Metallurgy of Mo in alloy steel & iron

The purpose of quenching steel after heating is hardening, i.e. to produce a hardened microstructure over the full cross section of the workpiece.

A round steel bar quenched in water from a temperature up to 900°C will cool faster near the surface than in the center (Fig 2 below).

Quenching round bar

Fig 2: Quenching round bar

Simulating cooling of steel sections

Fig 3: Simulating cooling of steel sections

On a laboratory scale this is simulated by the Jominy end quench test (Fig 3 above).

The standard sample is heated and then water quenched on one end. The cooling rate in the sample decreases from the water sprayed end where it is highest to the opposite end where it is lowest. When the sample is cool, the surface is ground and the hardness profile is taken. The change of hardness along the sample reflect variations of the microstructure brought about by the different cooling rates.

The curves in Fig 4 compare the hardness profile of steels with different alloy contents. The carbon manganese steel without molybdenum has only been hardened near the quenched end of the sample and the hardness drops quickly when moving away from the quenched end. With increasing molybdenum content the hard microstructure is maintained at increasing distances from the quenched end. That means that it is possible to harden a steel with higher molybdenum content with slower cooling rates: the hardenability is improved.

Jominy hardenability curves

Fig 4: Jominy hardenability curves: Hardenability improves with increasing Mo content (after W.W. Cias1)

The hardenability indicates the depth, to which a steel grade can be hardened.

In standard Quenched and Tempered Steels a combination of alloying elements is usually used, including manganese, chromium, molybdenum, nickel and silicon.

Hardenability Multiplying Factor

Fig 5: The Hardenability Multiplying Factor shows the rate at which the hardening depth is increased with the percentage of the alloying element (after Honeycombe2)

The basis of steel hardening lies in the fact, that iron exists in two crystal structures:

Below 912°C and from 1394°C to its melting point iron is body centered cubic – bcc – called ferrite. In the lower temperature range ferrite is also referred to as alpha iron, in the higher temperature range as delta iron.

At a temperatures from 912°C to 1394°C iron is in the face centered cubic crystal structure – fcc – called gamma iron or austenite. Heating pure iron above 912°C transforms the structure from Ferrite into Austenite. Cooling the iron from the austenitizing area below 912°C results in the original bcc iron structure, no matter what cooling rate is applied.

Pure iron can not be hardened.

The addition of carbon converts iron into hardenable steel. (Alloying elements such as manganese, molybdenum and chromium enhance the hardenability).

Carbon is present in iron both in solid solution and in the form of carbides. It is significant that the sides of the face centered cubes of the austenite are about 25% larger than the sides of the body centered cube of the ferrite. The solubility for carbon is therefore much greater in austenite than in ferrite.

When a steel with say 0.4% carbon is heated above the ferrite.austenite (alpha-gamma) transformation point, carbon and the other alloying elements can go into solid solution in the spacious austenitic fcc structure. Subsequent cooling through the gamma – alpha transformation point leads into the narrow ferrite structure. There is not enough space in this structure to keep carbon in solid solution.

So, if the cooling rate is low, carbide is formed in connection with the transformation process. As a result the microstructure at room temperature consists of ferrite and carbide. (The fine lamellar structure of ferrite and iron carbide is called pearlite - see Fig 6).

Ferrite - Pearlite microstructure

Fig 6: Ferrite – Pearlite microstructure – soft and ductile
Courtesy of metallograph.de

Martensite microstructure

Fig 7: Martensite microstructure – hard and brittle
Courtesy of metallograph.de

The critical factor is, that there is enough time available for the carbon atoms to move through the lattices to form carbides, which results in the soft microstructure of ferrite and pearlite.

Increasing the cooling rate progressively reduces the carbide formation. A very high cooling rate is achieved with water quenching, which completely supresses the carbide formation. In that case carbon is uncomfortably forced into narrow spaces in the ferrite structure. The microstructure which is generated that way is called martensite. This is the hardest and most brittle form of steel. (See Fig 7)

In plain carbon steels the high cooling rates required for the formation of martensite are only achieved near the quenched surface. Inside the work piece the structure remains soft. Water quenching larger sections also involves the risk of quench cracking.

This is where molybdenum and the other alloying elements enter the scene. Alloying elements slow down the diffusion of carbon atoms through the iron lattice., which delays the transformation from austenite into ferrite. The hardenability of the steel is thus improved since martensite can be produced at slower cooling rates. As shown in Fig 5, molybdenum is very effective in that respect.

Also, in larger cross sections at intermediate cooling rates a structure called Bainite is formed particularly in Mo alloyed steel. In that case, some nucleation of carbides has taken place during cooling before the austenite- ferrite transformation.

In practice, the microstructure of quenched and tempered steel components consists of a combination of martensite and bainite.