SNCM439 Steel Research Articles

Multi-Criteria Decision Making in the Milling Process Using the PARIS Method

The Multi-Criteria Decision-Making (MCDM) process of milling SNCM439 steel is presented in this study. In this experimental study, 3 cutting tool parameters, namely the number of pieces, cutting piece material, and tip radius were considered and 3 cutting mode parameters, i.e. cutting speed, feed rate, and depth of cut changed in each experiment. SR and MRR are selected as the output parameters of the milling process. The PARIS method was used for MCDM, in which, the weights of SR and MRR were determined by 3 methods, namely AW, EW, and MW. Twenty-seven sets of ranking results for 27 alternatives (experiments) are presented. The GINI index was used to evaluate the stability of ranking alternatives. The results have determined the value of 6 input parameters to ensure the minimum SR and the maximum MRR simultaneously.

Crack Propagation Behavior of SNCM439 Steels in High-pressure Hydrogen Gas

Crack Propagation Behavior of SNCM439 Steels in High-pressure Hydrogen Gas

Effects of inclusions on the fatigue life of SNCM439 steel in high-pressure hydrogen gas

Four heats of commercially available JIS SNCM439 steel are prepared, and fatigue tests are conducted in air and hydrogen gas. The materials evaluated are all martensitic steel with a tensile strength of 900 MPa or less and contain nonmetallic inclusions of different sizes. A decrease in the fatigue limit is observed in the specimens with large nonmetallic inclusions, but the fatigue limit in air is approximately equal to the fatigue strength at 300,000 cycles in hydrogen. However, in the finite life region, the fatigue life in hydrogen significantly decreases owing to the presence of large nonmetallic inclusions. It was observed that hydrogen considerably affects the fatigue life even at low stress amplitudes close to the fatigue limit. This effect is considered to be dependent on the size of the initial crack originating from the nonmetallic inclusions; large nonmetallic inclusions accelerate the hydrogen-induced fatigue crack growth rate.

Effect of hydrogen on gigacycle fatigue properties of SNCM439 steel used for storage cylinder in 70 MPa hydrogen station

Effect of hydrogen on gigacycle fatigue properties of SNCM439 steel used for storage cylinder in 70 MPa hydrogen station

Fatigue Crack Initiation and Propagation Behaviors in Rotating Bending of SNCM439 Steel in Very High Cycle Regime

In order to ensure the long term durability of mechanical structures, the fatigue property of structural components should be clarified in the long life region such as the gigacycle regime. The rotating bending fatigue tests in very high cycle regime were carried out for a nickel chromium molybdenum steel for structural use of machines (JIS Material Code: SNCM439) in this study. Based on the initiation site of the fatigue crack, fracture modes were classified into the following typical three modes: (1) usual surface fracture, (2) surface defect-initiated fracture and (3) interior inclusion-initiated fracture, respectively. InS-Ndiagram, experimental data in the usual surface fracture mode appeared at higher stress levels with fewer loading cycles, whereas the data in the other two fracture modes appeared at lower stress levels with more loading cycles. Thus, the duplexS-Nproperty was confirmed for this steel in the very long life regime. In order to clarify the fatigue mechanism of the interior inclusion-initiated fracture, the quantitative evaluations were made by applying the stress intensity factor range. The fatigue crack initiation and propagation processes in the interior inclusion-initiated fracture were divided into four stages: formation of the fine granular area (FGA) due to initiation and coalescence of micro-debondings, formation of the fish-eye due to penny-shape crack propagation, crack propagation as surface crack and final catastrophic fracture.

OS1002 水素ステーション蓄圧器用SNCM439鋼のギガサイクル疲労特性に及ぼす水素の影響

OS1002 水素ステーション蓄圧器用SNCM439鋼のギガサイクル疲労特性に及ぼす水素の影響

Formation of Al and Cr Dual Coatings by Pack Cementation on SNCM439 Steel

Two stage pack cementation processes are developed to form dual Fe–Al–Cr layers on surfaces of SNCM439 steels. The first 550°C treatment assists to modulate adequate aluminum activity for the formation of iron-rich intermetallics. In the second 750°C treatment stage, simultaneous chromizing and aluminizing treatments are achieved by first forming a FeAl ferritic layer and then a surface layer with higher Cr content at later time. The current study examines the effects of second stage 750°C holding time, activator concentration, and Cr:Al ratio on coating structures. Fe–Al coatings consisting of Fe3Al and FeAl intermetallic phases are observed to form initially. This Fe–Al layer accounts for 25 to 32 μm thickness of the coatings and show good adherence with the substrates. The coating thickness increases parabolically with 750°C holding time. With prolonged treatment at 750°C, surface concentration of aluminum in powder packs drops with treatment time and increasing concentration of activator. A peak concentration exists at a depth below substrate surface. Aluminum is back diffused from the steel surface into the powder packs. The growth of Fe–Al intermetallics slows down. Surface layer then forms a thickness of 6 μm coating with 2–5 wt.% of chromium. Samples treated for longer than 6 h with over 12 wt.% of NH4Cl activator concentration or with Cr:Al ratio higher than 90:10 induce earlier chromium infusion and lead to porous coating structure due to Kirkendall effect. Eventually chromium carbide forms to cease further growth of the dual coating structures.

The effect of hydrogen on the fatigue life of Ni–Cr–Mo steel envisaged for use as a storage cylinder for a 70 MPa hydrogen station

SNCM439 steel specimens cut out from an 80 MPa storage cylinder prototype for a 70 MPa hydrogen station were used for inclusion rating analysis, hydrogen desorption analysis and tension–compression fatigue tests. The fatigue tests were conducted for hydrogen gas-exposed and unexposed specimens. Some specimens contained a small artificial hole, while the others were smooth. The testing frequencies were f = 0.2, 2, 20 and 500 Hz, and the stress ratios were R = −1, 0, 0.3, 0.5 and 0.7. In the smooth specimens, Al 2O 3 inclusion clusters were the fracture origins. There was an approximately 10-fold acceleration of the fatigue crack growth rates for the hydrogen gas-exposed specimens compared to the unexposed specimens. There was however no clear effect of hydrogen on the fatigue limit. The area parameter method was used to estimate the fatigue strength, using the upper limit of the fatigue crack growth acceleration with hydrogen.

Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengths

Gigacycle fatigue behaviors of two SNCM439 steels with different tensile strengthes were experimentally studied by rotating bending tests, to investigate the effects of the tensile strength obtained by different heat treatment processes on very high cycle fatigue failure mechanisms. The material with higher tensile strength of 1 710MPa exhibited typical gigacycle fatigue failure characteristics, whereas one with lower tensile strength of 1 010MPa showed only traditional fatigue limit during the tests and no gigacycle failure could be found even when the specimen ran up to more than 108 cycles. Metallographic and fractographic analysis were carried out by an optical microscope (OM) and scanning electron microscope (SEM). It showed two different crack initiation mechanisms that for the specimen with lower tensile strength the crack prefers surface initiation and for that with higher strength the crack initiates from subsurface inclusions revealed by a fish-eye like microstructure.

Fish-eye shape prediction with gigacycle fatigue failure

In this paper, a model using the energy approach was established to investigate the formation of fish-eye shape in the gigacycle fatigue failure. The model was used to predict the size of the fish-eye formed by a given subsurface inclusion. The predicted results were comparable with the fractography of SNCM439 steel and other experimental results of gigacycle fatigue available in the literature. The current model can capture the fish-eye shape and can be used to predict the life of crack initiation during fatigue failure.

Phenomenological Aspect of Hydrogen Enviroment Embrittlement of SNCM439 Steels

The hydrogen environment embrittlement of SNCM439 steels (Ni–Cr–Mo steels) was investigated in 20-MPa hydrogen gas at room temperature at different tensile strengths ranging from 800 to 1200 MPa by heat treatments. The results of tensile tests revealed that hydrogen environment embrittlement was observed as decrease of ductility, whereas the tensile strength of the specimens did not change. Although the uniform elongation of the specimens was the same in air and hydrogen, local contraction decreased with increasing tensile strength in hydrogen. In contrast to the specimens with smooth surface, the notch tensile strength of the specimens having a tensile strength of over 1000 MPa remarkably decreased. The analysis of fractures in the specimens revealed that a crack was initiated from the external of the specimens in hydrogen, whereas it was initiated from the center of the specimens in air. The fracture mode was the same in both the tensile tests with notched and notchless specimens, and the presence of hydrogen induced quasi-cleavage fracture. It was observed that intergranular fracture was dominant in highly embrittled materials, and quasi-cleavage fracture was ahead of intergranular fracture. Even in the notched specimens whose intergranular fracture was dominant, quasi-cleavage fracture was observed beneath the notch. These behaviors indicate that the susceptibility to hydrogen embrittlement was more remarkable in specimens with high tensile strength, and the large plastic deformation required for the formation of quasi-cleavage fracture induced hydrogen penetration and hydrogen environment embrittlement.

612 SNCM439鋼低温焼戻し材の回転曲げ荷重下における超高サイクル疲労特性(GS.F-2 疲労)

Fatigue specimens of SNCM439 steel tempered at 160deg.C and 180deg.C were prepared and fatigue tests in gigacycle regime were performed in rotating bending. These steels tempered at both temperatures indicated significant dual S-N characteristics consisting of S-N curves for surface-induced fracture and interior inclusion-induced fracture. In the case of interior inclusion-induced fracture, fish-eye pattern can be observed depending on the depth of the inclusion played a role as the crack starter. Fine granular area(FGA) was found around the inclusion as the crack starter and the SIF range at FGA front was almost constant.

ラジカル窒化したSNCM439鋼の疲労特性(S14-4 高強度鋼の疲労と水素の影響,S14 金属材料の疲労特性と破壊機構)

Rotating bending fatigue tests were carried out for radical nitrided SNCM439 steels in order to investigate the influence of nitriding time on the fatigue strength. The hardend layer was increased with increase in nitriding time. However, there was not or little influence of nitriding time on the fatigue strength, though the fatigue strength was increased by nitriding. Fracture at low stress levels occurred from internal inclusions near the boundary between hardened and non-hardened layers. By removing the compound layer, the origin of fracture was changed from subsurface to surface and fatigue strength was decreased.

105 高強度鋼の超高サイクル疲労特性と破面性状に関する研究

In order to investigate facture mechanism of super-high cycle fatigue and subsurface inner failure, high cycle fatigue test was conducted for rotating bending on SUJ2 steel and SCM435 steel, and for pure bending and pure torsion on SNCM439 steel. FRASTA analysis to simulate the process of fatigue crack propagation using 3D configuration of coupled areas of opposing fracture surface via computer was conducted.

912 WC-Co 溶射 SNCM439 鋼のフレッティング疲労特性

912 WC-Co 溶射 SNCM439 鋼のフレッティング疲労特性

Influence of Compound Layer on Fatigue Strength of Radical Nitrided SNCM439 Steel

Rotating bending fatigue tests were carried out for a radical nitrided SNCM 439 steel in order to investigate the influence of compound layers on the fatigue strength and fracture mechanism. By nitriding at 480°C to the steel quenched at 835°C and tempered at 160°C, the fatigue strength increased, though the internal matrix softened and the static strength decreased. In both of the nitrided steel and the quench-tempered steel, fish-eye fracture occurred in long life region, and the fracture initiated from a specimen surface in short life region. By removing the compound layers in the nitrided steel, the fatigue strength decreased dramatically and the fracture origin changed from a subsurface to a specimen surface. That is, compound layers make the fatigue life increase through both of the suppression of the initiation of a surface crack and the change of fracture origin.

712 20kHz 及び 600Hz でのギガサイクル疲労試験

Fatigue tests were carried out at frequencies of 20kHz, 600Hz and 100Hz for low-temperature-tempered SNCM439 steel. Ultrasonic, high-speed servohydraulic and electromagnetic fatigue test machines were used for 20kHz, 600Hz and 100Hz tests, respectively. The fatigue tests showed that fatigue properties were independent of the frequencies, while fatigue lives were scattered on the S-N diagram. Almost all specimens were broken from internal Al_2O_3 inclusions of various sizes between 9 and 82μm. ODAs were observed in all specimens broken over (10)^7 cycles. ODA sizes were also independent of the frequencies. The scattered S-N diagram was found to be expressed by an unique curve by normalizing stress amplitudes σ_a with fatigue limits σ_w estimated by Murakami's equation. The modified S-N diagram showed a clear fatigue limit between (10)^9 and (10)^<10>cycles.

Effect of Frequency on Giga-Cycle Fatigue Properties for Low-Temperature-Tempered SNCM439 Steel.

Fatigue tests were carried out at frequencies of 20 kHz, 600 Hz and 100 Hz for low-temperature-tempered SNCM439 steel. Ultrasonic, high-speed servohydraulic and electromagnetic fatigue test machines were used for 20 kHz, 600 kHz and 100 Hz tests, respectively. The fatigue tests showed that fatigue properties were independent of the frequencies, while fatigue lives were distributed in wide range on the S-N diagram. Almost all specimens were broken from internal Al2O3 inclusions of various sizes between 9 and 82 μm. Fatigue lives were found to depend on the inclusion sizes. ODA, i.e. optically dark area, was observed in all specimens broken over 107 cycles. ODA sizes were also independent of the frequencies. The scattered S-N diagram was found to be expressed by an unique curve by normalizing stress amplitudes σa with fatigue limits σ'w estimated by Murakamis equation. The modified S-N diagram showed a clear fatigue limit between 109 and 1010 cycles.

710 SNCM439 鋼の超長寿命疲労挙動とショット・ピーニングの影響

710 SNCM439 鋼の超長寿命疲労挙動とショット・ピーニングの影響

Gigacycle fatigue properties for high-strength low-alloy steel at 100 Hz, 600 Hz, and 20 kHz

Fatigue tests were carried out at frequencies of 100 Hz, 600 Hz, and 20 kHz for low-temperature-tempered JIS SNCM439 steel. Almost all specimens were broken from internal inclusions and frequency effects were not found on the gigacycle fatigue properties. The fatigue limit appeared at over 10 9 cycles.