动态再结晶对亚温形变淬火15钢和15Mn2Nb钢组织与性能的影响

研究了动态回复和动态再结晶对亚温形变淬火15钢和15Mn2Nb钢的组织与性能的影响。结果表明:选择适当的亚温形变淬火温度,控制动态回复与动态再结晶程度,可使15钢获得良好的强塑性匹配。合金元素铌能阻碍15Mn2Nb钢动态回复与动态再结晶,同时具有细化晶粒的作用。

Wear Performance of Laser Surface Hardened GCr15 Steel

In order to assess the new tribological properties of laser surface hardened GCr15 steel, the wear resistance between specimens treated with laser and those of conventionally hardened under dry sliding conditions was compared. The change of wear mechanisms in laser hardened GCr15 resulted in a distinct difference in wear rates. The results showed that quenched zones not only had sufficient depth of hardening and higher hardness, but had more retained austenite and finer carbides because of a higher degree of carbide dissolution. Laser surface hardened GCr15 steel specimens exhibited superior wear resistance to their conventionally hardened specimens due to the effects of the microstructure hardening, high hardness and toughness. The wear mechanism for both the laser quenched layer and conventionally hardened layer was highly similar, generally involving adhesive, material transfer, wear-induced oxidation and plowing. When conventionally hardened block specimens rubbed against the laser hardened specimens, the surface of conventionally hardened block specimens was polished. The microstructural thermal stability was increased after laser surface treatment.

Hot deformation behaviors and flow stress model of GCr15 bearing steel

The hot deformation behaviors of GCr15 bearing steel were investigated by isothermal compression tests, performed on a Gleeble-3800 thermal-mechanical simulator at temperatures between 950 ℃ and 1 150 ℃ and strain rates between 0.1 and 10 s-1. The peak stress and peak strain as functions of processing parameters were obtained. The dependence of peak stress on strain rate and temperature obeys a hyperbolic sine equation with a Zener-Hollomon parameter. By regression analysis, in the temperature range of 950-1 150 ℃ and strain rate range of 0.1?10 s?1, the mean activation energy and the stress exponent were determined to be 351kJ/mol and 4.728, respectively. Meanwhile, models of flow stress and dynamic recrystallization (DRX) grain size were also established. The model predictions show good agreement with experimental results.

Effect of electromagnetic stirring on metallurgical quality of GCr15 electroslag ingot

To further improve the metallurgical quality of electroslag remelting,remove the large inclusions in electroslag ingot and refine the solidification structure of electroslag ingot,an electroslag remelting furnace with electromagnetic stirring was designed,and the influence of different magnetic induction intensities on metallurgical quality of GCr15 electroslag ingot was studied.Inclusions with different sizes and types were analyzed by an ASPEX scanning electron microscope,and the morphology and composition of inclusions were further observed by a JSM-6510LV scanning electron microscope.The distribution of alloying elements on the cross section of electroslag ingot was analyzed by original position analysis(OPA).The results show that whether or not electromagnetic stirring is used,the inclusions in electroslag ingot are mainly composed of Al_(2)O_(3),MnS,MnS-oxide and other oxides,among which Al_(2)O_(3) is the most.Compared with an electroslag ingot without electromagnetic stirring,the number of inclusions decreases considerably,and the proportion of small inclusions increases while the proportion of large inclusions decreases when the electromagnetic stirring with remelting current of 1.1 kA and magnetic induction intensity of 62 Gs is applied.However,excessive electromagnetic force will cause the number and diameter of inclusions to increase again.Electromagnetic stirring has different effects on the segregation of different elements.Under the experimental conditions,the weak electromagnetic force with 1.1 kA and magnetic induction intensity of 108 Gs has little effect on the segregation of C,but decreases the segregation of P,and the excessive electromagnetic force aggravates the segregation of alloy elements.

Microstructural evolution of GCr15 steel during austenitizing and quenching considering C and Cr content

Microstructural evolution of GCr15 steels with different C and Cr contents during austenitizing and quenching was studied. Thermodynamic analysis of cementite dissolution was implied to obtain the critical temperature. The coordination number x in Fe_xCr_(3-x)C and the volume fraction of undissolved cementite were computed according to element conservation and equilibrium phase diagram. The M_S(martensite transformation temperature) was calculated by using empirical formula. The retained austenite content was calculated with further consideration of quenching temperature. The results showed that the coordination number and the undissolved cementite content were promoted by the austenitizing temperature and carbon content of the steel. Increasing Cr element reduced the coordination number.GCr15 steels with different components had nearly the same M_S when austenitization at 830 °C to 860 °C. The interaction of C and Cr complicated the evolution of M_S and retained austenite content. The results were in good agreement with the literature, which could guide to obtain specified retained austenite and/or carbides.

Alloying Effect of Lanthanum in GCr15 Bearing Steel with Tin and Antimony

Effect of tin and antimony on hot ductility of GCr15 bearing steel as well as interaction between lanthanum and tramp elements, were studied by simulator Gleeble-1500 and the fractures for the tested steels were analyzed using SEM and EDS. The results show that with increasing contents of tin and antimony elements, the hot ductility of GCr15 bearing steel was decreased. Lanthanum can reduce the harmful effect of tramp elements on the hot ductility of GCr15, which can react with antimony to form compound and segregate at grain boundary in steel. Lanthanum can refine the recrystallized austenite grains after deformation and subsequent martensite structure.

Effects of Suspension Casting on Solidification Process of GCr15 Steel Ingot

The mechanism of inoculation in the case of suspension casting process has been studied through solidification kinetics. The effect of suspension casting process on temperature field, solidification rate, temperature gap of crystallization, effective distribution coefficient of solute and nucleation frequency during solidification process in steel ingot were discussed on the base of experiments. It has been found that the suspension casting process can increase both cooling at and solidification rate of steel ingot, improve the temperature field and solute distribution, narrow the temperature gap of crystallication, and increase the nucleation frequency. Thus, the solidification time can be shorten, the solute can be well distributied, the shrinkage porosity can be reduced and the grain of crystallization can be fined.

The microstructural nanoindentation characteristics of different phases in precision annealed GCr15 steel

This study characterizes the mechanical properties and volume fractions of the different phases in precision annealed GCr15 steel using nanoindentation technology. Experimental results indicate that the nanoindentation hardness of cementite grains is between 14.15 GPa and 17.61 GPa,with a mean value of 15.40 GPa. This hardness is much higher than the hardness of ferrite grains. The nanoindentation hardness of ferrite is between 2.78 GPa and 4.89 GPa, with a mean value of 3.35 GPa. The volume fractions of the different phases were also determined using nanoindentation technology, and the volume fraction of cementite in the steel was identified as 15%.

ON THE RHEOLOGICAL MODEL OF STEELS AT ELEVATED TEMPERATURES

Mechanical properties of steels ZG15CrMoV and 35 in the temperature range of 500 to 1400℃ have been experimently researched by means of Gleeble-1500.The experi- mental results is used as input-output responses,a rhelological model with five elements of the mechanical behaviour of steels ZG15CrMoV and 35 at elevated temperatures, i.e[M]=[H_1]-[H_2/[N_2]-[S/N_3],is determined by a system identification method,and the parameters of the model in the temperature range are also identified.

MAGNETIC ACOUSTIC EMISSION CHARACTERISTICS OF HYDROGEN ATTACKED LOW CARBON STEEL

Hydrogen attack occurred in low carbon steel and steel 25CrMo which had been exposed in hydrogen under 18MPa at 450 and 500℃ for 240,480 and 720 h.The methane bubbles and microcracks grow along grain boundaries.The degree of hydrogen attack increases with increasing exposure time and temperature.Magnetic acoustic emission(MAE) was used to detect the degree of hydrogen attack.The results show that the characteristics of MAE for samples of low carbon steel and steel 15CrMo with hydrogen attack have changed obviously comparing to the samples without hydrogen attack,and the MAE signals was sensitive to the degree of hydrogen attack at the last stage of hydrogen attack.The magnetic detection way,as a new method of nonrestrictive testing of hydrogen attack,can be used to detect the hydrogen attack in practice.

Analysis on SH-CCT curves of HD15Ni1MnMoNbCu steel for nuclear power station

The microstructural evolution and Vickers hardness measurement in the welding heat-affected zone （HAZ） of HD15 Nil MnMoNbCu steel for nuclear power station were investigated by Gleeble-3180 thermal mechanical simulator, and the simulated HAZ continuous cooling transformation curves （SH-CCT） were measured simultaneously. With ts/5 inereasing from 3.75 s to 15 000 s, the product was obtained martensite, bainite, ferrite and pearlite, successively. The result of microstructure and Vickers hardness in the heat-affected zone was in good agreement with those measured by SH-CCT diagram with the heat input 16. 2 kJ/cm as an example to weld the HD15Ni1MnMoNbCu steel pipe using TIG/SMAW/SAW welding methods.

Effect of Heat Treatment on Gradient Microstructure and Tensile Property of Laser Powder Bed Fusion Fabricated 15-5 Precipitation Hardening Stainless Steel

This work investigated the gradient microstructure evolution and tensile property of LPBF fabricated 15-5 precipitation hardening stainless steel in post-process direct ageing(DA)and solution treating&ageing(STA).The varied microstructures for austenite and small-sized oxide inclusions at different sample heights in the as-built(AB)condition was generally preserved after DA treatment.However,austenite was almost disappeared,and oxide particle grew significantly after the STA treatment.As a result,the tensile property differences in sample top and bottom for AB and DA conditions did not occur in the STA samples.For the influence of post-process heat treatment,the STA condition had the highest yield strength due to the highest volume fraction of nano-sized Cu precipitates.However,the DA specimen had the highest ultimate tensile strength and elongation owing to the considerable amount of austenite phase and associated transformation induced plasticity effect.

Three-step learning strategy for designing 15Cr ferritic steels with enhanced strength and plasticity at elevated temperature

15Cr ferrite steels are urgently required in advanced Ultra-supercritical power plants but meet design challenges in balancing excellent strength and plasticity at high temperatures.We developed a three-step learning strategy based on mutually driven machine learning and purposeful experiments to complete this multi-objective task.Compared with traditional adaptive learning and local-interpolation learning,this step-by-step modular manner provides good transparency and interpretability of the information flow,which is ensured by identifying essential factors from an exquisitely prepared composition-microstructure dataset,and learning valuable knowledge about the composition-property relationship.The requirement of only two groups of experiments indicates the low cost and high efficiency of the strategy.Performing the strategy,we found that Ti is another key element affecting the Laves phase besides Mo and W,and their effects on ultimate tensile strength(UTS)and elongation were also uncovered.Importantly,several low-cost steels free of Co were successfully designed,and the best steel exhibited 156%,31%,and 62%higher UTS and elongation at 650°C than the typical 9Cr,15Cr,and 20Cr steels,respectively.Based on the advantages and success of the strategy in terms of alloy improvement,we believe the strategy suits other multi-objective design tasks in more materials systems.

Effect of Aging on Hardening Behavior of 15-5 PH Stainless Steel

Microstructure transformation and aging hardening behavior of 15-5 PH stainless steel were studied by optical microscopy （OM）, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. The results showed that the 15-5 PH stainless steel consists of NbC precipitates and lath matensite with a high dislocation density after solution treatment. With increasing aging temperature and aging time, the martensitic laths were resolved gradually. Meanwhile, the nanometric-sized Cu precipitates gradually coarsened and lost their coherency with＇the martensite matrix, which exhibited an elliptical shape finally. Fine Cu precipitates can lead to significant dispersion hardening effect, while the coarsened Cu precipitates have no contribution to strengthening. The reversed austenite was observed in the speci- mens aged at 550 ℃ and above; moreover, the amount of reversed austenite increased as aging temperature in- creased. The precipitation hardening behavior of 15-5 PH stainless steel may depend on the balance between the softening caused by the formation of reversed austenite and the hardening caused by the precipitation of copper.

Secondary hardening behavior of 15CrMnMoV thin rolled tube

15CrMnMoV rolled tube is applied to many critical parts of helicopter. The secondary hardening behavior of the steel which varied with the treating process is essential for further usage. In this paper, the secondary hardening behavior of water quenched, oil quenched and air cooled steel are compared. The precipitations of alloy carbides are analyzed by electrolytic extraction, X-ray diffraction, optical microscopy as well as scanning electron microscopy. Results show that the secondary hardening effect in the air cooled steel is higher than in the water quenched and oil quenched steel, but the toughness decreases more seriously, due to more precipitation of Mo2C and VC.

Effect of axial static magnetic field on cleanliness and microstructure in magnetically controlled electroslag remelted bearing steel

The effect of the axial static magnetic field(ASMF)on cleanliness and microstructure in magnetically controlled electroslag remelted GCr15 bearing steel ingots was investigated experimentally.The results show that a magnetically controlled spin-vibration induced by the interaction of the ASMFs and the remelting current exists at the consumable electrode tip,resulting in thinner liquid melt film and smaller droplets.With the increase in magnetic flux density,the optimization effect of ASMFs on electroslag remelting process increases and reaches the peak with a 40 mT ASMF,then decreases.The cleanliness of the ingots was improved,and the count of inclusions larger than 5μm was reduced.The microstructure of the ingots processed with a 40 mT ASMF was significantly refined.The depth of the metallic molten pool was reduced from 45.2 to 17.5 mm with the application of 40 mT ASMF.The tensile strength,impact toughness,and Rockwell hardness of the ingots obtained under the 40 mT ASMF were significantly improved.The mechanisms of the spin-vibration occurring at the electrode tip end were interpreted in detail to elucidate the effect of ASMFs.

M_(23)C_(6)precipitation and Si segregation promoted by deep cryogenic treatment aggravating pitting corrosion of supermartensitic stainless steel

The microstructure evolution and the pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment process were clarified through X-ray diffraction,field emission scanning electron microscopy,transmission electron microscopy(TEM)and electrochemical methods.The results showed that the microstructure of supermartensitic stainless steel mainly consisted of reversed austenite,tempered martensite,and M_(23)C_(6)carbides after tempering.The deep cryogenic treatment promoted the refinement of the martensite laths and the precipitation of the carbides in comparison with the traditional process.TEM analysis indicated that the segregation of Si atoms at the boundary was found at the interface between carbide and martensite.The pitting corrosion potential of the specimens subjected to deep cryogenic treatment decreased with the elevated tempering temperature,and the lowest pitting corrosion potential was found at the tempering temperature of 650℃.The sensitivity of the pitting corrosion potential was attributed to the precipitation of M_(23)C_(6)carbides and Si atoms segregation.Si atoms segregation engendered the formation of Cr-depleted zone near M_(23)C_(6)and impeded the recovery of Cr-depleted zone.

Hot Deformation Behavior of GCr15 Steel

Hot deformation behavior of GCr15（ASTM 52100） steel was investigated using single-hit compression tests on Gleeble-1500 simulator at the temperature range of 850-1 100 ℃ and strain rate range of 0.1-10 s-1.The flow stress constitutive equation of GCr15 steel during hot deformation was determined by stress-strain curves analysis on the basis of the hyperbolic sine equation.And the models of dynamic recrystallization fraction and dynamic recrystallization grain size of GCr15 steel were established by the measured curves and microstructure observation in different experimental conditions.The mean activation energy and the time exponent of dynamic recrystallization kinetics equation in the range of experimental conditions were determined to be 356.2 kJ/mol and 2.12,respectively.Meanwhile,the flow stress model was also established by the method of allocating flow stress curve with three main stress values,the saturation stress,the steady state stress and the stress when strain is 0.1.The flow stress curves predicted by the developed models under different deformation conditions are in good agreements with the measured ones.

Effect of heat treatments on Charpy impact properties of 15Cr12MoVWN ferritic/martensitic steel

The Charpy impact properties of 15Cr12MoVWN ferritic/martensitic steel for sodium-cooled reactors with variation in heat treatment factors and parameters are reported.The results show that the ductile-to-brittle transition temperature(DBTT)increased and the upper shelf energy(USE)decreased with increase in normalizing temperature.However,the variation tendency of DBTT and USE was the opposite with increase in tempering temperature.The tempering temperature showed a greater influence on USE than the normalizing temperature,and normalizing and tempering temperatures had the equally significant effects on DBTT,but the cooling method was not a significant factor for DBTT and USE.The prior austenite grain and M_(23)C_(6) size were the main influences on DBTT,and the dislocation density was the main factor affecting the variation of USE.The heat-treatment regime recommended for 15Cr12MoVWN steel was composed of normalizing at 1000-1050℃ for 0.5 h followed by water quenching or air cooling and tempering at 760℃ for 1.5 h.

Corrosion Performance of High Strength 15Cr Martensitic Stainless Steel in Severe Environments

High pressure and high temperature corrosion performance of high strength 15 Cr martensitic stainless steel was studied in different severe environments—live acid（10%HCl＋1.5%HF＋3%HAc＋5.1% corrosion inhibitor）,spent acid and formation water containing CO2.The results show that the corrosion of high strength 15 Cr martensitic stainless steel in live acid is most serious,and the uniform corrosion rate is far greater than those in spent acid and formation water containing CO2 corrosion environments,but all of them can be acceptable for oilfield.Acidizing corrosion inhibitor displays a good matching ability with the high strength 15 Cr martensitic stainless steel in terms of decreasing the uniform corrosion rate,which changes mainly the anodic process of high strength 15 Cr martensitic stainless steel.The corrosion potential moves to the positive direction,thus the corrosion current density decreases significantly.There are some different degrees of pitting of high strength 15 Cr martensitic stainless steel after corrosion tests in live acid,spent acid and formation water containing CO2,and the pitting density aggravates significantly and the maximum pit depth decreases in the corrosion sequence.