Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/48—After-treatment of electroplated surfaces
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment

Definitions

• the invention relates to a method for producing workpieces from ferritic steel according to the preamble of patent claim 1.
• the workpieces should have a high level of hardness and wear resistance. They should also be corrosion-resistant. Carbon steels are not suitable for these applications because they are particularly sensitive to corrosive media. It is therefore known to use rust-resistant alloy steels containing, for example, chromium, nickel, cobalt, molybdenum and the like. Such Alloyed steels are usually relatively inert and can be adjusted to the desired hardness because of their hardenability. Nevertheless, chromium steel is not permitted for the production of fasteners in the construction sector, for example for the production of screws, especially self-drilling screws. The reason is the hydrogen-induced cracking (hydrogen embrittlement).
• the hydrogen contained in the lanyard can lead to said crack formation.
• the hydrogen that is applied to the connecting means from the outside for example, in the event of a cathodic reaction in the event of corrosion - such a process affects the fatigue strength of a screw, for example, which affects the safety of a building structure.
• coatings such as galvanizing, which represents a barrier for the hydrogen.
• the protective layer will be damaged during use or that the formation of cracks will not be completely excluded by nature, so that hydrogen embrittlement cannot be stopped in this way.
• the invention has for its object to provide a process for the production of workpieces made of ferritic steel, in which the workpieces with high hardness and strength have a high corrosion resistance that comes close to that of austenitic chromium-nickel steels and in which hydrogen-induced crack formation is avoided .
• the workpiece is formed from non-convertible chrome steel with a chrome content of at least 13%.
• workpieces made of chrome steel are known per se.
• the workpiece is provided with a coating of nickel or an alloy essentially containing nickel or cobalt with a thickness of at least 5 m.
• the workpiece is then brought to the desired hardness in the absence of oxygen at at least 850 ° C. by heat treatment, the diffusion layer with the special properties being formed simultaneously from the coating and the base material.
• the treated workpieces have a nickel jacket with an underlying chromium-nickel-iron layer of varying composition.
• the existing hydrogen is expelled during the heat treatment.
• Hydrogen which can arise during a corrosion process, for example, cannot penetrate the nickel-iron diffusion layer.
• the core of the workpiece also consists of a high-quality, non-convertible steel and that the diffusion of nickel into the chromium steel or vice versa of chromium in the nickel plating results in a relatively thick austenitic intermediate layer which provides an effective barrier against the penetration of Represents hydrogen.
• no hardness-producing substances have to diffuse through the nickel layer in order to achieve the usability of the workpiece.
• the hardening and the generation of the corrosion resistance as a result of austenitic coatings containing chromium and possibly molybdenum on an inherently inert material takes place in a single step. If the chromium steel contains molybdenum in one embodiment of the invention, the molybdenum content of the austenitic intermediate layer has a favorable effect on the repassivation.
• a coating of chromium, cobalt, molybdenum or copper with a thickness of at least two m is applied to the nickel coating or the coating made of nickel or an alloy containing cobalt.
• an austenitic chromium-nickel iron layer is formed on the chrome steel, which seamlessly merges with the martensitic chrome steel.
• the method according to the invention leads to workpieces which can be adjusted to the desired hardness with approximately the same good corrosion resistance.
• the workpiece is formed from steel containing nickel or molybdenum, the workpiece is then provided with a chrome coating with a thickness of min at least 5 m and then in the absence of oxygen at least 850 ° C by heat treatment to the desired hardness. In this process, a corrosion protection coating similar to that described above is obtained.
• additional corrosion protection by galvanizing or cadmium coating can be dispensed with.
• additional layers can be applied to improve the sliding behavior (reduction of friction when using the connecting means).
• metallic coatings are also conceivable.
• the method according to the invention can be used for various high-quality workpieces, for example for knife blades, surgical instruments or other highly wear-resistant tools and components. It is particularly advantageous for the production of fasteners, such as screws or the like, in particular self-drilling screws, in construction, for which a high level of corrosion resistance and great hardness is required for safety reasons.
• the hardness is required, for example, for self-drilling screws so that they are able to drill a core hole even in relatively hard carrier material.
• Chrome steel Material according to DIN 17006: 1.1% carbon; 15% chromium; 0.5 molybdenum Material No. 4112 according to DIN 17006: 0.9% carbon; 18% chromium; 1.2 molybdenum
• a self-drilling screw for example made of the above chrome steel, is electroplated with a nickel coating of 10 m thick or a coating of 5 m thick nickel and about 2 m thick chrome.
• Electroplating conditions for the nickel layer Temperature: 50 to 70 ° C pH: 6 to 3 Current density: 2 to 9 A / d cm2 Nickel bath: Nickel sulfate: 300 g / l Nickel chloride: 40 g / l Nickel boric acid: 40 g / l
• Electroplating conditions for the chrome layer Temperature: 55 ° C Current density: 40 A / d cm2 Chrome bath: Chromic acid: CrO3 350 g / l Sulfuric acid: density 1.84 2.5 g / l
• electroplating can also be carried out using the currentless method.
• the heat treatment takes place in the oven with the exclusion of oxygen at a temperature of, for example, 1000 ° C over a period of 10 minutes.
• the temperature depends on the core material used and the time on the hardness to be set.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Electroplating Methods And Accessories (AREA)

Applications Claiming Priority (1)

Publications (1)

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Citations (5)

• 1988
  • 1988-02-17 DE DE19883804824 patent/DE3804824A1/de active Granted
• 1989
  • 1989-07-26 EP EP89113767A patent/EP0410033A1/fr not_active Withdrawn
  • 1989-08-04 AU AU39318/89A patent/AU3931889A/en not_active Abandoned

Patent Citations (5)

Non-Patent Citations (3)

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