Second, the tensile and fatigue properties of the steel pipe are not qualified
Annealing, normalizing and quenching of steel pipes are the most widely used integral heat treatment processes. Annealing and normalizing of steel pipe are mainly used as preparatory heat treatment. Its purpose is to eliminate the defects of casting and forging, improve the cutting performance of the workpiece, and prepare the organization for the final heat treatment. The defects caused by annealing and normalizing of steel pipes are mainly caused by heating, such as oxidation, decarbonization, overheating and overburning.
2.1 The tensile property of the steel pipe is unqualified
The heat treatment quality of quenched and tempered steel pipe is usually controlled by hardness, and it is worth noting that the final hardness of the steel pipe does not mean that other mechanical properties are the same. Steels with different degrees of quenching can obtain the same hardness by changing the tempering temperature, but their mechanical properties are very different.
The tensile strength of the steel pipe depends on the final tempering hardness and is almost unaffected by the quenching hardness. The yield strength, elongation and section shrinkage of the steel pipe not only depend on the tempering hardness, but also have a great relationship with the quenching hardness, under the condition that the tempering gets the same hardness, these performance indicators increase with the quenching hardness. Therefore, the tempered steel pipe can not only control the hardness after tempering, but also check the hardness after quenching
2.2 Steel pipe fatigue performance is not qualified
When quenching is insufficient or unquenched due to low heating temperature, large workpiece size or insufficient quenching cooling speed, even if the hardness after tempering meets the technical requirements, the fatigue strength often cannot meet the requirements of use and may lead to early failure of the workpiece. This situation should be regarded as a heat treatment defect, and the workpiece should be re-heat treated or other measures taken to remedy it.
When gas carburizing and carbonitriding are carried out in endothermic atmosphere, black network non-martensitic structure appears on the surface after carburizing and quenching, which is called internal oxidation. The inner oxide layer depth is generally not more than 0.05mm. The internal oxidation reduces the surface hardness of the carburized workpiece and forms residual tensile stress on the surface, thus greatly reducing the fatigue strength of the steel pipe. When the inner oxide layer depth is less than 0.013mm, the fatigue strength is not affected. When it exceeds 0.016mm, the fatigue strength can be reduced by 25%.
When the activity of carburizing agent is too large, the carburizing time is too long and the cooling rate after carburizing is too slow, the network or large block carbide is easy to form in the carburizing layer, the formation of carbide leads to the local alloying element dilution and hardenability decline around it, and the non-martensitic structure is easy to form after quenching. The formation of network and large massive carbide and non-martensitic structure reduces the favorable residual compressive stress in the permeated layer, which can reduce the fatigue strength of carburized steel pipe parts by 25%~30%.
The serious decarburization of the steel pipe will make the surface non-martensitic structure, reduce the surface hardness, make the surface show residual tensile stress state, and reduce the fatigue strength of the steel pipe.
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