Abstract: During quenching, the length direction of 20 steel I-shape gauge shrinks strongly.The results show that the main reason for shrinkage is that during high temperature cooling, the high carbon martensite on the surface is compressed to the center, and the ferrite with good plasticity in the center is extruded, the length is shortened, showing obvious thermal stress characteristics.After reducing the quenching temperature and using the nitrate with small deformation as the quenching medium, the process is optimized.After the optimized process, the I-gauge hardness is 61 HRC, the shrinkage is 0.24 mm and the flexural deformation is 0.05 mm.The metallographic structure of the carburized layer on the surface is cryptoacicular martensite and the center is lath martensite, which can meet the technical requirements.