Aqualoy is available in 4 different grades and is stocked in shaft sizes ranging from 3/4 inches to 8 inches in diameter with lengths up to 31-1/2 feet. Longer shafting is available for larger boats through precision rotary forging. This capability lets us manufacture Aqualoy propeller shafting with exceptional as-forged straightness up to 38 feet long and up to 15 inches in diameter.
Comparison of Marine Grade Materials with Standard ASTM Materials
In order to understand the differences in the materials we need to first understand the environment in which the materials are expected to perform. Aqualoy 19 and 22 were created as specific upgrades to their standard counterparts in both strength and corrosion resistance. Table 1 shows the chemical composition of Aqualoy 19, 304, Aqualoy 22 and 316.
As you can see, the chemical compositions of the alloy pairs are different with Aq 19 having higher Mn, Cr and N than standard 304. Aq 22 also has higher Mn, Cr, N as well as higher Ni and Mo.
These chemical differences serve to improve alloy performance in two ways. First the increase in Mn and Cr increase the solubility of nitrogen which is important as nitrogen helps the alloys to attain higher strength. Secondly, the increase in Cr, Mo and N help to increase the pitting resistance equivalent number which is calculated as follows:
PREN = %Cr + 3.3 x %Mo + 16 x %N
Two of the most common forms of corrosion of stainless steel in marine environments are pitting and crevice corrosion. Typically alloys which are more resistant to one mechanism are more resistant to both. This equation has been used for many years to rank austenitic stainless steels and their relative resistance to pitting in chloride type environments such as sea water. Studies have shown that higher PREN’s correspond to increased resistance to pitting. If we are to calculate the PREN of our 4 alloys above one can easily see the superiority of the marine grades.
From the chemistry modifications one would expect and indeed one does see improved corrosion resistance with the marine grades in marine environments.
As far as mechanical properties are concerned, the differences are also dramatic. In fact the original design criteria for Aqualoy 22 were to double the strength of 316 at the same time as the corrosion resistance was improved. As was mentioned earlier, the primary modifications to the chemistry was aimed at increased nitrogen content and when one looks at the annealed yield strength of the 4 alloys the improvements are obvious.
One further modification found on the marine grades is that in addition to the chemical modifications, there area also processing modifications which are designed to increase the strength dramatically above the annealed properties listed above. Depending on the size it is possible to increase the yield strength of both AQ 19 and AQ 22 in excess of 105,000 psi. This will allow more power to be transferred through marine grades of the same size when compared to their standard counterparts.