In the hot-rolled conditions (Fig.l), the microstructure of the steels consisted of elongated coarse ferritic grains (~220fLm long x 40fLm thick), with elongated thin martensitic grains and grain-boundary carbides at the ferrite grain boundaries. Tempering at 750°C produced no change in the ferrite and resulted in the martensite tempering to a fine dispersion of carbides (Fig.2). Normalizing at 950°C caused the martensitic areas to transform to Y which on cooling retransformed to give martensite islands which have a more equiaxed appearance (Fig.3). The volume fraction of the second phase was greater than for the hot-rolled condition but the difference was small (~3 %), suggesting that the major effect of this heat treatment was to change the shape of the martensitic areas. The ferrite/ martensite grain boundaries can now be seen to be broken up (see Figs. 1-3), the boundary having changed from planar to undulating. Tensile results (Table 2) showed th~t whereas the 0·2% proof stress (the steels are·'non-straln The ferritic stainless steel examined had the composition given in Table 1. The steel had been hot rolled to 25mm plate and was examined in three conditions: (i) hot rolled (ii) hot rolled and tempered at 750°C. for 1h and air cooled (iii) hot rolled and normalized for 1h at 950°C. Longitudinal Charpy V-notch impact transition curves were obtained for the three conditions (testpieces notched in the through-thickness direction). Longitudinal tensile testpieces 4·4mm dia. with a 16mm gauge length were also obtained and tested on an Instron machine at a crosshead speed of 0·5mmmin -1. Microhardness measurements were taken within the ferrite grains using a 50g load, and the grain elongation of the ferritic grains in the rolling direction was measured by the method of mean linear intercept. The fracture appearance was examined and photographs taken of the fissures. Impact testpieces of controlled-rolled steels notched in the through-thickness direction often reveal fissures on the fracture surfaces after breaking, the fissures forming parallel to the plate surfaces. There appear, therefore, to be 'planes of weakness' in these steels parallel to the rolling plane which fracture ahead of the notch to form fissures, which by relieving the triaxial stress system associated with a notch reduce the possibility of brittle fracture. 1 A considerable amount of work2-6 has been carried out in recent years into the origin of these splits or' fissures and it is noticeable that the controlled-rolled ferrite-pearlite steels which give rise to fissures generally have elongated ferrite grains and a preferred orientation as a result of deformation in the ferrite temperature range. Both these factors, indeed, have been quoted as playing a part in controlling fissure formation since normalizing generally prevents their formation.2,6 However, work by Mintz et a1.7 has shown that grain shape rather than texture is the important factor in controlling fissure formation and this has been substantiated further by the recent metallographic observations of Morrison et al.2 In the latter work, the fissures were observed to nucleate at seCond-phase particles, generally non-metallic inclusions, in both controlled-rolled and normalized steels. However, whereas in normalized steels the holes associated with second-phase particles became blunted, those in the controlled-rolled steel were able to initiate cracks which propagated in a brittle manner giving the characteristic appearance of fissures. The growth of a crack large enough to propagate in a brittle manner appeared to take place by intergranular decohesion. Although inclusions are favoured initiation sites, fissure formation has been shown not to be critically dependent on the cleanliness of the steel, sufficient inclusions always being present to initiate a crack even in the cleanest of steels.5,8 The critical event appears to be the ability to decohese a grain boundary for a sufficiently long enough length to promote brittle fracture. It would therefore be expected that the longer and more closely aligned are the ferrite grain boundaries, the easier it would be to decohese along them. It is not surprising, therefore, that the tendency to fissure formation increases with increase in grain elongation. 2,3, 7 For example, normalized steels which have equiaxed grains do not give rise to fissures and it is only after the grains have been elongated to a critical amount that fissures are first observed. 6, 7 Increasing the dislocation density by warm w,orking has also been shown to favour their formation 7 and, because of this, it has be((n suggested that raising the strength of the grain matrix concentrates the strain on to the grain boundaries thereby encouraging decohesion. However, the reason for the apparent weakness of the grain-boundary regions after warm working is by no means clear and further studies are required. During a recent study into the impact behaviour of Fe-13Cr ferritic steels, some interesting observations were made with regard to the manner in which grain boundaries