Abstract: A method of making a composite tubular product which comprises inner and outer tubes of different chemical compositions and in which the stresses produced at the interface between the outer and inner tubes during cooling from a diffusion bonding temperature are such that the interface between the two tubes remains intact during such cooling. The method comprises the step of inserting into the interior of the inner tube a core of a material whose transformation and/or contraction characteristics are sufficient to counteract at least to a substantial amount interfacial stresses which arise during cooling because of differences in the transformation and contraction properties of different materials from which the inner and outer tubes of the composite product are formed.