A numerical study of quenching (in oil and water) of steel cylinders is carried out to determine the effect of composition changes on the radial profile of the residual stresses. The numerical model takes into account only the martensitic transformation. As a first approach, the composition change has been limited to the carbon content and this has been considered to influence only the M, temperature, the martensitic transformation strain, the yield stress and the strain hardening coefficient (these last two, just for the martensite phase). The composition oscillations representative of micro-segregation bands have been simulated by a sinusoidal fluctuation between 0.35 and 0.45 %C, the average value being 0.4 %C. The sinusoidal composition oscillations have led to very significant fluctuations of the axial residual stress around the calculated value for the constant carbon content (equal to 0.4 %C). The present study clearly shows that the analysis of residual stress measurements in comparison with calculated results must not neglect the disturbing influence of localised composition changes, even when these seem to be not relevant.