Cavitation Erosion of Cast Irons in Engine Coolants
Interactions and Damage Mechanisms
Time: Fri 2025-06-13 10.00
Location: D3 , Lindstetsvägen 5
Video link: https://kth-se.zoom.us/j/69078439839
Language: English
Subject area: Materials Science and Engineering
Doctoral student: Marcio Freitas de Abreu , Materialvetenskap
Opponent: Professor Emeritus Braham Prakash, Luleå tekniska universitet
Supervisor: Professor Peter Hedström, Hultgren Laboratoriet för Materialkarakterisering; Docent Jessica Elfsberg, Scania CV AB
Abstract
In the heavy-duty automotive industry, cavitation erosion is a recurring issue for the components of the engine cooling system. It is caused by the repeated implosion of bubbles in a liquid, such as the coolant, and can seriously damage exposed components through surface roughening, pitting and debris generation. Severe wear is common in cylinder liners and retarder pumps, for example, and leads to high maintenance costs, safety risks and vehicle downtime. The research presented here was therefore elaborated to supply missing knowledge on the damage mechanisms of engine component materials, mostly cast irons, and what metallurgical factors affect their performance. Some of their interactions with different coolant formulations and with operational parameters that emulate the cooling system environment were also investigated.
Using an ultrasonic test rig based in ASTM G32, samples of several cast iron grades were exposed to various coolant mixtures. Different test setups with the direct and indirect methods, amplitudes and temperatures were also investigated. The analyses comprised sample weight change and surface damage documentation by scanning electron microscopy.
The indirect test method and lower amplitudes led to much lower mass loss. Higher glycol concentrations and the presence of inhibitors lead to less damage. Surprisingly, however, used coolants collected from serviced trucks were less aggressive than their fresh counterparts. A boron nitride suspension in fresh coolant led to an outstanding reduction in mass loss, potentially granting long-lasting protection by dissipating the impact load. This finding is promising for the development of unconventional solutions and for a deeper understanding of bubble dynamics in complex environments.
Analyses of damage initiation at very early test times suggest that most of the impact load originates in the cavitation cloud as pressure waves, imparting stresses on the whole surface, whereas damage from individual bubble implosions may be rare and less important.
For the cast irons, hardness and microstructure are, together, a strong predictor of cavitation resistance. Compacted and lamellar graphite are detrimental graphite forms; ferrite also lowers cavitation resistance; pearlite, steadite and ausferrite are beneficial. Damage evolution consists of graphite removal, matrix chipping around voids and pit expansion, with contributions from surface and subsurface cracks. With no graphite in their structure, steels usually perform significantly better than irons. Fatigue cracking was found to be the predominant fracture mechanism in milder cavitation loads. These findings open further possibilities for optimizing materials for cavitation-intensive applications in prolonged exposures for which other solutions, such as coatings, are not possible.