IMPROVEMENT ON SUSCEPTIBILITY OF ULTAR-HIGH STRENGTH STEEL TO STRESS CORROSION CRACKING BY HIGH TEMPERATURE QUENCHING

The effect of quenching temperature on the stress corrosion cracking of 30Cr3SiNiMoV ultra-high strength steel in 3.5% NaCl aqueous solution has been studied.The threshold K_(ISCC) may continuously increase with the quenching temperature raised from 870 to 1200℃ . All of the fractures are intergranular.The analyses of the segregation along prior austenitic grain boundaries,grain size and other microstructural factors reveal that the inerease of K_(ISCC) is mainly due to the coarsening of prior austenitic grains.

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

SCC evaluation of ultra-high strength steel in acidic chloride solution

The stress corrosion crack （SCC） susceptibility of ultra-high strength steel AerMet 100 was investigated by slow strain rate technique （SSRT）, tensile with polarization and surface analysis technique. The curves of tf^Cl/tf^W -strain rate are divided into three regions： stress-dominated region, SCC-dominated region, and corrosion-dominated region, so as the curves of εf^Cl/εf^W - strain rate and tm/tf-strain rate. The results of tensile tests with polarization show that the main SCC mechanism of AerMet 100 is anodic dissolution, which controls the corrosion process. The three regions have been discussed according to the relationship between the rate of slip-step formation and the rate of dissolution. Fracture appearances in different environments were analyzed by scanning electron microscopy （SEM）. SCC fracture appears as a mixture of intergranular and dimples, while it is totally dimples in the inert environment. The εf becomes the parameter to predict tf because the relationship between εf^Cl/εf^W and tf^Cl/tf^w is a straight line for AerMet 100.

Manifestations in Corrosion Prophase of Ultra-high Strength Steel 30CrMnSiNi2 A in Sodium Chloride Solutions

The corrosion behaviors of ultra-high strength steel 30CrMnSiNi2A in sodium chloride solution were studied by weight loss and electrochemical methods. The morphology of corrosion products was observed using scanning electron microscopy（SEM） and the composition was analyzed using an energy dispersive spectroscopy（EDS） and X-Ray diffraction （XRD）. The experimental results showed that the corrosion came from pitting corrosion and the rust layer was composed of outer rust layer γ-FeOOH and inner rust layer Fe_2O_3 with a little β-FeOOH. The correlation between corrosion rate and test time accorded with exponential rule. The corrosion current measured by polarization methods was higher than that calculated by weight loss method after a long-time immersion, the main reason was that,β-FeOOH and γ-Fe_2O_3 transformed by γ-FeOOH led to overestimating corrosion rate. The processes of corrosion prophase were obtained from XRD and EIS results. The corrosion product, Fe（OH）_2 formed at the initial stage stayed at a non-steady state and then consequently transferred to γ-FeOOH, γ-Fe_2O_3 or β-FeOOH.

Microstructure and Mechanical Properties of Aer Met 100 Ultra-high Strength Steel Joints by Laser Welding

AerMet100 ultra-high strength steel plates with a thickness of 2 mm were welded using a COz laser welding system. The influences of the welding process parameters on the morphology and microstructure of the welding joints were investigated, and the mechanical property of the welding joints was analyzed. The experimental results showed that the fusion zone of welding joint mainly consisted of columnar grains and a fine dendrite substructure grew epitaxially from the matrix. With the other conditions remaining unchanged, a finer weld microstructure was along with the scanning speed increase. The solidification microstructure gradually transformed from cellular crystal into dendrite crystal and the spaces of dendrite secondary arms rose from the fusion line to the center of the fusion zone. In the fusion zone of the weld, the rapid cooling caused the formation of martensite, which led the microhardness of the fusion zone higher than that of the matrix and the heat affected zone. The tensile strength of the welding joints was tested as 1 700 MPa, which was about 87% of the matrix. However, the tensile strength of the welding joints without defects existed was tested as 1832 MPa, which was about 94% of the matrix.

Analysis of the effect of geometrical parameters on fatigue performance of spot-weld joint for ultra-high strength steel

This paper studied the spot welding structure of ultra-high strength steel 22MnB5.ANSYS software was adopted to simulate its static strength;BS5400 algorithm was used to calculate the fatigue life;and the grouping method was used to test the fatigue performance of tensile shear spot weld specimens.The simulation results were in good agreement with the experimental values.Based on the validation of the simulation method,influences of different structural parameters on static strength and fatigue life were explored by adopting single factor.The results showed that within the selected structure parameter range,increase of the sheet thickness,nugget diameter,sheet width and overlapping length can lead to longer fatigue life.Besides,the fatigue life of spot weld took on a linear relationship with the overlapping length,a DoseResp relationship with the sheet thickness,and a single exponential decay relationship with the sheet width and the nugget diameter.Moreover,in order to estimate the impact from various parameters on the fatigue life of the specimens,the Taguchi orthogonal design method was applied in the simulation design.The simulating result indicated that influence of the sheet thickness on fatigue life was the most significant.In addition,the effects of nugget diameter,sheet width and overlapping length on fatigue life were reduced in turn.

Effect of anions on stress corrosion cracking behaviors of ultra-high strength steel 23Co14Ni12Cr3Mo

The effects of chloride,sulfate and carbonate anions on stress corrosion behaviors of ultra-high strength steel 23Co14Ni12Cr3Mo were studied by stress corrosion cracking(SCC)test method using double cantilever beam(DCB)specimens.The SCC morphology was observed by using scanning electron microscopy(SEM)and the composition of corrosion products was analyzed by using energy dispersive spectrometer(EDS).The results show that the crack propagates to bifurcation in NaCl and Na2SO4 solution,while the crack in Na2CO3 solution propagates along the load direction.The SCC rate in NaCl solution is the highest,while lower in Na2SO4 solution and little in Na2CO3 solution.From the SEM morphologies,quasi-cleavage fracture was observed in NaCl and Na2SO4 solutions,but intergranular features in Na2CO3 solution.The mechanism of anion effect on SCC of steel 23Co14Ni12Cr3Mo was studied by using full immersion test and electrochemical measurements.

Research and development of hot-rolled ultra-high strength steel at Baosteel

The effects of the composition and cooling process on the microstructures and properties of hot-rolled ultra-high strength low alloy （HSLA） steel, complex phase steel and martensite steel were studied in the laboratory. And S700MC and MP1200 ultra-high strength steels were trial produced at the 1 880 mm hot-rolling line of Baosteel. Compared with conventional hot-rolled high strength products,the idea that water is alloy was applied in the newly developed hot-rolled ultra-high strength steel. By the use of the economical composition design and controlled cooling after hot-rolling effectively,ultra-high strength steel of different steel grades can be obtained.

Internal oxidation of hot-rolled ultra-high strength steel and its effect on the surface quality of cold-rolled sheets

In this study,the scale and internal oxidation of hot-rolled ultra-high strength steel sheets were characterized.It was found that both the formation of the scale and the internal oxidation of Si and Mn depended on the coiling temperature and position of the steel sample on the strip coil.At a relatively high coiling temperature,a large amount of internal oxidation was observed on the samples cut from the middle of the coil.The depth of the internal oxidation zone exceeded 10 μm and a thin iron layer covering the scale was observed in some cases.Pickling and cold-rolling experiments were conducted on selected samples.Scale pickling was found to be greatly delayed by the formation of an iron layer,which frequently resulted in under-pickled defects.In addition,pickling of the entire internal oxidation zone was difficult,except at the grain boundaries,where the degree of internal Si and Mn oxidation was enriched.The surface of the cold-rolled steel sheet was ruined by the remaining oxidation zone in the subsurface of the pickled steel.The internal oxidation of hot-rolled ultra-high strength steel must be precisely controlled to improve the subsequent surface quality of cold-rolled steel.

Corrosion Behavior of Ultra-high Strength Steel 300M in Different Simulated Marine Environments

Corrosion behavior of 300M in neutralcorrosion environments containing Na Clsimulated by totalimmersion（TI）,salt spraying（SS）and periodic immersion（PI）,was investigated by surface analysis techniques,corrosion weight-loss method,and electrochemicalmeasurements.In totalimmersion environment,rust on the steelconsisted of a porous outer rust layer with main constituent of γ-Fe OOH,and an inner rust layer of dense Fe_3O_4 film with network broad cracks.In salt spraying environment,outer rust with main composition of γ-Fe OOH/α-Fe OOH/Fe_3O_4 was compact,and inner rust showed dense Fe_3O_4 film.Rust formed by periodic immersion exhibited a compact outer rust layer with constituent of α-Fe OOH/γ-Fe OOH/Fe_3O_4 and an inner rust layer with composition of α-Fe OOH/α-Fe_2O_3;inner rust showed a ultra-dense film adherent to the steel.The corrosion rate showed a rule of vss（salt spraying）〉vti（totalimmersion）〉〉vpi（periodic immersion）in 0-240 h,and vss≈vti？vpiin 240-720 h.The rust formed by periodic immersion was dense and compact,with stable electrochemicalproperties,and had excellent protection on the steel.Humidity and oxygen concentration in allthe environments played major roles in rust formation.

Simulation about hot stamping of ultra-high strength steel on the basis of lightweight technology

With the development of automobile lightweight,it is very necessary to apply the ultra-high strength steel parts manufactured by hot stamping,which offers the possibility to reduce the weight of automobiles and maintain the safety requirement.In order to complete hot stamping,it is important to design the structure of parts reasonably,which is related with reasonable matching of strength.The objective of this paper is to guide the design of parts manufactured by hot stamping and find the forming technical requirements of vehicle performance.Through experiments,the paper obtains the stress and strain curves at different deformation temperatures and strain rates.Based on experimental data, the constitutive relationship model is established which can reflect the deformation capacity of ultra-high strength steel during the process of hot stamping.Combined with finite element simulation results of hot stamping by commercial software AUTOFORM,transfer path of load and matching law of strength,the paper determines the design criteria and forming technical requirements of parts manufactured by hot stamping.At the same time,the impact performance of front cross member internal plate is taken into consideration.

Feasibility of 0.02% Nb-Based Microalloyed Steel for the Application of One-Step Quenching and Partitioning Heat Treatment

To attain an enhanced combination of mechanical properties for low alloyed steel, the current study has been made to fulfill that growing need in the industry. Its results are introduced within this paper. One step Quenching and Partitioning (Q&P) heat treatment has been applied on Niobium-based microalloyed steel alloy with 0.2 %C, in the form of 2 mm thickness sheets. The target of this study is to investigate the viability of applying that significantly recommended, results-wise, heat treatment on the highly well-suited alloy steel samples, to achieve the main target of enhanced properties. A single temperature of 275°C was used as quenching and Partitioning temperature. Four Partitioning periods (30, 200, 500, and 1000 Seconds) were used for soaking at the same temperature. The results were analyzed in the light of microstructural investigation and mechanical testing. All applied cycles did not enhance the strength but moderately improved the ductility and toughness, mainly caused by the slightly high soaking temperature used. Niobium impact of grain refining was apparent through all cycles. The cycle of 500 Seconds Partitioning time obtained optimum values at that particular temperature. The 1000 Seconds Cycle obtained the worst combination of properties. A set of recommendations are set. More research is required at this point, where a lower Partitioning temperature is advised. In the light of the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds. A high probability that periods closer to 500 than 1000 Seconds will produce better results. More research is needed between those two values of Partitioning time to precisely determine the optimum time at that temperature on that specific alloy.

Laser Beam Welding of 600 MPa Quenched and Tempered High-Strength Steel

Conventional fusion arc welding of high-strength quenched and tempered steel can be improved through the use of non-conventional laser beam welding. This article presents the investigations of autogenous bead on plate and butt CO2 Laser Welding (LW) of 7 mm thick high-strength quenched and tempered low alloy SM570 (JIS) steel plates. The influence of laser welding parameters, mainly welding speed, defocusing distance and shielding gas flow rate on the weld profile, i.e., weld zone penetration depth and width, microstructure and mechanical properties of welded joints was determined. All welded joints showed smooth and uniform weld beads free from superficial porosity and undercuts. The selected best welding conditions were a laser power of 5.0 kW, welding speed of 500 mm/min, argon gas shielding flow rate of 30 L/min and a defocusing distance of -0.5 mm. It was observed that these conditions gave complete penetration and minimized the width of the weld bead. The microstructure of the welded joints was evaluated by light optical microscopy. The weld metal (WM) and heat-affected zone (HAZ) near weld metal achieved maximum hardness (355 HV). The tensile fractured samples showed the ductile mode of failure and ultimate tensile strength of 580 MPa.

Study on welding process performance of HG70D high strength steel plate

In this paper, the performance of HG70D welded joint of ultra-high strength steel plates is presented, and the performance of HG70D and Q345B welded joints is studied. The high strength steel plate HG70D showed excellent weldability. Through the X-ray inspection of welded joints, tensile strength, impact test and bending performance test, the comprehensive performance of the joint was excellent. The macroscopic and microscopic metallographic analysises of the welded joints show that the welding seams included pearlite, sorbite, ferrite, etc. The influence of stress annealing temperature on HG70D of high strength steel plate was analyzed by heat treatment.

Evaluation of Mechanical Properties of Medium Carbon Steel Quenched in Water and Oil

Samples of medium carbon steel were examined after heating between 900℃ - 980℃ and soaked for 45 minutes in a muffle furnace before quenching in palm oil and water separately. The mechanical behavior of the samples was investigated using universal tensile testing machine for tensile test and Vickers pyramid method for hardness testing. The microstructure of the quenched samples was studied using optical microscope. The tensile strength and hardness values of the quenched samples were relatively higher than those of the ascast samples, suggesting improved mechanical properties. However, samples quenched in palm oil displayed better properties compared with that of water-quenched samples. This behavior was traced to the fact that the carbon particles in palm oil quenched samples were more uniform and evenly distributed, indicating the formation of more pearlite structure, than those quenched in water and the as-received samples.

Investigation on microstructure and properties of a combined precipitation hardening ultrahigh strength steel

The microstructure and properties of a combined precipitation hardening ultrahigh strength steel with nano-sized carbides and intermetallics were studied systematically.The results show that after tempering at 300℃lots ofε-carbides are precipitated in the martensite,the strength rises and the toughness falls slightly.After tempering at 430℃,much coarser cementite lamina are precipitated in martensitic laths,which causes the impact toughness falls to the minimum value.With temperature further increasing the cementites are dissolved and M_2C carbides,β-NiAl intermetallics and reverse austenite begin to precipitate.The tensile strength and yield strength achieve the peak value at 470℃,490℃respectively.The tested steel achieve a tensile strength of 2 120 MPa,a yield strength of 1 950 MPa and impact energy of 54 J/cm^2 after optimum tempering at 510℃.When tempering temperature is above 530℃the M_2C carbides and reverse austenite is coarsening.After tempering at 560℃the reverse austenite reaches the maximum volume fraction in present work.

Development of ultra high strength special steels and wrought nickel base superalloys

Ultra-high strength special steels and wrought Ni-base superalloys are been ever increasingly applied in aircraft and aerospace,power generation,oil-gas and other industries.The development of those materials is driven by tougher working conditions,higher efficiency,environment consideration,resource limitation and cost reduction. This presentation will focus on the development of ultra high strength special steels and wrought Ni-base superalloys in aircraft industry. The examples of ultra-high strength steels include: (1) Ultra-high strength stainless steels for landing gear and other structures; (2)High speed and high temperature main shaft bearing steels; (3) Ultra-high strength steels for jet engine main shaft. New and modified Ni-base superalloys will be discussed by examples of (1 )Low cost,process-friendly superalloys with higher performance; (2)Modification of hard-to-processed superalloys. Future development of those materials will be addressed.

Microstructural analysis of the softened zone in the welding joint of 100 kg class hot-rolled extra-high-strength steel

Using the Gleeble 3500 thermal-mechanical system to simulate thermal cycles with different peak temperatures, the hardness and microstructure in the heat-affected zones of two kinds of 100 kg class hot-rolled extra-high-strength steel were compared. When the peak temperature of the thermal cycle was 800℃ ,incomplete transformation occurred during quenching in both steels, and massive martensite and bainite grains were formed. The hardness was determined by the composition and distribution of the microstructure. The concentration of massive martensite was low, and hence the hardness was low,in steel #1. Conversely,the massive martensite content in steel #2 was high and uniformly distributed,resulting in a high hardness. These findings can provide a reference for improving the mechanical properties in the softened zone.

Effect of short-time low-temperature austenitizing on microstructure and mechanical properties of DT300 ultra-high strength steel fabricated by laser powder bed fusion

To address the inhomogeneous microstructure and improve the mechanical properties of DT300 ultra-high strength steel specimens fabricated by laser powder bed fusion,different post-heat treatment schedules are performed.With the increase in austenitizing temperature and time,the migration rate of austenite grain boundaries continuously increases with the dissolution of nano-carbides,and the formation of nano-oxides and twin martensite is also inhibited accordingly.The rapid growth in the size of prior austenite grains and martensite laths,as well as the decrease in the content of nano-oxides and twin martensite,led to a rapid decrease in the strength(yield strength and ultimate tensile strength)from HT2 to HTF specimens.The HT1 specimens(austenitizing at 830℃for 30 min,then oil quenching and tempering at 300℃for 120 min and finally air cooling)display excellent mechanical properties of yield strength of 1572 MPa,ultimate tensile strength of 1847 MPa,elongation of 9.84%,and fracture toughness of 106 MPa m^(1/2),which are counterparts to those of conventional DT300 steel forgings after heat treatment.

Study on Quenching and Tempering Process of Q960 High Strength Steel for Construction Machinery

To develop the Q960 high-strength quenched and tempered steel plates for construction machinery,the effects of quenching and tempering treating regime on the microstructures and mechanical properties were investigated.The results show that the perfect austenization and fine grain size can be achieved by the optimum quenching process that is quenching temperature 900℃ and holding time 20min.Considering performance and production efficiency,the optimum tempering process parameters are found that tempering temperature 600℃ and holding time 40min.The excellent overall properties of specimens with tempered sorbite microstructure can be ultimately obtained.The yield strength is 1030MPa,tensile strength 1080MPa,percentage elongation 16.8% and the Charpy impact energy 144J at-40℃.All these indexes come up to the National Standard GB/T 16270-2009.