Ausforming Research Articles

A heterostructured bainitic steel produced by two-step austempering and low-temperature ausforming

Heterogeneous microstructure has been become a very effective scheme for evading the yield strength (YS)-ductility trade-off in metallic materials, however, strategies of fabricating heterostructured bainite (HeteroB) in carbide-free bainite have not yet been explored. Herein, we successfully produced a HeteroB composed of submicron-scale primary bainite and nano-scale low-temperature bainite by combining two-step austempering (AT) and low-temperature ausforming (AF). Compared with the homogeneous bainite (HomoB) in the conventional AT sample, the HeteroB in the AT&AF sample exhibits a wider lath size distribution due to its bimodal size variation. The difference in average nano-hardness between the primary bainite and nanobainite reaches 0.78 GPa. The HeteroB renders an impressive enhancement in strength (∼310 MPa of YS) and ductility (∼26.1 % of uniform elongation) in the AT&AF sample compared to the conventional AT sample. This synergistic improvement in YS and ductility relies on the sustained transformation-induced plasticity and heterogeneous deformation-induced strengthening effect.

Ultra high strength in steel.

Very high strength levels in steel, not obtained by conventional methods of hardening by quenching and low temperature tempering, are obtained by mew hardening techniques involving the thermal-mechanical treatment of metastable austenite.The earlier attempts on such hardening techniques and the development of new processes such as 'Ausforming' investigated in the Ford Motor Company Research Laboratories and ''Maraging'' developed and patented by the International Nickel Company (Mond)Limited, are briefly reviewed here.

Influence of Ausforming Treatment of 0.4wt%C Steel Modified With Nb On The Microstructure And Hardness Properties

In the present work the relationship between the microstructure and the hardness of the Steel both with and without an addition of niobium was studied. Investigation by direct observation using optical microscopy was carried out. The purpose of this work was to analyze the effects of the low thermomechanical treatment of ausforming on the final microstructure and hardness of steel and of the same steel modified with niobium. It was found that the deformation during low thermomechamical treatment of ausforming introduces changes in the microstructure, such as carbide precipitation, which affect the hardness of the steel. The results show that the mechanical properties of the ausformed Steel depend on the deformation temperature, amount of deformation. The addition of niobium lead to change in the hardness of the Steels investigated.

Ausforming of NiTi

Lattice defects are introduced into stable austenite (β) by hot rolling up to 70% (in one pass) in a temperature range between 900 > TAF > 340 °C (ausforming (AF)). The changes in microstructure, thermal transformation behavior, as well as normal and anomalous properties in stress-, strain-, and temperature-space were studied. Martensitic transformation temperatures are lowered with decreasing deformation temperature and increasing amount of deformation. A two-stage reaction is induced at TAF ≤ 400 °C. Deformation twins are the predominant microstructural feature, in addition to dislocations at TAF ≤ 700 °C. Highly elongated grains indicate that long-range recrystallization is absent at all ausforming temperatures. Semicoherent pecipitation in addition to high dislocation densities are likely to be responsible for the premartensitic anomalies of thermal transformation for TAF ≤ 400 °C. Ausforming leads to a considerable increase in conventional strength properties (yield stress, tensile strength, elongation) without loss of transformability and, consequently, shape memory or pseudo-elasticity.

18Ni-Co-Mo鋼のオースフォーム

18Ni-Co-Mo鋼のオースフォーム