The paper presents a theoretical-experimental analysis of possible causes for fracture of a responsible machine part. During exploitation the part is exposed to short, but high dynamic loading and pressure. Because the original material (low-alloyed tool steel) of the broken part is unavailable, the manufacturer is forced to use the substitute material of similar properties. Two massive steel blocks, accompanied by necessary certificates of material quality have been delivered by a very renowned European steel manufacturer. The manufacturer of the final part – the user has also performed the prescribed control of the mechanically– and heat-treated part. Despite all this, the part has broken into several pieces during the very first test run. Then the pieces of broken part were tested. At first, the chemical composition of the base material was investigated and then its mechanical-technological and physical-metallurgical properties. Three possible causes of fracture of the working part have been analysed: structure design solution, error during machining or heat treatment and the disagreement of the declared and the real chemical composition of the steel.
The objective is to point out caution to those who are forced, due to the lack of supply, to substitute the steel type of the manufactured responsible parts, many times verified in operation, with new steel material. Besides standard chemical, mechanical and metallographic investigations, additional tests need to be performed – destructive and non-destructive (e.g. test for non-metallic inclusions, their shape, size and distribution), what in this case has not been done.
Keywords: fracture, theoretical analysis, experimental investigations, non-metallic inclusions