440B and 440C martensitic stainless steel are commonly used materials in MIM manufacturing, but their liquid-phase sintering temperature range is very narrow, and there are many defective products such as deformation and local collapse during the sintering process.?

The high hardness and corrosion resistance requirements limit the selection of MIM materials

When MIM parts are used in manual tools, cutting and grinding tools (such as cutting tools, razors, surgical tools), small hardware (such as mobile phone parts), and gasoline nozzle components in automotive engines, it is usually required that the material has a hardness greater than 55HRC and a certain degree of corrosion resistance.

There are relatively few materials in MIM technology that can meet these properties. The commonly used martensitic stainless steels are 440B and 440C. However, these materials are difficult to sinter; The temperature range of liquid-phase sintering is very narrow, which leads to uneven density distribution, large size shrinkage changes, and partial melting and collapse of parts in the same sintering furnace.

In order to reduce deformation and partial collapse, 440B and 440C are usually sintered by solid-state sintering to a medium density with closed pores, and then compacted by non mold hot isostatic pressing. The additional process greatly increases the overall manufacturing cost.

Note: HRC is the hardness obtained using a 150kg load and a 120 ° diamond cone indenter, and is used for materials with extremely high hardness. The shallower the indentation, the higher the HR value, and the higher the material hardness.

Modified martensitic stainless steel 440C Nb

A modified martensitic stainless steel "440C Nb" has emerged, which has a wider sintering window.

The 440C Nb system can obtain a sintering window with a width of approximately 25 ° C (defined as a temperature range where the sintering density is not less than 7.63g/cm 3 without causing collapse and partial melting). This temperature range is relatively easy to achieve for standard industrial sintering furnaces, whether they are batch furnaces or continuous furnaces.

Why does 440C Nb have a wider sintering window?

During the sintering process at 440C (1.0% C), when the temperature exceeds the solidus temperature of 1285 ° C, the following eutectic reaction occurs:

γ+M7C3 ? liquid phase

The eutectic reaction resulted in the instantaneous formation of a 7.5% liquid phase. As the temperature increases, the amount of liquid phase continues to increase: at 1289 ° C, the amount of liquid phase is 10.5%, and at 1294 ° C it reaches 12.6%.

Correspondingly, the solidus temperature in the 440C Nb (1.2% C) system is 1281 ° C, only slightly lower than 440C. When the temperature exceeds the solidus line, only 4% of the liquid phase forms. As the temperature continues to rise to 1294 ℃, the amount of liquid phase remains at 4% (due to the presence of NbC, a large amount of C is fixed in the form of NbC, and the eutectic reaction of γ+M7C3 ? liquid phase is inhibited). When the temperature rises to 1350 ℃, exceeding the liquidus temperature of 70 ℃, the liquid phase content of 440C Nb is still lower than 440C.

This explains the problem encountered in 440C sintering very well: the amount of liquid phase produced by slight temperature changes at different positions or even different positions of the same part in the sintering furnace is different. For 440C Nb liquid phase, the amount is even smaller, and the increase in liquid phase with temperature is not significant, so such problems rarely occur.

In summary, this new type of material can effectively serve as a high hardness and corrosion-resistant material required in MIM processes.

Note: Solid phase line temperature: the temperature at which melting begins.