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Austenitic Stainless Steel
Description of material
Marinox16 is a low-carbon austenitic stainless steel with Molybdenum and has good general and pitting resistance, as well as good intergranular corrosion resistance after welding processes. Marinox16 is the older and popular stainless steel for marine and boat shafting applications first used before the appearance of more high performance grades.
Many marine applications and boat propeller shafting. In sea water, this grade is more resistant to pitting and crevice corrosion than type 304/304L.
Generally, Marinox16 has the same overall corrosion resistance of type 304 series steels, but offers a better resistance to pitting and crevice corrosion. However, in spite of its Molybdenum content, which is able to supply a certain improvement of resistance to localized corrosion, this grade is still susceptible to pitting and crevice corrosion in stagnant sea water if the propeller shaft stays motionless for lengthy intervals of time and doesn’t operate continuously. In this case, a periodical rotation of the shaft should be done, because this movement of sea water (flow) avoids a stagnant situation able to prime pitting and crevice corrosion. Cathodic protection is recommended in order to avoid these kinds of corrosion. The choice of sacrificial anodes, such as zinc, magnesium and aluminum, depends on the kind and nature of water where the propeller shaft works. Marinox16, as all standard austenitic grades, could suffer from stress corrosion cracking above certain levels of stress and sea water temperature. Strain hardened structures increase the risk of this form of corrosion.
Marinox16 has the typical machinability of austenitic stainless and no particular difficulties occur in turning, threading and milling processes. However, operators should know that Austenitic grades are different from PH-martensitic (Marinox17) and Martensitic (Marinox18) steels and require more rigid and powerful machines, in addition to the correct choice of tools, coatings and cutting fluids. The Austenite structure is prone to transform in to α’Martensite caused by strain hardening of the tool on the surface of the work piece. Even if Marinox16 has a hardening factor lower than Marinox 19 /22/ 22HS, the knowledge of this behavior must be correctly considered when a piece requires two or several cutting steps to be finished. The layer of α’Martensite is very hard and, if the subsequent turning or milling processes work on this hardened layer, a rapid tool wear could happen. The tool must work under this layer. Particular care is recommended to obtain a shaft with very low surface roughness and a good geometry of its keyways avoiding sharp corners and poor interior roughness. Top and interior corners must have a suitable filleting and sled-runner type keyway end should be used to increase the service life. Machining parameters must avoid operations that cause stresses in the shaft. A precision straightening after machining operations is also very important to increase the service life, improving the fatigue resistance of the boat shaft propeller.
Marinox16 is supplied in precision straightened cold finished bars ready for fabricating into boat propeller shafts. In the case of limited welding repairs, no preheating is required but it is important to point out that any localized heating may cause a certain amount of bending of the shaft and this amount depends on the width/ thickness of the heated zone. Matching fillers should be used for this operation but other austenitic fillers may be an alternative.