Failure analysis of hot sinter screens of a sinter plant

Failure analysis of a sinter screen in a sinter plant has been presented. The sinter screen is used for removing fines (<5 mm size) form the crushed hot sinter (temperature around 1000 °C) thereby facilitating the cooling of sinter as well as smooth running of blast furnaces. The screens are cracking prematurely within 15 days. The material of the failed screens is found to be ASTM A297 HK 40 which is basically a heat resistant cast austenitic stainless steel alloy. Based on the failure analysis, a new material HK 40 (Nb modified) has been recommended which increases the service life of the component by 6 times. Samples of both HK 40 and HP 40 steels from unused and used screens are collected for analysis. The analysis consists of plant visit and visual observation, chemical analysis, fractography, microstructural examination using optical and scanning electron microscopes, characterization with EDS analysis, and measurement of hardness, impact toughness and wear properties. The analysis reveals that HK 40 screens are cracking because of material degradation at elevated service temperature due to formation of sigma phase along with chromium carbide network leading to intergranular corrosion. Further, HK 40 screen exhibit significant increase in hardness and decrease in impact toughness after service compared to that of HP 40 due to material degradation. Microstructural examination of HP 40 steel does not exhibit any sigma phase formation and intergranular corrosion. A comparative study of HK 40 and HP 40 steels before and after service as hot sinter screen has been carried out.