High alloy steel precision small diameter seamless steel pipe almost does not use direct quenching process; When the quantity of retained austenite is strictly required, the low alloy steel will not be directly quenched. A typical problem is the appearance of ripples on the surface of a precision small diameter seamless steel pipe (such as spiral or hypoid gears) under the working load.
In order to reduce the amount of retained austenite and deformation during direct quenching, pre cooling can be used to approach Ar1 temperature after carburizing and then quenching. The precision small diameter seamless steel pipe can also be quenched first when it is transferred to a furnace slightly higher than Ar1 temperature. After carburizing and quenching, the precision small caliber seamless steel pipe can also be heated to a temperature above the surface Ac1, further refining the grain and reducing the amount of residual austenite.
The precision small diameter cold rolled seamless steel pipe is directly quenched to obtain the highest strength and hardness, but the toughness is not high. The hardness of the carburized layer can be measured with a file. The hardness can be reduced due to the presence of residual austenite in the precision small seamless pipe of high alloy steel. This method can also reduce the cementite on the grain boundary. When machining the carburized surface of precision small caliber seamless steel pipe is required, slow cooling or annealing shall be adopted.
If high hardenability steel is selected or affected by equipment, even if the cooling is slow, the surface of small diameter precision bright thick wall pipe still has a high hardness. In this case, softening treatment should be adopted. To avoid network carbide precipitation, medium cooling rate must be adopted. Take some precautions to prevent decarburization during slow cooling. Water quenched steel shall be used subsequently. After carburizing, slow cooling or oil quenching method can be used to refine the grains in the center of precision small seamless steel pipe without serious deformation or cracking.
In the heat treatment process of multiple heating quenching, carburizing and cooling followed by reheating quenching, surface quenching can be realized by surface quenching, and these methods can also be used. For precision small seamless steel pipes with high alloy content, it is better to adopt the process method closest to direct quenching, that is, reheat to 15~25 ℃ (25 ℃~50 ℃) above the central transformation point temperature (of course, beyond the surface transformation range).
The heat treatment process maximizes the strength of the core of the small diameter high-precision seamless pipe and has good toughness. Almost all the remaining carbides on the surface are dissolved. Although the residual austenite content of such process precision small diameter cold rolled seamless steel pipe is less than that of direct quenching, it will be partially retained. The deformation is larger than that of direct quenching, but within the acceptable range. As with direct quenching, this method is hardly used for ordinary carbon steel.
After slow cooling or quenching, the liquid steel can be reheated to a phase change temperature slightly higher than that of the carburized layer. For example, the small diameter alloy precision seamless pipe can be reheated after carburizing and slow cooling, because it can not refine grains and quenching, so that its hardness is low, deformation is small, and toughness is moderate; The surface layer has undissolved carbides, so it has high hardness and brittleness; For example, the precision small seamless steel pipe is reheated after carburizing and quenching, and the grain is fully refined. The center has high toughness, low hardness, and the surface has no network cementite, so the hardness is high and the toughness is good, but the disadvantage is that the deformation is large.
Post time: Dec-06-2022