The degasser operation imposes severe stress into the refractory lining system owing to rapid changes in temperature.
In the recirculatory degassing operation, liquid steel is forced from the ladle into an evacuated refractory chamber by atmospheric pressure. The low pressure in the chamber then allows the entrapped gases to expand and rise to the surface, resulting in the denser degassed steel returning to the ladle. The action of the degassing process also results in high turbulence within the steel volume giving rise to homogenisation. Gases from the chamber are removed through off takes and coolers.
RH AND DH DEGASSING VESSELS
Vacuum degassing is carried out in two distinct recirculatory units, the DH -Dortmund-Hoerde utilising a single snorkel leg whereby steel is drawn into the chamber and after degassing leaves through the same leg. RH - Ruhrstaal - Heraeus using an upleg snorkel leg through which steel is drawn into the chamber and the denser degassed steel leaves through the down leg.
The DH degassing system is typically used in the production of high alloy and speciality steels from low tonnage electric arc furnace shops, whereas the RH degassing system is associated with high tonnage BOS shops producing low-carbon aluminium-killed steel.
RH degassing is generally preferred owing to the metallurgical advantage of downstream refining processes to produce large tonnage's of high quality lower cost continuously cast steel.
Development of the RH degassing system has led to
RH-OB - oxygen blown to produce low carbon (<0.015%) steel.
RH-PB - powder injection of synthetic slag desulphurisers to remove or modify sulphur inclusions.
RH-OB - aluminium heating by the addition of aluminium metal.
REFRACTORY WEAR PROCESSES
Refractory selection for the lining of vacuum degassers is invariably determined by consideration of the various features of the process.
TEMPERATURE
Temperatures in the degassing vessel vary from 1480 to 1760oC, with the temperature often being sustained at 1480oC between heats, and increased up to 1760oC during treatment.
EROSION
The action of molten steel and slag entering the vacuum chamber places a highly erosive action upon the refractory lining.
ABRASION
Abrasive forces exists in the gas off takes owing to the action of fine steel particles entrapped in the exiting gases.
THERMAL CYCLING
Owing to the intermittent nature of the process there is inevitably temperature cycling in the vessel, this coupled with invasion into brick matrices leads to disruptive spalling of the refractory lining.
REQUIREMENTS OF THE REFRACTORY LINING
The performance of the working lining is totally governed by the presence of basic slags and iron oxide, demanding a basic refractory lining. The greatest wear occurs in the snorkel legs and bottom of the chamber.
Refractory requirements are high strength, good slag resistance, and high thermal shock resistance.
SNORKEL LEG
Materials based upon direct bonded sintered and fused co-clinker have been shown to give optimum performance.
Snorkel leg materials are supplied with all mating faces diamond ground to tight tolerance to allow construction without the use of mortar joints.
SNORKEL LEG OUTER LINING
Many plants provide the snorkel leg with an outer cast using a high quality refractory concrete. Dyson Refractories prefer to enhance the refractory concrete with metal fibres to increases the resistance to material loss by Thermo-mechanical damage.
WORKING BOTTOM AND LOWER SIDEWALL
The working bottom of the degasser is normally constructed using a soldier course design, with materials based upon fused magnesia chrome clinker. In this area of the vessel, high demand is placed upon the materials ability to resist both erosion and slag attack.
ALLOY CHUTE
The alloy chute demands refractory materials with high resistance to thermal shock and abrasion, and here again fused grain magnesia chrome clinker are preferred.
UPPER VESSEL WORKING LINING
The upper vessel working is less prone to attack by either erosion or slag, but places special demands upon the refractory material. The upper working lining is primarily affected by temperature variation, and to a lesser extent by metal and or slag contact. As such a refractory lining offering high resistance to thermal shock is required.
SAFETY LININGS
All areas of the vessel require a high quality economical safety lining capable of resisting metal at high temperature.
Materials based upon Andalusia have proven to be the most sensible choice