>I was flattening the backs and sharpening several sets of Marples chisels when I noticed one kept forming an unusually large burr that kept breaking off. I tried the chisel on some pine and the chisel edge caved in like it was made of cheese. To be fair all 19 of the other chisels that I sharpened sliced through the pine like a hot knife through butter. My guess is that this chisel was not heat treated.
>You could take it back. Most vendors will make good on bad tools. If you don't want to do that, you could probably heat treat it yourself and make it serviceable.