ON THE ISSUE OF IMPROVING THE RELIABILITY OF QUALITY CONTROL OF WELDED JOINTS

G. E. Povelitsyn

At present, nondestructive methods for quality control of materials and products have become widely developed in industry. There are a large number of methods that make it possible to detect defects of various types, oriented in different ways and located at different depths in the material being inspected. The use of these methods is determined by the type of material being inspected (magnetic or nonmagnetic), its heat treatment (grain size and attenuation of oscillations), the type of defects, the depth at which they occur (internal, subsurface, and surface defects), and the economic feasibility of introducing one method or another. All these methods have their advantages and shortcomings and make it possible to reveal only a particular kind of defect. For more reliable detection of defects in structures, multistage inspection is used: first the stock material is inspected by the penetrating-radiation method or by the ultrasonic method; then the surface and subsurface layer of semifinished products and parts is inspected at early manufacturing stages by magnetographic, electromagnetic, and electroinductive methods; and finally, final inspection of surface defects in finished products is carried out by the penetrant-liquid method. Modern requirements imposed on materials of products operating under conditions of sharp temperature and pressure gradients, prolonged static and dynamic loading, and other complex conditions, in many cases cannot be satisfied by introducing any single method of nondestructive quality control of materials and products at any operation of their manufacture. In order to detect defects reliably, materials are inspected by yet another method, which, however, entails additional costs.

The present work is devoted to selecting the optimal use of combined X-ray and ultrasonic quality control of welded seams in closed structures made of steels 30KhGSA and 30KhGSNA, with a seam cross section of 12 mm and a 4 mm backing strip, with one-sided access for measurement. Radiographic inspection of the welded seams was carried out through two walls using an RUP-150/300-10 X-ray unit with a 2.5 BПM4-250 tube on RT-5 film in two stages: first the root of the seam was radiographed, and then the entire seam after it had been completely filled (arc welding, automatic under a flux layer in 10–11 passes). The radiographic conditions were \(U_{1,2} = 145\), \(I_{1,2} = 7\), \(t_1 = 4.5\) min, \(t_2 = 55\), \(F = 750\) mm. In this case, lack of fusion in the root of the seam, cracks, and inclusions were detected, which pod-

This was confirmed by metallographic studies. In addition, metallographic studies made it possible to detect individual discontinuities with an area of no more than 2–5 mm², representing lack of fusion of individual weld layers and small slag inclusions, which indicates the insufficiency of using only the radiographic method for inspecting the upper zone of the weld. To fully reveal defects located in the upper zone of the weld, the possibility was tested of applying ultrasonic inspection with a UDM-1M flaw detector, using prismatic search heads at a frequency of 2.5 MHz, with ultrasonic vibrations introduced at \(40^\circ\) according to a single-probe scheme. During inspection, the flaw detector’s search head was positioned in the near-weld zone in such a way that the direction of radiation was perpendicular to the length of the weld. The search head was moved automatically at a speed of 0.75 m/min. The zone being inspected on the flaw detector screen was bounded by strobe pulses, the leading edge of which coincided with the single and then with the triple reflection of the beam from the inner surface of the weld. The depth of occurrence of defects was determined using the flaw detector’s depth-measuring device, adjusted against a reference standard with a reflector diameter of 1.5–2 mm.

Metallographic studies of defective zones showed that the developed ultrasonic inspection procedure makes it possible to reliably detect lack of fusion between individual weld layers, incomplete penetration, cracks, and slag inclusions located in the upper and middle parts of the weld. Detection of defects in the weld root by the ultrasonic method is difficult because of the difficulty of interpreting echo signals reflected from the backing strip.

Comparison of the results of radiographic and ultrasonic inspection showed that the ultrasonic method reliably detects defects of the type involving lack of fusion of individual layers in the upper and middle zones of the weld, while the radiographic method detects incomplete penetration in the weld root. Thus, the two methods complement one another, and only their combined application ensures high inspection reliability.