Causes and Temperature Requirements of Low Temperature Embrittlement of Cold Rolled Bright Tubes

The cold brittleness (or low temperature brittleness tendency) of cold rolled bright tubes in use is expressed by the ductile brittle transition temperature Tc. The Tc of high purity iron (0.01% C) is at - 100C, below which it is completely brittle. Most alloy elements in the cold rolled precision bright steel pipe increase the ductile brittle transition temperature of the cold rolled precision bright steel pipe, and increase the cold brittle tendency. When ductile fracture occurs above room temperature, the fracture of cold-rolled precision bright steel pipe is dimple fracture, and when brittle fracture occurs at low temperature, it is cleavage fracture.

The reasons for low temperature embrittlement of cold rolled bright tubes are:

(1) When the dislocation generated by the dislocation source during deformation is blocked by obstacles (such as grain boundary and second phase), the local stress exceeds the theoretical strength of the cold-rolled precision bright steel pipe, resulting in microcracks.

(2) Several stacked dislocations form a microcrack at the grain boundary.

(3) The intersection of two slip bands reacts, causing fixed dislocation% 26lt; 010% 26gt, it is wedge-shaped microcrack, which can split along the cleavage plane.

The factors of cold brittleness of cold rolled bright tubes include:

(1) Solution strengthening elements. The ductile brittle transition temperature is stronger with the increase of phosphorus; Molybdenum, titanium and vanadium; The elements that have little effect when the content is low but increase the ductile brittle transition temperature when the content is high are silicon, chromium and copper. The elements that decrease the ductile brittle transition temperature are nickel. The elements that decrease the ductile brittle transition temperature first and then increase the ductile brittle transition temperature are manganese.

(2) Elements forming the second phase. Carbon is the more important element to increase the cold brittleness of cold rolled precision bright steel pipes by the second phase. With the increase of carbon content in cold rolled precision bright steel pipes, the pearlite content in cold rolled precision bright steel pipes increases. The ductile brittle transition temperature increases by 2.2 ¡æ on average for every 1% increase in pearlite volume.

(3) It refers to the effect of carbon content on brittleness in ferritic pearlitic steel. The addition of microalloying elements such as titanium, niobium and vanadium forms dispersed nitrides or carbonitrides, which causes the ductile brittle transition temperature of cold rolled precision bright steel tubes to rise.

(4) The grain size affects the ductile brittle transition temperature, which increases with grain coarsening. Grain refinement reduces the cold brittleness tendency of cold rolled precision bright steel tubes, which is a widely used method.