Microstructure Control and Property Optimization

of High-strength Weldable Steels Strengthened by

Nanoparticles for Construction Applications

B.C. Zhou, M.C. Niu, Z.B. Jiao

Department of Mechanical Engineering

Steel is essential for modern society

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Steel production brings pollution problems

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“Green steels”

How to make steels strong?

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Strengthening mechanisms

Lu, Ke; et al. science, 2009, 324.5925: 349-352.

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❖ High strength: nanoprecipitates with sizes of 1-5 nm are effective in

strengthening steels

❖ High ductility/toughness: strengthening without a significant reduction

of ductility and toughness

❖ Low material cost: less consumption of alloying elements

❖ Good weldability: low carbon content facilitates good weldability

❖ Easy to process: solutioning and aging treatment

Precipitation strengthening has been recognized as the most

effective method to strengthen steels.

Benefits of nanoprecipitation strengthening

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Precipitation strengthening model

8Wiki: strengthening mechanisms of materials

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Thermodynamic calculation-aided design

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Precipitation hardening kinetics

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Nanoscale Ni3Ti precipitate

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Nanoscale ω and Laves precipitates

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Nanoscale ω and Laves precipitates

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Nanoscale CrFe precipitates

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Mechanical properties

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Strengthening mechanisms

Conclusions

➢ Through the partially replacement of expensive Ni and Co with

inexpensive Cr while optimizing the Ti and Mo concentrations, a new

class of ultrahigh strength steels was developed, which achieves

comparable mechanical properties as conventional maraging steels

but with a substantially reduced cost.

➢ The newly developed steels exhibit a yield strength of 1.8 GPa and a

ductility of more than 10%. Strength modeling indicates that the

strength increment originates mainly from the precipitation

strengthening of Ni3Ti, Mo-enriched, and Cr-rich particles.