Hot crack
A hot crack is damage to the workpiece caused by welding . The crack occurs during solidification at temperatures between the solidus and liquidus temperatures , and is therefore a typical defect in the solidus weld .
definition
According to data sheet DVS 1004-1, the term “hot crack” is understood to mean cracks that are caused in the presence of low-melting, brittle substances on the grain boundaries . They are caused by high temperatures during or after the welding process has ended. Based on their fracture behavior in hot tensile tests, hot cracks are also referred to in the specialist literature as “brittle”, ie deformation-free material separations, in the weld metal and in the heat-affected zone of the base material.
Differentiation from the cold crack
Cold cracks occur in the solid state after the solidus temperature has fallen below , for example due to large differences in hardness, welding shrinkage stresses or hydrogen embrittlement .
Mechanism of hot cracking
Materials with a large solidification interval between liquidus and solidus temperature, high phosphorus, sulfur and carbon contents tend to hot cracks just like materials with low toughness at high temperatures. In the case of alloys , a eutectic or non-metallic phase forms in front of or on the solidifying dendrites . During grain growth, these impurities are pushed ahead of the growing grain and remain in the melt. The segregation of the residual melt with non-metallic substances reduces the solidification temperature of the remaining melt and causes a relatively long period of a liquid phase to remain between the already solidified dendrites. Consequently, the risk of hot cracking increases with the increase in this steel contamination, since the solidus temperature increases with the degree of contamination. This melt can then be torn apart by the stresses that arise during welding. The formation of hot cracks (previously also known as "hot cracks") is therefore linked to the presence of liquid, semi-liquid, low-melting or brittle phases on the grain boundaries, which can arise or are already present as a result of solidification and transformation processes in the temperature range of the solidus temperature . Brittle phases are not able to transfer occurring shrinkage stresses (see also terrace breakage ). Even after the welded component has cooled down, there may still be hot cracks, usually as microcracks, as a result of shrinkage stresses or other external loads. High-alloy austenitic steels are particularly at risk of hot cracking . The ability of austenitic dendrites to dissolve sulfur, phosphorus and carbon is significantly lower than that of ferritic steels. In addition, the austenites usually have a particularly coarse-grained and therefore brittle grain structure without additional material technology measures.
Types of hot cracks
Depending on the cause of their formation, a distinction is made between solidification and melting cracks.
Solidification cracks
Solidification cracks lie in the weld metal and arise when the material crystallizes from the liquid phase. In the interior of the weld bead, microcracks initially appear, which can extend to the surface as the solidification continues and become visible there as macrocracks. In this case, the crack surfaces are not bare metal, but coated with oxides. The solidification cracks are basically perpendicular to the strongest shrinkage deformation. These cracks can usually be seen with a magnifying glass, but often with the naked eye (for example: seam center cracks, end crater cracks). If these extend to the surface, they can be detected using surface crack testing methods.
To avoid hot cracks, it is recommended that the component be preheated and small, shallow weld pools. Small weld pools can be produced, for example, by using the multi-layer welding technique or by selecting favorable seam shapes. These measures reduce shrinkage stresses and avoid pronounced dendritic solidification of the weld pool.
Melting cracks or remelting cracks
Melting cracks, also called remelting cracks, arise in the heat-affected zone (HAZ) adjacent to the weld pool. This can then be both the base material or, in the case of multi-layer welding, also the heat-affected zone of weld beads located below or next to it. They arise preferentially at the transition point between the liquid weld metal and the HAZ and they often extend into the base material. The cause of remelting cracks is the reliquefaction of the precipitates located at the grain boundaries. At the same time, tensile stresses occur as a result of the cooling of the weld metal due to its reduction in volume. The precipitations were a direct consequence of the welding process, but had not led to the solidification cracks described above.
As with solidification cracks, the risk of hot cracks during welding can be reduced by reducing the heat input, for example through line beads and multi-layer welding.
Cracks due to a drop in deformability
Cracks caused by the reduction in the deformability of the material as a result of the weld are located in the HAZ or directly next to it. These are intergranular separations that occur after the material has cooled down. The grain boundaries did not melt.
Detection of hot cracks
Usually, hot cracks are small and rarely extend over several millimeters or even centimeters. This applies in particular to remelting cracks and cracks as a result of a drop in ductility or toughness (Ductility Dip Cracking "DDC"), which mainly occur as microcracks and often do not reach the surface. Microcracks can therefore not always be found with non-destructive testing methods.
The cause of the crack and its demarcation from cold cracks can be made using a metallographic section. The direction of solidification can usually be seen clearly in the micrograph. An examination with the scanning electron microscope offers a better demarcation of solidification cracks from cold cracks and cavities. The fracture surface of a solidification crack in an austenitic weld metal then runs along the differently oriented dendrite packages. The rounded dendrite tips clearly show that a melt lying on the surface of the dendrites has solidified freely after it has been torn open.
Individual evidence
- ↑ Leaflet DVS 1004-1 of the "Technical Committee of the German Association for Welding Technology": Hot crack testing method - basics.
- ↑ Formation of hot cracks
- ↑ IWT Ingenieurbüro für Werkstofftechnik: Crack phenomena in steels I and II ( Memento of January 22, 2005 in the Internet Archive ), sheet 4: Subdivision of the crack types; Sheets 5 to 7: Scanning electron microscope images of hot cracks
- ↑ Volker Schmidt, Martin Möser, TH Magdeburg, page 11, image 21 + image 22, scanning electron microscope images of hot cracks (PDF; 981 kB).