Slag-free Self-shielded Flux-cored Wire For Wear-resistant Hardfacing And Its Matellurgical Behaviour

Slag-free self-shielded flux cored wires have several advantages, such as simple weldingprocedure (not need shielding gas, flux and slag clearing), high deposition rate as compared to thecommon flux-cored wires and solid wires, which offer effective support to the automatic hardfacingfor heavy equipments. In this paper, a new type of slag-free self-shield flux-cored wire was developed.This wire possessed excellent processing properties and wear resistance. The hardness were among58～67HRC. On this basis, the processing properties and wear resistance of slag-free self-shieldedflux cored wires were studied.The spatter of slag-free self-shielded flux cored wires typically classified into four types as arcrepulsion force spatter，electrically exploding spatter, gas precipitation spatter and metal vaporpressure. The mathematics model of quantitative relation between spatter and cored-wirecompositions was established using an orthogonal regression design. The mechanism of spatter wasanalyzed from surface tension, electrical conductivity of arc, gas precipitation and exothermicreaction. The cored-wire compositions which reduced surface tension, increased electricalconductivity of arc, reduced the amount of gas precipitation and exothermic reaction contributed tothe decrease of spatter rate. The smallest spatter rate (0.57%) was got by adjusting flux-coredcomposition. For the environmentally aware, the slag-free self-shielded flux cored wires were alsostudied. By comparising the shielded metal arc welding and CO2gas shielded arc welding, slag-freeself-shielded flux-cored contributed to energy saving and emission reduction.The dropt transfer of slag-free self-shielded flux cored wires was studied using self-made highspeed photography apparatus and arc physics monitoring system. The results showed that the slagspattered continually in the welding process, which named “slag spatter”. It was discussed that theformation of slag spatter was caused by the great difference of coefficience of linear expansionbetween slag and droplet, small volume of slag and rapid rotation of droplet. The feature of slagspatter was successful in explaining oxidation products where they gone, it was what caused weldbond without slag.Moreover, the droplet transfer classified into four types, namely repelled transfer, surface tensiontransfer, drop transfer and explosive transfer. Among them, repelled transfer and surface tensiontransfer were the main droplet transfer. In addition, the effect of mode of droplet transfer on arc wasstudied. Finally, the effects of welding parameter and cored-wire compositions on the mode of droplettransfer were also studied. The alloy system of slag-free self-shielded flux cored wire was determined as Fe-Cr-B-C.Especially, the effects of graphite, Fe-B and Fe-B/graphite on the microstructure of Fe-Cr-B-Chardfacing alloys were studied. The graphite promoted the growth of primary carbides perpendicularto the surface of base metal and restrained the growth of eutectic carbides. The hardness wasincreased rapidly with the carbon content increased to6%, and then increased more slowly. When theB content was about5%, the volume of carbides exceeded90%. Moreover, the average diameter ofM7(C, B)3(M＝Cr, Fe mainly) carbide was increased from9to20um and carbide volume fraction(CVF) was increased from14.10%to36.00%with the boron content increasing from0to1.4%inFe-Cr-Ti-C alloy. When the total content of graphite and Fe-B remained the same, the size of thecarbides was reduced, the shape of the carbides became grain-like and the distribution of the carbidesbecame diffuse with the increase of the proportion of Fe-B.Fe-Nb and Fe-Ti were added into Fe-Cr-B-C slag-free self-shielded flux-cored wire and theeffects of Ti and Nb on the alloy microstructure and wear resistance were investigated. TiC existed inthe iron-based hardfacing alloy due to the addition of Fe-Ti, and it acted as the nucleus of the primaryM7(C, B)3carbides (M=Cr, Fe, Mn). TiC decreased the amount of M7(C, B)3carbides when titaniumwas added into the alloys. When24wt.%Fe-Ti was added, the alloy microstructure was changedfrom a hypereutectic structure to a hypoeutectic one due to the formation of TiC, which consumed amass of carbon. The addition of titanium into iron-based hardfacing alloy was beneficial to the wearresistance with respect to the higher hardness and refined microstructure. The wear loss of the samplewith24wt.%Fe-Ti was the smallest (14.9mg). On the other hand, NbC acted as the nucleus of theprimary M7(C, B)3carbides and decreased the amount of M7(C,B)3carbides when niobium was addedinto the alloys. When18wt.%Fe-Nb was added, the microstructure of hardfacing alloy transformedfrom a hypereutectic structure to a eutectic one due to the formation of NbC, which consumed a massof carbon. Moreover, the microstructure changed into a hypoeutectic structure when the Fe-Nbcontent was up to24wt.%. The hardness of Nb-free hardfacing alloy was58.9HRC. When theFe-Nb content was increased to18%, the hardness was64.3HRC. However, its hardness wasdecreased to62.7HRC when the Fe-Nb content reached to24%. The wear loss of the hardfacingalloy with18wt.%Fe-Nb addition was the smallest among all the alloys.With the addition of Fe-Ti or Nb-Fe, the MC-type carbide (M=Ti/Nb) with small size and highhardness was precipitated, the amount and size of M7(C, B)3carbides were both reduced, and thematrix was enhanced by solid solution strengthening of Cr. The main abrasive wear mechanismchanged from microcracking to microcutting and microploughing resulting from times of plastic deformation due to the formation of MC and the reduction of primary M7(C, B)3carbides.Hardfacing processing of roller press was simulated by the finite element method (FEM). Theworking stress distribution in roller press was analyzed considering welding residual stress. Theresults showed that when the hardfacing layer has three buffer layers (60mm in thickness) and onewearing layer (20mm in thickness), the maximum stress was558MPa and occurred at the region15mm below the hardfacing surface. It was the optimal hardfacing processing. The results providefundamental base for the further application for hardfacing roller press by using slag-free self-shieldedflux cored wire.