High-strength iron alloys

Abstract

The yield stress of a single crystal of high-purity iron is less than 4000 Lb. /in. 2 (1·7 tons/in. 2 ) (Allen 1959). The theoretical strength of iron is greater than 1200 000 Lb. /in. 2 (540 tons/in. 2 ) and strengths of this magnitude have been obtained in iron whiskers (Brenner 1958). Pure polycrystalline iron is not much stronger than the single crystal but it can be strengthened to a remarkable degree by a variety of mechanisms. The yield stress σ y increases with decreasing grain size (Petch 1953) σ y ═ σ i + k y d –½ . The proportionality constant k y increases with the extent of Cottrell segregation of carbon and nitrogen atoms to dislocations, but saturation is quickly achieved at the temperatures to which steel is subjected during processing and generally k y has the maximum value and is a constant independent of composition and temperature. By changing the ferrite grain size of annealed mild steel from the coarsest to the finest obtainable industrially the yield stress is increased three times (Cracknell & Petch 1955). The strength of quenched steel also increases with decreasing size of the crystal of the transformation product (martensite), but it is not clear which microstructural dimension should be identified with d . The lattice frictional stress σ i , which increases rapidly with decreasing test temperature, is greater than the Peierls stress by increments due to dislocation interaction, solid-solution and precipitation (dispersion) hardening. Strain hardening can produce very high strength. Steel wires cold-drawn to small diameter can develop strengths greater than 600 000 Lb. /in. 2 (268 tons/in. 2 ). The contribution of strain hardening to the yield strength of annealed steel is negligible, but the effect may be of some importance in quenched steels in which the ferrite grains have a high dislocation density. At a constant testing temperature, the rate of increase of σ i with increasing solute concentration is much greater for interstitial solute atoms producing a non-symmetrical distortion of the ferrite lattice than for substitutional solutes (Fleischer & Hibbard 1963); ∆σ i is between 6000 and 7500 Lb. /in. 2 (2·7 and 3·5 tons/in. 2 ) per 0·01% carbon or nitrogen (Wert 1950; Stephenson 1962) compared with about 45 Lb. /in. 2 per 0·01% nickel (Lacy & Gensamer 1944; Rees, Hopkins & Tipler 1954). The lattice frictional stress can be increased also by precipitation hardening, but until recently the only precipitation treatment commonly applied to steels, tempering, has been used as an overageing treatment to decrease the strength and increase the ductility of quenched steels.