DK174707B1 - Case-hardening of stainless steel article by gas including carbon and/or nitrogen, involves applying top layer including metal which is catalytic to decomposition of gas, on activated surface of article - Google Patents

Abstract

The case-hardening of stainless steel article by gas including carbon and/or nitrogen atoms which diffuse through the surface of article, involves applying top layer on activated surface of steel article to prevent repassivation. The top layer includes a metal which is catalytic to the decomposition of gas. The top layer includes metal(s) like iron, nickel, ruthenium, cobalt or palladium, preferably nickel. The article is austenitic stainless steel article. An independent claim is included for case-hardened stainless steel article.

Description

l DK 174707 B1

The invention relates to a method according to the preamble of claim and a stainless steel blank according to claim 8.

Thermochemical coating of steel using carbon or nitrogen-bearing gases is well-known processes and is called insert curing, carburizing (carbon insertion) or nitriding. Carbonite extraction is a process using a gas which carries both carbon and nitrogen. These processes are traditionally used to increase the hardness and wear resistance of iron or low-alloy steel items. The steel article is exposed to a carbon and / or nitrogen-bearing gas at an elevated temperature for a period of time, with the gas decomposing and carbon and / or nitrogen atoms diffusing through the steel surface into the steel material. The outermost material close to the surface is converted into a layer of increased hardness, and the thickness of this layer depends on the treatment temperature and the treatment time.

15 Stainless steel has good corrosion properties, but is relatively soft and has poor wear resistance, especially against adhesive wear. Therefore, there is a need to increase the stainless steel surface properties. Gas carburetor, nitration, and carbon titration of stainless steel poses certain difficulties, the passive layer being the cause of the good corrosion properties acting as a barrier layer preventing carbon and / or nitrogen atoms from diffusing through the surface. The elevated treatment temperatures also promote the formation of chromium carbides or chromium nitrides. The formation of chromium carbides and / or chromium nitrides reduces the free content of chromium in the material, thereby weakening the corrosion properties.

Several methods have been proposed for stainless steel insert hardening, where these drawbacks are minimized or reduced.

It is known that a pretreatment in a halogen-containing atmosphere provides an effective activation of the surface.

30 2 DK 174707 B1

EP 0588458 discloses a method in which fluorine is used as an active component in a gas pretreatment, in which the passive layer of the stainless steel surface is converted into a fluorinated surface layer which is permeable to carbon and nitrogen atoms.

5 Plasma-assisted thermochemical treatment and ion implantation have also been proposed. In this case, the passive layer of the stainless steel is removed by cathode sputtering, which is an integral part of the process.

From EP 0248431 B1 a method is known in which a blank of austenitic stainless steel 10 is electrolytically coated with iron before gas nitration. The nitrogen atoms can diffuse through the iron layer and into the austenitic stainless steel. After gas nitration, the iron layer is removed and a hardened surface is obtained. According to the sole example of this patent, the process is carried out at 575 ° C for 2 hours. At this temperature, chromium nitrides are formed, thereby weakening the corrosion properties.

15

EP 1095170 discloses a carburizing process in which a stainless steel workpiece is electrolytically coated with a surface layer before carburizing. A passive layer is avoided and the carburization can be carried out at a relatively low temperature without the formation of carbides.

From NL 1003455 there is known a method in which a piece of iron or low alloy steel is coated with a catalytic layer of e.g. nickel before gas nitration. Nickel protects the iron from oxidation and serves as a catalytic surface for the decomposition of NH3 gas. The process can be carried out at temperatures below 400 ° C and the purpose is to obtain a pore-free iron nitride layer.

25

The object of the invention is to provide a new and improved method for inserting hardening stainless steel. The object of the invention is achieved by a method according to the preamble of claim 1, wherein the top layer is a metal catalyzing for the decomposition of the gas carrying the carbon and / or nitrogen atoms and is a layer of nickel, ruthenium, palladium. or cobalt. The metal layer protects the stainless steel surface against oxidation and acts as a catalytic surface for the decomposition of the gas.

As a result, the process temperature can be kept below the temperature at which carbides 5 and / or nitrides are formed and the process can be completed within a reasonable time. After the thermochemical treatment, the catalytic metal layer can be removed to expose the hardened stainless steel surface and repass.

When carbon atoms, nitrogen atoms or both diffuse into stainless steel, the 10 metastable S phase is formed. S phase is also called "extended austenite" and has carbon and / or nitrogen in a solid solution at an upper stable temperature of about 450 ° C when nitrogen stabilized and about 510 ° C when carbon stabilized. Thus, the process of the invention can be carried out at temperatures up to 450 ° C and 510 ° C to obtain S phase.

15

To date, stainless steel S phase has been achieved almost exclusively by plasma assisted or ion implantation processes. Tests have shown that the formation of S phase at the surface has no adverse effect on the corrosion resistance of the stainless steel. With nitrogen stabilized S phases, an improvement in the corrosion resistance is achieved.

When stainless steel is treated with the method of the invention, the hardness and abrasion resistance are significantly improved without adversely affecting the corrosion properties. Avoiding iron in the top layer prevents the stainless steel blank from being contaminated as iron atoms are diffused into the stainless steel blank, while this has been coated with a iron coating. Such contamination destroys the corrosion properties.

According to the invention, the metal layer may be a nickel layer. Nickel is easy to apply and contributes 30 very well to the decomposition of carbon or nitrogen containing gases. Furthermore, nickel is easy to remove, e.g. by etching after the thermochemical treatment.

4 DK 174707 B1

According to a preferred embodiment, the calculated average thickness of the nickel layer does not exceed 300 nanometers, preferably 200 nanometers. A nickel layer of this thickness is sufficient to prevent oxidation and to allow carbon and / or nitrogen to diffuse through the nickel layer into the stainless steel to form a satisfactory S phase layer.

According to a further embodiment of the invention, the nickel layer on the surface of the stainless steel blank can be coated chemically or electrolytically, e.g. in a Wood's nickel bath.

10

According to a preferred embodiment, the workpiece is made of austenitic stainless steel, e.g. AISI304 or AISI316.

According to another embodiment of the invention, the gas contains carbon, e.g. CO, and the 15 process temperature is kept below 510 ° C. When a temperature close to, but not above 510 ° C and CO is used as gas, a sufficient thickness of the S-phase layer can be obtained at the surface of a stainless steel blank within a reasonable time, e.g. six hours.

According to an embodiment of the invention, the catalytic metal layer is applied only to parts 20 of the surface of the stainless steel blank. This may be advantageous if the insert hardened steel blank is to be welded together with other workpieces, since the insert hardened surface is not suitable for welding due to sensitization, the non-insert hardened areas can be used for this purpose.

The following examples with accompanying figures explain the invention.

In the following examples, stainless steel washers with a diameter of 2 cm and a thickness of 0.35 cm were all pretreated as follows.

30 Depassivation in a solution of 100 ml of 15% w / w hydrochloric acid + 1 ml of 35% hydrogen peroxide for 15 seconds.

5 DK 174707 B1

Electrolytic precipitation of a catalytic nickel layer, thickness <200 nanometers (calculated average) in a Wood's nickel bath, which is an acidic halide-containing electrolyte.

The insertion curing was carried out in a furnace which was infused with pure NH3 or pure CO.

Example 1 10 Nitrogen purification in pure NH3 gas, austenitic stainless steel AISI304

Austenitic stainless steel AISI 304 was nitrated in pure NH3 gas (maximum nitriding potential) for 17 hours and 30 minutes at 429 ° C. Heating to the nitriding temperature was carried out in a hydrogen atmosphere (H2), after which the supply of hydrogen gas was interrupted and nitration gas was fed Cooling to room temperature was carried out in an argon gas (Ar) in less than 10 minutes. The subject was analyzed using light optical microscopy and X-ray microanalysis (probe). The layer formed was nitrogen S phase and had a maximum layer thickness of 9 μπι. The maximum concentration of nitrogen in the S phase was more than 20 atomic%. The analysis showed that no nitrides were precipitated.

Example 2:

Nitration in pure NH3 gas, austenitic stainless steel AISI 316, Figures 1 and 2 25

Austenitic stainless steel AISI 316 was treated as described in Example 1, but at a temperature of 449 ° C for 20 hours. The subject was analyzed with light optical microscopy (LOM), X-ray diffraction analysis (XRD) and microhardness measurements. The LOM results are shown in Figure 1. The layer formed consisted of nitrogen S phase and had a layer thickness of 12 µm. Microhardness was measured to more than 1500 HV (load 100 g). The stainless steel 6 DK 174707 B1 had a hardness of between 200 and 300 HV. No nitrides were precipitated.

An austenitic steel blank heated in ammonia to 480 ° C and maintained at this temperature for 21 hours showed development of chromium nitrides ON (and ferrite) close to the surface as well as locally in the S phase layer (the dark areas of Figure 2 ). This result indicates that a high temperature of 480 ° C should be avoided to obtain monophase S phase layers.

Example 3: 10

Carbonation in pure CO gas, austenitic stainless steel AISI316, Figure 3

Austenitic stainless steel AISI 316 was carburized in pure CO gas for 6 hours at 507 ° C to form carbon S phase. The heating was carried out in a hydrogen atmosphere (H2) until the carburizing temperature was reached, after which the supply of hydrogen was interrupted and CO gas supplied. Cooling to room temperature was performed in argon gas (Ar) in less than 10 minutes. The subject was analyzed by optical microscopy, X-ray diffraction analysis, and microhardness measurements. The LOM results are shown in Figure 3. The layer formed was carbon S phase with a layer thickness of 20 μιη (see Figure 3). The micro hardness of the 20 surface was more than 1000 HV (load 100 g). No carbides were precipitated.

Example 4:

Carburization + nitration, austenitic stainless steel AISI 316 25

Austenitic stainless steel AISI 316 was carburized as described in Example 3 but at a temperature of 500 ° C for 4 hours. Then, the blank was nitrated as described in Example 1, but at a temperature of 440 ° C for 18 hours and 30 minutes. Thus, two separate thermochemical processes were used, with carbon being added to one and nitrogen to the other. The subject was analyzed by light optical microscopy analysis and microhardness measurements. The layer formed was carbon S phase and nitrogen S phase.

7 DK 174707 B1

The layer thickness was a maximum of 35 μηι. The outermost layer was nitrogen S phase and the inner layer was carbon S phase. The micro hardness was more than 1500 HV. Neither nitrides nor carbides were precipitated.

Example 5:

Nitration in pure NH3 gas, duplex stainless steel A1SI329, Figures 4 and 5

Samples were nitrated for 23 hours and 20 minutes at 400 ° C. Metallurgical studies of the nitrated subjects involved X-ray diffraction analysis (XRD) and light optical microscopy analysis (LOM). The AISI 329 stainless steel is a duplex steel consisting of ferrite and austenite. After nitration at 400 ° C, ferrite was converted to austenite (and S phase) in the insert cured zone. A LOM image of the subject after treatment at 400 ° C is shown in Figure 4; the corresponding XRD diagram is shown in Figure 5. It is clear that the 15 S phase is developed along the surface of the duplex stage.

Example 6:

Nitration in pure NH3 gas, austenitic stainless steel AISI 316, Figure 6 20

The AISI 316 steel blank was treated at 400 ° C, 425 ° C and 450 ° C for 23 hours and 20 minutes. The diffraction pattern shown in Figure 6 clearly shows that the S phase is the only phase formed during the nitration.

The insert cure temperature of the above examples is between 400 ° C and 507 ° C. However, it is likely that the S phase can also be obtained at lower temperatures, e.g. 300 ° C or 350 ° C at high nitration / carburizing potentials within a reasonable time.

30 Initial experiments have shown that the S phase can also be obtained with AISI 420, a martensitic stainless steel, and AISI 17-4 PH, a martensitic steel for precipitation hardening.

8 DK 174707 B1

Experiments have established that the nitration treatments carried out in a small laboratory oven can easily be transferred to an industrial oven.

In the examples, the catalytic nickel layer was electrolytically precipitated in a Wood's nickel bath. Alternatively, chemical nickel plating, e.g. contact plating is used.

Other metals, e.g. ruthenium, palladium and cobalt, all of which are catalytic for the decomposition of NH3 gas and CO gas, can also be used. Palladium and ruthenium can be precipitated by ion exchange plating.

The process according to the invention is suitable for "n / 7" nitration or carburization in a plant. Stainless steel pipes and tanks can be nickel plated before installation. After installation, parts of the system subject to wear can be heated and flowed through NH 3 or other nitrogen or carbon containing gases.

A very suitable method for applying a layer of electrolytic nickel to parts of a surface is brush plating.

Claims (8)

9 DK 174707 B1 A method of inserting hardening a stainless steel blank by gas comprising carbon and / or nitrogen, wherein carbon and / or nitrogen atoms diffuse through the surface of the blank, which comprises activating the blank surface, applying a top layer to the activated surface to prevent repassivation, characterized in that the top layer comprises components which are catalytic for the decomposition of the gas. 10 The method of claim 1, wherein the top layer is a nickel layer. The method of claim 2, wherein the maximum average thickness of the nickel layer is 300 nanometers, preferably 200 nanometers. 15 The method of claim 2 or 3, wherein the nickel layer is applied by a chemical or electrolytic plating process, e.g. by electroplating in a Wood's nickel bath. A method according to any one of the preceding claims, wherein the blank is of austenitic stainless steel. A process according to any one of claims 1 to 5, wherein the gas used contains carbon, preferably CO, and wherein the temperature is kept below 510 ° C. A method according to any one of the preceding claims, wherein the catalytic metal layer is applied only to parts of the surface of the stainless steel blank. A stainless steel blank treated by a method according to any one of the preceding claims. 30