Analysis of cracking characteristics and mechanism of high-strength steel after chemical oxidation

Abstract

The tooth surface of the 20CrNi2Mo worm underwent cracking after chemical oxidation. The appearance inspection results showed that there were multiple cracks on the tooth surface of the worm slice, and the direction of the cracks was perpendicular to the machining pattern. There were many microcracks on both sides of the main crack. The profile morphology shows that the crack depth reaches 1100 μm. The crack extends perpendicular to the tooth surface to a certain depth and then propagates in two transverse directions. After opening the crack on the tooth surface, observation shows that the overall section is black, and the artificially opened area is silver-white. At the interface between the artificially opened area and the initial fracture surface, the fracture surface changes from intergranular morphology to ductile dimple morphology. The energy spectrum shows that the oxygen content in the intergranular cracking area is much higher than that in the ductile dimple area, indicating that the worm has undergone intergranular corrosion cracking. After the worm tooth surface is ground, there will be certain tensile stress on the surface. During the chemical oxidation process, the worm will be exposed to many corrosive media such as OH−, and NO2 −. Under the combined action of tensile stress and corrosive ions, the tooth surface will undergo stress corrosion cracking. To effectively avoid cracking on the worm gear surface, three measures have been proposed to reduce the risk of cracking: (1) improving machining parameters to reduce grinding feed, (2) increasing destressing before chemical oxidation to reduce residual tensile stress, (3) improving chemical oxidation process to reduce solution concentration, treatment time, and treatment temperature.