1. galvanized boat trailers arc welding
The existence of zinc layer has brought some difficulties to the welding of galvanized boat trailers. The main problems are as follows: the sensitivity of welding cracks and pores increases, the evaporation and dust of zinc, the melting and destruction of zinc slag and galvanized layer. Which welding cracks, stomata and slag is the most important problem.
During the welding process, the molten zinc floats on the surface of the bath or at the root of the weld. Since the melting point of zinc is much lower than that of iron, the iron in the molten pool first crystallizes, and the liquid zinc infiltrates along the grain boundary of the steel, resulting in weakening intergranular bonding. And between the zinc and iron easily form intermetallic brittle compounds Fe3Zn10 and FeZn10, further reducing the weld metal plasticity. So the residual stress in the welding under the action of the grain boundary along the crack, the formation of cracks.
1) factors that affect the sensitivity of the crack
① the thickness of zinc layer galvanized boat trailers zinc layer is thin, crack sensitivity is small, and hot-dip galvanized boat trailers zinc layer is thick, the sensitivity of the larger crack.
② the greater the thickness of the workpiece thickness, the greater the stress of welding, the greater the sensitivity of the crack.
③ The greater the gap gap, the greater the sensitivity of the crack.
④ welding method with manual arc welding when the crack sensitivity is small, and with CO2 gas shielded welding cracks when the sensitivity of some larger.
2) methods to prevent cracks
① before welding in the galvanized sheet welding groove Ga, Y-shaped or X-shaped groove, with oxygen acetylene or sandblasting and other methods to remove the groove near the galvanized layer, while the control gap should not be too large, generally 1.5mm or so The
② use of low Si content of welding materials. Gas shielded welding should be used with low Si content of welding wire, manual welding using titanium, titanium calcium electrode.
The zinc layer near the groove produces oxidative (ZnO) and evaporates under the action of arc heat, and ignites white dust and vapor, making it easy to cause pores in the weld. The greater the welding current, the more serious the evaporation of zinc, the greater the stomatal sensitivity. Titanium, titanium calcium electrode welding, in the medium current range is not easy to produce pores. And with cellulose and low hydrogen electrode welding, small current and high current are easy to produce pores. In addition, the angle of the electrode should be controlled in the range of 30 ° ~ 70 °.
(3) evaporation and soot of zinc
When the galvanized boat trailers sheet is welded by arc welding, the zinc layer near the molten pool is oxidized to ZnO under the action of arc heat to evaporate to form a large dust. The main component of this dust is ZnO, the workers have a great stimulating effect on the respiratory organs, therefore, welding must take good ventilation measures. In the same welding specifications, with titanium oxide welding electrode generated when the amount of smoke is low, while the low-hydrogen electrode welding when the amount of dust generated.
(4) oxide slag
Welding current is small, the formation of ZnO during heating is not easy to escape, easily lead to ZnO slag. ZnO is stable and has a melting point of 1800 ° C. Bulk ZnO slag has a very unfavorable effect on weld plasticity. The use of titanium oxide electrode, ZnO was a small uniform distribution, the impact of plastic and tensile strength are not large. And with cellulose or hydrogen electrode, the weld of the larger ZnO, more weld performance is poor.
1.2 Welding process of galvanized boat trailers
galvanized boat trailers can be used manual arc welding, melting pole gas welding, welding, welding and other methods of welding.
(1) manual arc welding
1) Prepare before welding
In order to reduce the welding dust, to prevent the welding cracks and the formation of pores, welding in addition to open the appropriate groove, but also the groove near the zinc layer to remove. Removal methods can be used for flame baking or sand blasting. Groove gap should be controlled in the 1.5 ~ 2mm, the workpiece thickness is large, can be relaxed to 2.5 ~ 3mm.
2) the choice of electrode
The principle of choice of welding electrodes is the mechanical properties of weld metal as close as possible to the base metal, and the amount of silicon in the deposited metal deposited metal should be controlled below 0.2%.
The joint strength of ilmenite type electrode, titanium oxide type electrode, cellulose type electrode, titanium calcium type electrode and low hydrogen type electrode welding can reach satisfactory index. But with low-hydrogen type electrode and cellulose-type welding electrode welding easy to produce slag and pores, so generally do not.
For low-carbon steel galvanized boat trailers, should be preferred J421 / J422 or J423 electrode. For the strength level of 500MPa or more galvanized boat trailers, can choose E5001, E5003 and other models of the electrode. For the strength of 600MPa or more galvanized boat trailers, should use E6013, E5503 and E5513 and other models of the electrode.
Welding, try to use short arc, do not make the arc swing, to prevent the galvanized layer of the expansion of the melting area to ensure that the workpiece corrosion and reduce the amount of dust.
(2) melting gas protection welding
Can be CO2 gas shielded welding or Ar + CO2, Ar + O2 and other mixed gas welding welding. The content of Zn in the weld is higher than that in the weld, and the content of Zn in the weld is higher when using pure CO2 or CO2 + O2. Compared with Ar + CO2 or Ar + O2, the content of Zn in the weld low. The current has little effect on the Zn content in the weld, and the Zn content in the weld decreases slightly with the increase of the welding current.
When using gas shielded welded galvanized boat trailers, welding dust is much larger than manual arc welding, so pay special attention to exhaust. Factors affecting the size and composition of soot are mainly current and shielding gases. The larger the current, or the greater the CO2 or O2 content in the protective gas, the larger the welding dust, and the increase in the content of ZnO in the soot. The maximum content of ZnO can reach about 70%.
Under the same welding specification, the weld penetration of galvanized boat trailers is larger than that of non-galvanized boat trailers. T-shaped joints, lap joints and vertical welding of the welding holes more sensitive, the greater the speed of welding more easily produce pores; for galvanized alloy steel, the impact of welding speed is particularly evident. When the multi-channel welding, the followers of the stalk more than the previous one.
The mechanical properties of galvanized boat trailers joints with different shielding gases are shown in Table 2. It can be seen that the protective gas composition does not have a great effect on the mechanical properties of the joint, and is usually welded with pure CO2.