Refractories for a Reheating Furnace

Refractories for a Reheating Furnace

Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. They are the materials which are resistant to heat and exposure to different degrees of mechanical stress and strain, thermal stress and strain, corrosion/erosion from solids, liquids and gases, gas diffusion, and mechanical abrasion at various temperatures. In simplified language, refractories are considered to be materials of construction which are able to withstand high temperatures.

The general requirements from the refractories for are (i) ability to withstand high temperatures and trap heat within a limited area such as a reheating furnace, (ii) ability to withstand sudden changes of temperature, (iii) ability to withstand load at service conditions, (iv) ability to withstand chemical and abrasive action of the materials such as liquid metal, liquid slag, and hot gases etc. coming in contact with the refractories, (v) ability to resist contamination of the material such as scale etc. with which it comes into contact, (vi) ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling, (vii) ability to conserve heat, (viii) ability to withstand load and abrasive forces, and (ix) to have low coefficient of thermal expansion.

Properties of the refractories can be classified to resist four types of service stresses namely (i) chemical, (ii) mechanical, (iii) thermal, and (iv) thermo-technical. A suitable selection of the refractories for the lining of the reheating furnace can only be made with an accurate knowledge of the refractory properties and the stresses on the refractories during service. The relationship between service stresses and important properties of the refractories are at Tab 1. 

Tab 1 Relationship between type of stress and refractory property
Sl.No. Type of stress Important refractory property
1 Chemical Chemical composition
  Mineralogical composition and crystal formation
  Pore size distribution and types of pores
  Gas permeability
  Resistance to slag, glass melts, gasses, and vapours
2 Mechanical Crushing strength
  Abrasion resistance
  Cold modulus of rupture and deformation modulus
  Porosity and Density
3 Thermal Pyrometric cone equivalent
  Refractoriness under load
  Refractoriness under load (differential)
  Thermal expansion under load (creep)
  Hot modulus of rupture
  Thermal expansion
  Reheat change (after shrinkage and after expansion)
  Thermal shock resistance
4 Thermo-technical Thermal conductivity
  Specific heat
  Bulk density
  Thermal capacity and temperature conductivity

The main function of a reheating furnace is to raise the temperature of the semi-finished steels (billets, blooms, slabs or rounds) typically to temperatures between 1100 deg C and 1250 deg C, until it is plastic enough to be rolled to the desired section, size or shape in the hot rolling mill. The reheating furnace must also meet specific requirements and objectives in terms of the heating rates for metallurgical and productivity reasons. In the reheating furnace, there is a continuous flow of material which is heated to the desired temperature as it travels through the furnace.

The reheating furnace mainly consist of (i) a chamber constructed of refractory and insulating materials for retaining heat at the high operating temperatures, (ii) a furnace hearth for supporting and carrying the steel, and (iii) a set of burners which may use solid, liquid, or gaseous fuel for raising and maintaining the temperature in the chamber, and (iv) a set of doors for charging and discharging of steel materials as well as for inspection of the furnace inside during its operation.

Refractories play a key role in containing energy consumption and in improving the efficiency of the reheating furnace. The life of the refractory lining and the efficiency of the reheating furnace depend largely upon the suitable choice, the quality, and the correct installation of the refractory materials in the furnace. In fact, a well-established relationship exists between all these factors.

In a reheating furnace (especially in a pusher type furnace) the severity of operational conditions increases in the direction of steel material movement and maximizes in soaking zone. In the side-discharge units, the ejection out of the steel charge material introduces additional abrasion on the paving. The operational hazards in the heating and soaking zone include (i) impact which follows drop of piled up charge, (ii) frequent cooling for maintenance resulting In thermal stresses, and (iii) excessive abrasion. Mechanical abrasion accentuated by the presence of scales at a high temperature, is another problematic issue. The amount and the nature of the scale formed are also of vital importance in determining the life of the heating and soaking zone hearth refractories.

The refractories for the reheating furnace are to be able to withstand the action of abrasive solids, and gases at high temperatures.  Due to the various combinations of operating conditions in which the refractories are used in the furnace it becomes a necessity that a range of refractory materials with different properties are used.

The purpose of the refractory lining in a reheating furnace is to prevent the heat loss from the furnace and to ensure attainment of the skin temperature of furnace close to ambient temperature so as to achieve targeted specific fuel consumption. Refractories and insulation have a major role in minimizing heat losses from the side walls and roof.

Reheating furnaces for rolling mills are traditionally lined with hard (dense) refractory such as bricks, castable, gunite, shotcrete, etc. Such dense refractory do not fare well when exposed to intense and continuous flame impingement, thermal cycling and alkali attack. Routine processes like ramp-ups, ramp-downs, and opening and shutting the furnace doors to charge or remove product can cause severe thermal shock and damage to the refractory lining.

Once damaged, hard-refractory repair and patchwork is cumbersome, time-consuming, labour intensive, and expensive. Reheating furnaces often have to be shut down for days because the refractory first have to be completely cooled, then the repairs to be made followed by the and specific dry-out schedules.  Finally, the reheating furnace is very slowly ramped back up to the operating temperature (sudden ramp-ups create thermal shock). A large portion of this energy is absorbed by the refractory lining, and, due to the high thermal conductivity of dense refractories, a large percentage of this heat is conducted through to the steel casing of the reheating furnace. Furthermore, traditional linings of hard refractory are heavy, putting undue strain on the mechanical components and structure of equipment. The resulting wear and tear of these mechanical components further adds to maintenance problems as well as the costs.

The reheating furnace is mainly consists of three zones namely (i) preheating zone, (ii) heating zone, and (iii) soaking zone. Furnace chamber consists of two side walls, two end walls one at soaking zone side and the other at preheating zone side, a hearth and a roof. The width of the furnace is required to have sufficient dimension to accommodate the length of the steel material to be heated along with adequate clearance on either side so that the steel material does not rub the side walls during its forward movement.

Since the atmospheric conditions within the reheating furnace existing in three zones differ widely, there is necessity to have different types of refractories for these zones. Similarly, the qualities of refractories needed for hearth and roof are to meet the operational requirements of the area.

The refractory lining of a reheating furnace can be done in several alternative ways. The alternative solutions which can be adopted for a reheating furnace can include (i) the classical brick lining, or (ii) combination of brick lining with monolithic refractory materials. The installation techniques can be different depending on the refractory material used, i.e. laying, casting of monolithics, ramming, gunning, or shotcreting. In some reheating furnaces, precast refractory slabs are also used.

The objectives of the refractory lining solutions for a reheating furnace (Fig 1) are (i) to meet the requirements imposed by the operating conditions, (ii) to meet requirements of the adopted technology for the refractory solutions,  (iii) to achieve the targeted life of the refractory lining, (iv) to meet the targeted time frame for the refractory lining , (v) to meet the environmental conditions imposed by the type of reheating furnace, and (vi) to ensure that the refractory solution adopted is within budget available for the lining. These six objectives influence the engineering solution for the refractory lining.

Fig 1 Objectives for the refractory lining solutions for a reheating furnace

Sidewalls of the reheating furnace are required to be lined with multi-layer of refractories. Towards the furnace shell insulating refractories of needed qualities and in required thicknesses are to be used to ensure that the heat of the furnace is not transmitted to the furnace shell and the temperature of the furnace shell is not higher than the ambient temperature of the area. The refractories of the side wall needed towards the steel material side needs high refractoriness, high resistance to thermal spalling, high resistance to thermal shock, low permanent linear change, good abrasion resistance and good resistance against chemical attack. In the three zones of the reheating furnace different grades of alumina refractories are normally used for the inner layers of the side walls.

For the end walls also multi-layer lining of refractories is used. Lining towards the outer atmosphere consists of insulating refractories while for the inner layers towards the furnace different quality of refractories are suitable for the preheating zone side and soaking zone side end walls. The preheating zone side end wall refractories need to have good refractoriness as well as good abrasion resistance while the soaking zone side end wall refractories need to have high refractoriness, good thermal spalling characteristics and good emissivity to radiate heat. Presently monolithic refractories are preferred for this application.

The hearth of the reheating furnace continuously faces high temperature abrasion as well as build-up of scale.  The refractories chosen for this area is to withstand these two adverse conditions. It should also have volumetric stability. Low cement medium to high alumina castables with stainless steel reinforcement are preferred solutions for the lining of the hearth. In the soaking zone, where steel material is pushed out or taken out by an ejector, lining with fused cast high alumina blocks are preferred since these blocks have got very high resistance to abrasion. Lining with basic castables also provides good performance for the hearth refractories.

In case of refractories for the roof of the reheating furnace, the choice of refractories depends on the type of roof the furnace has. Roof refractories need to have good thermal spalling resistance, good refractoriness as well as good abrasion resistance. Since roof refractories are required to radiate heat to the surface of the steel being heated, it should have good emissivity. Presently roofs cast with low cement alumina castable refractories are preferred over brick lined alumina refractories. Behind these refractories there is necessity to install insulation refractories of adequate thickness to prevent the loss of heat through roof.

Burner blocks are normally made of castable refractories. For burner blocks normally castings are made, and then they are dried and fired. In case of furnace doors also castable based refractory solutions are preferred for better life of door refractories.