New Generation Bainitic Steels

Bainitic Steel is known for high impact strength and high hardness, metallurgically two properties difficult to bond.

Irving and Pickering invented and developed a series of low carbon (Mo B, 0.5%) Bainitic steels in the 1950's, however product development from the invention was tardy.

Researchers at Tsinghua University identified in recent years that a number of technical and economical advantages can be exploited by examining again the field of Bainitic steels and by including the low to medium-high carbon steels suitably alloyed with Manganese and Boron.

When this work was combined with advanced air cooling techniques from the Austenite stage to the final product a new Bainitic steel called Bainite/Martensite Duplex Structure Steel was invented and patented under Australian Patent no. 624201.

Although transformation behaviour of low alloy steels during continuous air cooling is quite involved, some general statements can be made in order to position Bainitic steels on the quality scale.

Regrading the micro-structure of steel it can be said, the smaller the structure of the grain, the higher the toughness of the steel.

To achieve this, the type and amount of alloy and the distribution pattern of iron carbide in the steel are crucial.

Bainite is formed from Austenite at temperatures below the 'nose' of the Time-Temperature-Transformation Diagram (TTT). It has feathery or acicular micro-structure. Bainite receives its centre of growth from ferrite, a process that is accompanied by formation and dispersion of iron carbide in a matrix of Ferrite. With lower transformation temperatures the distribution of iron carbide is finer and harder than pearlite.

Transformation of Austenite to Martensite at temperatures lower than Bainite temperatures occurs as function of temperature only. Martensite exhibits lath or plate-like micro-structure. Carbon supersaturation and lattice deformation from shearing contribute to the thigh hardness of Martensite.

The impact toughness of Bainitic Steels originates from Granular Bainite which was confused with Granular Structure until 1991 when Professor Fang and his team discovered that Granular Bainite and Granular Structure although made of the same ingredients (ferrite matrix plus islands) are in fact two (2) different structures because of their different formation.

Granular Bainite is distinguished by directional distribution of islands forcing crack propagation into single channels. With this structure, cracks must detour the islands to continue propagation requiring increasing propagation energy, thus resulting in enhanced impact toughness of the steel.

Granular Structure has no directional distribution of islands, and its crack propagation is multi-channel which reduces propagation energy requirements and hence toughness of the steel.

To optimise steel quality, the process of manufacture must be able to reduce the size of the islands in the steel structure and at the same time provide for a sufficient quantity of islands. The process must also pay attention to reduced carbon content and low rolling temperatures and the process shall be able to provide for a suitable tempering treatment if required.

Bainitec Steels manufactured by Beijing Bainite Steel Corporation (BBSC) take care of these stipulations and in so doing, have moved to the forefront of the market in low cost high quality steels.

Bainitic Steels represent the optimum combination of hardness and toughness in steel and as such have a high potential to improve the financial results of the user's business provided they are employed in accordance with the recommendations of the manufacturer.

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