Delayed Fracture Properties Research Articles

Effect of Carbon Density at Grain Boundary on Delayed Fracture Properties of Martensitic Steels

Effect of Carbon Density at Grain Boundary on Delayed Fracture Properties of Martensitic Steels

Evaluation of Delayed Fracture Properties of Hydrogenated Surface Hardened JIS SCM420H

Evaluation of Delayed Fracture Properties of Hydrogenated Surface Hardened JIS SCM420H

Study on the improving effect of Nb-V microalloying on the hydrogen induced delayed fracture property of 22MnB5 press hardened steel

Press-hardened steel (PHS) is widely used in manufacturing automobile parts and is sensitive to hydrogen embrittlement (HE). A novel 0.04Nb-0.04 V (wt%) PHS (22MnB5NbV) was developed based on 22MnB5. The constant-strain bending test shows the duration of no-cracking for 22MnB5NbV in 0.5 mol/L HCl is > 300 h even though the pre-bending stress is 1500–1600 MPa, and that for 22MnB5 is 12 h. The prior austenite grain size and martensite lath width in 22MnB5NbV decrease from 13.4 to 8.2 μm and from 0.35 to 0.15 μm due to Nb-V microalloying, respectively, resulting in the hydrogen trapped in reversible hydrogen traps increases by 2.09 times according to thermal desorption spectrometry analysis. The leaching experiment presents that 88% Nb and 25% V exist in the form of precipitates rather than in solution in 22MnB5NbV. The nano-precipitates (< 10 nm) are mainly (Nb, Ti, V)C, in which Nb-bearing precipitates account for the largest proportion and supply the heterogeneous nucleation sites for V-bearing precipitates, this is confirmed by atom probe tomography (APT). APT confirms that the hydrogen is mainly trapped at the interface of Fe matrix/carbide. The dispersed nano-carbide and refined microstructure resulting from the Nb-V microalloying are the key factors improving the HE of 22MnB5NbV.

Influence of Thermomechanical Treatment on Delayed Fracture Property of Mo-Bearing Medium-Carbon Steel

Influence of Thermomechanical Treatment on Delayed Fracture Property of Mo-Bearing Medium-Carbon Steel

Effects of Nb or (Nb + Mo) alloying on Charpy impact, bending, and delayed fracture properties in 1.9-GPa-grade press hardening steels

In this study, effects of Nb or (Nb + Mo) alloying into a 1.9-GPa-grade 32MnB5 press hardening steel on Charpy impact, bending, and delayed fracture properties were investigated. All the properties were enhanced in the Nb- or (Nb + Mo)-alloyed steels, especially in the (Nb + Mo)-alloyed steel. The Nb alloying worked for the precipitation of nano-sized complex carbides, thereby leading to the refinement of prior austenite grains by a Zener pinning effect. Mo also contributed to the grain refinement, while Mo affected it mostly in a solid-solution state by a solute drag effect. This grain refinement resulted in the more deviated crack propagation path and consequently in the more improved Charpy impact energy. The size reduction of Ti(C,N) inclusion due to the attachment of a small amount of Nb and Mo also improved the impact toughness. As well as microstructural effects, the lower residual stress due to the lower peak bending load by the Nb or (Nb + Mo) alloying contributed to the increase in H-induced delayed cracking time. Therefore, this study suggests that synergic interactions of Nb and Mo alloying would provide promising solutions for improving various properties, which should be more importantly considered under actually using environments such as mechanically forming and crashing for press hardening components, while satisfying the 1.9-GPa-strength 32MnB5 specification.

The Research Committee Report on Evaluation of Delayed Fracture Properties for Spring Steels Second Activity Part 2 —Grasping Delayed Fracture Pattern of Coil Spring and Establishing Test Method Used Test Piece Reproduced it—

The risk of delayed fracture tends to increase by increasing hardness of coil spring, and to establishing suitable and safe evaluation method test method is essential. Once the evaluation method is standardized, automotive maker and spring maker can make fair judgement based on common criteria. It is expected that spring maker can reduce evaluation cost. Previous report was to establish hydrogen analysis method to separate hydrogen occurred in the rust remained on material surface. And was to estimate maximum hydrogen content entered from automotive environment. What are the dominant factor of spring to promote hydrogen penetration caused by grinding was also discussed. This study reports experiment results to confirm an evaluation method of test piece to monitor delayed fracture by holding coil spring simulating fracture pattern in the market. This paper proposes an evaluation method using test piece to reproduce fracture pattern in the market.

Evaluation of delayed fracture property and fractography of notched specimens for high tensile strength steel

Evaluation of delayed fracture property and fractography of notched specimens for high tensile strength steel

The Research Committee Report on Evaluation of Delayed Fracture Properties for Spring Steels Second Activity Part 1 —Maximum Diffusible Hydrogen in Actual Environment and Standardizing Chemical Analysis Method of Hydrogen—

The Research Committee Report on Evaluation of Delayed Fracture Properties for Spring Steels Second Activity Part 1 —Maximum Diffusible Hydrogen in Actual Environment and Standardizing Chemical Analysis Method of Hydrogen—

Effects of Alloying Elements Addition on Delayed Fracture Properties of Ultra High-Strength TRIP-Aided Martensitic Steels

To develop ultra high-strength cold stamping steels for automobile frame parts, the effects of alloying elements on hydrogen embrittlement properties of ultra high-strength low alloy transformation induced plasticity (TRIP)-aided steels with a martensite matrix (TM steels) were investigated using the four-point bending test and conventional strain rate tensile test (CSRT). Hydrogen embrittlement properties of the TM steels were improved by the alloying addition. Particularly, 1.0 mass% chromium added TM steel indicated excellent hydrogen embrittlement resistance. This effect was attributed to (1) the decrease in the diffusible hydrogen concentration at the uniform and fine prior austenite grain and packet, block, and lath boundaries; (2) the suppression of hydrogen trapping at martensite matrix/cementite interfaces owing to the suppression of precipitation of cementite at the coarse martensite lath matrix; and (3) the suppression of the hydrogen diffusion to the crack initiation sites owing to the high stability of retained austenite because of the existence of retained austenite in a large amount of the martensite–austenite constituent (M–A) phase in the TM steels containing 1.0 mass% chromium.

Delayed fracture properties of 1.8 GPa-class ultra-high strength fail-safe bolt

Delayed fracture properties of 1.8 GPa-class ultra-high strength fail-safe bolt

Evaluation of Delayed Fracture Property of Notched Specimens for High Tensile Strength Steel

Evaluation of Delayed Fracture Property of Notched Specimens for High Tensile Strength Steel

Effects of Annealing Treatment Prior to Cold Rolling on Delayed Fracture Properties in Ferrite-Austenite Duplex Lightweight Steels

Tensile properties of recently developed automotive high-strength steels containing about 10 wt pct of Mn and Al are superior to other conventional steels, but the active commercialization has been postponed because they are often subjected to cracking during formation or to the delayed fracture after formation. Here, the delayed fracture behavior of a ferrite-austenite duplex lightweight steel whose microstructure was modified by a batch annealing treatment at 1023 K (750 °C) prior to cold rolling was examined by HCl immersion tests of cup specimens, and was compared with that of an unmodified steel. After the batch annealing, band structures were almost decomposed as strong textures of {100}〈011〉 α-fibers and {111}〈112〉 γ-fibers were considerably dissolved, while ferrite grains were refined. The steel cup specimen having this modified microstructure was not cracked when immersed in an HCl solution for 18 days, whereas the specimen having unmodified microstructure underwent the delayed fracture within 1 day. This time delayed fracture was more critically affected by difference in deformation characteristics such as martensitic transformation and deformation inhomogeneity induced from concentration of residual stress or plastic strain, rather than the difference in initial microstructures. The present work gives a promise for automotive applications requiring excellent mechanical and delayed fracture properties as well as reduced specific weight.

Residual Stress Analysis in Deep Drawn Twinning Induced Plasticity (TWIP) Steels Using Neutron Diffraction Method

In Twinning Induced Plasticity (TWIP) steels, delayed fracture occurs due to residual stresses induced during deep drawing. In order to investigate the relation between residual stresses and delayed fracture, in the present study, residual stresses of deep drawn TWIP steels (22Mn-0.6C and 18Mn-2Al-0.6C steels) were investigated using the finite element method (FEM) and neutron diffraction measurements. In addition, the delayed fracture properties were examined by dipping tests of cup specimens in the boiled water. In the FEM analysis, the hoop direction residual stress was highly tensile at cup edge, and the delayed fracture was initiated by the separation of hoop direction and propagated in an axial direction. According to the neutron diffraction analysis, residual stresses in 18Mn-2Al-0.6C steel were about half the residual stresses in 22Mn-0.6C steel. From the residual strain measurement using electron back-scatter diffraction, formation of deformation twins caused a lot of grain rotation and local strain at the grain boundaries and twin boundaries. These local residual strains induce residual stress at boundaries. Al addition in TWIP steels restrained the formation of deformation twins and dynamic strain aging, resulting in more homogeneous stress and strain distributions in cup specimens. Thus, in Al-added TWIP steels, residual stress of cup specimen considerably decreased, and delayed fracture resistance was remarkably improved by the addition of Al in TWIP steels.

Precision Piercing and Blanking of Ultrahigh-strength Steel Sheets

The paper proposes new technologies able to improve the surface quality of sheared products by means of a combined process of finish blanking and press shaving applied to materials having very high strength. Experimental researches were developed and the best combination of process conditions was identified. Furthermore, the technology was checked of so-called delayed fracture properties of the products by subjecting them into a solution of hydrochloric acid. They also have proved to possess good anti-delayed fracture properties.

Effects of Inclusions on Delayed Fracture Properties of Three TWinning Induced Plasticity (TWIP) Steels

In the present study, delayed fracture properties of a high-Mn TWinning Induced Plasticity (TWIP) steel and two Al-added TWIP steels were examined by dipping tests of cup specimens in the boiled water, after which the microcrack formation behavior was analyzed. The TWIP steels contained a small amount of elongated MnS inclusions, spherical-shaped AlN particles, and submicron-sized (Fe,Mn)3C carbides. Since MnS inclusions worked as crack initiation sites, longitudinal cracks were formed along the cup forming direction mostly by MnS inclusions. These cracks were readily grown when high tensile residual stresses affected the cracking or hydrogen atoms were gathered inside cracks, which resulted in the delayed fracture. In the Al-added steels, MnS inclusions acted as crack initiation and propagation sites during cup forming or boiled-water dipping test, but residual stresses applied to MnS might be low for the crack initiation and growth. Thus, longitudinal cracks formed by MnS inclusions did not work much for delayed fracture. AlN particles present in the Al-added steels hardly acted as crack initiation or growth sites for the delayed fracture because of their spherical shape.

Evaluation of Delayed Fracture Property of High Strength Bolt Steels

Experimental studies attempting to evaluate the delayed fracture property of high strength bolt steels by using slow strain rate tests (SSRT) of circumferentially notched round bar specimens are reviewed in this paper. The relationship between the notch tensile strength (NTS) of the hydrogen-precharged specimens and the hydrogen content was approximated by power law relationship. It has been found that the local stress and the local diffusible hydrogen concentration control the occurrence of delayed fracture. To take into account the effect of hydrogen uptake from the environment, the notched bar specimens were subjected to cyclic corrosion test (CCT) and outdoor exposure, and their NTSs were measured using SSRT. The susceptibility of high strength steels to delayed fracture estimated from the decrease of NTS was correspondent to the fracture ratio of high strength bolts of the steels obtained by outdoor exposure tests, and was successfully evaluated with consideration of hydrogen uptake. Hydrogen entry behavior under CCT was monitored by electrochemical hydrogen permeation test, and continuous increase in hydrogen entry with growth of rust layer was observed. It is suggested that the “delay” of delayed fracture is the time required for the enhancement of the hydrogen entry efficiency.

Study on the Hydrogen Delayed Fracture Property of TRIP Steel by Slow Strain Rate Testing Method

The demands of high-strength steel have been steadily increased to reduce the weight of vehicles. Although the TRIP steel has been the promising candidate material for the purpose, high strength hinders the application due to the susceptibility to hydrogen delayed fracture in the corrosive environment. Moreover, the testing method was not specified in the ISO standards. In this work, the test method to evaluate the susceptibility of hydrogen delayed fracture was studied by slow strain rate testing technique. The four test experimental parameters were studied : strain rate, hydrogen charging time, holding time after hydrogen charging, and holding time after cadmium plating. The steel was fractured by hydrogen in case the strain rate was in the range of . It was confirmed that the slow strain rate test is effective method to evaluate the susceptibility to hydrogen delayed fracture. The holding time over 24 hrs after hydrogen charging, nullified the hydrogen effect, that is, the specimen was no more susceptible to hydrogen after 24 hrs even though the specimen was fully hydrogen-charged. Moreover, cadmium electroplating could not prevent from diffusing out the hydrogen from the steel in the experiment. The effective experimental procedures were discussed.

Evaluation of delayed fracture property of outdoor-exposed high strength AISI 4135 steels

Delayed fracture properties of AISI 4135 high strength steels with 1490 and 1310 MPa of tensile strength, represented as B15 and B13, respectively, have been studied by means of slow strain rate test (SSRT) of notched bar specimens after outdoor exposure at rural and coastal areas. The exposed specimens were kept at humid medium before SSRT to reproduce active hydrogen entry influenced by the rust layer and to homogenize hydrogen distribution. The influences of exposure site and exposure time on fracture stress have been investigated. The susceptibility of B15 to delayed fracture was obviously higher than that of B13.

Hydrogen Delayed Fracture Properties and Internal Hydrogen Behavior of a Fe–18Mn–1.5Al–0.6C TWIP Steel

The hydrogen delayed fracture (HDF) properties and internal hydrogen behavior were investigated in a Fe–18Mn–1.5Al–0.6C steel, a representative twinning induced plasticity (TWIP) aided steel. Slow strain rate tests (SSRT) were employed on both smooth and notched specimens to evaluate the effects of diffusible hydrogen on the HDF properties of the steel. Results showed that the fracture stress, fracture strain and time to fracture of the hydrogen pre-charged specimens were relatively insensitive to the amount of diffusible hydrogen. Fracture surface exhibited a ductile dimple fracture mode regardless of the diffusible hydrogen concentration. It was found that most hydrogen became non-diffusible after SSRT. The major trapping sites of hydrogen were dislocations, grain boundaries and twins. The activation energies for detrapping of hydrogen were estimated 35 kJ/mol for dislocations or grain boundaries, and 62 kJ/mol for twins. A comparison of the HDF properties of the present steel with those of other high strength steels revealed that the TWIP steel appeared to be relatively immune to hydrogen delayed fracture. This was due to the combined effects of (a) higher hydrogen solubility of austenite matrix (b) negligible portion of diffusible hydrogen to the total hydrogen, (c) decrease of diffusible hydrogen content during the deformation, and (d) no transformation of austenite to either e or α′ martensite.

Microstructural influences on hydrogen delayed fracture of high strength steels

This study was aimed at investigating the effect of the microstructural constituents of high strength steels on their hydrogen delayed fracture properties. For this purpose, a series of constant loading tests, slow strain rate tests, cyclic corrosion tests, and thermal desorption spectrometry analysis was conducted on the hydrogen pre-charged specimens with tempered martensite or full pearlite showing a similar tensile strength level of 1600 MPa. Constant loading tests and slow strain rate tests revealed that the tempered martensitic steel was more susceptible to hydrogen delayed fracture than the fully pearlitic steel. In slow strain rate tests, the maximum tensile strength decreased with increasing diffusible hydrogen content in a power-law manner. The content of hydrogen inflowing from environment was also simulated by cyclic corrosion tests. It was found that the fully pearlitic steel has the higher equilibrium hydrogen content than the tempered martensitic steel. The primary trapping sites were prior austenite grain boundaries for the tempered martensitic steel, and ferrite/cementite interfaces and dislocations for the fully pearlitic steel. SEM fractographs revealed that the cracks induced by hydrogen propagated along the prior austenite grain boundaries resulting in brittle intergranular fracture for the tempered martensitic steel while the fully pearlitic steel was fractured in a ductile manner.