BS ISO 18203:2016 pdf download – Steel — Determination of the thickness of surface-hardened layers
7.2 Preparation of the surface to be examined Cut specimens for preparation with the gentlest possible technique, using only laboratory equipment. Mount specimens with a high-quality resin designed to provide excellent edge retention. Grind and polish using a proper sequence of abrasives to remove the damage from sectioning and grinding while maintaining edge retention. Exercise care to ensure that the region of interest is not altered by the sample preparation. The lower the test load in hardness testing, the higher the preparation quality must be. After polishing, etch the specimen with an appropriate solution to determine if the preparation was adequate and to see if there is a modification to the surface. Repeat one or more of the final preparation steps, if the quality is inadequate. Carefully clean off any films or residue on the surface. Do not touch the surface with fingers. For microindentation hardness testing, it is recommended to test an as-polished, non-etched surface. For the micrographic method, use the appropriate etch to reveal and discriminate between the different constituents present.
8 Method of measurement
8.1 Hardness testing method Make hardness impressions along one or more parallel lines normal to the surface and within a band of width, W, of 1,5 mm (see Figure 3). The distance between these lines shall meet the requirement of ISO 6507-1. The distance, Δd, separating two adjacent impressions shall be not less than three times their diagonal (see Figure 3). The difference between the successive distances of each impression from the surface (e.g. d 2 – d 1 ) shall not exceed 0,1 mm and the cumulative distances from the surface shall be measured to an accuracy of ±25 μm.
The diagonals of the impression shall be measured with the accuracy specified in ISO 6507-2. The centre of the first impression shall be at a distance, d 1 , from the surface of at least 2,5 times its diagonal.
Tests using Vickers or Knoop indents are performed at forces from 0,980 7 N to 9,807 N. Measurements of the impressions shall be made with a high quality light optical microscope and proper illumination, with or without a camera system (and possible by image analysis), at a high enough magnification so that the diagonals are between 25 % and 75 % of the eyepiece or screen width or height and the indent tips can be focused without distortion. NOTE Due to the spacing of indents shown in Figure 3, test forces of 0,980 7 N to 2,942 N are normally used for this work. Make the measurements on the prepared surface in two or more bands, the location of which shall be agreed between the parties concerned, and for each band plot the results in order to obtain the curve representing the variations in hardness as a function of distance from the surface.
8.2 Microscopic methods
8.2.1 Total thickness of surface hardening depth (THD) Examine the surface of the hardened part with the specimen etched with the best reagent for discrimination between the constituents present. Scan parallel to the surface at low magnification to get an overall impression of the uniformity of the total thickness of surface hardening layer. If it is non-uniform, measure the depths to what appears to be the minimum and maximum total thickness of surface layer. If the total thickness of surface hardening layer appears to be reasonably uniform, make at least five measurements of the total thickness of surface hardening layer (spaced apart from each other) and determine the average depth. NOTE 1 For some specimens, it may be possible to visually estimate the location where the microstructure is 50 % martensite and 50 % other constituents, such as ferrite, pearlite and/or bainite. If so, measure the depth to 50 % martensite in at least five locations (spaced apart from each other) and determine the average hardened depth. NOTE 2 Microstructural measurement of the surface hardening depth, that is, the depth to some % of martensite (such as 80 %), may be quite difficult to do without making quantitative measurements of the microstructure by lineal analysis on lines parallel to the surface at known depths.
8.2.2 Compound layer thickness (CLT) The compound layer thickness shall be determined from a sample cut perpendicular to the surface layer. When preparing the sample for the metallographic evaluation, special caution is needed to avoid damaging the layer. The layer can be protected during grinding and polishing, for example with a mechanical screen or with an electrolytic deposit. The determination should be carried out at a magnification that displays the layer as 1/3 to 2/3 of the view field. It is recommended to make 10 measurements within 5 mm band and average (see Annex B). [5]