A heat-resistant assembly to shield boiler tubes which does not entail a thick heat-resistant block. This structure would be interposed between the boiler tubes and the combustion gases. Such a structure would be distinguished by the fact that it comprises heat-resistant block 16, itself composed of curved portion 16a, whose inner surface at one point comes in contact with the aforesaid boiler tube 11, and connecting portions 16b, and the aforesaid boiler tube assembly 12. The interlocking attachment structure which interlockingly secures these two components together in such a way that the block can be mounted or removed comprises recess 17, 58 or 68 and tongue 18, 59 or 69.