Precision steel pipes: Precision bright pipes (precision steel pipes) are a kind of high-precision steel pipe material that is processed by fine drawing or cold rolling of ordinary seamless steel pipes (or de-diameter welded steel pipes). Because the inner and outer walls of precision bright pipes have no oxide layer, can withstand high pressure without leakage, high precision, high finish, no deformation after cold bending, no cracks after expansion and flattening, etc., they are mainly used to produce pneumatic or hydraulic components, such as cylinders or oil cylinders. They can be seamless pipes or welded pipes.
The chemical composition of precision bright pipes includes carbon C, silicon Si, manganese Mn, sulfur S, phosphorus P, and chromium Cr. High-quality carbon steel, fine rolling, non-oxidation bright heat treatment (NBK state), non-destructive testing, the inner wall of the steel pipe is brushed with special equipment and washed under high pressure, anti-rust oil is applied to the steel pipe for rust prevention, and the two ends are sealed for dust prevention. The inner and outer walls of the steel pipe are of high precision and high finish. After heat treatment, the steel pipe has no oxide layer and the inner wall is clean. The steel pipe can withstand high pressure, does not deform when cold bent, and has no cracks when expanded or flattened. Precision steel pipes can be subjected to various complex deformations and mechanical processing. Steel pipe color: white with bright color, with high metallic luster.
Main uses of precision steel pipes:
Automobiles, mechanical parts, and other machinery have high requirements for the precision and finish of steel pipes. Precision steel pipe users are not the only users with high requirements for precision and finish. Because precision bright tubes have high precision and the tolerance can be maintained at 2-8 wires, many mechanical processing users are slowly converting seamless steel pipes or round steel into precision bright tubes to save labor, materials, and time.
The effects of elements in precision bright tubes on high temperature temper brittleness are divided into:
(1) Impurity elements such as phosphorus, tin, antimony, etc. that cause high temperature temper brittleness of precision bright tubes.
(2) Alloy elements that promote or slow down high temperature temper brittleness in different forms and degrees. Chromium, manganese, nickel, silicon, and other elements play a promoting role, while molybdenum, tungsten, titanium, and other elements play a retarding role. Carbon also plays a promoting role.
Generally, carbon precision bright tubes are not sensitive to high-temperature temper brittleness. Binary or multi-element alloy steels containing chromium, manganese, nickel, and silicon are very sensitive, and their sensitivity varies depending on the type and content of alloy elements.
The original structure of tempered precision bright tubes has a significant difference in sensitivity to high-temperature temper brittleness of steel. The martensite high-temperature tempering structure is the most sensitive to high-temperature tempering brittleness, followed by the bainite high-temperature tempering structure, and the pearlite structure is the smallest.
The essence of high-temperature tempering brittleness of precision bright tubes is generally believed to be the result of impurity elements such as phosphorus, tin, antimony, and arsenic segregating at the original austenite grain boundaries, leading to grain boundary embrittlement. Alloy elements such as manganese, nickel, and chromium co-segregate with the above impurity elements at the grain boundaries, promoting the enrichment of impurity elements and aggravating embrittlement. Molybdenum, on the contrary, has a strong interaction with impurity elements such as phosphorus, which can produce precipitation phases in the crystal and hinder the grain boundary segregation of phosphorus, which can reduce high-temperature temper brittleness. Rare earth elements also have similar effects to molybdenum. Titanium more effectively promotes the precipitation of impurity elements such as phosphorus in the crystal, thereby weakening the grain boundary segregation of impurity elements and alleviating high-temperature temper brittleness.
Measures to reduce the high-temperature temper brittleness of precision bright tubes include:
(1) Use oil cooling or water rapid cooling after high-temperature tempering to inhibit the segregation of impurity elements at the grain boundary;
(2) Use molybdenum-containing precision bright tubes. When the molybdenum content in the steel increases to 0.7%, the tendency of high-temperature temper embrittlement is greatly reduced. When this limit is exceeded, special molybdenum-rich carbides are formed in the 20# precision steel tube, the molybdenum content in the matrix decreases, and the embrittlement tendency of the precision bright tube increases instead;
(3) Reduce the content of impurity elements in 20# precision steel tubes;
(4) For parts that work in the high-temperature temper embrittlement zone for a long time, it is difficult to prevent embrittlement by adding molybdenum alone. Only by reducing the content of impurity elements in 20# precision steel tubes, improving the purity of precision bright tubes, and supplemented by composite alloying of aluminum and rare earth elements, can high-temperature temper brittleness be effectively prevented.
The main delivery states of precision steel pipes: NBK (+N), GBK (+A), BK (+C), BKW (+LC), and BKS (+SR) After quenching the precision bright tube to obtain martensite structure, tempering at a temperature range of 450-600℃; or after tempering at 650℃, slowly cooling through 350-600℃; or after tempering at 650℃, heating for a long time at a temperature range of 350-650℃, all of which will cause the precision bright tube to become brittle. If the brittle 20# precision steel pipe is reheated to 650℃ and then quickly cooled, the toughness can be restored, so it is also called “reversible temper brittleness”. High-temperature temper brittleness is manifested as an increase in the toughness-brittleness transition temperature of the precision bright tube. High-temperature temper brittleness. Sensitivity is generally expressed by the difference (%Delta; T) between the toughness-brittleness transition temperature of the toughened state and the brittle state. The more severe the high-temperature temper brittleness is, the higher the proportion of intergranular fracture on the fracture of the precision bright tube is.
Post time: Oct-31-2024