EP3850121A1 - Stainless steel object having a surface modified with chromium - Google Patents

Stainless steel object having a surface modified with chromium

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Publication number
EP3850121A1
EP3850121A1 EP19769790.7A EP19769790A EP3850121A1 EP 3850121 A1 EP3850121 A1 EP 3850121A1 EP 19769790 A EP19769790 A EP 19769790A EP 3850121 A1 EP3850121 A1 EP 3850121A1
Authority
EP
European Patent Office
Prior art keywords
stainless steel
chromium
steel object
weight
process according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP19769790.7A
Other languages
German (de)
French (fr)
Inventor
Mads Brink LAUERSEN
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Trd Surfaces Aps
Original Assignee
Trd Surfaces Aps
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Filing date
Publication date
Application filed by Trd Surfaces Aps filed Critical Trd Surfaces Aps
Publication of EP3850121A1 publication Critical patent/EP3850121A1/en
Pending legal-status Critical Current

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    • 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
    • C23C10/06—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases
    • 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
    • C23C10/06—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases
    • C23C10/08—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases only one element being diffused
    • 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
    • C23C10/06—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases
    • C23C10/08—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases only one element being diffused
    • C23C10/10—Chromising
    • 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
    • C23C10/06—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases
    • C23C10/08—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases only one element being diffused
    • C23C10/10—Chromising
    • C23C10/12—Chromising of ferrous surfaces
    • 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
    • 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
    • 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
    • C23C10/24—Salt bath containing the element to be diffused
    • 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
    • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
    • C23C10/30—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes using a layer of powder or paste on the surface
    • C23C10/32—Chromising
    • 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
    • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
    • C23C10/34—Embedding in a powder mixture, i.e. pack cementation
    • 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
    • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
    • C23C10/34—Embedding in a powder mixture, i.e. pack cementation
    • C23C10/36—Embedding in a powder mixture, i.e. pack cementation only one element being diffused
    • 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
    • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
    • C23C10/34—Embedding in a powder mixture, i.e. pack cementation
    • C23C10/36—Embedding in a powder mixture, i.e. pack cementation only one element being diffused
    • C23C10/38—Chromising
    • 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
    • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
    • C23C10/34—Embedding in a powder mixture, i.e. pack cementation
    • C23C10/36—Embedding in a powder mixture, i.e. pack cementation only one element being diffused
    • C23C10/38—Chromising
    • C23C10/40—Chromising of ferrous surfaces
    • 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
    • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
    • C23C10/34—Embedding in a powder mixture, i.e. pack cementation
    • C23C10/36—Embedding in a powder mixture, i.e. pack cementation only one element being diffused
    • C23C10/38—Chromising
    • C23C10/40—Chromising of ferrous surfaces
    • C23C10/42—Chromising of ferrous surfaces in the presence of volatile transport additives, e.g. halogenated substances
    • 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
    • C23C10/60—After-treatment

Definitions

  • the invention relates to a process for the manufacture of a stainless steel object having a surface modified with chromium. Furthermore, the invention relates to a stainless steel object having a surface modified with chromium obtainable according to the process.
  • Stainless steel is widely used in applications where components are exposed to both corrosion and wear at the same time .
  • GB 586,241 discloses a method for diffusion of metals such as chromium, into iron and steel objects.
  • the method includes providing the iron or steel object in a closed reaction vessel and packing the object with powders comprising ammonium chloride and one or more chromium compounds capable of diffusing chromium into the iron or steel object.
  • the powder mixture may also comprise sand or refractory powder.
  • the closed reaction vessel was subjected to a process involving heating to a temperature of between 700 and 1200°C. During the heat treatment, chromium diffuses into the surface and reacts with carbon if available.
  • US 2,536,774 discloses a process using an improved packing mixture which functions for sustained chromium diffusion into the ferrous tool.
  • the packing mixture uses a substantial proportion of kaolin which effectively absorbs or fixes the reaction-inhibiting ferrous halide vapor formed as the reaction product of the chromizing reaction.
  • the process is used on high carbon steel and cast iron to provide a surface which is hard and abrasion resistant.
  • GB 672,275 discloses a method for manufacture of tools made from ferrous metal having chromium diffused into the surface thereof to improved wear resistance.
  • the tools are usually manufactured from high carbon (cast) steel or special ferrous alloys which depend for their hardness upon the presence of carbon.
  • the diffusion of a high percentage of chromium into the surface results in the formation of a chromium carbide in the surface.
  • the ferrous metal having chromium diffused into its surface may be produced by packing the tool in a closed reaction vessel containing a packing mixture of ferro- chromium powder, kaolin as an inert filler, and potassium iodide as a catalyst, and heating the reaction vessel to 750°C-1100°C .
  • the treated tools could be re-heated in a controlled atmosphere furnace at 800°C and quenched in oil.
  • the produced tools showed an improved cutting efficiency and a reduced wear of the cutting edges.
  • the chromium diffuses into the surface of the stainless steel object during the thermochemical surface treatment. Due to the low concentration of carbon in the stainless steel object the chromium will not form chromium carbide in any substantial amount. Instead, the chromium will be present as an intermetallic phase often referred to as a sigma (s) phase.
  • the sigma phase is normally not desired in the stainless steel matrix as it is one of the main reasons for the deterioration of stainless steels' properties, for example, decreased corrosion resistance, decreased toughness, decreased elongation, and decreased weldability. However, in the presence as a surface coating, the sigma phase may improve wear and corrosion properties to some extent.
  • the present inventor has realized that a layer comprising the sigma phase can be transformed to a layer comprising a solid solution of chromium by subjecting the stainless steel object to a certain temperature and subsequently quenching the stainless steel object at a certain cooling rate to a temperature below the sigma phase transformation temperature.
  • the layer comprising the solid solution shows a higher corrosion resistance than the as-produced sigma phase .
  • carbon is present only in a low amount if present at all in the stainless steel object to prevent reaction of carbon and the diffused chromium to form chromium carbide.
  • a minor amount, such as 0.1 by weight or less of carbon, is acceptable because a major part of the chromium will still be present in the sigma-phase and thus available for the subsequent conversion to the solid state phase. It is suitable, that the concentration of carbon in the stainless steel object of step a is 0.07% by weight or less.
  • the stainless steel object may contain further components, such as manganese (Mn) , silicon (Si), vanadium (V), rhenium (Re), thallium (Tl) , aluminium (Al) , tungsten (W) , phosphor (P) , Nitrogen (N) , and sulfur (S) .
  • Mn manganese
  • Si silicon
  • V vanadium
  • Re rhenium
  • Tl thallium
  • Al aluminium
  • W tungsten
  • P phosphor
  • Nitrogen N
  • sulfur sulfur
  • Stabilising components such as titanium (Ti) , niobium (Nb) , zirconium (Zr) , and tantalum (Ta) , may be added in the manufacture of the stainless steel objects of the present invention, as such components tend to react faster with carbon than chromium.
  • Ti titanium
  • Nb niobium
  • Zr zirconium
  • Ta tantalum
  • it is not desired to include stabilising components in the composition because of increased costs, increased complexity, limited type of supply forms and/or the necessity for further process steps.
  • the produced reaction products between carbon and one or more of the stabilising components may provide for unpredictable or undesired properties of the final object.
  • the stainless steel object comprises austenitic or low-carbon martensitic stainless steel.
  • Austenitic stainless steel is also known as gamma-phase iron (g-Fe) and is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element.
  • Low-carbon martensitic stainless steel is a ferro-magnetic, relatively hard and tough allotrope of iron with a body centered tetragonal (BCT) structure.
  • the stainless steel is selected from the group consisting of AISI 316L, AISI 304L, and AISI S165M.
  • AISI 304L is 1.4307, EN X2CrNil8-19, UNS S30403.
  • Alternative names of AISI 316L is 1.4404, SS 2348, EN X2CrNiMol7-12-2 , UNS S31603.
  • Alternative names of AISI S165M is 1.4418, SS 2387, EN X4CrNiMol 6-5-1.
  • the composition of these stainless steel types are shown in the table below:
  • Table 1 Composition of stainless steel types. All figures are shown in % by weight.
  • the amount of chromium generally is below 25% by weight it is preferred in a suitable embodiment of the invention that the amount of chromium does not exceed 22% by weight and preferably does not exceed 20% by weight.
  • the content of chromium in the starting object is generally not less than 12% by weight, suitably not less than 14% by weight.
  • the amount of nickel in the composition may vary within the broad range of 2-25% by weight. Embodiments in the range 3- 20% by weight, typically 4-15% by weight, are generally preferred. Molybdenum may be present or absent depending of the type of stainless steel. When present, the amount of molybdenum does not exceed 4% by weight and usually the amount of molybdenum does not exceed 3% by weight of the composition. The amount of manganese is generally below 3% by weight, such as in the range of 0.5-2% by weight. The components silicon, phosphor, sulfur, and nitrogen are generally present in an amount of 1% by weight or less.
  • the stainless steel object is generally enriched in chromium by a thermochemical process. While an electrochemical process in principle would be suitable it is currently considered less preferred as an electrochemical process tend to provide a layer of chromium or chromium alloy on the surface rather than having the chromium diffused into the surface of the stainless steel object.
  • the stainless steel object having been subjected to the thermochemical treatment obtains a layer or a zone into which the chromium has diffused.
  • the stainless steel object is enriched in chromium to a concentration of 27% by weight or more in the surface layer.
  • the concentration of chromium in the surface layer is 30% by weight or above, such as 35% by weight or above.
  • the layer into which the chromium has diffused is clearly visible and distinguishable from the body part of the stainless steel object.
  • the surface layer of the stainless steel object being enriched in chromium has a thickness of 5-100pm. While a thickness of less than 5pm can be used in certain applications, the thickness of the layer is usually not less than 10pm, such as not less than 15pm to obtain a more pronounced corrosion reducing effect. Usually, it is sufficient that the thickness of the layer is 70pm or less, such as 50pm or less.
  • the layer of the stainless steel object that has been subjected to the diffusion of the chromium may be produced by a number of thermochemical processes. Notably, the stainless steel object having a surface layer enriched in chromium is produced by pack cementation, out-of-pack cementation, a salt bath method, or gas phase chemical vapor deposition.
  • the stainless steel object having a surface layer enriched in chromium is produced by pack cementation, comprising the steps of:
  • the pack cementation may also be referred to as pack chromizing in the literature.
  • the powder mixture contains a chromium donating powder also known as master alloy, i.e. a powder that during heating is able to deliver vaporized chromium for deposition in the surface of the stainless steel object.
  • the chromium donating powder may be selected among a number of components including but not limited to ferro- chromium (FeCr) , elemental chromium, and silicon-chromium (Cr-Si) .
  • the activator powder is usually a halide-salt activator, such as a fluoride, chloride, bromide, or iodide salt.
  • the cation of the halide salt is usually selected among sodium, potassium, ammonium, and methylammonium .
  • Specific materials useful for the activator powder may be selected among NaCl, NaF, NH4CI, AIF3, NH4I, KC1, and methylamine hydrochloride.
  • the activator powder is NH4CI.
  • the inert filler powder may be selected among a number of particle materials including alumina (AI2O3) , S1O 2 , SiC, and kaolin .
  • the powder mixture is packed around the stainless steel object in an enclosure, such as a steel box. After the packing of the stainless steel object the enclosure is sealed to minimize the escaping of gases.
  • the steel box is subsequently positioned in a furnace and heat treated.
  • the activator reacts with the chromium to form a gaseous halide compound, which is transferred to the surface of the stainless steel object.
  • the halogen activator is released and the chromium is deposited in the surface layer of the stainless steel object, leaving the activator to return to the pack and react with the source metal again.
  • the temperature should be sufficient for the chromium in the chromium donating powder to react with the halide for the formation of a gaseous compound.
  • the specific choice of temperature depends largely on the circumstances but is usually above 700°C, suitably above 800°C. To avoid substantive side reactions, the temperature generally does not exceed 1150°C, i.e. the temperature does generally not exceed 1050°C.
  • the treating time depends among other elements on the desired thickness of the chromium-enriched layer, the stainless steel type, and the shape of the stainless steel object. Generally, it is desired to treat the stainless steel object at the predetermined temperature for at least 15 minutes, such as at least one hour, and preferably at least two hours. Generally, it is not required to treat the is stainless steel object for more than 16 hours to obtain the desired effect. Typically, the treatment time at the predetermined temperature is between 4 and 8 hours. After the heat treatment, the enclosure is generally allowed to cool in the furnace, or outside the furnace dependent on the methods used. The enclosure may be opened after the temperature inside the enclosure has decreased to about 200°C or less to unpack the stainless steel object .
  • the stainless steel object in the surface of which the chromium has been deposited obtains a hard surface.
  • the Vickers hardness is 1000-1200HV and the surface is brittle.
  • the chromium is present in the surface layer as an intermetallic sigma phase.
  • step c. of the present invention the stainless steel object is heat treated in an inert atmosphere for transforming the sigma phase to a solid solution.
  • the specific choice of temperature depends largely on the circumstances but is usually above 900°C, suitably above 1000°C. To avoid substantive side reactions, the temperature generally does not exceed 1180°C, i.e. the temperature does generally not exceed 1150°C.
  • the stainless steel object in step c. is treated at a temperature of between 1000°C to 1150°C.
  • the inert atmosphere is non-reactive in the sense that it does not in any substantial degree react with the chromium- enriched surface of the object.
  • the inert atmosphere may be selected from the group comprising nitrogen (N2) , argon (Ar) , hydrogen (3 ⁇ 4) , helium (He), neon (Ne), krypton (Kr) , xenon (Xe) , and radon (Rn) .
  • a preferred inert gas is argon.
  • the inert atmosphere may also be attained in a vacuum condition, preferably less than 5xl0 -2 mbar of pressure. If discoloration of the treated stainless steel object can be accepted, a higher pressure can be applied, such as a vacuum at or below 1 mbar .
  • the treating time in step c. depends among other elements on the thickness of the chromium-enriched layer. Thus, in general, the thicker the chromium-enriched layer the longer the treating time. Generally, it is desired to treat the stainless steel object at the predetermined temperature for at least 10 minutes, such as at least 15 minutes, and preferably at least 25 minutes. Generally, it is not required to treat the stainless steel object for more than 2 hours to obtain the desired effect. Typically, the treatment time at the predetermined temperature is between 25 minutes and 1 hour .
  • the cooling rate is essential. If the cooling rate is slow, the chromium tends to return to the sigma phase, whereas a cooling rate of 0.5°C/s or above the chromium enriched region will be kept as a solid solution phase. In a preferred aspect the cooling rate is not above 50°C/s to avoid stress in the object. In a preferred aspect the cooling rate of step c. is 0.5°C/s to 15°C/s.
  • the thickness of the layer enriched in chromium is generally increased during the heat treatment of step c.
  • the increased thickness of the layer is due to the transformation of the sigma phase to a solid solution phase.
  • the thickness of the layer increases 10% or more, such as 20% or more.
  • the layer does not increase to a thickness greater than 200% of the surface layer obtained in step b.
  • the layer usually does not increase to a thickness greater than 100% of the surface layer obtained in step b, i.e. usually the thickness of the surface layer is increased between 30% and 70% compared to the chromium enriched surface layer obtained in step b.
  • the increase of the surface layer thickness can be above 100%, such as in the range of 100% to 200% of the surface layer obtained in step b .
  • the stainless steel object having a surface modified with chromium produced as described herein obtains exceptional high corrosion resistance.
  • the pitting corrosion is less than 1 g/m 2 , such as less than 0.5 g/m 2 , and preferably less than 0.1 g/m 2 .
  • the corrosion is 0.01 g/m 2 , i.e. virtually non-existing.
  • the hardness measured as the Vickers hardness is decreased by the heat treatment of step c. In general the hardness is reduced at least 30% and preferably to a value below 600HV. The reduced hardness provides for an object being more ductile and less brittle.
  • the stainless steel objects having been subjected to the process described herein find applications in various areas, including but not limited to food preparation equipment, chemical containers, pharmaceuticals, marine applications, architectural constructions, medical implants (pins, screws, and orthopaedic implants like total hip and knee replacements), fasteners, and equipment for beer-brewing and wine-making .
  • Example 1 Stainless steel type 1.4404 (316L)
  • Coupons were made of 20x20x5mm austenitic stainless steel 1.4404.
  • 1 st step The coupons were subjected to chromium powder-pack treatment (powder pack: pure chromium powder, alumina powder and NEUCl activator) at 980°C for 8 hours in a closed retort furnace with subsequent slow cooling (4 hours) to room temperature.
  • the treated coupons had a 14pm Cr-enriched layer in the surface made of sigma-phase. The surfaces showed a micro vickers hardness of 1100-1200HV.
  • 2 nd step The coupons were heated at a heating rate of 5°C/min in a vacuum heat treatment furnace with a vacuum level of 10 ⁇ 2 mbar . A solution treatment was performed at 1060°C for 45 min. The treated coupons were quenched with 5 bars of overpressure of nitrogen gas with a cooling rate of 1°C per second .
  • Fig. 1 shows the profiles for the chromium concentrations for the 1 st and the 2 nd step. It is noted that the solution treatment of step 2 dissolved the sigma-phase formed during the 1 st step and produced a 21pm diffusion zone of chromium in solid solution with an average micro vickers hardness of 410HV.
  • Untreated coupons, coupons treated by the 1 st step only, and coupons treated by the 1 st step as well as the 2 nd step were subjected to ASTM G48 corrosion test.
  • the test involves subjecting the coupons to a pitting corrosion test by immersion of the coupons in 6% FeCl3 aqueous solution for 24 hours .
  • the test results are shown in Fig. 2.
  • the untreated coupons showed a corrosion rate of 73,39 g/m 2 .
  • the treatment according to the first step resulted in the corrosion rate being decreased from 73,39 g/m 2 to 5,47 g/m 2 , while after performing the second solution step, corrosion rate is practically zero.
  • the test results validate the pitting corrosion resistance, which is the most common form of corrosion. This type of corrosion is the dominant in environments with chloride ions present (e.g. seawater).
  • Example 2 Stainless steel type 1.4307 (304L)
  • Cubes were made of 40x40x40mm austenitic stainless steel 1.4307.
  • 1st step The cubes were subjected to chromium powder-pack treatment (powder pack: ferrochromium powder, alumina powder and NEUCl activator) at 980°C for 8 hours in a closed retort furnace with subsequent slow cooling (4 hours) to room temperature.
  • the treated cubes had a 22pm Cr-enriched zone in the surface made of sigma-phase.
  • the surface showed a micro vickers hardness of 1100-1200HV.
  • 2nd step The cubes were heated at a heating rate of 5°C/min in a vacuum heat treatment furnace with a vacuum level of 10 ⁇ 2 mbar . A solution treatment was performed at 1020°C for 30 min. The treated cubes were quenched with 5 bars of overpressure in nitrogen gas with a cooling rate of 1.5°C per second .
  • step 2 dissolves the sigma-phase and produces a 33 pm diffusion zone of chromium in solid solution with an average micro vickers hardness of 410HV.
  • Discs were made of 030x5mm martensitic stainless steel 1.4418
  • 1st step The discs were subjected to chromium powder-pack treatment (powder pack: pure chromium powder, alumina powder and NH4CI activator) at 980°C for 8 hours in a closed retort furnace with subsequent slow cooling (4 hours) to room temperature.
  • the treated discs had a 20pm zone made of sigma- phase with a micro vickers hardness of 1200-1300HV and a further chromium diffusion zone of 20pm ranging from 32 wt% to 28wt% chromium.
  • the base material will have a hardness of 380-420HV .
  • 2nd step The discs were heated at a heating rate of 5°C/min in a vacuum heat treatment furnace with a vacuum level of 10 ⁇ 2 mbar . A solution treatment was performed at 1090°C for 30 min. The treated discs were quenched with 5 bars of overpressure of nitrogen gas with a cooling rate of 1.5°C per second. The solution treatment dissolves the sigma-phase and produces, together with the original diffusion zone, a total of 50 pm chromium diffusion zone in solid solution with an average micro vickers hardness of 480HV.

Abstract

Disclosed herein is a process for the manufacture of a stainless steel object having a surface modified with chromium, comprising the steps of providing a stainless steel object comprising 12-25% by weight chromium, 2-25 % by weight nickel, 0-4% by weight molybdenum, 0.1% by weight or less carbon, and to 100% by weight iron; enriching a surface layer of the stainless steel object in chromium by a thermochemical surface treatment; treating the stainless steel object having a surface enriched in chromium at a temperature of 900°C to 1180°C in an inert atmosphere or vacuum; and subsequently, quenching the stainless steel object at a cooling rate of 0.5°C/s to 50°C/s to a temperature of 500 °C or less. The surface-modified stainless steel objects obtained by the process shows high corrosion resistance.

Description

TITLE
Stainless steel object having a surface modified with chromium TECHNICAL FIELD
The invention relates to a process for the manufacture of a stainless steel object having a surface modified with chromium. Furthermore, the invention relates to a stainless steel object having a surface modified with chromium obtainable according to the process.
BACKGROUND
Stainless steel is widely used in applications where components are exposed to both corrosion and wear at the same time .
GB 586,241 discloses a method for diffusion of metals such as chromium, into iron and steel objects. The method includes providing the iron or steel object in a closed reaction vessel and packing the object with powders comprising ammonium chloride and one or more chromium compounds capable of diffusing chromium into the iron or steel object. Optionally, the powder mixture may also comprise sand or refractory powder. The closed reaction vessel was subjected to a process involving heating to a temperature of between 700 and 1200°C. During the heat treatment, chromium diffuses into the surface and reacts with carbon if available.
US 2,536,774 discloses a process using an improved packing mixture which functions for sustained chromium diffusion into the ferrous tool. The packing mixture uses a substantial proportion of kaolin which effectively absorbs or fixes the reaction-inhibiting ferrous halide vapor formed as the reaction product of the chromizing reaction. The process is used on high carbon steel and cast iron to provide a surface which is hard and abrasion resistant.
GB 672,275 discloses a method for manufacture of tools made from ferrous metal having chromium diffused into the surface thereof to improved wear resistance. The tools are usually manufactured from high carbon (cast) steel or special ferrous alloys which depend for their hardness upon the presence of carbon. The diffusion of a high percentage of chromium into the surface results in the formation of a chromium carbide in the surface. The ferrous metal having chromium diffused into its surface may be produced by packing the tool in a closed reaction vessel containing a packing mixture of ferro- chromium powder, kaolin as an inert filler, and potassium iodide as a catalyst, and heating the reaction vessel to 750°C-1100°C . The treated tools could be re-heated in a controlled atmosphere furnace at 800°C and quenched in oil. The produced tools showed an improved cutting efficiency and a reduced wear of the cutting edges.
The diffusion of chromium into iron or steel having a significant carbon content results in the formation of a hard and brittle material due to the formation of chromium carbide. While a hard and brittle material may be desirable for many objects it also has disadvantages due to low processability and ductility. Furthermore, the material may be susceptible to corrosion by acids due to cracks in the brittle chromium carbide coating. It is an object of the present invention to provide a stainless steel in which the disadvantages of the ferrous materials of the prior art are reduced.
SUMMARY
It is an object to provide a process for the manufacture of a stainless steel object having a surface modified with chromium, comprising the steps of:
a. providing a stainless steel object comprising
i. 12-25% by weight chromium,
ii. 2-25 % by weight nickel
iii. 0-4% by weight molybdenum
iv. 0.1% by weight or less carbon, and v. to 100% by weight iron
b. enriching a surface layer of the stainless steel object in chromium by a thermochemical surface treatment,
c. treating the stainless steel object having a surface enriched in chromium at a temperature of 900°C to 1180°C in an inert atmosphere or vacuum, and
d. subsequently, quenching the stainless steel object at a cooling rate of 0.5°C/s to 50°C/s to a temperature of 500 °C or less.
The chromium diffuses into the surface of the stainless steel object during the thermochemical surface treatment. Due to the low concentration of carbon in the stainless steel object the chromium will not form chromium carbide in any substantial amount. Instead, the chromium will be present as an intermetallic phase often referred to as a sigma (s) phase. The sigma phase is normally not desired in the stainless steel matrix as it is one of the main reasons for the deterioration of stainless steels' properties, for example, decreased corrosion resistance, decreased toughness, decreased elongation, and decreased weldability. However, in the presence as a surface coating, the sigma phase may improve wear and corrosion properties to some extent.
The present inventor has realized that a layer comprising the sigma phase can be transformed to a layer comprising a solid solution of chromium by subjecting the stainless steel object to a certain temperature and subsequently quenching the stainless steel object at a certain cooling rate to a temperature below the sigma phase transformation temperature. Surprisingly, the layer comprising the solid solution shows a higher corrosion resistance than the as-produced sigma phase .
DETAILED DECRIPTION OF THE INVENTION
Generally, carbon is present only in a low amount if present at all in the stainless steel object to prevent reaction of carbon and the diffused chromium to form chromium carbide. A minor amount, such as 0.1 by weight or less of carbon, is acceptable because a major part of the chromium will still be present in the sigma-phase and thus available for the subsequent conversion to the solid state phase. It is suitable, that the concentration of carbon in the stainless steel object of step a is 0.07% by weight or less.
Besides the component mentioned in step a. the stainless steel object may contain further components, such as manganese (Mn) , silicon (Si), vanadium (V), rhenium (Re), thallium (Tl) , aluminium (Al) , tungsten (W) , phosphor (P) , Nitrogen (N) , and sulfur (S) .
Stabilising components, such as titanium (Ti) , niobium (Nb) , zirconium (Zr) , and tantalum (Ta) , may be added in the manufacture of the stainless steel objects of the present invention, as such components tend to react faster with carbon than chromium. However, generally, it is not desired to include stabilising components in the composition because of increased costs, increased complexity, limited type of supply forms and/or the necessity for further process steps. Furthermore, the produced reaction products between carbon and one or more of the stabilising components may provide for unpredictable or undesired properties of the final object.
In a preferred aspect of the invention, the stainless steel object comprises austenitic or low-carbon martensitic stainless steel. Austenitic stainless steel is also known as gamma-phase iron (g-Fe) and is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. Low-carbon martensitic stainless steel is a ferro-magnetic, relatively hard and tough allotrope of iron with a body centered tetragonal (BCT) structure.
In a preferred embodiment of the invention the stainless steel is selected from the group consisting of AISI 316L, AISI 304L, and AISI S165M. Alternative names of AISI 304L is 1.4307, EN X2CrNil8-19, UNS S30403. Alternative names of AISI 316L is 1.4404, SS 2348, EN X2CrNiMol7-12-2 , UNS S31603. Alternative names of AISI S165M is 1.4418, SS 2387, EN X4CrNiMol 6-5-1. The composition of these stainless steel types are shown in the table below:
Table 1: Composition of stainless steel types. All figures are shown in % by weight.
Other types of stainless steels may also successfully be subjected to the process of the present invention. While the amount of chromium generally is below 25% by weight it is preferred in a suitable embodiment of the invention that the amount of chromium does not exceed 22% by weight and preferably does not exceed 20% by weight. To obtain the desirable properties of the present invention the content of chromium in the starting object is generally not less than 12% by weight, suitably not less than 14% by weight.
The amount of nickel in the composition may vary within the broad range of 2-25% by weight. Embodiments in the range 3- 20% by weight, typically 4-15% by weight, are generally preferred. Molybdenum may be present or absent depending of the type of stainless steel. When present, the amount of molybdenum does not exceed 4% by weight and usually the amount of molybdenum does not exceed 3% by weight of the composition. The amount of manganese is generally below 3% by weight, such as in the range of 0.5-2% by weight. The components silicon, phosphor, sulfur, and nitrogen are generally present in an amount of 1% by weight or less.
The stainless steel object is generally enriched in chromium by a thermochemical process. While an electrochemical process in principle would be suitable it is currently considered less preferred as an electrochemical process tend to provide a layer of chromium or chromium alloy on the surface rather than having the chromium diffused into the surface of the stainless steel object.
The stainless steel object having been subjected to the thermochemical treatment obtains a layer or a zone into which the chromium has diffused. Generally, the stainless steel object is enriched in chromium to a concentration of 27% by weight or more in the surface layer. Suitably, the concentration of chromium in the surface layer is 30% by weight or above, such as 35% by weight or above.
When viewed in a microscope, the layer into which the chromium has diffused is clearly visible and distinguishable from the body part of the stainless steel object. Generally, the surface layer of the stainless steel object being enriched in chromium has a thickness of 5-100pm. While a thickness of less than 5pm can be used in certain applications, the thickness of the layer is usually not less than 10pm, such as not less than 15pm to obtain a more pronounced corrosion reducing effect. Usually, it is sufficient that the thickness of the layer is 70pm or less, such as 50pm or less. The layer of the stainless steel object that has been subjected to the diffusion of the chromium may be produced by a number of thermochemical processes. Notably, the stainless steel object having a surface layer enriched in chromium is produced by pack cementation, out-of-pack cementation, a salt bath method, or gas phase chemical vapor deposition.
Generally, the stainless steel object having a surface layer enriched in chromium is produced by pack cementation, comprising the steps of:
• packing the stainless steel object in a vessel containing a powder mixture comprising a chromium donating powder, an activator powder, and an inert filler powder,
• treating the vessel at a temperature between 700°C to 1150°C for diffusion of chromium into the surface of the stainless steel object, and
• cooling and unpacking of the stainless steel object .
The pack cementation may also be referred to as pack chromizing in the literature. The powder mixture contains a chromium donating powder also known as master alloy, i.e. a powder that during heating is able to deliver vaporized chromium for deposition in the surface of the stainless steel object. The chromium donating powder may be selected among a number of components including but not limited to ferro- chromium (FeCr) , elemental chromium, and silicon-chromium (Cr-Si) . The activator powder is usually a halide-salt activator, such as a fluoride, chloride, bromide, or iodide salt. The cation of the halide salt is usually selected among sodium, potassium, ammonium, and methylammonium . Specific materials useful for the activator powder may be selected among NaCl, NaF, NH4CI, AIF3, NH4I, KC1, and methylamine hydrochloride. Suitably, the activator powder is NH4CI.
The inert filler powder may be selected among a number of particle materials including alumina (AI2O3) , S1O2, SiC, and kaolin .
The powder mixture is packed around the stainless steel object in an enclosure, such as a steel box. After the packing of the stainless steel object the enclosure is sealed to minimize the escaping of gases. The steel box is subsequently positioned in a furnace and heat treated.
At the treating temperature, the activator reacts with the chromium to form a gaseous halide compound, which is transferred to the surface of the stainless steel object. As this gas decomposes, the halogen activator is released and the chromium is deposited in the surface layer of the stainless steel object, leaving the activator to return to the pack and react with the source metal again.
The temperature should be sufficient for the chromium in the chromium donating powder to react with the halide for the formation of a gaseous compound. The specific choice of temperature depends largely on the circumstances but is usually above 700°C, suitably above 800°C. To avoid substantive side reactions, the temperature generally does not exceed 1150°C, i.e. the temperature does generally not exceed 1050°C.
The treating time depends among other elements on the desired thickness of the chromium-enriched layer, the stainless steel type, and the shape of the stainless steel object. Generally, it is desired to treat the stainless steel object at the predetermined temperature for at least 15 minutes, such as at least one hour, and preferably at least two hours. Generally, it is not required to treat the is stainless steel object for more than 16 hours to obtain the desired effect. Typically, the treatment time at the predetermined temperature is between 4 and 8 hours. After the heat treatment, the enclosure is generally allowed to cool in the furnace, or outside the furnace dependent on the methods used. The enclosure may be opened after the temperature inside the enclosure has decreased to about 200°C or less to unpack the stainless steel object .
The stainless steel object in the surface of which the chromium has been deposited obtains a hard surface. Usually, the Vickers hardness is 1000-1200HV and the surface is brittle. The chromium is present in the surface layer as an intermetallic sigma phase.
In step c. of the present invention, the stainless steel object is heat treated in an inert atmosphere for transforming the sigma phase to a solid solution. The specific choice of temperature depends largely on the circumstances but is usually above 900°C, suitably above 1000°C. To avoid substantive side reactions, the temperature generally does not exceed 1180°C, i.e. the temperature does generally not exceed 1150°C. Generally, the stainless steel object in step c. is treated at a temperature of between 1000°C to 1150°C.
The inert atmosphere is non-reactive in the sense that it does not in any substantial degree react with the chromium- enriched surface of the object. The inert atmosphere may be selected from the group comprising nitrogen (N2) , argon (Ar) , hydrogen (¾) , helium (He), neon (Ne), krypton (Kr) , xenon (Xe) , and radon (Rn) . A preferred inert gas is argon. The inert atmosphere may also be attained in a vacuum condition, preferably less than 5xl0-2 mbar of pressure. If discoloration of the treated stainless steel object can be accepted, a higher pressure can be applied, such as a vacuum at or below 1 mbar .
The treating time in step c. depends among other elements on the thickness of the chromium-enriched layer. Thus, in general, the thicker the chromium-enriched layer the longer the treating time. Generally, it is desired to treat the stainless steel object at the predetermined temperature for at least 10 minutes, such as at least 15 minutes, and preferably at least 25 minutes. Generally, it is not required to treat the stainless steel object for more than 2 hours to obtain the desired effect. Typically, the treatment time at the predetermined temperature is between 25 minutes and 1 hour .
For the chromium to be transformed from the sigma phase to a solid solution phase the cooling rate is essential. If the cooling rate is slow, the chromium tends to return to the sigma phase, whereas a cooling rate of 0.5°C/s or above the chromium enriched region will be kept as a solid solution phase. In a preferred aspect the cooling rate is not above 50°C/s to avoid stress in the object. In a preferred aspect the cooling rate of step c. is 0.5°C/s to 15°C/s.
The thickness of the layer enriched in chromium is generally increased during the heat treatment of step c. The increased thickness of the layer is due to the transformation of the sigma phase to a solid solution phase. Generally, the thickness of the layer increases 10% or more, such as 20% or more. Usually, the layer does not increase to a thickness greater than 200% of the surface layer obtained in step b. For austenitic types of stainless steel, the layer usually does not increase to a thickness greater than 100% of the surface layer obtained in step b, i.e. usually the thickness of the surface layer is increased between 30% and 70% compared to the chromium enriched surface layer obtained in step b. For low-carbon martensitic stainless steel, the increase of the surface layer thickness can be above 100%, such as in the range of 100% to 200% of the surface layer obtained in step b .
The stainless steel object having a surface modified with chromium produced as described herein obtains exceptional high corrosion resistance. Thus, when measured according to the ASTM G48 corrosion test (immersion of the object in 6% FeCl3 (aq) for 24 hours) the pitting corrosion is less than 1 g/m2, such as less than 0.5 g/m2, and preferably less than 0.1 g/m2. In a specific example reported herein the corrosion is 0.01 g/m2, i.e. virtually non-existing. The hardness measured as the Vickers hardness is decreased by the heat treatment of step c. In general the hardness is reduced at least 30% and preferably to a value below 600HV. The reduced hardness provides for an object being more ductile and less brittle.
The stainless steel objects having been subjected to the process described herein find applications in various areas, including but not limited to food preparation equipment, chemical containers, pharmaceuticals, marine applications, architectural constructions, medical implants (pins, screws, and orthopaedic implants like total hip and knee replacements), fasteners, and equipment for beer-brewing and wine-making .
EXAMPLES
Example 1: Stainless steel type 1.4404 (316L)
Coupons were made of 20x20x5mm austenitic stainless steel 1.4404.
1st step: The coupons were subjected to chromium powder-pack treatment (powder pack: pure chromium powder, alumina powder and NEUCl activator) at 980°C for 8 hours in a closed retort furnace with subsequent slow cooling (4 hours) to room temperature. The treated coupons had a 14pm Cr-enriched layer in the surface made of sigma-phase. The surfaces showed a micro vickers hardness of 1100-1200HV.
2nd step: The coupons were heated at a heating rate of 5°C/min in a vacuum heat treatment furnace with a vacuum level of 10~ 2 mbar . A solution treatment was performed at 1060°C for 45 min. The treated coupons were quenched with 5 bars of overpressure of nitrogen gas with a cooling rate of 1°C per second .
Fig. 1 shows the profiles for the chromium concentrations for the 1st and the 2nd step. It is noted that the solution treatment of step 2 dissolved the sigma-phase formed during the 1st step and produced a 21pm diffusion zone of chromium in solid solution with an average micro vickers hardness of 410HV.
Untreated coupons, coupons treated by the 1st step only, and coupons treated by the 1st step as well as the 2nd step were subjected to ASTM G48 corrosion test. The test involves subjecting the coupons to a pitting corrosion test by immersion of the coupons in 6% FeCl3 aqueous solution for 24 hours .
The test results are shown in Fig. 2. The untreated coupons showed a corrosion rate of 73,39 g/m2. The treatment according to the first step resulted in the corrosion rate being decreased from 73,39 g/m2 to 5,47 g/m2, while after performing the second solution step, corrosion rate is practically zero. The test results validate the pitting corrosion resistance, which is the most common form of corrosion. This type of corrosion is the dominant in environments with chloride ions present (e.g. seawater). Example 2: Stainless steel type 1.4307 (304L)
Cubes were made of 40x40x40mm austenitic stainless steel 1.4307.
1st step: The cubes were subjected to chromium powder-pack treatment (powder pack: ferrochromium powder, alumina powder and NEUCl activator) at 980°C for 8 hours in a closed retort furnace with subsequent slow cooling (4 hours) to room temperature. The treated cubes had a 22pm Cr-enriched zone in the surface made of sigma-phase. The surface showed a micro vickers hardness of 1100-1200HV.
2nd step: The cubes were heated at a heating rate of 5°C/min in a vacuum heat treatment furnace with a vacuum level of 10~ 2 mbar . A solution treatment was performed at 1020°C for 30 min. The treated cubes were quenched with 5 bars of overpressure in nitrogen gas with a cooling rate of 1.5°C per second .
The solution treatment of step 2 dissolves the sigma-phase and produces a 33 pm diffusion zone of chromium in solid solution with an average micro vickers hardness of 410HV.
Example 3: Stainless steel type 1.4418 (S165M)
Discs were made of 030x5mm martensitic stainless steel 1.4418
1st step: The discs were subjected to chromium powder-pack treatment (powder pack: pure chromium powder, alumina powder and NH4CI activator) at 980°C for 8 hours in a closed retort furnace with subsequent slow cooling (4 hours) to room temperature. The treated discs had a 20pm zone made of sigma- phase with a micro vickers hardness of 1200-1300HV and a further chromium diffusion zone of 20pm ranging from 32 wt% to 28wt% chromium. The base material will have a hardness of 380-420HV .
2nd step: The discs were heated at a heating rate of 5°C/min in a vacuum heat treatment furnace with a vacuum level of 10~ 2 mbar . A solution treatment was performed at 1090°C for 30 min. The treated discs were quenched with 5 bars of overpressure of nitrogen gas with a cooling rate of 1.5°C per second. The solution treatment dissolves the sigma-phase and produces, together with the original diffusion zone, a total of 50 pm chromium diffusion zone in solid solution with an average micro vickers hardness of 480HV.

Claims

1. A process for the manufacture of a stainless steel object having a surface modified with chromium, comprising the steps of:
a. providing a stainless steel object comprising
i. 12-25% by weight chromium,
ii. 2-25 % by weight nickel
iii. 0-4% by weight molybdenum
iv. 0.1% by weight or less carbon, and v. to 100% by weight iron
b. enriching a surface layer of the stainless steel object in chromium by a thermochemical surface treatment,
c. treating the stainless steel object having a surface enriched in chromium at a temperature of 900°C to 1180°C in an inert atmosphere or vacuum, and
d. subsequently, quenching the stainless steel object at a cooling rate of 0.5°C/s to 50°C/s to a temperature of 500 °C or less.
2. The process according to claim 1, wherein the concentration of carbon in the stainless steel object of step a is 0.07% by weight or less.
3. The process according to claim 1 or 2, wherein the stainless steel object comprises austenitic or low- carbon martensitic stainless steel.
4. The process according to anyone of the claims 1 to 3, wherein the stainless steel is selected from the group consisting of AISI 316L, AISI 304L, and AISI S165M.
5. The process according to anyone of the claims 1 to 4 , wherein the stainless steel object is enriched in chromium to a concentration of 27% by weight or more in the surface layer.
6. The process according to anyone of the claims 1 to 5, wherein the surface layer of the stainless steel object being enriched in chromium has a thickness of 5-100pm.
7. The process according to claim 1 to 6, wherein the stainless steel object having a surface layer enriched in chromium is produced by pack cementation, out-of-pack cementation, a salt bath method, or gas phase chemical vapor deposition.
8. The process according to claim 7, wherein the stainless steel object having a surface layer enriched in chromium is produced by pack cementation, comprising the steps of :
• packing the stainless steel object in a vessel containing a powder mixture comprising a chromium donating powder, an activator powder, and an inert filler powder,
• treating the vessel at a temperature between 700°C to 1150°C for diffusion of chromium into the surface of the stainless steel object, and
• cooling and unpacking of the stainless steel object .
9. The process according to claim 1 to 8, wherein the stainless steel object in step c. is treated at a temperature of between 1000°C to 1150°C.
10. The process according to anyone of the claims 1 to 9, wherein the cooling rate of step c. is 0.5°C/s to 15 °C/s .
11. A stainless steel object having a surface modified with chromium obtainable according to anyone of the claims 1 to 10.
EP19769790.7A 2018-09-14 2019-09-16 Stainless steel object having a surface modified with chromium Pending EP3850121A1 (en)

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