temperatures shows the hardness that can he expected in my improved steel after quenching from a temperature of 2225 degrees Fahrenheit. Tools hardened at this temperature will have a comparatively small grain.

Tampering temperature ggg A qiinnnhpd 66 800 F- 62. 5 900 F 63. 5 950 F. 64. 5 1000 F- 65 1025 F. 66 1050 F. 64. 5 1075 F 63 1100 F 62 1125 61 1150 F 59 1200 F 54- 5 About the same hardness results will be obtained with tools quenched from 2200 degrees Fahrenheit and slightly higher results with tools quenched from a temperature of 2250 degrees Fahrenheit. The tools should be heated slowly to the tempering temperature and should be held at this temperature for about one and one-half hours, after which they may be cooled in air.

Superficial surface hardening.During recent years there has been a tendency for many manufacturers of high speed steel to impart a superficial surface hardness after the tools have been finish ground. Standard nitriding treatments have been used but, in general, this has not been found satisfactory as apparently the concentration of the nitrides is too great at the surface of the steel causing unusual brittleness. By immersing the finished tool in a molten bath composed of about 50% sodium cyanide and 50% potassium cyanide at a temperature of approximately 1025 degrees Fahrenheit a superficial hardness is imparted which is probably not over several thousandths in depth but which for certain classes of tools may even double their life. The time in the cyanide bath may be from ten minutes to one and one-half hours, according to the requirements desired. When this treatment is used the tools shouldfirst be given the regular treatment,. finlsh ground and then slowly heated to the temperature at which they should be immersed in the cyanide bath.

This application may be considered a continuation-in-part of my application Serial No. 38,059, filed August 27, 1935.

Having thus described the invention, what I claim as new and desire to secure by Letters Patent of the United States is:

A tool composed of a readily forgeable and ma chinable high speed alloy steel of the molybdenum type, said steel containing from .75% to 1.20% carbon, from 3.75% to 4.75% chromium, from 8.25% to 10.00% molydenum, andfrom 2.00% to 4.25% vanadium, the ratio of the vanadium to carbon being in excess, of two and one-half to one and below three and one-half to one, with the remainder of the alloy substantially all ir'on.

JAMES P. GILL. 10