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Stress relaxation tests were carried out over the temperature range 800-1000°C to study the precipitation of Nb(C,N) in a family of high strength low alloy steels. The effects of addition of boron ('" 0·003 wt-%), amount of deformation (0, 5, and 25%), and holding time on the kinetics, nature, and size distribution of the strain induced Nb( C,N) precipitates were investigated. Specimens were quenched at various stages of the stress relaxation process and then examined using transmission electron microscopy and electron energy loss spectroscopy (EELS). The evolution of the size distribution indicated that the presence of boron accelerates the kinetics of precipitation and leads to a finer and more dense distribution. The presence of boron in the large undissolved TiN particles was not detected using EELS; by contrast, a boron peak was observed in the strain induced Nb (C,N) precipitates. The latter were of cuboid shape and were heterogeneously distributed. The particle size measurements are consistent with a parabolic law for the growth of the precipitates. The diffusion coefficient of niobium, in the presence and absence of boron, was estimated in this manner and it is concluded that boron does not significantly affect the growth and coarsening of the precipitates. Instead, boron intervenes in the nucleation stage and increases the rate of nucleation.MST/1279 List of symbolsd diameter of precipitates a mean diameter of precipitates D diffusion coefficient DNb diffusion coefficient of niobium DNb+B diffusion coefficient of niobium in presence of boron E Young's modulus Np particle density Ps precipitation start time Pf precipitation finish time R gas constant t relaxation time T absolute temperature X~b concentration of niobium dissolved in austenite XNb concentration of niobium in precipitates X&b equilibrium concentration of niobium in remaining matrix (:J. relaxation constant f3 relaxation constant E prestrain e strain rate 11 precipitate growth coefficient (J stress (J 0 initial stress