IL46078A - Method of case-alloying metals such as steel or cast iron - Google Patents

Description

Method of case alloying metals such as steel or cast iron AVCO ΕΥΕΕΕΪΤ SESEARCH LABORATORY, IMC.

C:~ 44065 ΐη© present invention relates to me hods of case-alloying metals such as steel or cast iron. fher© are many known and long practised methods for improving the resistance of fabricated or semifabricated metal (including elements, alloys and compounds) to wear, galling, deformation, corrosion, heating and/or erosion. T ese include overcoating the soarfac© of the metal and modification of the composition and/or micros rue ore of the surface through such techniques as car urizing, nitriding, siliconizing, diffusion hardening, hard surfacing (welding a high-alloy layer to the surface), flame hardening, induction hardening and physical modification (e.g. peening). S?he overcoating methods include electroplating chromium or nickel on to the surface, plasma spraying or flame spraying refractories on to the surface and roll cladding (for sheet form or wire form mill products).

It is an important aim of the present invention to provide an improvement in known methods of orming a surface layer casing on metal, whereby a superior case-alloyed product results.

It is a further aim of the invention to convert a surface layer on a substrate to a form which includes a significant proportion of the substrate as a separate and distinct phase and/or as a constituent of a newly formed compound.

It is a further aim of the invention to convert a surface layer on a substrate to a casing form by melting and mixing the surface layer with alloying ingredients in a very short time.

According to the invention, there is provided a comprising applying to the surface thereof a coating layer of tinuous -wave laser, said beam having a power density of froa 50 to 10,000 kilowatts per square inch and a traverse rate which cause said coating layer and a predetermined depth of the immediately underlying metal to melt, intermix and resolidify along said linear work path, thereby to produce an alloy casing along aaid path having said predetermined depth and having throughout said depth a substantially uniform composition and a finer grain structure than the substrate grain structure.

In carrying out the method, a metal substrate is coated with the alloying ingredient or ingredients to be introduced into the predetermined depth of the substrate to orm an intimate mixture and/or compounds therewith wherein the metal of the substrate comprises a significant volume percent, preferably in the range of 10-90 volume percent, of the mixture. The coating may thereafter be supplemented or replaced by other reservoirs of alloying ingredients, and the method repeated. he surface layer of the substrate is melted to the preselected depth through application of the focused output beam of the continuous wave laser to limited surface area regions of .01 to 0.4 sq. inches or less and relative scanning of the laser beam and surface to sequentially melt and resolidify a series of such areas to define a d eeired pattern of surface modification, fhe conditions of melting are controlled to induce forced mixing of the coating material with the molten substrate material to the extent that mass transfer through such mixing is predominant over diffusion in the molten surface layer region. Any such region is maintained at molten impacting laser beam.

The method is preferably conducted at atmospheric^or superatmospheric pressure to suppress volatilization of mixture ingredients and to avoid the fixturing, cleanliness and setup time requirements of vacuum processing.

Preferably, the transitory zone of laser beam application for melting is oscillated locally at 100 to 1000 Hertz to further promote mixing of the molten metal and alloying ingredients. Such oscillation may comprise local sweeping of the laser beam and/or modification of the beam contour such as switching between rectangular and round beam shapes.

These and other aims, features and advantages of the invention will be more fully understood from the following detailed description with reference to the accompanying drawings, in which: FIG. 1 is a cross-section sketch of a sheet form substrate part at an early stage of surface modification; FIG. 2 is a similar section view of the part after completion of modification; FIG. 3 is a diagrammatic sketch of a preferred apparatus arrangement for carrying out the method of the present invention; FIGS. 4 and 5 are isometric views of parts undergoing surface modification; FIGS. 6 and 6A-6D are photomicrographs of cross-sections of a substrate subjected to surface modification; FIGS. 7-9, 11, 12, 15 and 19 are concentration vs. case depth curves for alloying ingredients introduced into the surface layer of workpieces modified by the method according to the invention; and FIGS. 10, 13, 14, 16 - 18 and 20 - 21 are hardness vs. or a fabricated part such as a steel valve seat. In or er to increase the wear resistance of its upper surface, a layer of minority ingredients (e.g. chromium and manganese) for convert^ a surface layer of the part to high alloy composition is coatecf" thereon. A preselected depth line 16 is established below the surface of the part 10 to define a surface layer and an amount of the substrate metal which, together with the alloying ingredient (s) coating 12, will form the desired alloy. An energy absorbing layer 14 may be placed on the substrate part as an undercoat for the coating 12 or admixed with the coating 12. A beam of radiant energy 18 is provided from continuous wave laser to produce a molten zone 19 down to depth line 16 and through relative displacement of the substrate part 10 and beam 18, the molten zone is scanned with linear continuity or skips as desired to form a time sequence array of such molten zones. During the maintenance of molten zone 19 in any given point, heat transferred therefrom to the large heat sink provided by the substrate part 10 is very rapid and as soon as the impacting beam 18 is relatively displaced therefrom, the molten zone cools and solidifies.

For treating metals according to the present method, a 5-20 kilowatt laser beam focused to a .025 to 0.7 inch diameter circle, or areal equivalent of other form, providing a beam power density of about 50 to 10,000 kilowatts per square inch, is scanned along a linear work path across thi surface to be modified at a rate of from 5 to 50 inches per minute. Typical times of residence in the molten state for any given region of surface layer are from 0.1 to 1.5 seconds and cooling time for the molten region to below the applicable solidus temperature for the alloy composition therein is typically from 0.1 to 1.5 sec.

During the melting, thermal gradients alone induce a substantial degree of mixing of the allowing ingredient or wave further promotes vigorous mixing. Such mixing can be further substantially enhanced by local oscillation of the laser bea!tt as described below in connection with FIGS. 4 and 5.

Faster scanning rates are possible where the substrate is preheated. See Example 7.

Referring now to FIG. 2 there is shown a substrate part 10 with hardened surface layer casing 20 and an interfacial layer 22. The thickness of layer 20 is to a depth coinciding essentially with the preselected depth line 16 (FIG. 1) .

Referring now to FIG. 3r there is shown an apparatus arrangement for practice of the invention. The workpiece 10 is placed on a conventional milling machine base having ways and traversing controls for displacing the workpiece in orthogonal directions as indicated by the double headed arrows 11 and 13. Such x and y traverses may be simultaneous or one of them may be intermittent.

The beam 18 for working as described above in connection with FIG. 1 is provided by a continuous wave (CW) laser 30. A suitable laser of this type is disclosed in U.S. Patent Nos. 3,721,915, 3,702,973, 3,577,096 and 3,713,030. The laser 30 in FIG. 3 is coupled to beam splitter 31. One or more beam tunnels 36 exiting from the beam splitter 31 are used to divert the laser beam to one or more points of application and the beam may be time-shared among such applications very quickly, and even multiplexed, using tiltable mirror means and rotating beam chopper wheels. Mirror means indicated schematically at 38, may be provided in adjustable form to receive the laser beam from the optics module for producing a sweep of beam 18 as a supplement to or in lieu of work-piece displacement on table 39 and for the local oscillation purposes described below in connection with FIG. 5. / 12/14 thereon as described above in connection with FIG. lg^ The workpiece is displaced relative to the beam in a longitudinal scanning direction indicated by arrow 11A in FIG. 4 and arrow 11B in FIG. 5 and intermittently displaced as indicated by arrow 13 in FIGS. 4 and 5 to produce a series of adjacent lines.

One of the beam tunnels 36 is coupled to an optics module 37 where the laser beam is configured for heating a substrate.

The beam is switched off during retrace or alternatively may be kept on to do adjacent lines (20A in FIG. 4 or 20B in FIG. 5) during a pass and retrace. The beam may also be switched off to allow skipping surface area regions of the coating 12/14 to form a desired pattern of hard surfaced and non-hard surfaced areas. ^ The relative displacement traverse indicated by both of arrows 11A and 11B in FIGS. 4 and 5 respectively, is along a linear work path and typically done at a relative scan rate of about 20 inches per minute. However, there may be superimposed on the scanning in FIG. 5 a local lateral oscillation of the beam relative to the scan direction as indicated by the oscillating wave form 11C superimposed on arrow 11B. The working spot size would typically coincide with the full width of lines 20A in the first processing mode indicated in FIG. 4 and be substantially less than the full width of lines 20B in the second processing mode in FIG. 5 and in the latter case the oscillations of the spot relative to the surface of part 10 would provide full line coverage while producing repeated bursts of energy input to the same surface area region thereby substantially promoting mixing of the molten surface layer of part 10 with the locally melted portion of coating 12. Such gradients can also be induced with a large spot size of focused n s forms. Longitudinal oscillations of beam center may be used in addition to or in lieu of lateral oscillations.

The practice of the invention is further illustrated by the following non-limiting examples .

Example 1 Mixtures of metal powder were coated on the surfaces of metallic products by spraying. The coated surfaces were scanned by a high powered laser beam causing the surface and the powder to melt and alloy uniformly. By this process, the hardnesses of the melted zones were found to be increased substantially. In one test, the surface of AISI C 1018 steel was coated with a 1/4 to 1/2 mil layer of manganese phosphate by a commercial manganese phosphating process. This optional coating of manganese phosphate facilitates absorption of a laser beam. Other possible choices of heat absorbing material are zinc phosphate, aluminum oxide, and carbon black. The choice will also depend on the wave length of the heat source. Then, a mixture containing 6 grams of 45 micron sized carbon power, 3 grams of 10 micron sized chromium powder, and 3 grams of 45 micron sized manganese powder suspended in 40 milliliters of isopropyl alchol was sprayed uniformly over the manganese phosphate coating. The thickness of the loosely packed powdered metal coating was 1/2 mil. The surface of the steel containing the mixture of metal powder was scanned at 20 inches per minute under an 11-11.5 kilowatts dithered (locally oscillated) laser beam. The laser beam size was 0.1 inches by 0.5 inches in a rectangular configuration and was elongated perpendicular to the direction of traverse because of local oscillation being performed along this direction to cover the complete line width (20B, Fig. 5). The oscillation rate was 690 Hertz. Under these conditions, the surface of the steel was melted and uniformly allo ed with the carbon chromium and man anese owder. The FIGo 6 is a 200 times magnified photomicrograph of a workpiece treated in accordance with this Example 1 above and FIGS . 6A, 6B, 6C, 6D are 3500 times magnification scanning electron microscope photomicrographs of small regions at depths of 2, 3, 4 and 5 mils, respectively,, below the surface of the workpiece of the region shown in FIG. 6.

Referring first to FIG. 6, the unaffected substrate is indicated at 10, the alloyed surface layer thereof at 20 and interfacial layer 22 defines an inner boundary between surface layer 20 and substrate 10. Layer 20 has a finer grain structure than the substrate. FIGS. 6A-6D reveal a two-phase structure in layer 20 of martensitic dendrites surrounded by carbides in the interdendritic regions.

FIGS. 7, 8 and 9 show minor alloy ingredient concentration in the finished product for carbon, chromium, and manganese, respectively , produced by processing in accordance with Example 1. The curves of FIGS. 7, 8 and 9 are plots of concentration of the respective minor alloy component against cased depth. These curves are fitted to original data points in accordance with conventional statistical practice. These curves show an increase in the minority ingredients of the ferrous alloy composition. The depression at portion 93 of curve 91 (FIG. 9) reflects volatilization of manganese at the surface? in the absence of volatilization, the curve portion would be indicated in dashed lines at 95 or somewhat higher. If the case depth is much deeper (see Example 7) , then the surface effect becomes less significant.

FIG. 10 shows hardness profiles in the finished product subjected to laser working with local oscillation. The melted zone was 5 mils deep and in this zone the hardness was Rockwell affected zone was approximately 0.05 inches. ^ Example 2 The surface of AISI C 1018 plate was coated with a 1/4 to 1/2 mil. layer of manganese phosphate by a commercial manganese phosphating process. A small portion of a mixture containing 10 grams of 45 micron-sized aluminum powder suspended ir 50 milliliters of isopropyl alcohol was uniformly brushed on to the manganese phosphate surface. Aluminum powder was applied to prevent gas evolution during melting. Then a mixture containing 12 grams of 45 micron-sized carbon powder, 6 grams of 10 micron-sized chromium powder, and 6 grams of 45 micron-sized manganese powder, suspended in 40 milliliters of isopropyl alcohol was sprayed 20 times uniformly over the aluminum powder coating. The thickness of the loosely packed metal powder coating was 15 to 20 mils.

The coated surface was then subjected to laser working of separate specimens thereof in the fashions disclosed in connection- with FIGS. 4 and 5, respectively, above. That is, the surface of one specimen was hardened by melting down to a limited depth to intermix minor ingredients of an alloy with a major ingredient derived from the substrate without local oscillation and other specimens were so treated with oscillation. In both modes, the melting time for any given surface area was 0.3 seconds and the power output from the laser was 13-14 kilowatts. In working without local oscillation, the traverse speed was 50 linear inches per minute and in working with local oscillation, the traverse speed was 20 inches per minute. Average power density applied in melting in both cases was 250-300 kilowatts per square inch of work piece surface area. In the non-local oscillation working, the beam size of the impinging laser beam was 0.25 inch in diameter in a circular configuration and in the line width (20B, FIG. 5). The oscillation rate was 690 Hertz-.

FIGS. 11 and 12 show minor alloy ingredient profiles in the finished product for chromium and manganese respectively. The upper curves for chromium and manganese were obtained with local oscillation and the lower curves were obtained without local oscillation.

FIG. 13 shows hardness profiles in separate finished products subjected to laser working without local oscillation and with local oscillation respectively. Laser working without local oscillation gives hardness values ranging between Rockwell C 27 to 44 and laser working with local oscillation gives hardness values ranging between Rockwell C 46 to 58, up to a depth of nearly 0.03 inches in both cases. The hardness value for the core of the steel was Rockwell B 93, and the hardness value for the heat affected zone was Rockwell B 90. The depth of the heat affected zone was approximately 0.05 inches.

Example 3 The surface of AISI C 1018 plate was coated with a 1/4 to 1/2 mil. layer of manganese phosphate by a commercial man-ganese phosphating process. A small portion of a mixture containing 10 grams of 45 micron-sized aluminum powder suspended in 50 milliliters of isopropyl alcohol was uniformly brushed on to the manganese phosphate surface. The aluminum powder coating was applied over the manganese phosphate coating to prevent gas evolution during melting. Then a mixture containing 12 grams of 45 micron-sized carbon powder, 20 grams of 10 micron-sized chromium powder, and 8 grams of 45 micron-sized tungsten powder, suspended in 40 milliliters of isopropyl alcohol was sprayed 20 times uniformly over the aluminum powder coating. The thickness of the loosely packed metal powder coating was 25 to 30 mils. 0.5 inches in a rectangular configuration elongated perpend^Tular to the direction of traverse because of the local oscillation being performed along this direction. The oscillation rate was 690 Hertz. The surface layer of the steel which was melted and resolidified was intimately alloyed with the carbon, chromium, tungsten, and aluminum powder.

FIG. 14 shows hardness profile in the finished product subjected to laser working with local oscillation as described above in this Example 3. The melted zone was 44 mils deep and in this zone, the hardness was Rockwell C 48 to 53, while the hardness value for the core of the steel was Rockwell B 93, and the hardness value for the heat affected zone was Rockwell B 90. The depth of the heat affected zone was approximately 0.12 inches.

Example 4 The surface of a grey cast iron part containing approximately 0.2 weight percent chromium was coated with a 1/4 to 1/2 mil layer of manganese phosphate by a commercial manganese phos-phating process. Then a mixture containing 5 grams of 10 micron-sized chromium powder suspended in 40 milliliters of isopropyl alcohol was sprayed 10 times uniformly over the manganese phosphate coating. The thickness of the loosely packed metal powder coating was 1/2 to 1 mil. The surface of the cast iron part containing chromium powder was scanned at 30 inches per minute under an 11 kilowatt locally oscillated laser beam. The laser beam size was 0.1 inches by 0.5 inches in a rectangular configuration and being elongated perpendicular to the direction of traverse because of local oscillation being performed along this direction. The oscillation rate was 690 Hertz. Thus, the surface of the cast iron part melted and intimately alloyed with the chromium powder.

FIG. 15 shows minor alloy ingredient concentration in chromium in the melted zone which is 10 mils deep.

PIG. 16 shows hardness profiles in the finished product subjected to laser working with local oscillation. In the melted zone, the hardness was Rockwell C60 to 65 while the hardness value for the cast iron part which was not subjected to laser working was Rockwell B 95. The hardness value of the heat affected zone was Rockwell C 56 to 61.

Example 5 The surface of a grey cast iron plate was coated with a 1/4 to 1/2 mil layer of manganese phosphate by a commercial manganese phosphating process. Then a mixture containing 5 grams of 10 micron-sized chromium powder and 5 grams of 45 micron-sized silicon powder suspended in 40 milliliters of isopropyl alcohol was sprayed 5 times uniformly over the manganese phosphate coating. The thickness of the loosely packed metal powder coating was 1/2 to 1 mil. The cast iron part containing chromium and silicon powder was preheated to 750 °F in a furnace, and then its surface was scanned at 60 inches per minute under a 6 kilowatt locally oscillated laser beam. The laser beam size was 0.5 inches by 0.5 inches. The oscillation rate was 690 Hertz perpendicular to the direction of traverse and 125 Hertz along the direction of traverse. Under these conditions the surface of the cast iron part melted and intimately alloyed with the chromium and silicon powder.

FIG. 17 shows hardness profiles in the finished product subjected to laser working with local oscillation. The melted zone was about 5 mils deep and the hardness in this zone was Rockwell C 56 to 60 while the hardness value for the cast iron part which was not subjected to laser working was Rockwell B 95. The hardness in the heat affected zone was Rockwell C 45.

Example 6 of 10 micron-sized chromium powder suspended in 40 of isopropyl alcohol was sprayed 5 times uniformly over the manganese phosphate coating. The thickness of the loosely packed metal powder coating was 1/2 to 1 mil. The surface of the cast iron part containing chromium powder was scanned at 30 inches per minute under an 8 kilowatt locally oscillated laser beam.

The laser beam size was 0.5 inches by 0.5 inches. The oscillation rate was 690 Hertz perpendicular to the direction of traverse and 125 Hertz along the direction of traverse. Under these conditions, the surface of the cast iron part melted and intimately alloyed with the chromium powder.

FIG. 18 shows the hardness profiles in the finished product subjected to laser working with local oscillation. The melted zone was 10 mils deep and the hardness in this zone was Rockwell C 58 to 67 while the hardness value for the cast iron part which was not subjected to laser working was Rockwell B 98. The hardness in the heat affected zone was Rockwell C 51.

Example 7 The surface of an AISI 4815 steel plate was coated with a 1/4 to 1/2 mil layer of manganese phosphate by a commercial manganese phosphating process. 10 micron-sized chromium powder was uniformly sprinkled over the manganese phosphate coating and the metal powder was densely compacted. The depth of the densely compacted chromium powder coating was approximately 0.025 inches. 45 micron-sized carbon powder was uniformly sprinkled over the chromium powder coating and the carbon powder was densely compacted. The depth of the densely compacted carbon powder coating was approximately 0.01 inches. The steel plate containing carbon and chromium powder was preheated to 900°F, and then its surface was scanned at 9 inches per minute under a 14 kilowatt inches in a rectangular configuration elongated perpendicula^ Eo the direction of traverse because of the local oscillation being performed along this direction. The oscillation rate was 690 Hertz. Under these conditions, the surface of the steel plate melted and intimately alloyed with the carbon and chromium powder. Immediately after laser working, the steel plate was post heated at 900 °F for 1/2 hour. The preheating and post heating were performed in a furnace and these treatments eliminated cracking of the melted zone.

FIG. 19 shows minor alloy ingredient concentration in the finished product for chromium. The curve is fitted to original data points (not shown) in accordance with conventional statistical practice. This curve shows a substantial increase in the concentration of chromium in the melted zone which is 50 mils deep. The concentration of chromium was 21 weight percent up to a depth of 50 mils.

FIG. 20 shows hardness profiles in the finished product subjected to laser working with local oscillation. In the melted zone, the hardness was Rockwell C 53 to 57 while the hardness value for the steel plate which was not subjected to laser working was Rockwell C 20. The hardness in the heat affected zone was Rockwell C 30.

FIG. 21 shows hardness profiles in the finished product subjected to laser working with local oscillation, the laser worked steel plate having been further subjected to a furnace heat treatment at 1200 °F for two hours and air-cooling at the conclusion of heat treatment. In the melted zone, the hardness was Rockwell C 55 to 58. The hardness in the heat affected zone was Rockwell C 25. The curve in FIG. 21 indicates the resistance of the melted zone to high temperature tempering. 46078/2 are low* The disturbance of underlying substrate pro er ies^ is minimal. Alloys and other mixtures are formed by introducing minority alloying or mixture components into the substrate. The resultant surface layer casing may be single phase or multiphase as described above in connection with FIGS. 6A-6D. The casing may be areally continuous within its areal outline or discontinuous therein. The casing will, in most instances, comprise a concentration gradient of alloying ingredients decreasing towards the underlying substrate but having areal compositional uniformity at any given depth within said gradient.

The invention is applicable to ferrous metals and alloys including all types of cast iron and all types of steel. It can also be applied to non-ferrous metals and alloys.

The element or elements to be alloyed on the surface of the metallic product may be applied as a powder or a mixture of powders or an alloyed powder or any suitable combination of the above.

Clearly, the invention is not limited to achieving alloying of the surface layer by means of a single pass of the laser heat source ove the orkpiece. After the first pass , additional iloying material may be added to the substrate and additional heat added in the manner described to further modify the surface layer in the manner heretofore described.

Claims (7)

46078-3- CASE AEKL-148

1. A method of case-alloying metal such as steel or cast iron, comprising applying to the surface thereof a coating layer of one or more alloying ingredients and traversing the coated surface along a linear work path with an output beam of a continuous wave laser, said beam having a power density of from 50 to 10,000 kilowatts per square inch and a traverse rate which cause said coating layer and a predetermined depth of the immediately underlying metal to melt, intermix and resolidify along said linear work path, thereby to produce an alloy casing along said path having said predetermined depth and having throughout said depth a substantially uniform composition and a finer grain structure than the substrate grain structure.

2. A method according to claim 1, wherein the laser beam is deflected to scan the coated surface successively along several linear work paths arranged in side-by-side relationship.

3. A method according to claim 1 or 2, wherein the laser beam is oscillated laterally of its work path or paths while traversing said coated surface.

4. A method according to claim 3, wherein the laser beam lateral oscillation is of a frequency of from 100 to 1,000 Hertz.

5. A method according to any of claims 1 to 4, wherein said beam is focussed to a circle of from 0.025 to 0.7 inch diameter or to an areal equivalent thereof of other form, the power of the laser output beam is from 5 to 20 kilowatts and the traverse rate thereof is frcm 5 to 50 inches per minute.

6. A method according to any one of claims 1 to 5, wherein the power density of the laser beam is from 250 to 300 kilowatts per square inch.

7. A method of case-alloying metal such as steel or cast iron, substantially as hereinbefore described. For the Appiicanis DR. EWHQ^ COHN AND PARJHiRS