Welding and cutting the cutting
titanium plate is inevitable, but due to some characteristics of the cutting titanium plate itself, it also has its own particularity when welding and cutting, and it is easier to produce various kinds of heat in its welded joints and heat-affected zone (HAZ). shortcoming. Pay special attention to the physical properties of the cut titanium plate when welding. For example, the thermal expansion coefficient of austenitic cutting titanium plate is 1.5 times that of low carbon titanium plate and high chromium series cutting titanium plate; the thermal conductivity is about 1/3 of low carbon titanium plate, while the thermal conductivity of high chromium series cutting titanium plate is It is about 1/2 of the low-carbon titanium plate; the specific resistance is more than 4 times that of the low-carbon titanium plate, and the high-chromium cutting titanium plate is 3 times that of the low-carbon titanium plate. These conditions plus the density, surface tension, magnetic properties and other conditions of the metal all affect the welding conditions.
Welding properties of cut titanium plates
To sum up, the welding performance of cutting titanium plates mainly reflects these aspects:
(1) High-temperature cracks: The high-temperature cracks mentioned here refer to cracks related to welding. High-temperature cracks can be roughly divided into condensation cracks, micro-cracks, HAZ (heat-affected zone) cracks, and reheating cracks.
(2) Low-temperature cracks: Low-temperature cracks sometimes occur in martensitic cut titanium plates and some ferritic cut titanium plates with martensitic structures. Since the main reason for its occurrence is hydrogen dispersion, the degree of restraint of the welded joint and the hardened tissue therein, the solution is mainly to reduce the dispersion of hydrogen during the welding process, properly perform preheating and post-weld heat treatment, and reduce the degree of restraint.
(3) Resistance of welded joints: In order to reduce the sensitivity of high-temperature cracks in austenitic cut titanium plates, 5%-10% ferrite is usually left in the composition drawing. However, the presence of these ferrites leads to a decrease in low temperature resistance. When the duplex cut titanium plate is welded, the amount of austenite in the welded joint area is reduced, which affects the resistance. In addition, with the increase of ferrite, the resistance value tends to decrease significantly.
It has been proved that the resistance of welded joints of high-purity ferritic cut titanium plates is significantly reduced because of the inclusion of carbon, nitrogen, and oxygen. Among them, the oxygen content in the welded joints of some titanium plates increases to generate oxide-type inclusions, and these inclusions become the source of cracks or the way cracks propagate reduces the resistance. However, some titanium plates are due to the air mixed in the protective gas, and the nitrogen content in it increases to produce lath-like Cr2N on the cleavage surface {100} of the substrate, which hardens the substrate and reduces the resistance.
(4) σ phase embrittlement: Austenitic cut titanium plate, ferrite cut titanium plate and duplex titanium plate are prone to σ phase embrittlement. Because a few percent of the α phase is separated in the structure, the resistance is significantly reduced. "The phase is usually separated in the range of 600-900 °C, especially at around 75 °C. As a preventive method to avoid" phase occurrence, the content of ferrite should be reduced as much as possible in the austenitic cutting titanium plate.
(5) Embrittlement at 475°C, when held for a long time near 475°C (370-540°C), the Fe-Cr alloy is decomposed into α solid solution with low chromium concentration and α' solid solution with high chromium concentration. When the chromium concentration in the α' solid solution is greater than 75%, the deformation changes from slip deformation to twin deformation, and then 475°C embrittlement occurs.
