The present work characterized the 9 pct Ni steel heat-affected zone (HAZ). HAZ thermal cycles (peak temperatures— T p —of 500 °C, 600 °C, 650 °C, 700 °C, 750 °C, 850 °C, 1100 °C, and 1370 °C) were numerically (FEM model) and physically (Gleeble machine) simulated. The simulated HAZ was evaluated through optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), dilatometry, and Vickers hardness. T p  ≤ 650 °C had microstructure similar to base metal; T p  of 700 °C yielded the partial transformation ( A c1  <  T p  <  A c3 ) and presence of ferrite and martensite; T p  > 750 °C yielded martensite (majority) with maximum hardness (344 HV10) for T p  = 850 °C. The amount of austenite was maximum (3.1 pct) to T p  = 700 °C and decreased to T p ≥ 750 °C, becoming approximately zero for T p  ≥ 1100 °C. Besides, the austenite quantification methodology (electrolytic polished and 4 pct Picral etching + SEM image analysis) was validated through XRD measurement and used to analyze the roundness and number of austenite particle size of the BM and HAZ subzones. Its roundness and amount remained are close to the BM for T p ≤ 650 °C, increase in the T p = 700 °C, and decrease for T p ≥ 750 °C.