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The methods of measuring the fractal dimension of a Sierpinski carpet are summarized. The Sierpinski fractal dimension of a metallograph, consisting of regions of martensite and retained austenite in a Fe-29 wt% Ni-0.16 wt% C alloy by cooling to - 125[ddot]C, is determined and its physical significance illustrated.

acad sinica,inst met res,fatigue & fracture mat lab,shenyang 110015,peoples r china.;long, qy (reprint author), acad sinica,int ctr mat phys,72 wenhua rd,shenyang 110015,peoples r china

ACAD SINICA,INT CTR MAT PHYS,SHENYANG 110015,PEOPLES R CHINA.;ZHU, ZM (reprint author), ACAD SINICA,INST MET RES,STATE KEY LAB FATIGUE & FRACTURE MAT,72 WENHUA RD,SHENYANG 110015,PEOPLES R CHINA

ACAD SINICA,INT CTR MAT PHYS,SHENYANG 110015,PEOPLES R CHINA.;LONG, QY (reprint author), ACAD SINICA,INST MET RES,STATE KEY LAB FATIGUE & FRACTURE MAT,72 WENHUA RD,SHENYANG 110015,PEOPLES R CHINA

ACAD SINICA,INST MET RES,SHENYANG 110015,PEOPLES R CHINA.;FU, R (reprint author), ACAD SINICA,INT CTR MAT PHYS,WENHUA RD 72,SHENYANG 110015,PEOPLES R CHINA

The effects of processing parameters on austenite retention in intercritically annealed HSLA steels have been investigated. Low-temperature tempering was found to be necessary for successful tint etch of high-carbon martensite in dual-phase structure with sodium metabisulfite. Cold working before the intercritical anneal increases the nucleation sites for transformation to austenite and, thereby, the amount of retained austenite is generally increased. The microstructural evolution during postanneal cooling is detrimental to austenite retention. Both the particle size effect and the chemical stabilization by carbon are responsible for the suppression of martensitic transformation.

The martensitic transformation of retained austenite particles in an intercritically annealed high-strength low-alloy steel has been studied using the acoustic emission technique. It was found that the stability of austenite particles was mainly dependent on the particle size and to a lesser extent on the enrichment of austenite by carbon and manganese. We have demonstrated that a substantial fraction of the retained austenite particles in the as-annealed condition already contains martensite embryos which can initiate martensitic transformation during cooling below 300 K. The martensite embryos are believed to form from a suitable group of dislocations. Those austenite particles which do not contain such dislocations will not transform by supercooling alone. The particle size stabilization effect is viewed as a decreasing probability of finding dislocations in a particle with decreasing particle volume. Plastic deformation can bring about the transformation by introducing dislocations in the austenite particles. The transformation of the retained austenite particles is shown to enhance the ductility of the HSLA steel.