Simplified Determination of Weldment Toughness
Simplified Determination of Weldment Toughness
In the AASHTO/AWS D1.5 Bridge Welding Code, and to a lesser extent in AWS D1.1 Structural Welding Code, the Weld Procedure Specification Qualifications rely on test plates which are full penetration weld joints with backing bars that require side bend, reduced tension and all-weld metal tensile specimens. CVN specimens have their notches only in the weld metal. In D1.5, these tests are designated as Test Plates A and B, along with the welder qualification test plate for unlimited thickness. In D1.1, the qualification test is called the test plate for unlimited thickness. The dynamic testing of fillet weld toughness does not exist in either code. Primarily the test consists of bending over a fillet weld on one side of a T-specimen and noting if it “…bends flat upon itself. If the fillet weld fractures, the fracture surface shall show complete fusion into the root of the joint…”, and no porosity larger than 2mm in the largest dimension. Completed testing of Plates A or B is very costly; typically $6,000 or more, due to the extensive cutting and machining involved, particularly on the all-weld metal round tensile bar and CVN tests. Moreover, they principally focus on the weld metal; only the transverse reduced tension specimen measures the resistance of the fusion and heat-affected zone (HAZ) to fracture, but it is accomplished at the slow strain rates of standard ASTM tensile tests. In the side bend test, also conducted at slow strain rate, the greatest strain is placed on the weld metal, especially if a backing bar is used, and less strain is exerted on the fusion and HAZ portions of the weldment. All these existing tests primarily focus on the weld metal, which is usually a very low-carbon steel deposit, with silicon and manganese additions for deoxidation and slagging purposes.
Although these tests may have been appropriate in the 1930s to the 1960s, welding electrodes have markedly improved, and these tests end up as being duplicative regulatory requirements for fabricators that consumable manufacturers have already certified. They do not measure the toughness of the fusion zone or the HAZ in dynamic mode as found in the CVN test. These are often the most vulnerable fracture paths, because base metals with carbon contents at 0.20% or more and alloying elements which increase carbon equivalent typically do not have the same toughness as weld metal.
In most structures, there are a lesser number of full penetration weldments compared to the more widespread use of fillet and partial penetration welds. There is a need for tests which measure the dynamic toughness of partial penetration weld joints and fillet welds of plates and shapes that are actually going to be used in a structure. The test would have a portable restraint device for use in a fabrication shop environment that measures the effects of different parameters such as heat input, different electrodes, and the joining of different metals, such as stainless steels to carbon steels, and whether the fusion zone or HAZ has sufficient toughness or a minimum of porosity/slag episodes. Fortunately tests have been developed which measure the dynamic toughness of weldments and the toughness of the HAZ. These include, but not limited to, the welded notch toughness test (Welding Journal, Vol 70, No. 2, Feb 1991, pp 47-54) and the HAZ notch test (Structural Materials Conference, Atlantic City, NJ, Feb 28-Mar 3, 2000, pp 371-379), and have been previously evaluated for application to fillet and partial penetration welds. The advantage of these welding tests is that test plates are held in a portable restraint device, and place the Charpy V-notch at the root of the weld, and have no fixed plate thickness, unlike the code tests which fix plate thickness at either 1” or 3/8” for welder qualification and use backing bars. Many of the existing tests used in the code have limited usefulness because they do not actually measure weld joint toughness. Instead, they concentrate on weld metal toughness, which is typically the least fracture prone. The above-cited tests concentrate on the weld root and the fusion or HAZ areas, which are the principal sources of cracking and rupture in most weldments.
This project will concentrate on where these tests best fit into the D1.5 code, either as primary tests or as alternates to the Plate A and B tests and the fillet weld bend-over test. This will provide the D1.5 code with more flexibility for fabricators and welding engineers and inspectors with the ability to evaluate of various electrodes and base materials and other welding parameters. These tests can approximate the dynamic behavior of many approved weld joints, including full penetration welds not employing backing bars where melt-through at the root may not be complete, or where there may be potential problems in the HAZ or fusion zone due to base metal composition.