Powder metallurgy stainless steel vacuum sintering process

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Overview of powder metallurgy stainless steel

Stainless steel manufactured by powder metallurgy is a powder metallurgy material that can be made into steel products or parts.

Advantage

1.Reduce the segregation of alloy elements;

2.Refine the microstructure and improve performance;

3.Save raw materials, save energy consumption and reduce costs.

Corrosion resistance is achieved using powder metallurgy stainless steel.

Commonly used stainless steel

304L: Basic austenitic stainless steel, the most economical austenitic stainless steel.

304LSC: Adding Sn on the basis of 304L, it has all the properties of 304L and improves the corrosion resistance of sintering at 1120°C.

410: Martensitic stainless steel, which has the lowest corrosion resistance among all powder metallurgy stainless steels, but has low cost.

316L: Austenitic stainless steel, with better corrosion resistance and machining performance than 304L. With proper production control, its corrosion resistance can meet higher requirements.

Definition of stainless steel

There is a sufficient amount of chromium in the iron-based alloy, which will form a stable passivation film on the surface. An iron-based alloy with a chromium content of more than 10.5% can be called stainless steel. Stainless steel still has good oxidation resistance at high temperatures. With sufficient chromium content > 10.5%, a passivation protective film can be formed on the surface of the material in aqueous environments. Hydrated oxides require the presence of oxygen; the film is dynamic and can be repaired automatically, mainly containing Cr, Mo, and Si; chloride ions and acidic conditions are destructive to it; if there are oxides, precipitates, and lattice defects near the film, The corrosion resistance will be reduced; the stability of the film determines the corrosion resistance of the material.

Production Process

suppress

Green density: 6.2-6.4 g/cm³

Green density: 6.4-6.6 g/cm³

Green density: 6.6-6.8 g/cm³

(Different molding densities benefit from different lubricants and different powder specifications)

Industrial sintering furnace

The most widely used sintering furnaces in powder metallurgy are mesh belt furnaces 1120-1150℃, push rod furnaces 1150-1250℃, and vacuum furnaces 1200-1350℃. Sintered parts have the best corrosion resistance.

Sintering atmosphere: decomposed ammonia, H2/N2, mixed gas, pure hydrogen, vacuum.

If the temperature rises too quickly, the C in the lubricant will be absorbed by the parts.

Case: 1% lubricant in the green body = 0.7% C. If the dewaxing efficiency is 90%, the dewaxed part still contains 0.07% C before entering the high temperature zone. The impact is not big. For 316L, if it is sintered at 1150℃, the corrosion resistance will be very poor.

Solution: Make sure the parts are fully dewaxed before entering the high temperature area.

Vacuum sintering

Factors affecting the corrosion resistance of powder metallurgy stainless steel
Related to sintering: 1. Formation of oxides 2. Formation of carbides 3. Formation of nitrides
The C, N, and O content in sintered parts is very important
Powder performance requirements
C<0.03%
O < 0.3%
N < 0.05%
Minimum requirements for normal sintered parts -> acceptable corrosion resistance
C<0.05%
O < 0.4%
N < 0.3%
Higher requirements -> good corrosion resistance
C<0.03%
O < 0.25%
N < 0.2%
Carbon content control
1. Remove lubricant, conventional dewaxing range: 600-850℃.
2. There should be no decomposition products of lubricant and graphite in the vacuum sintering furnace. It should be run without load for at least 24 hours before use.
3. Cool as quickly as possible to prevent the formation of Cr7C3 and Cr23C6.

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