The high-temperature cracks mentioned here refer to the cracks related to welding. High-temperature cracks can be roughly divided into solidification cracks, microcracks, HAZ (heat affected zone) cracks and reheating cracks.
Low-temperature crack
Low-temperature cracks sometimes occur in martensitic stainless steel and ferritic stainless steel with martensitic structure locally. Since the main causes are hydrogen diffusion, the constraint level of welded joints and the hardened tissue, the treatment method is mainly to reduce the hydrogen diffusion in the welding process, and stop preheating and thermal disposal appropriately after welding and reduce the constraint level.
The toughness of welded joints
In order to reduce the sensitivity of high-temperature cracks in austenitic stainless steel, the composition design usually makes the remaining ferrite 5% to 10%. However, the presence of these ferrites results in a drop in low-temperature toughness. When the welding of duplex stainless steel stops, the austenite in the welded joint area decreases, and the toughness is affected. In addition, the toughness value of ferrite tends to drop significantly with the increase of ferrite.
It has been proved that the reason why the toughness of welded joints of Sanitary Fittings of high purity ferritin drops significantly is the mixing of carbon, nitrogen, and oxygen. The increase of oxygen content in the welded joints of some steels results in oxide inclusions, which has become the source of crack initiation or the way of crack propagation and make toughness drop. Besides, some steels are mixed with air in the maintenance gas, and the increase of nitrogen content produces strip Cr2N on the cleavage plane {100} of the matrix, which hardens the matrix and makes the toughness drop.
Embrittlement of the σphase
In austenitic stainless steel, the composition design usually makes the remaining 5% to 10% ferrite to reduce the sensitivity of high-temperature cracks. As a few percent σphase was precipitated from the tissue, the toughness decreased significantly. Ferrite content in austenitic stainless steel should be minimized as a preventive measure to avoid the generation of σphase.
