Use of bainitic steels

There are large markets for steels with strengths less than 1000 MPa, and where the total alloy concentration rarely exceeds 2 wt.%. Bainitic steels are well suited for applications within these constraints. However, alloy design must be careful in order to obtain the right microstructures. Steels with inadequate hardenability tend to transform to mixtures of allotriomorphic ferrite and bainite. Attempts to improve hardenability usually lead to partially martensitic microstructures. The solution therefore lies in low-alloy, low-carbon steels, containing small amounts of boron and molybdenum to suppress allotriomorphic ferrite formation. Boron increases the bainitic hardenability. Other solute additions can, in the presence of boron, be kept at sufficiently low concentrations to avoid the formation of martensite. A typical composition might be Fe-0.1C-0.25Si-0.50Mn-0.55Mo-0.003B wt.%. Steels like these are found to transform into virtually fully bainitic microstructures with very little martensite using normalising heat treatments.

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The most modern bainitic steels are designed with much reduced carbon and other alloying element concentrations. They are then processed using accelerated cooling in order to obtain the necessary bainitic microstructure. The reduced alloy concentration not only gives better weldability, but also a larger strength due to the refined bainitic microstructure.

The range of bainitic alloys available commercially includes ultra-low carbon bainitic steels for high weldablity, very high strength steels competing with the quenched and tempered martensitic alloys, creep resistant steels which have now been used for decades in the power generation industries, forging steels which are better than martensitic alloys because they require mush less processing, inoculated steels in which the bainite is induced to nucleate intragranularly on particles to produce a chaotic microstructure which resists the propagation of cracks, etc. The ultra-high strength steels consist of mixtures of bainitic ferrite, martensite and retained austenite. They have an enhanced hardenability using manganese, chromium and nickel, and usually also contain a large silicon concentration (about 2 wt.%) in order to prevent the formation of cementite ( Figure). High strength steels are made with very low impurity and inclusion concentrations, so that the steel then becomes suceptible to the formation of cementite particles, which therefore have to be avoided or refined.


 Alloy                | C       Si     Mn   Ni   Mo    Cr    V   B       Nb   Other 

 Early bainitic steel | 0.10   0.25   0.5   -    0.55   -    -   0.003   -    

 Ultra low carbon     | 0.02   0.20   2.0   0.3  0.30   -    -   0.010   0.05  

 Ultra high strength  | 0.20   2.00   3.00  -    -      -    -   -       -   

 Creep resistant      | 0.15   0.25   0.50  -    1.00   2.3  -   -       -   

 Forging alloy        | 0.10   0.25   1.00  0.5  1.00   -    -   -       0.10   

 Inoculated           | 0.08   0.20   1.40  -    -      -    -   -       0.10   0.012 Ti 

Chemical composition, wt.%, of typical bainitic steels

Medium strength steels with the same microstructure but somewhat reduced alloy content have found applications in the automobile industry as crash reinforcement bars to protect against sidewise impact. Another major advance in the automobile industry has been in the application of bainitic forging alloys to the manufacture of components such as cam shafts. These were previously made of martensitic steels, by forging, hardening, tempering, straightening and finally stress-relieving. All of these operations are now replaced by controlled cooling from the die forging temperature, to generate the bainitic microstructure, with cost savings which on occasions have made the difference between profit and loss for the entire unit.

Creep resistant bainitic steels have been used successfully in the power generation industry since the early 1940's. Their hardenability has to be such that components as large as 1 m in diameter can be cooled continuously to generate a bainitic microstructure throughout the section. The alloys utilise chromium and molybdenum, which serve to enhance hardenability but also, during subsequent heat-treatment, cause the precipitation of alloy carbides which greatly improve the creep resistance.

By inoculating molten steel with controlled additions of nonmetallic particles, bainite can be induced to nucleate intragranularly on the inclusions, rather than from the austenite grain surfaces. This intragranularly nucleated bainite is called "acicular ferrite". It is a much more disorganised microstructure with a larger ability to deflect cracks. Inoculated steels are now available commercially and are being used in demanding structural applications such as the fabrication of oil rigs for hostile environments.

Advances in rolling technology have led to the ability to cool the steel plate rapidly during the rolling process, without causing undue distortion. This has led to the development of "accelerated cooled steels" which have a bainitic microstructure, can be highly formable and compete with conventional control-rolled steels.