After the cold-worked seamless steel pipe is cold-worked, the strength and hardness are increased, and the plasticity is decreased. This phenomenon is called cold work hardening, referred to as chilling phenomenon. This change in the performance of thick-walled stainless steel pipes is caused by the plastic deformation of thick-walled stainless steel pipe materials during processing. The greater the deformation, the more severe the chill phenomenon.
The strength and plasticity of mild steel vary with increasing deformation. In the cutting process of thick-walled stainless steel tubes, the thick-walled stainless steel tube is subjected to large plastic deformation under the action of the cutter, so the chill phenomenon is also serious. It is the microhardness distribution of 304 stainless steel thick-walled tube and stainless steel lCr18N19Ti after cutting.
According to the hardness distribution after processing, such a rule can be found that the hardness of the chip is greatly improved compared with the original hardness of the cut thick-walled stainless steel pipe, which occurs because the thick-walled stainless steel pipe of the cutting layer becomes a chip. Great plastic deformation. The degree of hardening of the chips directly affects the continuity and breakage of the chips.
The greater the plastic deformation, the higher the degree of hardening, and the worse the ability of the chips to continue to deform, the easier it is to break. In addition to the basic deformation, the thick-walled stainless steel pipe of the chip bottom layer also produces frictional deformation. Therefore, the chip bottom layer is harder than other parts of the chip. This is clearly reflected in the microhardness distribution of the 304 stainless steel thick-walled tube.
On the microhardness distribution diagram of stainless steel lCr18N19Ti, the hardness of the bottom layer of the chip is not as high as that of the other parts of the 304 stainless steel thick-walled tube. This is due to the low thermal conductivity of the material and the friction between the chip and the rake face of the tool. The heat is concentrated on the underside of the chip, which causes the underside of the chip to soften to some extent.
The machined surface will also undergo plastic deformation during the cutting process. Because the basic deformation zone sometimes penetrates below the machined surface; the blade front deformation zone includes a certain depth below the machined surface; before the machined surface is formed, plastic deformation will occur again due to the friction of the flank face. Therefore, chilling occurs in a certain depth range below the machined surface.
