Quality control of cement-weld joints

Abstract

620.179.16 Cement-weld joints obtained by spot welding along a layer of liquid c e m e n t are distinguished by their strength and tightness, and the p r o c e s s itself is efficient [1]. However, if the technological operating conditions are not maintained during welding, the following defects m a y appear: porosity, high internal s t r e s s e s , partial or complete destruction, thickened joints, unsatisfactory adhesion to metal, and so on, which are c a u s e d by nonuniform application of the cement, use of poor quality components, bad surface treatment, and nonobservance of the cementing procedure; and partial or no fusion with the presence of gas layers, small fusion depth or small core diameter, burning of the metal, excessive electrode dents, etc. -these a r e due to insufficient electrode p r e s s u r e , contamination of the element or electrode surface, i r r e g u l a r shape of the contact surface, increase in h a m m e r p r e s s u r e and welding current, and so on. The defects due to nonfusion, overheating, and excessive electrode dents cause deformations in the mating elements; as a result, the specific p r e s s u r e on the cement film diminishes, gaps in cementing occur, or joints a r e thickened.The g r e a t e r number of the c a u s e s for these defects m a y be eliminated in the p r o c e s s of operational control, which consists in ensuring the p r o p e r quality of the cement compounds, controlling the surface t r e a tment, aligning the elements being cemented, observing the cementing and welding p r o c e s s conditions, and compulsory visual checking of the cement-weld joints even after the job has been completed. Nevertheless, operational control and subsequent visual checking do not fully rule out defects in the joint and at the weld points. The m o s t dangerous defect in cement-weld joints is noncontinuous cementing. This leads to leakage and reduced strength of the structure, which b r e a k s down owing to the s m a l l e r a r e a of good-quality cementing and s t r e s s concentration at the noncemented spots.The work conducted in the V. A. Kucherenko Central Scientific R e s e a r c h Insitute for Structural Designs (TsNIISK)* showed that c e m e n t -w e l d s can be effectively tested by nondestructive methods. The choice of the method depends, first of all, on the physical p r o p e r t i e s and thickness of the m a t e r i a l s and the designs of the joints. Metal elements have practically constant physical properties. The elements a r e usually 1-2 m m thick. The physical p r o p e r t i e s of the cement joints differ sharply from those of the elements being joined and m a y v a r y widely. The cement-joint thickness at the same place of a cement-weld joint is ext r e m e l y variable. In some c a s e s the cement-joint thickness may be equal to or even g r e a t e r than the * It was p e r f o r m e d under the guidance of Prof. Yu. A. "Nilender and Prof. A. V. Gubenko.