EP2055801B1 - Method for hardening stainless steel surfaces on workpieces and fused salt for performing the method - Google Patents

Description

The invention relates to a method for hardening surfaces of workpieces made of stainless steel and a molten salt for carrying out the method.

Due to its excellent corrosion resistance, stainless steel is used in chemical apparatus engineering, in food technology, in the petrochemical industry, in the offshore sector, in shipbuilding and aircraft construction, in architecture, in building construction and equipment construction and in many other industrial sectors.

Corrosion-resistant stainless steel is used when at least 13% by weight chromium is alloyed to an iron material. In most cases nickel, titanium and molybdenum are additionally contained in the iron alloy, as for example in "Stahl Merkblatt 821 Stainless Steel - Properties Information Office Stainless Steel, PF 102205, 40013 Düsseldorf www.edelstahl-rostfrei.de" and in " P. Gümpel et al. Stainless Steels, Expert Verlag, Volume 349, Renningen Malmsheim 1998 ", is operated.

• 1.4301: C 0,05 Si 0,5 Mn 1,4 Cr 18,5 Ni 9,5 Gew.%
• 1.4571: C 0,03 Si 0,5 Mn 1,7 Cr 17,0 Ni 11,2 Mo 2,2 Ti 0,1 Gew.%

Typical austenitic stainless steels are the alloys of steels 1.4301 or 1.4571 having the following compositions:

• 1.4301: C 0.05 Si 0.5 Mn 1.4 Cr 18.5 Ni 9.5 wt%
• 1.4571: C 0.03 Si 0.5 Mn 1.7 Cr 17.0 Ni 11.2 Mo 2.2 Ti 0.1 wt%

If the chromium content is less than 13% by weight, the steel is generally not sufficiently resistant to corrosion to be considered "stainless steel". The content of metallic chromium in the steel is thus an important criterion for corrosion resistance, as stated in particular in the cited publication by P. Gümpel.

A major disadvantage of most common stainless steels such as 1.4301, 1.4441, 1.4541 or 1.4575 is that they are quite soft and thus susceptible to scratching the surface by hard particles such as dust, sand and the like. Most stainless steels - apart from the so-called martensitic stainless steels - are not hardenable by physical methods such as annealing and quenching. The low surface hardness often hinders the use of stainless steel. Another disadvantage of most stainless steels is their strong tendency to seize, that is, to weld the surface of two mutually sliding surfaces due to adhesion.

To address this problem, it is known to perform a thermochemical treatment of stainless steel workpieces. Nitriding or nitrocarburizing in the gas (under ammonia atmosphere), in the plasma (under nitrogen / argon) or in the molten salt (in molten cyanates) enriches the surface of stainless steel with nitrogen to form iron and chromium nitrides. The resulting layers form out of the material, so they are - unlike galvanic or physical layers - not applied from the outside and therefore extremely adherent. Depending on the duration of treatment, hard layers of 5 to 50 μm thickness are formed. The hardness of such nitrided or nitrocarburized layers on stainless steel reaches over 1000 units on the Vickers hardness scale due to the high hardness of the resulting iron and chromium nitrides.

The problem with the practical use of such nitrided or nitrocarburized layers on stainless steel is that while these layers are hard, they lose their corrosion resistance. The reason for this is the relatively high treatment temperature, which is around 580 ° C during nitriding or nitrocarburizing. At this temperature, the diffusing elements nitrogen and carbon form chromium-stable chromium nitrides (CrN) or chromium carbides (Cr ₇ C ₃ ) in the area of the component surface. In this way, the corrosion resistance-free free chromium is removed from the stainless steel matrix to a depth of about 50 μm below the surface and converted into chromium nitride or chromium carbide. The surface of the component becomes hard due to the formation of iron and chromium nitride, but it is susceptible to corrosion. In use, such layers are rapidly worn away due to corrosion.

To avoid this problem, the following procedures exist.

It is known that the surface hardness on stainless steel can be improved by galvanic coatings, for example by nickel plating, or physical coatings, for example by means of PVD (Physical Vapor Deposition) coating. However, a foreign substance is applied to the surface of the steel. The surface in contact with the abrasive or corrosive medium is no longer the steel surface itself. Problems of adhesion and corrosion resistance result. These methods are therefore not very common for improving the hardness and wear behavior of stainless steel.

A hard and at the same time corrosion-resistant layer can be thermochemically produced by the so-called Kolsterisieren on stainless steel. This process is mentioned, for example, in "Kolsterizing - Corrosion-resistant O-surface hardening of austenitic stainless steel - Information Sheet of Bodycote Hardiff bv, Parimariboweg 45, NL-7333 Apeldoorn, info@hardiff.de". fo@hardiff.de "However, the conditions of the process are neither described in the patent literature nor in the generally accessible scientific literature.These components treated have a hard, wear-resistant layer between 10 and 35 μm thick, the corrosion resistance of the base material is retained. Kolsterised components must not be heated above 400 ° C, otherwise they will lose their corrosion resistance.

By plasma nitriding, as in " H.-J. Spies et al. Mat.-Wiss. u. Materials Engineering 30 (1999) 457-464 " and in " Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp Plasma Nitriding "or by vacuum carburizing at low temperatures, as in" D. Günther, F. Hoffmann, M. Jung, P. May: Surface hardening of austenitic steels while maintaining the corrosion resistance Hardening Techn. Mitt. 56 (2001) 74-83 described, one can produce a supersaturated solution of nitrogen and / or carbon in the surface of components made of stainless steel, which has the desired properties, ie higher hardness with unchanged corrosion resistance.

However, both methods require a high expenditure on equipment and high investment and energy costs, to operate the equipment is particularly trained, usually even scientifically trained staff required.

The DE 10 2006 026 883 B3 relates to a method for hardening stainless steel workpieces by diffusing the elements carbon and / or nitrogen into the workpiece surfaces. The workpieces are immersed in a molten salt and this exposed at temperatures below 450 ° C for a period of 15 minutes to 240 hours. In addition to potassium chloride and lithium chloride, the molten salt contains an activator substance consisting of barium, strontium, magnesium and / or calcium chloride and a carbon-donating substance of a free or complex cyanide.

The US 3,840,450 A relates to an electrolytic process for surface hardening of metals. The electrolyte solution may contain acetates.

The invention has for its object to provide a method by means of which a hardening of workpieces made of stainless steel is made possible, in which at the same time a high corrosion resistance of the workpieces is obtained.

To solve this problem, the features of claim 1 are provided. Advantageous embodiments and expedient developments of the invention are described in the subclaims.

The inventive method is used for hardening surfaces of workpieces made of stainless steel. The workpieces are immersed in a molten salt and this exposed for a period of 24 hours to 240 hours. The temperature of the molten salt is less than 400 ° C. The molten salt has the following composition: potassium acetate with a proportion greater than or equal to 60% by weight and less than 100% by weight sodium <100% by weight metal salt ≤ 2% by weight

• Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ti^3+/4+, V^2+/3+/4+/5+, Cr^2+/3+, Mn^2+/4+, Fe^2+/3+, Co^2+/3+, Ni^2+/3+, Cu^+/2+, Zn²⁺, Mo^4+/5+/6+, Ru^2+/3+, Rh^1+/3+, Pd²⁺, W⁶⁺, Os⁴⁺, Ir^+/4+

The metal salt used consists of at least one of the following cations:

• Li ⁺ , Na ⁺ , K ⁺ , Cs ⁺ , Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , Ti ^{3 + / 4 +} , V ^{2 + / 3 + / 4 + / 5 +} , Cr ^{2 + / 3 +} , Mn ^{2 + / 4 +} , Fe ^{2 + / 3 +} , Co ^{2 + / 3 +} , Ni ^{2 + / 3 +} , Cu ^{+ / 2 +} , Zn ²⁺ , Mo ^{4 + / 5 + / 6+} , Ru ^{2 + / 3 +} , Rh ^{1 + / 3 +} , Pd ²⁺ , W ⁶⁺ , Os ⁴⁺ , Ir ^{+ / 4 +}

Furthermore, the metal salt used consists of at least one of the following anions: F ^- , Cl ^- , Br ^- , I ^- , O ^2- , CH ₃ COO ^- , C ₂ O ₄ ^2- , CN ^- , NCO ^- .

In the process of the present invention, since the treatment temperature of the workpieces, that is, the temperature of the molten salt used in the present invention is smaller than the formation temperature of chromium carbide which is in the range of 420 ° C to 440 ° C, the formation of carbides in the steel matrix becomes is called the lattice structure of the stainless steel, avoided.

Since the formation of chromium carbides is largely avoided, this means that the essential for the corrosion resistance of stainless steel workpieces free chromium is not displaced from the surface area of the workpieces. The workpieces thus have hard, wear-inhibiting and easily sliding surfaces with simultaneously high corrosion resistance.

Essential for achieving these advantageous effects is the use of the molten salt according to the invention, which contains constituents from which diffusible carbon can be released and suitable activators which cause the release of diffusible carbon at low temperatures.

The concentration of active carbon donors (acetates or intermediate carbides) in the molten salt of the invention is very high compared to the concentration of corresponding substances (ammonia, methane, carbon monoxide) in gas atmospheres or in the plasma. The relative Long treatment times of the workpieces in the molten salt are based on the fact that the diffusion rate of carbon is a function of the temperature and significantly decreases at temperatures below 450 ° C. At the low temperatures required to avoid formation of chromium carbide, long diffusion times of 24 to 240 hours must be used. However, the resulting long treatment periods are not critical because stainless steels, especially austenitic stainless steels or so-called. Duplex steels (ferritic - austenitic steels) are very insensitive to such long heat treatment periods, that is, they change their other mechanical properties or the structure so good as not.

Stainless steel is usually in the form of an austenitic steel, that is, the iron matrix has the structure of austenite, a face-centered cubic lattice, such as in "Steel Leaflet 821 Stainless Steel - Properties Observatory Stainless Steel, PF 102204, 40013 Dusseldorf www.edelstahl-rustproof .de "and in" P. Gümpel et al. Stainless Steels, Expert Verlag, Volume 349, Renningen Malmsheim 1998 "is described.

1. (a) Wenn Kohlenstoff unterhalb der Bildungstemperatur des Chromcarbids (420 - 440°C) eindiffundiert, bilden sich keine Carbide des Chroms. Demzufolge wird der Legierungsmatrix im Bereich der Diffusionsschicht kein Chrom entzogen und die Korrosionsbeständigkeit des Edelstahls bleibt erhalten.
2. (b) Die eindiffundierten Elemente dehnen das austenitische Gitter und führen zu einer starken Druckspannung im Bereich der Diffusionszone. Dies wiederum führt zu einer beträchtlichen Härtesteigerung. In der wissenschaftlichen Literatur spricht man von expandiertem Austenit oder einer so bezeichneten S-Phase, die eine Härte bis 1000 auf der Vickers Skala annehmen kann. Dies ist in " Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp. Plasma Nitriding " erwähnt.

In this grid, a non-metallic element such as carbon may be in solid solution. If it is possible to introduce carbon into the surface of an austenitic stainless steel and keep it in a solid saturated or even supersaturated solution, two effects occur:

1. (a) When carbon diffuses below the formation temperature of the chromium carbide (420-440 ° C), no chromium carbides are formed. As a result, no chromium is removed from the alloy matrix in the region of the diffusion layer and the corrosion resistance of the stainless steel is retained.
2. (b) The diffused elements stretch the austenitic lattice and lead to a strong compressive stress in the area of the diffusion zone. This in turn leads to a considerable increase in hardness. In the Scientific literature is called expanded austenite or S-phase, which can reach a hardness of up to 1000 on the Vickers scale. This is in " Y. Sun, T. Bell et al. The Response of Austenitic Stainless Steel to Low Temp Plasma Nitriding " mentioned.

• Li⁺ Na⁺, K⁺,Cs⁺, Mg²⁺, Ca^2+,Sr²⁺, Ba²⁺, Ti^3+/4+, V^2+/3+/4+/5+, Cr^2+/3+, Mn^2+/4+, Fe^2+/3+, Co^2+/3+, Ni^2+/3+, Cu^+/2+, Zn²⁺, Mo^4+/5+/6+, Ru^2+/3+, Rh^1+/3+, Pd²⁺, W⁶⁺, Os⁴⁺, Ir^+/4+,

und wobei das verwendete Metallsalz aus wenigstens einem der folgenden Anionen besteht: F^-, Cl^-, Br^-, I^-, O^2-, CH₃COO^-, C₂O₄ ^2-, CN^-, NCO^-.In the present invention, these considerations using a molten salt are used as a reactive medium and as a heat transfer, wherein the molten salt has the following composition: potassium acetate with a proportion greater than or equal to 60% by weight and less than 100% by weight sodium <100% by weight metal salt ≤ 0-2% by weight wherein the metal salt used consists of at least one of the following cations:

• Li ⁺ Na ⁺ , K ⁺ , Cs ⁺ , Mg ²⁺ , Ca ^2+, Sr ²⁺ , Ba ²⁺ , Ti ^{3 + / 4 +} , V ^{2 + / 3 + / 4 + / 5 +} , Cr ^{2+ / 3 +} , Mn ^{2 + / 4 +} , Fe ^{2 + / 3 +} , Co ^{2 + / 3 +} , Ni ^{2 + / 3 +} , Cu ^{+ / 2 +} , Zn ²⁺ , Mo ^{4 + / 5 + / 6 +} , Ru ^{2 + / 3 +} , Rh ^{1 + / 3 +} , Pd ²⁺ , W ⁶⁺ , Os ⁴⁺ , Ir ^{+ / 4 +} ,

and wherein the metal salt used consists of at least one of the following anions: F ^- , Cl ^- , Br ^- , I ^- , O ^2- , CH ₃ COO ^- , C ₂ O ₄ ^2- , CN ^- , NCO ^- .

The base melt is a salt mixture of potassium acetate, sodium acetate and a metal salt. Due to the holding time at a fixed temperature, which is in any case below 400 ° C., and thus below the formation temperature of chromium carbide, and which preferably lies in the range between 320 ° C. and 380 ° C., the acetate decomposes and forms free carbon. The added metal salt can also cause a catalytic decomposition of the acetate to a metal carbide, which in turn decomposes at the existing temperature and releases "atomic" carbon to the stainless steel.

The present invention avoids high equipment and energy costs and makes use of a light, easy for less qualified personnel easily executable procedure.

By the invention, the tendency of the stainless steel for eating, that is, for cold welding and thus also the adhesive wear is substantially reduced. The hardness of the surface of the stainless steel is increased from 200 to 300 Vickers to values up to 1000 Vickers, resulting in a high scratch resistance.

The metal salt contained in the molten salt of the invention preferably has the cations and anions recited in claims 3 and 4.

In a particularly inexpensive and simple embodiment of the invention, the molten salt is operated under air. The disadvantage here, however, is that by air contact an accelerated decomposition of the acetates in the molten salt is carried out by oxidation processes, whereby the efficiency in the treatment of the workpieces is reduced in the molten salt.

This disadvantage can be avoided if the molten salt is operated in a protective gas atmosphere. N ₂ , Ar, CO, CO ₂ or mixtures of these gases can be used as protective gases. In this case, the acetates decompose only due to heat, no longer additionally by oxidation processes, that is, the rate of decomposition of acetates is significantly reduced.

The production of a protective gas atmosphere requires a considerable design effort, since the molten salt must be stored in a retort, in which the protective gas must be introduced, wherein the introduction of the protective gas must be repeated after each opening of the retort.

The decomposition of the acetates can also be reduced with little design effort by the fact that the protective gases introduced into the molten salt, that is initiated. This results in a simultaneous circulation of the molten salt, which leads to a uniform distribution of the salts in the molten salt. In general, a circulation can also take place by introducing air into the molten salt.

Alternatively, the molten salt can also be moved mechanically, for example by stirring or tumbling.

Abb. 1:

Querschliff eines mit einer ersten Salzschmelze behandelten Werkstücks.

Abb. 2:

Ortsabhängiger Verlauf der Kohlenstoffkonzentration im Oberflächenbereich des Werkstücks gemäß Abb. 1.

Abb. 3:

Querschliff des mit einer zweiten Salzschmelze behandelten Werkstücks.

Abb. 4:

Ortsabhängiger Verlauf der Konzentrationen von Fe, Cr, C im Oberflächenbereich des Werkstücks gemäß Abb.3.

The invention will be explained below with reference to embodiments and figures. Show it:

Fig. 1:

Cross section of a treated with a first molten salt workpiece.

Fig. 2:

Location-dependent course of the carbon concentration in the surface region of the workpiece according to Fig. 1 ,

Fig. 3:

Cross section of the treated with a second molten salt workpiece.

Fig. 4:

Location-dependent course of the concentrations of Fe, Cr, C in the surface area of the workpiece according to Fig.3 ,

The following examples show the results of the treatment of the same workpiece, namely a bolt consisting of the material X5 Cr Ni Mo 17-12-12, with two different variants of the molten salt according to the invention.

example 1

A mixture of 120 g of potassium acetate and 0.2 g of NiCl ₂ is melted in a crucible and at 380 ° C for 53.5 h, a bolt (material: X5 Cr Ni Mo 17-12-2) immersed. After treatment, the bolt is quenched in water. Layer thicknesses of 11 μm to 13 μm are formed. The GDOS analysis according to Fig. 2 shows a significant increase of carbon (up to 16%) in this layer ( Fig. 2 shows the carbon content in wt.% depending on the distance from the surface of the workpiece). Fig. 1 shows a cross-section of the workpiece (bolt) in the region of this layer.

Example 2

A mixture of 120 g potassium acetate and 0.2 g NiCl ₂ is melted in a crucible and at 380 ° C for 100 h a bolt (material: X5 Cr Ni Mo 17-12-2) immersed. After treatment, the bolt is quenched in water. Layer thicknesses of 17 μm to 21 μm are formed. Fig. 4 in this case shows the concentration of Fe, C, Cr in the workpiece in% by weight as a function of the distance from the surface of the workpiece. How out Fig.4 can be seen, in turn, a significant increase of carbon is obtained in the layer, wherein in the layer, the proportion of Cr, Fe is reduced. Fig. 3 shows a cross-section of the workpiece (bolt) in the region of this layer.

Claims (10)

1. Method of hardening surfaces of workpieces of stainless steel, characterised in that the workpieces are immersed in a molten salt bath and exposed to this for a period of 24 hours to 240 hours, wherein the temperature of the molten salt bath is less than 400° C and wherein the molten salt bath has the following composition: potassium acetate 60 to 100 weight % with a proportion equal to or greater than 60 weight % and smaller than 100 weight % sodium acetate < 100 weight % metal salt ≤ 2 weight % wherein the metal salt used consists of at least one of the following cations:

   Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ti^3+/4+, V^2+/3+/4+/5+, Cr^2+/3+, Mn^2+/4+,Fe^2+/3+, Co^2+/3+, Ni^2+/3+, CU^+/2+, Zn²⁺, Mo^4+/5+/6+, Ru^2+/3+, Rh^1+/3+, Pd²⁺, W⁶⁺, Os⁴⁺, Ir^+/4+,

   and wherein the metal salt used consists of at least one of the following anions: F^-, Cl^-, Br^-, I^-, O^2-, CH₃COO^-, C₂O₄ ^2-, CN^-, NCO^-.
2. Method according to claim 1, characterised in that the temperature of the molten salt bath lies in the range of 330° C to 380° C.
3. Method according to one of claims 1 and 2, characterised in that the molten salt bath is used under air.
4. Method according to one of claims 1 and 2, characterised in that the molten salt bath is used in a protective gas atmosphere.
5. Method according to one of claims 1 and 2, characterised in that air or a protective gas is conducted through the molten salt bath.
6. Method according to one of claims 4 and 5, characterised in that N₂, Ar, CO and/or CO₂ is or are used as protective gas.
7. Method according to one of claims 1 and 2, characterised in that the molten salt bath is covered by powder or granulate containing carbon.
8. Method according to any one of claims 1 to 7, characterised in that the molten salt bath is set in motion.
9. Method according to claim 8, characterised in that the molten salt bath is stirred or circulated.
10. Use of a molten salt bath for carrying out the method according to any one of claims 1 to 9, characterised in that this has the following composition: potassium acetate with a proportion equal to or greater than 60 weight % and smaller than 100 weight % sodium acetate < 100 weight % metal salt ≤ 0 - 2 weight % wherein the metal salt used consists of at least one of the following cations:

    Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Ti^3+/4+, V^2+/3+/4+/5+, Cr^2+/3+, Mn^2+/4+, Fe^2+/3+, Co^2+/3+, Ni^2+/3+, Cu^+/2+, Zn²⁺, Mo^4+/5+/6+, Ru^2+/3+, Rh^1+/3+, Pd²⁺, W⁶⁺, Os⁴⁺, Ir^+/4+,

    and wherein the metal salt used consists of at least one of the following anions: F^-, Cl^-, Br^-, I^-, O^2-, CH₃COO^-, C₂O₄ ^2-, CN^-, NCO-.

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