Measures to reduce the brittleness of seamless pipes during high temperature tempering

Posted by chase Liu on February 17th, 2023

The essence of the high-temperature tempering brittleness of seamless pipes is generally believed to be the result of segregation of impurity elements such as phosphorus, tin, and antimony at the grain boundaries of prior austenite, resulting in grain boundary embrittlement. Alloying elements such as manganese, nickel, chromium and the above-mentioned impurity elements co-segregate at the grain boundary, which promotes the enrichment of impurity elements and aggravates embrittlement. On the contrary, molybdenum has a strong interaction with impurity elements such as phosphorus, which can cause precipitation in the crystal and hinder the grain boundary segregation of phosphorus. Rare elements that can reduce high temperature temper brittleness also have similar effects to molybdenum. Titanium can more effectively promote the precipitation of impurity elements such as phosphorus in the crystal, thereby weakening the grain boundary segregation of impurity elements and slowing down the high temperature temper brittleness.

Measures to reduce the high temperature temper brittleness of seamless pipes:

1. After high temperature tempering, use oil cooling or water cooling to suppress the segregation of impurity elements at the grain boundary;

2. Use molybdenum-containing seamless pipe types. When the molybdenum content in steel increases to 0.7%, the high-temperature tempering embrittlement tendency of seamless pipes decreases. Beyond this limit, molybdenum-rich special carbides are formed in the seamless pipe, the molybdenum content in the matrix decreases, and the embrittlement tendency of the seamless pipe increases instead;

3. Reduce the content of impurity elements in seamless pipes;

4. It is difficult to prevent embrittlement by adding molybdenum alone for components that have been working in the high-temperature tempering embrittlement zone for a long time. Only by reducing the content of impurity elements in the seamless pipe, improving the purity of the seamless pipe, and supplemented by the compound alloying of aluminum and rare earth elements, can the high temperature temper brittleness be effectively prevented.

Seamless pipe refers to a steel pipe that is perforated from a whole round steel and has no welds on the surface. According to different production methods, it is divided into hot-rolled pipe, cold-rolled pipe, cold-drawn pipe, extruded pipe, pipe jacking, etc., all of which have their own process regulations. Among them, due to the small width-to-thickness ratio of the steel plate, compared with the cold drawn seamless tubing, the dimensional accuracy of the hot-rolled seamless pipe is relatively low, and the plate shape problem is not easy to occur, and the convexity is mainly controlled. If there are requirements for the structure, it is generally realized by controlled rolling and controlled cooling, that is, controlling the starting temperature and finishing temperature of finishing rolling.