Martensitic Advanced High-strength Steels Research Articles

Modelling of room temperature outgassing and diffusion in a martensitic advanced high-strength steel

Hydrogen embrittlement (HE) is among the limiting factors for the employment of advanced high-strength steels in the automotive industry. One of the most relevant manifestations of HE for those components is delayed fracture. To perform HE risk assessment against delayed fracture in structural components wherein hydrogen accumulates near notches or bends after manufacturing or assembly, continuum models can be used to predict hydrogen diffusion and accumulation. To this aim, it is crucial to identify both the correct model parameters and boundary conditions (BCs). This study provides the required modelling and experimental framework to estimate the constitutive diffusion, trapping and outgassing parameters for a commercial martensitic advanced high-strength steel grade 1300. Permeation testing and simple room outgassing tests showed that a natural BC with one-parameter effectively reproduces the experimental results. The proposed model can be used to predict a reasonable hydrogen distribution, both inside of the component and near to the surface, which is valuable for HE risk assessments. Finally, the same BC can be employed with the interpretation of ex-situ hydrogen charged tensile tests.

Proposal of a hydrogen embrittlement index for a martensitic advanced high-strength steel

This paper proposes a definition of a Hydrogen Embrittlement Index for assessing structural components made by a martensitic advanced high-strength steel. A material model, including a damage model and a fracture criterion, was developed to reproduce the tensile behaviour until fracture nucleation under different hydrogen concentrations. The analysis of the tests with different specimen geometries demonstrated that the maximum principal strain at fracture is correlated to hydrogen concentration, and it can be used to define the embrittlement index for quantifying the hydrogen susceptibility of material in a wide range of notch severity.

Effect of shearing prestrain on the hydrogen embrittlement of 1180 MPa grade martensitic advanced high-strength steel

This work investigated the effect of shearing prestrain on hydrogen embrittlement. The strength and ductility values were similar to those in air for hydrogen precharging before tensile testing, but the ductility decreased significantly for tensile testing with in-situ hydrogen charging due to intergranular fracture initiation at the specimen surface. In contrast, there was a significant decrease in ductility for the hydrogen precharged steel after prestrain. The hydrogen-facilitated fracture was transgranular. The hydrogen fracture grew by fracture tip blunting, new fracture initiation, and coalescence, consistent with a hydrogen enhanced localized plasticity (HELP) mechanism.

Microstructure and Mechanical Performance of Resistance Spot Welded Martensitic Advanced High Strength Steel

Resistance spot welded 1.2 mm (t)-thick 1400 MPa martensitic steel (MS1400) samples are fabricated and their microstructure, mechanical properties are investigated thoroughly. The mechanical performance and failure modes exhibit a strong dependence on weld-nugget size. The pull-out failure mode for MS1400 steel resistance spot welds does not follow the conventional weld-nugget size recommendation criteria of 4t0.5. Significant softening was observed due to dual phase microstructure of ferrite and martensite in the inter-critical heat affected zone (HAZ) and tempered martensite (TM) structure in sub-critical HAZ. However, the upper-critical HAZ exhibits obvious higher hardness than the nugget zone (NZ). In addition, the mechanical properties show that the cross-tension strength (CTS) is about one quarter of the tension-shear strength (TSS) of MS1400 weld joints, whilst the absorbed energy of cross-tension and tension-shear are almost identical.

Hydrogen-induced fast fracture in notched 1500 and 1700 MPa class automotive martensitic advanced high-strength steel

Hydrogen embrittlement of notched martensitic advanced high-strength steels was studied. Notched specimens had (i) higher tensile strength and lower ductility, and (ii) increased hydrogen sensitivity, manifested by reductions in ductility and strength. Hydrogen-induced fast fractures (HIFF) initiated when the load-controlled specimen became mechanically unstable. The HIFF velocity (61−130 m/s) was greater than the velocity of ductile fracture (46 m/s). HIFF susceptibility increased with increasing steel strength, increasing hydrogen fugacity, and decreasing stress rate. HIFF exhibited brittle features (quasi-cleavage, transgranular and intergranular fracture) in the initiation zone, suggesting hydrogen-enhanced plasticity-mediated decohesion (HEPD). These features increased in area with increasing hydrogen embrittlement.

The Role of HAZ Softening on Cross-Tension Mechanical Performance of Martensitic Advanced High Strength Steel Resistance Spot Welds

The Role of HAZ Softening on Cross-Tension Mechanical Performance of Martensitic Advanced High Strength Steel Resistance Spot Welds

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

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.

Hydrogen embrittlement of an automotive 1700 MPa martensitic advanced high-strength steel

The influence of hydrogen on a 1700 MPa martensitic advanced high-strength steel was investigated and compared with similar steels. MS1700 exhibited some hydrogen embrittlement for cathodic hydrogen charging in 0.1 M NaOH and 0.1 M HCl, but minimal hydrogen sensitivity for hydrogen charging in 3.5 wt% NaCl at the open circuit and zinc potentials. The hydrogen influence was manifested as a reduction of ductility and the occurrence of shear fracture. The heat treatment associated with paint baking increased the hydrogen embrittlement susceptibility. The higher hydrogen sensitivity of MS1700 was attributed to enhanced hydrogen trapping due to the higher carbon and martensite content.

Correlation of Local Constitutive Properties to Global Mechanical Performance of Advanced High-Strength Steel Spot Welds

Recent results have shown that the strength of resistance spot-welded joints made from advanced high-strength steels (AHSS) do not increase linearly with their base metal strength. While the martensite tempering and subsequent softening in a narrow heat affected zone (HAZ) of these joints has been reported as a primary cause for this degradation, the quantitative effects of HAZ softening on above nonlinearity in different steels have not been explored. In this research, the role of material heterogeneity on load-displacement characteristics of dual-phase and martensitic AHSS with initial martensite volume fractions of 16, 58, and 100 pct during tension-shear (TS) and cross-tension (CT) testing was modeled with finite element method and compared with experimental measurements of global deformation and fracture behavior. Results from low-strength steels showed that the location of HAZ failure transitions from hardened to softened regions, as the nugget diameter increases from 4 to 6 and 8 mm, even with the presence of softening in the HAZ. At the same time, the results from higher strength steels showed more sensitivity to the softened region in larger nugget diameters. This result elucidates that the nonlinearity in strength of high-strength AHSS spot welds in comparison to lower strength of mild steel, is due to their intrinsic brittleness, as well as the overall geometry of the weld nugget. Our results also suggest that, while HAZ softening plays a detrimental role in DP steels, it helps to improve the ultimate load and global extension in high strength grades. Finally, several uncertainties related to finite element simulations and experiments have been highlighted. The results from this study can help in optimizing the resistance spot-welding process parameters and the design of the part made from AHSS.

Evaluation of automobile service performance using laboratory testing

ABSTRACTA methodology is presented to evaluate martensitic advanced high-strength steels in auto service. There was essentially no influence of hydrogen for (i) linearly increasing stress tests of specimens with hydrogen contents much greater than for automobile service, and (ii) for tensile tests with simultaneous hydrogen charging and with a substantial hydrogen content. These results allow evaluation of the hydrogen influence for the tested steels for auto service. In contrast, electrochemically hydrogen charged martensitic advanced high-strength steels showed an influence of hydrogen on mechanical properties, manifest as (i) a decrease in yield strength, by hydrogen enhanced macroscopic ductility, and (ii) a change of the micro-fracture mode, by hydrogen-assisted micro-fracture.

The influence of microstructure on the hydrogen embrittlement susceptibility of martensitic advanced high strength steels

This work addresses the influence of microstructure on the hydrogen embrittlement of advanced high-strength steels. With sufficient hydrogen, fracture initiates at the ultimate tensile stress, but initiation is at the specimen surface. Fracture initiation is typically intergranular, transgranular or quasi-cleavage. These micro-fracture modes suggest that planar defects are important, such as prior austenite grain boundaries and lath and block boundaries. Fracture propagation is by shear fracture indicating a major influence of hydrogen-dislocation interactions. These hydrogen assisted fractures occur at a fast rate, probably approaching the speed of sound, that is at fracture velocities greater than 1000 m/s.

Further study of the hydrogen embrittlement of martensitic advanced high-strength steel in simulated auto service conditions

This work examines the influence of hydrogen on the mechanical and fracture properties of martensitic advanced high-strength steels under conditions relevant to automotive service: (i) in 3.5 wt% NaCl at different cathodic potentials, (ii) in acidified 3.5 wt% NaCl and (iii) at substantial stress rates. The hydrogen embrittlement susceptibility of the steels increases at (i) increasingly negative potentials and at lower pH in 3.5 wt% NaCl, and (ii) at high charging potentials in 0.1 M NaOH at substantial stress rates. The hydrogen influence is manifested by a reduction in ductility, and the presence of brittle features on the fracture surface.

Resistance spot welding of MS1200 martensitic advanced high strength steel: Microstructure-properties relationship

This paper addresses the microstructure and tensile-shear mechanical performance of MS1200 Giga-grade martensitic advanced high strength steel resistance spot welds. The key phase transformations in MS1200 welds were lath martensite formation in the fusion zone (FZ) and upper-critical heat affected zone (HAZ), new ferrite formation in the inter-critical HAZ and martensite tempering in the sub-critical HAZ. The MS1200 welds were featured by a near matching hardness in the fusion zone and under-matching hardness in the heat affected zone (HAZ) compared to the base metal. At certain process window a complete nugget pullout and separation was observed with high post-necking tearing energy. The interfacial to pullout failure mode transition was explained in the light of FZ hardness as well as the HAZ softening associated with martensite tempering in the sub-critical HAZ. The load bearing capacity of MS1200 welds failed at interfacial mode was strongly depends on the FZ size as well as the FZ hardness. However, the peak load of welds failed at pullout mode was a function of HAZ softening as well as the plastic constraint in the HAZ associated with the hard upper-critical HAZ/FZ and martensitic BM.

Equivalent hydrogen fugacity during electrochemical charging of some martensitic advanced high-strength steels

A new thermal desorption spectroscopy apparatus was used to study hydrogen in four martensitic advanced high-strength steels (designated MS980, MS1180, MS1300 and MS1500) after gaseous hydrogen charging, and after hydrogen charging electrochemically in 0.1M NaOH and in 3.5wt.% NaCl. The hydrogen concentration did not correlate strongly with steel strength. For MS1500, a relationship was derived between equivalent hydrogen fugacity and the applied charging overpotential. Also for MS1500, the trap binding energies were evaluated to be 26.4kJmol−1 and 61.0kJmol−1, which were interpreted as dislocation and interface hydrogen traps.

Welding metallurgy of martensitic advanced high strength steels during resistance spot welding

ABSTRACTThe paper addresses the process–microstructure–performance relationships in resistance spot welding of a martensitic advanced high strength steel. Significant softening was observed in the heat affected zone (HAZ) due to allotriomorphic ferrite formation in the inter-critical HAZ and tempering of martensite in sub-critical HAZ (SCHAZ), with the latter plays more important role in mechanical properties of the spot welds. The strain concentration associated with the HAZ softening promotes initiation of pullout failure from the soft SCHAZ. While, the peak load in the interfacial failure mode is governed by the fusion zone size, that of the pullout mode is significantly affected by the HAZ softening. To improve weldability of martensitic steels, the HAZ softening should be minimised via modifications in welding process or steel chemistry.

A review of hydrogen embrittlement of martensitic advanced high-strength steels

AbstractThe martensitic advanced high-strength steels (MS-AHSS) are used to create fuel-efficient, crashworthy cars. Hydrogen embrittlement (HE) is an issue with high-strength steels; thus, the interaction of hydrogen with MS-AHSS needs to be studied. There are only a few published works on the HE of MS-AHSS. The current literature indicates that the HE susceptibility of MS-AHSS is affected by (i) the strength of the steel, (ii) the applied strain rate, (iii) the concentration of hydrogen, (iv) microstructure, (v) tempering, (vi) residual stress, (vii) fabrication route, (viii) inclusions, (ix) metallic coatings, and (x) specific precipitates. Some of the unresolved issues include (i) the correlation of laboratory results to service performance, (ii) establishing the conditions or factors that lead to a certain HE response, (iii) studying the effect of stress rate on HE, and (iv) a comprehensive understanding of hydrogen trapping in MS-AHSS.

Influence of hydrogen on the mechanical and fracture properties of some martensitic advanced high strength steels in simulated service conditions

This work investigated the influence of hydrogen on the mechanical and fracture properties of four martensitic advanced high-strength steels in simulated service conditions: (i) immersed in 3.5wt% NaCl solution, and (ii) at substantial applied stress rates. There was little influence of hydrogen for the four MS-AHSS in 3.5wt% NaCl. Similarly, there was little influence of hydrogen for hydrogen-precharged MS1300 and MS1500 subjected to tensile tests at substantial stress rates. The diffusivities of hydrogen in MS980, MS1300 and MS1500 were similar. The use of a Pt counter electrode during cathodic hydrogen charging is not recommended.

The influence of hydrogen on the mechanical and fracture properties of some martensitic advanced high strength steels studied using the linearly increasing stress test

The influence of hydrogen on the mechanical and fracture properties of four martensitic advanced high strength steels was studied using the linearly increasing stress test and electrochemical hydrogen charging. The hydrogen influence increased with steel strength, decreasing charging potential, and decreasing applied stress rate. Increased hydrogen influence was manifest in (i) the decreased yield stress attributed to solid solution softening by hydrogen and (ii) the reduced macroscopic ductility, and by the change from ductile cup-and-cone fracture to macroscopically brittle shear fracture, attributed to a dynamic interaction of hydrogen with the dislocation substructure somewhat similar to the HELP mechanism.

Similar and dissimilar resistance spot welding of martensitic advanced high strength steel and low carbon steel: metallurgical characteristics and failure mode transition

Safety, emissions and fuel economy of vehicles have always been of great importance to automakers. Advanced high strength steels (AHSSs) have proved to be effective in fulfilling these needs of the industry, and this has led to great opportunities for increasing usage of AHSSs. The present work aimed to investigate the resistance spot welding characteristics of martensitic steel (M130) having the thickness of 2 mm, utilising metallographic observation, microhardness and tensile–shear testing. The results were compared for different combinations of martensitic steel and low carbon steel (LCS). In the study of interfacial to pull-out failure mode transition, dissimilar M130/LCS welds exhibited the lowest tendency to fail in the interfacial failure mode. Weld nugget diameter is proposed as a main variable controlling the peak load and energy absorption of the welds along with other factors such as electrode indentation and hardness/strength of the failure location.

Development of rapid heating and cooling (flash processing) process to produce advanced high strength steel microstructures

Flash processing of an AISI8620 steel sheet, which involves rapid heating and cooling with an overall process duration of <10 s, produced a steel microstructure with a high tensile strength and good ductility similar to that of advanced high strength steels. Flash processed steel [ultimate tensile strength (UTS): 1694 MPa, elongation: 7·1%], showed at least 7% higher UTS and 30% greater elongation than published results on martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5·1%). The underlying microstructure was characterised with optical, scanning electron, transmission electron microscopy as well as hardness mapping. A complex distribution of bainitic and martensite microstructures with carbides was observed. A mechanism for the above microstructure evolution is proposed.