Effects of Molybdenum on Austenitic Stainless Iron

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Generally speaking, simple chrome-nickel (Cr, Mn, and nitrogen) austenitic stainless iron is only applied in requirements for stainless and oxidation-resistance medium (such as nitric acid, etc.), and the adding of molybdenum, an important alloying element in austenitic stainless iron, has widened its application range. The main role of molybdenum is to improve iron’s corrosion resistance in reductants (H2SO4, H3PO4, some organic acids, and urea environment as well), and also improves its resistance to pitting and crevice corrosion as well.

Molybdenum’s influence on structures

Molybdenum and chromium both are the elements that can form and stabilize ferrites and extend ferrite phase, and their abilities of forming ferrites are almost equal. Molybdenum also promotes precipitation of iron intermetallic phases in austenitic stainless, such as σ phase, κ phase, and Laves phase. It also adversely influences the corrosion resistance and mechanical properties of iron, leading to the ductility and toughness decrease. In order to have Austrian austenitic stainless iron remain in a single austenite structure, an increase of austenitic forming elements (nickel, nitrogen and manganese) must be met with the increase of molybdenum in iron so as to keep balance between ferrites and austenitic forming elements.

Molybdenum
Molybdenum’s impact on performance

Molybdenum doesn’t have quite significant oxidation on austenitic stainless iron, so when chrome-nickel austenitic stainless austenitic iron remains single austenitic structure with no precipitation of intermetallic phases, adding molybdenum has trivial impacts on its room-emperature mechanical properties. However, with the increase of molybdenum powder, the high temperature strength of iron gets improve, so do other properties, such as its persistence and creep properties. Thus, molybdenum-containing stainless iron can also be applied at a high temperature. However, the adding of molybdenum in iron also increases its high temperature deformation resistance, coupled with the presence of a small amount of δ ferrite stainless steel in iron, containing molybdenum iron has poorer hot workability than those free of molybdenum, and the more molybdenum it contains, the worse its hot-workability becomes. In addition, the κ (σ) phase precipitates molybdenum-containing austenitic stainless iron might have will significantly deteriorate the ductility and toughness of iron. Therefore during the production, equipment manufacturing and application of molybdenum-containing austenitic stainless iron, special attention must to be paid in order to prevent the formation of intermetallic phases in iron.

The foremost role of molybdenum in austenitic stainless iron is to enhance iron’s reduction ability and its resistance to pitting or crevice corrosion and other properties. Judging from molybdenum’s corrosion resistance influences on chromium-nickel austenitic iron in media such as nitric acid, sulfuric acid, acetic acid, phosphoric acid and urea, it is easy to notice that molybdenum is of many benefits except in oxidizing medium HNO3.

In terms of stress corrosion resistance in high concentrated chloride solution, although the exact reaction mechanisms of the alloying element molybdenum has on austenitic stainless steel iron’s resistance to reducing, pitting or crevice corrosions are not entirely clear, but a large number of experiments have indicated that molybdenum’s corrosion resistant ability only happens when there is a higher amount of chromium in iron. Molybdenum mainly strengthens corrosion resistant ability of chromium, while the corrosion inhibition of acid salt formed by molybdenum has also been confirmed by experiments.

Molybdenum can be harmful to the stress corrosion resistance of austenitic stainless steel iron, but due to the fact that the common chromium-nickel austenitic stainless steel iron was more used in aqueous medium containing traces of chlorides and oxygen saturation, and its stress corrosion starts with pitting corrosion, therefore chromium-nickel austenitic stainless steel iron containing molybdenum usually has better resistance to chloride stress corrosion in practical applications due to its higher resistance to pitting corrosion.