In general, the evaluation of steel cleanliness consists of the following steps.

  • Sampling and specimen preparation
    Sectioning, hardening, grinding, mounting (not mandatory), and several polishing steps are performed. Usually, n-representative locations of one lot are cut out. The specimen size and the location are dependent on the steel product to be analyzed. Note that the polished surface must be parallel to the main deformation direction, otherwise the generated measurement results are not meaningful.
  • Specimen acquisition and analysis
    Images of the prepared steel specimen are acquired, usually six test objects, originating from one lot. The inclusions are classified by size and type. Inclusions are rated based on standard specific test methods, usually several standards with the corresponding methods are applied.
    Note: The main deformation direction must be considered for a correct placement of the individual specimen on the microscope stage. In doing so, the elongated inclusions are oriented either horizontal or vertical related to the x-axis of the stage. Otherwise the generated measurement results are not meaningful.
  • Inspection and documentation of results

General Workflow of the Operator

Step

Operator task

1

Place specimen (often up to six) on the scanning stage of the microscope using a stage insert with a steel specimen holder.
Note: Consideration of the main deformation direction is required.

2

Select job template for NMI Analysis and executes job.

3

Fill in data in the overarching and specimen form.

4

Optional: Adjust acquisition pre-settings.

6

Inspect the results using the interactive result views and the report document view.

7

Optional: Modify results using the interactive views, (Global Result view and Field Based Inspection), i.e. changing the inclusion type or excluding artifacts.

8

Perform the final inspection of the test methods results in the Report view.

9

Optional: Re-open and/or export job results via Browse Job Results.

Best Practice

General remarks for image acquisition:

  • Perform specimen acquisition in brightfield contrast, usually with a total magnification of 100x.
  • Acquire specimen with oxides and sulfides in BW mode. Appropriate job template: Non-Metallic Inclusion Analysis.
  • For nitrides, the image must be acquired in color mode. Appropriate job template: Non-Metallic Inclusion Analysis Nitrides.
  • Adjust image brightness and exposure time so that also the light gray inclusions are well contrasted.

Best practice for manual thresholding setting (segmentation)

  • Use the auto threshold as base setting.
  • Select a specimen area which shows both Oxides and Sulfides.
  • Zoom in using the mouse wheel until finer grey inclusions are clearly visible.
  • Adjust the Tolerance Upper Sulfide Threshold to avoid artifacts from potential matrix background detection.
  • Toggle with Oxide, Sulfide and All buttons to inspect the segmentation result; this at various positions.

If the result of thresholding is as expected, no further action is needed, otherwise adjust the thresholds for oxides and sulfides manually.

The upper threshold value for the oxides determines automatically the lower threshold value for sulfides.

For this purpose, start adjusting the oxide threshold range and continue afterwards with setting of the upper sulfide threshold value. Fine tuning of the segmentation results is possible with the Tolerance Sulfide Range.

For more information, see Basics of Running a Job.