爐渣處理設備破損原因分析

Failure Cause Analysis of BOF Slag Processing Equipment

本文旨在分析探討煉鋼製程所使用之滾筒式轉爐渣處理設備（BSSF, Baosteel’s short flow）之腐蝕破損原因。某公司使用BSSF滾筒式設備進行轉爐渣之處理，高溫鋼渣首先倒在塗有石灰膏之滾筒襯套上，以水直接冷卻後再落入下方之滾筒內進行研磨粉碎。該襯套先前使用鑄鋼材料壽命約5 ~ 6個月，換用球墨鑄鐵襯套後壽命僅約26天，因襯套更換不易，嚴重影響設備運作，為了解襯套之破損成因，乃展開原因分析。
分析方法包括現場設備破損外觀觀察、材質成分分析、SEM及OM金相分析、表面SEM/EDS分析、腐蝕生成物分析等。分析結果顯示襯套破裂原因為熱衝擊與腐蝕共同作用所導致。襯套在倒渣後因局部高溫使得表面受到壓應力，當大量水倒在襯套後表面冷卻較快，內部溫度較高，此時表面受到拉應力，造成熱衝擊現象。另外由於強鹼及氯離子共同侵蝕表面氧化層，形成局部蝕坑，造成應力集中現象，更易形成表面裂縫之起始處，在加熱、冷卻及腐蝕循環作用下，裂縫逐漸往內部發展而導致襯套開裂。

The purpose of this paper is to study the failure cause of BSSF slag processing equipment. The BSSF was used to process BOF slag. Molten BOF slag was dumped on the feeding hopper surface coated with lime paste, and then the cooling water was sprayed on it to cool down the slag. After that, the chilled slag fell into rotary drum to be granulated into solid particles. The service life of the previous cast iron feeding hopper was about 5−6 months, but life expectancy of the new placed ductile cast iron was only about 26 days, which was being used because the feeding hopper was difficult to be replaced, and might affect the equipment operation efficiency seriously.
For speculating the failure cause, some investigations and studies were performed. Including appearance examination, material composition, metallography analysis, surface SEM/EDS analysis and corrosion products analysis etc.. From the analysis results it was speculated that the failure cause of the BSSF feeding hopper was resulted from both thermal shock and corrosion phenomena. After slag dumping, compressive stress arose at local high temperature area of the feeding hopper surface, while cooling water was sprayed on it. The feeding hopper surface would suffer from tensile stress and resulted in thermal shock phenomenon. In addition, the surface and oxide would be corroded by alkaline solution and chloride ion, which led to the formation of local pits. These pits would further cause the stress arise and resulted in surface cracks initiation more easily. Through the repeated heating, cooling and corrosion effects, cracks developed gradually towards internal and led to the failure of the feeding hopper.