How to leave expansion joints during the construction of refractory castables?

During the construction of refractory castables, how to leave expansion joints mainly depends on the following 5 key points:

01 Contraction and expansion of refractory castables

During the first heating process, refractory castables will shrink twice in volume when dehydrated at 50℃～200℃ and sintered at 900℃～1000℃. In other temperature ranges, heating will cause the castable to expand. After the first heating, the castable generally no longer shrinks.

In order to prevent the stress caused by volume change from damaging the castable, the lining must be divided into small areas with a diagonal size of no more than 1.5m, and the lining must be cast in sections and leave gaps for expansion and contraction at the boundary of each casting area.

02 Influence range of expansion joints

The size of the gap should ensure the free expansion of the castables in each section, and the expansion joints in the right position can also become control joints. In the high temperature section (>600℃), the width of the expansion joint should be controlled at 3～4mm according to the spacing of the expansion joints. The expansion joint and control joint are set on the plane about 200mm away from the convex corner, but not in the convex corner and the pointed cone. The cracking tendency of the castables on both sides of the expansion joint and the control joint is relatively large, and the density of the nails can be appropriately increased.

03 Width control of expansion joints

The width of the expansion joint is related to the working temperature and the length of the control line. When the temperature is high and the control length is long, the expansion joint can be appropriately widened. In areas with relatively low temperatures (<400℃), the castable can be used directly on the equipment shell without insulation. Under such conditions, inserting 2mm thick cardboard or plastic film every 1.5m can meet the requirements of the expansion space.

04 Optimization of the location of the expansion joint

When determining the expansion position, the reasonable arrangement of the vibration process should be considered at the same time. The vibration of the castable on one side of the gap should not affect the castable that has been poured and vibrated and has begun to harden initially. The location of the expansion joint avoids the stress-bearing parts, the holes of the furnace frame and the lining. In areas with complex shapes, such as corners, convexities, etc., expansion joints adapted to their surfaces should be set at convex angles and curved surfaces with small curvature radius. When two casting surfaces intersect to form a concave angle, an L-shaped expansion joint should be set at the concave angle.

The lining diagram should show the location and size of the expansion joints in the main parts. If the working temperature of the casting material exceeds 1200℃, an expansion joint with a width of more than 3mm should be set, and a ceramic fiber blanket should be inserted into it.

05 Setting of control joints

Since the casting material will cause cracking due to shrinkage, it is necessary to set a control joint at about 200mm at the convex corner, so that the shrinkage cracking can occur in a predetermined and safer position. The setting of the control joint can be filled with paper, paraffin or thin wood in the predetermined gap. Under high temperature conditions, flammable or flowing materials are cast. After these flammable materials are burned or lost, control joints are left.

The control joint must be set in the middle of adjacent nails, and the distance from the nails on both sides is roughly equal. It should be set on continuous curved surfaces, conical surfaces and pipes, and should be set on flat and curved surfaces where the risk of shrinkage cannot be controlled. It should be avoided to set it in areas where expansion joints cannot be set, such as setting expansion joints on small casting surfaces and convex corners.