In a masonry fireplace, firebrick is used to construct the firebox. Refractory brick panels line the firebox of a prefabricated metal fireplace. In both cases, the bricks and fireclay mortar are designed to withstand extreme temperatures. However, over time, the brick, mortar, or panels can crack and crumble, creating a serious fire hazard.
Here’s what you need to do to address these problems:
If a brick in a masonry fireplace cracks, you need to patch it. If the brick is crumbling, have it replaced with a new firebrick embedded in refractory mortar.
If the mortar joints in a firebox are crumbling, chisel out the old mortar and replace it with new mortar. This process, known as tuckpointing, is the same one you follow to replace or repair mortar joints in any brick structure. The only difference is that in a firebox, you must use refractory mortar, which is specially designed to withstand extreme temperatures.
If the integrity of the majority of the firebrick and mortar in the firebox is in question, have a qualified chimney sweep or masonry contractor inspect it. If replacement is in order, the job is best left to a pro.
If a panel of a prefabricated metal fireplace develops extensive cracks or is beginning to crumble, replace it with a new panel. This is a job that most do-it-yourselfers can handle: Simply remove the old panel by unscrewing the screws that hold it in place and install the new panel; it should fit snuggly against the adjoining panels. When replacing a rear panel, remove the side panels first. Refractory mortar is not generally needed, because the corners are designed to fit snuggly against one another.
Tip : To make the job of finding a replacement panel easy, jot down the fireplace make and model number. (You can find these numbers on a metal plate just inside the opening of the firebox.) Give this information to the manufacturer or an installing dealer. Although replacement panels are often a stock item, a special order may be required, which can take from a few days to a couple of weeks. Don’t use the fireplace until a full and final repair has been made.
Tip : Repairing minor cracks and mortar joints in prefabricated metal fireplaces is essentially the same as with a brick firebox.
Source : http://www.dummies.com/home-garden/exteriors/brick-repair/how-to-fix-fireplace-cracks/