الملخص
إن إنجاز عمليات اللحام بمعدلات حرارة دخل منخفضة يقلل من مدى التغيرات الميتالورجيه والإجهادات المتبقية لمعدن الأساس. لحام القطاعات السميكة يتطلب حرارة دخل عالية والتي تؤثر على خواص وصلات اللحام. يتميز اللحام بالحزمة الإلكترونية على قدرته العاليـة على التغلغل وعدم اتساع درزة اللحام (Weld bead) مع انخفاض معدل حرارة الدخل (Heat input) المستخدمة مقارنة بطرق اللحام الأخرى. من المعروف أن عمق التغلغل في وصلات اللحام يزداد كلما انخفضت سرعة اللحام (ازدياد حرارة الدخل). هذه الورقة تعرض نتائج علاقة سرعة اللحام بعمق التغلغل في لحام الحزمة الإلكترونية لوصلات من الصلب الكربوني ومقارنة النتائج مع عمق التغلغل المتوقع نظرياً. أوضحت النتائج أن تقدير عمق التغلغل نظرياً في وصلات اللحام السميكة لا يتطابق مع عمق التغلغل الفعلي والذي يقل عن العمق المتوقع أما في وصلات اللحام الأقل سمكاً فإنه يمكن حساب عمق التغلغل نظرياً والذي وافق النتائج المعملية في لحام الحزمة الإلكترونية.
ABSTRACT
This paper presents the results of a work carried out on Electron Beam Welding (EBW) of carbon steel thick sections. It was aimed to study the effect of varying the welding speed of EBW on weld penetration. It is well known that reducing the welding speed will increase the weld depth. However, it is worth investigating the manner the penetration depth varies with welding speed and the possibility of using the Rosenthal equation to predict the weld penetration in EBW.
Welding thick sections using butt joints encountered technical problems in this study including the precise fit and tolerance of weld joints for EBW. Using seam weld T-joint is more practical regarding the research aim of this work. T-joints used in this work were consisting of two plates with the same thickness. Three joints with plate thicknesses of 10 mm, 15 mm and 20 mm were welded by EBW using welding speeds of 400, 175 and 100 mm/min respectively with welding current of 160 mA and voltage of 65 kV. Penetration depths obtained at these welding parameters were compared with theoretical predictions based on Rosenthal equations. The criterion to define the plate thickness depends on the manner of heat dissipation from the fusion zone to the base metal. Heat input as well as plat thickness determines the critical thickness above which a joint is dealt with as a thick plate. The adopted solution is either the two dimension solution applied for joints considered as thin or the three dimension solution when the plate thickness of the joint is considered thick. In this work the two dimension solution is applied for the first joint and the three dimension solution for the other two joints.
Results showed agreement between theoretical predictions and real observations for the first joint with welding speed of 400 mm/min. For the other two joints with lower welding speeds penetration was overestimated by the theoretical prediction. Discrepancy between observed penetration and predicted values increased with decreasing welding speed (increasing heat input). Better prediction of weld penetration was obtained with a modified three dimension solution of Rosenthal equation.
Weld depth using EBW can be predicted with Rosenthal equations when welded sections are considered as thin joint. For thick sections the three dimension solution can be adopted with small modification on Rosenthal equation.
