Depending on the amount of carbon in unaltered steels, the two major phases that can be found in the microstructure are ferrite and pearlite. The primary microstructural characteristics that affect the final mechanical properties are:
- Proportion of phases
- Grain size of phases
When there are more alloying elements and heat-treatment steps, there are more phases forming in the steel. This adds to the information that can be collected from a microstructure. Some of the well-known phases studied through metallography are – austenite, martensite, retained austenite, bainite. In some cases, such as weld analysis, there can be a range of phases in the sample requiring careful analysis after polishing and etching.
Cutting
Cutting wheels
For all ferrous alloys, Lamplan provides 4 abrasive cutting wheels depending on the precision necessary and the hardness of the alloy. Lamplan Excellence wheels have coarser abrasives and are thicker. Hence, they are suitable for coarser and faster cutting. The Precision wheels contain finer abrasives and are thinner to provide a smoother surface finish making polishing easier.
| Excellence H1 | |
| Excellence H2 | |
| Red | |
| Blue |
Machine parameters
The rotational speed of the wheel and feed rate are input depending on the alloy being cut. The chart below presents the optimal rotational speed based on the diameter of the cutting wheel.
When there are fragile phases or brittle coatings in the workpiece to be cut, the precision cutting wheels have to be used with a feed rate less than 0.3 mm/s to avoid damage.
Specifically using the automatic Cutlam machines, the burning and deformation can be completely avoided using the motor threshold control. The wheel advance will pause whenever the load on the motor crosses the set limit. This allows the user to change the parameters appropriately to have a burn-free cut.
Generally, steels are resistant to temperatures greater than 180oC. This allows them to be hot mounted in mounting presses. Depending on the functionality necessary the right hot mounting resin can be used.
Bakelite is widely as it is cheap and easy to use. However, we recommend an improved alternative – Lamplan Phenofree resin which is a phenol-free thermosetting resins.
Lamplan Epoxy 633 and 634 resins are used when edge-retention becomes a critical parameter. When delicate coatings such as carbonitride layers are to be analyzed, Epoxy resins are to be used to avoid damage while grinding and polishing.
Grinding and Polishing
The first step is usually coarse grinding to render the samples flat. Cameo Platinium disks are very effective while coarse grinding steels. When precision cutting wheels are used, the Cameo Platinium 2 is optimum. When more material has to be removed to make the samples flat(when the samples are highly skewed), Cameo Platinium 1 is used as it is more aggressive.
Once the samples are flat, they can be pre-polished. Cameo Disk Silver and Gold are recommended with diamond abrasive slurries (Biodiamant Neodia series) for pre-polishing steel alloys.
Once the samples are pre-polished, polishing pads are used with finer diamond slurries to remove all the scratches from pre-polishing. Often, this polishing step is supplemented by a finer polishing step to refine the scratches further to analyse the microstructure are higher magnifications(>200x) or even an SEM(Scanning Electron Microscope).
More general information on Lamplan polishing consumables can be found here:
Two of our most effective polishing methods are presented below with some results they have produced.
Polishing Method 1
For basic microstructural analysis of soft and medium hard steels the following 4-step procedure can be sufficient.
CW – Clockwise / CCW – Counter Clockwise / Head direction: CW for all steps (the procedure is presented of polishing 6 mounted samples of diameter 30 mm)
Microstructures
Below are few microstructures resulting from the aforementioned method
Microstructure of a surface hardened steel etched with Nital 5%- The dark region of the microstructure contains a high carbon pearlite phase with traces of ferrite in the grain boundaries whereas the bright region contains a toughened tempered martensitic phase.
Microstructure of a rolled low carbon with regular but discontinuous bands of peralite.
Macrostructure of weld captured after quick manual polishing with Cameo Platinium 1 and Cameo Silver and etching with Nital 5%
For weld analysis where weld shape and macro-defects are of the only interest which is generally studied by macro-etching, the fourth step can be skipped.
Polishing Method 2
The Method 2 is optimized for having very flat polished surface on harder and stainless steels with hard and brittle coatings.
CW – Clockwise / CCW – Counter Clockwise / Head direction: CW
(the procedure is presented of polishing 6 mounted samples of diameter 30 mm)
Microstructures
Microstructure of rolled and welded Stainless steel 316L (Slightly etched with Kalling’s reagent.
Austenitic grains in Stainless steel 316L from Additive Manufacturing (etched with Kalling’s reagent).
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