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.

Microstructure and Fracture Morphology in the Welding Zone of T91 Heat-resisting Steel Used in Power Station

Microstructure performance in the welding zone of T91 heat-resistant steel under the condition of TIG welding was researched by means of metallography, X-ray diffraction and scanning electron microscope (SEM). Experimental results indicated that microstructure of T91 weld metal was austenite + a little amount of S ferrite when using TGS-9cb filler wire. Substructure inside the austenite grain was crypto-crystal lath martensite, on which some Cr23C6 blocky carbides were distributed. The maximum hardness (HRC44) in the welding zone is near the fusion zone. There existed no obvious softening zone in the heat-affected zone (HAZ). For T91 steel tube of $63 mmx5 mm, when increasing welding heat input (E) from 4.8 kJ/cm to 12 5 kJ/cm, fracture morphology in the fusion zone and the HAZ changed from dimple fracture into quasi-cleavage fracture (QC). Controlling the welding heat input of about 9.8 kJ/cm is suitable in the welding of T91 heat-resistant steel.

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 ℃.

Evolution of nonmetallic inclusions in 80-t 9CrMoCoB large-scale ingots during electroslag remelting process

In combination with theoretical calculations,experiments were conducted to investigate the evolution behavior of nonmetallic inclusions(NMIs)during the manufacture of large-scale heat-resistant steel ingots using 9CrMoCoB heat-resistant steel and CaF_(2)–CaO–Al_(2)O_(3)–SiO_(2)–B_(2)O_(3)electroslag remelting(ESR)-type slag in an 80-t industrial ESR furnace.The main types of NMI in the consumable electrode comprised pure alumina,a multiphase oxide consisting of an Al_(2)O_(3)core and liquid CaO–Al_(2)O_(3)–SiO_(2)–MnO shell,and M_(23)C_(6)carbides with an MnS core.The Al_(2)O_(3)and MnS inclusions had higher precipitation temperatures than the M_(23)C_(6)-type carbide under equilibrium and nonequilibrium solidification processes.Therefore,inclusions can act as nucleation sites for carbide layer precipitation.The ESR process completely removed the liquid CaO–Al_(2)O_(3)–SiO_(2)–MnO oxide and MnS inclusion with a carbide shell,and only the Al_(2)O_(3)inclusions and Al_(2)O_(3)core with a carbide shell occupied the remelted ingot.The M_(23)C_(6)-type carbides in steel were determined as Cr_(23)C_(6)based on the analysis of transmission electron microscopy results.The substitution of Cr with W,Fe,or/and Mo in the Cr_(23)C_(6)lattice caused slight changes in the lattice parameter of the Cr_(23)C_(6)carbide.Therefore,Cr_(21.34)Fe_(1.66)C_(6),(Cr_(19)W_(4)C_(6),Cr_(18.4)Mo_(4.6)C_(6),and Cr_(16)Fe_(5)Mo_(2)C_(6)can match the fraction pattern of Cr_(23)C_(6)carbide.The Al_(2)O_(3)inclusions in the remelted ingot formed due to the reduction of CaO,SiO_(2),and MnO components in the liquid inclusion.The increased Al content in liquid steel or the higher supersaturation degree of Al_(2)O_(3)precipitation in the remelted ingot than that in the electrode can be attributed to the evaporation of CaF_(2)and the increase in CaO content in the ESR-type slag.

DETERMINATION AND APPLICATION OF LARSON-MILLER PARAMETER FOR HEAT RESISTANT STEEL 12CrlMoV AND 15CrMo

Based on the analysis and processing on relative empirical formula and data, C-values in Larson-Miller (P) expression, P= T(C + Igt), have determined for pearlitic heat resistant steel 12Cr1MoV and 15CrMo(20.62 and 20.30). The simulation experiments of high temperature aging, heated from 1.5 to 873 hours, have been designed and performed for its verification. And in combination with published information and the present nearly quantitative works, it has further been verified that both the degradations of microstructures and mechanical properties show a good accuracy and practicability using the Larson-Miller parameter with the present determined C-values. Finally, the effects of carbon content on C-value are analyzed by the empirical electron theory of solids and molecules (EET).

Diffusion bonding between nickel－base superalloy K24 and heat－resisting steel 1Cr11Ni2W2MoV

The Vacuum diffusion bonding between nickel-base superalloy K24 and heatresisting steel 1Cr11Ni2W2MoV was studied and the effects of welding parameters,including bonding temperature T,pressure P,time t,vacuum V and interlayer thickness h ,on the microstructure and properties of the joint were investigated.The optimum parameters for the diffusion bonding were found to be:T=1100￣1200℃,t=5￣30 MPa,V≥1×10-2 Pa.When the welded specimens are tensioned at room temperature,ductile fracture takes place at the nickel-base superalloy part or at the joint and the tensile strength reaches 850 MPa,When the tensile tests are carried out at 500℃ ,all the bonded specimens fractured at heat-resisting steel part,and tensile strength is 750 MPa.The interlayer thickness greatly affects the joint properties,and the interlayer thickness depends on the size of the bonded specimen.The impeller of nickel-base superalloy K24 and shaft of heat-resisting steel 1Cr11Ni2MoV,which are used in aerospace engine,has been successfully jointed jointed by means of high frequency induction diffusion bonding.

VARIATION OF SUBSTRUCTURES OF PEARLITIC HEAT RESISTANT STEEL AFTER HIGH TEMPERATURE AGING

The observations of dislocations, substructures and other microstructural details were conducted mainly by means of transmission electron microscope (TEM) and scanning electron microscope (SEM) for 12CrlMoV pearlitic heat-resistant steel. It is shown that during the high temperature long-term aging, the disordered and jumbled phase-transformed dislocations caused by normalized cooling are recovered and rearranged into cell substructures, and then the dislocation density is reduced gradually. Finally a low density linear dislocation configuration and a stabler dislocation network are formed and ferritic grains grow considerably.

Effects of Austenite Stabilization on the Onset of Martensite Transformation in T91 Steel

The influences of thermal stabilization of austenitic on the onset temperature for a martensite transformation in T91 ferritic heat-resistant steel were studied by high-resolution differential dilatometer. The phase transformation kinetic information was obtained by adopting lever rule from the recorded dilatometric curves. The results show that an inverse stabilization, featured by the damage of ＂the atmosphere of carbon atoms＂ and the increase of the starting temperature for martensite transformation takes place when the T91 ferritic steel is isothermally treated above the Ms point, and it becomes strong with increasing the holding time. While the continued temperature for martensite transformation decreases gradually when isothermally holding at a temperature below Ms point. The observed inverse stabilization behavior could be attributed to the relatively high temperature of Ms point in the explored T91 ferritic heat-resistant steel.

Effects of laser heat treatment on salt spray corrosion of 1Cr5Mo heat resistant steel welding joints

The surface of 1Cr5 Mo heat-resistant steel welding joint was processed with CO2 laser, and the corrosion behaviors before and after laser heat treatment(LHT) were investigated in the salt spray corrosion environments. The microstructures, phases, residual stresses and retained austenite content of 1Cr5 Mo steel welding joint before and after LHT were analyzed with optical microscope and X-ray diffraction, respectively. The cracking morphologies and chemical compositions of corrosion products after salt spray corrosion were analyzed with field emission scanning electron microscopy(FESEM) and energy disperse spectroscopy(EDS), respectively, the polarization curves were measured on a PS-268 A type electrochemical workstation, and the mechanism of corrosion resistance by LHT was investigated as well. The results show that the passive film of original sample is destroyed owing to the corrosive media penetrating into the subsurface, resulting in the redox reaction. The content of residual austenite in the surface and the self-corrosion potential are increased by LHT, which is contributed to improving the capability of salt spray corrosion resistance.

Evaluation and modification of inclusion characteristics in HK40 heatresistant cast steel

Specifications of inclusions such as the type, morphology, number, composition, size, and place of inclusions in HK40 heat-resistant casting steel were studied by optical microscopy and scanning electron microscopy equipped with an energy-dispersive spectroscopy system. The effects of calcium silicide （CaSi） addition on these characteristics were evaluated at two pouring temperatures of 1420℃ and 1470℃. It was found that most of the appeared inclusions were in the type of chromium oxide. CaSi addition had a significant effect on the morphological modification, size and distribution of inclusions as well as changing the composition to oxide-sulfide compounds.

RELATIONSHIP BETWEEN CREEP PROPERTIES AND STRUCTURE OF STEEL 20Cr11MoVNbNB

Studies were made of the creep properties and structure change of steet 20Cr11MoVNbNB at 550-650℃.It is found that at 550℃ when creep stress>180 MPa there exists linear rela- tion with two different slopes β_1 and β-2 between transitional creep strain ε_β and creep time t^(1/3),and β_1(in initiol period)>β_2(in later period).The Creep rate of stable stage at 550℃ can be expressed by =Aσ exp(-Q_ /RT),where,n=4.7.A exp(-Q_ /RT)= 5.37×10^(-16).Apparent creep activation energy Q_ =430 kJ/mol at 550-650℃,which is much higher than the self-diffusion activation energy of atoms for matrix Fe.In creep process,as the creep stress and time increase,the total weight of precipitated phases and rela- tive weight of Cr,Mo and V increase,but those of Fe and Nb decrease in precipitated phases,

Introduction of creep-rupture testing capability and proficiency at Baosteel

To characterize the long-term service performance and predict the lifetime of heat-resistant steel applications,more than 300 test pieces can be loaded simultaneously at Baosteel to determine the relationship between stress and rupture duration at specific temperatures. In addition, versatile testing methods have been established that evaluate creep-rupture, stress-relaxation, and creep-fatigue, as well as state-of-the-art tensile testing at hyper-high temperatures exceeding 2 100 ~2. Based on the Larson-Miller method, according to API 530, the accumulation of large volumes of data with respect to stress, temperature, and rupture time of actually tested steels ensures the reliability of predicting the allowable working stress over 100 000 h at any given temperature. For examples ,the stress tendencies against rupture time of T/P91 and T/P92 steel tubes, as obtained by Baosteel, are consistent with those from NIMS and ECCC. As an accredited provider of the Proficiency Testing schemes in ISO/ IEC 17025 and ISO/IEC 17043, Baosteel has periodically organized national proficiency testing with respect to creep and rupture, thereby providing an efficient way for all parties to assess their technical competence when performing uniaxial creep testing by the methods in ISO 204 and ASTM E139.

Electrochemical corrosion failure mechanism of M152 steel under a salt-spray environment

The corrosion failure mechanism of M152 was studied using the neutral salt-spray test to better understand the corrosion behavior of 1Cr12Ni3Mo2VN（M152）, provide a basis for the optimization of material selection, and prevent the occurrence of failure. Moreover, the mechanism was investigated using the mass loss method, polarization curves, electrochemical impedance spectroscopy（EIS）, stereology microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy（EDS）. The results show that M152 steel suffers severe corrosion, especially pitting corrosion, in a high-salt-spray environment. In the early stage of the experiment, the color of the corrosion products was mainly orange. The products then gradually evolved into a dense, brown substance, which coincided with a decrease of corrosion rate. Correspondingly, the EIS spectrum of M152 in the late test also exhibited three time constants and presented Warburg impedance at low frequencies.