US3727290A - Method for processing hot-rolled metal bodies and the like - Google Patents

Abstract

A hot-rolled metal body, e.g. a bar, rod, billet, bloom, rail or wire, emerges from between the last rolls of the rolling-mill train in a hot condition and is subjected to a high-pressure water jet to remove all traces of scale and prepare the surface of the body for receiving an enamel coating without substantial cooling of the body. The workpiece is then coated with a powder fusing at a temperature at or below the temperature of the body to form a liquid layer or film which solidifies upon cooling to provide the enamel coating. Since the enamel coating ruptures, cracks, ''''stars'''' or otherwise breaks in the region of surface flaws of the body, the coating serves to indicate the presence of such flaws while protecting the body against oxidation or scaling as the body cools.

Description

United States Patent [191 Schaumburg METHOD FOR PROCESSING HOT- ROLLED METAL BODIES AND THE LIKE [76] Inventor: Georges Schaumburg, 1 bis rue de Londres, Montigny-Les-Metz,

France [22] Filed: Mar. 2, 1971 [21] Appl.No.: 120,084

Related US. Application Data [63] Continuation-impart of Ser. No. 835,359, June 23,

1969, Pat. No. 3,648,349.

[30] Foreign Application Priority Data Nov. 14, 1970 Germany ..P 20 56 135.7

[56] References Cited UNITED STATES PATENTS 8/1914 Chartener ..29/81 1451 Apr. 17, 1973 Primary Examiner-Charles W. Lanham Assistant Examiner-V. A. Dipalma Attorney-Karl F. Ross 57 ABSTRACT A hot-rolled metal body, e.g. a bar, rod, billet, bloom, rail or wire, emerges from between the last rolls of the rolling-mill train in a hot condition and is subjected to a high-pressure water jet to remove all traces of scale and prepare the surface of the body for receiving an enamel coating without substantial cooling of the body. The workpiece is then coated with a powder fusing at a temperature at or below the temperature of the body to form a liquid layer or film which solidifies upon cooling to provide the enamel coating. Since the enamel coating ruptures, cracksfstars or otherwise breaks in the region of surface flaws of the body, the coating serves to indicate the presence of such flaws while protecting the body against oxidation or scaling as the body cools.

1 1 Claims, 7 Drawing Figures PAIENTEUAPR H915 v 3, 727, 290

v SHEET 1 [1F 4 INVENTORI Georges Schaumb urg r\ (Ross ATTORNEY PATENTED APR 1 71073 mm 2 OF 4 I v iada Jut zu b ur Eb W9 GUN Georges Schaumburg INVENTOR.

ATTORNEY PAI T APR 1 7 m3 SHEET 3 BF 4 FIG HIGH PRESSUKE NATE/2 JET IFIG.5

Georges Schaumburg mvwvrom PATENTED 3.721290 SHEET U UF 4 reducing qua Georges Schau urg INVENI BY gap.

A TT'UR NI'IY METHOD FOR PROCESSING HOT-ROLLED METAL BODIES AND THE LIKE CROSS-REFERENCE TO COPENDING APPLICATION The instantapplication is a continuation-in-part of application Ser. No. 835,359, filed 23 June 1969 and entitled METHOD OF AND APPARATUS FOR TREATING A HOT-ROLLED METAL BODY AND THE LIKE, now US. Pat. No. 3,648,349.

FIELD OF THE INVENTION My present invention relates to the production of rolled metal bodies and, more particularly, to the treatment of hot-rolled metal bodies. In addition, the invention relates to a method of and a system for detecting surface flaws in hot, preferably hot-rolled, metal bodies.

BACKGROUND OF THE INVENTION The metallurgical art provides numerous metalforming processes among the most significant of which is, of course, the hot-rolling technique. In the hotrolling of a metal body, a heated ingot, billet, bloom or slab, is passed between a pair of rolls to elongate the workpiece and flatten or shape the latter. In general, the workpiece is heated in a soaking pit or in the furnace to a temperature just below its melting point or is rolled in the hot state at maintenance for some time after casting. A significant problem in the processing of heated workpieces is the formation of scale or crust upon the body. The scale may derive from the presence of internal contaminants but generally is a result of the interaction of the surface metal with ambient oxygen. Such crust or scale is relatively hard, is only partly sloughed during rolling and in many cases must be removed by mechanical, chemical or other means to avoid damage to the rolling-mill equipment or to prevent the formation of products which are unsightly or mechanically unsatisfactory.

As the body leaves the last pair of rolls in the rolling train, i.e. the succession of roll stands through which the workpiece is passed for stepwise reduction in its cross-section and corresponding increases of its length, it is hot but relatively scale-free as a result of the aforementioned mechanical compression which effectively descales the body to an extent which suffices for most purposes. However, this scale-free state is only transient since the body immediately begins to develop scale at its high temperature in the ambient oxygencontaining atmosphere.

The scale-formation is, of course, detrimental because it roughens the interior of the workpiece and otherwise may negatively affect the metallurgical process. In addition and, possibly of greater significance, is the fact that the scale may obscure or hide any surface flaws on the workpiece, thereby precluding scale is also abrasive and hard so that handling of scalecovered workpieces damages rolling-mill equipment and may cause breakage until descaling is complete or some other means is taken to prevent the scale from coming into contact with sensitive equipment.

OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved method of treating rolling-mill workpieces to avoid the aforementioned disadvantages and simultaneously extend the principles originally set out in my application Ser. No. 835,359.

It is a corollary object of the invention to provide an apparatus for carrying out the method of the instant invention and for efficiently treating hot-rolled metal bodies.

Still another object of the invention is to provide an improved method of and apparatus for treating a hotformed metal body to eliminate the adverse effects of scale formation both upon the body and upon the plant.

A more specific object is to provide a method and apparatus for the purposes described which can overcome the above-mentioned difficulties particular to hot-rolled metal bodies.

It is another object of this invention to provide an improved method of detecting surface flaws in hotformed bodies and especially hot-rolled steel workpieces.

Still a further object of the invention is to provide a method of protecting hot-rolled steel bodies against scaling and, therefore, improve markedly the quality of hot-rolled steel obtainable from a rolling mill.

SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter, are attained in accordance with the invention, in a method of treating a hotrolled metal body, preferably a hot-rolled steel workpiece, as it emerges in a hot, solid and relatively scale-free condition from a hot-forming stage, specifically a rolling mill. As described in the aforementioned application, the invention involves passing the workpiece directly into a treatment chamber, i.e. without an intervening sojuorn in an oxidizing atmosphere, a powder being applied to the surface of the body in this chamber to produce an enamel-like film. The powder consists of a mixture of reducing agents (to remove any slight scale whilch may remain by chemical action) and glass-forming substances which fuse at the temperature of the body.

According to the invention, this powder is blown against the exterior of the body as it passes through the chamber, whereupon the powder fuses to form a liquid film around the body which is both thin and continuous; on cooling of the body, the film solidifies to form the glassy enamel-like coating which ruptures in the region of surface flaws on the body, thereby enabling their observation. The enamel-like coating also seals the surface of the body from the atmosphere to prevent any further scaling.

In accordance with another aspect of the invention, the method involves the steps of hot-forming a steel body (hot-rolling) prior to any substantial formation of scale thereon or after an initial descaling treatment of the hot slightly scaled body with a reducing agent. It has already been noted that the enamel-like film should be thin and substantially translucent or transparent. When the coating hardens, it not only seals the surface against chemical attack and further scale formation, but itself is capable of indicating the presence of surface flaws as noted below. These flaws, not readily discernible otherwise, can be eliminated by conventional techniques.

The treatment chamber, according to still another aspect of the invention, is fitted with at least one powder-spray or powder-dispensing nozzle which is connected through at least one blower to a hopper containing the powder described above. This noule is formed with a venturi restriction and a pipe communciating with the source of compressed air. This pipe, which may also be sprayed with the reducing gas or an inert gas, serves to accelerate the flow of fluids through the chamber and projects the powder out of the nozzle with greater force.

The powder can be a mixture of oxides of silicon, magnesium, aluminum, calcium and iron along with metallic sodium, boron or potassium, or a compound thereof. The use of these elemental metals has, of course, the advantage that they act as reducing agents and will not only make a contribution to the formation of the enamel or glass layer, but will act to strip any residual scale from the surface of the workpiece. Other compounds have been found to be satisfactory and compounds such as zinc borate are most satisfactory in that the breakdown of the zin compound yields a zinc coating on the metal body. Hence the glassforming substance, in this case, consists of a material adapted to leave a residue of a permanent coating substance. The coated and cooled body can, of course, be stripped of its temporary glassy coating, for instance in a straghtening station, and may subsequently be recoated for greater permanence by spraying with paint, lacquer, synthetic-resin protective coatings or the like. The method is quite advantageous in that the glassy coating may be removed quite easily, i.e. more rapidly and conveniently than the customary scale, so that the entire body can be completely processed in one continuous operation.

Prior to the application of the enamel coating, it is possible to treat the body with a reducing agent such as hydrogen or methane to reduce any slight scale which may have developed thereon upon emergence of the workpiece from the rolling mill. Of course, where reduction is carried out by the reaction of a gas-phase reducing agent with the solid phase, there is no need to mix reducing agents, such as the powdered reducing metals, with the glass-forming powders.

In an alternative embodiment of the invention, the' body (which can be a continuous rod, wire, sheet or the like), is simply passed through a fluidized bed of the above-described metallic and metal-oxide powders to coat the workpiece. In any case, the coating must be thin enough to rupture in the region of any flaws or to allow the flaws to be seen through the coating while being tough enough not to flake off immediately as the workpiece is transported.

Thus, a key feature of the present invention is the formation of a glass-like coating upon the surface of a hot-formed body while it is still in the heated condition and susceptible to scaling, of a relatively thin and brittle character upon cooling. This enamel layer, which may be composed of any conventional enameling composition, dispensed in the liquid or solid state and merely solidifiable on the body and/or fusible thereon utilizing the heat of the body, surprisingly is capable of providng a visible indication of surface flaws or defects in the body. While the mechanism of this system is not fully understood, it would appear that the enamel film ruptures, fractures or breaks in the presence of surface flaws, e.g. pits, striations, projections, contamination areas, cracking and similar defects, either as a consequence of thermal stress during cooling, or mechanical movement of various portions of the defect area, etc. or as a result of an inability to adhere effectively to these regions, thereby producing a fracture zone in the region of the defects. This marking or indication of the defects permits the defective material to be removed or otherwise monitored to ensure that the finished product will be free from the flaws. The enamel layer does not remain as tenaciously upon the body as would a slag layer or other scaleforming material, but rather can be broken away with ease as will be apparent hereinafter.

While reference has been made above to enamel and glass-like material as capable of fulfilling the requirements of the present invention, it may be noted that numerous enamel compositions which are capable of melting at a wide range of temperatures including the temperatures at which the metal (steel) body emerges from the hot-forming stage, are known and may be used in accordance with the present invention as long as the material is applied to the body with a vicosity sufficiently low to enable the liquid material to flow into a uniform thin film coating the body. This film, which may have a thickness of the order of microns, e. g. 5-10 microns, may either be applied in the form of a liquid coating or glass or may be applied in a powder stream, e.g. designed to fuse or flow along the surface at the temperature of the body, but in either case will be a glassy substance.

When powders are used, it is preferred to direct them against the body in high-velocity aerosol-type jets, the velocity being controlled by compressed air. At sufficiently high velocity, the powder effectively penetrates the surface skin of the metal, which can be assumed to be substantially free from scale, and is rapidly heated by its intimate contact with the metal to the melting point of the powder and thereabove, whereby the molten droplets coalesce to form a liquid glass film upon the surface of the metal. Preferably, this treatment is carried out in a tubular chamber enclosing the body and open at its ends to permit the body to pass through the chamber. The latter is provided with a configuration geometrically similar to that of the body, i.e. a chamber of circular cross-section when the body is a continuously cast rod of circular cross-section.

As noted earlier, reducing agents may be used in the treatment process to eliminate any scale which may have been formed on the body prior to coating with the enamel layer. Thus, the body may be treated with a reducing gas or reducing agents, e. g. sodium, potassium and like reducing metals may be employed, the reduction products and the oxidized metal compounds resulting from the reducing reaction being incorporated in the subsequently formed enamel coating. In this respect, it may be observed that scale layers, when composed of the slag former present in metallurgical systems are of a glassy nature and are soluble in the glass-like enamel coating applied in accordance with the present invention. I

In applying the powders, preferably in the form of a powder mixture of the character set forth above, it has been found to be advantageous to provide a tandem array of blowers, the first receiving a mixture of powders and air and constituting a loosening means whereby the powders are dispersed in the air stream to form a flowable mixture. In the second or downstream blower, the powder/air mixture is accelerated to a high velocity and propelled to a discharge nozzle trained upon the hot-metal surface. Between this nozzle and the second blower, there may be provided a venturi injector to which compressed air is fed to regulate the velocity of the powder/air mixture directed against the workpiece surface.

I have now found, further, that the workpiece can be prepared to accept the enamel coating to a greater extent and can be treated effectively without interruption of the coating process and the rolling operation if a jet of high-pressure water is directed (trained) upon the hot workpiece surface, preferably perpendicularly thereto. Surprisingly, the high-pressure jet, which is ap plied immediately upstream of the treatment chamber and the application of the enameling coating, operates to remove all traces of scale and prepares, in ways which are not completely clear, the surface of the workpiece to better receive the enameling layer without substantial cooling of the workpiece and without scale formation resulting from the fact that water is considered an oxidizing agent in some metallurgical processes.

The water-jet system may be used in place of the mechanical or chemical descaling mentioned earlier or in addition thereto. More specifically, it has been found that all traces of scale, oxide coatings and the like are removed, surface defects or flaws rendered more visible and the enamel coating applied with greater uniformity when the workpiece is subjected to a highpressure water jet from one or more nozzles trained perpendicularly to the workpiece surface, the water having a pressure at the nozzle of at least 100 atmospheres gauge and preferably of about 200 atmospheres gauge. The volume rate of flow of the water is in excess of 200 liter/minute, preferably about 400 liter/minute, and the nozzle aperture is circular with a diameter of 0.2 to 3mm, preferably about 2.1mm. Another critical parameter is the water velocity which should range from to I00 m/second and preferably is about 80 meter/second at contact with the workpiece.

The highpressure water not only mechanically breaks up and removes the scale but also appears to etch or otherwise modify the surface so that the effects become more visible upon application of the enamel coating of glass-forming or slag-forming materials. The surface of the workpiece, as it enters the treatment chamber, is totally free from scale and is particularly receptive to the enameling film. Furthermore, no substantial cooling takes place and the powder may be applied without concern that the temperature of the workpiece has been lowered. Drying is substantially instantaneous so that, in this respect, the powder is ap plied to a clean dry surface.

DESCRIPTION or THE DRAWING The above and other objects, features and ad vantages will become more apparent from the following description, reference being made to the accompanying drawing, in which:

FIG. 1 is a perspective view of a rolling mill embodying the principles of the present invention;

FIG. 2 is a vertical section through the rolling mill of FIG. 1;

FIG. 3 is a perspective view of a continuous casting installation embodying the principles of the present invention;

FIG. 4 is a vertical section through the installation of FIG. 3;

FIG. 5 is a vertical section through another apparatus embodying the present invention; and

FIGS. 6 and 7 are sections through a metal body at two separate stages in its treatment according to the present invention.

SPECIFIC DESCRIPTION As seen in FIGS. 1 and 2, a slab or sheet 3 is formed between two rolls 23 and 23 of a last rolling stage 1 of a rolling train. This slab 3 emerges from between these rolls 23 and 23 in a very hot, solid and relatively scalefree condition. It is transported by driven rollers 2 through a chamber 22 where it is subjected to the reduction effects of a reducing gas, e.g. hydrogen, carbon monoxide, methane or mixtures thereof, to remove any slight scale formations which might have come into being thereon.

The slab 3, still hot, then enters an inlet 4a of a treatment chamber 4. As it passes through this chamber 4 it is contacted on all its surfaces by a powder P which is a mixture of reducing-metal and metal oxide particles that fuse on the surface and form a very hard, inflexible enamel-like glassy coating (see FIG. 6). In addition, other substances (e.g. zinc) needed for the later surface treatment of this body can be added to this powder in the form of the material itself or as another substance (e.g. zinc borate) which produces the desired coating or alloying substance in situ.

These powders are held in a hopper 11 which is closed by a vane 17 and connected through a conduit 12 to a funnel 26 feeding a first aerating blower 9 in turn connected to a second driving blower 10. A conduit 13 leading from this second blower 10 is formed with a venturi restriction 15 just downstream from the end of a narrow pipe 14 into which compressed air is fed. Thence the aerosol particles P are expelled through a nozzle 8 onto the slab 3 all around the latter. Thus another nozzle 18 can be connected also to the blower 10 and the pipe 14 to cause the fine particles P to impinge against the underside of the slab 3 in a region 27 free of rollers 2.

After the treated slab 3 exits through an outlet opening 4b of the chamber 4, it passes along a region 5 where it cools and the coating becomes very hard. FIG. 6 shows the slab 3 with its hardened coatings C.

Further downstream the slab 3 passes through a straightening station 6 where three rollers 28a c bend the slab 3 sufficiently to breakfree the coating C. Due to the regularity of this coating, it is easily removed. This coating is, however, very tight on the metal body 3, since the surface of the body 3 reacts to some extent with the wetted powders P to form the coating C.

Finally, a coating station 7 having a plurality of nozzles 7a sprays a more flexible protective coating M, e.g. paint, upon the surface of the cooled slab 3. This coating M is shown in FIG. 7.

Because of the thinness of the coating C, any flaws (as shown at F in FIG. 6) will not be covered; the coating C will break in the region of these flaws F. Thus, an observer in the region can clearly see them and mark the slab 3 in this region so that it can be later discarded.

In this manner, the flaws are easily recognized and there is no need to do a major descaling of the sheet 3, since any slight scale is removed by the gas in the chamber 22 or the oxides sprayed against it in the chamber 4 and, for the rest of the cooling time, the sheet 3 is covered by a very tight, nearly continuous glassy coating. Removal of this coating C has been found to be very simple once the sheet 3 is fully cooled and further scaling is no longer a problem.

FIGS. 3 and 4 show an alternative embodiment of the present invention. Here, molten metal is poured from-a ladle 16 into a funnel 1a on top of a mole l of a continuous casting installation. A metal body in the form of a thick wire 3' leaves the base of this mold 1' and is bent through 90 by driven rollers 3 of hyperbolic profile and through a chamber 22' similar to the chamber 22 of FIGS. 1 and 2.

A box-like chamber 4 is equipped with two nozzles 18' and 8' connected through a powder-aerating apparatus to a hopper 11 just as in FIGS. 1 and 2, common reference numerals referring to common structure. This wire 3 is here coated according to the method and principles set out in connection with FIGS. 1 and 2.

As the wire 3' leaves the box 4', it is gripped between rollers 5'.

In this embodiment, the enamel coating is not stripped off the wire 3, since it serves to protect the wire during shipping and handling, and relatively easily removed when desired. This coating also serves to accentuate and make more visible flaws in the surface of the wire 3, as described above for FIGS. 1 and 2.

FIG. 5 shows a further embodiment of the invention having in this case a treatment chamber 4". Here a fluidized bed is formed around a continuous workpiece 3" riding on rollers 2". The workpiece 3" is very hot and relatively scale free. A blower 21 having an output connected through a conduit b to the bottom of the chamber 4" and an input conduit 20a connected to the top of this chamber 4" serves to fluidize minute metallic and metallic oxide particles P fed into the chamber through a conduit 11a" connected to a hopper 11" above the chamber 4". In this way the workpiece 3" is very efficiently covered by the powder, so that it can move at high speeds through the chamber 4" and still be adequately coated.

SPECIFIC EXAMPLES EXAMPLE A A steel body is hot rolled and enters the treatment chamber at a temperature (900C) somewhat below its melting point. It is sprayed with a mixture of powders such that substantially equal parts of Si0 A1 0 Fe,0 Ca0, Mg0, Na, K and B mixed with and carried by air strike the surface of the body. This mixture reduces the slight scale on the surface and fuses to form a glassy enamel-like coating, over the entire exterior of the body, the fused mixture flowing to cover all of its surface to a thickness on the order of say 5 to 10 microns. As the body cools, the enamel coating breaks in the region of any flaws on it.

EXAMPLE B A steel wire is formed by hot rolling and enters a treatment chamber as above. In this chamber it is sprayed with a powder whose prime consitutuent is zinc borate. The hard, enamel-like coating is formed as above. Then the body is fed through a straightening apparatus which removes the coating and is found to be zinc coated in the sense that zinc is alloyed with surface zones of the metal body.

In all of the aforedescribed embodiments, a water jet arrangement is provided as described earlier. In FIGS. 1 and 2, for example, there is shown a high-pressure water-spray arrangement 30 having nozzles 31 trained at the workpiece immediately upstream of the treatment chamber 4. Each of the nozzles has a discharge aperture with a diameter of 0.2 to 3mm as stated above and the apparatus 30 may be provided on opposite sides of the sheet-like workpiece 3 and may each include a high-pressure pump designed to project the water from the nozzles with a pressure upwards of 100 atmospheres gauge and a velocity of 15 to 100 m/sec. A collecting rough 32 may beprovided below the workpiece to collect the water dispensed by the nozzles and return it to the pumps of the high-pressured jet installation 13.

EXAMPLE C The steel body of Example A, after hot-rolling at a temperature below its melting point and above about 700C is treated with high-pressure water sprayed from nozzles uniformly spaced along the upper and lower surfaces of the flat body which has the configuration illustrated in FIGS. 1 and 2. Each of the nozzles has a circular aperture of a diameter of 2.1mm and is spaced between I and 5 cm from the workpiece. The water is ejected from the nozzles at a pressure of 200 atmospheres gauge and a velocity of m/second. The steel body then passes into the treatment chamber in which it is sprayed with a mixture of powders consisting of substantially equal parts by weight of silicon dioxide, alumina, calcium oxide, magnesium oxide and boric anhydride to form a glassy film which, when cooled, has a thickness of 8 microns. It is found that this film reveals surface flaws as described with respect to Example A without any obscuration resulting from residual scale. Some obscruation of the surface flaws is present when the reducing metals of Example A are omitted from the powder mixture. With respect to FIGS. 3-5, which deal with the formation of a continuously extruded rod or wire, a spray arrangement 30 with nozzles 31 is provided here as well immediately ahead of the treatment chamber.

EXAMPLE D The method of Example B is followed except that water is directed at the wire from two nozzles disposed at diametrically opposite sides of the wire and having apertures of 2.1mm. The nozzles are spaced at about 3cm from the wire and direct the respective water jets against the workpiece with a velocity of 80 m/sec. at a pressure of 200 atmospheres gauge. Again, an improvement in surface-flaw detection is obtained over the system of Example B.

The improvement described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the invention except as limited by the appended claims.

I claim:

1. A method of accenting and removing surface flaws in a hot metal body emerging from a forming stage comprising the steps of:

progressively advancing said body along a trans-port path;

applying a glassy coating to the surface of said body by continously depositing on said surface of said body as it emerges from said forming stage a glassforming substance fusible at least at the tempera ture of said body to form an enamel-like coating thereon;

directing at least one high-pressure jet of water against said body immediately prior to the application of said glass-forming substance thereto to erode residual scale and prepare the surface of said body for receiving said enamel-like coating; selectively cracking said coating in the region of surface flaws of said body by cooling said coating on said body; and removing said flaws from said body directed against said body at a volume rate of flow in excess of 200 I/minute.

5. The method defined in claim 4 wherein said jet is projected against said body at a volume rate of flow of about 400 I/min.

6. The method defined in claim 4 wherein the jet has a diameter between 0.2 and 3mm.

7. The method defined in claim 6 wherein said jet is directed at said body from a nozzle having an aperture diameter of about 2. 1mm.

8. The method defined in claim 6 wherein said jet is directed at said body with a velocity of 15 to 100 m/sec.

9. The method defined in claim 8 wherein said jet is directed against said body with a velocity of about m/sec.

I 10. The method defined in claim 1 wherein said coating is formed in part by interaction between said substance and said body.

11. The method defined in claim 1 wherein said substance consists at least partially of a material desirable for later surface treatment of said body.

Claims (11)

1. A method of accenting and removing surface flaws in a hot metal body emerging from a forming stage comprising the steps of: progressively advancing said body along a trans-port path; applying a glassy coating to the surface of said body by continously depositing on said surface of said body as it emerges from said forming stage a glass-forming substance fusible at least at the temperature of said body to form an enamel-like coating thereon; directing at least one high-pressure jet of water against said body immediately prior to the application of said glass-forming substance thereto to erode residual scale and prepare the surface of said body for receiving said enamel-like coating; selectively cracking said coating in the region of surface flaws of said body by cooling said coating on said body; and removing said flaws from said body at re-gions thereon visually accented by cracks in said coating.

2. The method defined in cliam 1 wherein said jet has a pressure in excess of 100 atmospheres gauge.

3. The method defined in claim 2 where said pressure is about 200 atmospheres gauge.

4. The method defined in claim 2 wherein said jet is directed against said body at a volume rate of flow in excess of 200 l/minute.

5. The method defined in claim 4 wherein said jet is projected against said body at a volume rate of flow of about 400 l/min.

6. The method defined in claim 4 wherein the jet has a diameter between 0.2 and 3mm.

7. The method defined in claim 6 wherein said jet is directed at said body from a nozzle having an aperture diameter of about 2.1mm.

8. The method defined in claim 6 wherein said jet is directed at said body with a velocity of 15 to 100 m/sec.

9. The method defined in claim 8 wherein said jet is directed against said body with a velocity of about 80 m/sec.

10. The method defined in claim 1 wherein said coating is formed in part by interaction between said substance and said body.

11. The method defined in claim 1 wherein said substance consists at least partially of a material desirable for later surface treatment of said body.

Images