Sample Imaging


Stereomicroscopy is a variation of optical microscopy specific for low level magnification in the range of 5X. For Apollo-Clad, stereomicroscopy is routinely used to evaluate the quality of the surfaces of our coatings. This form of imaging is also used for level magnification images of cross sections of laser welds to observe penetration and degree of fusion as well as cross sections of laser clad coatings to observe the quality of metallurgical bonding.

Figure 1: (left): Photo of a laser clad test piece under the stereomicroscope; (top right): Photo of a nickel-tungsten carbide laser clad overlay; (bottom right): High magnification view of the surface roughness of the same laser clad overlay. Scale markings for both photos are in millimeters

Optical Microscopy

For high magnification imaging, Apollo is equipped with an optical microscope capable of 1000X magnification. At these levels, features at the sub millimeter scale such as precipitates, microstructural phases, grain size, inclusions, and porosity can be observed and measured. Apollo’s analysis capabilities include differential interference contrast (DIC) imaging, which allows for enhanced image contrast for superior photomicrographs. Shown below are some sample micrographs of dissimilar material laser clad layers applied to steel and nickel-based materials.

Figure 2: (top ;eft): Nickel-tungsten carbide laser clad overlay applied to 4145 steel; (Top Right):  Laser clad stellite layer onto 4140 steel; (Bottom Left): Nickel-tungsten carbide laser clad overlay applied to high chromium white cast iron with a buffer layer; (Bottom Right): Inconel 625 laser clad layer applied to Inconel 718 base material

Hardness Testing

Apollo-Clad offers two types of hardness testing: macro indentation with Rockwell hardness testing and Vickers microhardness testing. Both work on the principle of resisting permanent deformation of an indenter under an applied load: softer materials result in a larger indent (reduced resistance to the load for a standard period of time) and harder materials produce smaller indents (higher resistance). These two types of hardness testing are described below:

Rockwell Hardness Testing

Rockwell hardness testing is the industry standard for measuring representative bulk hardness. From a metallurgical perspective, Rockwell hardness testing occurs over a relatively large area capturing a large scale or “macro” region of the sample. A Rockwell indentation combines the resistance of many grains, phases, and/or secondary particles in the sample revealing their overall contribution to the bulk hardness. The test is performed on a flat, polished surface, and a minimum of 3 points are taken to report an average value. Apollo’s machine uses both the “B” Scale (HRB) to test softer materials along with the higher hardness “C” scale to evaluate our wear and corrosion resistant products produced through laser cladding and laser heat treating.

Figure 3: (left) Apollo’s Rockwell Tester; (center) Indenter load applied to a polished steel surface (HRC scale); (right) Photomicrograph of a Rockwell hardness indent in carbon steel

Vickers Microhardness Testing

Microhardness testing is an essential tool for measuring hardness of individual, microscopic aspects of a material microstructure such as individual grains or reinforcing particles. Tests are typically gram force (gf) loads and the resulting indents from the test must be measured using a microscope. Vickers microhardness testing is also used to map hardness transitions from a treated surface or through dissimilar materials. The size of microstructurally susceptible regions such as the heat affected zone (HAZ) of a weld can be mapped by regular spacing of microhardness indents. This procedure is a standard for laser cladding, laser heat treating and laser welding processes revealing typical heat affected zone sizes of less than a millimeter and superior hardness of surface layers from the fast cooling rates of laser processes.

It is typical that a sample hardness evaluation will include both Vickers microhardness testing for a surface treatment and Rockwell hardness testing to evaluate the average base material hardness. View a sample report for a typical case depth analysis >

Figure 4: (top) Apollo’s Vickers microhardness testing machine and computer; (center) Zoomed in view of a microhardness test with a 200gf load applied using a diamond tipped indenter; (right) Optical micrograph of a Vickers microhardness test point in an etched nickel based material. The diamond shaped indent is on the same scale as the surrounding grains and is 25 times smaller than the Rockwell indent in Figure 3.
Figure 5: (left) Shows a hardness traverse on an Inconel 625 laser clad repair of an Inconel 718 base material. The hardness traverse is combined with optical microscopy to understand the true extent of the laser clad repair on the substrate. In the figure, the heat affected zone (HAZ) is only 0.36 mm or 0.014 in. (right) Hardness traverse of an Inconel 625 laser clad repair on an Inconel 718 substrate. The microstructural relationship to measured microhardness values is apparent. Reported HRC values have been converted from HV using standard ASTM conversion methods.