Hardness Mapping of a DP980 Steel Sample

The automotive industry requires steel with a high strength, and this need is continually being met by ongoing developments. One fruit of this research is advanced dual-phase (DP) steel. This is steel with a microstructure characterized by a soft phase made of ferrite, in which grains of a much harder and stronger mineral called martensite are dispersed.

The older DP steels combined a phase with low yield strength with one that had a high strength. The newer ones, however, utilize the phenomenon called solid-solution hardening within the soft ferrite phase to produce enhanced yield strength.

As a result, these are used to make structural and safety components for automotive applications, including longitudinal beams, cross beams, and reinforcements. Before they enter commercial production, it is vital that their mechanical attributes are studied in full.

The microstructure of steels determines how strong and ductile they are. Steels which have a required mechanical property to a specific value can now be designed by regulating the factors that influence their microstructure. These include the elements introduced while making the alloy, and the type of mechanical or heat treatment used to process them.

During their characterization, several techniques go into assessing the traits that have measurable effects upon their mechanical properties such as the distribution of grain sizes, any precipitates present, and the average distribution of orientation of the steel, which is called its texture. In this article the use of hardness mapping to characterize the soft and hard phases of a DP steel sample is described.

This helps to assess the properties of the phases in a quantitative manner provided the indentation test has a small enough deformed volume to fit into only one phase, and if there are enough indentations to ensure that both the phases are tested.

Figure 1. Topography image of the 100x100 indent pattern. The SPM image is showing a 15x15 μm scan of the 60x60 μm pattern area.

Procedure

For this test, the DP980 steel sample was prepared by grinding and polishing with OPS (oxide polishing suspensions) to obtain a smooth surface. This was then ready for nano-hardness testing. The sample was mounted in the Hysitron^® TI 980 TriboIndenter^® and tested for hardness with XPM™ using a 100x100 indentation grid made with a sharp Berkovich probe.

The map acquisition took one and a half hours and covered an area of 60x60 μm, at the half-thickness position. Each load cycle consumed 400 ms, with the probe penetrating to a depth of approximately 50 nm. A single subset of the indentation array of 10,000 points is shown in the SPM image in Figure 1.

Results

Figure 2 shows the hardness map which makes it possible to quickly identify the ferrite and martensite phases by the measured hardness values. In Figure 3 the plot shows how the hardness is distributed for the 10,000 indents (shown as red triangles). If a Gaussian distribution is used then the relative amounts of the different phases can be calculated.

The distribution of ferrite is fitted to the data points for hardness between 0 and 5.8 GPa while that of martensite is fitted to hardness values ranging upwards from 8.4 GPa. The data values between 5.8 and 8.4 GPa show a significant overlap and are hence not used for this fit. If the material is mapped using electron backscatter diffraction (EBSD), about 33% of the surface area (as seen by the black area in Figure 2) is attributed to martensite, leaving the peak area of ferrite to martensite to be 2:1.

Figure 3 shows a black curve which represents the combination of both the Gaussian fit curves, but it does not correlate with the actual distribution determined experimentally within the range of values between 5.8 to 8.4 GPa. In this way the model takes too little account of the measurement data in this transition range.

This deviation from the experimental value is a measure of how many indents were carried out on the phase interface, since these caused deformation of both phases without becoming part of either Gaussian curve. This helped to find that 2,800 of 10,000 indents fell at the interphase. The hardness values for ferrite and martensite were 4.8 GPa and 8.3 GPa respectively.

Figure 2. (Top) Results of a 60x60 μm EBSD map of DP980. Colors indicate the size and orientation of the ferritic grains. Black areas show that the martensitic phase distribution in the alloy is stretched in the rolling direction. Measurements are taken at the half thickness of the steel sheet. (Bottom) Results of a 60x60 μm hardness map with 100x100 indentation grid. As indicated by the color scale, the hardness of martensite is much higher than the hardness of ferrite.

Figure 3. Hardness distribution on sample DP980 as determined by 10,000 hardness measurements.

Conclusions

XPM nano-hardness testing provides speedy and highly informative characterization of the mechanical properties of DP steels. Its important advantages include being able to indent to depths up to 50 nm, with the plastic zone being under good control, which meant that steels with small grains could be tested; automation of hardness mapping so that large amounts of data could be acquired for the least amount of actual hands-on time; and powerful data analytic software.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.