Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
  • C23C10/18—Solid state diffusion of only metal elements or silicon into metallic material surfaces using liquids, e.g. salt baths, liquid suspensions
  • C23C10/20—Solid state diffusion of only metal elements or silicon into metallic material surfaces using liquids, e.g. salt baths, liquid suspensions only one element being diffused
  • C23C10/22—Metal melt containing the element to be diffused
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9265—Special properties
  • Y10S428/927—Decorative informative
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9335—Product by special process
  • Y10S428/934—Electrical process
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9335—Product by special process
  • Y10S428/937—Sprayed metal
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/922—Static electricity metal bleed-off metallic stock
  • Y10S428/9335—Product by special process
  • Y10S428/939—Molten or fused coating
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12729—Group IIA metal-base component
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12861—Group VIII or IB metal-base component

Definitions

• This invention relates to a novel metal diffusion process in which various elements are diffused into a ferrous metal article from a melt of certain metal transfer agents and to novel articles of manufacture comprising a ferrous metal substrate having a ferritic iron-chromium alloy coating prepared by said metal diffusion process.
• Coated metals are commonly used materials which have their surfaces protected against corrosion, oxidation, and wear. Many of these metal coatings are produced by electroplating, by hot dipping, or metal spraying. These coatings as a class are alike in that the deposit is a distinct adjunct to the metal base and adhere essentially by means of mechanical bonds. Such coatings usually suffer from the disadvantage that the adhesive forces are not great enough to maintain an integral bond between the coating and base metal when deformation is applied. Consequently, these coatings are generally applied only after the base metal is formed int-o the desired shape.
• Coatings can also be applied to metal surfaces through diffusion processes.
• the object of such processes is to enrich or alloy the surface of a metal with certain ele ments which provide desirable properties such as corrosion resistance not possessed -by the base metal itself at a minimum expenditure of the foreign or enriching element.
• plain carbon steel can be made corrosion resistant to certain depths.
• An outstanding feature of articles prepared by diffusion processes is that the coating is metallurgically bonded to the substrate metal of the article so that an integral bond is formed between coating and base metal.
• diffusion methods in the past have been found to be of limited commercial value due to apparatus limitations, inferiority of the coatings obtained, or economic reasons.
• the prior art chromium diffusion methods for treating ferrous articles generally employ solid packs or gaseous treatment schemes in which chromium transfer is accomplished by gaseous compounds of chromium. As soon as chromium is deposited on the substrate surface, it diffuses inward to form a diffusion coating.
• Articles produced by such prior art processes have found limited commercial acceptance in areas where resistance to wear or high temperature oxidation is paramount. However, articles prepared by such processes have attained no significant commercial acceptance in the industrially more important area where corrosion resistance and formability are the Skilled workers in this art have long recognized the control of carbon in the coatings of articles prepared as being essential to gaining these property requirements.
• a further goal for the control of carbon in chromium diffusion coatings having a chromium concentration at the surface in excess of about 12% by weight has been to eliminate the presence of iron-chromium carbides at the surface of the coating.
• the presence of such carbides has been recognized as reducing the formability of chromium diffusion coatings as well as impairing the decorative value of an otherwise bright metallic surface.
• the chromium diffusion coated articles prepared from such base metals are extremely soft and have unsuitable strength characteristics for most applications.
• a further desirable goal in the control of carbon in coatings of chromium diffusion coatings on ferrous metal articles has been to prevent the occurrence of intergranular precipitates of iron-chromium carbides in the coating.
• It is still another object of the present invention to provide a novel article of manufacture comprising an unstabilized ferrous metal substrate containing an amount of carbon in excess of 0.01% by weight, having a diffusion coating of a ferritic iron-chromium alloy containing at least 12% by weight chromium at the surface thereof, wherein the surface of said coating is free of ironchromium carbides.
• It is still another object of the present invention to provide a novel article of manufacture comprising an unstabilized ferrous metal substrate having a diffusion coating of a ferritic iron-chromium alloy containing at least 12% by weight chromium at the surface thereof, wherein said coating is characterized in being free of intergranular precipitates of iron-chromium carbides.
• the diffusion method hereof is applicable to any ferrous metal article such as cast iron, mild steel, stainless steel, or the like, wherein iron is the predominant element.
• any of the named diffusing elements or combinations thereof canbe diffused into the I ferrous articlefrom the molten transfer agent toform alloy coatings of predetermined compositions distinct from the original alloy material.
• the process can also .be adapted to remove or. decrease the amount of any of thenamed diffusing elements present in a'ferrous article treated in order to alter its surface alloy composition.
• Metals that serve as suitable transfer agents in the transfer bath may be completely in the molten state 7 with the diffusing element in solution in the molten transfer agent.
• the diffusing element has a limited solubility in the transfer agent such as, for example, chromium
• an excess of the solid diffusing element may be incorporated so that there is both a solid and liquid phase present. It is to be understood that it is the transfer agent in the liquid phase of the 7 bath that is the medium for the movement of the difprocess are calcium, barium, strontium, and magnesium,
• Illustrative examples of such diluents are copper, lead, I
• Solid phases that may be present in thebath either are present as inert diluents or as a convenient reservoir of the diffusing element's.
• the amount of solid phase in the transfer bath becomes excessive an undesirable result is the embedding ofparticles in the surface of the coating.
• the liquid phase will constitute over byweight of the bath. While the content of' transfer agent in the bath may vary between wide limits, a practical, lower limit for most coating operations within the invention will be above about 10% by Weight.
• the metal transfer bath can be prepared in a'nurnber
• the transfer agent and one or more of the diffusing elements can be heated up together toprocess temperature.
• one or more of the diffusing elements in selected concentrations can be prepared and added to the molten transfer agent; the mixture then being heated to. process temperature.
• the diffusing elements may be added periodically'to replenish the bath or added solid transfer occurs when the transfer agent in the molten bath is saturated with the diffusing element and when the diffusing element is not present in the solid article, though capable of complete solution therein.
• the liquid-to-solid transfer of chromium to iron in a Ca Cr melt readily reaches an optimum thermodynamic tendency since chromium is completely miscible in ironand only slightly solublein the transfer agent, the amount 7 of chromium soluble in calcium,'for example, at 1100 C.
• Cobalt and manganese are similar to chromium ing a relatively low solubilityrinthe transfer agent" and a high solubility in ferrous metal article's.
• Nickel onthe other hand, is an example of a diffusing elemcnt'that is highly soluble in both the transfer agents and the ferrous metal articles. Consequently, higherconcentrations of this latter element in a melt of the transferagent is required in order to reach the greatest. thermodynamic tendency for liquid-to-solid transfer to occur.
• Liquid-to-solid transfer results in the incorporation of At the high temperatures employed in the contacting, further'inward diffusionof the. elementthen causes coating growth.
• alloy coatings may be prepared, on a ferrous metal'article such coatings containing'up to about -90% by 'Weight'cobalhup to about 60% by weightchromium or :nianganese, andup to about 50-55% byweightnickelf V
• the metal transfer bath comprisesthe metaltransfer 7 agent or agents," the diffusing element or elements, and? any diluent materials .- ⁇ yhich may be present, The metal able being fully satisfactory in the process.
• the diffusing elements other than the elemental form of continuously in controlled amounts to facilitateprolonged coating operation.
• the diffusing elements may be added in almost'any particle form.
• diffusing elements that are slightly soluble in the metal transfer agent, such as, chromium, manganese, and cobalt that improved results are obtained if the diffusing element is added in the form ofa finely divided powder.
• the diffusing elements are generallyintroduced to the diffusing bath in their elemental form, the metals as commercially avail- Sources of themetals may also beused, such as, for example, a Cr.Ni alloy or a Fe-Cr alloy may serve as a suitable source of chromium in the process of theinvention.
• a Cr.Ni alloy or a Fe-Cr alloy may serve as a suitable source of chromium in the process of theinvention.
• compounds which are reducibl'e'by the transfer agent to the metallic form of the diffusing element may be employed, such as, for example, CrO or Cr O which 7 metal transfer bath is desirable but not necessary since the bath may be operated under carefully controlled conditions in the open atmosphere. It is desirableto agitate the bath The use of a blanket of inert gas over the this again is notessential.
• the o'pera'tingtemper'ature oflthe bath for the process IS selected to'favorably affecttherate'of diffusion of the elements and to maintain the transfer agent or combinatrons thereof in the molten state; Generally, temperatures less thana'oout 800 C. are not considered practical for metal diffusion because the rate of diffusionis too slow. 'Howeven'smce the transfer agents either alone 01'- in combination can be maintained in a molten state at V 8005C, this may be considered as an approxim'ateminimum pract1cal...,operating temperature for the process.
• a preferred operating temperature for the process is fromabout 1000 120090.
• the maximum practical operating temperature may be consideredtobe the normal fboili'ngpoint'of the transfer: agent used but, in any event,
• the residence time of the ferrous article in the metal transfer bath for diffusing in any particular diffusing element influences the thickness of the coating obtained and may vary widely. Coatings of appreciable thickness for some of the diffusing elements may be formed in as little as one minute of treatment in the metal transfer bath. For example, in the case of a calcium bath operated at 1100" C. which is saturated with chromium, a 0.3 mil coating may be obtained in one minute.
• coiled steel sheet or shaped ferrous metal articles may be passed continuously through the metal transfer bath at a rate to provide the required residence time for a desired coating, or articles may be immersed batchwise in the metal transfer bath to provide the required residence time for a desired coating and then withdrawn.
• the ferrous articles treated in accordance with the hereinbefore described method of the invention are termed coated articles, although it must be appreciated the diffusing elements migrate into the solid surface of the ferrous articles and thus alter the characteristics of the articles.
• the coating is characterized by different concentrations of the diffusing elements at its outer surface than are found in the interior.
• the coating is characterized by greater concentrations of the diffusing elements at its outer surface and a decreasing concentration of the difusing elements with increasing distance from the surface.
• a distinct advantage of the method of the invention resides in the fact that several diffusing elements may be effectively diffused into a ferrous base metal simultaneously. Therefore, by selecting various amounts of diffusing elements and by proper control of process conditions, the concentrations of the elements present in the coatings can be controlled and ferrous alloys containing two or more of the diffusing elements of predetermined composition may be formed.
• Other diffusion techniques such as commercial vapor diffusion processes are usually limited in that the diffusion of more than one element is not practical unless diffusion of each element is carried out separately. Stepwise diffusion in this manner makes it difficult if not impossible to control alloy composition at the surface of the coating since during diffusion of each succeeding element, the earlier diffused elements continue to diffuse inwardly from the surface under the influence of heat.
• iron-chromium-nickel alloy coatings on ferrous substrates which will protect the substrate against corrosive attack.
• coatings of iron-chromium-nickel alloy may be formed on inexpensive ferrous base metals which will impart surface alloy compositions which are equivalent to the commercial iron-chromi-um-nickel austenitic stainless steels.
• Coatings may also be formed of iron-chromium-nickel alloys which are fully equivalent in surface composition to nickel bearing ferritic stainless steels.
• a salient feature of the method of the invention is the potent purifying action on the coatings by the transfer agent particularly when calcium is used, which occurs simultaneously with the formation of coatings on ferrous base metals whereby the concentration of carbon and nitrogen are effectively reduced to limits in the coatings lower than heretofore obtained.
• Other interstitial impurities such as, oxygen, sulfur, phosphorous, etc., are also reduced in concentration.
• the amount of the various ingredient-s are given in terms of percent by [weight unless otherwise indicated.
• the transfer agent comprises more than 60% by weight of the diffusing bath.
• the thicknesses of the coatings formed on the ferrous metal articles were determined by metallographic examination or by measuring the thickness of the strip film after the substrate has been dissolved away.
• the compositions reported for the surface of the coating were determined by X-ray fluorescence.
• a first series of experiments demonstrates the preparation of ferrous alloy coatings with a single diffusing element in the diffusing bath.
• EXAMPLE 1 An iron container holding a bath composed of 2400 grams of calcium and grams of powdered chromium (100 mesh) was heated to 1100 C. The bath was agitated by a mechanical stirrer and was protected from the atmosphere by an inert atmosphere of argon. A mild steel coupon (4" x 1" x .06") containing about 0.06% carbon was introduced and withdrawn after 60 minutes treatment. A coating 2.1 mils thick was formed having a surface concentration of 40% chromium.
• EXAMPLE 8 a A bath was formed in an iron crucible containing 150 grams barium and grams of cobalt powder. The bath be seen that as the treating temperature is increased, the
• V e 7 EXAMPLE 2 An iron container holding a bath preparedfrom 1500 grams ofcal'cium-and 501 grams of CrO was heated to 1100 C. The bath was agitated by a mechanical stirrer and was covered :by' an inert'atr'nosphere of argon. A mild steel coupon containing about 0.06% carbon was treated in the bath for 45 minutes. A coating of 1.6 mils was formed, having a surface concentration of 36% chromium. 'Cr O may be substituted for CrO as the source for chromium with substantially equivalent results.
• EXAMPLE 3 An iron container holding a bath composed of 100 that used in theruns of Example 1 was introduced and withdrawn after two hours of treatment. Analysis showed that the'surface of the coating contained 22% chromium EXAMPLE 4 was agitated and operated under argon. A sample of iron (0.0025% G) was. immersed in this bath for 15 minutes at 1100 .C. A coating 0.2 mil thick was obtained on the base metal containing approxiamtely 30% cobalt at the surface.
• EXAMPL 0 7 A bath was formed in amolybdenum crucible containing 90 grams strontium and 10' grams powdered nickel.
• the bath was agitated and operated under argon. A sam- 7 grams strontium and 5. grams of cobalt powder. The bath A bath comprising 500 grams calcium and 100 grams ferrochrome 100 mesh) was heated toYl060". C. in
• Ahigh carbon (about 0.8% carbon): steel sample /s" 'x 2") was treated for 16 minutes and'was found to have a chromized coating 0.40 mil thick; The surface concentration of chromium was 18%.
• sample Iij was 0.62 mil thick, having a surface concentration of,37 Ni;
• sample '4 was 0.81 mil thick, having a surface concentration of 39% Ni.
• a bath was formed containing 500 grams calcium :and'
• EXAMPLE 14 A bath of 500 grams calcium and 5 grams chromium powder was heated to 1100 C. Then 88 grams nickel shot were added after the calcium-chromium had been stirred (under argon) for 20 minutes. The temperature was 1100 C. After 30 minutes additional stirring, a mild steel sample was treated for one hour, then removed, water quenched, and cleaned. A coating of 0.51 mil was formed. The surface composition was 17% chromium, 23% nickel, and 60% iron. X-ray diffraction data showed that the surface was pure austenite.
• EXAMPLE 15 A bath containing 500 grams calcium, 30 grams powdered chromium and 70 grams nickel shot was heated to 1100 C. A mild steel sample immersed in this bath for 4 hours had a coating 2.78 mils thick. The diffusion was carried out under an argon atmosphere with agitation.
• EXAMPLE 16 In a bath prepared from calcium, powdered chromium and nickel shot in the weight ratio of 400240170, a mild steel sample was treated for one hour. The surface composition was analyzed as about 45% chromium and 5% nickel. X-ray diffraction analysis and metallographic cross section examination showed that the coating contained no austenite but only ferrite.
• Examples 12-16 show that concentrations of the elements present in the alloy coatings can be controlled and that alloys of widely varying contents can be formed by varying the amounts of the diffusing elements in the bath while the thickness of the coating formed is primarily dependent on the time of treatment at a given temperature and whether the coating is ferritic or austenitic at the treatment temperature.
• EXAMPLE 17 A bath of 500 grams calcium and a ten-gram lump of cobalt was heated to 1050-1100 C. 20 grams of chromium powder was added after the calcium-cobalt had been stirred under argon for one hour. After minutes additional stirring, a mild steel sample was treated for 30 minutes at 1050-1100 C. and a coating containing cobalt and chromium was formed. The outer surface of the coating analyzed about 27% cobalt and 14% chromium.
• a new bath was made up having 500 grams calcium, 5 grams cobalt powder, and 20 grams chromium powder. After being immersed for five hours in the bath at 1050- 1100" C., the surface of a sample was analyzed. The alloy had about 20% chromium and about cobalt. Under the same conditions a bath having 500 grams calcium, grams chromium, and 5 grams cobalt yielded coatings having a surface concentration of about 22% chromium and 16% cobalt after one hour.
• the diffusion process can be advantageously employed to alter the surface characteristics of austenitic stainless steel so as to prevent stress corrosion cracking.
• Stress corrosion cracking generally occurs under residual or applied stress and the cracking of austenitic stainless steel can be observed when the steel is stressed and its surface is exposed to solutions containing chlorides.
• annealing may relieve prior residual stresses, additional stresses introduced by quenching from the annealing treatment can cause cracking under corrosive conditions.
• Diffusing baths used in -1 and 2 of Table I contained 600 grams calcium and 60 grams chromium and were operated at about 1060 C. In 1 and 2 it will be seen that the nickel concentration in the steel article decreased. Since no nickel was present in these baths, the distribution of nickel in the baths and in the articles was not an equilibrium distribution. Thus part of the nickel diff-used from the articles surface into the bath.
• the bath used in 3 contained 800 grams calcium, 112 grams powdered chromium and 336 grams nickel shot and was operated at about 1100 C.
• the stainless steel samples of 2 and 3 were treated separately in the order listed in Table I.
• the samples treated in baths of 1-3 were rapidly cooled and the most resistant sample to cracking ad a ferritic layer exposed to Mgcl EXAMPLE 19
• the diffusion process can be advantageously employed to obtain intricately shaped forms by diffusion coating shaped or machined ferrous parts for a sufficient length of time to impart a coating (usually 3-10 mils) of the desired thickness.
• the coating can be punctured and the ferrous substrate containing less than 12% chromium can be removed by dissolving. in this way, light-weight complex parts can be produced.
• the essentially infinite throwing power (i.e., the ability to coat inside small recesses or cavities) of the metal transfer baths permits intricate shapes to be evenly coated.
• a A-inch section of one inch diameter iron bar had a A-inch hole drilled through the axis of the bar and three /l-lIlCh holes drilled radially perpendicular to the axis.
• This part was then immersed in a diffusion bath containing molten calcium and an excess of chromium and the bath -was maintained .at 1060" Cpfor 45 hours. All exposed surfaces were uniformly alloyed with chromium and the coating was found to extend more than ten mils in depth from the surface.
• the top of the part was sawed off and the ferrous base metal completely dissolved out by hot nitric acid leaving a thinshell of stainless steel having an intricate shape. It has been found that hot nitric acid removes the ferrous metal containing less than about 12% i2% chromium.
• novel articles of manufacture comprising a ferrous metal substrate having a diffusion coating of a Pferritic iron-chromium alloy which exhibit physical and metallurgical characteristics long sought in the art that lead to significant improvements in corrosion resistance, appearance, and formability over previously known chromium diffusion articles. 7 V 7
• control of carbon concen-' tration in the chromium diffusion coating on a ferrous metal substrate in the past has presented a special problem due to the strong tendency for the carbon in the sub-v strate metal to migrate, at the temperatures necessary for forming a diffusion coating, into the chromium-rich coating and concentrate there as carbides of iron and chromium.
• unstabilized ferrous metal substrate refers to' ferrous metal substrates which have not been alloyed withcarbon stabilizing elements and'must 'ings formed on an unstabilized ferrous metal substrate of metallic elements that can be found in steels as a consequence of usual refining procedures-
• carbon stabili'zing elements such as titanium,.niobium, tantalum, zirconium, and vanadium' are typically present in only small amounts unless purposefully added to steel since they a'repreferentially oxidized andslagged away during refining.
• elements such as chromium, manganese, molybdenum, and tungsten which may stabilize carbon'can be found in larger amounts introduced from scrap iron and incompletely removed during refining.
• unstabilized ferrous metal substrate as used herein and in the claims is intended to include ferrous metal substrates having no more than about 0.2%. by weight of' titani'urmniobium, tantalum, zirconium, or vanadiumor combinations thereof and no more than about 2%"by weight of chromium, manganese, molybdenum, or tungsten, or combinations thereof.
• the'-carbon values ofthe coating and the'bulk carbon concentration of the article were determinedby analysis andfthe carbon value of the. substrate calculated therefrom.
• the bull; carbon concentration was obtained by running an analysis on th'e'article as prepared consist- ]ing of the coating and substrate material.
• the coating carbon concentration was obtained by stripping off the coating from the article as prepared and running an analysis on the coating material thus isolated.
• the stripping technique employed involved cutting along one edge of V the article as prepared to expose. the substrate material and then immersing in boiling 30% nitric acid for l to 4 hours. The acid reagent dissolves away the substrate material leaving the chromium diffusion coating material substrate.
• a novel article of manufacture can be r surface thereof,-'wherein the surface of said coating is Ca and 72 ga'ofpowdered.chromium':('+325 mesh) was heated to. 11-40 .C.
• Type lofl- Al-killed steel Wasplaccd in the bath 'for "a treating time "of '9minutes, The coated coupon wa's'then removed from-the bath and rapidly quenched from a temperature of approximately -1000 C.
• a coating 1.0.mi1 was formed on the base steel analyzing 3 S% Cr at the surface of'the coating.
• the coating and bulk carbonconcent'ration' were determined by analysis I is to be 93 and 401 p.p.m., respectively.
• the carbon concentration of the substrate was calculated to be 407 p.p.m.
• EXAMPLE 21 An iron container holding a bath composed of 2300 g. Ca and 115 g. of powdered chromium -325 mesh) was heated to 1140 C. The bath was agitated by a mechanical stirrer. A coupon 20 mils thick of SAE type 1070 steel was placed in the bath for a treating time of minutes. The coated coupon was then removed from the bath and rapidly quenched from a temperature of approximately 1000" C. A coating 0.65 mil was formed on the base steel. The coating and bulk carbon concentrations were determined by analysis to be 240 and 1860 p.p.m., respectively. The carbon concentration of the substrate was calculated to be 1973 p.p.m.
• EXAMPLE 22 An iron container having a bath composed of 2000' g. of Ca and 100 g. of powdered chromium (100 mesh) was heated to 1100" C. The bath was agitated by a mechanical stirrer. A coupon 60 mils thick of SAE type 1008 rimmed steel was placed in the bath for a treatment time of 45 minutes. The coated coupon was then removed from the bath and rapidly quenched from a temperature of approximately 1000 C. A coating of 1.7 mils was formed on the base metal analyzing 43% Cr at the surface of the coating. The coating and bulk carbon concentrations were determined by analysis to be 110 and 336 p.p.m., respectively. The substrate carbon concentration was calculated to contain 350 p.p.m.
• EXAMPLE 23 A variety of other samples were coated in a similar manner after which the carbon concentration of the chromium diffusion coating and the chromium diifusion coating plus the substrate were analyzed. From these values, the carbon concentration of the substrate was calculated. The results are reported in Table 11 below and show that while there may be a wide variation in substrate carbon concentration, the coating carbon concenration of these samples is invariably less than the subable to rapidly quench the article recovered from the coating bath. To obtain optimum benefits the coating process will be operated above 1000 C. so that the article can be quenched from at least above 900 C.
• rapidly quenched has reference to a quick transfer of the article, for example, in the order of from 3 to econds, from the molten bath to a cooling medium which will quickly dissipate heat.
• Oil provides a suitable cooling medium. While most quenching oils are satisfactory in this respect, it is desirable to test any particular oil to insure that it does not carbonize in contact with the hot metal surface and thereby introduce additional carbon into the coating.
• Immersion or" the article in a fluidized bed or in a high velocity stream of gas, such as helium, has also proven successful for rapidly dissipating heat after coating. Water is also satisfactory as a cooling medium provided that rigorous precautions are taken to avoid the ignition of hydrogen liberated when water contacts calcrum.
• the coating is considered to be free 'of intergr anular precipitates of iron-chromium carbides if no grain within the field of view is found to be completely surrounded. by a ditched grain boundary. Coatings having a ditched grained boundary are found to exhibit significantly inferior corrosion resistance compared to the other types of microstructures observed in the application of the test.-
• the. coated article should generally be immersed in the quenching medium within a time of about 3 to 40 seconds after it is removed from the coating bath, it will be within the skill of one. versed'in the art of heat treating to determine the optimum condition using the micro- A structural characterization and the carbon analysis de- To determine if the chromium diffusion coatingofa stainless steels is applied to the sample prepared for testing in a particular manner.
• Electrolytic Oxalic Acid Etching Test This test has heretofore been used to distinguishbetween stepped and ditched grain'boundaries' as they affect corrosion resistance of austeniticstainless steel. Ithas now been determined that correlation can be made beappears to be necessitated by the peculiar etch character of the high chromium regionof'thecoatings in oxalic acid and by'the presence of. surface iron-chromium carbides on chromium diffusion coatings of the prior art.
• the thickness of the article as well as the efifective thermal conductivity of the transfer environment and the quenching'medium will determine in any particular case the limiting cooling condition to avoid intergranular preci itation andto insure that the carbon concentration'of the coating is maintained below that of the underlying substrate.
• EXAMPLE 24 A series of five samples,"each comprising an unstabilized ferrousmetal substrate having a diffusion coating of an iron-chromium alloy, were prepared by the process ing times as indicated at temperatures: above 500 C.
• the sample preparation procedurelbeforefthe oxalic acid etching test is conducted, therefore, consistsof elec z tropolishing the coating to remove so'muchof the coating 7 as is necessary to expose a layer containing a chromium by X-ray fluorescence.”
• sampleSLserVingf as an anode' are electrolyticallyetched in 10% by weight.
• the corrosion performance of the samples 130 is outstanding after extended exposure to the accelerated 50 CASS oooooion tooo- 25.5 IIIIIIII""1 37
• relatively inexpensive base metal 17 may be provided the surface characteristics of a superior None ferritic chromium steel with the use of only a very thin None surface coating.
• a ferritic chromium coating of any 293 None finite thickness may be useful in improving the corro- 0 29 5 None sion resistance of a base metal of mild steel, for example. None Usually, however, the ferritic iron-chromium alloy coat- 3L0 None ing will be of a thickness approximately 0.5 mil or 32.1 None greater.
• ferritic iron-chromium alloy used to de- Egg scribe the diifusion coating of the novel articles of the None invention is, of course, intended to include other alloy- None ing elements in addition to chromium and iron so long None as the structure of the alloy coating remains essentially None ferritic.
• nickel or nickel in combination with other elements may be incorporated in the iron-chromium alloy coating formed.
• articles of the invention may be formed having a wide range of chromium concentration, it is desirable that the chromium concentration at the surface of the coating be at least 12% by weight so as to impart a stainless quality to the surface. It is particularly preferred that the novel articles of the invention have a concentration of chromium at the surface of the diffusion coating in excess of 28% by weight. It has been found that articles of the invention in which the chromium concentration at the surface of the diffusion coating is in excess of 28% by weight are remarkably resistant to a well-known insidious type of corrosion, namely, pitting corrosion as illustrated by the following example.
• EXAMPLE 25 A series of samples comprising an unstabilized steel having a diffusion coating of a ferritic iron-chromium It is quite apparent from Table IV above that pitting corrosion is significantly reduced when the surface chromium concentration exceeds 24% by weight and essentially eliminated when the surface chromium concentration exceeds 28% by weight.
• the maximum chromium concentration at the surface of the coating for articles of the invention may well exceed the 41.7% shown above. Usually, however, there is little advantage in exceeding 60% by weight chrominum which is approximately the maximum attaintable surface concentration by the process of the invention.
• corrosion resistance has been shown for the novel articles hereof even further improved corrosion resistance can be obtained by Well-known post treatment techniques for this purpose.
• corrosion resistance can be markedly improved by passivating the article after coating in 50% nitric acid or 20% nitric acid2% sodium dichrornate solutions.
• the novel articles of the invention find utility in :a'wide variety of shaped forms and applications.
• the chromium diffusion coating may be formed on preshaped ferrous articles.
• automobile bumpers forms: automobile bumpers; automotive bright. hardware, such as, brake handles, door hardware, radio antennae, roof racks, windshield Wiper arms, dash board metal work, marine hardware; machinery, such as, busi' ness machine, hardware, gears, spray nozzles, valves, pumps, cams, conveyor parts, wire cables, springs, nuts, bolts, and screws; appliances, such as, irons, washing machine tubs,stationary tubs, and dish racks for dish washers; sporting equipment, such as, golf club heads, ice skates, fishing reel gears; and various consumer articles, 'such as, cutlery, screening, spades, flash light cases, etc. V
• the chromium diffusion coating can be readily formed on a flat rolled sheet of formable iron or steel after which the coated sheet may be formed into shaped articles.
• articles of the. invention can 'bemade into the following. forms: automobile bumpers; grilles, moldings, hub caps,. wheel covers, mufllers,'tail pipes; appliance trim, such as on refrigerators, ranges, toasters, coffee pots, etc.; kitchen cabinets; water heaters, water-softeners, water cooler tops; shower stalls; bath tubs; lavatories; sinks, splash boards, stove
• the articles of the invention can thus be made in the following shaped transfer agent;
• a process for the diffusion. coating of a ferrous metal article comprising contacting said article with a substantially carbon free molten 'bath containing at least about 10% by weight of calcium. and a source of chromium, said contacting being carried out at a temperature between; about 800 C. and the melting point of said article.
• a process for the diffusion coating of. a mild steel article comprising contacting. said article with a substantiallycarbon free molten bath containing at least about '10% by weight of calcium and a source of chromium,
• tially carbon free molten bath containing at least about 10% by weight of calcium and a source of chromium and nickel, said contacting being carried out at a temperature of from 1000 to 1200 C. and then quenching the diffusion coated article from above a temperature of 900 C.
• a process for the diffusion coating of a ferrous metal article comprising ,immersing'isaid'article in a substantially carbon free molten bath containing at least 60% by weight of a metal transfer agent selected from the group consisting of calcium, barium, strontium, andmag- V n'esium and at least one diffusing element selected from v the group consisting of chromium, 'nickel,manganese, and cobalt, said bath being maintained at a temperature bemetal transfer agent.
• a metal transfer agent selected from the group consisting of calcium, barium, strontium, andmag- V n'esium and at least one diffusing element selected from v the group consisting of chromium, 'nickel,manganese, and cobalt
• a process for the difiusion coating of a 'mild steel article comprising immersing said article in a substantially carbon free molten bath containing at least 10% by weight calcium and a sourceof chromium, said bath While other modifications of this invention, which inay be employed Within the scope of the invention havenot been described, the invention is intended to include all such as may be comprised within thefollowing claims.
• tially carbon-freemolten bath containing at least'about 10% by weight of a metal transferagent selected from f the group consisting iofcal'cium, barium, strontiumpand magnesium and atleast one diffusingblerhent selected 7 fromthe group/consisting of chromium, nickel, man ga- '"r' nese,'and1cobalt, said contacting being carricdi'o'ut atfa beingmaintained at a temperature between 1000 to li200 C, Withdrawing the 'difiusioncoated article from said bath and thereafter quenching said' coated from above a t'emperature of 900 C.
• a metal transferagent selected from f the group consisting iofcal'cium, barium, strontiumpand magnesium and atleast one diffusingblerhent selected 7 fromthe group/consisting of chromium, nickel, man ga- '"r' nese,'and1cobalt, said contacting being carric
• a process for the diffusion coating of a ferrous rnetal article comprising immersing said article in a substantially carbon free molten bath containing at least 10% V by weight calciumand a source of chromium and nickel, said bath being maintained at a temperature between 800 C., and themelting'point of said article.
• a process for thefdiflfusion coating of a mild-steel article comprising immersing-said article in a substantially carbon free 'molten *bath'jcontaining at'least 10% by weight'calcium anda source of chromiumand nickel, said bathfbeing ruaintainedi at atemp'er'ature between 1000 to 1200? C., withdrawing the diffusion coated article from 21 said bath and thereafter quenching said coated article 2,817,141 from above a temperature of 900 C. 3,058,841 3,061,462 References Cited by the Examiner 3,086,386

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• 1963
  • 1963-12-16 US US330540A patent/US3184331A/en not_active Expired - Lifetime
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