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MARINOX18- Martensitic Stainless Steel

Steel data sheets

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Valbruna Grade


Steel type

Martensitic Stainless Steel

Description of material

Marinox18 is a low Carbon martensitic stainless steel with Molybdenum. It has high strength, excellent toughness, and a good resistance to stress corrosion cracking. Its structure of tempered martensite with finely dispersed stable austenite creates high values of toughness even at very low temperatures, and an improvement of resistance to SCC. See grade X164M to know the properties and metallurgical behavior of Marinox18.


Many marine applications and boat propeller shafting where the Marinox18 offers a combination of high strength – toughness- corrosion resistance, allowing the use of a smaller shaft diameter and, consequently, a reduction of its weight.

Melting practices

Argon Oxygen Decarburization

Corrosion resistance

Generally, Marinox18 has the same general corrosion resistance of type 304 series steels, but offers a better resistance to stress corrosion cracking. In spite of its Molybdenum content, which is able to improve a little its resistance to localized corrosion, this grade is 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 highly suggested and 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.


Marinox18 has a good machinability in turning, threading and milling processes. 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 able to 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.


Marinox18 is supplied in precision straightened cold finished bars ready for fabricating boat propeller shafts. In the case of limited or localized welding repairs, no preheating is required but it is important to point out that untempered martensite results in FZ and HAZ reducing ductility in this zone. Matching fillers should be used for this operation but Austenitic fillers may be an alternative. In this case a reduction of strength in the repaired zones should be well evaluated. In addition, it is useful to underline 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.


W.N. 1.4418
EN X4CrNiMo16-5-1
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