| The mechanical behavior and corrosion resistant property of the high nitrogen austenitic stainless steels could well meet the requirement of the non-magnetic drill collar, so it is quickly applied in the non-magnetic drill collars. In China, High nitrogen austenitic stainless steel used for non-magnetic drill collar was very late, the development also was backward. At present, the non-magnetic drill collar steels of the wide employment are W1813N and N1310B, their many performances are not ideal, such as strength, corrosion resistance, so they don’t appropriate to be used in the severe environments like high corrosion, hard layer. Therefore, it is significative to study the new non-magnetic drill collar using high nitrogen austenitic stainless steel.Based on the thermo-calc software, the alloy composition of the new non-magnetic drill collar using high nitrogen austenitic stainless steel was reasonably design, and obtained the three experimental steels. Compared with the1#experimental steel, the composition of the2#experimental steel added0.5-1.0%Nb; the composition of the3#experimental steel increased by0.1%N. After solution treatment and aging precipitation, the organization of the experimental steels of the different hot working states was studied by the Leica optical microscope and the S-4300scaning electron microscope. The room temperature tensile and impact experiments were conducted by the WE-300hydraulic tensile testing machine and the JBM-300impact testing machine, and the room temperature tensile and impact data were analyzed. The influence of N content, Nb elements to microstructure, aging precipitation and mechanical property were discussed. With hot compression simulation experiment, the microstructure and mechanical data of the compression samples were analyzed, and the optimized schemes of the hot working process of the three experimental steels were acquired.N as the interstitial solid solution elements is easier to gather to defects such as grain boundary, which leads to uneven composition near grain boundary and the grain boundary migration will has a certain hinder when grains grow. When the high nitrogen austenitic stainless steel is liquid, Nb with N forms NbN, which increases solidification nucleation points; The precipitations of NbN nanoparticles during the cooling process have apparently pinning function of grain boundary migration, so Nb can significantly refine grains. Moreover, the formation of NbN compounds reduces the solid solution nitrogen content and obviously restrains Cr2N intergranular precipitate with aging. The impact toughness of the2#experimental steel is low, which mainly is the influence of the massive NbN formations in the high temperature smelting process. The Nb content was appropriately reduced during the metallurgical design, or at the low temperature casting and speeding up the cooling rate, which could avoid or reduce the formation of the abundantly massive NbN, eliminating its influence on impact toughness.The following data were acquire through the room temperature tensile experiments, under the several hot working conditions, The1#experimental steel:Rp0.2≥762MPa, Rm≥988MPa; the2#experimental steel:Rp0.2≥764MPa, Rm≥1006MPa; the3#experimental steel:Rp0.2≥875MPa, Rm≥1066MPa. The tensile strength, yield strength of the three kind experimental steels of the different hot working states are significantly higher than the related standard of the mechanical properties of the non-magnetic drill collar in china, reaching the international advanced level of the non-magnetic drill collar using steels.The optimized schemes of the1#,2#and3#experiment steels are follows:the hot forging temperature range is1050-1150℃, the thermal forging deformation range is35-50%, the suitable strain rate should not be less than1/s, but it should not be too high. Because of the influence of NbN precipitate particles, the forging of the2#experimental steel is relatively sensitive to strain rate. |
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