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.

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 ℃.

Predictability of Hardness of the Heat Affect Zone in Aluminum Weldments Cooled in Palm Oil Based on Hardness of Similarly Cooled Mild Steel and Cast Iron Weldments

The predictability of hardness of the heat affected zone (HAZ) in aluminum weldments cooled in palm oil, based on hardness of similarly cooled mild steel and cast iron weldments has been ascertained. The general model: α = 1.2769? indicates that HAZ hardness of aluminium weldment is dependant on the ratio of product to sum of HAZ hardness of mild steel and cast iron weldments cooled in palm oil under the same conditions. The maximum deviations of the model-predicted HAZ hardness values α, μ and β from the corresponding experimental values αexp, μexp and βexp were less than 0.04% indicating the reliability and validity of the model.

Effect of Hydrogen Charging on the Tensile and Constant Load Properties of an Austenitic Stainless Steel Weldment

The effect of cathodic hydrogen charging on the tensile and constant load properties was deter- mined for an austenitic stainless steel weldment comprising a 304L steel in the solution treated condition as a base metal and a 308L filler steel as a weld metal. Part of the 304L solution treated steel was separately given additional sensitization treatment to simulate the microstructure that would develop in the heat affected zone. Tests were performed at room temperature on notched round bar specimens. Hydrogen charging resulted in a pronounced embrittlement of the tested materials. This was manifested mainly as a considerable loss in the ductility of tensile specimens and a decrease in the time to failure and threshold stress of constant load specimens. The 308L weld metal exhibited the highest, and the 304L solution treated steel the lowest, resistance to hydrogen embrittlement. Hydrogen embrittlement was associated with the formation of strain induced martensite as well as a transition from brittle to ductile fracture morphology onwards the centre of the specimens.

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.

Characterization of the Type IV crack in high Cr steel weldments

The weldments for several types of high Cr ferritic steels were investigated to understand the fracture mechanism of the Type IV crack. The creep testing shows that the Type IV cracking is more likely to occur at the lower stress level, whereas at the higher stress level the crack shifts towards parent metal. The microstructures observed in the HAZ for all the materials are mostly tempered lath martensites. There was no δ ferrites found in all creep specimens.

不同蠕变条件下耐热钢焊接件的空洞损伤演化

针对两种马氏体耐热钢焊接接头(F92/Co3W2和Co3W2/Co3W2)在105~250 MPa和873~948 K条件下开展蠕变拉伸测试,利用Larson-Miller参数法外推不同温度下焊接接头服役100000 h寿命对应的蠕变断裂许用应力,研究其在不同蠕变条件下的空洞损伤规律.随着外加应力的降低,两种焊件的断裂位置、断裂模式及断裂机制均发生了转变,从位于母材的穿晶塑性断裂变为细晶热影响区的沿晶脆性断裂(即Ⅳ型断裂).针对断口区域的空洞损伤变化规律的研究表明:(1)当断裂模式由塑性断裂转变为脆性断裂时,断口附近空洞会出现突变,塑性断口空洞“个头大数量少”而脆性断口空洞“个头小数量多”;(2)在相同断裂模式下,随蠕变应力的减小,空洞按照“尺寸变大、数量密度增大及面积分数增大”的规律变化;(3)Ⅳ型断口处空洞存在合并链接现象,其方向与应力方向垂直.

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.

Effect of cerium on microstructure,eutectic carbides and Laves phase in electroslag remelted 15Cr-22Ni-1Nb austenitic heat-resistant steel

The dendrites,eutectic carbides,Laves phase and microsegregation of alloying element in electroslag remelted 15Cr-22Ni-1Nb austenitic heat-resistant steel with varying cerium contents were studied.The liquidus and solidus temperatures of the steel were determined to reveal the effect of cerium on solidification temperature interval and local solidification time of the steel.The secondary dendrite arm spacing decreases from 57.10 to 40.18μm with increasing the cerium content from 0 to 0.0630 wt.%.The eutectic NbC and Laves phase in as-cast ingots exhibit blocky and honeycomb morphology,respectively.The area fractions and sizes of eutectic NbC and Laves phase in as-cast ingots decrease with the increase in cerium content.The atomic percentage of Laves phase-forming element(Ni,Nb,Cr,Mo and Si)decreases with the increase in cerium content of the steel.The microsegregation of Mo,Ni,Si,Cr and Nb decreases with increasing the cerium content,which is favorable to reducing both the amount and sizes of eutectic NbC and Laves phase in as-cast ingots.The solidification temperature interval and local solidification time of the steel decrease as the cerium content is increased from 0 to 0.0630 wt.%,which inhibits the growth of dendrites,eutectic NbC and Laves phase.

Optimization of gas tungsten arc welding parameters for the dissimilar welding between AISI 304 and AISI 201 stainless steels

This present study applied gas tungsten arc welding in order to join AISI 304 and AISI 201 stainless steels.The objective was to find the optimum welding condition that gave a weld bead in accordance with DIN EN ISO 25817 quality level B, pitting corrosion potential of the weld metal of not less than that of the AISI304 base metal and a ratio of delta-ferrite in austenite matrix of the weld metal of not lower than 3%.Such a ratio is a criterion widely accepted to protect the weld metal from solidification cracking. At the welding current of 75 A and by using pure argon as a shielding gas 0 to 8 vol.% and applying a welding speed in the range of 2-3.5 mm·s^(-1) was found to give a complete weld bead with an increased depthper-width ratio(promote weldability). For welding speed in the range of 3 and 3.5 mm·s^(-1)(promote corrosion resistance). Increasing the welding speed in such a range decreased the amount of delta-ferrite in the austenite matrix, and increased the pitting corrosion potential of the weld metal to be 302 mV_(SCE).This value was still lower than the pitting corrosion potential of the AISI 304 base metal. Mixing nitrogen in argon shielding gas increased the nitrogen content in the weld. The optimum condition was found when using a welding speed of 3 mm· s^(-1) and mixing 1 vol.% of nitrogen in the argon shielding gas(promote weldability and corrosion resistance). Pitted areas after potentiodynamic test were observed in the austenite in which its Cr content was relatively low.

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).

Creep Behavior and Life Assessment of a Novel Heat-Resistant Austenite Steel and Its Weldment

In the present study, creep activation energy for rupture was obtained as 221-348 kJ/mol for 22Cr15Ni3.5 CuNbN due to the precipitation-hardening mechanism. The extrapolation strength of creep rupture time of 10~5 h at 923 K for22 Cr15 Ni3.5 CuNbN is more valid(83.71 MPa) predicted by the Manson-Haferd method, which is superior to other commercial heat-resistant steels. The tensile creep tests ranging from 180 to 240 MPa at 923 K were conducted to investigate creep deformation behavior of welded joint between a novel heat-resistant austenite steel 22Cr15Ni3.5 CuNbN and ERNiCrCoMo-1 weld metal. Apparent stress exponent value of 6.54 was obtained, which indicated that the ratecontrolled creep occurred in weldment during creep. A damage tolerance factor of 6.4 in the weldment illustrates that the microstructural degradation is the dominant creep damaging mechanism in the alloy. Meanwhile, the welded joints perform two types of deformation behavior with the variation in applied stress, which resulted from the different parts that govern the creep processing. Also, the morphology evolution of the fracture surfaces confirms the effects of stress level and stress state.

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.