Analysis of cracking on the surface of mechanical blades

Mechanical paper cutting knives are made of bimetallic pressing. For example, the body of the blade is Q235, and the cutting edge is CrWMn steel. The process is: rolling round steel into steel strips - planing the side - rolling and pressing with Q235 steel at 1000℃ - straightening - quenching - 180℃ nitrate tempering - straightening and grinding. After grinding, many fine cracks were found on the blade surface.

After color flaw detection, it can be seen that the cracks on the CrWMn steel surface are slender, perpendicular to the grinding marks, and tend to be mesh-shaped. The crack surface is open, and there is no crack on the surface of the Q235 steel base of the blade.

When the sample is cut horizontally at the crack, it can be seen that the crack develops vertically from the surface, then forks, and the deep crack is tortuous. There are also several discontinuous fine cracks on both sides of the tail end, which develop along the grain. After the sample was etched with 4% nitric acid alcohol solution, an uneven white bright layer (secondary quenching layer) can be seen on the top layer; below it is a dark tempering area; the light-colored area in the middle is an insufficient tempering area. The matrix structure is martensite, residual austenite and granular carbide, and the martensite is relatively coarse (about level 5). After further deep etching, it is obvious that the carbides are distributed in a network or intermittent network.

From the above analysis, it can be seen that the heat treatment process of the CrWMn steel of the blade is improperly controlled, resulting in a coarse structure and a network of carbides, and the abnormal grinding process is the main reason for the cracking of this blade.