Difference in the effect of cold working and tempering on the microstructure of Mod.9Cr-1Mo steel was investigated by means of X-ray diffraction (XRD), electron channeling contrast image (ECCI) and electron backscatter diffraction (EBSD) technique. For simulating the cold work, tensile tests were conducted at room temperature and interrupted for preparing some specimens with 0-20% strain. Tempering was conducted at 700-760°C for 0.25-6 h in order to reach the same hardness level as interrupted tensile specimens.

X-ray diffraction was carried out to measure the dislocation density. ECCI and EBSD technique were applied to measuring lath width, the spacing of precipitates and the spacing of high angle boundaries, separately.

In the specimens subjected to pre-strain and the specimens subjected to tempering, lath width, precipitates and dislocation spacing were different in spite of the same hardness. It was found that the dislocation substructure for the specimens subjected to pre-strain is completely different from those of the specimens subjected to tempering.