FI104566B - Process for alloying a metal powder with nitrogen - Google Patents

Description

104566

METHOD FOR DAMPING METAL POWDER

The invention relates to a process for nitrogenizing a metal powder.

By doping metals and alloys, such as steels, nitrogen can significantly improve their mechanical, tribological and corrosion properties. However, conventional melt-metallurgical manufacturing processes have only limited success in nitrogen alloying and the possible concentrations of nitrogen are quite low. In powder metallurgy production, the nitrogen content can be raised significantly above this by nitrogenizing the steel powder. The general procedure then is to anneal the powder as a solid mattress in an enclosed space or in an airtight furnace in a nitrogen gas atmosphere. However, the solidification of the solid powder mattress is uneven and the process is white to control. Digestion also requires high temperature, steel powder 850 -1000 ° C.

The object of the invention is to eliminate the above-mentioned drawbacks. In particular, it is an object of the present invention to provide a novel nitrification process which is faster and simpler than the present ones and which provides a significantly higher nitrogen content.

: 25 As to the features which characterize the invention, reference is made to the claims section.

In the process of nitrogenizing the metal powder according to the invention, the metal powder is nitrogenized in a fluidized bed with ammonia gas at a temperature of 400-750 ° C.

As the metal powder in the process of the invention, for example, steel powder can be used and the typing temperature can typically be 570-670 ° C.

In the process according to the invention, ammonia NH3 flowing in a powder bed decomposes on the metal particle-35 to hydrogen H2 and atomic nitrogen N, whereby nitrogen is dissolved in the iron lattice. At low temperature, about 600 ° C, nitrogen gas N 2 in molecular form 104566 2 is unable to nitrogen the steel. Depending on the level of the nitrogen potential in the atmosphere, the degree to which the steel is depleted and the nitride layer on the steel surface is formed.

5 When the composition of steel contains strong nitride formers such as chromium, vanadium and aluminum, nitrides are readily formed even at low nitrogen potentials. The nitrogen potential depends on the amount of ammonia gas in the atmosphere and can be reduced by adding nitrogen or hydrogen to the fluidizing gas.

The surface of the metal particles can be removed from the nitride layer and its high nitrogen content reduced by diffusion annealing in an inert gas such as nitrogen gas after the nitrification step, whereby the new nitrogen is not transferred to the particles but nitrogen to the inner particles.

Since the type of process according to the invention occurs at a temperature of several hundred degrees, the nitrogen to be powdered must first be heated to a sufficient temperature, preferably in an inert gas such as nitrogen gas, whereby only at a sufficient temperature the nitrogen gas is partially or completely converted into ammonia gas.

Similarly, hot, nitrogenated powder cannot be directly removed from the treatment, but must be cooled in a fluidized bed, e.g., 30 ° C, preferably in an inert gas such as nitrogen gas, which is replaced in the fluidized bed with hot ammonia gas when cooling begins.

The process according to the invention was investigated by triturating steel powder with ammonia gas and fluidized bed technology. Batch-type fluidized bed apparatus was used in the experiments.

35 The reactor space was a steel retort with a diameter of 100 mm and a height of 275 mm. The inventive gas is fed to the powder bed from the bottom of the retort through the distribution plate i 104566 3. The retort is heated externally by electric resistors with a heating power of 2.5 kW.

After trituration, it is possible to raise the retort to cool in a housing adjacent to the furnace 5 with a compressed air flow to accelerate cooling. The flow of fluidizing gases is controlled by flowmeters, for example rotometers. In the case of a gas mixture, each gas component is individually regulated and the gases are combined after the flow meters. Put-10 tread pressure before flowmeters is 2 bar excess pressure.

In the experiments performed, AISI 316 L steel powder with a particle size of 45-250 μπ> was nitrogenized. The powder was tested with four different type-15 programs. Samples were taken at the center of the fluidized bed during sampling. After sampling, the sample and sample container were cooled in a sealed container under N2. After cooling, the samples were analyzed for nitrogen and oxygen. combustion technology.

Samples were prepared from heels, and the microstructure of the particle was observed optically and with an scanning electron microscope.

Example 1 ; Nitrogen gas was introduced into the furnace and a sample of steel powder was poured into the retort. The temperature of the powder bed was raised to 570 ° C and the N2 fluidized gas was changed to NH3. It took about one hour for the powder mat to warm up. The length of the trituration step, where the NH3 fluidization was allowed to remain on for 45 min, and τ samples were taken from the powder at the time t = 0, 10, 25 and 45 min from the time of conversion to NH3 gas. Thereafter, the NH3 gas was changed back to the N2 gas, i.e. diffusion annealing was started. Diffusion annealing lasted 90 min and 35 samples were taken after 30 and 90 min diffusion annealing, i.e. at times t = 75 and 135 min.

After diffusion annealing, the retort was raised into a cooling jacket in which the floating powder mattress was allowed to cool in N2 gas until it reached a temperature of about 30 ° C.

After cooling for 80 min, a final sample of the mattress was taken.

5

Example 2

The experiment investigated the effect of time on powder fading. The nitrification program was a replication of Example 1 except that the duration of the nitrification lie was extended to 120 to 10 minutes. The diffusion annealing time was 90 min and samples were taken for NH3 gas from the time of conversion at t = 0, 30, 60, 90, 120, 150, 180, 210 min and still at the end of the experiment when the powder temperature was n.

30 ° C, i.e. t = 290 min.

15

Example 3 Herein, the dissolution of the nitride layer on the particle surface was studied. Initial operations were as in the previous two examples. The pi-20 of the nitrification step was shorter, only 30 min, and the diffusion annealing length was 90 min. Samples were taken for NH3 gas at the time of switching at t = 30, 60, 90, 120 min and still at the end of the test at a temperature of about 30 ° C, i.e. t = 200 min.

-; 25

Example 4

The effect of elevated temperature on the dissolution of the nitride layer during diffusion annealing was investigated. Initial operations were the same as in the previous 30 experiments. When the powder bed reached 570 ° C, the fluidizing gas was changed to NH3. The powder was triturated with NH3 gas for 30 min, after which the fluidization gas was changed to 95% N2 / 5% H2 gas and the temperature of the mattress * was raised to 670 ° C. The mattress reached 35 temperature in 20 minutes. The purpose of the hydrogen addition was to reduce the oxygen content of the powders during the diffusion annealing step. Diffusion heating lasted 90 min. Samples 104566 were taken at the time t = 30, 50, 80, 110, 140 min from the time of NH3 gas exchange and after cooling of the powder bed, i.e. t = 220 min.

5 The following results were obtained in the experiments performed. During heating to 570 ° C, the nitrogen content of the powder did not rise under N2. When NH3 gas was introduced into the mattress, the nitrogen concentration increased rapidly. In ten minutes, a nitrogen content of 0.4%, 1.5% in 30 minutes and 2.2% in 45 minutes was achieved, the percentages being by weight.

After 2 hours of nitrogenization, the nitrogen content was 6.8%.

At the beginning of diffusion annealing, immediately after the dip step, nitrogen was strongly concentrated on the surface layer of the particles. In diffuse sludge annealing, the nitrogen content stabilized towards its core and the nitride layer on the surface of the particles increased to 20. The average nitrogen concentration seemed to decrease somewhat in diffusion annealing. At a nitrogen content of 2 to 2.5%, the nitrogen content of the powders reduced to 1.8%.

At the beginning of nitrogenization always up to nitrogen content of approx.

: Up to 0.4%, no nitride layer was visible on the surface of the powder particles. As the nitrogen progressed and the nitrogen content increased, a nitride layer began to form on the particle surface. Initially, the nitride layer was thin, about 5 -10 μηι after 30 min of nitrogenization, with a nitrogen content of about 1.5% in the powder and a face! slowly as the nitrogen progresses, being 20 -30 μπι after two hours of nitrogenization, the measured nitrogen content in the powder is about 6.8%.

At the beginning of the diffusion annealing at 570 ° C / N 2, the nitride layer is on the face. For example, 1.5 hour diffusion annealing after 2 hours of nitrogenization had increased the nitride layer and some of the powder particles were completely nitride.

104566 6

Diffusion annealing at 670 ° C under N 2 + 5% H 2, after one hour of NH 3 nitrogenization at 570 ° C, increased the diffusion layer and resulted in nitride precipitates in the substrate just below the center of the particles. The pores are also visible on the formed surface layer.

Summarizing the experiments performed, fluidized bed technology and ammonia gas were able to rapidly increase the nitrogen content of AISI 316 L powder. At 570 ° C, the nitrogen concentration increased from the initial value of 0.018% to 1.5% in 30 minutes and to 2.5% in 45 minutes and as much as 6.8% at 2 hours of the nitrogenization step.

Analytical results showed that the nitrogen concentration increased until diffusion annealing in the nitrogen atmosphere began, and shortly thereafter. When the total nitrogen content of the powder exceeded 0.4%, the nitrogen transferred from ammonia to steel formed a layer containing chromium and iron nitrides on the surface of the powders. On the surface of the particles, the solubility limit of nitrogen in the austenite was exceeded and the nitride layer grew rapidly due to the slow diffusion of nitrogen at the treatment temperature. During diffusion annealing, nitrogen was diffused inside the particle and nitrides of the surface layer were partially decomposed.

Because the nitrogen atmosphere did not increase the nitrogen level of the particles during heating of the powder bed, the nitrogen gas could not nitrogen AISI 316 L powder at the test temperatures. In diffusion annealing, the nitrogen content of the nitrated powders to 2 to 2.5% was reduced to 1.8%. Thus, from the nitride layer of the powder particles, nitrogen was likely to transfer slightly to the nitrogen atmosphere at 570 ° C. *

In light of the above test results, it will be noted that the process of the invention can nitrify metal powders at substantially lower temperatures than the prior art, and additionally, 1045667 nitrogen can be bonded to the metal powder in very large amounts. Low nitrification temperature combined with short processing time ensures low energy consumption. Thus, the invention enables a very economical way to produce large quantities of suitably nitrogenated metal powder, since a small amount of very high nitrogen content powder can first be prepared and mixed in a suitable proportion with the corresponding non-nitrogenous powder. This avoids the dulling of large volumes and the use of relatively small equipment in the dyeing equipment.

The invention has been described in detail above by means of examples, with various embodiments of the invention being possible within the scope of the claimed invention.

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Claims (6)

104566. A process for the alloying of a metal powder with nitrogen, characterized in that: - before the nitrogenizing, the metal powder is heated to a nitrogenization temperature in an inert gas, such as nitrogen gas, . Process according to Claim 1, characterized in that steel powder is used as the metal powder. Process according to Claim 1 or 2, characterized in that the nitrogenization takes place at a temperature of 500 to 700 ° C, preferably 570 to 670 ° C. Method according to one of Claims 1 to 3, characterized in that the nitrogen content of the metal particles is equalized by diffusion annealing in an inert gas. Method according to one of Claims 1 to 4, characterized in that the nitrogenization and the nitrogen diffusion are controlled by the composition of the nitrogen time and / or the gas mixture. Process according to claim 5, characterized in that hydrogen or an inert gas such as nitrogen is mixed with the ammonia gas. i, 4 »I * I 1 ι: I 9 104566