Feature 2 | METHODS AND MATERIALS
for hull construction is based on a classification society’s list of approved consumables. Welding consumables for hull steels are classified or graded on the basis of toughness and minimum yield strength. The grade is usually indicated by a numerical character for toughness and an alphabetical character for the strength level. For higher strength steels the alphabetical character is supplemented by a number representing the yield strength, traditionally in Kg/ mm2
. For example, in the description
2Y40, ‘2’ indicates the toughness grade, ‘Y’ indicates the higher tensile steel, and ‘40’ indicates yield strength; in this instance 400N/mm [2]. The selection of welding consumables
is based on the weld metal Charpy impact toughness requirements matching the minimum Charpy V-notch (CVN) impact toughness levels specified for hull materials. Table 1 summarises the welding consumable grades appropriate to the steel grade. You will note from the above table that
the required test temperature is 20°C higher than that required for the parent plate. This is an historical requirement, rather than a technical one, and dates
from the time of the early development of welding consumables when low temperature properties in weld metals were difficult to achieve when compared to the wrought plate. However, the consumable manufacturers did not get too much of a concession, the weld metal had to achieve a higher energy value. Today, welding consumables would have no problem in achieving the lower temperature used for the wrought product, yet the historical difference remains.
Weld procedure The establishment and qualification of welding procedures in any fabrication is an essential function and has become standardised throughout the industry [4] and, indeed, throughout the world [5]. No matter how good the base material’s mechanical and fracture properties are, the process of fusion welding will cause a reduction of these properties, and the amount of this reduction needs to be controlled. It is the welding procedure specification (WPS) that confirms that the minimum required properties can be achievable, provided of course, the welder follows the welding procedure.
Table 2: Examination and testing of weld procedure test assemblies. Weld assembly
Test
Butt joint with full penetrat ion
Visual
Radiographic or ultrasonic Surface crack detection Transverse tensile test
transverse bend test Impact test hardness test Macroscopic examinination
Fillet welds
Visual Surface crack detection Fracture tests Hardness test Macroscopic examination
44
100% 100% 100% 2 4
Weld & HAZ 1 1
Extent of testing
The welding procedure specification
is qualified by a series of destructive and non-destructive tests. A typical example of the requirements for such tests taken from Lloyd’s Register’s Rules for Materials is shown in Table 2. Where a non-approved welding
• manufacturer
consumable is used, a longitudinal tensile test and chemical analysis of the weld metal are also required. The record of the test results, any retests and the details of the welding procedure specification are maintained and recorded in a separate document, The Welding Procedure Qualification Record (WPQR). This document also outlines a series of essential variables that include the:
• welding process and technique • welding position • joint type
• thickness and diameter • parent material type • welding consumable • shielding gas • heat input • current type
• preheat temperature
• interpass temperature. The essential variables are very
specific, such as the welding process, or have a specified range to which the welding procedure specification can be applied, such as with thickness. It should also be noted that a welding procedure qualification record may be used to qualify multiple welding procedure specifications.
100% 100% 2 1 1
Welder qualification Whereas the qual i f ied welding procedure specification is the control necessary to ensure adequate control of the metallurgy and mechanical properties of the welded joint, the testing and qualification of the welder to the welding procedure specification is essential in terms of a workmanship standard. The welder must be shown capable of using a welding procedure specification by producing a test weld of good quality in terms of freedom from deleterious imperfections.
The Naval Architect July/August 2010
Feature 2
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