بررسي كاهش سختي سطحي موضعي و آزاد‌سازي تنش داخلي فولاد روتور

STRESS RELIEVING OF LOCALLY HARDENED ROTOR STEEL

آزادسازي تنش فولاد روتور كه به‌صورت موضعي دچار افزايش سختي شده، با تعيين پارامترهاي عمليات حرارتي همچون دما و زمان مورد توجه قرار گرفته است. پس از تعيين ارتباط بين سختي اندازه‌گيري شده و تنش پسماند داخلي ماده، با حل يك تحليل المان محدود حرارتي ـ سازه‌يي و جفت‌كردن آن با پديده‌ي خزش، مشخصات و روش اجراي عمليات حرارتي به‌منظور حذف تنش‌هاي داخلي مشخص شد. در روش دوم كاهش مستقيم سختي موضعي با توجه به تغييرات سختي با دما و زمان مورد تحليل قرار گرفت و نتايج با روش المان محدود و اجراي عمليات حرارتي مقايسه شد. هر سه روش با دقت بالايي آزاد‌سازي تنش و كاهش سختي را در محدوده‌ي دمايي حدود 685 درجه سانتي‌گراد و زمان 5 تا 6 ساعت تعيين كردند. نتايج به‌دست آمده براي اجراي عمليات حرارتي در بخش تعميرات صنعت مربوطه مورد استفاده قرار گرفت و نشان داد كه روش ارايه‌شده در تعيين شرايط دقيق عمليات حرارتي مورد نياز و كاهش قابل توجه زمان بسيار مفيد است .

Residual stresses can change parts performance, mechanical properties and also have no desired influence on life time. Some mechanical and thermal effects can be the reason for this stress. A heat treatment process, in order to decrease the surface hardness of locally hardened rotor steel by stress relieving, has been specified. In order to set the main heat treatment parameters, such as time and temperature, two methods were employed using creep principles. In the first method, after equalizing local hardness by internal stress, a coupled explicit thermal-structural finite element analysis implementing creep stress relieving was carried out. This method created a good opportunity to determine the influence of heating rate and constant temperature time on stress relaxation. The second method employed creep equations related directly to hardness. In the finite element modeling (FEM), residual stresses on rotor steel are simulated by preapplying stresses. The advantage of this method (FEM) is in applying the influence of the presentation of bulk material around the heating section of the model. Results obtained from both methods were compared with experimental data. Our methods showed stress relieving at 684 to 690C° in 5-6 hours, which is in good agreement with experimental results. Performing this experiment has some special considerations; the temperature should be increased, and the model must be kept in balanced form and prevented from any corrosion and oxiding, which are shown in the pictures and descriptions in this paper. Our results showed that the analytical method is very quick and effective, and can be employed in practice. The presented method can be applied to other industrial models to determine final residual stresses and surface hardness after a tempering heat treatment.