Embrittlement in HSSs Limits Use in Lightweight Body

n today’s automotive industry, the goal to Materials evaluated Matthias Loidl I lower CO2 emissions is the driving force in The materials tested consist of four differ- O. Kolk material development in a wide range of ap- ent advanced HSSs representing different types BMW Group plications. This ecological stimulus is also in- of microstructural concepts and different München, Germany fluencing the body-in-white design where full strength levels. Dual-phase steels consist of a exploitation of light weight potential becomes soft ferritic matrix with up to 20% hard marten- Steels having increasingly important. When using high sitic phase. Complex phase steels exhibit better a tensile strength steel (HSS), a reduction in thickness bending properties due to the presence of leads to a weight reduction of the overall car bainitic phases, and martensitic steels consist strength body frame while maintaining the crash-wor- of nearly 100% martensitic phase. In addition, a greater than thiness of the construction. However, steels TRIP steel containing retained austenite that 1000 MPa can having a tensile strength greater than 1000 MPa transforms into martensite during deformation fail due to (145,000 psi) may fail due to hydrogen embrit- was tested. The chemical compositions of ma- hydrogen tlement (HE) under certain circumstances (hy- terials tested are shown in Table 1. drogen content vs. stresses vs. microstructure). embrittlement This still hinders the use of the highest strength Quasi-static tensile testing under certain steels in the body in white. Tests were performed on a servo-hydraulic circumstances, These steels include dual phase (DP), com- loading machine equipped with a high pressure which hinders plex phase (CP), martensitic phase, and TRIP gas test chamber [1]. The material was tested ei- (transformation induced plasticity) steels. This ther under or hydrogen atmosphere. Be- the use of the article describes a methodology to rank the dif- fore starting the experiment in hydrogen, the test high strength ferent steels regarding their susceptibility to hy- chamber was evacuated and flushed several steels in the drogen embrittlement with respect to their times to ensure that no gas other than hydrogen body in white microstructure and respective strength level. was in the test chamber. The gas pressure for design. Several approaches, such as static and quasi- tests under both atmospheres was 10 MPa and static tensile testing, under various atmos- the purity of the gases was 6.0 each. pheres and hydrogen loading conditions are All tests were performed at room tempera- illustrated. The investigations a better un- ture using a constant cross-head speed of 0.1 derstanding of microstructural influence on the mm/min. Tensile samples (Fig.1) were cut out failure mode due to hydrogen embrittlement in of sheet steel via the wire-erosion process. The general, and will help in designing guidelines final thickness of the specimens was defined for the use and selection of high strength steels after grinding both surfaces to remove the zinc in body in white development and manufacture layer and to establish a homogeneous surface processes. condition for the various test parameters. To evaluate the effect of hydrogen embrit- tlement, the change in ductility was considered according to equation (1):

S = (A – A )/A (Eq. 1) bHe bH2 bHe

where AbHe is the breaking strain under helium atmosphere and AbH2 is the breaking strain under hydrogen atmosphere. In addition, the surface was analyzed via scanning elec- tron microscopy to differentiate between dif- ferent fracture modes. One sample per variant was also analyzed by means of thermal desorp- tion analysis to determine the hydrogen con- tent of the sample after failure.

Static tensile testing Representative automobile body in white. The sample geometry used for the static

22 ADVANCED MATERIALS & PROCESSES • MARCH 2011 in White Design

TABLE 1 — CHEMICAL COMPOSITION OF STEELS TESTED Chemical composition, wt% Steel grade C Si Mn P S Al Ti Nb Cr Mo Ni V Cu B HC600X 0.16 0.19 2.2 0.012 <0.001 0.24 0.003 0.022 0.481 0.004 0.009 0.008 0.013 0.0004 HC700X 0.15 0.21 1.85 0.016 <0.001 0.022 0.019 0.003 0.216 0.003 0.021 0.005 0.022 0.0003 HC900X 0.14 0.22 1.88 0.019 <0.001 0.031 0.022 0.003 0.22 0.003 0.013 0.005 0.011 0.0002 HC600C 0.14 0.19 2.12 0.01 <0.001 0.042 0.002 0.024 0.288 0.004 0.009 0.006 0.013 0.0003 HC800C 0.17 0.042 2.02 0.008 <0.001 0.046 0.023 0.003 0.238 0.004 0.01 0.008 0.015 0.0027 HC950C 0.15 0.23 1.96 0.008 <0.001 0.039 0.031 0.004 0.25 0.003 0.01 0.006 0.014 0.0019 HC500T 0.21 1.87 1.67 0.012 <0.001 0.041 0.005 0.003 0.025 0.003 0.02 0.004 0.024 0.0004 HD900MS 0.11 0.095 1.39 0.008 <0.001 0.028 0.034 0.002 0.231 0.004 0.047 0.004 0.036 0.0012

tensile testing (Fig. 2.) was originally developed Fig. 1 — Geometry of 10 23 10 for testing steel sheet in the electroplated, as- quasi-static tensile specimen; thickness delivered condition to determine its suscepti- varies depending on the bility to hydrogen-induced delayed cracking as 11 steel grade. All defined in the standard SEP1970 [2]. The sam- 5 5 20 dimensions in mm. 11 ples used in this work were punched out of un- coated steel to allow hydrogen introduction R 12 into the material via electrochemical charging. The specimens were cathodically charged using a dc power supply in a solution of 6% H2S04 and 75 0.5g/L As203 as a hydrogen-recombination poi- son. A current density of 5 mA/cm2 was used Fig. 2 — Geometry of to charge the samples at loading times of 1, 5, 85 static tensile specimen; thickness varies 10, and 20 minutes. After the hydrogen charg- R 10 depending on the steel ing, the samples were mounted into the me- R 5.5 grade. All dimensions in chanical loading device and after a time interval mm. 30 of 5 minutes loaded with a constant stress of 100% Rp0.2 and 80% of Rp0.2. Three samples were tested for each parameter combination.

Results of quasi-static testing 45 Table 2 lists the mechanical properties ob- tained by quasi-static testing under helium and gation decreases from 30 to 10% resulting in an hydrogen atmospheres. Straining the material HE-index of 66%. This is of interest because the under hydrogen atmosphere decreases the tensile strength level of this steel is well under overall elongation. This fact is emphasized by 1000 MPa, which is deemed to be a critical the hydrogen embrittlement (HE) index, which limit for hydrogen embrittlement to occur. is described by equation (1). This value can be Both DP and CP steels show a considerable loss in the range of 0 to 100%, where 0% means that of ductility under hydrogen atmosphere. How- no hydrogen embrittlement occurs and a value ever, a comparison of different strength levels of 100% describes a material without any break- among these two steel types shows that the DP ing strain under hydrogen atmosphere. steels have a higher susceptibility to hydrogen Considering the HE-index, HC500T steel embrittlement than CP steels. This can be ex- shows the highest degree of HE. The total elon- plained by the presence of hard martensitic

ADVANCED MATERIALS & PROCESSES • MARCH 2011 23 TABLE 2 — MECHANICAL PROPERTIES OF MATERIALS TESTED UNDER H AND HE ATMOSPHERES Yield Tensile strength, MPa strength, MPa Elong., % Steel HE-index, grade He H He H He H % HC600X 760 760 1006 1002 12 7 38 HC700X 789 787 1026 1026 12 7 40 HC900X 1048 1047 1282 1265 9 5 47 HC600C 680 654 807 810 15 12 19 10 μm HC800C 1007 1005 1026 1044 9 6 30 Fig. 3 —Ductile shear dimples in HC950C HC950C 1061 1054 1274 1257 10 6 38 steel after testing in helium atmosphere. HC500T 542 533 883 798 30 10 66 HC260LAD 310 314 366 366 33 29 13

phase in the soft ferritic matrix in DP steels, which causes a stress concentration between the interfaces of these two phases promoting the occurrence of HE. CP steels also con- tain bainitic phases, lowering the stress gradient between ferrite and martensite. Typical fracture surfaces of material after tensile testing in helium and hydrogen atmospheres are shown in Fig. 3 and Fig. 4, respectively. A higher magnification of the brittle-fracture surface shown in Fig. 5 shows classical transgranular quasi-cleavage fracture. As a reference, low- was tested to put the results of the high strength steels into perspective. Even in this case, a decrease of ductility was measured. Figure 6 shows 10 μm the fracture surface near the edge of the sample. Brittle fracture can be seen starting from the surface of the material. After 20 mm, the fracture surface morphology changes Fig. 4 — Brittle transgranular fracture of to pure ductile shear dimples. This phenomenon yields a HE index of even 13%; this oc- HC950C in hydrogen atmosphere. curs because the surface of the samples was ground to remove the zinc layer causing work hardening, which further caused hydrogen embrittlement.

Results of static tensile testing HC900X, HC950C, and HD900MS samples were cathodically charged with hydro- gen and subsequently loaded with a force representing the stress at the yield strength of each material. Diffusible hydrogen content after charging measured using hot gas extraction (1000°C) is shown below. Hydrogen content, ppm Time, min HC900X HC950C HD900MS 0 0.17 0.15 0.18 1 0.45 0.33 0.34 1 μm 5 1.43 0.99 0.70 10 1.72 1.27 1.85 Fig. 5 — Higher magnification of fracture 20 1.88 1.69 2.52 surface shown in Fig. 4 showing surface in greater detail. There was a considerable difference in the response to hydrogen charging among the three steel types tested. After 5 minutes of charging, the DP steel lasted for 0.3 h, and for charging times of 10 or more minutes, the material failed in the elastic region while applying the load. In contrast, the complex phase steel did not break after 96 hours. The response to charging and loading of the martensitic phase steel resembles the different hydrogen levels. Test results after hydrogen charging for different times and loading at 100% Rp0.2 are shown below.

Hydrogen Time to fracture, h charging time, min HC900X HC950C HD900MS 5 0.3 No fracture after 96 h 5.6 10 0 3.4 20 0 0.4

24 ADVANCED MATERIALS & PROCESSES • MARCH 2011 Summary Additional static tensile testing will be performed on the same materials tested via the quasi-static tensile test- ing under hydrogen atmosphere to further investigate the phenomenon of hydrogen embrittlement and delayed cracking. Additionally, thermo-desorption analysis of dif- ferent steels after hydrogen charging will be done to char- acterize the response to cathodic charging for different alloying concepts. This will yield a framework for further investigations on the influence of the body in white man- ufacturing process on hydrogen uptake and influence on AHSS. 20 μm References Fig. 6 — Transition zone from brittle to 1. P. Deimel and C. Hanisch, Tests on the steels 15 MnNi 6 3 ductile fracture mode in low-alloy steel. and X 56 TM in high pressure hydrogen gas of high purity, Intl. J. Hydrogen Energy, Vol 14, No. 2, p 147-151, 1989. 2. Standard SEP1970: Test of the resistance of advanced high strength steels (AHSS) for automotive applications against production related hydrogen-induced brittle fracture, May 2009.

For more information: Matthias Loidl, BMW Group, Struk- turwerkstoffe, EG-511, Knorrstraße 147, 80788 München, Germany; tel: +49-89-382-14631; Mobil: +49-176-601-14631; email: [email protected]; www.bmwgroup.com.

www.asminternational.org/CTSO [email protected]

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