Metallography Recommandations
Recommendations for the preparation of metallographic samples
CUTTING-OFF
The choice of cut-off wheel is primordial. It depends on how hard and ductile the material to be cut is as well as on the fragility of the coatings making up the sample.
How to choose a cut-off wheel?
Choose a wheel :
- with A1203 abrasive for ferrous materials,
- with Sic for non ferrous materials.
The harder the substance to be cut, the softer the mounted point binder must be.
For example, to carry out precision cutting on treated steel with a hardness of 50 HRc, we recommend the LAM PLAN BLEU cut-off wheel, for which the bakelite binder is relatively soft.
Recommendations for use
ICheck that the lubrication is correct. This prevents any risk of overheating. Indeed, this phenomenon could alter the structure of the material. LAM PLAN recommends 721 sectioning fluid for all metallographic cutting.
EMBEDDING
The purpose of embedding a sample in a resin is to limit the edge effects. In addition, this operation will allow you :
- During manual polishing: to facilitate the grip on the sample.
- During automatic polishing: to gather a number of samples in the sample holder in optimum tightening conditions.
Recommendations for use
ABefore any embedding procedure, remove any grease from the samples so that the resin adheres to the contours of the part.
Ensure that that there is no burr from the cut-off process. If necessary, deburr the corners of the sample using abrasive paper
How to choose a resin?
Chose a resin whose hardness is close to that of the material to be embedded.
There are two types of embedding resin :
Cold resins :
Recommended for the preparation of individual samples. Requires only investment in the resins and the embedding moulds.
Hot resins :
For laboratories which prepare a large number of samples. With this process, the embedding size is uniform. The resin price is reasonable but an embedding machine must be purchased.
Procedure for embedding with cold resins
Place the sample in the mould. Mix the resin components respecting the doses stipulated in the technical data sheet. Mix gently to avoid creating air bubbles.
The embedding time is generally close to fifteen minutes, except for RESINA CRISTAL (epoxy resin) which takes around 8 hours.
Procedure for embedding with hot resins
The sample is placed in the lower piston of the embedding press. The embedding is made possible by the pressurisation of the resin and the sample as well as by the heating process. This is followed by a cooling step using water to finalise the hardening of the resin. The quantity of resin must be optimised. It depends on the volume of the part to be embedded.
POLISHING
Polishing is defined by the use of increasingly fine abrasive particles making it possible to obtain an ideal surface for microscopic observation.
This procedure can be divided into two stages: PRE-POLISHING and POLISHING
PRE-POLISHING
The objective of this stage is to obtain a flat surface on samples.
So-called “fixed abrasive” aids are used for this :
- for soft materials, abrasive paper made from silicon carbide
- for other materials, the CAMEO PLATINIUM disc range.
Following the pre-polishing, there is a transitory work phase, carried out with SILVER or GOLD CAMEO DISKs. These are used without added abrasives, but guarantee a very good flatness. A diamond liquid is therefore used on these two discs.
POLISHING
The objective of the polishing is to eliminate any damages left by the preceding stages.
We advise you to use a diamond abrasive, gradually decreasing in particle size, which will be sprayed onto increasingly soft polishing pads (LAM 4x4 (woven) or MM 431 (felt) until perfect polishing is obtained.
It should be noted that for certain materials, colloidal abrasive will be used rather than diamond (for very ductile materials)
