Abstract: The following piston material of some models is GCr15 steel, the thickest part of the part next to 20mm.The soft point appears in the hardness test after vacuum oil quenching. Therefore, the original tissue, the surface state during heating, the cooling capacity of the quenching medium and the operation process may lead to the soft points after the quenching are analyzed one by one. The main reasons for the soft points after the quenching are inappropriate methods, such as improper furnace loading method, and insufficient oil surface pressure during vacuum quenching, etc. After the improved process, the surface pressure of the blind hole confronts the furnace and the quenching oil is adjusted to above 1kPa, and the surface hardness of the piston is distributed uniform after requenching.