RECYCLING OF SLAG IN SUBMERGED ARC WELDING

Abstract

Flux used in submerged arc welding converts into slag during welding, which is considered as waste. Such a large quantity of slag has to be disposed of in a land fill space. Since it is non-bio-degradable in nature, it will not decay with time. It cannot be used as a filling material in building construction because it is brittle and glassy. Due to this reason, cost will increase, apart from environmental pollution. Non renewable resources may be exhausted due to continuous mining. Thus, slag generated during submerged arc welding is a waste and imposes a number of problems. The higher the amount of slag generated, the higher the wastivity. It is not possible to stop the generation of slag because it is a by-product of the process, but slag can be reused as a flux in the same submerged arc welding process. Wastivity can be decreased by recycling the slag, thus resulting in higher productivity. Furthermore, the slag can be processed according to one’s requirements and applications. Therefore, an attempt has been made to develop technology for the recycling of slag so as to use it for commercial purposes. The process of recycling has been performed in various stages so as to confirm the overall soundness of the slag developed. In the first stage, pure slag has been collected from the industry, then washed, crushed and sieved to the same grain size as that of fresh flux, and then a weld pad has been obtained using pure slag. After performing spectro analysis, it was found that the weld metal composition was not in accordance with ASME SFA 5.17. Therefore, it was decided that slag should be recycled. In order to replenish the slag, pulverised form (100 mesh size) slag was added with deoxidisers, i.e., CaCO3; Al powder; Mn powder with K2TiO3, which acts as an arc vii stabiliser. K2SiO3 was added as a binder to bind the mixture. They were then mixed and agglomerated. After which this mixture was air dried and baked at 850˚C. This baked mass, after crushing and sieving, is known as recycled slag. After performing spectro analysis of the weld pad prepared using recycled slag, it was found that, the weld metal composition of recycled slag was in accordance with ASME SFA 5.17. The weld pad obtained using recycled slag was then investigated to evaluate its mechanical properties by performing visual inspection, dye penetrant and radiographic tests. Later, the specimen was removed from the middle of the weld pad to perform mechanical testing. It was found that, the tensile test and impact strength were in accordance with ASME SFA 5.17. To study the effect of welding parameters on weld quality, it is necessary to consider the transfer of elements such as C, Mn, Si, S, and P into the weld metal using recycled slag. A design matrix was developed using two-level half factorial Design. Empirical models were developed for the responses obtained. These models were then checked for adequacy and significance of model using the ‘F’ test and ‘t’ test respectively. It was observed that the transfer of C, Mn, and P was directly related to arc voltage, whereas it was inversely related to the wire feed rate. Moreover, the transfer of Si decreased with an increase in wire feed rate but remained the same with varying arc voltage, welding speed, and contact tube to plate distance. Furthermore, it was noticed that welding speed and contact tube to plate distance have insignificant effect on weld metal chemistry. It was concluded that recycled slag gives satisfactory results and can be used as a fresh flux.