The stress corrosion cracking (SCC) behaviour of low alloy steel AISI 4140 was investigated in 33% sodium hydroxide solution at 80 degreesC under freely corroding conditions. Tempering temperatures of 450, 500, 550, 600 and 650 degreesC were used to produce a range of micro structures and strength levels. The relation between the crack growth rate and the stress intensity factor was obtained by constant displacement rate tests conducted on pre-cracked compact tension specimens. The crack growth behaviour and fracture mode were found to be sensitive to the presence of inclusions and to the microstructure of the steel. In case of insufficient tempering, intergranular cracking was observed in the band zone adjacent to the sulfide inclusions. This was attributed to hydrogen embrittlement of martensitic band structure. Disappearance of intergranular cracking in specimens tempered above 550 degreesC was explained by the difference of hydrogen diffusivity in various microstructures of quenched and tempered steels. The results were also interpreted with a model developed for low alloy steels in which the relation between the grain size and the plastic zone size was emphasized. (C) 2001 Elsevier Science B.V. All rights reserved.