Analysis of the causes of softening defects caused by normali

Drill pipe is an important part of oilfield drilling machinery and equipment. The drill pipe is friction welded by the drill pipe joint and the pipe body. The drill pipe joint size is 132mmx23mm in diameter (wall thickness), and its material is 36CrNiMo4 high-strength steel. Normalizing after welding of high-strength steel is carried out using a medium-frequency induction heating power supply. If the induction heating width is too large, a wide heat treatment softening zone will be formed around the welding zone. The strength of the softening zone will be significantly reduced, and softening failure of the welded joint will easily occur. To this end, the induction heating zone narrowing normalizing process test was carried out on the 36CrNiMo4 steel friction welded joint, and good results were achieved.

In order to prevent the magnetic field lines from escaping and leaking magnetic flux, and increase the heating efficiency, magnetic shielding is used to constrain and control the magnetic field lines of the inductor. To this end, we developed a yoke-type inductor, which is designed with two turns. Due to the single turn The inductor will produce a non-closed ring during heat treatment, which can easily cause uneven temperature distribution in the circumferential direction of the drill pipe. Multi-turn inductors can easily make the heating zone of the workpiece too wide, causing defects in the wide softening zone of the heat treatment. Through experiments, we know that the surface temperature of the friction welding joint has exceeded the normalizing process temperature, resulting in a heating temperature overshoot phenomenon, which makes the induction normalizing constant temperature poor, fluctuating between ±22 and ±44°C. When the power of the medium frequency induction heating power increases, the heating speed and heating efficiency of the joint surface are significantly improved, and the width of the induction heating zone of the workpiece is reduced; however, on the other hand, the peak temperature overshoot is increased, resulting in poor temperature control accuracy of the workpiece. It is easy to cause coarse grains in the workpiece.

The peak temperature when the normalizing temperature overshoots is related to the power of the medium frequency induction heating power supply and the design structure of the inductor. The use of a yoke-type inductor with magnetic restraint control and matching with larger heating power can suppress the adverse effects of temperature overshoot and improve the accuracy of temperature control of the workpiece. Tests have shown that after the 36CrNiMo4 high-strength steel friction welding joint adopts the above-mentioned induction heating zone narrowing technology, when the surface temperature of the workpiece normalized by medium-frequency induction heating reaches a temperature above 700°C, the temperature zone width is narrowed by about 20% compared with conventional induction heating, achieving The effect is obvious and can be used in heat treatment production.

This article briefly introduces the technology of friction welding induction heating normalizing softening defects of high-strength steel and narrowing the width of induction heating zone. Many manufacturers use the above process for heat treatment, and the drill pipes produced are of good quality and meet their working needs.