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Brittleness of precision tube

Precision tube is a kind of cold drawn or cold rolled seamless steel tube production process of high precision and high brightness by. The inside and outside diameter sizes can be accurate to 0.2mm, and the utility model can be widely used for the manufacture of precision mechanical parts and engineering structures when the bending and torsional strength are the same. Also used for production of various conventional weapons, barrel, shells, bearing etc..

According to the tempering temperature range of brittleness, it can be divided into low temperature temper brittleness and high temperature temper brittleness.

After tempering the martensite steel, the brittleness of the low temperature tempered brittle alloy steel is brittle at the temperature range of 250~400 DEG C. The brittle precision tube can not be eliminated by the method of low temperature tempering heating, so it is also called%26ldquo; irreversible temper brittleness%26rdquo;. It occurs mainly in alloy steel and low alloy ultra high strength steel precision tubes etc.. The fracture has brittle precision tube is intergranular fracture is intergranular or mixed fracture and quasi cleavage. The cause of low temperature temper brittleness:

1). It is closely related to the grain boundary embrittlement of cementite during the low temperature tempering.

2). One of the reasons for the low temperature temper brittleness is the segregation of impurity element phosphorus in the original austenite grain boundary. Less than 0.005% of the phosphorus containing high purity precision tube does not produce low temperature temper brittleness. The austenite grain boundary segregation occurs when phosphorus is heated in the fire, and it is retained after quenching. In the original austenite grain boundary segregation and cementite tempering, the phosphorus precipitates in the original austenite grain boundary, and these two factors cause brittle fracture along the crystal, which results in the occurrence of low temperature temper brittleness.

 

The effect of alloying elements on the low temperature temper embrittlement of precision tubes. Chromium and manganese impurities promote phosphorus segregation at austenite grain boundaries, thus promoting the low temperature tempering brittleness, tungsten and vanadium basically has no effect, reduce the low temperature tempering precision tube molybdenum ductile brittle transition temperature, but it is not enough to prevent low temperature temper brittleness. Silicon can delay the precipitation of cementite and improve the formation temperature, so it can improve the temperature of low temperature temper brittleness.

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