The “three major problems” in welding high-strength weathering steel and their solutions

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Welding cold cracks in high-strength weathering steel. There are three main factors that cause welding cold cracks: (1) the hardening tendency of the steel； (2) Welding tensile stress ； (3) Hydrogen content and its distribution in the welded joint. Regarding the influencing factors in the above 3 aspects, the main measures that can be taken are: (1) Using low-hydrogen electrodes and fluxes with low alkalinity. Weld joints made with basic low-hydrogen electrodes and fluxes have a low hydrogen content, excellent desulfurization and dephosphorization properties, and high impact toughness. Before use, it must be dried at a temperature of 350°C to 420°C for 1 to 2 hours, with the aim of effectively removing the moisture contained within, thereby reducing the oxygen content in the welded joint and minimizing the tendency for cold cracking. (2) Welding spare parts must pass inspection before they can be assembled. Do not force the assembly to avoid excessive assembly stress. (3) Clean the weldment before welding. The area to be welded on the steel, as well as a 20 mm range on each side of it, must be thoroughly cleaned to remove moisture, rust, oil, and other contaminants. (4) Arrange the welding sequence reasonably. The principle is to ensure that most welds can be welded under conditions of relatively low stiffness, thereby further reducing welding stress. (5) Preheating before welding, slow cooling after welding, or heat treatment. Preheating before welding is usually an important process measure to prevent cold cracking in high-strength steel welding. Post-weld slow cooling or heat treatment allows the diffused hydrogen to escape fully, reducing welding residual stresses, improving the microstructure, and decreasing hardenability, thereby reducing the tendency for welding cold cracking. Preheating and post-weld slow cooling are generally not required when welding high-strength weathering steel. Preheating before welding is required in the following situations: when the welding environment temperature is below 5°C, the weldment should be preheated to 75°C–125°C℃ ； When welding defects such as cracks on the base metal of high-strength weathering steel with a weld repair thickness of not less than 8 mm, the welded area must be locally preheated to 100°C–150°C prior to welding. Where preheating before welding is carried out, the interpass temperature during welding shall not be lower than the local preheating temperature of the weldment. (6) Select an appropriate welding line energy. During manual arc welding and mixed gas shielded welding, the welding line energy is generally low. In such cases, appropriately increasing the welding line energy can prolong the cooling time of the weld joint, reduce or prevent the formation of quenched structures in the heat-affected zone. It also facilitates the escape of hydrogen, thereby reducing the tendency for cold cracking. (7) Select appropriate welding methods and welding procedures. Among several commonly used welding methods for medium and thick plates, the low-temperature impact toughness of the weld joints is best achieved by back-grinded double-sided multi-pass mixed gas shielded welding; followed by back-grinded double-sided multi-pass manual electrode welding; and then by double-sided single-pass submerged arc automatic welding without back-grinding. Welds produced by mixed gas shielded welding have a low hydrogen content and good resistance to cold cracking, so they should be given priority. During multi-layer welding, the previous layer of weld serves to preheat the subsequent layer ； The subsequent weld pass then serves to provide post-heating, slow cooling, and tempering for the preceding weld pass; therefore, multi-layer welded joints have higher crack resistance than single-layer welded joints. When the weld cross-section is large, multi-layer and multi-pass welding should be employed. Manual electrode welding requires short-arc operation. For manual electrode welding and mixed-gas shielded semi-automatic welding, arc initiation should be performed 20–30 mm away from the end of the weld; after the arc becomes stable, it should be moved to the weld end for normal welding. The end of the weld must be finished using the backstep technique; the length of the backstep should be 25–40 mm ； Weld craters must be filled. Welding hot cracks in high-strength weathering steel: The welding hot cracks in high-strength weathering steel are mainly crystalline cracks in the weld. Existing welding practices have shown that high-strength weathering steel welds have a lower tendency to heat cracking than ordinary weathering steel, which is estimated to be related to the lower sulfur and phosphorus contents, as well as the higher manganese content in high-strength weathering steel, along with the use of basic electrodes in shielded metal arc welding. To avoid welding hot cracks in high-strength weathering steel, the main measures taken include: (1) using basic electrodes and fluxes. (2) Arrange the welding sequence reasonably to minimize welding stress. (3) Control the shape of the weld. In concave-centered and flat fillet welds, as well as narrow and deep butt welds, during weld crystallization, substances with low melting points tend to accumulate on the central surface of the weld. Under the effect of welding tensile stress, crystalline cracks are very likely to occur. In wide and shallow butt welds, as columnar crystals grow upward, most of the impurities are pushed to the surface and dispersed there, which significantly reduces stress concentration; as a result, such welds have higher resistance to thermal cracking. Therefore, the shape factor (width-to-thickness ratio) of the butt welds is generally kept between 1.3 and 2, with a weld bead height of 12 mm (applicable only to the vehicle body welds). The shape of butt welds and fillet welds should be slightly convex, with the ends of the welds finished using a back-welding technique; the crater formed at the end of welds made by manual arc welding or semi-automatic gas-electric welding must be completely filled. Submerged arc automatic welding should be equipped with an arc starting plate and a lead-out plate. (4) Adopt reasonable welding specifications. The greater the welding current, the greater the welding penetration and the higher the fusion ratio. Appropriately reduce the welding current and increase the arc voltage. The problem of weld embrittlement in high-strength weathering steel: During low-temperature V-notch impact tests on welded joints made of high-strength weathering steel, it was found that the area with the lowest impact toughness was located in the weld zone, which is different from the case in ordinary high-strength steel welded joints where the area with the lowest impact toughness is found in the fusion zone. Another regular phenomenon is that the impact toughness of welds produced by welding with high parameters (high wire energy) is lower than that of welds produced by welding with low parameters (low wire energy). When welding high-strength weathering steel, it is advantageous to use a lower wire energy in order to improve the impact toughness of the welded joint. Of course, the effect of reducing the welding line energy (i.e., increasing the cooling rate) on improving impact toughness is limited. On the other hand, reducing the welding line energy also has an adverse effect on the cold crack resistance and plasticity properties of the joint. Therefore, the selection of welding wire energy must be considered comprehensively. Existing welding tests have shown that although the impact toughness of the welds produced by GMAW and semi-automatic gas-electric welding using high-strength weathering steel at higher linear energy levels is relatively low, it remains above the impact toughness requirements of the base material. Therefore, when using manual arc welding and semi-automatic gas-electric welding, there is no need to consider the impact toughness of the joints separately. However, when using thick-wire submerged arc automatic welding, the effect of welding heat input on the impact toughness of the joint must be considered. While ensuring full penetration and complete fusion, the welding line energy must not exceed 30 kJ/cm.