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MARINOX25- Duplex

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Valbruna Grade

MARINOX25

Steel type

Duplex Stainless Steel

Description of material

Marinox25 is a ferritic-austenitic stainless steel with Molybdenum and Nitrogen, whose duplex structure, containing approximately the same amounts of Ferrite and Austenite, is able to provide a good pitting and uniform corrosion resistance, as well as very good stress corrosion cracking resistance, together with high mechanical properties and toughness.

Applications

Many marine applications and boat propeller shafts where corrosion fatigue resistance is required. In sea water, this grade is more resistant to pitting and crevice corrosion than types 316L/304L.

Melting practices

Argon Oxygen Decarburization

Corrosion resistance

Marinox25 shows better results in general and pitting corrosion than type 316 and similar austenitic stainless steels, and has very good resistance to crevice corrosion thanks to high Chromium, Nitrogen and Molybdenum contents. Stress corrosion resistance is guaranteed thanks to presence of the Ferritic part of the duplex structure. However, although this grade has a very high resistance to localized corrosion, a periodical rotation of shafts should be done, because this movement of sea water (flow) avoids a stagnant situation that could prime pitting and crevice corrosion. In any case, a cathodic protection is recommended. The choice of sacrificial anodes, such as zinc, magnesium and aluminum, depends on the kind and nature of water where the propeller shaft works.

Machinability

Marinox25, such as all duplex stainless steels, is more difficult to machine if compared to the typical austenitic grades or ferritic ones. A careful choice of machining parameters should partially reduce the gap. 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 very important to increase the service life, improving the fatigue resistance of the boat shaft propeller.

Weldability

Marinox25 is supplied as precision straightened cold finished bars ready for fabricating into boat propeller shafts. In the case of limited and localized 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 heated zone. The structure of HAZ should show an acceptable austenite content if the right welding parameters are applied. Matching filler duplex alloys could be used if the composition of the FZ and HAZ is to able to supply the expected results. Over -alloyed fillers may be a right alternative in order to obtain comparable properties of the base metal or, at least, an austenite content more than 40%. In very aggressive environments, fillers of Nickel alloys or high alloy Austenitic grades should be used.

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