Details of precision steel pipes

First, what is a precision steel pipe?
Precision steel pipe is a kind of high-precision steel pipe material after fine drawing or cold rolling of ordinary seamless steel pipe (or de-diameter welded steel pipe). Because the inner and outer walls of precision steel pipes have no oxide layer, no leakage under high pressure, 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 steel pipes is 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 high-precision and high-finish. After heat treatment, the steel pipe has no oxide layer, the inner wall is clean, the steel pipe can withstand high pressure, no deformation after cold bending, and no cracks after expansion and flattening. Steel pipe color: white with bright color, with high metallic luster.

Second, the 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 steel pipes 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 to precision steel pipes to save labor, materials, and time.

The effects of elements in precision steel pipes 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 steel pipes.
(2) Alloy elements that promote or slow down high-temperature temper brittleness in different forms and degrees. Chromium, manganese, nickel, silicon, etc. play a promoting role, while molybdenum, tungsten, titanium, etc. play a delaying role. Carbon also plays a promoting role.
Generally, carbon precision steel pipes are not very effective in high-temperature temper brittleness. Sensitive, 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 alloying elements.

The sensitivity of the original structure of tempered precision steel pipes to the high-temperature temper brittleness of steel varies significantly. The martensite high-temperature tempering structure is 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 the high-temperature tempering brittleness of precision steel pipes 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-mentioned 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 tempering 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 slowing down high-temperature temper brittleness.

Measures to reduce the high-temperature temper brittleness of precision steel pipes are:
(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 steel pipes. When the molybdenum content in the steel increases to 0.7%, the tendency of high-temperature temper brittleness is greatly reduced. When this limit is exceeded, special molybdenum-rich carbides are formed in 20# precision steel pipes, the molybdenum content in the matrix decreases, and the brittleness tendency of precision steel pipes increases instead;
(3) Reduce the content of impurity elements in 20# precision steel pipes;
(4) For parts that work in the high-temperature temper brittleness zone for a long time, it is difficult to prevent brittleness by adding molybdenum alone. Only by reducing the content of impurity elements in 20# precision steel pipes, improving the purity of precision steel pipes, and supplementing by composite alloying of aluminum and rare earth elements can high-temperature temper brittleness be effectively prevented.