Investigation of Mechanical Tests for Hydrogen Embrittlement in Automotive PHS Steels

The problem of hydrogen embrittlement in ultra-high-strength steels is well known. In this study, slow strain rate, four-point bending, and permeation tests were performed with the aim of characterizing innovative materials with an ultimate tensile strength higher than 1000 MPa. Hydrogen uptake, in the case of automotive components, can take place in many phases of the manufacturing process: during hot stamping, due to the presence of moisture in the furnace atmosphere, high-temperature dissociation giving rise to atomic hydrogen, or also during electrochemical treatments such as cataphoresis. Moreover, possible corrosive phenomena could be a source of hydrogen during an automobile’s life. This series of tests was performed here in order to characterize two press-hardened steels (PHS)—USIBOR 1500® and USIBOR 2000®—to establish a correlation between ultimate mechanical properties and critical hydrogen concentration.


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The priority of maintaining and, if possible, increasing the safety of drivers and passengers 25 collides with the necessity of reducing the CO2 emissions and so the weight of vehicles. In this contest 26 the research and characterization of new materials with the possibility of emissions reduction are 27 more and more diffused. To reduce thicknesses of sheets, higher tensile properties are required and 28 a major hydrogen susceptibility is the natural consequence.

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Advanced high strength steels (AHSS) exhibit considerable mechanical properties and a good 30 formability and during the production process hydrogen could be adsorbed in various phases: 31 pickling, electroplating, cataphoresis and phosphating or even moisture during welding or heat 32 treatment, can cause an introduction of hydrogen in the material. the first one assumes that hydrogen gives a contribution to the tensile strength: the higher the 46 hydrogen concentration higher the hydrogen pressure and decohesion of steel atoms; the latter suggests that hydrogen, moving towards crack tips where hydrostatic stress is higher, promotes 48 dislocations motion and reduces the interaction energy of them with internal obstacles with 49 consequent formation of micro-voids.

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Two different conditions were studied and compared in this work: on one side the Slow Strain Rate 51 Tests, so a very slow imposed deformation of the material, in order to give hydrogen necessary time 52 to hydrogen to concentrate in the plastic zone; on the other hand there was the study of static behavior 53 of materials investigated by means of Four Point Bending Tests.

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In addition, a permeation test campaign was performed to evaluate the diffusion coefficient and its 55 dependence on thermal treatment.

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The aim of this activity was to determine a practical and easy-applicable methodology to study the 57 susceptiblity to hydrogen of Ultra High Strength Steels for automotive industry.

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These materials are subjected to hot forming process and they are used in structural and safety 63 automobile components.

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After the thermal treatment the microstructure is completely martensite and thus susceptibility to hydrogen embrittlement increases.

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In this work NH4SCN varied in the range of 0,03-0,3 % and current in the range of 0,25-1 mA/cm 2 to 83 avoid surface damage.
Every sample was further treated by introducing it in the laboratory furnace and heating material at 85 the temperature of 150 °C for 10 minutes simulating paint baking industrial process: in this way a uniform hydrogen distribution was achieved inside the material and, according to [4], not a 87 considerable amount of hydrogen was desorbed.

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In Figure 1   The loading consists of increasing the stress to which the material is subjected progressively, always 109 in the elastic range, starting from 50% of Rp,02; this load was maintained for 24 h, then it was 110 increased every 2 hours up to 90%.
It must be noticed that in case of rupture the hydrogen content was immediately evaluated by hot 112 extraction method to correlate the concentration of hydrogen to the critical load.

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Once reached the 90% of Rp,02, the final test was to keep the sample in this condition for other 24 114 hours. If the sample didn't fail it was declared safe.

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In Figure 4 is shown the device to carry out the four point bending tests.

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The absorbed hydrogen reaches the outside surface where the sensor's probe is applied, leaves the 137 sample and passes through the probe and is detected: as a result, the typical permeation curve is 138 obtained.

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In Figure 6 and Figure 7 were reported the two characteristics of the two different materials.

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For both pictures the magnification is 5000x and the hydrogen content, absorbed by the samples,

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The FE model was simplified representing a quarter of the sample considering symmetry planes.
geometry (big displacements). For holed specimens FEM simulation returned a stress concentration 196 factor value close to 2.

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In Figure 12 is shown the quarter of sample and with a red dot is highlighted the node  Where ∆7 is equal to 30 kJ/mol, R the gas constant and T absolute temperature.

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The hydrogen concentration tends to accumulate next to the crack tip because of higher hydrostatic

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In automotive industry very thin thickness (0,5-2E-03 m) sheets are used and for this reason a more 258 practical approach is reasonable.

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The extrapolation of a regression curve, figure 6 and figure 7,

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The same was done for the Four Point Bending Tests and from the mathematical expressions, 266 critical hydrogen concentration was found for both materials, summarized in Table 4: 267 268

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In Figure 13 is represented the evolution of hydrogen concentration at the crack tip as a function of