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Welding Repair of Steel Workpiece For Diesel Generator Set

Dec. 25, 2018

Welding repairs on diesel generator steel workpieces are mainly for repairing cracks and compensating for wear dimensions. Due to the wide variety of steels, various elements contained in the weld will have a certain influence on the weld repair, so the weldability varies greatly. Among them, the change in carbon content is the most significant. Low carbon steel and low carbon alloy steel have less tendency to harden during weld repair and have good weldability; weldability decreases with increasing carbon content; high carbon steel and high carbon alloy steel are welded After the replenishment, the temperature tends to decrease, and the tendency of hardening tends to occur, and the martensite is embrittled due to the infiltration of hydrogen in the weld zone, and cracks are easily formed. The following is a note for the welding of low carbon steel and medium carbon steel.


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1. Low-carbon steel

 

Its welding performance is good and no special technical measures are required. For manual arc welding, J42 type electrode is generally used to obtain satisfactory results. If the base metal or electrode component is unqualified, the carbon is too high or the sulfur is too high, or the workpiece with high rigidity is welded under low temperature conditions, cracks may occur. In this case, it is necessary to select high-quality welding rods, such as J426 and J427, J506, J507, etc., at the same time use a reasonable welding repair process, preheat the workpiece if necessary. Pay attention to the arc start, the welding rod, the beginning and the end of the weld. When determining the process parameters, the current, voltage, electrode diameter, power source type and polarity, and welding speed should be considered to avoid defects. Gas welding generally uses materials similar to the metal to be welded, without gas welding powder. Pay attention to flame selection and adjustment, ignition and flameout, welding sequence and operation method to prevent defects.

 

2. Carbon steel

 

The main difficulty in the repair of medium carbon steel is that it is very prone to hot cracks in the weld, especially at the crater. The main reason is the high content of carbon and sulfur in the weld, especially the presence of sulfur. The low-melting iron sulfide produced during crystallization exists in the intercrystalline layer in a liquid or semi-liquid state to form a very fragile interlayer, which causes cracks upon shrinkage. Cold cracks can also occur at the weld, especially on the base metal in the near weld zone. It occurs when it is cooled to about 300 ° C or lower after welding, and some even after a certain period of time after cooling. The main reason is that after the carbon content of the steel increases, the quenching tendency increases accordingly, and the near-weld area of the base metal is affected by heat, and the heating and cooling rates are large, resulting in a low-plastic hardened structure. In addition, the hydrogen content of the weld and the heat-affected zone diffuses toward the hot zone as the weld is cooled, where the hardened structure is carbonized by the action of hydrogen, that is, cracks are generated due to shrinkage stress.

 

It is worth noting that the heat treatment state of the diesel generator steel workpiece before welding is also affected by the quality of the weld repair. Materials with high carbon or alloying elements need to be heat treated before use. If damaged, directly weld without welding. It is difficult and prone to cracks. Cracks in steel parts can be classified into two types: thermal cracking of the weld metal during cooling and cold cracking of the base material in the near-weld zone due to embrittlement.

 

One of the weld repairs of diesel generator steels is a thermal stress crack. The cracked part is often on the weak section of the large stiffness weldment, such as the first two welds, the locating weld or the base metal in the slab groove. At the time, the time generated is during the cooling process. The main cause of the crack is that the rigidity of the weld repair zone is too large, so that it cannot be freely shrunk, and a large weld repair is required. The weldment is weakly broken and cracks due to the failure to withstand the weld repair stress.

 

First, preheating is required. Preheating is the main measure to prevent cracks in carbon steel workpieces in welding. Especially when the workpiece is stiff, preheating is beneficial to reduce the maximum hardness of the affected zone and prevent cold cracks and thermal stress cracks. Improve joint plasticity and reduce post-weld residual stress. Typically, 35 and 45 steels have a preheat temperature of 150 to 200 °C. It is best to preheat the whole.

 

Secondly, according to the working conditions and performance requirements of steel parts, select the appropriate welding rods, and try to choose alkaline low-hydrogen welding rods or chrome-nickel stainless steel welding rods with strong JH crack resistance.

 

Third, try to reduce the proportion of the base metal weld metal weld, adopt V-shaped groove, the first layer weld with small current, slow welding speed, pay attention to the penetration of the base metal, avoid the defects such as slag inclusion and unfusion. . Fourth, avoid excessive heat in the weld repair zone and excessive temperature difference between the weld repair zone and the weldment. Fifth, after the welding, it should be cooled as slowly as possible, and the low temperature and the same fire should be kept at the temperature of 150~200 °C in the electric furnace or other dense furnace for 2~4h, and the furnace will be cooled to eliminate some welding compensation stress.

 

In addition, there is a welding of high carbon steel, which is basically similar to medium carbon steel. Due to the higher carbon content, the post-weld hardening and cracking tendency is greater, and the weldability is worse, so the welding rod is more demanding during the welding repair. The general choice is J506 or J507; the high requirement is J607 or J707. Preheating must be carried out at a temperature not lower than 350 °C. In order to prevent defects, reduce the melting of the base metal and apply welding at a slow current with a small current. Heat treatment after welding.

 


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