Cracking in a multiple gas-nitrided H13 aluminum extrusion mandrel

A failure analysis was carried out to investigate the root cause of cracking in a mandrel used in a local aluminum extrusion plant. The tool was made of AISI H13 and received multiple gas-nitriding case hardening during its service life. The scraped mandrel showed several microcracks in the filleted bridges. Izod impact tests were conducted over a range of temperatures (23-550. °C) for un-nitrided and single nitrided H13 specimens to examine the possible susceptibility of the tool steel to temper embrittlement. The effect of single and multiple nitriding cycles on the surface fracture resistance of H13 specimens was characterized using Vickers indentation test. The impact test results showed a low impact-energy regime at temperatures at or below 350. °C and a high impact-energy regime at higher temperatures. Temper embrittlement was ruled out as a likely cause of cracking since the extrusion temperature of 425. °C is within the high impact-energy regime. Vickers indentations typically showed sharp cracks initiated from the corners of the indentation for a sample taken from the mandrel. No cracks emanated from indentations applied on un-nitrided and single-nitrided specimens. Crack initiation in the mandrel is primarily attributed to multiple gas-nitriding and the subsequent thickening of the brittle nitrided layer which promoted brittle crack initiation in the mandrel bridges under the applied extrusion pressure.