Study On Welding Procedure Of Corrosion Resistant Layer On Low-alloy Steel Surface And Properties Of Hardfacing Layer

Austenitic stainless steel is widely used in the manufacturing process of high temperature boiler parts and pipes which is attributed to its super physical performance, such as good corrosion resistance and high temperature resistance. However, the cost of austenitic stainless steel fabrication process is relatively higher than other counterparts which largely hinder the large scale application of this kind of materials. Generally, a common approach, overlaying a austenitic stainless steel layer onto the low alloy steel surface, was developed to substitute the use of total austenitic stainless steel materials. Unfortunately, due to the big performance differences between these two kinds of materials, defects(eg. cracks, pore) tend to appear in the materials junction which largely influence the performance and service life of workpieces.To satify the actual demand of productivity and tackle the scientific reasearch project of enterprise, differences in the thickness of transition layer approach was performed in the work. Consumable electrode active gas shielded welding(MAG) technique was adopted to overlay an austenitic stainless steel material onto the low alloy steel Q345 surface, the material for the transition and anti-corrosion layer is ER309 and ER347 L stainless steel wire, respectively. Bending, shear, microhardness, microstructure, shear fracture, molten salt corrosion morphology and corrosion products were applied on the bead welding sample to determine the experimental analysis and assess the mechanical and corrosion resisting property comprehensively. Experimatal results suggest that cracks can be observed in the sample without transition layer while there is no cracks in the sample with transition layer, it is deduced that the use of transition layer is extremely helpful to the physical performance of bead welding layer. Shear ports exhibit a ductile fracture morphology and dimple shape predominantly and toughening nest shape of transition layer seem to be large and deep, it is implied that the sample has a better ductility and toughness property. The change tendency of microhardness of samples are the same. The hardness value of the surfacing layer is minimum(about 125HV), while the hardness value of the parent material is the largest and the hardness value of the fusion zone is also greater, which is about 190 HV. The reason to this phenomenon due to the carbon migration in fusion zone. We can find by the metallographic experiment that the structure of the surfacing layer is mainly made of ferrite, austenite and the brittleness carbide phase with trace of Cr element. Most of the ferrite is in the shape of strip and block distribution, which can significantly improve the plastic toughness of the surfacing layer. The SEM image of molten salt corrosion shows that the hot corrosion of non-transition layer sample is quite serious. The crack is obvious near the fusion line, the corrosion layer is loose and the micro-structure is not uniform. And while, the hot corrosion degree of transition layer sample is smaller. The XRD pattern of corrosion products shows that corrosion resistant Cr2O3 oxide film is of integrity in transition layer samples, it can effectively prevent the corrosion further deepening. In all, the connectivity and corrosion resistance of the surfacing layer and base material with transition layer sample is better, but the thickness of the transition layer has little influence.In this paper, the finite element software MSC.Marc was carried on to simulate the numerical parameters.Through using the Marc unit and heat-structure coupling function analysis for simulation of welding process, the weld residual stress distribution in different directions of three dimensional curve were calculated and then comparing different transition layer thickness of residual stress and strain to determine the welding residual stress distribution rule of different transition layer thickness. It is turned out that transitional layer of the specimens show a larger residual stress when the bead w-elding thickness in a same condition, the maximum value is 409 MPa and 429 MPa, 432 MPa and 446 MPa, respectively, that is to say, surface residual stresses were increased with the increasing thickness of transition layer.