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Surface hardening of powder metallurgy parts

Low pressure carburizing technology has been used for many years in the field of products requiring high performance, especially in the automotive industry. The application of this process in powder metallurgy products is not very extensive, but because this process can avoid the use of quenching and can safely and effectively deal with oxygen sensitive materials, its future application prospect is very interesting. This paper introduces the specificity of powder metallurgy parts, the results obtained by low pressure carburizing process, and the new research and development direction recently.

Key parameters of low pressure carburizing for powder metallurgy parts

One of the key parameters for P/M parts to achieve mechanical and metallurgical properties is the final density or, more recently, the type of porosity, compared with the surface hardened parts machined. The density depends on the compression pressure, the type of the powder and the sintering temperature.
For thermal diffusion processes such as carburizing, the type of pores is also a key parameter to be considered. Density is generally considered acceptable around 7, but in practice it is not appropriate when mesh-like penetrating or open porosity occurs.
It is obvious that a single open pore and/or reticulated pore provide a more active surface and a heterogeneous structure, resulting in different results. In fact, the carbon enrichment occurs at different depths during intensive carburization and is enhanced by surface diffusion treatment. (see chart 1)

influence of net penetrating pore on strong permeability

Fig. 1: influence of net penetrating pore on strong permeability

As a result, this uncontrollable carbon enrichment in a network of penetrating pores leads to carbide precipitation.
The morphology of pores also depends on the content of alloying elements in mixed powders. Adequate alloying elements, suitable compaction pressure and/or sintering temperature are essential for obtaining closed-type pores, which are also necessary for chemical heat treatment processes.

Development of low pressure carburizing process for powder metallurgy parts

Vacuum technology provides various benefits for surface hardening of powder metallurgy parts.
In addition to the following known benefits:
No oxidation
High temperature treatment to reduce processing time
Low energy consumption
Vacuum furnaces also have the following characteristics
Better temperature distribution
Better quenching uniformity and less deformation after hardening.
Improving final sintering density of powder metallurgy parts
Clean and environmentally friendly technology
Develop a new process that includes all processes from sintering to quenching in a single device.
Preheating and purifying
High temperature sintering (in an anoxic atmosphere)
High temperature carburizing
High pressure gas quenching.

Fig. 2: all processes are completed in a ECM double chamber vacuum furnace.

Results of low pressure carburizing of powder metallurgy parts

More specifically, the low pressure carburizing treatment developed at Astaloy 85Mo and chromium grade (Astaloy Cr L&A) yielded good results.
After treatment, the surface hardness of all powder grades can reach 800HV, and the surface hardening depth of Ast85Mo grade can reach 1 mm.
Following these results, the industrial application gearbox spiral gears manufactured by powder metallurgy of Ast820Mo material were further tested. Very good profiles of hardened layers were obtained on the tooth surface and heel, even though the hardened layers on the tooth surface were deeper than those on the heel (shown below). The results clearly demonstrate the effect of shape on Carburizing penetration depth.

Fig. 3: dia carburizing distribution on helical gear tooth profile

Fig. 4: corresponding hardness gradient distribution
Latest research findings
The volume of the heating chamber is reduced, and the temperature uniformity is very good. In addition, the perfect distribution of quenching speed and the pressure and gas flow rate can be controlled.

This quenching interruption re creates the conditions of quenching in hot oil bath, and goes to a self tempering. At the same time, the carbonitriding process can also be applied in this furnace type, thereby improving the fatigue resistance.
In summary, considering various development trends (furnace type and process development) and the special requirements of powder metallurgy parts, the conclusion is that low pressure carburizing process is very promising for powder metallurgy parts.

Fig. 5: quenching interruption