Abstract:

The aging and failure of sealing rubber in liquid propellant environments is a critical factor affecting the reliability of launch vehicle propulsion systems.Accelerated aging tests serve as a crucial tool for evaluating the long-term storage performance of these materials by studying their degradation patterns and predicting service life.This paper systematically reviews the research progress on accelerated aging of typical sealing rubbers exposed to propellant media.It summarizes the underlying aging mechanisms，the evolution of material properties，and existing lifetime prediction methods.Future research directions in this field are also discussed.The aim of this review is to provide a reference for material optimization and engineering applications of media-resistant rubber.

Abstract:

The combined cooling method represented by active-passive hybrid cooling was an efficient cooling technique for combustion chambers.Nevertheless， this cooling mode was subject to numerous influencing factors and intricate internal interactions.This paper aimed to investigate the influence of composite flame tubes on film cooling performance under an active-passive hybrid thermal protection configuration integrating composites and film cooling.A physical model and numerical method for film cooling on composite plates were established， and simulations were conducted by varying the film cooling inlet angle and the fiber orientation of fiber-toughened composite materials.The results show that with the increase of x，the comprehensive cooling efficiency drops significantly at large fiber angles， whereas the cooling performance degradation is relatively mild at small fiber angles.Within a certain region downstream of film holes，the comprehensive cooling efficiency corresponding to large fiber angles decreases sharply，while the cooling performance degradation at small fiber angles remains relatively moderate.A smaller inlet angle leads to weaker influence of fiber orientation on cooling performance downstream of film holes.The difference in cooling performance upstream of film holes caused by fiber angles is more pronounced than that downstream，and the impact of fiber angles is far less significant than that of inlet angles.

Abstract:

TFM is one of the current research hotspots in array ultrasound imaging algorithms，the noise and disturbance of the original signal will affect its final imaging quality.In this regard，this paper combined the CF-based adaptive weighting method，which was applied maturely in the field of medical ultrasound imaging，with the full focus full matrix imaging algorithm in the field of industrial ultrasound imaging，proposed the TFM-CF algorithm，and conducted detection imaging verification experiments using phased array standard test blocks.In the meantime，it used quantitative indicators to evaluate the TFM-CF and the original TFM algorithms.The result shows that the resolution and contrast under TFM-CF processing are better than the original algorithm.For non-uniform depth transverse holes，the average improvement in lateral resolution is that of 20 μm，and the average improvement in contrast CR is 2.71 dB.In the test of non-uniform cross transverse holes，the average improvement in longitudinal resolution is 41 μm，and the average improvement in contrast is 5.26 dB.The adaptive weighting method based on CF can effectively improve the imaging quality of TFM can be proved.

Abstract:

Aiming at the problem of low tolerance of electronic products under high overload environments，an epoxy encapsulant with excellent performance was investigated and prepared.ANSYS 19.2 software was employed to conduct simulation analysis on the product assembly under a 20 000 g impact condition，and the equivalent stress distribution of BGA solder balls under different reinforcement schemes was calculated.The results show that the equivalent stress exerted on the solder balls is minimized in the assembly using both underfill and encapsulant simultaneously.The simulation results are verified through reliability tests and analyses.Experimental results demonstrate that the combined application of underfilling and encapsulation reinforcement can effectively improve the reliability of electronic products in a 20 000 g high overload environment.

Abstract:

Fatigue tests were conducted on DD6［001］ single crystal alloy specimens with the same service state as aeroengine gas turbine blades，and sufficient fatigue test data were acquired.Based on data transformation，four subsamples for fatigue life testing and one subsample for fatigue strength testing under 10⁷ cycles were designed.The safety screening subsample method was adopted to optimize all subsamples.Fatigue P-S-N curves corresponding to survival probabilities of 0.13%，50% and 99.87% were fitted with fatigue stress-life data via Origin software.According to the random fatigue limit model，the relational equations between fatigue strength （stress） and fatigue life of DD6［001］ alloy under the three survival probabilities were established，and the reliability evaluation system for aeroengines was constructed.The research conclusions can provide basic fatigue reliability data for the modeling，testing and probabilistic design of endurance tolerance of DD6［001］ alloy structural components in aeroengines.They can also be applied to predict the service life and fatigue strength of DD6［001］ alloy parts，and serve as quantitative indexes for the acceptance and performance evaluation of fatigue properties of aeroengine components made of DD6［001］ alloy.

Abstract:

Corrosion，fatigue and impact often occured simultaneously during the service of joints which play a vital role in engineering structures.It was essential to study the mechanical behavior of joints under their combined effect.This paper investigated the tensile behavior of metal-composite bolted joints under the combined condition of corrosion，fatigue and impact by experimental and simulation methods.The research results show that the simulation results are in good agreement with the experimental results，which proves the effectiveness and accuracy of the simulation model in predicting the mechanical behavior of the connection under the combined action of corrosion，fatigue and impact.The combined effect of corrosion，fatigue and impact significantly reduces the tensile strength of the bolted joints.The peak value of the load-displacement curve of bolted joint becomes lower with the extension of the salt spray corrosion time.

Abstract:

This study used composite patches to repair circular penetration damage on 2A12 metal sheets，and compared the tensile strength of four repair forms of test specimens：intact，circular penetration damage，single-sided repair and double-sided repair.Based on the single-sided repair process of composite patches，the tensile strength of three different aperture circular penetration damage adhesive repair test specimens was studied.Meanwhile，considering the influence of the intrinsic elastic strength and bonding strength of damaged aluminum alloys on the strength recovery rate of repaired parts，a correlation formula between the damage ratio and the required bonding area was proposed.The research results indicate that the repair effects of single-sided and double-sided patches on composite materials meet the acceptance criteria for the strength of repaired structures，with strength recovery rates reaching 99.68% and 109.43%，respectively.The tensile strength of three types of damaged pore size adhesive repair parts varies less with pore size.The fractures all occur at the cross-section where the metal base metal through-hole is located，and the composite patch shape is intact.The main failure modes are tensile failure of the mother plate and shear failure of the adhesive layer.

Abstract:

To meet the structural stiffness and lightweight requirements of satellite engine brackets，the applicability and reliability of 3D printing technology for manufacturing such components were investigated.Taking the 3D printed flange and upper joint of a satellite engine bracket as the research objects，the macroscopic mechanical properties and internal quality of the coupons were evaluated through mechanical testing，X-ray inspection，and fluorescence testing.Adhesive bonded performance and structural stability were assessed via tensile-shear tests，tube shear tests，and high temperature static loading tests.The verification results show that the mechanical properties of the coupons meet the design requirements （Yield strength ≥180 MPa，elastic modulus 65-85 GPa，elongation at break ≥7.0%）.The internal and surface quality of the parts meet acceptance criteria.The tensile-shear strength of the adhesive joints is ≥16 MPa，and tube shear failure loads of both room temperature and high temperature significantly exceed qualification level loads.Vibrationtest data for the 3D printed joint support and engine assembly are superior to those of the conventional braided joint bracket.Overall，the satellite engine bracket manufactured using 3D printing technology satisfies application requirements in terms of mechanical performance，adhesive bond quality，and structural stability，demonstrating strong engineering reliability.

Abstract:

Power VDMOS was widely used in various secondary power supply systems in aerospace field.However，the mechanical damage of chip existed in bonding process.Taking power VDMOS devices as the research object，it was clear that bonding failure was the damage caused by different degrees of mechanical action，the main factors of mechanical damage are chip structure，bonding material and bonding parameters.The results show that the damage degree can be divided into polysilicon damage，crack and crater from light to severe.The Al layer cushioned，and its thickness increases from 4 μm to 6 μm，which can reduce the damage degree.Microstructure analysis and simulation show that the damage occurs at both ends of the bonding point，the stress distribution of the bonding interface can be changed by adjusting the downward movement and amplitude.Reducing the bonding pressure and ultrasonic power can avoid bonding damage caused by stress concentration.

Abstract:

To solve the connection failure problem of pin A3 of the CCGA-packaged FPGA chip in a certain aerospace product，this paper constructed a "multi-dimensional detection+multi-disciplinary coupling analysis" system and conducted research by combining X-ray detection，SEM analysis，and mechanical and thermal simulation.The results show that the failure is not caused by defects in the chip''s solder column or batch problems in the welding process，but by the dual weak links formed between the solder and the solder mask，and between the copper strip and the solder.Fatigue cracks initiate under the combined action of mechanical loads and temperature cycles，and the large voids distributed at close distances in the solder joint further accelerate the crack propagation，resulting in an occasional defect failure.Based on the failure mechanism，this paper proposes an immediate solution of replacing the spare board，and plans to adopt long-term optimization directions such as inert gas/vacuum welding to reduce the bubble rate and optimize PCB design to enhance mechanical support，providing technical references and feasible process improvement paths for the high-reliability application of aerospace-grade CCGA-packaged devices.

Abstract:

Using basalt fiber-reinforced polymer （BFRP） bars to replace traditional steel bars and developing BFRP bar-reinforced seawater and sea sand concrete structures provided a new solution to simultaneously address the shortage of river sand resources and the easy corrosion of steel bars in marine concrete structures. However， BFRP bars might still suffer from fiber-resin interfacial bonding degradation and resin matrix hydrolysis in highly alkaline environments，which further leads to the degradation of their mechanical properties.Based on the electromechanical impedance （EMI） technique，a piezoelectric patch-BFRP bar degradation monitoring sensor was developed.Combined with accelerated corrosion tests，the degradation process of the interlaminar shear strength of BFRP bars and the variation law of the sensor monitoring signals were measured and analyzed.The results show that under high temperature and strong alkaline environments，with the increase of corrosion age，the interlaminar shear strength of BFRP bars first increases slightly and then decreases continuously.In contrast，the characteristic peak frequency and various statistical damage indices of the sensor conductance response exhibit regular monotonic increasing or decreasing changes.By establishing regression equations between the sensor characteristic indices and the mechanical property indices of BFRP bars，the quantitative prediction of the degradation degree of BFRP bars is realized.

Abstract:

Aiming at the problem that the bolt connection structure was prone to loosening under transverse vibration，the axial force attenuation curves of bolts under different displacement amplitudes were obtained by transverse vibration test，and the influence of displacement amplitude on the loosening behavior of 30CrMnSiA bolt connection structure was studied.The attenuation curve of the bolt axial force was obtained through transverse vibration tests.The damage morphologies of the contact thread surfaces were characterized using optical microscopy，scanning electron microscopy，and white light interferometry，while the chemical composition of the damaged areas was analyzed via energy dispersive X-ray spectroscopy.The results indicate that the degree of bolt loosening increases significantly with the displacement amplitude.Specifically，the loosening degree rises from 2.7%（at A_f=0.10 mm） to 13.8%（at A_f=0.35 mm），accompanied by aggravated damage on the contact thread surfaces.When the displacement amplitude exceeds 0.25 mm，a secondary rapid decline stage emerges in the axial force during the loosening process.Moreover，the dominant wear mechanism of the thread surfaces transitions from being primarily abrasive wear and adhesive wear under small displacement amplitudes to being predominantly fatigue wear under large displacement amplitudes.Consequently，the loosening behavior of the bolted joint is governed by the interfacial wear mechanism，which is critically regulated by the displacement amplitude.

Abstract:

Spacecraft in medium Earth orbit was constantly exposed to the high-energy proton irradiation environment of the Earth''s radiation belts.The development of anti-radiation cover glass that possess both high electronic stopping power and low displacement damage was the key to reducing the impact of the space radiation environment on solar cells.In this paper，the energy deposition and damage evolution laws of lead-doped borosilicate glass （0-10% Pb） under 10 MeV proton irradiation were simulated using the SRIM program.The research results show that the lead doping significantly enhances the electron stopping power，with the total ionization energy loss ratio exceeding 99% and increasing linearly with the lead concentration.The average proton range expands from 743.5 μm（undoped） to 866.6 μm（10% lead doping），and the range dispersion increases by 32.3%.However，the lead doping also intensifies deep displacement damage，with the peak vacancy density migrating towards the end of the range，and the total number of vacancies increasing nonlinearly with the lead concentration.The growth rate slows down due to the reduction in nuclear stopping cross-section and the weakening of atomic binding energy caused by the loosening of the glass network.The study indicates that the lead doping suppresses shallow layer damage through enhanced electron interaction，but the accumulation of deep displacement defects requires a balance between optimizing the doping concentration and the densification of the glass structure.

Abstract:

This study investigated the corrosion behavior of Q235 carbon steel in the coastal atmospheric environment of Hainan.Through a four-year atmospheric exposure test，the corrosion kinetics，morphological evolution，and corrosion products of Q235 carbon steel were systematically analyzed.The results indicate that the corrosion rate of Q235 carbon steel in this environment increases over time，with the rust layer exhibiting no significant protective effect on the substrate.The corrosion products were predominantly composed of γ-FeOOH（lepidocrocite）.After prolonged exposure，β-FeOOH（akaganéite） and minor Fe₃O₄（magnetite） are additionally detected.This research provides critical insights into understanding the corrosion mechanisms of Q235 carbon steel in Hainan''s coastal-marine atmosphere and predicting its long-term service lifespan.

Abstract:

K424 alloy was a kind of cast nickel-base superalloy that was rich in Al and Ti element and low density.Turbine blades made of this alloy cracked after overheated condition.In order to investigate the failure mechanism of K424 alloy blades，the morphology，composition and microstructure was analyzed by means of stereoscopic microscope，optical microscope，scanning electron microscope（SEM），energy dispersive spectroscopy（EDS）and micro-hardness testing machine.The analysis results show that the cracks in the blades have interdendritic morphology and the oxidation layer thickness on the fracture is more than 1 μm；the fracture mode is creep fatigue and high temperature oxidation.After overheated condition，the cubic γ’ phase（size~0.4 μm）transforms into secondary spherical γ’ phase（size ~0.08 μm），thus impairing the anti creep property.As a result，cracks initiate at dendrite grain boundaries，insular γ/γ′ eutectic-matrix interface and bulk carbide-matrix interface where stress concentration，deformation uncoordination and strength weaken position exists.After crack initiation，the Al，Ti and Cr elements in the fracture surface endure selective oxidation at an oxidizing atmosphere，resulting in an increasing volume and thus promote the expansion of cracks.

Abstract:

Fracture occurred in a batch of 1Cr17Ni2 bolts during storage after motor assembly.Through methods such as macro/microscopic morphology analysis and energy-dispersive spectroscopy（EDS），the failure mode was identified as brittle fracture with a mechanism of hydrogen embrittlement.To investigate the cause of failure and evaluate the use safety of bolts from other batches of the same specification，comparative tests including microhardness，metallographic analysis，and hydrogen content measurement were conducted on the failed batch and two other batches.The step-loading method was employed to reproduce the failure mode.The results indicate that the internal hydrogen content of the failed batch is significantly higher than that of the other batches.Furthermore，its in homogeneously distributed martensitic microstructure with coarse grains lead to local hydrogen enrichment，which triggered delayed hydrogen embrittlement fracture under assembly stress.In a laboratory environment，the failure phenomenon of the failed batch is successfully reproduced using the step-loading method.It is also evaluated that the other two batches show no hydrogen embrittlement susceptibility and are safe for use.This study demonstrates that the step-loading method can effectively reproduce hydrogen embrittlement failures and is suitable for the rapid assessment of hydrogen embrittlement susceptibility in bolt products prior to their use.

Abstract:

In response to the application requirements of dual-phase high-strength steel FB590 in various structural and safety components，this study investigated the fatigue fracture morphology and failure mechanism of samples subjected to pre-bending deformation and subsequent annealing treatment.The results reveal that the fatigue fracture surfaces of FB590 samples consist of three distinct regions：the fatigue initiation zone，the crack propagation zone，and the final fracture zone，exhibiting typical characteristics of fatigue failure.In the original sheet samples，fatigue cracks predominantly initiate from surface defects or local stress concentration induced internal inclusions，with the instantaneous fracture zone characterized by large dimples，indicating good ductility.After pre-bending deformation，the R20 samples （r=20 mm） exhibit multi-source fatigue fractures，with cracks initiating from inner surface processing defects.Under high stress conditions，fatigue striations appear groove-ridge shaped，while under low stress conditions，they show cleavage-like morphology.The number and size of dimples decrease，reflecting a reduction in ductility.In the R10 samples （r=10 mm），fatigue cracks initiate mainly from inclusions，and the instantaneous fracture zone exhibits a mixed fracture mode with fewer dimples.Following annealing treatment，the R20an samples maintain a multi-source fracture pattern，initiating from surface defects or inclusions.At high stress condition，the fracture mode is predominantly transgranular fracture，while at low stress condition，intergranular fracture is observed.The final fracture zone comprises a small number of dimples and a large number of micro voids.The R10an samples exhibit single-source fatigue fractures，with evident plastic slip traces along grain boundaries in the crack propagation zone.The dominant fracture mode is transgranular fracture，accompanied by a relatively small number of dimples in the final fracture zone.A comprehensive analysis indicates that pre-bending deformation significantly degrades the fatigue performance of FB590 steel.Although annealing treatment partially improves fatigue resistance，its effectiveness in restoring the ductility loss induced by bending deformation is limited.

Abstract:

A failure analysis was conducted for the leakage of metal diaphragm in expulsion efficiency test of propellant tank by the methods such as macrostructure and microstructure analysis，EDS and metallographic analysis.Through the simulation of the metal diaphragm rolling process，the stress and strain behavior was studied.The results show that the major cause of the leakage is the gas porosity in diaphragm.The gas porosity diameter（about 0.45 mm） nearly penetrates the material wall thickness（about 0.45 mm），causing a reduction in the load-bearing capacity of the leakage area.Before the end of expulsion efficiency test，the material wall thickness reduction caused by diaphragm rolling is less than 0.1 mm.At this moment，the porosity defect has not leaked.At the end of expulsion efficiency test，the diaphragm is about to adhere to the inner wall of propellant tank，which increases the pressure difference on the diaphragm to 1.7 MPa.And 300 MPa stress and 0.2 stain exceeds the defect exposure limit.The gas porosity ruptured，which resulted in diaphragm leakage generated finally.

Abstract:

Through macro-observation，metallographic analysis，fracture surface examination，energy dispersive spectroscopy，and thermal simulation testing，an in-depth investigation was conducted on vacuum electron beam welds and diffusion welding cracks in stainless steel micro-flow controllers.Results indicate that the primary cause of welding cracks is the presence of phosphorus-containing low-melting-point eutectic phases generated by diffusion welding on the butt joint surfaces of vacuum electron beam welds.These low-melting-point eutectic phases not only increase phosphorus impurities within the weld pool，triggering crystallization-induced hot cracks，but also melt within the heat-affected zone of the vacuum electron beam weld，leading to diffusion weld cracking.Furthermore，employing a modified welding method with a large defocusing amount for the vacuum electron beam weld resulted in a wider weld with shallow penetration.The resulting columnar crystals grew nearly parallel to the weld joint plane，reducing the weld''s ability to resist tensile stresses during solidification and further increasing the risk of crystallization hot cracking initiation and propagation.Ultimately，during leak testing，crystallization-induced thermal cracks originating at the junction between the vacuum electron beam weld and the diffusion weld propagated outward，causing leakage.

Abstract:

Environmentally friendly and efficient liquid oxygen/kerosene was used as rocket propellant for the new generation of carrier rockets.The boiling point of liquid oxygen was as low as -183 ℃，which put forward strict requirements for the low-temperature sealing of the carrier rocket body valve disc.At present，the metal/non-metal（polytetrafluoroethylene） low-temperature valve disc was prepared by hot pressing process.The non-metal interior was prone to producing black impurity defects，which had the risk of seal failure.In view of the phenomenon of black impurity inside the non-metal，this paper analyzed the formation reasons of black impurity defects inside the non-metal in detail.It was proposed to reduce the black impurity phenomenon by non-metallic powder sieve process，improving the sealing reliability and safety of the valve disc.At the same time，the risk analysis of seals with black spot defects inside was carried out through mechanical test and valve assembly test.The black impurity inside the non-metal is confirmed by element analysis that the black impurity is caused by the oxidation of low molecular weight polytetrafluoroethylene under high temperature and high pressure.In addition，the mechanical test results show that the black spot inside the non-metal has no effect on its tensile strength，modulus，elongation at break and impact strength.The low-temperature valve assembly test with black spots inside the sealing surface ±1.5 mm also passes the benchmark test.

Abstract:

Metal tubings，owing to their structural characteristics and complex service environments，were prone to leakage failures，significantly compromising their reliability.Based on multiple typical failure cases，this study systematically analyzed the primary leakage modes and mechanisms of metal tubings.Research reveals that fatigue-induced leakage is the most prevalent failure mode，initiated by factors such as loosely braided wire mesh，manufacturing defects in the bellows，additional vibrations or resonance due to improper type selection，and stress concentration during assembly.Crack propagation exhibits multi-origin fatigue characteristics.Corrosion-induced leakage primarily stems from stress corrosion cracking and localized pitting，where the synergistic effect of corrosive media and residual stress leads to brittle fracture or penetrative corrosion pores.Leakage from welding defects is predominantly associated with process-related issues such as oxide film inclusions in TIG welding and undesired flow of brazing filler metal，resulting in the formation of micro-channels.Mechanical damage leakage involves wall thinning caused by puncture during manufacturing or interference wear during operation，while fretting wear arises from long-term contact between the wire mesh and the bellows，leading to progressive damage.

Abstract:

A summary of failure analysis cases for chip thick film resistors and resistor arrays is presented.The primary failure modes observed are open circuits and resistance value changes.Eight causes leading to open circuits and resistance variations are illustrated through practical examples.Utilizing testing methods such as optical microscopy，scanning electron microscopy，and energy dispersive spectroscopy，the analysis approach for thick-film resistors is categorized from failure phenomena to specific analytical processes.The fundamental sequence for failure analysis in these products is established as proceeding from the exterior to the interior.

Abstract:

The forming and expanding of cracks in high strength aluminum alloy self-piercing riveting process were studied，the forming simulation model of self-piercing riveting process was established，the forming and expanding of cracks in high strength aluminum alloy self-piercing riveting joint forming process was analyzed，and the self-piercing riveting process was verified by experiments.The results show that the stamping speed has an obvious influence on the structure and strength of the mechanical inner lock of 7075 aluminum alloy self-piercing riveting joint.With the increase of punching speed，the crack at the joint turns from crack to gap，and the mechanical inner lock size and strength of the joint decrease respectively.The punching speed is positively related to the equivalent plastic strain rate of the sheet.The equivalent plastic strain rate of sheet increases with the increase of punching speed，the lower sheet is more prone to crack and the damage of joint is more serious；For the self-piercing riveting of low ductility aluminum alloy materials，the punching speed shall be reduced to improve the forming quality of self-piercing riveting joints.

Abstract:

The non-homogeneous anisotropy of carbon fiber reinforced plastic （CFRP） was prone to causing processing defects and material damage during secondary processing，leading to deviations in material performance and affecting design standards.Currently，the characterization of CFRP processing quality and its influence on material mechanical properties were not fully studied.This paper adopt a multi-scale characterization method，selected burr height Δh，three-dimensional roughness Sa value，and surface porosity P as the evaluation indicators of CFRP milling quality，and explore the influence of milling parameters on the above evaluation indicators.Through the mechanical property tests of V-shaped shear specimens and open-hole tensile specimens，the influence laws of each index on mechanical properties were studied.The results show that the spindle speed and feed rate are the main factors affecting burr height and three-dimensional roughness，while the cutting depth has a significant impact on surface porosity.At the moment，it is found that the open-hole tensile strength is more sensitive to processing quality，with the highest strength degradation of 14.3%；three-dimensional roughness and surface porosity have a significant impact on the degradation of in-plane shear strength，with the highest strength degradation rate of 10.21%.This study can provide a basis for the research on the correlation between the processing quality and mechanical properties of CFRP structural components in aircraft.

Abstract:

Fe-Mn-Al-C series high Al steel was a new type of lightweight austenitic alloy steel with excellent mechanical properties.In this paper，the wear resistance of Fe-20Mn-6Al-1.5C-5Cr test steel in as-cast state （ZG-1-Z） and forging state（ZG-1-D） was studied by impact wear test，and compared with Mn13 steel.The test results show that the weight loss rate of Mn13 steel increases linearly with the increase of impact time，while the weight loss rate of test steel（ZG-1-Z，ZG-1-D） also increases with the increase of impact time，but impact has a relatively small effect on the growth of weight loss rate when the impact time is 1 h and 2 h.The wear form of Mn13 steel is mainly characterized by plastic deformation，and the wear surface of the test steel（ZG-1-Z，ZG-1-D） shows a large-area cutting furrow morphology，and a fatigue crack layer with a thickness of about 5 μm is formed on the surface.According to the nanoindentation test，different impact times have little effect on the hardness value of Mn13 steel，and have a great influence on the hardness value of as-cast（ZG-1-Z） test steel，while the hardness of forged （ZG-1-D） test steel changes steadily，with obvious hardening layer and high overall hardness value.