A TREATISE ON WELD EVALUATION

摘要

Welds are inspected by various techniques which include visual examination, surface examination and volumetric examination. While the above techniques would qualify a weld to workmanship criteria, they would not necessarily be indicative of weld properties. Preparation and qualification of welding procedures and testing of production welds are indicative that the weldment would probably provide a safe and satisfactory service life. However, weldments have to operate at their design conditions which may include high temperatures and ASME Codes do not necessarily stipulate tests for verification of high temperature properties. In addition, defective welds are often repaired by removing the originally deposited weld metal and re-welding. The effects of double heat input are not necessarily evaluated. In this paper, an insight is provided into the factors which provide assurance that weldments will perform satisfactorily in service and the combination of non-destructive evaluation methods which would enable effective detection of imperfections. Non-destructive volumetric examination method for welds has traditionally be radiography. With the advent of automated data acquisition methods in Ultrasonics, like Time of Flight Diffraction and Phased Array Ultrasonic Testing, these methods are rapidly replacing radiographic methods for weld inspection. Ultrasonic acceptance criteria in ASME Section Ⅷ Div. 1, ASME Section Ⅷ Div. 2 and ASME Section IX do not include evaluation of porosity as ultrasonic methods do not easily detect porosity. The result of all this is that today we are accepting welders qualified using Ultrasonic examination as per ASME Section IX but on the job there is still the option of inspecting the weld using Radiography in which, excessive porosity can be a cause for weld repair. Considering this and various other criteria, a comprehensive weld evaluation methodology is proposed taking advantage of the strengths of each inspection technique while welding technology used would ensure that welds have required properties at service temperatures. A proposal is also made to improve the detectability of imperfections using modifications of existing Ultrasonic A-scan Techniques. NOMENCLATURE, ABBREVIATIONS AND DEFINITIONS MT: Magnetic Particle Examination PT: Liquid Penetrant Examination QC: Quality Control SMAW: Shielded Metal Arc Welding t: nominal thickness of plate UTS: Ultimate Tensile Strength WEP: Weld Edge Preparation WPS: Welding Procedure Specification Type no. 1 butt joint: Butt joints produced by double welding or by other means which produce the same quality of deposited weld metal on both inside and outside weld surfaces Weld: A localized coalescence of metals or non-metals produced either by heating the materials to suitable temperatures, with or without the application of pressure, or by the application of pressure alone and with or without the use of filler metal Weldment: An assembly whose component parts are joined by welding. Weld defect: A geometric imperfection in a weld which exceeds permissible limits in the construction Code.