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Austenitic Stainless Steel
Description of material
Marinox19 is a Chromium-Nickel austenitic stainless steel whose Nitrogen content increases the strength with little influence on toughness and corrosion. Marinox19 is a variation of the well-known type 304 stainless steel but offers more resistance corrosion.
Many marine applications and boat propeller shafting. This grade has higher strength than Marinox16.
Argon Oxygen Decarburization
Generally, Marinox19 has the general corrosion resistance of type 304 series and Marinox17, but offers a better resistance to pitting and crevice corrosion. This grade is still susceptible to pitting and crevice corrosion in stagnant sea water, particularly if the propeller shaft stays motionless for a lengthy interval of time and doesn’t operate continuously. In this case, a periodical rotation of the shaft should be done, because this motion of sea water (flow) avoids a stagnant situation, able to prime pitting and crevice corrosion. Cathodic protection is always 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. Marinox19, 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.
Marinox19 has the typical machinability of austenitic structures strengthened by Nitrogen and some difficulties could happen in turning, threading and milling processes due to their capacity to cold work harden. 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, coating 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. 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 keyway 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 able to cause stresses in the shaft. A precision straightening after machining operations is also much important to increase the service life, improving the fatigue resistance of the boat shaft propeller.
Marinox19 is supplied in precision straightened cold finished bars, ready to be fabricated 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.