An Investigation on the Cracking of Pallet Side Walls at a Sinter Plant

Abstract

A failure analysis on the cracking of pallet side walls of a sintering machine in an integrated steel plant is presented. The pallets moving at a constant speed carry the base mix for sintering and enter an ignition hood furnace (temperature ≈ 1150°C) at a regular interval of time. The pallet side walls of a sintering machine are therefore subjected to continuous thermal cycling. The material of the pallet side wall is spheroidal graphite (SG) cast iron. Ten cracked side walls are collected and analyzed. The failure investigation involves field visit, visual observation of the cracked side walls, fractography, chemical analysis, microstructural characterization, tensile and impact tests. Most of the cracks are observed between the bolt–hole locations of the lower side walls; bolt–hole locations act as obstructions to thermal movement of the casting. The chemical analysis shows higher level of sulfur while the materials must be of higher purity for SG iron. Fractography shows predominantly intergranular fracture. Examinations of microstructures at the cross sections of the samples show the presence of primarily intergranular cracks. Matrix structure reveals pearlite along with ferrite surrounding the embedded graphite nodules. The amount of pearlite in the matrix is measured around 30–35% whereas predominantly ferrite matrix is desirable at the elevated temperature application. Determinations of tensile and impact properties exhibit low values of elongation (10%) and impact energy (7 J), respectively, indicating poor toughness properties of the casting. The presence of pearlite and lower amount of graphite nodules deteriorate the thermal conductivity of the casting, thereby generating more thermal stress. The analyses show that the pallet side walls start cracking under cyclic high thermal stress due to embrittlement because of improper material.