This will be the first in a series of smaller articles on Powder metal gear technology. Each article will focus around a topic that is related to gearing and how it is affected by the use of powder metal (PM). Gear technology using powder metal instead of wrought steel sometimes requires different practices. There are also manufacturing related advantages with PM that is there, free of charge. So with these articles the ambition is to increase awareness of what is possible, what is not possible and how PM can be tailored to better perform under different conditions.
This first article will discuss the material itself. The other five articles will discuss design, NVH and manufacturing affected by the fact that the material is powder metal.
When looking at the material from a gear perspective, the most obvious difference is the porosity. Figure 1 is a cross section of a gear tooth, polished and magnified. All the black spots are pores. This is a little frightening and goes against the trend towards cleaner materials to increase performance and reliability, a proven concept by the bearing manufacturers. But the pores are there, they will not go away without additional processes that will be discussed in a later article. Since there are all these millions of pores it is quite certain that failures such as tooth root breakage or pitting will initiate from a pore provided that the rest of the manufacturing process is under control. This actually creates a form of reliability; the numbers of outliers in the S-n data are less and the pores don’t create any surprises.
Solid steel on the other hand should be as clean as possible because an inclusion or pore in a highly stressed volume will drastically reduce the performance level. With PM you could argue that you always have this scenario due to the pores, and this is what gives lower performance compared to a gear steel. The more pores, or actually the bigger the pores, the lower the performance. This is why high density is so important for the fatigue and impact life of PM gears. Density is a crude parameter; it is more the pore size and shape that plays a role in determining performance. But higher density normally leads to smaller pores, and to make life easy mechanical properties can often be seen as a function of density.
The performance level for PM gear is dependent on several different factors, as always there are no straight answers that are always true but some pointers are given below.
PM can match any steel performance and I would stick out my neck and say that it can be even stronger than SAE 8620 steel for instance.
But porosity is also a blessing in disguise; it may be used to lower weight and inertia. This leads to lower rotational losses and less heat dissipation in a synchronizer mesh. The pores have a dampening effect that reduces transfer of vibrations to other parts in the system.
The pores could be used for impregnation with a lubricant and the gears could work in dry mode in certain applications. The very fact that the material is in a powder form gives unique opportunities in the shaping process. If the designer is aware of this and utilizes the possibilities; additional value is created that will reduce the number of process steps, simplify assembly and give a cost reduction.
The relevant fatigue data for PM is very much related to the process. Alloying content cannot make up for a bad process. The material has to deliver the right case hardening parameters meaning residual stress and hardness profile. Residual stress and hardness is again connected to density and the recommendation is to get as high as possible, 7.25-7.3 g/cm3 in single compaction and single sintering. Higher density is possible and will be discussed in a later article.
Our experience is that the fatigue limit for PM gears starts at 650MPa in bending and 1100MPa in contact pressure, 99% survivability when tested according to ISO 6336. Higher is possible as well as lower if processes are not properly controlled.
The next article will discuss NVH and PM gears. NVH is a tricky subject; because of few standards, it’s difficult to test, difficult to analyze, and everyone uses their own methods of testing, so it’s like a bit of a black art.