Fracture of a Metal-Injection Molded Steel Firearm Component
The failed component met the chemical requirements for MIM-4605, a metal injection molded low alloy steel, which is similar in composition to an AISI/SAE wrought 4XXX-series molybdenum (low alloy) steel like AISI/SAE 4140 steel. The apparent hardness of the part (44 HRC) was slightly low compared to the typical apparent hardness value for MIM-4605 in the quench and tempered condition (48 HRC), and significantly lower than typical apparent hardness value for MIM-4605 in the hot-isostatically-pressed quenched and tempered condition (54 HRC). Based on the test data, the part was made of MIM-4605 in the quench and tempered condition, rather than the hot-isostatically-pressed quenched and tempered condition.
Failure analysis was conducted to determine the failure mechanism and contributing factors. In addition, OEM and exemplar components machined from AISI/SAE 4140 steel bar stock were impact tested and examined for comparison to the failed MIM component. Visual examination of the subject MIM component fracture surface revealed dark and bright metal regions. The dark metal regions were indicative of either oxidation or blood residue. SEM examination of the bright metal regions revealed a mixed fracture surface morphology. Near the edges of the fracture surface, the morphology was microvoid coalescence (aka: dimpled rupture). The central portion of the fracture surface displayed an intergranular morphology. Numerous circular or spherical pores ranging up to approximately 10 microns in diameter were present across the entire fracture surface. Examination of impact tested OEM components revealed the same fracture surface appearance, whereas impact testing of wrought metal parts resulted in mixed fracture morphology with dimpled rupture along the edges and cleavage in the central region.