US3254867A - Ingot mold - Google Patents

Description

June 7, 1966 R. F. HARVEY ETAL INGO'I MOLD Filed 001;. 24, 1965 THE-=2- @Qfi TE 4A- Columnar Equiaxed Grains Grains Flu-.1 4B- Ca/umnar Equiaxed Grains Grains N VE N TORS.

RICHARD E HARVEY and CARL H. BRYANT Attorneys United States Patent 01 3,254,867 INGOT MOLD Richard F. Harvey, Ross Township, Allegheny County,

and Carl H. Bryant, Aliquippa, Pa., assignors, by mesne assignments, to Cyclops Corporation, a corporation of Pennsylvania Filed Oct. 24, 1963, Ser. No. 318,607 6 Claims. (Cl. 249-474) The present invention relates generally to ingot molds and more particularly to molds of the big-end-up type employed in casting ingots of high alloy tool and specialty steels from molten metal.

Molds for casting steel ingots are old in theart and have long engaged the attention of both the practical and technical personnel of the industry in endeavoring to control the physical and metallurgical qualities of the steel ingots cast therein. As a consequence a vast number of mold constructions have been employed in an endeavor to provide steel ingots to facilitate further working of the steel into the desired end product. As of this date the perfect ingot mold for all purposes has not been found.

One of the remaining unsolved problems is to provide a mold for producing ingots of high alloy tool and other specialty steels with the desired control of dimensional changes and grain structure during subsequent working into heat treated tools. All ingot molds are provided with some tape-r in the cavity inner walls to facilitate stripping of the ingot from the mold. conventionally, the crosssectional contour of the mold cavity has been either square or rectangular. It is also conventional to provide such mold cavities with the larger cross-sectional area at the top of the mold, such molds being known as big-end-up molds. The degree of taper in these square and rectangular mold cavity walls has varied.

The structural changes which molten metal undergoes during solidification in a mold are generally well understood. The molten metal entering the mold, at its area of contact with the mold cavity side walls, solidifies or freezes very rapidly to form a relatively narrow surface zone or chill zone composed of relatively small randomly oriented crystals. Adjacent the chill zone and inwardly of the mold cavity, freezing of the molten metal occurs at a progressively slower rate in producing a zone of elongated columnar crystals. This latter region has strongly directional properties resulting from the columnar crystals being oriented generally in a direction perpendicular to the mold cavity walls. Such columnar type of crystallization may persist to the center of the ingot with some alloys, wit-h suflicient temperature gradient between the solid and molten liquid. Generally, however, the columnar type of crystallization stops at low temperature gradients and random solidification of approximately equiaxed crystals occurs in the central region of the ingot. Also, solidification in ingots occurs subs-tantially vertically from the bottom tothe top thereof along the vertical axis of the mold, as Well as inwardly or substantially horizontally from the sides of the mold. If the horizontal solidification is completed before the vertical solidification, a solid bridge of metal may form to entrap molten metal below the bridge. This results in deep cavities, unsoundness, porosity and segregation.

The present invention has advantages in casting ingots of all steels, but is of special advantage in casting tool steels and in particular, high alloy tool steels, such as high speed steel, high-carbon-high-chromium die steels, and the air hardening die steels. A large and important class of tools, including unground hobs, form relieved cutters and thread rolling dies made from such high alloy tool steels, must be heat treated for hardening, after machining, .and of necessity must have restricted dimensional changes or distortion, such as minimum out of roundness 3,254,867 Patented June 7, 1966 ice and elongation within predictable limits, as a result of such heat treatment.

Elongation of the steel during heat treatment may be calculated. However, in commercial heat treating practice the elongation is not always uniform and consistent, which results in tools which are out of tolerance. Out of 'roundness is undesirable in form relieved cutters, such as unground hobs, and must therefore be held to a minimum value. Tools and other parts made from steep tapered octagonal ingots produced in accordance with the teachin-gs of the present invention have been found to be superior in holding concentricity during heat treatment over those produced from ingots produced in the conventional square ingot mold.

An object of the invention is to provide improved ingot molds for producing steel having improved dimensional and out of roundness control over that produced in con-. Ventional square and rectangular molds.

Another object of the invention is to provide a non square fluted ingot mold for casting high alloy tool steels which insures minimum carbide and sulphide segregation, with improved soundness and freedom from adverse solidification effects.

Another object of the invention is to provide a fluted, big-endup steep tapered ingot mold for casting high alloy tool steels, including high speed and high-carbomhighchromium tool steels characterized by a high degree of uniformity and minimum distortion after heat treating.

A still further object of the invention is to provide a fluted, big-end-up mold for casting high alloy tool steels wherein the mold cavity side wall taper, cavity wall thickness and length of fluted cavity wall bear a proportional relation which insures the improved results claimed herein.

These and other objects of the invention will be made apparent from the following description and the drawing forming a part thereof, wherein:

FIG. 1 shows a side elevation of a fluted ingot mold of the invention;

FIG. 2 shows a plan view of the top of the fluted mold of FIG. 1;

FIG. 3 shows a horizontal section taken on lines III-- III of FIG. 1; and

FIGS. 4A and 4B show-diagrammatically a comparison of solidification of metal in a conventional square mold and a mold of the invention.

Referring nowin detail to the drawing, particularly FIGS. 1 to 3 inclusive, the big-end-up ingot mold indicated generally as 1 is provided with a preferably octagonally shaped mold cavity 2, each face 3 thereof being convex and preferably tapering uniformly from the top of the cavity to adjacent the bottom of the mold cavity. The bottom of the cavity, below the line III-III, termimates in a suitable arcuate shape. The exterior surface 4 of the mold is preferably circular and tapers from top to bottom thereof. It will be noted, as hereinafter discussed, that the wall thickness of the octagonal mold cavity is greater at the bottom than at the top thereof. Suitable conventional lifting lugs 5 may be provided at both sides of the mold.

For better identification of the proportional dimensioning of the mold surfaces of FIGS. 1 to 3, as hereinafter referred to, D, indicates the minor diameter of the top of the polygonal mold cavity, D indicates the minor diameter of the bottom of the polygonal mold cavity; MD, indicates the major diameter of the top of the mold cavity, MD, the major diameter of the bottom of the mold cavity at line A-A, T indicates mold cavity taper, RF indicates the radius of the cavity flutes at the top of the cavity, and RE, the radius of the cavity flutes at the bottom of the cavity.

Since the principal purpose of the mold of the invention is to control the solidification of the ingot metal throughout the length thereof, it has been found desirable to proportion the critical dimensions of the mold cavity in relation to the fluted face dimensions thereof, the thickness and the taper in the cavity side wall. We have found that optimum results are obtained when the octagonal mold cavity walls have a taper of 1.30 to 1.60 inches per foot, the ratio of the top area of the ingot to the top area of the mold is maintainedbetween 1.05 and 1.35, with the ratio of the bottom area of the fluted ingot to the adjacent area of the mol-d maintained between 1.80 and 2.25. Additionally, we have found that a ratio between length of fluted mold cavity to diameter at the top of the cavity of 1.75 to 2.20 is also critical.

Although the primary object of the invention is to provide ingot molds effecting controlled solidification of high alloy tool and specialty steels for obtaining improved dimensional control and minimum out of roundness in tools produced therefrom, We are not limiting our invention solely to octagonal fluted mold cavity. That is, the results referred to herein as improved dimensional control and minimum out of roundness, with minimum carbide and sulphide segregation are also obtainable with similarly fluted polygonal mold cavities containing six to ten sides, employing the critical dimensional relationship hereinbefore referred to.

As previously discussed, solidification of molten metal in an ingot mold occurs in three different zones which comprise the cross-section of the ingot, namely chill, columnar crystals and equiaxed. Within the chill zone solidification proceeds as relatively small randomly oriented crystals. The fluted polygonalmolds of the invention provide greater surface area in the mold cavity and consequently a deeper penetration of the chill zone. Inwardly of the chill zone the solidifying metal customarily forms more slowly into elongated columnar crystals and in a somewhat dendritic pattern. Our design of polygonal mold cavity presenting a multiplicity of angularly disposed cavity faces creates interference between the adjacent forming columnar crystals impeding both their longitudinal growth and dendritic pattern. Additionally, the increased surface area of the cavity accelerates dissipation of heat from the ingot, further impeding growth of the columnar crystals, and increases the area of the ingot central region comprised of equiaxed crystals of random orientation. The steep taper of the walls of the mold cavity coupled with mold wall thickness tends to accelerate vertically upward solid-ification and impedes bridging over as above referred to.

Reference is now made to FIGS. 4A and 4B graphically comparing progress of metal solidification in a conventional square or rectangular mold cavity with the polygonal mold cavity of the invention. In the respective FIGS. 4A and 4B, the relative sizes of the columnar crystal outer zones to the equiaxed randomly oriented crystals of the central zones in ingots of M2 high speed steel cast in the said conventional molds and the polygonal molds of the invention, under normal conditions of tapping, are shown. It is recognized that the columnar crystals of a dendritic type of solidification are harmful from the standpoints of segregation and directional effects.

In FIG. 4A the columnar crystals or grains are indicated by lines generally perpendicular to the ingot outer walls and the zone of equiaxed randomly oriented crystals or grains are defined by a dotted area. Centerline a-a through the ingot shows the depth of columnar crystals as 1-2 and 3-4 along line aa, with the extent of equiaxed crystals indicated as 2-3. The ratio of columnar zone depth to equiaxed zone depth, as determined from AISI grade M2 high-speed steel ingots from conventional square molds is 1-2 or between 1 and 2.

In FIG. 4B, the polygonal mold cavity providing greater interference between the forming columnar crystals results in relatively shallow zones of columnar crystals (5-6) and 7-8) measured on centerline bb and a larger cross-sectional area 6-7 of equiaxed randomly oriented crystals. The ratio of columnar zone depth (5-6) +(7-8) to the equiaxed zone (6-7) is substantially less than 1, as determined from AISI grade M2 high speed steel ingots cast in the molds of the invention.

A practical example of the advantages of commercially produced AISI grade M2 high speed tool steel ingots exemplified in FIGS. 4A and 4B and converted into tools to be heat treated as found by the following comparison:

(1) Three inch diameter hobs made from AISI M2 high speed steel produced from conventional square ingots illustrated by FIG. 4A exhibited an out of roundness of 0.0025 inch when heat treated.

(2) Three inch diameter hobs made from AISI M2 high speed steel produced from octagonal ingots illustrated by FIG. 4B exhibited an out of roundness of 0.0004 inch when heat treated.

(3) While it is customary to allow 0.002 of an inch per inch of length for elongation on AISI M2 high speed steel, We find that with conventional square ingots this will actually vary between 0.0015 to 0.0025 inch per inch of length.

(4) AISI M2 high speed steel produced from octagonal ingots made in accordance with the teachings of this invention have an actual elongation on heat treating of between 0.0018 to 0.0022 inch per inch of length.

EXAMPLE I Since optimum results have been so far obtained with fluted octagonal mold cavities We submit herewith, what appears at the present time to .provide satisfactory ratios and range of ratios of the critical dimensions of octagonal shaped mold cavities embodying the principles of our invention.

Preferred Limits D Minor Diameter Top of Mold Cavity.

L Length of Ingot to Radius at 1.97 D 1. /2. 15 D Bottom of Ingot.

Db Minor Diameter Bottom of 0.76 D 0. 65/0. D

Mold Cavity.

T* Mold Taper, Inches Per Foot. 1. 49 1. 30/1. 60

L/D* Ratio Length/Minor Diameter 1. 97 1. 75/2. 20

Top of Cavity.

MD: Major Diameter Top of Mold 1.12 D 1. 05/1. 20 D am y.

MDi, Major Diameter Bottom of 0. 86 D 0. 70/1. 00 D Mold Cavity.

WTRL* Wall Thickness Ratio Top 1. 20 1.05/1. 35

Area Ingot/Area Mold.

WTRb*"- Wall Thickness Ratio Bottom 2. 12 1. 80/2. 2

Area Ingot/Area Mold.

ODt Outside Diameter Mold Top 1 54 D 1. 40/1. 70 D ODb Outside Diameter Mold 1 41 D 1. 30/1. 65 D t a lus u'es, op D 0.701.201) RFM Radius Flutes, Bottom 0 73 D 0. 60/0. D OLC Overall Length Mold Cavity.. 2 20 D 2. 00/2. 35 D *Critieal mold dimensions.

EXAMPLE II An octagonal shaped mold cavity provided in an ingot mold such as disclosed in FIG. 2 has been used in commercial production and has consistently produced high alloy tool steels which, when embodied in heat treated tools, provided unusually satisfactory results. The mold to these dimensions provided an ingot weighing 1,206 pounds, based upon 0.26 pound per cubic inch.

D 16% in. L 32 in.

D in. T 1.49 irL/ft.

L/D 1.97. MD 18% in MD 14 m WTR WTR 212 D 25% in. OD in. RF, 16 /2 in. RF 11% in. OLC 36 in.

A 3 inch diameter sulphurize'd M2 high speed steel hob made from steel cast in the mold of Example II when austenitized at 2200 F. and double tempered at 1025 F. to a hardness of Rockwell C65 exhibited only 0.0004 inch of out of roundness. Identical tools from the same steel cast in conventional square molds exhibited an out of roundness of 0.0026 inch.

EXAMPLE III The ingot mold whose dimensions appear below, made in accordance with the invention, provided an octagonal ingot weighing 2,500 lbs. at .26 lb./ cu. in.

D 19% in.

L 37 in. D 14 /2 in. T 1.53 in./ft. L/D 1.92. MD, 21 /2 in. MD, 16 /2 in. it?

b 1 l OD, 29 /2 in. 0D,, 27 /2 in. RF, 18% in. RF in. OLC 41 in.

EXAMPLE IV A smaller ingot mold whose dimensions are in accordance with the invention provide-d an octagonal ingot weight of 940 lbs. at .26 lb./ cu. in.

EXAMPLE V Ingots weighing 1,650 lbs. and comprised of AISI D2, high carbon-high chromium tool steel, were cast in a mold of the present invention having a mold cavity side wall taper of 1.5 inches per foot and a conventional square type mold having a mold cavity side wall taper of 0.90 inch per foot. Ingots from each type mold were processed into 5 /2 in. and 3% round bars, then checked for quality by standard reflectoscope testing and by standard procedure of hot acid etching discs from both ends of each of the bars. These etched discs were then rated for porosity and unsoundness. As a result of these tests it was established that steep tapered octagonal ingots from molds of the invention provided 16.3% improved yield over steep tapered ingots from otherwise conventional square mold.

The foregoing examples have been based upon an octagonal or eight fluted side mold cavity. It will be understood, however, that the invention is applicable to polygonal ingots of 6 to 10 sides. Although the elongation and out of round characteristics of heat treated tools formed from ingots produced in the hereinbefore discussed molds, have been emphasized, the carbide distribution and distribution of sulphides in free machining tool steels is also greatly improved when cast in polygonal ingot molds of the invention. All type steels cast in molds of the invention provide minimum carbide and chemistry segregation and have enhanced soundness and are free from adverse solidification effects.

As an example of lack of uniformity in conventionally processed steels, it is common to find a distinct ingot pattern after hot acid etching of a cross-sectional specimen, as illustrated at page 415 of Metals Handbook, 1948 edition, published by the American Society for Metals. Ingotism, or the presence of a distinct ing ot pattern, has not been observed by us in a large number of etched specimens prepared from tool steels cast in molds of the invention.

We claim:

1.'A steep tapered fluted octagonal ingot mold characterized by producing steel with minimized out of roundness and freedom from segregation and having the following dimensional relationship limits:

2. A steep tapered fluted octagonal cavity ingot mold having a length to minor diameter ratio of 1.75 to 2.20 and a taper of about 1.3 to 1.6 inches per foot.

3. A steep tapered fluted octagonal ingot mold having the following mold dimension relationships:

T 1.30/1.60. L/D 1.15/220. wTR, 1.05/1.35. WTR 1.80/2.25. RF, 0.70/ D. RF 0.60/095 D.

4. A steep tapered fluted octagonal cavity ingot mold characterized by producing steel with a minimum of segregation, said mold having a cavity wall taper of about 1.5 inches per foot from top to bottom of the fluted cavity and a wall thickness to cavity ratio at its top of about 1.20 and at its bottom of about 2.12.

5. A steep tapered fluted polygonal cavity ingot mold provided with six to ten convex flutes, said ingot mold having a ratio of fluted cavity length to minor diameter at top of cavity of about 2, a cavity taper of about 1.5 inches per foot and a cavity wall thickness to cavity area ratio at its top of about 1.20 and at its bottom of about 2.12.

6. A steep tapered fluted polygonal cavity ingot mold provided with six to ten convex flutes, said ingot mold having a flute taper of about 1.30 to 1.60 inches per foot of length, a ratio of fluted cavity length to minor diameter at top of cavity of 1.75 to 2.20, a mold wall thickness to area of cavity ratio at its top of about 1.05 to 1.35 and at its bottom of about 1.80 to 2.25.

References Cited by the Examiner UNITED STATES PATENTS 1,892,569 12/1932 Gathmann 22-139 2,092,551 9/ 1937 Gathmann 22-139 MARCUS U. LYONS, Primary Examiner.

Claims (1)

1. A STEP TAPERED FLUTED OCTAGONAL INGOT MOLD CHARACTERIZED BY PRODUCING STEEL WITH MINIMIZED OUT OF ROUNDNESS AND FREEDOM FROM SEGREGATION AND HAVING THE FOLLOWING DIMENSIONAL RELATIONSHIP LIMITS: L 1.80/2.15 D. DB 0.65/0.85 D. T 1.30/1.60 IN./FT. L/D 1.75/2.20. MDT 0.95/1.20 D. MDB 0.70/1.00 D. WTRT 1.05/1.35. WTRB 1.80/2.25. ODT 1.40/1.70. ODB 1.30/1.65 D. RFT 0.70/1.20 D. RFB 0.60/0.95 D. OLC 2.00/2.35 D.

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