Metallurgy of Mo in Stainless Steel

8/10/2019 Metallurgy of Mo in Stainless Steel

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Metallurgy of Mo in stainless steel

Molybdenum adds corrosion resistance and high temperature strength

Molybdenum primarily increases the corrosion resistance of stainless steels (see Grades and Properties).Molybdenum containing grades of stainless steels are generally more corrosion resistant than

molybdenum-free grades. They are used in applications that are more corrosive, such as chemical

processing plants or in marine applications. There are many grades of stainless steels with different

molybdenum (and chromium, nickel, nitrogen, etc.) contents. The best grade for a given application is

selected based on the corrosivity of the service environment.

As a large atom, molybdenum increases the elevated temperature strength of stainless steels through solid

solution hardening. This effect is used in heat exchangers and other elevated temperature equipment such

as in automotive exhaust systems.

Molybdenum is a ferrite former

To discuss the influence of molybdenum on the metallurgy of stainless steels it is useful to look at the

metallurgy of stainless steels in general. Based on their microstructure, stainless steels are divided into the

following families:

austenitic

ferritic

martensitic

duplex

precipitation hardenable

The division based on microstructure is useful because the members within one family tend to have similar

physical and mechanical properties. However, the properties for one family can be very different from the

properties for another family. For example, austenitic stainless steels are non-magnetic, while ferritic andduplex stainless steels are magnetic.

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The difference between the families is fundamental on an atomic level. The arrangement of atoms in theferrite crystal is different from the one in the austenite crystal:

In the ferritic stainless steel, the iron and chromium atoms are arranged on the corners of a cube and in the

center of that cube. In the austenitic stainless steels the atoms, here iron, chromium and nickel, are

arranged on the corners of the cube and in the center of each of the faces of the cube.

This seemingly small difference profoundly affects the properties of these steels.
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Fig 1: The ferritic stainless steel on the left has a body centered cubic (bcc) crystal structure. By adding nickel to this stainless

steel the structure changes from bcc to face centered cubic (fcc), which is called austenitic.

Select properties of austenitic and ferritic stainless steels

Properties Austenitic Ferritic

Toughness Very high Moderate

Ductility Very high Moderate

Weldability Good Limited

Thermal expansion High Moderate

Stress corrosion cracking resistance Low Very high

Magnetic properties Non-magnetic Ferro magnetic

Because of their good mechanical properties and the ease of fabrication, austenitic stainless steels are

much more widely used than ferritic stainless steels. About 75% of all stainless steel used worldwide is

austenitic and about 25% is ferritic. The other families, martensitic, duplex and precipitation hardenable

stainless steels each represent less than 1% of the total market.

Besides nickel there are other elements that tend to make the structure austenitic. These elements are

called austenite formers. Alloying elements that tend to make the structure ferritic are called ferrite formers.

Ferrite and austenite formers

Ferrite formers Austenite formersIron Nickel

Chromium Nitrogen

Molybdenum Carbon

Silicon Manganese

Copper

Molybdenum is a ferrite former. That means that when molybdenum is added to improve the corrosion

resistance of an austenitic stainless steel, there has to be an austenite former such as nickel or nitrogen

added in order to keep the structure austenitic.

Duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure; hence they

have a duplex structure. This effect is achieved by adding less nickel than would be necessary for making

a fully austenitic stainless steel.


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Molybdenum is mainly used for added corrosion resistance in austenitic and duplex stainless steels. In

austenitic stainless steels between two and seven percent are added, in duplex stainless steels, between

three and five percent. The addition of one or two percent molybdenum to ferritic stainless steels also

significantly increases the corrosion resistance and the elevated temperature strength of these stainless

steels.

Fig 2: Adding 8% nickel to a ferritic chromium stainless steel makes an austenitic chromium-nickel stainless steel, for example

Type 304 stainless steel. If less nickel is added to a chromium steel, about four or five percent, a duplex structure, a mixture of

austenite and ferrite, is created as in 2205 duplex stainless steel.


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Molybdenum grade Alloy Steels & Irons

Molybdenum is used efficiently and economically in alloy steel & iron to

improve hardenability

reduce temper embrittlement resist hydrogen attack & sulphide stress cracking

increase elevated temperature strength

improve weldability, especially in high strength low alloy steels (HSLA)

In the present section the focus is on grades and properties of Mo containing alloy steel and iron. End uses

cover the whole world of engineered products for:

Automotive, shipbuilding, aircraft and aerospace

Drilling, mining, processing

Energy generation, including boilers, steam turbines and electricity generators

Vessels, tanks, heat exchangers

Chemical & Petrochemical processing

Offshore; Oil Country Tubular Goods (OCTG)In most cases molybdenum is needed to meet the high end of the application properties, which is

accomplished with comparatively small molybdenum additions. In fact, with the exception of High Speed

Steel and Maraging Steel the Mo content often ranges between 0.2 and 0.5% and rarely exceeds 1%.

Properties of Molybdenum

Typical % Mo content

Heat Treatable Engineering Steel 0.25 - 0.5

Case Hardened Steel 0.15 - 0.5

High Temperature Steel 0.3 - 1.2Oil Country Tubular Goods (OCTG) 0.3 - 1.0

HSLA Steel 0.15 - 0.25

Maraging Steels 4.0 - 5.0

Tool & High Speed Steel 0.5 - 9.0

Cast Iron 1.0 -3.0
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Duplex stainless steel

Duplex stainless steels are called duplex because they have a two -phase microstructure consisting of

grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as islands

surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquidphase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic

grains transform to austenitic grains (islands). The result is a microstructure of roughly 50% austenite and

50% ferrite.

Duplex stainless steels have a two-phase microstructure of austenite and ferrite grains.

The duplex structure gives this family of stainless steels a combination of attractive properties:Strength:Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.

Toughness and ductility:Duplex stainless steels have significantly better toughness and ductility than

ferritic grades; however, they do not reach the excellent values of austenitic grades.

Corrosion resistance:As with all stainless steels, corrosion resistance depends mostly on the

composition of the stainless steel. For chloride pitting and crevice corrosion resistance, their chromium,

molybdenum and nitrogen content are most important. Duplex stainless steel grades have a range of

corrosion resistance, similar to the range for austenitic stainless steels, i.e from Type 304 or 316 (e.g. LDX

2101) to 6% molybdenum (e.g. SAF 2507) stainless steels.

Stress corrosion cracking resistance:Duplex stainless steels show very good stress corrosion cracking

(SCC) resistance, a property they have inherited from the ferritic side. SCC can be a problem under

certain circumstances (chlorides, humidity, elevated temperature) for standard austenitics such as Types304 and 316.

Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic

counterparts of similar corrosion resistance. Due to the lower alloying content, duplex stainless steels can

be lower in cost, especially in times of high alloy surcharges. Additionally, it may often be possible to

reduce the section thickness of duplex stainless steel, due to its increased yield strength compared to

austenitic stainless steel. The combination can lead to significant cost and weight savings compared to a

solution in austenitic stainless steels.

Metallurgy of Mo in Stainless Steel

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