(A) hot cracking
Hot cracking refers to crack under high temperatures generated, also known as high-temperature cracking or solidification cracking, usually generated internally welds may sometimes occur in the heat affected zone, manifestations are: longitudinal cracks, transverse cracks, root crack crater cracks and heat-affected zone cracking.
Its causes is due to the existence of the weld puddle segregation during crystallization, and impurities in the low melting point eutectic crystallization process in the form of a liquid layer between the existence of segregation to form a solidified after a lower intensity, welding stress is large enough when , they will be among just solidified liquid layer or near-solid metal opened a crack.
In addition, if the grain boundaries of the base material there is also a low melting point eutectic and impurities, when welding the tensile stress is large enough, it will also be opened. In short, the thermal cracking is the result of metallurgical factors and mechanical factors working together.
For its causes, and its prevention measures are as follows:
(1) limit the base metal and welding materials (including rod, wire, flux and shielding gas) easily segregation element content and harmful impurities, in particular, should control the sulfur and phosphorus content and lower carbon content, generally used for welding in steel sulfur content should not exceed 0.04 5% phosphorus content should not exceed 0.055%; in addition the carbon content of the steel from the welding performance worse, general weld carbon content of 0.10% or less in control, thermal cracking susceptibility It can be greatly reduced.
(2) to adjust the chemical composition of the weld metal to improve weld, the weld refined grain products to increase its plasticity, or reduce the degree of dispersion of segregation, control the harmful effects of a low melting point eutectic.
(3) using basic electrodes or flux to reduce the weld containing impurities taken to improve the degree of crystallization of segregation.
(4) an appropriate increase in the shape factor of the weld, using multi-pass welding method avoids the centerline segregation prevented centerline cracks.
(5) the use of reasonable welding sequence and direction, a smaller weld lines can be super, overall warm-up and hammering method, fill the crater at Crater other process measures.
(B) cold cracking
Generally refers to the weld cold cracking during cooling down to the martensite transformation temperature of the temperature range (300-200 deg.] C or less) is produced, can occur immediately after welding, it can also occur in a long time after the welding, it is also called delayed cracking.
There are basic conditions for the formation of 3: welded joint formation of hardened tissue; the presence and concentration of hydrogen diffusion; and there is a large welding tensile stress.
For its causes, and its prevention measures are as follows:
(1) welding specifications and choose the right line can improve the weld heat affected zone and state organizations, such as preheating before welding, the control layer temperature, slow cooling after welding or the like to accelerate the hot hydrogen molecules from escaping.
(2) using basic electrodes or flux to reduce the oxygen content in the weld diffusion.
(3) electrode and flux before use should be strictly in accordance with the requirements for drying (low hydrogen electrodes 300 ℃ ~ 3 50 ℃ incubated lh; acid electrode l 00 ℃ ~ l50 ℃ incubated lh; solder 200 ℃ ~ 250.C insulation 2h), carefully cleaning the groove and the wire, remove oil, dirt, moisture and rust, etc., in order to reduce the source of hydrogen.
(4) post-weld heat treatment in a timely manner. First, an annealing treatment to eliminate stress, so that quenching and tempering to improve their toughness; two: eliminate the hydrogen treatment is carried out, the hydrogen from the weld joint fully escape.
(5) improve the quality of steel, reducing the steel layered inclusions.
(6) can be taken to reduce stress in a variety of welding process measures.
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