DD296967B5 - Process for the pretreatment of ferrous material surfaces for nitriding in gaseous ammonia-containing gas mixtures - Google Patents

Description

Field of application of the invention

The invention relates to a process for the pretreatment of ferrous material surfaces for nitriding in gaseous ammonia-containing gas mixtures for producing nitride-containing surface layers.

Characteristic of the known state of the art

In the thermal-chemical surface treatment of iron materials by nitriding in gaseous ammonia-containing gas mixtures, in particular at medium and higher alloy contents of chromium or chromium and nickel as a result of inhibiting surface states problems in the production of nitride-containing surface layers in the way that these materials are not or only nitrate after a long period of treatment and then usually very unevenly. For the removal of these nitriding in gaseous ammonia-containing gas mixtures inhibiting surface conditions has been proposed a variety of solutions. They concern mechanical or chemical pretreatments (Sidan, H .: Nitriding of stainless and acid-resistant steels, Technische Rundschau [1966] No. 24, pp. 9-13, no. 28 pp.37, no. 42 p.33) Gerscha, A., and HG Weidinger: Metalloberfläche 22 [1968], Issue 11, p.336-341), the addition of halogen compounds (DE 882097) or hydrogen sulfide (DE-AS 1229814) to the reaction gas, activating the reaction atmospheres by touching the nitriding gas with a special paste (DE-OS 1521162), the introduction of compounds into the reaction space (DE-OS 1521139-carrier materials with amino acids or amino salts) and the application of coatings Copper (Jones, B .: Stahl and Eisen 58 [1938] p.40) or coatings for releasing layer-destroying compounds (DD 156 718 dichloromethane-acetone-containing polyvinyl chloride solution) or for layer conversion (DD 152 947 conversion layer of, for example, ferrooxalate ). The results of all these methods for the elimination of the nitriding in gaseous ammonia-containing gas mixtures inhibiting surface conditions are unsatisfactory because no general effect on all materials is achieved by the respective method and the individual

Methods are not reproducible. In addition, for example, additives to the reaction gas do not allow controlled process control or pretreatments such as the combined bath / gas nitriding too expensive.

Inhibiting surface states due to production-related residues on surfaces to be nitrided also cause the formation of an uneven nitride-containing surface layer (Chatterjee-Fischer, R .: "Heat treatment of iron materials, nitriding and nitrocarburizing", expert-Verlag, Sindelfingen, 1986, pp. 267-276; Schreiner, A .: Härterei-Techn.-Mitt. 41 [1986] H.6, pp.399-410) The often proposed measure for the elimination of this type of inhibiting surface states by an oxidation preceding the nitriding in gaseous ammonia-containing gas mixtures Treating surfaces above 300 ⁰ C is also only partially effective.It also requires an additional operation and extends the treatment time when integrated into the process flow.

Object of the invention

The aim of the invention is to produce by nitriding in an ammonia-containing gas mixture nitride-containing surface layers also on difficult nitrorizable iron materials.

Explanation of the essence of the invention

The invention is based on the technical object of different causes, the thermal-chemical treatment in gaseous ammonia-containing gas mixtures inhibiting surface layers easily and comprehensively.

According to the invention the technical object is achieved in that prior to nitriding in gaseous ammonia-containing gas mixtures to be nitrided iron material surfaces in the degreased state with a mixture of 0.1-0.9 parts by weight of derivatives of melamine (2,4,6-triamino-1, 3,5-triazine) and / or organic compounds which form in the temperature range of 100-600 ⁰ C derivatives of melamine, and 0.9 to 0.1 parts by mass of carbonates and / or bicarbonates are coated. In this case, per cm ² iron material surface> 0.1 mg coating mixture is applied.

As derivatives of melamine melamine itself or thermal conversion products of melamine, in particular melam, Meiern and melon, are used. Compounds which form in the temperature range of 100-600 ⁰ C derivatives of melamine are particularly dicyandiamide and / or cyanamide. The carbonates used are alkali metal carbonates or alkaline earth metal carbonates, in particular sodium carbonate and potassium carbonate or magnesium carbonate and calcium carbonate, and bicarbonates used are alkali metal bicarbonates, in particular sodium hydrogen carbonate and potassium bicarbonate.

The coating of the surface to be nitrided is carried out using water-soluble carbonates and / or bicarbonates, in particular when using sodium and / or potassium carbonate and / or sodium and / or potassium bicarbonate, and melamine with an aqueous solution having a temperature of 20-100 ⁰ C containing 0.005-0.06 parts by weight of melamine and 0.005-0.01 parts by weight of carbonates and / or bicarbonates by dipping, spraying or brushing. In this case, it is advantageous to dip the coated surfaces in a low-boiling, water-miscible organic liquid, in particular in ethanol, methanol or acetone, thereby accelerating the drying of the material surface.

The advantage of the method according to the invention is that inhibiting and gas nitriding obstructing surface layers are activated by a simple coating for gas nitriding. This makes it possible to provide otherwise difficult or non-nitridable iron materials such as chromium and chromium-nickel steels reproducibly with a nitride-containing surface layer under normal nitriding conditions.

embodiment

The invention will be explained in more detail with reference to the following 3 examples:

example 1

The polished surfaces of two samples from the steel X45CrNiW 18.9 were prepared after degreasing as follows:

Sample 1 - without coating

Sample 2 - by dipping twice in an aqueous solution containing 0.03 parts by weight of melamine and 0.015 parts by weight of sodium carbonate and 0.015 parts by weight of potassium carbonate at a temperature of 95 ° C and with a dipping time of 30 seconds, the sample was mixed with 1 mg solid / cm ² coated. After each dipping, the sample was dipped in ethanol to accelerate the drying.

Both samples were then gasoxynitriert together for 18 hours at a temperature of 570 ⁰ C at a composition of the nitriding gas atmosphere according to the nitriding of the third The following nitration result has been achieved:

Sample 1 Sample 2

nitriding

h (kH + 50HV0.1) in mm - 0.1

Example 2

Two samples of steel 38CrMoV21,14 in the hardened and tempered material condition were ground on both sides using the production aid KC20. After degreasing in perchlorethylene, it was prepared as follows:

Sample 1 - without coating

Sample 2 three times to immersion in an aqueous suspension of 0.045 parts by mass of melamine, 0.005 parts by mass of melon, 0.045 Masäeanteilen sodium carbonate and 0.005 parts by mass of potassium hydrogen carbonate at a temperature of the suspension of 9O ⁰ C for an immersion duration of 20 seconds, the sample with 0.8 mg Solid / cm ² coated. Sample 1 and Sample 2 were then gasoxynitriert together at a temperature of 550 ⁰ C for 24 hours at a composition of the nitriding atmosphere in accordance with the nitriding of the sixth The following nitration result has been achieved:

Sample 1 sample

Surface hardness HV10 621 855

Thickness of the bonding layer

(VS) in цт 0-1 4

nitriding

h (kH + 50HV0.1) in mm 0.12 0.3

Example 3

The ground surfaces of 2 samples of steel X40Cr 13 were prepared after degreasing as follows:

Sample 1 -without coating

Sample 2 - By spraying a hot aqueous solution containing 0.03 parts by weight of dicyandiamide and 0.03 parts by weight of sodium potassium carbonate over a period of 1 minute and then drying in 8O ⁰ C hot air flow, the surfaces were treated with 0.8 mg solid / cm ² coated.

Both samples were then gas oxynitrided together for 8 hours at a temperature of 59O ⁰ C with a composition of the nitriding gas atmosphere corresponding to the nitriding number of 3. The following nitration result has been achieved:

Sample 1 sample

nitriding

h (kH + 50HV0.1) in mm 0.05 0.1

Claims (9)

1. A process for the pretreatment of ferrous material surfaces for nitriding in gaseous ammonia-containing gas mixtures by the material surfaces are coated in the degreased state, characterized in that the nitriding iron surfaces with a mixture of 0.1-0.9 parts by weight of derivatives of melamine (2 , 4,6-Tria'mino-1,3,5-triazine) and / or organic compounds which form in the temperature range of 100-600 ⁰ C derivatives of melamine, and 0.9 to 0.1 parts by mass of carbonates and / or Hydrogen carbonates are coated. 2. The method according to claim 1, characterized in that per cm ² iron material surface> 0.1 mg coating mixture is applied. 3. The method according to claim 1 and 2, characterized in that suspensions of melamine derivatives and / or dicyandiamide and / or cyanamide and carbonates and / or bicarbonates in water or polar organic solvents, such as methanol, ethanol or acetone, are used for coating , 4. The method according to claim 1, characterized in that are used as derivatives of melamine melamine itself or thermal conversion products of melamine, especially melam, Meiern and melon. 5. The method according to claim 1, characterized in that are used as compounds which form derivatives of melamine in the temperature range of 100-600 ⁰ C, in particular dicyandiamide and / or cyanamide. 6. The method according to claim 1, characterized in that as carbonates alkali carbonates or alkaline earth metal carbonates, in particular sodium carbonate and potassium carbonate or magnesium carbonate and calcium carbonate, are used. 7. The method according to claim 1 to 3, characterized in that are used as bicarbonates Alkalihydrogencarbonate, in particular sodium bicarbonate and potassium bicarbonate. 8. The method according to claim 1 and 2, characterized in that the use of water-soluble carbonates and / or bicarbonates and melamine with an aqueous solution having a temperature of 20-100 ° C, the 0.005-0.06 mass fractions melamine and 0.005- Contains 0.01 parts by weight of carbonates and / or bicarbonates, is coated by dipping, spraying or brushing. 9. The method according to claim 1 and 2, characterized in that the melamine derivatives and / or dicyandiamide and / or cyanamide and the carbonates and / or bicarbonates are applied separately in alternating thin layers on the iron material surface.