US3589695A

US3589695A - Device for the absorption of the expansion of a charge in a retort used in a nitriding process

Info

Publication number: US3589695A
Application number: US755219A
Authority: US
Inventors: Arne Ivar Backstrom
Original assignee: Avesta Jernverks AB
Current assignee: Outokumpu Stainless AB
Priority date: 1967-09-11
Filing date: 1968-08-26
Publication date: 1971-06-29
Anticipated expiration: 1988-06-29
Also published as: DE1796153B1; GB1242424A; SE320655B; FR1581305A; NO122239B

Abstract

A nitriding retort containing a means which forms pockets to absorb expansion of the nitrided material during the nitriding process.

Description

during the nitriding process.

SIWWWWIS United States Patent [1113,589,695

, [72] Inventor Arne Ivar Backslrom [56] R fe Cit d I N gfg UNITED STATES PATENTS f 26 1968 799,696 9/1905 Ward 266/20X [45] Patented June' 1971 3,116,842 1/1964 Mohr... 266/27 X Assignee Aves Jemverks Akfiebolag 3,252,601 /1966 isaacs 266/27 X Avesm Sweden FOREIGN PATENTS [32] Priority Sept. 11, 1967 1,068,021 /1959 Germany 266/ [33] Sweden 779,871 7/1957 Great Britain.. 266/20 [31] 12495/67 204,065 5/1966 Sweden 266/2 Primary Examiner-J. Spencer Overholser [54] DEVICE FOR THE ABSORPTION OF THE Assistant Examiner-R. Spencer Annear EXPANSION OF A CHARGE IN A RETORT USED IN Attorney-Brumbaugh, Graves, Donohue & Raymond A NITRIDING PROCESS 8 Claims, 8 Drawing Figs.

[52] US. Cl 266/2 R [51] C23cll/l4 ABSTRACT: A nitriding retort containing a means which Field of Search 266/2, 2A, forms pockets to absorb expansion of the nitrided material DEVICE FOR THE ABSORPTION OF THE EXPANSION OF A CHARGE IN A RETORT USED IN A NITRIIDING PROCESS Nitrogen is included as an alloying element in types of steel possessing certain qualities, i.e. in stainless, austenitic steel. In steel production the nitrogen is added in a combined form as metal nitride, in which the metal may consists of manganese, ferromanganese, chromium or ferrochromium.

Nitriding the metal is carried out in the solid phase, nor mally at 900 to 1,000 C. The nitrogen supply source is generally nitrogen gas. Since the nitrogen diffuses into the metal extremely slowly, the latter must present a large surface area if the time the treatment takesis not to be unreasonably long. The metal is therefore treated in the shape of flakes of a thickness of about I to 2 mm., for instance manganese metal produced electrolytically or in the shape of a crushed product produced with a grain size less than mm., for instance crushed ferrochromium, manganese or ferromanganese produced pyrometallurgically.

Suitable equipment for the nitriding operation is a retort of heat-resistant steel which during the treatment is placed vertically in an electric pit furnace. The retort should preferably have a circular cross section, and it should be somewhat coneshaped, in order to facilitate the discharging of the material.

In the nitration process the volume of the charge increases by about 5 to percent, the charge will thus exert a considerable pressure on the wall of the retort. Since the strength of the retort material is rather poor at 900 to 1,000 C. the

result will imply that the retort is deformed and/or that cracks appear in it.

The deformation appears as a bulge which is often largest at about half the height of the retort and which may be so extensive that the retort has to be discarded after a comparatively short time of operation. The risk of cracking increases in proportion to the heating cycles performed, the welds are especially delicate areas.

The object of the present invention is to prevent the charge in the course of the volume increase in connection with the nitriding from exerting such a pressure on the wall of the retort that the retort is deformed or that cracks appear in it.

The present invention is characterized in that the retort is provided with a number of discs preferably orientated centrally, horizontally and in a vertical pile. When the retort is charged vacant spaces develop under the discs. In the nitriding process these vacant spaces are entirely or partly filled by the expanding material. In this case the discs may suitably be arranged at a distance from each other by means of spacer members. According to a preferred embodiment of the present invention the spacer members consist of centrally arranged sleeves, preferably welded onto one disc each, the discs being provided with holes so that the discs and sleeves may be threaded on a central removable stand attached to the retort.

The present invention will be described in the following, reference being made to the attached drawings where FIG. 1 shows a vertical cross section of a retort with an expansion absorber according to the present invention,

FIG. 2a schematically shows the above absorber during charging of the retort,

FIG. 2b schematically shows part of the retort after the charging.

FIG. 20 schematically shows part of the retort during heat treatment,

FIG. 2d schematically shows part of the retort after completed heat treatment,

FIG. 3 shows a disc with an attached spacer member, partly in section,

FIG. 4 shows a view from above of the disc in FIG. 3, and

FIG. 5 shows another embodiment of the disc, partly in section.

FIG. 1 shows retort l with a circular section. The retort has a circular-cylindrical body 2, a cone-shaped lower part 3 and a rounded bottom part 4. In operation the retort is covered by lid 5 which, via flanges 6 on body 2, may be fastened to the re tort body.

An expansion absorber 7 has been placed centrally in the retort. The absorber consists of a vertical metal stand 8 provided with a number of rounded metal discs 9 horizontally placed and with their centers coinciding with the vertical stand. A device should be provided with one or several supporting devices which maintain expansion absorber 7 in its vertical, central position. At the top the stand ends in a loop for hoisting expansion absorber 7 and the nitrated cake after the final reaction. For the supply of nitrogen gas there are holes (not shown) in the bottom.

The way in which metal material 10 which is to be nitrided is fed into the retort is illustrated in FIG. 2a. The material flows into the retort down vertically, then it is symmetrically distributed in the retort. Under each circular disc 9 vacant space 11 is formed in the shape of an inverted cone or a frustum of a cone whose top rake angle is minus twice the angle of repose a of the metal material, FIG. 2b.

When the retort is heated to reaction temperature the metal starts to react with the nitrogen atmosphere forming metal nitride or nitrides. In the process of expansion of the material which is then starting, the material is pressed or expands radially in a direction towards the center shaft of the retort. The vacant space 11 under discs 9 are then reduced according to FIG. 2c.

After completed reaction and expansion vacant spaces 11 under discs 9 have been practically filled by the expanding material according to FIG. d, provided that the total volume of the vacant spaces corresponds to or somewhat exceeds the expansion of the charge expressed in volume units.

FIGS. 3, 4 and 5 show examples of the way in which a disc and a spacer member may be designed in practice. According to FIGS. 3 and 4, circular disc 12 is plane but provided with edge bevelling on the upper surface. Onto the lower surface of the disc guide sleeve 14 has been welded, which serves as a spacer member for keeping the desired distance to the underlying disc not shown, and three stiffening ribs IS. The expansion absorber consists of a number of such disc threaded onto central stand 16, which may, for instance, consist of a pipe or a round bar. The wall of the retort is marked 17.

FIG. 5 shows a construction where disc I8 has been shaped into an obtuse-angled cone with a top rake angle chosen so that an assumed line drawn from central stand 19 and in the plane of the disc towards wall 20 of the retort forms a right angle to the latter in the vertical plane. As to the rest, this embodiment corresponds with the one shown in FIGS. 3 and 4.

The present invention is not restricted exclusively to the constructive embodiments described in greater detailabove.

For instance, the discs need not necessarily be of a circular shape. Square, pentagon or hexagon discs, etc. may very well be used even if stereometrically the circle is the logical shape.

I claim:

1. A retort for nitriding metal particles at elevated temperatures comprising a vertically elongate container for retaining the metal particles therein in a substantially stationary mass during nitriding, means for reducing forces to the container by accommodating expansion of the mass of metal particles therein, the force reducing means including a plurality of means for forming pockets within the mass of particles to permit inward expansion of the mass, the pocket forming means including a plurality of members spaced-apart, positioned to interrupt the flow of particles into the container during filling to produce said pockets beneath each of the members, and supported throughout the fill depth to form an axially extending series of pockets throughout the particulate mass.

2. The retort according to claim ll, wherein the spaced members are a plurality of discs supported on a shaft, the shaft extends axially and supports the disc throughout the fill depth of the retort, and the shaft and discs are removable from within the retort following nitriding to facilitate removal of the nitrided mass of particles.

comprise sleeves attached perpendicularly and centrally between the discs such that a through hole is formed for the insertion of a supporting stand which may be attached to the retort.

8. A retort according to claim 7 wherein the discs and sleeves may be positioned on the stand by means of threads cut into the exterior surface of the stand and corresponding threads cut into the interior surface of the sleeves and the interior surface of the holes provided in the discs.

Claims

2. The retort according to claim 1, wherein the spaced members are a plurality of discs supported on a shaft, the shaft extends axially and supports the disc throughout the fill depth of the retort, and the shaft and discs are removable from within the retort following nitriding to facilitate removal of the nitrided mass of particles.
3. A retort according to claim 2, wherein said discs are essentially circular in shape.
4. A retort according to claim 2 wherein said container has an essentially circular horizontal cross section.
5. A retort according to claim 4 wherein said container is essentially cone-shaped with a rounded base.
6. A retort according to claim 2 wherein the horizontal discs are separated from each other by means of spacer members.
7. A retort according to claim 6 wherein the discs are provided with a centrally located hole and said spacer members comprise sleeves attached perpendicularly and centrally between the discs such that a through hole is formed for the insertion of a supporting stand which may be attached to the retort.
8. A retort according to claim 7 wherein the discs and sleeves may be positioned on the stand by means of threads cut into the exterior surface of the stand and corresponding threads cut into the interior surface of the sleeves and the interior surface of the holes provided in the discs.