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Catalytic reforming of pyrolysis tar over metallic nickel nanoparticlesembedded in pyrochar

Catalytic reforming of pyrolysis tar over metallic nickel nanoparticles embedded in

pyrochar

Yafei Shen a ,?a School of Environmental b

School of Environmental RHC Ni can inhibit the formation of both heavy and light tars including poly-cyclic aromatic hydrocarbons (PAHs).

a r t i c l e i n f o Article history:

Received 17March 2015

Received in revised form 3July 2015Accepted 4July 2015

Available online 17July 2015Keywords:

Biomass pyrolysis

Tar catalytic reforming

Metallic nickel nanoparticles Rice husk char

Carbothermal reduction

a b s t r a c t

Metallic nickel (Ni 0)nanoparticles could be in situ generated in the carbon matrix of rice husk char (RHC)via a facile one-step pyrolysis.The synthesized RHC Ni has a considerable performance on tar reforming.Tar reforming ef?ciency is increased with the increases of the used catalyst weight and reforming tem-perature.In particular,tar reforming ef?ciency can reach up to 90.5%and 99.8%by using 5g and 10g of RHC Ni,respectively.Tar reforming ef?ciency can also keep stable after 5cycles.Besides,RHC Ni showed high tar conversion ef?ciency,increasing from 92.3%to 100%,when the reforming temperature was increased from 500°C to 900°C.RHC Ni showed a high catalytic activity even at the relatively lower tem-peratures.Furthermore,the yield of liquid products was decreased from 30.2%to 10.7%,corresponding to tar reforming.Accordingly,the gas yield was increased from 37.5%to 58.0%,in which the main compo-nents of syngas are CO and H 2.It is noted that the PAHs compounds in tar could be signi?cantly reduced by using the RHC Ni.The surface areas of the used RHC Ni were increased due to the char gasi?cation

rate higher than deposition rate.The RHC Ni has a high potential to be used for tar catalytic reforming.

1.Introduction

The application of

thermal processes such as pyrolysis or gasi?-cation for the energy recovery from bio-wastes has attracted a growing attention.In general,pyrolysis can be optimized to obtain liquid oils and solid char,while gasi?cation is more favor of syngas production at higher temperature [1–5].Compared with the partial oxidative gasi?cation,the high-temperature pyrolysis can produce fuel gas with a high heating value [4].Also,pyrolysis-reforming is effective for the thermal conversion of biomass to improve gas yield,reduce tar contents,and enhance conversion rates [6–10].

https://www.sodocs.net/doc/c914071659.html,/10.1016/j.fuel.2015.07.007

?Corresponding authors.

E-mail addresses:jsshenyafei@https://www.sodocs.net/doc/c914071659.html, (Y.Shen),sunth@https://www.sodocs.net/doc/c914071659.html, (T.Sun).

In the two stage process,biomass is decomposed in the?rst stage, and then the derived vapors and tar are reformed in the second stage at higher temperatures(T P800°C).The addition of different catalysts with/without the steam in the second stage has shown a positive effect on tar reduction and syngas reformation.

Biomass pyrolysis tar is a complex mixture of condensable hydrocarbons,including1-ring to5-ring aromatic organic com-pounds along with other oxygenated hydrocarbons and polycyclic aromatic hydrocarbons(PAHs).Aromatic compounds present in tars,such as benzene and PAHs,are toxic and represent environ-mental hazards.Tar can deposit on surfaces of?lters,heat exchangers and engines.Moreover,tar could polymerize to form more complex structures.Catalytic reforming is the one of promis-ing tar removal techniques from gaseous products by transforma-tion of tar into H2and CO in the presence or absence of steam[11].

A variety of catalysts such as Fe-,Co-,Ni-based catalysts,dolo-mites,olivine and catalyst-loaded zeolites have been widely used for tar catalytic conversion at600–900°C[12–14].Among these supports,char,olivine,and dolomite appear particularly attractive, since they are all inexpensive and relatively abundant in different regions.Olivine is less active in gasi?cation,but it shows the opti-mal hardness required for the?uidized-bed reactor.Dolomite has high catalytic activity in tar reforming and considerable CO2cap-ture performance.This characteristic in particular results in the gasi?cation process possible at relative low temperatures(650–700°C),namely adsorption enhanced reforming process,than the more conventional range of850–900°C,without a signi?cant increase in the downstream tar content.However,the catalytic activities of dolomite and olivine for tar conversion leave room for improvement,so the motivation is the search for catalytic addi-tives[15].Recently,chars derived from coal or biomass have been used as low-cost carbonaceous catalysts[16–20]and adsorbents [21–23]in tar elimination.Char itself exhibits a fair catalytic activ-ity for tar reforming,which is often in?uenced by pore size,surface area and mineral contents.Moreover,char can act as a catalyst support to disperse the active clusters at nanoscale[24].The deac-tivated char can be gasi?ed without the need of frequent regener-ation(e.g.,(R1))[25].

Ct2H2O!CO2t2H2eR1TIn the previous work,it has been proved that rice husk char (RHC)supported nickel catalysts shows good catalytic activity on in-situ conversion of tar derived from biomass pyrolysis[26,27]. Catalysts placed in contact with the feedstock inside the pyrolysis reactor(in situ)can inhibit the nascent tars polymerization,reduc-ing the macromolecular tar formation and condensation.However, the nickel catalysts in the RHC could be unfeasible to be frequently recycled from the solid residues.Likewise,catalysts can be also used in the downstream of the primary reactor(ex situ)for tar con-version and vapors upgrading[28,29].In this work,the catalytic performances including reaction activity,cycle tests and service life(i.e.,deactivation)of the RHC nickel catalysts will be prelimi-nary studied for ex situ catalytic conversion of pyrolysis tar.

2.Materials and methods

2.1.Biomass and char characterization

The biomass feedstock of RH was collected from Thailand.The as-received RH was dried in an oven at105°C overnight.After that, the dry-based RH was devolatilized by pyrolysis at700°C for 30min for rice husk char(RHC)production.Furthermore,the rice husk ash(RHA)was obtained by burning the as-prepared RHC in a muf?e furnace in air for1h.Table1shows the proximate and ultimate analyses of RH,RHC and RHA,which were determined by the elemental analyzer(Vario MICRO Cube,Elementar, Germany)and the thermogravimetric(TG)and differential TG (DTG)analysis(DTG-50,Shimadzu,Nakagyo-ku,Japan),respec-tively.The chemical composition of RHA was determined by the X-ray?uorescence analysis(XRF,Shimadzu,Rayny EDX700, Japan).The total speci?c surface area(S BET)accordingly calculated by the Brunauer–Emmett–Teller(BET)method was measured by a Nova-2200e surface area and pore size analyzer(Quantachrome Instruments,USA).

2.2.Catalysts preparation

RH has a highly anisotropic cellular structure,which is used as a natural biotemplate to generate the RHC with the ordered micro-, meso-,and macro-porous structures.The metallic Ni nanoparticles could be generated in the carbon matrix of RHC via one-step pyrol-ysis.Initially,RH(20g)was added into the Ni(NO3)2solution (0.1mol/L)as a nickel precursor and then dried in an oven at 105°C.In consideration of the economic ef?ciency,the nickel elec-troplating wastewater can be considered as an alternative. Subsequently,the dry-based RH Ni was pyrolyzed at750°C in an inert gas(e.g.,N2)atmosphere for10min.After that,the RHC Ni was obtained and dry-stored for the further use.

2.3.Biomass pyrolysis-catalytic reforming

The experimental setup(as shown in Fig.1)is composed of a gas supplying system,a gas cleaning system and a pyrolysis-reforming facility,which was divided into three parts, an outer tube,a top cover with a feeding port and also a gas inlet and an inner tube.Two sintered quartz porous plates were?xed in each tube to support the biomass or the catalysts.The reactor was surrounded by a two-zone electric furnace.At?rst,the feed-stock of RH was prepared by crushing and sieving with the particle size below 5.0mm.Before adding the catalysts and biomass feedstock,the nitrogen(N2)with a?ow rate of$1.0L/min was continuously injected into the reactor to clear away the residual gases in the reactor.The pyrolyzer was heated up to750°C,while the temperature of reformer was controlled at a range of500–900°C.The fresh RHC Ni catalyst was initially put in the reformer. After that,biomass feedstock(RH,20g)could be fed into the pyrolyzer,and the volatile matters were released mainly in the form of vapor,including the nascent tars,which could be cracked into the small molecular gases over the RHC Ni by catalytic reactions.The condensable tar was collected by isopropanol(IPA) in the gas cleaning unit.

2.4.Sampling and analysis

The liquid products derived from biomass pyrolysis were deter-mined by weighing,mainly including tar and water[30].Biomass tar is a complex organic mixture composed of hundreds of condensable hydrocarbon compounds with other oxygenated hydrocarbons,which is determined by weighing[31].The detect-able composition of tar is determined by a gas chromatography-mass spectrometer(GC–MS,Shimazu,GCMS-QP2010,Japan). Additionally,the yield of producer gas was estimated by the mass balance.The producer gas mainly composed of H2,CH4,CO,CO2, and light hydrocarbons(i.e.,C2H4,C2H6)was measured by a micro gas chromatograph(Micro GC,Agilent,3000A,USA),which is?tted with thermal conductivity detector(TCD).Each trial was kept for ten minutes to ensure a good mass balance.Meanwhile,the repeatability experiments were performed to ensure the reliability of the system.Therefore,the collected tar sample was the total quantity of tars generated from the repeatability experiments. The Ni concentration in the RHC Ni was determined by the

Y.Shen et al./Fuel159(2015)570–579571

Inductively Coupled Plasma analysis(ICP,Iris Advantage1000, Thermo king-cord Co.).In addition,the fresh and the used RHC Ni catalysts were characterized by the TG/DTG analysis,the X-ray diffraction analysis(XRD,Rigaku,XRD-DSC II,Japan),the scanning electron microscopy(SEM,JSM-6610,JEOL/EO,Japan), and the transmission electron microscopy-energy dispersive spec-troscopy(TEM-EDS,JEM-2010F,JEOL,Japan),respectively.

3.Results and discussions

3.1.Effect of catalyst weight

The weight of spent catalyst is an important parameter for the catalytic reforming of tar.If excessive catalysts are used,it is uneconomical for the tar catalytic reforming.Moreover,the gas percolation resistance to pass through the catalyst zone might be increased.In contrast,if insuf?cient catalysts are used,tar reform-ing ef?ciency is relative low.Moreover,the catalyst is easily deac-tivated due to the short retention time;accordingly,the excessive catalyst can increase the contact frequency and reaction time of active sites with tar molecules,enhancing the catalytic perfor-mance.Fig.2shows the effect of the used catalyst weight on tar reforming ef?ciency at$700°C.Tar reforming ef?ciency increased with the increase of the weight of used RHC Ni.Tar reforming ef?ciency can reach about50.1%by using3g of RHC Ni,while it signi?cantly increased,up to90.5%and99.8%by using5g and 10g of RHC Ni,respectively.It might be attributed to the enhance-ment of catalytic reaction and retention time resulted from the

Fig.1.Schematic diagrams of experimental setup. 572Y.Shen et al./Fuel159(2015)570–579

increase of catalyst weight.Furthermore,tar reforming ef?ciency cannot decrease after5cycles,indicating a high stability perfor-mance on tar conversion.

3.2.Effect of catalytic temperature

Catalytic temperature is another signi?cant parameter,since it directly affects the reaction activity.During the thermo-chemical reactions,high temperature can enhance the catalytic reactivity. Fig.3shows the effect of the reforming temperature on tar conver-sion.Tar conversion ef?ciency generally increased with the increase of reforming temperature.If the RHC was used,tar con-version ef?ciency can signi?cantly improve from39.8%to78.7% in a temperature range of500–900°C.However,tar conversion ef?ciency slightly improved from92.3%to100%in the same tem-perature range by using the RHC Ni.It means that the temperature

can signi?cantly in?uence the tar conversion ef?ciency,if no cata-lyst or only the RHC is used.In addition,it suggested that RHC exhibited a higher catalytic activity for tar conversion even at the relatively lower temperatures(e.g.,500°C).

3.3.Syngas yield and composition

Gas,liquid and char are the predominate products derived from biomass pyrolysis or gasi?cation.Liquid including water and tar are not considerable products during biomass gasi?cation,while the char could be fabricated into the value-added carbonaceous materials.Fig.4A shows the products composition from RH gasi?-cation at750°C.It can be found that the yield of pyrolysis char was not affected.The char yield from RH pyrolysis at750°C was around 31.3–32.5%.However,the yields of liquid and producer gas had been changed signi?cantly.In particular,if the RHC Ni was used for catalytic reforming,the yield of liquid products reduced from 30.2%to10.7%,most likely caused by a series of the thermo-chemical reactions,such as water–gas-shift reaction,char gasi?cation,and tar reforming.Accordingly,the yield of product gas greatly increased from37.5%to58.0%(Fig.4A),in which the main components of syngas are CO and H2.From Fig.4B,it can be found that using the RHC Ni,the volume fractions of CO and H2in syngas increased from21.2%to33.5%and from18.5%to

24.3%,respectively.Besides,the volume fractions of CH4,C n H m

(i.e.,C2H4,C2H6)and CO2reduced due to the catalytic reforming over the RHC Ni.In particular,RHC exhibited a stable catalytic activity for vapor conversion most likely due to the presence of minerals in it[17].

3.4.GC–MS analysis of the condensed tar

Tars can be classi?ed by their solubility and condensability[32], categorized into?ve classes.Class1refers to the GC undetectable heavy tars,which can condense at relative high temperatures and very low concentrations;Class2refers to the heterocyclic aromatic compounds with high water solubility(e.g.,phenol and cresol); Class3refers to the light hydrocarbons single-ring aromatic com-pounds(e.g.,toluene and xylene);Class4refers to the light poly-cyclic aromatic hydrocarbons(2–3rings),which can condense at the relatively high concentrations and intermediate temperatures (e.g.,indene and naphthalene);Class5refers to the heavy poly-cyclic aromatic hydrocarbons(4–7rings),which can condense at relatively high temperatures and low concentrations(e.g.,pyrene and coronene).Fig.5A shows the GC–MS spectra of the condensed tar derived from RH pyrolysis at750°C.It could be observed that more than117absorbance peaks were determined.Likewise, Fig.5B shows the GC–MS spectra of the condensed tar derived from RH pyrolysis–catalytic reforming by the RHC Ni at the same tem-perature of750°C.In this case,the peak numbers are decreased to37,indicating that the tar compounds was signi?cantly reduced by the catalytic reforming.Meanwhile,these MS spectra were identi?ed by the MS database.The most abundant and important organic components in the condensed tars were phenol,benzene, naphthalene,biphenylene,and their derivates.In particular,the naphthalene and its derivates occupied more than30.26%in the tar sample1and44.33%in the tar sample2,respectively. However,the aromatic organic compounds consisting of PAHs, such as phenanthrene,pyrene,?uoranthene,existed in both of tar samples,indicating that it is dif?cult to convert them by the catalytic reforming.Furthermore,compared with the tertiary tars of PAHs,it is much easier to crack/reform the nascent tars by RHC Ni.Moreover,the tertiary tars except for naphthalene com-pletely disappeared after the heterogeneous gasi?cation process by the RHC and RHC Ni.More signi?cantly,the total amounts of tar by-products consisting of the tertiary tars were signi?cantly reduced.CO2dry reforming can also reduce parts of nascent tar and convert them into thermally stable tertiary tar components such as toluene,naphthalene and styrene.Because of the corrosion, condensation,and deposition effects in syngas utilization,it is of great importance to reduce the PAHs content,especially in terms of these troublesome tars decomposed in the gasi?cation zone by the char.In the reforming zone,the homogeneous partial oxidation and heterogeneous char conversion are two signi?cant factors to ensure that the low-tar syngas is achieved.The selectively

2.Effect of the catalyst weight[3g;5g;10g;10g(5cycles)]on tar reforming.

Fig.3.Effect of the reforming temperature on tar reforming(10g).

159(2015)570–579573

properties of RHC Ni in eliminating tertiary tars greatly increased application prospects of the gasi?er[30].In this work,the major tar compounds came from tar Class4;sometimes referred tertiary tars containing non-oxygenated organic compounds

Among the compounds grouped in tar Class4, contribution came from the naphthalene and its deri-indicated that RHC Ni showed the promising tar ef?ciency on the GC–MS detected tars.

of catalysts

identi?cation of crystal phases was performed by the XRD Rigaku D/Max3400powder diffraction system with

=0.1542nm)at45kV and40mA with a scan-

Fig.6presents the XRD patterns of the fresh

catalysts.Silica in all samples is amorphous.

typical silica characteristic is observed at a broad

around2#=22.5°,which can be attributed to the presence of disordered cristobalite[33,34].The main chemical

states of nickel in the RHC Ni catalysts were the nickel oxides (e.g.,NiO,bunsenite)and metallic nickel(Ni0)by the reactions (R2)–(R5).Initially,the nickel cations(Ni2+)in the aqueous solution were transformed into the relative stable form of Ni(H2O)62+(i.e.,octahedral water coordination complexes)[35],which was there-fore decomposed into the bunsenite(NiO).With the increase of pyrolysis time,more nickel oxides(i.e.,NiO)embedded in the car-bon matrix of RHC were reduced into the metallic nickel(Ni0)by

4.(A)Mass fraction of pyrolyzed products,(B)volume fraction of gas products derived from RH pyrolysis at750°C(reforming temperature:700°C,catalyst weight:

g).

Fig.5.GC–MS spectra identi?ed for main organic compounds in the condensed tar from RH pyrolysis(A)combined with RHC Ni catalytic reforming(B).

Fig.6.XRD patterns of the RHC,RHC Ni1(fresh)and RHC Ni2(used)catalysts.

the carbothermal reduction((R6)and(R7))[27]along with the hydrogenation reduction(R8).As shown in Fig.6,the crystalline Ni0was appeared after the pyrolysis of RH Ni[36].After used for catalytic reforming,the intensity of Ni0characteristic peaks in the RHC Ni2enhanced,indicating well crystallization.However, the particle size of Ni0was unchanged.

Ni2tt2eàSiOHT$NieàSiOT

t2HteR2T

5NieNO3T

2t24H2O!Ni2tt4?NieH2OT

2tt10NOà

eR3T

?NieH2OT

6 2t!NieOHT

t2H3Ot

t2H2Oethermal decompositionTeR4T

NieOHT

!NiOtH2OeR5TCarbothermal reduction:

NiOtC!NitCOeR6TNiOtCO!NitCO2eR7THydrogenation reduction:

NiOtH2!NitH2OeR8T

3.5.2.SEM analysis

Rice is one of the most widespread food crops for human suste-nance.The content of the RH reaches$20wt%of the entire rice kernel,a very large amount,considering the massive scale of global rice production.In addition,RH contains a variety of components such as lignin(20–30wt%),cellulose(55–65wt%),and silica (15–20wt%),which originates from monosilicic acid that is initially introduced into rice plants through their roots and is then moved to the rigid outer epidermal walls of the plants where it is converted into silica.Silicon(Si)exists along the outer rugged sur-faces of RH[37].RH has a typical globular structure,of which its main components are in the lemma/palea form,tightly interlocked with each other[38].The corrugate structural outer epidermis is highly ridged,while its ridges are punctuated with the prominent globular protrusions.However,RH is assembled around the Si-O carcass,which is concentrated in the protuberances and hairs (trichomes)on the outer and inner epidermis,adjacent to the rice kernel.Many cavities having varying particle sizes were dis-tributed within the char samples,as evidenced of the intercon-nected porous network and large speci?c surface area[39].With the development of reaction,the surface texture of char becomes irregularity ascribed to the shrinkage of the globular structure, which might be caused by devolatilization[40].Evaporation of the volatile materials could create new pores on the particle with rough surface and irregular outlet(Fig.7A).The external surface is found covered mostly with smooth open pores of different sizes (Fig.7B–D).Moreover,it appears that the possibility of fragmenta-tion since several cracks passing through the particle can be observed clearly.From all of the used RHC Ni,small particles with globular shape could be observed by magnifying the above micro-graphs(Fig.8).Some particles derived from dust attached on the parent RH.In addition,other formed larger clusters(Fig.8B)was most likely attributed to the soot formation and carbon deposit resulting from hydrocarbons cracking[40,41].

3.5.3.TEM analysis

Metallic nickel(Ni0)nanoparticles could be dispersed uniformly in the carbon matrix of the wood char obtained at pyrolysis tem-peratures from500to700°C.However,the very wide dispersion of the monocrystalline metallic nickel(Ni0)particles with the par-ticle sizes of2–4nm could form at the pyrolysis temperatures ranging from400to500°C and their nanometric size con?rmed the high dispersion of metal precursor in the wood obtained after the impregnation step[35].It can also be seen that nickel nanopar-ticles were embedded and highly dispersed in the carbon matrix of RHC with the particle size of$10nm(Fig.9A);whereas these par-ticle sizes became relative non-uniform after the reaction(Fig.9B), in terms of the slight decrease of particle size.Herein,the aggluti-nated nickel nanoparticles with the size range of10–20nm might be attributed to the coagulation with the nanosized amorphous sil-ica.Furthermore,carbon deposition on the surface of nickel parti-cles could result in the increase of particle size.It was observed that some nanoparticles were encapsulated by the dark-colored matters after use(Fig.9B).The elemental constitutes were the semi-quantitatively determined by EDS analysis.The main ele-ments in the RHC Ni were carbon(C),silicon(Si)and nickel(Ni), etc.Also,a higher content of nickel(Ni)and a lower content of car-bon(C)were detected in the used RHC https://www.sodocs.net/doc/c914071659.html,ly,the atomic ratio of Ni/C was increased after use.It might be caused by the carboth-ermal reduction,thereby enhancing the carbon consumption and the formation of nickel nanoparticles.Since C,Si,and Ni are the predominate elements inside the used RHC Ni,it can also be recy-cled for the fabrication of silica-based nickel after carbon conversion.

3.5.

4.TG analysis

The thermal behavior of the fresh and the used RHC Ni catalysts were characterized by TG analysis at a heating rate of20°C/min under the air?ow of150mL/min.The DTG curve of RHC had one sharp peak at$400°C due to devolatilization and the air combus-tion of volatiles and char(Fig.10).Signi?cantly,the?xed carbon combustion was the dominant combustion process for biochar [42].A larger mass loss of the fresh RHC Ni occurred at the temper-ature range of300–500°C,suggesting the presence of volatile mat-ters in it,detectable even at the high temperature of600°C.Still, the total mass loss in these three samples was only4–6%at the temperature range of100–800°C,indicating the thermal stability of the fresh and used RHC Ni catalysts.In the process of biomass catalytic pyrolysis,the aromatic compounds deposited on the cat-alyst can be generally eliminated over350°C[43].It showed a stable mass loss rate for the used RHC Ni from400to500°C,pos-sibly ascribed to the?xed carbon burn off.Basically,the burnout temperature of char and coke(R9)was over500°C[44],where the mass loss rate of the used10g-RHC Ni decreased,compared to the used3g-RHC Ni and5g-RHC Ni.It is also indicated that hydrocarbons decomposition(R10)and carbon deposition(R11) are easier take place when a small amount of catalyst is employed. CtO2!CO2ecoke combustionTeR9T

C n H m!CtH2tCH4tC x H yeR10T

CH4!Ct2H2ecoke formationTeR11T3.5.5.Surface characteristics

The fresh RHC Ni(72.75m2/g)has a lower BET surface area (117.08m2/g)compared with the RHC,indicating that metal cata-lyst(i.e.,nickel)loading decrease the surface area of char.The iso-therms suggest the signi?cant formation of mesopores during the reactions with the RHC Ni,because the isotherm curves resemble Type II isotherms after catalytic reforming.In contrast,the RHC Ni1shows the Type I isotherm,indicating a more typical microp-orous structure.Moreover,an increase in the hysteresis elbow is observed in the RHC Ni2,indicating the widened mesopores and the possibility of deeper pore formation(Fig.11);accordingly,its BET surface area and pore diameter increased from72.75m2/g to 200.50m2/g and0.12m2/g to0.15cm3/g,respectively(Fig.12).

Y.Shen et al./Fuel159(2015)570–579575

In general,the increase of surface areas for the used RHC Ni cata-lysts is most likely attributed to the formation of new pores by RHC and soot gasi?cation,corresponding to the soot/char gasi?ca-tion rate higher than deposition rate.3.6.Integrated catalytic pyrolysis and gasi?cation

The integrated strategy of catalytic biomass pyrolysis/gasi?ca-tion includes different key reaction steps:(step1)metal precursor

Fig.7.SEM images of the RHC catalysts[RHC(A),fresh(B)and used(C&D)RHC Ni particles].

Fig.8.Magni?ed SEM images of the fresh(A)and used(B&C)RHC Ni catalysts.

(e.g.,Ni2+)insertion into solid biomass,(step2)catalytic pyrolysis of biomass,(step3)the catalytic active nanoparticles(e.g.,Ni0) in situ generated and highly dispersed in the carbon matrix,(step 4)catalytic conversion of nascent tars by the formed nanocompos-ites,(step5)catalytic gasi?cation of the char residue,and(step6) recycling and reuse of the metal species in the ash(e.g.,Ni/SiO2). Each of these reaction steps requires a thorough fundamental understanding to develop a high-ef?ciency biomass pyrolysis–gasi?cation process at nanoscales.

In this work,the metallic nickel(Ni0)nanoparticles had been successfully embedded into the carbon matrix of RHC,which was employed for catalytic conversion of tar derived from biomass gasi?cation.The reactions in the second stage mainly include tar cracking and reforming.The tertiary tar in(R12)implies the total tar yields after the different conditions.Besides tertiary condensable gases such as CO,CO2,H2O,CH4,C n H m,and yielded.As for the important syngas components,these formation will increase the conversion ef?ciency of Reforming effects of CO2on tar evolution is likely hydroxy(OH)radicals produced from CO2during conversion of syngas.A higher concentration of OH enhance the oxidation process of tar[45].Over the reforming is signi?cantly enhanced(R13).In this study, (H2O)from the as-received biomass can enhance the reforming and inhibit the polymerization reactions, ascribed to more hydrogen produced by the

Fig.9.TEM-EDS analysis of(A)the fresh and(B)used RHC Ni catalysts.

Fig.10.DTG curves of the fresh and used RHC Ni.

Fig.11.N2adsorption–desorption curves of RHC,RHC Ni1and RHC Ni2.

(R14–R19).Additionally,the presence of CO 2or H 2O agents could in?uence the formation of other noncondensable gases.Nevertheless,gases in the outlet of gasi?er are present in a rela-tively stable composition,possibly attributed to these reversible reactions (R14–R18).In the reformer,char heterogeneous gasi?ca-tion is the dominant endothermic reaction,which is of bene?t to the char conversion ef?ciency (R19and R20)normally occurring at relatively high temperatures.Without suf?cient energy,the pro-ceeding of char/carbon gasi?cation reactions such as Boudouard reaction would be inhibited or occurred with low-ef?ciency.Moreover,the pyrolysis–catalytic reforming could not signi?cantly consume the RHC supported catalysts.However,the RHC Ni could be deactivated by carbon deposition on the surfaces of metal actives.Furthermore,the deposited carbon is directly gasi?ed into the additional syngas by steam or CO 2,thereby inhibiting its deac-tivation by producing new pores.Tar decomposition:

Nascent tar tCO 2=H 2O =RHC !Tertiary tar tCO tH 2

tCO 2tH 2O tC n H m tCH 4

eR12T

Nascent tar tCO 2=H 2O =RHCNi

!CO tH 2tCO 2tH 2O tC n H m tCH 4

eR13T

Gas reforming:

n CO 2tC n H m $2n CO tm =2H 2eR14Tn H 2O tC n H m $n CO ten tm =2TH 2eR15TCH 4tH 2O $CO t3H 2eR16TCH 4tCO 2$2CO t2H 2

eR17T

CO tH 2O $CO 2tH 2

eR18T

Char gasi?cation (char present):

C tCO 2$2CO eR19TC tH 2O $CO tH 2

eR20T

4.Conclusions

Metallic nickel (Ni 0)nanoparticles could be successfully in situ generated in the rice husk char via one-step pyrolysis.Ni 0nanopar-ticles are embedded and highly dispersed in the carbon matrix with the particle size of 5–10nm.The synthesized RHC Ni exhib-ited a high activity on the tar reforming.In particular,tar conver-sion ef?ciency increased with the increase of catalyst weight and reforming temperature.Tar reforming ef?ciency can reach 50.1%by using 3g of RHC Ni,while it signi?cantly increased,up to 99.8%by using 10g of RHC Ni,maintaining a high value of 99.3%after 5cycles.It might be attributed to the enhancement of cat-alytic reaction and residence time,which is caused by the increase of catalyst weight.Besides,if 10g of RHC Ni employed,tar conver-sion ef?ciency could be improved from 92.3%to 100%in the tem-perature range of 500–900°C.It is indicated that RHC Ni showed higher catalytic activity for tar reforming even at relatively lower temperatures (>500°C).More importantly,by using the RHC Ni,less PAHs can be formed from the nascent https://www.sodocs.net/doc/c914071659.html,pared to the tertiary tar compounds of PAHs,it is much easier to crack/reform the nascent tar compounds by RHC Ni.If the RHC Ni was used for the catalytic reforming,the yield of liquid products reduced from 30.2%to 10.7%,possibly caused by a series of thermo-chemical reactions,such as water–gas-shift reaction,char gasi?cation,and steam reforming of tar.Accordingly,the yield of gas product greatly increased from 37.5%to 58.0%.Among these gas molecules,the volume fractions of CO and H 2increased from 21.2%to 33.5%and from 18.5%to 24.3%,respectively,attributed to the catalytic reforming over the RHC Ni.Moreover,the RHC Ni could be deactivated by coke deposition on the metal actives.After catalytic reforming,the surface areas of the used RHC Ni were increased due to the char gasi?cation rate higher than deposition rate,contributing to the prolong of its service life.Consequently,the waste RHC Ni could be directly gasi?ed into the additional syn-gas by gasi?cation agents,such as steam and CO 2,accompanied by recycling of the silica-based nickel nanocomposites.All these results indicated that the RHC Ni has a potential to be used for cat-alytic reforming of tar during biomass pyrolysis or gasi?cation.Acknowledgements

The author would like to acknowledge the China Scholarship Council (CSC)for the ?nancial support under the Grant No.201206230168.The authors also thank the editors and anonymous referees for their helpful comments.References

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钢铁中的元素及作用

各种元素在钢铁中的作用钢铁是铁与C(碳)、Si(硅)、Mn(锰)、P(磷)、S(硫)以及少量的其他元素所组成的合金。其中除Fe(铁)外，C的含量对钢铁的机械性能起着主要作用，故统称为铁碳合金。它是工程技术中最重要、用量最大的金属材料。各种元素在钢铁中有什么作用碳（Carbon) 存在于所有的钢材，是最重要的硬化元素。有助于增加钢材的强度，我们通常希望刀具级别的钢材拥有0.6%以上的碳，也成为高碳钢。铬（Chromium) 增加耐磨损性，硬度，最重要的是耐腐蚀性，拥有13%以上的认为是不锈钢。尽管这么叫，如果保养不当，所有钢材都会生锈锰（Manganese）重要的元素，有助于生成纹理结构，增加坚固性，和强度、及耐磨损性。在热处理和卷压过程中使钢材内部脱氧，出现在大多数的刀剪用钢材中，除了A-2,L-6和CPM 420V。钼（Molybdenum）碳化作用剂，防止钢材变脆，在高温时保持钢材的强度，出现在很多钢材中，空气硬化钢（例如A-2,ATS-34）总是包含1%或者更多的钼,这样它们才能在空气中变硬。镍（Nickle）保持强度、抗腐蚀性、和韧性。出现在L-6\AUS-6和AUS-8中。硅（Silicon）有助于增强强度。和锰一样，硅在钢的生产过程中用于保持钢材的强度。钨（Tungsten）增强抗磨损性。将钨和适当比例的铬或锰混合用于制造高速钢。在高速钢M-2中就含有大量的钨。钒（Vanadium）增强抗磨损能力和延展性。一种钒的碳化物用于制造条纹钢。在许多种钢材中都含有钒，其中M-2，Vascowear，CPM T440V和420V A含有大量的钒。而BG-42与ATS-34最大的不同就是前者含有钒按钢的用途分类一、结构钢（1)建筑及工程用结构钢简称建造用钢，它是指用于建筑、桥梁、船舶、锅炉或其他工程上制作金属结构件的钢。（2)机械制造用结构钢--是指用于制造机械设备上结构零件的钢。这类钢基本上都是优质钢或高级优质钢，主要有优质碳素结构钢、合金结构钢、易切结构钢、弹簧钢、滚动轴承钢等根据含碳量和用途的不同﹐这类钢大致又分为三类﹕ 1. 小于0.25％C为低碳钢﹐其中尤以含碳低于0.10%的08F﹐08Al等﹐由于具有很好的深冲性和焊接性而被广泛地用作深冲件如汽车﹑制罐……等﹐20G则是制造普通锅炉的主要材料﹐此外﹐低碳钢也广泛地作为渗碳钢﹐用于机械制造业﹐ 2. 0.25～0.60％C为中碳钢﹐多在调质状态下使用﹐制作机械制造工业的零件。调质多少22~34HRC,能得到综合机械性能,也便于切削. 3. 大于0.6％C为高碳钢﹐多用于制造弹簧﹑齿轮﹑轧辊等﹐根据含锰量的不同﹐又可

低合金钢中合金元素作的作用

合金元素在钢中的作用随着现代工业和科学技术的不断发展，在机械制造中，对工件的强度、硬度、韧性、塑性、耐磨性以及其他各种物理化学性能的要求愈来愈高，碳钢已不能完全满足这些要求了。原因： ①由碳钢制成的零件尺寸不能太大。否则，因淬透性不够而不能满足对强度与塑性、韧性的要求。加入合金元素可增大淬透性。 ②用碳钢制成的切削刀具不能满足切削红硬性的要求。用合金工具钢、高速钢和硬质合金。 ③碳钢不能满足特殊性能的要求，如要求耐热、耐低温、抗腐蚀、有强烈磁性或无磁性等等，只有特种的合金钢才能具有这些性能。合金钢是以碳钢为基础，金相组织和相应的碳钢大体上是相似的。在钢中加入合金元素，钢的机械性能显著提高。弄清楚各种合金元素对钢材的影响对控制产品质量有非常大的作用。 1 合金元素在钢中的存在方式 1.1 合金元素与钢中的碳相互作用，形成碳化物存在于钢中按合金元素在钢中与碳相互作用的情况，它们可以分为两大类：(1) 不形成碳化物的元素(称为非碳化物形成元素)，包括镍、硅、铝、钴、铜等。由于这些元素与碳的结合力比铁小，因此在钢中它们不能与碳化合，它们对钢中碳化物的结构也无明显的影响。

(2) 形成碳化物的元素(称为碳化物形成元素)，根据其与碳结合力的强弱，可把碳化物形成元素分成三类。 1）弱碳化物形成元素：锰锰对碳的结合力仅略强于铁。锰加入钢中，一般不形成特殊碳化物(结构与Fe3C不同的碳化物称为特殊碳化物)，而是溶入渗碳体中。 2）中强碳化物形成元素；铬、钼、钨 3）强碳化物形成元素：钒、铌、钛有极高的稳定性，例如TiC在淬火加热时要到1000℃以上才开始缓慢的溶解，这些碳化物有极高的硬度，例如在高速钢中加人钒，形成V4C，使之有更高的耐磨性。 1.2 合金元素溶解于铁素体(或奥氏体)中，以固溶体形式存在于钢中。 1.3 合金元素与钢中的氮、氧、硫等化合，以氮化物、氧化物、硫化物和硅酸盐等非金属夹杂物的形式存在于钢中。 1.4 游离态，即不溶于铁，也不溶于化合物：铅，铜 2 合金元素对钢的平衡组织的影响表现在改变铁碳合金状态图。 2.1 合金元素对钢临界温度的影响锰、镍、铜使A3线降低，钼、钨、硅、钒使A3线升高。同样影响A1，影响程度更大。 2.2 合金元素对钢共析点（S点）位置的影响

常用合金元素的作用

1、钢的分类 1.1 一般分类碳钢也叫碳素钢，含炭量 WC 小于 2%的铁碳合金。碳钢除含碳外一般还含有少量的硅、锰、硫、磷按用途可以把碳钢分为碳素结构钢、碳素工具钢和易切削结构钢三类。碳素结构钢又分为建筑结构钢和机器制造结构钢两种按含碳量可以把碳钢分为低碳钢（WC ≤ 0.25%）,中碳钢（WC0.25%——0.6%）和高碳钢（WC>0。6%）。合金钢种类很多，通常按合金元素含量多少分为低合金钢（含量＜5％），中合金钢（含量 5％～10％），高合金钢（含量＞10％）；按质量分为优质合金钢、特质合金钢；按特性和用途又分为合金结构钢、不锈钢、耐酸钢、耐磨钢、耐热钢、合金工具钢、滚动轴承钢、合金弹簧钢和特殊性能钢（如软磁钢、永磁钢、无磁钢）等。 2、钢中合金元素分类 2.1 根据各种元素在钢中形成碳化物的倾向，可分为三类：强碳化物形成元素，如钒、钛、铌、锆等。这类元素只要有足够的碳，在适当的条件下，就形成各自的碳化物;仅在缺碳或高温的条件下,才以原子状态进入固溶体中。碳化物形成元素，如锰、铬、钨、钼等。这类元素一部分以原子状态进入固溶体中，另一部分形成置换式合金渗碳体,如(Fe，Mn)3C、(Fe，Cr)3C 等,如果含量超过一定限度（除锰以外），又将形成各自的碳化物,如(Fe，Cr)7C3、(Fe， W)6C 等。不形成碳化物元素，如硅、铝、铜、镍、钴等。这类元素一般以原子状态存在于奥氏体、铁素体等固溶体中。合金元素中一些比较活泼的元素，如铝、锰、硅、钛、锆等，极易和钢中的氧和氮化合，形成稳定的氧化物和氮化物，一般以夹杂物的形态存在于钢中。锰、锆等元素也和硫形成硫化物夹杂。钢中含有足够数量的镍、钛、铝、钼等元素时能形成不同类型的金属间化合物。有的合金元素如铜、铅等，如果含量超过它在钢中的溶解度，则以较纯的金属相存在。 2.2 钢中主要合金元素主要合金元素有硅、锰、铬、镍、钼、钨、钒、钛、铌、锆、钴、铝、铜、硼、稀土等。其中钒、钛、铌、锆等在钢中是强碳化物形成元素，只要有足够的碳，在适当条件下，就能形成各自的碳化物，当缺碳或在高温条件下，则以原子状态进入固溶体中；锰、铬、钨、钼为碳化物形成元素，其中一部分以原子状态进入固溶体中，另一部分形成置换式合金渗碳体；铝、铜、镍、钴、硅等是不形成碳化物元素，一般以原子状态存在于固溶体中。现分别说明它们在钢中的作用。碳（C）：是对钢的性能影响最大的基本元素，是决定钢力学性能的主要因素。不同的碳含量依据钢中杂质元素含量和轧后冷却条件的不同对于钢的性能影响是不同的。一般说来，随着钢中碳含量的增加，屈服点和抗拉强度升高，碳钢在热轧状态下的硬度直线上升，塑性和韧性降低。在亚共析范围内（碳含量小于 0.80％时），碳对抗拉强度的影响是，随着碳含量增加，抗拉强度不断提高；超过共析范围后（当碳含量大于 0.80％时），抗拉强度随碳含量的增加减缓，最后发展到随碳含量的增加抗拉强度降低。另外，含碳量增加时碳钢的

合金元素在钢中的主要作用

§5－1 合金元素在钢中的主要作用教学过程一、复习提问：碳素钢的性能特点二、新课教学：合金元素在钢中的主要作用（强化铁素体、形成合金碳化物、细化晶粒、提高钢的淬透性、提高钢的回火稳定性）三、课后小结：合金钢与碳素钢的区别四、作业安排：练习册P23，一、1、2；二、1、2、4；三、6 五、板书设计（见下页）：六、教学后记： §5－1 合金元素在钢中的主要作用 1、定义：为改善钢的性能，在冶炼时有目的地加入一种或几种合金元素的钢。 2、含碳量：＜2.11%。 3、常用元素： Cr铬、Ni镍、Mo钼、W钨、V钒、Ti钛、Al铝、B硼、Nb铌、Nd钕。 4、合金元素的影响：可以得到所需的力学性能，用于重要零件；特殊物理（熔点、磁性）、化学（耐热、耐腐蚀）性能；特殊工艺性能（焊接、热处理）；使C曲线右移，淬透性提高。一、强化铁素体（除铅外）： 1、存在形式：大多数合金元素溶于α-Fe，形成合金铁素体。 2、作用： 3、对韧性的影响： Si＜1.0%、Mn＜1.5%，F韧性不下降，超过此量，则F韧性下降。 Cr≤2%、Ni≤5%，明显强化F，提高F韧性。二、形成合金碳化物： 1、存在形形式（合金元素与碳亲和力不同）：

（1）非碳化物形成元素：镍、钴、铜、硅、铝、硼，不形成碳化物，溶于F 和A ，形成合金F 和合金A 。（2）弱碳化物形成元素：Mn 锰，与碳亲和力弱，大部分溶于F 或A ，少部分溶于Cm ，形成合金渗碳体。（3）中碳化物形成元素：Cr 铬、Mo 钼、W 钨，和碳亲和力强，形成合金渗碳体，硬度提高，明显提高低合金钢强度，组织比Cm 稳定。（4）强碳化物形成元素：V 钒、Nb 铌、Ti 钛，与碳形成特殊碳化物，比合金Cm 有更高的熔点、硬度和耐磨性，组织更稳定。 2、作用：碳化物种类、性能、在钢中分布状态，直接影响钢的性能、热处理相变。如果碳化物以弥散状分布，则强度↑、硬度↑、耐磨性↑，对工具钢有重要意义。三、细化晶粒（除Mn 外）： 1、元素作用： Mn 使晶粒长大倾向增大，即过热。其他元素加热时抑制A 长大，降低长大速度 V 、Nb 、Ti 形成的碳化物，铝在钢中形成的AlN 、Al2O3细小质点，相当于孕育剂，增加形核率。 2、结果：细化晶粒，使强度↑、韧性↑。使晶粒细化。四、提高钢的淬透性(除钴外)： 1、作用：合金元素溶于A ，使过冷A 稳定性增强，推迟珠光体转变，使C 曲线右移，V 临↓、淬透性↑。 2、结果：淬透性好，可采用冷却能力较低的介质，防变形、开裂，保持尺寸和形状精度。在同样淬火条件下，合金钢淬硬层较深，大截面零件组织均匀，综合力学性能提高。 3、常用元素：Mo 、Mn 、Cr 、Ni 、Si 、B 。 4、特例：微量的B （0.0005%～0.003%）可明显提高淬透性。五、提高钢的稳定性： 1、回火稳定性：钢在回火时，抵抗软化、抵抗硬度下降的能力。 2、产生原因：合金元素阻碍M 分解，且碳化物不易析出，即使析出也不易长大，保持较大弥散度，硬度下降慢。

合金元素在钢中的作用完整版

了合金化而加入的合金元素，最常用的有硅、猛、珞、線、钳、鹄、帆，钛，锐、硼、铝等。现分别说明它们在钢中的作用。 1、硅在钢中的作用：（1）提高钢中固溶体的强度和冷加工硕化程度使钢的韧性和塑性降低。（2）硅能显著地提高钢的弹性极限、屈服极限和屈强比，这是一般弹簧钢。（3）耐腐蚀性。硅的质量分数为15% — 20%的高硅铸铁，是很好的耐酸材料。含有硅的钢在氧化气氛中加热时，表面也将形成一层Si02薄膜，从而提高钢在高温时的抗氧化性。缺点：（4）使钢的焊接性能恶化。 2、镭在钢中的作用（1）镭提高钢的淬透性。（2）镭对提高低碳和中碳珠光体钢的强度有显著的作用。（3）镭对钢的高温瞬时强度有所提高。镭钢的主要缺点是，①含猛较高时，有较明显的回火脆性现象；②镭有促进晶粒长大的作用，因此镭钢对过热较敬感t在热处理工艺上必须注意。这种缺点可用加入细化晶粒元素如钮、飢、钛等来克服：⑧当镭的质量分数超过1%时, 会使钢的焊接性能变坏，④镭会使钢的耐锈蚀性能降低。 3、珞在钢中的作用（1）珞可提高钢的强度和硬度。（2）珞可提高钢的高温机械性能。（3）使钢具有良好的抗腐蚀性和抗氧化性（4）阻止石墨化（5）提高淬透性。缺点：①辂是显著提高钢的脆性转变温度②辂能促进钢的回火脆性。4、W 在钢中的作用（1）可提高钢的强度而不显著降低其韧性。（2）银可降低钢的脆性转变温度，即可提高钢的低温韧性。（3）改善钢的加工性和可焊性。（4）银可以提高钢的抗腐蚀能力，不仅能耐酸，而且能抗碱和大气的腐8 /I 蚀。 5、钮在钢中的作用（1）铝对铁素体有固溶强化作用。（2）提高钢热强性（3）抗氢侵蚀的作用。（4）提高钢的淬透性。缺点：钮的主要不良作用是它能使低合金钳钢发生石墨化的倾向。6、钩在钢中的作用（1）提高强度（2）提高钢的高温强度。（3）提髙钢的抗氢性能。

合金元素在钢中的主要作用

简述几种常见合金元素在钢中的主要作用为了改善和提高钢的某些性能和使之获得某些特殊性能而有意在冶炼过程中加入的元素称为合金元素。常用的合金元素有铬，镍，钼，钨，钒，钛，铌，锆，钴，硅，锰，铝，铜，硼，稀土等。磷，硫，氮等在某些情况下也起到合金的作用。 (1)铬(Cr) 铬能增加钢的淬透性并有二次硬化的作用，可提高碳钢的硬度和耐磨性而不使钢变脆。含量超过12%时，使钢有良好的高温抗氧化性和耐氧化性腐蚀的作用，还增加钢的热强性。铬为不锈钢耐酸钢及耐热钢的主要合金元素。铬能提高碳素钢轧制状态的强度和硬度，降低伸长率和断面收缩率。当铬含量超过15%时，强度和硬度将下降，伸长率和断面收缩率则相应地有所提高。含铬钢的零件经研磨容易获得较高的表面加工质量。铬在调质结构中的主要作用是提高淬透性，使钢经淬火回火后具有较好的综合力学性能，在渗碳钢中还可以形成含铬的碳化物，从而提高材料表面的耐磨性。含铬的弹簧钢在热处理时不易脱碳。铬能提高工具钢的耐磨性、硬度和红硬性，有良好的回火稳定性。在电热合金中，铬能提高合金的抗氧化性、电阻和强度。（2）镍（Ni）镍在钢中强化铁素体并细化珠光体，总的效果是提高强度，对塑性的影响不显著。一般地讲，对不需调质处理而在轧钢、正火或退火状态使用的低碳钢，一定的含镍量能提高钢的强度而不显著降低其韧性。据统计，每增加1%的镍约可提高强度。随着镍含量的增加，钢的屈服程度比抗拉强度提高的快，因此含镍钢的比可较普通碳素钢高。镍在提高钢强度的同时，对钢的韧性、塑性以及其他工艺的性能的损害较其他合金元素的影响小。对于中碳钢，由于镍降低珠光体转变温度，使珠光体变细；又由于镍降低共析点的含碳量，因而和相同的碳含量的碳素钢比，其珠光体数量较多，使含镍的珠光体铁素体钢的强度较相同碳含量的碳素钢高。反之，若使钢的强度相同，含镍钢的碳含量可以适当降低，因而能使钢的韧性和塑性有所提。镍可以提高钢对疲劳的抗力和减小钢对缺口的敏感性。镍降低钢的低温脆性转变温度，这对低温用钢有极重要的意义。含镍%的钢可在-100℃时使用，含镍9%的钢则可在 -196℃时工作。镍不增加钢对蠕变的抗力，因此一般不作为热强钢的强化元素。镍含量高的铁镍合金，其线胀系数随镍含量增减而显著变化，利用这一特性，可以设计和生产具有极低或一定线胀系数的精密合金、双金属材料等。此外，镍加入钢中不仅能耐酸，而且也能抗碱，对大气及盐都有抗蚀能力，镍是不锈耐酸钢中的重要元素之一。（3）钼（Mo）

合金元素在钢中的作用

第六章合金钢合金钢的优点：高的强度和淬透性第一节合金元素在钢中的作用常用合金元素：非碳化物形成元素——Co Ni Cu Si Al 碳化物形成元素——Zr Nb V Ti W Mo Cr Mn Fe 强中强弱一、合金元素对钢中基本相的影响 1、形成合金铁素体合金元素→溶入A →形成合金铁素体→固溶强化（Cr,Ni较好） 2、形成合金碳化物弱碳化物形成元素形成合金渗碳体(Fe,Mn)3C 中强碳化物形成元素形成合金碳化物(Cr23C6，Fe3W3C) 强碳化物形成元素形成特殊碳化物（VC，TiC）熔点、硬度和稳定性：特殊碳化物> 合金碳化物> 合金渗碳体> Fe3C 二、合金元素对Fe-FeC相图的影响合金元素对A相区影响扩大A相区元素（Mn）——E、S点左下移缩小A相区元素（Cr）——E、S点左上移奥氏体钢：1Cr18Ni9 铁素体钢：1Cr17 莱氏体钢：W18Cr4V 三、合金元素对热处理的影响 1、对加热的影响多数元素减缓A形成，阻碍晶粒长大 2、对冷却的影响多数元素溶入A后→过冷A稳定性↑→Vc↑→淬透性↑ →Ms点↓→残余A量↑提高淬透性的意义： ①增加淬硬层深度 ②减少工件变形、开裂倾向3、对回火的影响 ①回火稳定性→抗回火软化的能力 ②产生二次硬化（析出特殊碳化物，产生弥散强化；A残→M或B下）

一、低合金高强度钢碳素结构钢：Q195，Q215，Q235，Q255，Q275 低合金高强度钢：Q295，Q345，Q390，Q420，Q460 Q235+Me(<3%) →Q345 1、成分：~%C，合金元素2~3% 主加元素：Mn ——固溶强化辅加元素：Ti,Cr,Nb ——弥散强化使用状态：热轧或正火（F + P），不需最终热处理 2、性能：较高的σs ，良好的塑性韧性，焊接性，抗蚀性，冷脆转变温度低 3、常用钢号：Q295 （09Mn2），Q345 (16Mn) 用途：工程结构——桥梁，船舶，车辆外壳、支架、压力容器二、易切削结构钢牌号：Y12，Y12Pb，Y30，Y 40Mn 性能：良好的切削加工性（170~240HBS，塑性低）切削抗力小，刀具不易磨损，加工表面粗糙度低应用：成批、大量生产时，制作性能要求不高的紧固件和小型零件第三节合金钢的分类与牌号一、合金钢分类低合金钢——低合金高强度钢、易切削结构钢合金结构钢——渗碳钢、调质钢、弹簧钢、滚动轴承钢合金工具钢——合金工具钢、高速钢特殊性能钢——不锈钢、耐热钢、耐磨钢二、合金钢牌号 1、合金结构钢——20CrMnTi，60Si2Mn，25Cr2Ni4WA 2、滚动轴承钢——GCr15 3、合金工具钢——9Mn2V，CrWMn 4、高速钢——W18Cr4V，W6Mo5Cr4V2 5、不锈、耐热钢——4Cr13，0Cr18Ni11Ti，00Cr17Ni14Mo2 6、高锰耐磨钢——ZGMn13 学习思路：用途→工作条件→性能要求→成分特点→热处理特点→典型钢种应用

合金元素在钢中的作用

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合金元素在钢中的作用

了合金化而加入的合金元素,最常用的有硅、锰、铬、镍、钼、钨、钒,钛,铌、硼、铝等。现分别说明它们在钢中的作用。 1、硅在钢中的作用: (1)提高钢中固溶体的强度与冷加工硬化程度使钢的韧性与塑性降低。 (2) 硅能显著地提高钢的弹性极限、屈服极限与屈强比,这就是一般弹簧钢。(3)耐腐蚀性。硅的质量分数为15％一20％的高硅铸铁,就是很好的耐酸材料。含有硅的钢在氧化气氛中加热时,表面也将形成一层SiO2薄膜,从而提高钢在高温时的抗氧化性。缺点:(4)使钢的焊接性能恶化。 2、锰在钢中的作用 (1)锰提高钢的淬透性。 (2)锰对提高低碳与中碳珠光体钢的强度有显著的作用。 (3)锰对钢的高温瞬时强度有所提高。锰钢的主要缺点就是,①含锰较高时,有较明显的回火脆性现象;②锰有促进晶粒长大的作用,因此锰钢对过热较敏感t在热处理工艺上必须注意。这种缺点可用加入细化晶粒元素如钼、钒、钛等来克服:⑧当锰的质量分数超过1％时,会使钢的焊接性能变坏,④锰会使钢的耐锈蚀性能降低。 3、铬在钢中的作用 (1)铬可提高钢的强度与硬度。 (2)铬可提高钢的高温机械性能。 (3)使钢具有良好的抗腐蚀性与抗氧化性 (4)阻止石墨化 (5)提高淬透性。缺点:①铬就是显著提高钢的脆性转变温度②铬能促进钢的回火脆性。4、镍在钢中的作用 (1)可提高钢的强度而不显著降低其韧性。 (2)镍可降低钢的脆性转变温度,即可提高钢的低温韧性。 (3)改善钢的加工性与可焊性。 (4)镍可以提高钢的抗腐蚀能力,不仅能耐酸,而且能抗碱与大气的腐蚀。

5、钼在钢中的作用 (1)钼对铁素体有固溶强化作用。 (2)提高钢热强性 (3)抗氢侵蚀的作用。 (4)提高钢的淬透性。缺点:钼的主要不良作用就是它能使低合金钼钢发生石墨化的倾向。6、钨在钢中的作用 (1) 提高强度 (2)提高钢的高温强度。 (3)提高钢的抗氢性能。 (4)就是使钢具有热硬性。因此钨就是高速工具钢中的主要合金元素。7、钒在钢中的作用 (1)热强性。 (2)钒能显著地改善普通低碳低合金钢的焊接性能。8、钛在钢中的作用 (1)钛能改善钢的热强性,提高钢的抗蠕变性能及高温持久强度;(金属材料长期在高温条件下受热应力的作用而产生缓慢、连续的塑性变形的现象,叫金属的蠕变) (2)并能提高钢在高温高压氢气中的稳定性。使钢在高压下对氢的稳定性高达600℃以上,在珠光体低合金钢中,钛可阻止钼钢在高温下的石墨化现象。因此,钛就是锅炉高温元件所用的热强钢中的重要合金元素之一。 9、铌在钢中的作用 (1)铌与碳、氮、氧都有极强的结合力,并与之形成相应的极为稳定的化合物,因而能细化晶粒,降低钢的过热敏感性与回火脆性。 (2)有极好的抗氢性能。 (3)铌能提高钢的热强性 10、硼在钢中的作用 ; (1)提高钢的淬透性。 (2)提高钢的高温强度。强化晶界的作用。 11、铝在钢中的作用

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