EP0713926B1 - Process for the nitrocarburation of steel parts in a salt bath - Google Patents

Description

The invention relates to a method for nitrocarburization of components made of steel forming passive layers, in particular from steels with more than 10% by weight chromium and / or more than 4% by weight of nickel, in salt baths.

Alloyed steels, the larger quantities of chrome, nickel and / or other additives contain, form in the air Passive layers consisting mainly of oxides of Additional metals exist. These passive layers are conditional increased corrosion resistance of these steels but a hindrance when nitrocarburizing such Components in salt baths containing cyanide and cyanate, since the Passive layers the diffusion of nitrogen and Carbon from the salt bath into the steel surface complicate and lead to faulty nitriding layers.

So far, attempts have been made to overcome these disadvantages by by using high-alloy steels when nitrocarburizing uses high cyanide salt baths which reduce to the passive layers act. These high cyanide Salt baths, however, are extremely polluting.

In DD-C-94110 and in A.V. Kriulin et al., 2354 Metal Science and Heat Treatment 27 (1985) No. 1/2, 17-20 Procedures for nitrocarburizing steels recommended at an oxidizing pretreatment in the gas phase is provided. Yu.M. Lakhtin, 2354 Metal Science and Heat Treatment 36 (1994) No. 9/10, 445-451 indicates a procedure in which initially in the salt bath oxidized, then nitrocarburized and then oxidized again and recommends the Gas phase nitrocarburization followed by oxidation.

It was an object of the present invention Process for nitrocarburizing components Steels forming passive layers, in particular from steels with more than 10% by weight chromium and / or more than 4% by weight Nickel to develop in the salt bath, which is an oxidizing Pretreatment with which despite the existing Passive layers a perfect nitrocarburization is achieved.

This object is achieved in that the Parts in an oxidizing molten salt at 300 to 500 ° C be pretreated.

Preferably used as the oxidizing salt melt Mixture of alkali nitrate, alkali hydroxide and Alkali carbonate. Salt melts in particular have proven their worth with 5 to 30 wt.% Alkali nitrate, balance alkali hydroxide and Alkali carbonate.

The molten salt is advantageously operated at 330 up to 420 ° C, with a period of time for the treatment period 5 to 30 minutes has proven to be beneficial.

It is very surprising that despite an increase in Passive layers in an oxidizing molten salt then perfect nitrocarburizing layers in cyanide and salt baths containing cyanate.

Oxidizing salt baths are in the nitrocarburizing of Iron materials known per se (e.g. DE-PS 29 34 113), but so far they have only become after nitrocarburizing Increase in corrosion resistance used.

1. Bauteile aus einem Stahl mit 23 % Chrom und 8 % Nickel wurden in einem Salzbad (etwa 4 Gew.% Cyanid, etwa 37 Gew.% Cyanat, Rest Alkali) 90 Minuten bei 580° C nitrocarburiert. Es bildete sich eine fehlerhafte Nitrocarburierschicht aus, die unterschiedliche Schichtstärken aufwies. Wurden diese Bauteile vor dem Nitrocarburieren 25 Minuten bei 370° C in eine Alkalihydroxidschmelze getaucht, die 10 Gew.% Natriumnitrat enthielt, erzielte man beim Nitrocarburieren einwandfreie, überall die gleiche Dicke aufweisende Schichten.

2. Die gleichen Versuche wurden mit Bauteilen aus den Stählen 1.4028 (13 % Chrom) und 1.4112 (18 % Chrom) durchgeführt. Die metallographische Auswertung ergab bei den in der oxidierenden Salzschmelze vorbehandelten Proben eine gleichmäßige Ausbildung der Nitrierschicht, während die nichtvorbehandelten Proben eine ungleichmäßige, wellige Nitrierschicht zeigten.

The following examples are intended to explain the process according to the invention in more detail:

1. Components made of a steel with 23% chromium and 8% nickel were nitrocarburized in a salt bath (approx. 4 wt.% Cyanide, approx. 37 wt.% Cyanate, balance alkali) at 580 ° C. for 90 minutes. A defective nitrocarburizing layer was formed, which had different layer thicknesses. If these components were immersed in an alkali hydroxide melt containing 10% by weight of sodium nitrate for 25 minutes at 370 ° C. before nitrocarburizing, flawless layers were obtained which had the same thickness everywhere.

2. The same tests were carried out with components made of steels 1.4028 (13% chromium) and 1.4112 (18% chromium). The metallographic evaluation showed a uniform formation of the nitriding layer in the samples pretreated in the oxidizing salt melt, while the non-pretreated samples showed an uneven, wavy nitriding layer.

Claims (4)

1. Process for nitrocarburating components made from steels which form passive layers, in particular from steels with more than 10 wt.% of chromium and/or more than 4 wt.% of nickel, in a salt bath, which includes oxidising pretreatment,
   characterised in that
   the parts are pretreated in an oxidising molten salt at 300 to 500°C.
2. Process according to Claim 1,
   characterised in that
   the oxidising molten salt consists of a mixture of alkali metal nitrate, alkali metal hydroxide and alkali metal carbonate.
3. Process according to Claims 1 and 2,
   characterised in that
   the molten salt contains 5 to 30 wt.% of alkali metal nitrate, the remainder being alkali metal hydroxide and alkali metal carbonate.
4. Process according to Claim 1, 2 or 3,
   characterised in that
   the parts are treated at temperatures of 330 to 420°C in the oxidising molten salt.