Project Info

Liquid Metal Etching to Reveal Steel Microstructures

Emmanuel De Moor
edemoor@mines.edu
Owen Hildreth
ohildreth@mines.edu
Steel etching and prior austenite grain size measurements are important to microstructural characterization in a variety of applications such as line pipe steels, cryogenic steels, high toughness steels and challenges frequently arise with effective etching of grain boundaries. Further, certain chemicals helpful in revealing grain boundaries show Environmental Health and Safety concerns and have been discontinued in metallography laboratories. Low melting metals and alloys which can be used in a lab by e.g. hot plate melting may be a viable substitute and will be explored in the present project. The proposed approach will be to investigate low temperature melting metals to wet austenite grain boundaries and assess whether liquid metal etching can be used instead of organic chemicals etching. Liquid metal embrittlement is known to occur for steel substrates in contact with e.g. liquid Zinc where wetting and attack of PAG predominantly over the grain interiors locally weakens the grain boundary strength and cohesion. The wetting mechanism will be explored here to evaluate whether it can be employed towards etching and revealing the boundaries for PAGS measurements.
the research work will be conducted in the physical metallurgy laboratory and mechanical engineering departments and meetings will be conducted with professors De Moor and Hildreth and their group members.

More Information:

Grand Challenge: Not applicable
Prior austenite grain size (PAGS) measurements are frequently challenging due to weak etching response to reveal the boundaries. Further, some of the etchants contain chemicals that could be explosive under certain circumstances, notably dry/crystallized picric acid is explosive and Nital etchant preparation needs to be done carefully to avoid strong heat evolution that could lead to boiling and explosion. These environmental health and safety (EHS) hazards have led some laboratories to ban certain of these etching substances. The present approach proposes to investigate low temperature melting metals to wet austenite grain boundaries and assess whether liquid metal etching can be used instead of organic chemicals etching. Liquid metal embrittlement is known to occur for steel substrates in contact with e.g. liquid Zinc where wetting and attack of PAG predominantly over the grain interiors locally weakens the grain boundary strength and cohesion. The wetting mechanism will be explored here to evaluate whether it can be employed towards etching and revealing the boundaries for PAGS measurements. Anticipated challenges will be to identify metals and alloys with a low melting temperature (e.g. melt on a hot plate) that have a good environmental health and safety profile e.g. are not toxic, non-hazardous to dispose of, result in effective wetting with carbon steels, show preferred wetting of prior austenite grain boundaries over e.g. lath boundaries, etc. Some potential candidates may be tin, gallium, lithium, bismuth, and their alloys. Various wetting and (back) polishing iterations can be investigated in addition to a variety of steel grades in particular with very low residual contents (notably phosphor which sometimes is intentionally added to laboratory heats to assist in PAG etching). It is anticipated that energy dispersive spectroscopy (EDS) line scanning can help with imaging and determining the PAG boundary on the etched sample surface. Light optical metallography (LOM) can also be pursued should there be a satisfactory etching response from the liquid metal wetting. Further heat tinting may be helpful to assist in LOM as oxide growth on the low melting metal might be very different from the oxidation of the steel matrix. The work could be further extended to include sessile drop tests to detail the liquid metal wetting.

Primary Contacts:

Emmanuel De Moor and Owen Hildreth further the undergraduate researcher will work closely with graduate students from both Professors' groups

Student Preparation

Qualifications

Student must have taken MTGN 348 and MTGN 202.

TIME COMMITMENT (HRS/WK)

4

SKILLS/TECHNIQUES GAINED

physical metallurgy laboratory skills, acquired during MTGN 348

MENTORING PLAN

frequent meetings will be organized to review results and progress and discuss next steps

Preferred Student Status

Junior
Senior
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