Mechanical properties of nitrogen-ferrite
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I.
INTRODUCTION
THE mechanism of precipitation hardening by quenching and tempering, strain-aging, and interphase precipitation is extensively used to improve the mechanical properties of steels. The increased strength is associated with fine-scale precipitation of carbides, nitrides, or carbonitrides, but the exact nature of the strengthening mechanism and the initial stages of precipitation are not well understood due to the problems of working in a complex matrix. Problems of interpretation have been avoided recently by working with controlled stable microstructures obtained using constantactivity nitriding of simple Fe-X alloys, where X represents a substitutional alloying element. 1 The strength increases (-500 MPa) obtained in these annealed and nitrided Fe-X alloys which contain mixed substitutional atom-nitrogen clusters result from the high stresses required for the dislocation shearing of substitutional atom-nitrogen bonds and the creation of additional interfaces. Henderson 2 has shown that when the alloying addition is increased to two substitutional elements (molybdenum and titanium), the combined strengthening effect of the two solutes together with nitrogen is greater than that expected from the behavior of the respective single substitutional solutes. In a more recent development, 3 a technologically important class of HSLA steel has been produced by the nitriding of a cold-worked Fe-0.18 wt pct Ti alloy. A high strength of about 1100 MPa together with a good ductility makes this alloy attractive for structural applications. It has been suggested3 that one of the main uses of this formable and strengthenable alloy is automotive components where beneficial fuel economy is offered through weight reduction without the greatly increased cost associated with high alloying concentrations. The homogeneous structural transformations that occur on aging low-alloy ferritic steels containing mixed substitutional-nitrogen solute-atom clusters are very similar to those observed in classical age-hardenable systems. 4 The formation of the disc-shaped zones on {0131} matrix planes is followed by the formation of one or more intermediate phases, at first coherent with the matrix and then becoming incoherent. Finally, the equilibrium precipitate, usually incoherent, forms only after aging for prolonged times at temperatures of about 700 ~ The transformation sequence, accompanied by a decrease in hardness and an increase in toughness, is P. FERGUSON, formerly with the Wolfson Research Group for High-Strength Materials, Crystallography Laboratory, The University, Newcastle upon Tyne, United Kingdom, is now Personal Assistant to the Chairman, Anzon Limited, Cookson House, Wallsend, Tyne & Wear NE28 6UQ, United Kingdom. Manuscript submitted December 19, 1983. METALLURGICALTRANSACTIONS A
similar to the succession of phase changes that occurs during the quench-aging of nitrogen-ferrite at very much lower temperatures, i.e., 20 to 250 ~ The maximum solubility of nitrogen in ferrite of about 0.1 wt pct can be produced by
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