GMAW weld overlay and repair

Abstract:For the gmaw weld overlay and repair of large steel shafts, the selection of the welding method and the correct formulation of the process are the prerequisites for successful repair and good results. This paper describes the science of selecting welding methods and the main process methods to ensure welding quality.

 A large sugar factory, used to transport sugar liquid, transfer strong torque of two large dipping shaft, due to long-term wear and tear, the root of the coil is not sealed, the production of sugar liquid leakage, a serious waste of raw materials, affecting production. Due to the high cost of re-purchasing. To save money, it was decided that the two axes would be repaired by overlay welding and continue to be used.

I. Material and damage of the shaft

 Both axes are made of 40Cr steel. The composition is as shown in the table:(%)

    C         Si      Mn    Cr

   0.37–0.44 o.17–0.37 0.5–0.8 0.8–1.10

 The 40Cr steel for the shaft has been tempered to obtain a low alloy high strength steel with good overall performance, the specification is ψ350X4000mm, there is a hole of ψ60mm through the whole shaft in the center of the shaft, the weight of the whole shaft is 2T.

II. Analysis of the weldability of shaft materials

 40Cr steel for the carbon regulator steel, is a poor weldability of the steel, Cr, Mn content is relatively high. Therefore, the strong hardenability and hardening tendency, coupled with the large size of the shaft, the welding process, faster cooling, will exacerbate the above two tendencies.

 40Cr steel Mf point low, easy to produce a large number of martensitic tissue near the weld area, which has a strong tendency to cold crack, in order to prevent the generation of cold cracks and do not cause the shaft in the use of the process, cladding layer peeled off under the strong torque, in addition to off plasticity, good toughness of low hydrogen welding material, should also be preheated before welding, after welding in a timely manner.

 In addition, especially after welding, when the heat treatment cannot be restored, when welding in a tempered state, the line energy is too large, which leads to an increase in the high temperature tempering layer in the heat-affected zone, reducing the strength of the shaft cladding, affecting the performance of the shaft and causing repair failure.

 Likewise, the large line energy input, still can not avoid the formation of martensite, but will increase the degree of superheating and stability of austenite, the formation of coarse martensite tissue in the quenching area, increasing the tendency to embrittlement, brittle damage occurs.

 So, how to ensure that there is no excessive line energy and slow down the cooling of the heat-affected area after welding? The only way to weld is to use reasonable preheating and strict control of the interlayer temperature of the weld path, that is, to weld with a deep line and a narrow path, which can obtain small line energy. The choice of welding material should also consider materials with high resistance to cracking, low S, P and content.

iii. selection of welding methods

  The best welding method to avoid the problems described above and to satisfy the performance of the shaft is Co2 gas protection welding. Short-circuit transitions, narrow passes, and fast welding are used in the implementation. This helps to reduce the high temperature residence time in the heat-affected zone, reduce the degree of superheated embrittlement of the austenite in the zone, and improve the stability of its tissue.

 At the same time, with reasonable preheating before welding, strict control of the interlayer temperature, postheating and slow cooling measures, etc., to improve the organizational performance of the quenching area and improve the resistance to cold cracking. Under the above-mentioned circumstances, the use of small line energy welding is also conducive to reducing the softening area and reducing the local softening of the shaft to meet the use.

IV. GMAW weld overlay repair process

1. Make a bracket that allows the shaft to roll freely on it. Clean the wear parts of the shaft from dirt and expose the metal.

2, the volume in, fast heat dissipation, direct heating with acetylene flame heating slow, the use of the shaft of the central hole, through the steam, the overall heating of the shaft to more than 100 degrees, and then HO – 20 welding torque to wear the uniform heating to 200 – 250 degrees to begin welding.

3、CO2 gas protection welding, welding wire number H08Mn2SiA, diameter 0.8MM, welding current 100A, arc voltage 18-21V, using short circuit transition, first evenly rotate the large axis in the climbing position to ignite the arc welding, so that the welding path is spiraled forward, in order to achieve a week of welding path heat uniformity, to prevent shaft deformation and bending.

4、Weld 45 turns of each spiral layer and weld two layers. The surface temperature of the shaft is controlled to 200 degrees C per lap and between layers, and it is held for 2 hours after welding. The two axes were welded one by one in this way.

V. Conclusion

 The two shafts have been welded and operated normally, and no quality problems were found due to the gmaw overlay weld repair. When it comes to large 40Cr steel shafts, it is feasible to use small line energy and reasonable auxiliary processes to avoid the production of large amounts of martensite in the heat-affected zone, which causes the shaft to become brittle, and to reduce the softening zone to ensure that the strength of the shaft will not be affected.