EFFECT OF QUENCHING AND TEMPERING IN MICROALLOYED STEELS CONTAINING BORON AND TITANIUM

摘要

The OCTG (Oil Country Tubular Goods) steels are used in the production of casing and tubing for drilling system in petroleum industry. These steels usually contain niobium, titanium and vanadium, which ensure good toughness, high tensile strength and resistance to hydrogen embrittlement. It has been known that titanium nitride can promote the refinement of austenite grains with a uniformly dispersion of precipitates in the steel matrix. Nowadays, several researches are being done with boron addition in the microalloyed steels. Boron increases hardenability and for the maximum effect, the concentration should be in the range 0.001 to 0.003 wt%. In the present study the hardness of one microalloyed steel was investigated after quenching and tempering in different temperatures. At first, the steel containing boron and titanium was manufactured by hot rolled until a thickness of 7,2 mm. Afterward, samples in longitudinal orientation with 15×150 mm were cut and heated at 850, 950 and 1050 °C for 30 minutes. Thereafter, all specimens were tempered in the range from 150 to 650 °C for 60 minutes. Both cooling were fulfilled with water at 23 °C. In all specimens, the averages of five measurements were obtained with a load of 100 N using a Vickers hardness tester. The results showed no significant deviations of the values of hardness, after quenching in three temperatures and tempering in the established range. It can be conclude that the titanium nitrides precipitate, refine the austenitic grain during hot rolling and keep characteristics of the martensitic transformation, even in different quenching temperatures. In addition, these nitrides favor the solubility of boron and carbon in the matrix, with hardness increasing of the steel. Though, applying small amounts of boron and titanium in the microalloyed steels, there was no significant variation of hardness after quenching in different temperatures.