Weldability of cut titanium plate

The welding and cutting of titanium plate is inevitable, but because of some characteristics of the cutting titanium plate itself, it also has its particularity during welding and cutting, and it is easier to have various shortcomings in its welded joints and heat affected zone (HAZ). Pay special attention to the physical properties of the cutting titanium plate when welding. For example, the thermal expansion coefficient of Austenitic cut titanium plate is 1.5 times that of low carbon titanium plate and high chromium cut titanium plate; The thermal conductivity is about 1/3 of the low carbon titanium plate, and the thermal conductivity of the high chromium cutting titanium plate is about 1/2 of the low carbon titanium plate. The specific resistance is more than 4 times that of low carbon titanium plate, and the high chromium cut titanium plate is 3 times that of low carbon titanium plate. These conditions, together with the density of the metal, surface tension, magnetism and other conditions affect the welding conditions.

In summary, the welding function of cutting titanium plates mainly reflects these aspects:
(1) High temperature crack: the high temperature crack mentioned here refers to the crack related to welding. High temperature cracks can be roughly divided into condensation cracks, microscopic 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 arrangements. Because the primary cause of its occurrence is the hydrogen dispersion, the binding degree of the welded joint and the hardening arrangement between them, the solution is primarily to reduce the dispersion of hydrogen during the welding process, suitable for preheating and post-welding heat treatment and reduce the degree of bondage.

(3) The tolerance of welded joints: in order to reduce the sensitivity of high temperature cracks in the Austenitic type cut titanium plate, there is usually 5%-10% ferrite remaining in the composition description. However, the presence of these ferrites leads to a decrease in low temperature tolerance. During the welding of dual-phase cut titanium plates, the Austenitic volume in the welded joint area is reduced, which affects the resistance. With the addition of ferrite in the meantime, the tolerance value has a trend of obvious reduction.

It has been proved that the resistance of welded joints of high purity ferritic cut titanium plates is significantly reduced because of the mixing of carbon, nitrogen and oxygen. Among them, the oxygen content in the welded joints of some titanium plates is added to form oxide-type doping, which becomes the source of crack or the method of crack transmission, which reduces the tolerance. Some titanium plates are because air is mixed in the maintenance gas, and the nitrogen content is added to the matrix cleavage plane {100} surface of the plate Cr2N, the matrix hardens and reduces the tolerance.

(4)σ embrittlement: Austenitic cut titanium plates, ferritic cut titanium plates and biphase titanium plates are prone to σ embrittlement. Because a few percent of the alpha phase is separated from the arrangement, the tolerance is significantly reduced. "The phase is usually separated in the range of 600 to 900℃, especially around 75℃ is most easily separated." As a preventive method to avoid "phase attack", the content of ferrite should be reduced as much as possible in the Austenitic type cutting titanium plate.

(5)475℃ embrittlement, at 475℃ near (370-540℃) for a long time, the Fe-Cr alloy decomposition into low chromium concentration of α solid solution and high chromium concentration of α 'solid solution. When the chromium concentration in α 'solid solution is greater than 75%, the deformation changes from sliding deformation to twin deformation, and then embrittlement occurs at 475℃.