Achieving a high-strength dissimilar joint of T91 heat-resistant steel to 316L stainless steel via friction stir welding

The reliable welding of T91 heat-resistant steel to 316L stainless steel is a considerable issue for ensuring the safety in service of ultrasupercritical power generation unit and nuclear fusion reactor,but the high-quality dissimilar joint of these two steels was difficult to be obtained by traditional fusion welding methods.Here we improved the structure-property synergy in a dissimilar joint of T91 steel to 316L steel via friction stir welding.A defect-free joint with a large bonding interface was produced using a small-sized tool under a relatively high welding speed.The bonding interface was involved in a mixing zone with both mechanical mixing and metallurgical bonding.No obvious material softening was detected in the joint except a negligible hardness decline of only HV~10 in the heat-affected zone of the T91 steel side due to the formation of ferrite phase.The welded joint exhibited an excellent ultimate tensile strength as high as that of the 316L parent metal and a greatly enhanced yield strength on account of the dependable bonding and material renovation in the weld zone.This work recommends a promising technique for producing high-strength weldments of dissimilar nuclear steels.

Remaining Useful Life for Heavy-Duty Railway Cast Steel Knuckles Based on Crack Growth Behavior with Hypothetical Distributions

The current research on the integrity of critical structures of rail vehicles mainly focuses on the design stage,which needs an effective method for assessing the service state.This paper proposes a framework for predicting the remaining useful life(RUL)of in-service structures with and without visible cracks.The hypothetical distribution and delay time models were used to apply the equivalent crack growth life data of heavy-duty railway cast steel knuckles,which revealed the evolution characteristics of the crack length and life scores of the knuckle under different fracture failure modes.The results indicate that the method effectively predicts the RUL of service knuckles in different failure modes based on the cumulative failure probability curves for different locations and surface crack lengths.This study proposes an RUL prediction framework that supports the dynamic overhaul and state maintenance of knuckle fatigue cracks.

Micropores in G20Mn5N cast steel and their influence on stress distribution

To accurately analyze the impact of casting pores in steel,high-resolution 3D X-ray tomography technology was used to gather detailed statistical information about micropores.These micropores were classified as gas,shrinkage,and gas-shrinkage pores depending on their formation origin and morphology.Clustering tendencies and affinity parameters were defined to characterize the spatial correlations among these three types of pores.The 3D data from X-ray tomography scans were then integrated into finite element analysis(FEA)software to predict how micropore shape,size,and distribution influence stress distribution within the material.The results show that certain inflection points with small local radii within the cast pores are major contributors to stress concentration.Therefore,cast pores cannot be simply modeled as ideal spherical pores.The sphericity and volume of pores have a significant impact on the stress concentration of the model.Specifically,lower sphericity and larger pore volumes result in higher stress concentrations.Moreover,the internal pores of steel castings exhibit specific global distribution characteristics.Pores located on the surface of the specimen lead to significantly higher stress concentrations compared to those located inside the specimen.

Formation and control of the surface defect in hypo-peritectic steel during continuous casting:A review

Hypo-peritectic steels are widely used in various industrial fields because of their high strength,high toughness,high processability,high weldability,and low material cost.However,surface defects are liable to occur during continuous casting,which includes depression,longitudinal cracks,deep oscillation marks,and severe level fluctuation with slag entrapment.The high-efficiency production of hypo-peritectic steels by continuous casting is still a great challenge due to the limited understanding of the mechanism of peritectic solidification.This work reviews the definition and classification of hypo-peritectic steels and introduces the formation tendency of common surface defects related to peritectic solidification.New achievements in the mechanism of peritectic reaction and transformation have been listed.Finally,countermeasures to avoiding surface defects of hypo-peritectic steels duiring continuous casting are summarized.Enlightening certain points in the continuous casting of hypo-peritectic steels and the development of new techniques to overcome the present problems will be a great aid to researchers.

Influence of Creep Strength of Weld on Interfacial Creep Damage of Dissimilar Welded Joint between Martensitic and Bainitic Heat-Resistant Steel

The mechanical properties, creep rupture strength, creep damage and failure characteristics of dissimilar metal welded joint （DMWJ） between martensitic （SA213T91） and bainitic heat-resistant steel （12Cr2MoWVTiB（G102）） have been investigated by means of pulsed argon arc welding, high temperature accelerated simulation, mechanical and creep rupture test, and scanning electronic microscope （SEM）. The results show that there is a marked drop of mechanical properties of undermatching joint, and low ductility cracking along weld/G102 interface is induced due to creep damage. Creep rupture strength of overmatching joint is the least. The mechanical properties of medium matching joint are superior to those of overmatching and undermatching joint, and creep damage and failure tendency along the interface of weld/G102 are lower than those of overmatching and undermatching joint after accelerated simulation for 500 h, 1 000 h, 1 500 h, and the creep rupture strength of medium matching joint is the same as that of undermatching joint. Therefore, it is reasonable that the medium matching material is used for dissimilar welded joint between martensitic and bainitic steel.

Dissolution Behavior of Delta Ferrites in Martensitic Heat-resistant Steel for Ultra Supercritical Units Blades

The dissolution behavior of delta ferrites in martensitic heat-resistant steel was studied.And the reason why the dissolution rate of delta ferrites decreased with dissolution time was discussed.The experimental results show that the chemical compositions of delta ferrites negligibly change with dissolution time.The decrease of dissolution rate of delta ferrites with dissolution time should be attributed to the change of shape and distribution of delta ferrites.The shape of delta ferrites tends to transfer from polygon to sphere with dissolution time,causing the decrease of specific surface area of delta ferrites.The distribution position of delta ferrites tends to transfer from boundaries of austenite grains to interior of austenite grains with dissolution time,decreasing the diffusion coefficient of alloy atoms.Both them decrease the dissolution rate of delta ferrites.

High-temperature oxidation behavior of 9Cr‒5Si‒3Al ferritic heat-resistant steel

To improve the oxidation properties of ferritic heat-resistant steels,an Al-bearing 9Cr‒5Si‒3Al ferritic heat-resistant steel was designed.We then conducted cyclic oxidation tests to investigate the high-temperature oxidation behavior of 9Cr‒5Si and 9Cr‒5Si‒3Al ferritic heat-resistant steels at 900 and 1000℃.The characteristics of the oxide layer were analyzed by X-ray diffraction,scanning electron microscopy,and energy dispersive spectroscopy.The results show that the oxidation kinetics curves of the two tested steels follow the parabolic law,with the parabolic rate constant kp of 9Cr‒5Si‒3Al steel being much lower than that of 9Cr‒5Si steel at both 900 and 1000℃.The oxide film on the surface of the 9Cr‒5Si alloy exhibits Cr2MnO4 and Cr2O3 phases in the outer layer after oxidation at 900 and 1000℃.However,at oxidation temperatures of 900 and 1000℃,the oxide film of the 9Cr‒5Si‒3Al alloy consists only of Al2O3 and its oxide layer is thinner than that of the 9Cr‒5Si alloy.These results indicate that the addition of Al to the 9Cr‒5Si steel can improve its high-temperature oxidation resistance,which can be attributed to the formation of a continuous and compact Al2O3 film on the surface of the steel.

Microstructure and Corrosion Resistance of CrN and CrN/TiN Coated Heat-Resistant Steels in Molten Aluminum Alloy

The components of the equipment for processing the Al melts into the molded parts can be markedly corroded by the molten Al. In this study, a 4 μm CrN coating or CrN/TiN multilayer coating for providing the physical and chemical barriers between the molten reactive Al and the steel substrate were deposited by Cathodic Arc Evaporation onto 10 mm-thick heat-resistant steel plates. The dipping tests were conducted in a 700℃ A356 melt for 1 to 21 h at intervals of 3 h. The damage of the coated steel was eva...

EFFECT OF W ON MECHANICAL PROPERTIES OF 12%Cr HEAT-RESISTANT STEEL

The effect of W on mechanical properties of 12% Cr-W-V-Nb heat resistant steel at high temperatures and room temperature is reported.The experimental results indicated that if the W content was about 2.2—3.0 wt-%,there was no obviously change of R.T.tensile strength, but impact toughness decreased with the rise of W content.On the other hand,the increase of W content enhanced the short time stress rupture strength,but did not for the long time one. The increase of W have two effects on the precipitation behavior,promoting Laves formation of type Fe_2W,increasing the precipitated phase amount and speeding up the coarsening pro- cess of precipitated phase at high temperatures.The effect of W on the mechanical properties is closely associated with precipitation behaviors.When the rupture life is short,there has no enough time to coarsen the precipitated phases,so the increase of precipitated phases results in strengthening effect,i.e.the W increases the high temperature strength.After prolonged expo- sure,the evident coarsening took plaee,that decreased the effect of precipitation.

Development and Application of a Heat-resistant Low Ni Steel Modified by Rare Earth for Furnace Roller

The low Ni steel modified hy rare earth(3Cr24NiTSiN with an addition of 0.3% Ce)for furnace roller has been developed.Due to the RE(rare earth)addition,a dense oxide film is formed on the steel surface at high temperature,and the oxidation rate is decreased.This film has so good adhesion to the matrix that it will not be peeled off easily.The RE modified steel has excellent oxidation resistance and thermal strength even if being used continuously for a long period at high temperature.This steel roller has a service life of about 4 years com- parable to high Ni steel ones,so the low Ni steel can replace high Ni steel to make furnace roller.The Ni content of this material can be reduced by 65% in comparison with Cr25Ni20Si2 steel,The low Ni steel has better pro- eessing properties including melting,casting and working properties than that of high Ni ones.

Investigation on the strengthening mechanism of S30432 austenitic heat-resistant steel

From the viewpoint of energy-saving and environment protection,it is necessary to develop Ultra Super Critical(USC) fossil-fired power plants.In order to ensure the reliable operation of power plants under high steam conditions,good mechanical properties(particularly high creep strength),corrosion resistance and fabricability are generally required for the heat resistant steels used in USC boilers.Among these heat-resistant steels,S30432 austenitic heat-resistant steels are of interest due to high creep strength,excellent oxidation and corrosion resistance at temperatures up to 650 -700℃.In this paper,the strengthening mechanism of S30432 austenitic heat-resistant steel was investigated based on the precipitation behavior of S30432 during aging and creep at 650℃.Results show that the microstructure of as-supplied S30432 steel is austenite,the main precipitation consists of only Nb(C,N).After aged for 10 000 h or crept for 10 712 h,there is a slight increase in the size of fine Nb(C,N),but the transformation from Nb(C,N) to NbCrN does not occur.Aging and creep results in the precipitation ofε-Cu and M_(23)C_6.The coarsening velocity ofε-Cu particles diminishes greatly and they are still very fine in the long-term creep range.With the increase of aging and creep time M_(23)C_6 carbides tend to coarsen gradually.The size of M_(23)C_6 is larger and the coarsening is easier in contrast toε-Cu and Nb(C,N).Nb(C,N) precipitates in the as-supplied microstructure,while aging and creep result in the precipitation ofε-Cu and M_(23)C_6.High creep rupture strength of S30432 steel is attributed to the precipitation hardening ofε-Cu,Nb(C,N) and M_(23)C_6.Extremely,ε-Cu plays an important role in improving the creep rupture strength of S30432,and at least 61%of the creep rupture strength of S30432 at 650℃results from the precipitation hardening ofε-Cu particles.

Deformation behavior of 9Cr-3W-3Co martensitic heat-resistant steel

The deformation behavior of 9 Cr-3 W-3 Co heat-resistant steel at a high-temperature range of 1 060-1 260 ℃ and a strain rate of 0.3 s^(-1) was studied using a Gleeble 3800 heat-simulating test machine. The microstructure and precipitation phases of the steel at different temperatures were studied by optical microscopy,scanning electron microscopy,and transmission electron microscopy. The results show that due to its low melting point,coarse grain size,and the segregation of P,S,and Cu at the grain boundary,the thermoplasticity of 9 Cr-3 W-3 Co steel is poor at temperatures higher than 1 200 ℃.The bulk ferrite phase was the main factor affecting the thermoplasticity at 1 100-1 200 ℃.

Study on Key Joining Technology and Test Method of Steel/Al Hybrid Structure Body-in-White

Green and low carbon promote the application and development of light-weight materials in body-in-white. Large-scale die-casting Al alloy (DCAA) and high-strength thermo-formed steel sheet (TFSS) have put forward higher requirements for the application of joining technology of high-strength steel/Al dissimilar materials. Taking the new die-casting Al alloy body as an example, this paper systematically studies the progress of the latest joining methods of steel/Al dissimilar material with combination of two-layer plate and three-layer plate. By analyzing the joining technologies such as FSPR, RES, FDS and SPR, the technology and process characteristics of steel/Al dissimilar material joining are studied, and the joining technical feasibility and realization means of different material combination of the body are analyzed. The conditions of material combination, material thickness, material strength, flange height, preformed holes and joint spacing for achieving high-quality joining are given. The FSPR joining technology is developed and tested in order to meet with the joining of parts with DCAA and TFSS, especially for the joining of three-layer plates with them. It finds the method and technical basis for the realization of high quality joining of dissimilar materials, provides the early conditions for the application of large DCAA and TFSS parts in body-in-white, and meets the design requirements of new energy body. .

Research on the Landscape Utilization and Renovation of Industrial Wasteland in Panzhihua Steel-casting Factory

Renovation design of wasteland on the original Quarter A of Panzhihua Nongnongping Steelcasting Factory was taken for example in this study, natural conditions and biological landscapes in the study area were investigated. Principles and theories of the renovation design were introduced, it was proposed that protection and landscape renovation of industrial heritage had to be insisted to make regular and unique overall spatial layouts. On this basis, design approaches for the wasteland landscape renovation of the steel-casting factory were proposed as "maintaining original images, realizing the functional substitution; optimizing spatial structure; updating seriously-damaged industrial facilities with insignificant functionality; improving and recovering landscape soil; recovering and reconstructing vegetation".

Mechanical Properties of V-,Nb-,and Ti-bearing As-cast Microalloyed Steels

The influence of microalloying additions on the mechanical properties of a low-carbon cast steel containing combinations of V, Nb, and Ti in the as-cast condition was evaluated. Tensile and hardness test results indicated that good combinations of strength and ductility could be achieved by V and Nb additions. While the yield strength and UTS （ultimate tensile strength） increased up to the range of 378-435 MPa and 579- 590 MPa, respectively in the microalloyed heats, their total elongation ranged from 18% to 23%. The presence of Ti, however, led to some reduction in the strength. Microstructural studies including scanning electron microscopy （SEM） and optical microscopy revealed that coarse TiN particles were responsible for this behavior. The Charpy impact values of all compositions indicated that microalloying additions significantly decreased the impact energy and led to the dominance of cleavage facets on the fracture surfaces. It seems that the increase in the hardness of coarse ferrite grains due to the precipitation hardening is the main reason for brittle fracture.

Effects of Rare Earths on Toughness of 31Mn2SiRE Wear- Resistance Cast Steel

The toughness of 31Mn2SiRE wear-resistance cast steel were increased by means of RE compound modification and high temperature austenitizing. The results show that the microstructures can be refined, needle and network ferrite are eliminated, the dislocation density and the quantity of dislocated martensite are increased remarkably, and the shape and distribution of inclusions are improved by the addition of RE. Therefore, the mechanical properties of the modified steel can be greatly increased, especially the toughness (αK) by 44%, yield strength (σs) by 10%, and elongation (δ5) by 42%.

Effect of melting temperature on microstructural evolutions, behavior and corrosion morphology of Hadfield austenitic manganese steel in the casting process

In this study, the effect of melting temperature on the microstructural evolutions, behavior, and corrosion morphology of Hadfield steel in the casting process is investigated. The mold was prepared by the sodium silicate/CO_2 method, using a blind riser, and then the desired molten steel was obtained using a coreless induction furnace. The casting was performed at melting temperatures of 1350, 1400, 1450, and 1500°C, and the cast blocks were immediately quenched in water. Optical microscopy was used to analyze the microstructure, and scanning electron microscopy(SEM) and X-ray diffractrometry(XRD) were used to analyze the corrosion morphology and phase formation in the microstructure, respectively. The corrosion behavior of the samples was analyzed using a potentiodynamic polarization test and electrochemical impedance spectroscopy(EIS) in 3.5 wt% NaCl. The optical microscopy observations and XRD patterns show that the increase in melting temperature led to a decrease of carbides and an increase in the austenite grain size in the Hadfield steel microstructure. The corrosion tests results show that with increasing melting temperature in the casting process, Hadfield steel shows a higher corrosion resistance. The SEM images of the corrosion morphologies show that the reduction of melting temperature in the Hadfield steel casting process induced micro-galvanic corrosion conditions.

Effect of Alloying Elements on Thermal Wear of Cast Hot-Forging Die Steels

The effect of main alloying elements on thermal wear of cast hot-forging die steels was studied. The wear mechanism was discussed. The results show that alloying elements have significant influences on the thermal wear of cast hot-forging die steels. The wear rates decrease with an increase in chromium content from 3% to 4% and molybdenum content from 2% to 3%, respectively. With further increase of chromium and molybdenum contents, chromium slightly reduces the wear resistance and molybdenum severely deteriorates the wear resistance with high wear rate. Lower vanadium/carbon ratio （1.5-2.5） leads to a lower wear resistance with higher wear rate. With an increase in vanadium/carbon ratio, the wear resistance of the cast steel substantially increases. When vanadium/carbon ratio is 3, the wear rate reaches the lowest value. The predominant mechanism of thermal wear of cast hot-forging die steels are oxidation wear and fatigue delamination. The Fe2O3 and Fe3O4 or lumps of brittle wear debris are formed on the wear surface.

Solidification Structure of Low Carbon Steel Strips with Different Phosphorus Contents Produced by Strip Casting

In the present paper, low carbon steel strips with different phosphorus contents were produced using a twin roll strip casting process. The solidification structure was studied and its features were analyzed in detail. It was found that the strips possessed a fine microstructure compared with the mould cast steels. With increasing phosphorus content more ferrite has been formed with finer grains.

Analysis of cracking phenomena in continuous casting of 1Cr13 stainless steel billets with final electromagnetic stirring

Solidification cracking that occurs during continuous casting of 1Cr13 stainless steel was investigated with and without final elec- tromagnetic stirring （F-EMS）. The results show that cracks initiates and propagates along the grain boundaries where the elements of carbon and sulfur are enriched. The final stirrer should be appropriately placed at a location that is 7.5 m away from the meniscus, and the appropri- ate thickness of the liquid core in the stirring zone is 50 ram. As a stirring current of 250 A is imposed, it can promote colurnnar-equiaxed transition, decrease the secondary dendrite arm spacing, and reduce the segregation of both carbon and sulfur. F-EMS can effectively de- crease the amount of cracks in 1Cr13 stainless steel.