Hydrogen-induced delayed fracture of a 1180 MPa martensitic advanced high-strength steel under U-bend loading

Highlights

• MS1180 showed HIDF susceptibility only under conditions of high stress, significant plastic strain and substantial hydrogen.

• Bending limit diagrams identified conditions for safe car manufacturing processes.

• The hydrogen-induced fractures propagated in two stages.

• Hydrogen content increased with increasing (i) plastic strain, and (ii) hydrogen fugacity.

Abstract

Hydrogen-induced delayed failure (HIDF) of an 1180 MPa martensitic advanced high-strength steel (MS-AHSS) was studied using U-bend tests. HIDF susceptibility was increased by increasing stress, plastic strain and hydrogen content. Fracture initiated in the region of high tensile stress at the apex of the U-bend specimen. Hydrogen-induced fracture propagated in two stages, coinciding with the two stress regions of the specimen. The hydrogen content of the U-bend specimen increased with plastic strain and hydrogen fugacity. Delayed fracture is possible for specimens (and components) that contain substantial damage due to plastic strain and are subjected to a substantial stress.

Introduction

Martensitic advanced high-strength steels (MS-AHSSs) are used in lightweight crashworthy cars [[1], [2], [3], [4], [5], [6]]. MS-AHSSs have high-strength and are relatively inexpensive, although their ductility and formability are somewhat limited [[1], [2], [3], [4]]. MS-AHSSs are primarily used in the automotive body-in-white components that comprise the protective cage around passengers. These parts include front and rear bumpers, door beams, side sill reinforcement, and roof cross members [3,[7], [8], [9], [10]]. MS-AHSSs are available in different strength grades, such as MS980, MS1180, MS1300, MS1500, and MS1700. The number in each designation indicates the minimum ultimate tensile strength (UTS, MPa) of the steel [[11], [12], [13]].

Hydrogen embrittlement (HE) and hydrogen induced delayed fracture (HIDF) are issues for high-strength steels, particularly for strengths exceeding 700 MPa. HE degrades mechanical properties for hydrogen concentrations of ∼1 μg/g. HE has been reported for medium-strength steels [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]], conventional high-strength steels [[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]], and advanced high-strength steels (AHSS) [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51]]. HE requires an appropriate combination of three parameters, i.e. (i) a stress that exceeds a critical stress, (ii) a critical amount of hydrogen (often in the ppm concentrations), and (iii) a sensitive microstructure [52]. For steels, the martensite microstructure was the most susceptible to HE [53]. In addition, the applicable stresses may be residual stresses, such as may originate from different automotive forming processes [54].

HE can produce subcritical crack growth and the reduction of mechanical strength, toughness and ductility [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41],[44], [45], [46], [47], [48], [49], [50], [51], [52],55]. Alternatively, in some steels, HE manifests as a reduction in ductility without sub-critical crack growth and without a significant decrease in strength (i.e. yield strength, YS, and ultimate tensile stress, UTS) [56]. These two modes of HE may be designated as (i) hydrogen embrittlement with subcritical cracking (HESC), and (ii) hydrogen embrittlement with fracture at the tensile strength leading to some ductility loss (HEFT). HESC has the following mechanisms for sub-critical crack growth: (i) hydrogen-enhanced decohesion (HEDE), (ii) hydrogen-enhanced localized plasticity (HELP) [57], (iii) adsorption-induced dislocation emission (AIDE) [52] and (iv) hydrogen-enhanced strain-induced vacancies (HESIV) [58,59].

In contrast, HEFT was the manifestation of the hydrogen influence on some MS-AHSSs (MS980, MS1180, MS1300, MS1500 and MS1700) [39,[41], [42], [43]] studied using the linearly increasing stress test (LIST) and electrochemical hydrogen charging. Hydrogen had little influence on strength, but decreased overall ductility, and fracture occurred when the applied stress was equivalent to the UTS. The hydrogen-influenced micro-mechanisms were (i) hydrogen-enhanced ductility (HEMP) (attributed to hydrogen facilitating macroscopic dislocation motion and thereby decreasing the yield stress) and (ii) hydrogen-assisted micro-fracture (HAM) at the UTS [60].

Hydrogen-induced delayed failure (HIDF) is of particular concern during auto manufacture using advanced high-strength steels on the automobile assembly line.

The slow strain rate test (SSRT) and the linearly increasing stress test (LIST) are two popular methods to evaluate the influence of hydrogen on the mechanical properties of steels. The SSRT slowly strains to fracture the specimen exposed to a hydrogen environment. In contrast, the LIST slowly increases the load until fracture. The difference is that the SSRT is strain-controlled, while the LIST is stress-controlled.

Recently, the U-bend test has been used by the automotive industry to evaluate hydrogen-induced delayed failures of AHSS-sheet components [61,62] during auto production and in auto service. The U-bend test is a relatively simple test that was standardised by ASTM [63,64]. The stressed U-bend specimen is immersed in a hydrogen environment, such as a 1.0 M HCl solution, and the failure time is recorded. Because the U-bend test can identify deformation conditions that induce hydrogen-induced delayed failures, safe manufacturing conditions can be identified for the production of automotive parts (e.g., door impact beams and bumper reinforcements) that are formed by plastic bending. Furthermore, Louthan [65] maintained that the highest hydrogen sensitivity is produced for a condition of constant strain with a static load, such as in the U-bend test.

Toji et al. [66], Takagi et al. [67] and Li et al. [68] used the U-bend test to identify the conditions of mechanical stress and strain (bend radius) that led to hydrogen induced delayed fracture (HIDF). Such a bending limit diagram characterizes the formability of AHSS in the presence of hydrogen. Such information is critical for MS-AHSS because these steels possess limited ductility and are relatively susceptible to HIDF. Extending this approach to MS1180 is the reason for this present research. Mechanistically, Li et al. [68] showed that hydrogen-induced delayed fracture occurred only near the bending limit and was promoted by the presence of microvoids and micro-fractures, which were introduced by the severe plastic deformation of the U-bend specimens. This research explored the use of the U-bend test (i) to characterise bending conditions in the presence of hydrogen that led to hydrogen-induced delayed fracture, and (ii) to understand the mechanisms of hydrogen induced fracture of MS1180.

The present research used the U-bend test to evaluate hydrogen-induced delayed fracture (HIDF) susceptibility of MS1180 in HCl solutions; and builds on prior research [39,41,69] which studied the influence of hydrogen on a range of automotive MS-AHSSs using dynamic loading with LIST. The aims addressed herein were as follows:

• to characterise the role of stress, plastic strain and diffusible hydrogen content on the HIDF susceptibility of MS1180 using the U-bend test in HCl solutions with pH 1 and pH 3;

• to create bending limit diagrams that identify the delayed fracture conditions; and

• to understand the mechanisms for the hydrogen-influenced delayed fracture of MS1180.