Terrace break
As terrace break in is welding technique a in the thickness direction by a welded steel plate extending cold cracking referred caused by weld shrinkage stresses. The crack is typically stepped in terraces or is lamellar in the rolling direction.
Risk of terrace breakage
The terrace break is one of the most common component defects caused by welding. The German Stahleisenverlag published in its DAST 014, recommendations for the avoidance of terrace breaks in welded steel constructions . These recommendations have found their way into the German set of standards, including DIN 18800 and DIN EN 1993-1-10, and are to be regarded as state of the art .
Basically, only those components are at risk of terrace breakage which are loaded by welding shrinkage stresses perpendicular to the rolling plane. Welds that lie in the rolling plane, for example sheet metal butt joints or other full connections in which the entire sheet thickness is covered, are not problematic, since the shrinkage stresses occur exclusively or almost exclusively in the rolling direction. The risk of terrace breakage increases with:
- Poor strength value ( necking at break ) in the direction of the component thickness
- Welding without preheating , as the high cooling speeds do not allow the rearrangement processes to take time and the deformation literally freezes in the cooling phase.
- Large sheet thicknesses as this results in high cooling rates.
- Large weld seam thicknesses because of the high heat input
- Stiff constructions caused by unfavorable component and seam geometries that prevent free shrinkage.
Reduction of deformability in the direction of thickness
Rolled plates have a significantly lower deformability and tensile strength perpendicular to the rolling direction, i.e. in the direction of thickness, than in the rolling direction. This is due to inclusions of oxides , silicates and sulfides . These inclusions, which were originally located at points in the steel when the slab was cast , are flattened as a result of the rolling process. This has the consequence that these lie flat or in lines in the sheet metal plate. The bond between these lamellas is poor, which significantly reduces the strength of the panel in the direction of thickness. This can be illustrated with an analogy to a stack of paper: While individual sheets of the stack can be lifted off easily, the stack as a whole is difficult to tear up.
Cause of weld shrinkage stresses
As a result of the cooling after welding, the component reduces its volume. If the construction is "stiff", i.e. if the volume reduction is prevented by fixed clamping, tensions arise in the component, which can stress the component to its strength limit . The cause of these shrinkage stresses is the plasticization of the component. With increasing temperature, the steel loses its elasticity and strength. If the thermal expansion is hindered, the component is compressed in the area of the weld seam. With the relatively rapid cooling after welding, the workpiece then no longer has enough time to reverse this deformation. The reduction in volume of the liquid weld metal after cooling also contributes to the shrinkage stresses.
Constriction of the fracture
Since the deformation ability of the material is decisive for the terrace fracture tendency, the fracture constriction in the direction of the thickness is measured in the tensile test. The determination of the strength values in the direction of the thickness has little informative value, since a significant reduction in tensile strength can only be demonstrated at very low values of the fracture necking, below about 20%. The reduction in cross-sectional area up to the point of fracture of the sample is called fracture necking. The steel is divided into 3 classes based on the test result:
- Steel quality class 1: fracture necking on average at least 15%, lowest value 10%
- Steel quality class 2: fracture necking on average at least 25%, lowest value 15%
- Steel quality class 3: fracture necking on average at least 35%, minimum value 20%
The mean value of several samples is determined, whereby the specified minimum value must not be fallen below.
Selection of material grades to avoid terrace breaks
By weighting individual influences on the basis of simple graphs and table values, the DAST-014 can be used to determine which minimum requirements the steel should have with regard to the constriction of fracture. The minimum fracture constriction in percent can be read directly from the addition of the weighted influences and the required steel quality class can be determined. The DAST-014 has determined the required minimum value as decisive here. Fractional constrictions of up to 10% mean that no requirements are placed on the steel with regard to the verification of the quality class, fracture constrictions of up to 15% quality class 1 etc.
When it comes to welding technology, the paver must always take the risk of terrace breaks into account, especially with large sheet metal thicknesses and large seams. In many constructions, simple preheating to around 100 degrees Celsius is sufficient to drastically reduce this risk, as this significantly reduces the welding shrinkage stresses.
Individual evidence
- ↑ IWT Ingenieurbüro für Werkstofftechnik: Crack phenomena in steels, page 11, Fig. 20. Terrace fracture (here called lamellar fracture) at a nozzle weld ( Memento from January 22, 2005 in the Internet Archive )
- ↑ DAST guideline 014. Recommendations for avoiding terrace breaks in welded steel structures.
- ↑ Execution of steel structures. By Herbert Schmidt, Ulrich Schulte, Rainer Zwätz, Lothar on Google Books .
- ↑ Cause of weld shrinkage stresses ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. .
