US2327561A - Silver-containing free-machining die steel - Google Patents

Description

Patented Aug. 24, 1943 siLvnn-oon'ramnvo FREE-MACHINING DIE s'rnnr.

Howard W. Russell and Lloyd R. Jackson, Columbus, Ohio, assignors, by mesne assignments, to Heppenstall Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application May 8, 1942,

Serial No. 442,220

"18 Claims.

This invention relates to forming dies, extruding dies, dies for plastics, hot work dies, and others which are subjected to severe wear or abrasion due to the flow of material which is being formed. It is particularly adapted to dies which are used at temperatures above atmospheric, and especially to hot forging dies used for making metallic forgings.

Hot work, or hot forging, dies may be subjected to unusually severe conditions, not only because of the temperatures at which they must operate, but also because the abrasive action due to flow of metal in the die cavity is augmented by scale carried on the surface of the forging stock. Hence such dies may be referred to, by way of illustration only, particularly in describing the invention.

It is to the interest of the die user to obtain as high production life as possible because of the relatively high cost of these dies, especially those of large size or of intricate shape. For that reason they are hardened before use, and the .almost invariable practice is to harden before machining or cutting the die cavity so that it will be unnecessary to heat treat the die after the die sinking operations have been completed. The die steels used previous to the present invention have been rather difficult to machine when at a desirably high hardness, which has rendered the die sinking operations difficult or.

. in greater die life. Up to the present time; however, it hasbeen necessary to compromise, i. e.,

to use the dies at the highest hardness that permits reasonable ma'chinability, and to sacrifice the greater life which would attend higher hardness.

Various expedients have been proposed for improving the machining properties of steels. However, none of the known means has been satisfactorily applicable to the die steel field. For

example, sulfur in abnormally high amounts is' used for producing some types of freemachining steels. However, sulfur renders the steels hot short so that it can not be used for the production of' freely machinable dies. Lead is used also to confer free machining properties upon steels, but its use is attended by the evolutionf toxic fumes,

. in the case of die blocks, they must be forged or dies.

rolled. Selenium and tellurium are also used for the same purpose, but they likewise produce highly objectionable fumes and are otherwise not adapted for use in producing steels for hot work For instance, selenium acts like sulfur in rendering steel hot short, and it may adversely affect the mechanical properties of steels containing it.

It is among the objects of this invention to provide forming dies which possess free machining properties. I

Another object is to provide free machining forming dies, and particularly hot work dies, which can be produced easily and cheaply Wit/hr out the dangers and disadvantages which attend the use of such elements as sulfur, selenium, tellurium and lead, are capable of being hardened satisfactorily, are of improved machinability inthe hardened condition, and possess the same steels without silver, and to be machinable at commercially desirable rates at higher hardnesses than possible heretofore, and to do this without any substantial adverse effect on the steels small amounts of silver.

strength and related properties. v

Still another object is to provide a method of producing free machining grades of die steels containing silver.

A further object is to provide free machining steels suited particularly for making forming dies.

Other objects will be recognized from the following description..

This invention is predicated on the discovery that the machiningproperties of die steels can be improved substantially by incorporating in the Silver is soluble only to a greatly restricted extent in either liquid or solid steel, and we have found that use may 2 asamu be made of this fact to serve the objects of the invention by producing die steels to contain silver at least in excess of its solid solubility in the steel. As the steel cools from the liquid state the solubility of silver decreases and we believe that the amount in excess thereof is precipitated throughout the structure uniformly and in a state of high dispersion. We believe further that the improvement in machinability which attends the incorporation of silver in die steels according to this invention is due to such an action,

Forming dies of the type contemplated by the invention are made for the most part from low alloy steels, of which a variety are used for that purpose. We believe the invention to be applicable generally to die steels, but it is best suited to those containing chromium and molybdenum with or without nickel and vanadium, either or both, in which field it is adapted particularly to the die steels disclosed in Patent No. 2,085,676, issued on an application filed by John A, Succop and Patent No, 1,464,174, issued on an application by William F. Finkl. The steels disclosed in No. 2,085,676 contain about 0.4 to 0.7 per cent of carbon, about 0.6 to 1.25 per cent of chromium, about 0.6 to 1.0 per cent of manganese, about 0.3 to 0.8 per cent of molybdenum, and about 0.08 to 0.2 per cent of vanadium. Dies suited for some purposes may be made from steels of the foregoing analysis without vanadium. According to the Finkl patent, dies are made from steels containing from about 0.3 to 1.1 per cent of carbon, 0.25 to 1.1 per cent of chromium, 0.15 to 2.0 per cent of molybdenum, and 0.5 to 2.0 per cent of nickel. The remainder of such steels consists of iron. together with impurities in the amounts normally present in steels of these analyses, although other-elements may be present in amounts which do not adversely affect the properties which adapt these steels for use as hot work dies;

The amount of silver to be incorporated in the die steels will depend upon various factors. It should, of course, exceed the solid solubility of silver in the die steel because, as we believe, its

- effect upon machinability is dependent upon dis- .persion of the silver in finely divided form through the alloy, which makes it necessary that its solid solubility in the steel be exceeded. Most suitably it should not greatly exceed the liquid solubility because it is heavier than steel so that there is then a tendency for any excess to produce relatively large globules of silver, which we consider to be undesirable. Also, an excess of silver represents a waste due to the fact that the excess may appear at the surface of the billet or may actually sink toward the bottom of the furnace, ladle or ingot. The exact amount of silver to be used, bearing in mind such considerations, will be dependent further on the degree of itsdispersion in the steel, e. g., other things being equal, the more silver present the greater is the likelihood of it agglomeration and segregation. On the other hand, the more finely it is dispersed the less likely it is to ag lomerate and settle or segregate. We now believe that from about 0.02 or 0.03 to about 0.5 per cent of silver suflices for most purposes of the invention, and that preferably it should be present in an amount not over about 0.2 per cent. In steels of the compositions specified hereinabove, we believe that the best results are to be had when the steels contain about 0.1 per cent of silver. In no case should the silver content exceed about 2 per cent, at least for reaeons of economy.

The silver may be added in the furnace or to the ladle, and it may be introduced in various ways, for instance as metallic silver. We have discovered, and the invention is predicated in part on this, that especially desirable results are to be had by introducing in'some form other than as pure metallic silver, e. g., as reducible metallic compound, such as silver nitrate or other salt or compound which decomposes in contact with the molten metal to liberate metallic silver, or as an alloy with another metal. In this way the desired high degree of dispersion is obtained, and segregation and related losses are reduced. We now prefer to use copper-silver alloys. Suitable compositions are, for example, those containing 50 per cent each of copper and silver, or about 75 per cent of copper and 25 per cent of silver. Such alloys are produced easily and they serve not only to introduce the silver into the steel smoothly and without reaction or objectionable fumes, but also to accomplish the necessary uniform dispersion of the silver throughout the heat or ingot. The silvermay, however, be alloyed with other elements such as manganese, or with lead, bismuth or tellurium if the fumes produced are not objectionable. In all such cases the silver may be referred to as being in diluted form.

As exemplifying the benefits which attend the practice of the invention, reference may be made to actual tests of steels made in accordance with it. In one such test a 14-pound ingot of a'steel containing 0.56 per cent of carbon, 1.12 per cent of chromium,"0.55 per cent of manganese, 0.53 per cent of molybdenum, 0.1 per cent of silicon, .22 per cent of silver, and 0.14 per cent of vanadium was produced in a 35 kva. Ajax Northrup induction furnace, using a magnesia crucible. The ingot was a 2-inch round. l-inch rounds were forged at about 2050 F., and. they were then machined to %-inch rounds which were hardened by oil quenching from 1525 F. followed by drawing to a hardness of 43 to 45 Rockwell C (Rc) which is the hardness generally considered by die users as being the maximum for economically rapid machinability.

The hardened A-inch rounds were then subjected to a saw test which consists in measuring the time interval to saw through a standard bar compared with the bar under test. In this test a new Barnes 12-inch high-speed hack sawblade having 14 teeth per inch is operated at 80 strokes per minute in a gravity feed machine under an effective blade load of 53 /2 pounds.

' The time to cut through a 4-inch standard bar in sawing the standard bar to the total time required for sawing the test bar. The standard bar in this instance was a heat of die steel made to the same specification, but without silver, as the steel used in this test, and heat treatedin the same manner. Taking the machinability of the standard as 100, the machinability of the silver-containing steel was as measured by this test.

' In another testthere were made a series of ingots of steel containing from 0.54 to 0.56 per cent of carbon, from 0.98 -to 1.0 per cent of lets was detected.

chromium, from 0.82 to 0.88 per cent of manganese, from 0.55 to 0.57 per cent of molybdenum, from 0.23 to 0.28 per cent of silicon, from 0.10 to 0.13 per cent of siiver, and from 0.08 to 0.09 per cent of vanadium. The silver was introduced in the form of a 75 copper-25 silver alloy.

These heats were melted and the ingots were forged into bars and the bars heat treated and subjected to sawing tests in the manner just described. These tests showed that the hardened bars had a machinability index varying from 132 to 165.

As showing that the invention is applicable to commercial practice, reference may now be made to the application of the invention to two 1350- pound ingots produced in an electric induction furnace. The first of these was a steel which has been used satisfactorily and on a large scale for the manufacture of hot work dies. The second was made to the same specification except that silver was introduced in the form of a 50 per cent copper-silver alloy. The analyses of the steels were as follows:

Heat Cr Mn M0 Si I Ag v Standard 0.55 1.12 0.80 0. 54 0.22 0.08 Standard-l-Ag 0.55. 1.12 0.02 0.51 0.28 0.10 0.00

The ingots were forgedto 8-inch square billets approximately 54 inches in length, and the billets were annealed. A section l-inch thick was then sawed from each end of the billet followed by cutting a 12-inch length from each end, leaving a central block approximately 2'7 inches in length. No difference in sawability of the annealed bil- The two l2-inch sections were forged into 3-inch and 4-inch bars, while the 27-inch length was upset forged on allsix faces at about 2000 F. into a die block 12 x x 14 inches. These products were hardened by normalizing at 1600 F., air cooling, and quenching into oil from about 1550" F. The die blocks were tempered at about 1080 F. to the hardness commonly used with die blocks made from the commercial analysis, in which the Brinell ball diameter impression is between 2.95 and 3.00 mm., corresponding to a Shore hardness of 58 to 60. The round bars were tempered to a Shore hardness of about 62 Shore (444 Brinell; 46 Re).

These products were then subjected to tests to compare their machinability. Faces of the die blocks were machined to remove the surface skin, after which those faces were cut on a shaper and accurate record kept of the amount of metal removed with a feed of 0.053 inch and a depth of cut of inch. 63 square inches of metal were removed from the standard die block before the tool failed. In comparison, 189 square inches of metal had been removed from the block containing silver when the tool failed. On the basis of this test, the block made in accordance with this invention machined three times" as easily as the same block without silver. j r

The blocks were then given a saw-cut test in which a new blade was used for each cut and a cut was made across the entire section starting at one face of the block. The amount of sawing done before the blade became too dull to cut and broke was recorded. In the case of the standard block saw failure occurred after 30 minutes. The cut started to run out at 4 -inchdepth, and the blade broke at 5 A inches after having sawed a cross-sectional area of 68% inches. In the case of.the silver-treated steel the saw cut entirely through the cross section of the'block, amounting to 109.5 inches, in 43 minutes, and the blade was still in fairly good condition; Saw tests made as described in connection with the 14-pound ingots shows the silver-bearing heat to have a machinability index of 133.5.

The same blocks were then tested for machinability by drilling holes with a drill 1% inches in diameter, care being taken to see that the drills were as alike as posible. A hole 2% inches deep was drilled in the standard block in 4 minutes. A new hole was started but when it had progressed /2 inch the drill failed (after 30 seconds). A hole 2% inches deep. was drilled in the silver-bearing block in 3 minutes, 40 seconds, a second hole 3 inches deep in 3 minutes, 15 seconds, and a third progressed inch before the drill failed (after 15 seconds).

This test clearly demonstrates the superior machinability which attends the practice of this invention.

The 3-inch rounds tempered to 62 Shore (444 Brinell; 46 Re) were likewise examined for machinability cutting them on a lathe using a feed of 0.012 inch and a depth of cut of 3% inch at Bars tempered at 1080 F.

Heat T ensilc R 1 1. sm E1. um. Elong. t; B. n. N.

Psi Psi Per cent Per cent Standard 206, 750 167, 500 15. 5 47.8 415 Do 204, 250 163, 250 14. 5 49. 2 411 Standard+Ag 196, 000 165, 000 15. 0 41. 9 401 Do s 20 7, 500 170, 250 15.0 47.8 415 of free machining quality without the drawbacks and disadvantages which have heretofore attended attempts or expedients to do the same thing in the die block field. This means that the user of'die blocks can sink the die cavities more quickly and better at the same hardness than heretofore, or if desired the blocks can be machined at higher hardnesses than heretofore with consequent increase in production life. Such super-normal hardnesses can not be used at the present time because of the extreme difficulties which would be encountered in attempting to perform the necessary machining operations. Other advantages will be understood by those familiar with the art.

According to the provisions of the patent statutes, we have explained the principle and method of practicing our invention and have described what we now consider to represent its best embodiments. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

We claim: I

1. As a newarticle of manufacture, a die or the like formed from die steel containing silver in an amount in excess of the solid solubility of silver in said steel but not over about 2per cent, said block being characterized by having at least a portion of said silver distributed unito 2 per cent of molybdenum, and the remainder iron together with impurities and elements in amounts which do not substantially alter the die properties of the steel, and said block being in the heat treated and hardened condition.

4. As a new article of manufacture, a die block or the like formed from hot work die steel containing about 0.3 to 1.1 per cent of carbon, about 0.25 to 1.25 per cent of chromium, about 0.6 to 1.0 per cent of manganese, about 0.15 to 2 per cent of molybdenum, and silver in an amount in excess of its solid solubility in said steel but not over about 2.0 per cent, and the remainder iron together with impurities and elements in amounts which do not substantially alter the die properties of the steel, said block being in the heat treated and hardened condition and being characterized by having at least a portion of said silver distributed uniformly through the structure in highly dispersed form and by being of substantially better machinability than a similar die block made from the same steel without said silver.

5. A die block according to claim 4, said steel containing about 0.1 per cent of silver.

6. As a new article of manufacture, a die formed from die steel containing about 0.3 to 1.1

per cent of carbon, about 0.25 to 1.25 per centing of about 0.5 to 2 per cent of nickel, and

about 0.08 to 0.2 percent of vanadium, silver in an amount in excess of the solid solubility of silver in said steelbut not over about 0.5 percent, and the remainder iron together with impurities and elements in amounts which do not substantially alter the die properties of the steel, said die being in the heat treated and hardened condition and being characterized by having at least a portion of said silver distributed uniformly through the structure in highly dispersed form and by being of substantially better machinability than a similar die block made from the same steel without said'silver.

7. As a new article of manufacture, a die formed from die steel containing about 0.3 to 1.1 per cent of carbon, about 0.25 to 1.1 per cent of chromium, about 0.15 to 2 per cent of molybdenum, and about 0.5 to 2 per cent of nickel, silver in an amount in excess of the solid solubility of silver in said steel but not over about 0.5 per cent, and the remainder iron together with impurities and elements in amounts which do not substantially alter the die properties of the steel, said die being in the heat treated and hardened condition and being characterized by having at least a portion of said silver distributed uniformly through the structure in highly dispersed form and by being of substantially better machinability than a similar die block made from the same steel without said silver.

8. As a new article of manufacture, a die -formed from hot work die steel containing about um. silver. in an amount in excess of the solid' solubility of silver in said steel but not over about 0.5 per cent, and the remainder iron together with impurities and elements in amounts which do not substantially alter the die properties of the steel, said block being in the heat treated and hardened condition and being characterized by having at least a portion of said silver distributed uniformly through the structure in highly dispersed form and by being of substantially better machinability than a similar die block made from the same steel without said silver.

9. That method of improving the machinability of a die steel in the hardened condition which comprises adding to the molten steel silver in a diluted form and in an amount to provide the steel with silver in an amount in excess of its solid solubility but not over about 2 per cent by weight of the steel.

10. A method according to claim 9, the amount of said silver being between about 0.02 and 0.5 per cent by weight of the steel.

11. That method of improving the machinability of a die steel in the hardened condition which comprises adding to the molten steel an alloy ofsilver in an amount to provide the steel with silver in excess of its solid solubility but not over about 0.5 per cent by weight.

12. The method of improvin the machinability of a die steel in the hardened condition 13. The method of improving the machinabil ity of-a die steel in the hardened condition containing chromium and molybdenum, which comprises introducing into the molten steel silver in an amount in excess of the solid solubility of silver in said steel but not over about 2 per cent in the form of a copper-silver alloy containing from about 25 to about 50 per cent of silver.

14. The method of improving the machinability of a hot work die steel in the hardened condition containing about 0.3 to 1.1 per cent of carbon, about 0.25 to 1.25 per cent of chromium. about 0.6 to 1.0 per cent of manganese, and about 0.15 to 2 per cent of molybdenum, which comprises adding to the molten steel silver-in an amount in excess of the solid solubility of ganese, about 0.15 to 2 per cent of molybdenum, and silver in an amount in excess of-the solid solubility of silver in said steel but not over about 2.0 per cent, and the remainder iron-together with impurities and elements in amounts whichdo not substantially alterlthe die proper-,

ties of the steel.

' 1s. A steel according to claim'15,said steel' containing from about 0.05 to 0.5 per cent of silver.

That method of improving the machinability of a die steel in the hardened condition which consists in preparing said steel in the molten'state and adding thereto silver in an amount in excess of the solid solubility in said steel but not over about 2 per cent by weight 0f "the steel.

17. A' steel according to claim 15, said steel,

containing about 0.1 per cent .of silver.

HOWARD w. RUSSELL. LLOYD R. JACKSON.