Detection And Analysis On Cracks And Property Deterioration Of The 16MnR Welded Joints Caused By Copper Infiltrating Into The Weld

Bottom housing is the main steel construction component in the MPS-Ⅱ265 type of medium-fast coal mill. It is generally made of 16MnR steel plates according to European standards. In this thesis, on the basis of analysis on the weldability and influence factors of 16MnR parent metal, welding procedure qualification of main welded joints in bottom housing was carried out according to ENISO15614 standard. And characteristics and the reasons of the hot cracks caused by copper infiltrating into the weld were detected and analyzed. Finally, prevention measures were put forward.On the basis of comprehensive analysis of the welded joint forms in bottom housing, welding procedures about the flat butt joint, corner joint and T joint of thick 16MnR plates were qualified and the corresponding welding procedure specification documents were formulated. After the pre-welding procedure specification documents were established, the specific requirements were obtained about welding and assembling of the parent metals, welding consumable specification, weld pass arrangement, welding parameters, preheating and post-heating temperature, the temperature between layers, back gouging, welding quality and detection. And then, welding procedure specification documents used to guide the actual production were formulated by procedure assessment parameters.The splice welds in bottom housing cylinder were first welded by combined welding technology and then formed by roll bending. The backing welds were completed by mixed gas shielded arc welding process and the other welding layers were accomplished by submerged arc welding process. This welding procedure including two welding processes together is the combined welding technology. 16MnR parent metal was hot-rolled and its ribbon microstructures were composed of ferrite and pearlite. Fine F+P microstructures could be found in heat-affected zone (HAZ). But the weld microstructures were coarser and mainly composed of pro-eutectoid ferrite, bainite and pearlite. The copper content in weld metal was locally increased dramatically owing to wire guide and contact tubes melting and copper infiltrating into the weld. This resulted in not only weld microstructure change, higher hardness and deterioration in mechanical properties, but also hot cracks on the weld surface. This kind of hot cracks was caused by huge low melting Cu element infiltrating into the local weld places. In the later period of solidification of the weld metal, the less liquid metal in grain boundaries, that is liquid film, was fractured to form hot cracks, precisely named as solidification cracks, which distributed along the grain boundary. Its fractograph had the characteristic of intergranular cracking. In addition, there were some slag inclusion, poor fusion and other welding defects in the welded joints.The main reasons above welding defects were poor groove preparation by flame cutting, bad welding assembling quality and not strict quality detection. Especially, the sense of responsibility of welding workers was not strong. The welding torch was not adjusted to the centerline during submerged arc welding making the copper contact tube in touch with the parent metal to melt. The result was copper infiltrating into the weld to deteriorate the quality of the welded joints. After the strict management and strengthening supervision of production and welder training, especially strict implementation of welding procedure specification, products meeting quality requirements of foreign companies had been produced and manufacturing tasks of main components in medium-fast coal mill were well fulfilled.