Effect of aging at 700 °C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainl

Nuclear Engineering and Design235(2005)

2485–2494

Effect of aging at700?C on precipitation and toughness of AISI 321and AISI347austenitic stainless steel welds

Kaishu Guan a,?,Xiaodong Xu b,Hong Xu a,Zhiwen Wang a

a Research Institute of Process Equipment and Pressure Vessel,East China University of Science and Technology,Box402,

130Meilong Road,Shanghai200237,PR China

b Jinan Company,Sinope

c Corp.,Jinan250101,PR China

Received30January2005;received in revised form30May2005;accepted6June2005

Abstract

A detailed knowledge of changes in microstructures and mechanical behaviour that occur in austenitic stainless steels with or without Nb/Ti-stabilized weld during heat treatment is of great interest,since the ductility and toughness of the material may change drastically after long aging times.Two kinds of materials,i.e.AISI321base and without Ti-stabilized weld steel and AISI347base with Nb-stabilized weld steel,were compared during aging at700?C up to6000h.Both materials present increased amount of precipitate and decreased impact energy as the aging time increases.The decreased extent of impact energy with aging is almost the same for both base materials.However,it presents differences for347and321weld samples.The latter shows a more drastic decrease of impact energy than the former due to the different amount of precipitates.321weld sample precipitates more numerously than347weld sample due to the absence of stabilized Ti/Nb on the https://www.360docs.net/doc/6612201299.html,rge amount of carbides is formed on321weld sample immediately after welding.The carbides are transformed to sigma phase,which is mainly responsible for the much more sigma phase precipitation compared with other samples,after high-temperature aging. The fractographs showed,in general,brittle fracture mode in321weld impact-fractured specimens after aging at700?C for 6000h.However,other samples show ductile fracture mode in general.Several approaches should be employed to control sigma phase precipitation in weld material.These approaches include:decreasing content of ferrite and M23C6carbide in weld and selecting Nb added weld wire during welding.

1.Introduction

Austenitic stainless steels with the approximate composition18wt.%chromium,10wt.%nickel and

?Corresponding author.Tel.:+862164253055;

fax:+862164253810.

E-mail address:guankaishu@https://www.360docs.net/doc/6612201299.html,(K.Guan).additions of molybdenum,titanium or niobium are today widely used in components designed for high-temperature applications like nuclear power stations, boilers,superheaters,and chemical reactors(Schwind et al.,2000;Wachter and Brummer,1997;Erve et al., 1997a).Additions of titanium or niobium prevent pre-cipitation of grain boundary carbides and contribute to precipitation hardening by the formation of small MX-

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type carbides or carbonitrides.However,addition of Nb/Ti also brings about the austenite unstable due to contribution to formation of intermetallic phases dur-ing exposure to high-temperature(Wasnik et al.,2003). Formation of an intermetallic phase known as?-phase is a severe problem when using standard austenitic stainless steels at elevated temperatures.The presence of?-phase not only reduces corrosion resistance of materials by removing chromium and molybdenum from the austenitic matrix,but also deteriorates its mechanical properties(Schwind et al.,2000;Minami et al.,1985;Wilms et al.,1991;Chowdhury et al., 1988;Kallqvist and Andren,1999).Since mechanical and corrosion properties control the lifetime of com-ponents manufactured from the steel and materials,it is important to determine when and in what amount of different phases precipitate in materials serviced at elevated temperature.

A lot of papers have reported the precipitation of car-bides and sigma phase in AISI300series austenitic SS (Smith and Farrar,1993;Kallqvist and Andren,1999; Pechenkin and Epov,1993).However,most researches focused on the base metal,and little is known about the weld joints.Weld joints are extensively used in the fabrication industry,and failure usually occurs at these joints(Chabaud-Reytier et al.,2003;Erve et al., 1997b).Therefore,precipitation must be minimized for weld serving at high-temperature to control them according to the respective requirements.

In this work,two kinds of austenitic stainless steel (SS),both base and weld metals were investigated. One is AISI321base and with unstabilized weld sam-ple,another is AISI347base and with Nb-stabilized weld sample.The main aim of this work is to?nd the in?uence of Ti/Nb on the microstructure,precip-itation,impact energy of weld joints and offer the knowledge of processing-microstructure–properties relation.

2.Experimental details

The321steel was a plate material with12mm in thickness,whereas347steel was a tube mate-rial with89mm in diameter and9mm in thickness. Part of the base specimens were cut and prepared to welding.

347steel tube was cut into28mm coupons with a 30?level to provide60?groove angle for a single-V-groove butt joint con?guration.The root face was1mm with root opening of2.5mm.321steel plate was cut into28mm×50mm coupons with a30?level on each side to provide60?groove angle for an X-groove butt joint con?guration.Fig.1shows the schematic draw-ing of weld joints.The base materials were solution annealed before welding.

In order to compare the difference of welds with sta-bilized Nb and without Nb,304weld wire and347-Nb weld wire are used for321steel and347steel welding, respectively.So AISI321welding material contains unstabilized element and AISI347welding material contains Nb stabilized element.The chemical compo-sitions for the BMs and the welding wire are presented in Table1.All weld specimens were taken in the

longi-

Fig.1.Schematic welding joint of321and347materials.

Table1

Chemical composition of the base plates and the welding wire(wt.%)

Designation C Mn Si S P Cr Ni Ti Nb Fe AISI347base metal0.055 1.080.400.0140.03717.8910.500.72Balance Weld wire of AISI3470.12 1.90.900.030.0418140.78Balance AISI321base metal0.07 1.60.80.030.03517.8120.55Balance Weld wire of AISI3040.10 1.880.760.030.03518.413.2Balance

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Fig.2.Schematic Charpy-V-notch impact specimens.

Table2

Welding parameters

Arc voltage(V)36 Current(I)100 Travel speed(mm/s)9 Wire diameter(mm) 3.2 tudinal direction from the plates and tubes.All welding was performed using the manual metal arc welding pro-cess with the same welding procedure.No preheating was employed for the welding.In all cases,the shield-ing gas used had a purity of at least99.99%to minimize the probability of hydrogen-induced cold cracking in the weldments.Welding procedure data of all produced welded joints are presented in Table2.

To evaluate changes of microstructures and mechan-ical properties during high-temperature service,spec-imens were aged at700?C for a maximum of6000h in an electric resistance furnace.From these sheets, Charpy impact specimens were machined and sub-jected to a high-temperature and room temperature impact test in accordance with ASTM E8-79standard. The size of321material is10mm×10mm×55mm, while347materials is7.5mm×10mm×55mm due to the tube thickness.For the weld joints,all notches were located at the centre of the weld deposit as is shown in Fig.2.The notch direction on base samples is the same as that on the weld joints.

The average value is taken from test on three spec-imens per condition.The fracture surfaces through the impact test specimens were examined by scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS)in order to identify the fracture characterizations and precipitate composi-tions.Specimens for light optical microscopy(LOM) were prepared by a standard method including grind-ing,polishing and etching.The microstructure was revealed by etching with a solution of hydrochloric acid

and nitric acid.The volume ratio of hydrochloric acid and nitric acid is3:1.Ferrite phase content in the weld metal was measured using Ferritoscope instrument.

In order to evaluate the changes of energy impact toughness with aging time,the loss coef?cient of impact toughness is expressed as:

αt=

A KV

?A KV t

KV0

where A KV

is impact toughness of initial material

specimen,A KV

the impact toughness of specimen aged for t h.The maximum loss coef?cient is1,and the ini-tial state the loss coef?cient is0.

3.Results

3.1.Charpy impact toughness

The results of the Charpy-V-impact tests are shown in Fig.3(at room temperature)and Fig.4(at700?C). The tendency of impact toughness for both base and weld materials of the two kinds of tested samples shows reduction as the aging time is increased.The base metal shows better impact toughness than weld metal for both materials.The difference in impact toughness reduc-tion between the321and347bases is small,whereas the reduction tendency for both welds is signi?cantly different.The impact toughness is drastically decreased when compared to the original weld material.The loss of impact toughness for321weld is94%(at room tem-perature)and77%(at700?C),and for347weld is 82%(at room temperature)and65%(at700?C),com-pared to the original weld material,respectively.321 weld is the most brittle of all the samples after aging at 700?C.

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Fig.3.Variations of impact toughness at room temperature of 347and 321with aging time for (a)tested values and (b)loss coef?cient of impact

toughness.

Fig.4.Variations of impact toughness at 700?C of 347and 321with aging time for (a)tested values and (b)loss coef?cient of impact toughness.

3.2.Metallography and image analysis

Fig.5shows the typical as-welded microstructures for the two samples observed adjacent to the base interface.Weld metal consisted of primarily austenitic phase,little strip and vermicular morphology ?-ferrite phase along grain boundaries,and precipitation par-ticles in grains and along grain boundaries.?-Ferrite phase contents are 5.8vol.%in 321weld and 6.7vol.%in 347weld,respectively.EDS analysis shows

that

Fig.5.Optical micrographs of the weld metal of (a)321and (b)347.

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Fig.6.Base metal aging for4.5h at700?C of(a)321and(b)347.

Table3

Chemical composition of particles in347(wt.%)

Element

Fe Cr Ni Nb Content46.213.61 6.134.09

these precipitation particles are blocky type Cr-rich car-bides with88.43wt.%Cr–7.52wt.%Fe–4.05wt.%Ni. The chemical composition obtained with the EDS detector does not provide further information about the type of carbides,but M23C6is a common precipitate in weld.The precipitation of carbides in347weld sam-ples is signi?cantly less than in321weld samples.This is due to the fact that347weld contains stabilized Nb which inhibits the chromium precipitate(Wasnik et al., 2003).

Fig.6shows the LOM images obtained from a sample held for4.5h at700?C.A series of small sec-ondary carbides are found.These secondary carbides are M23C6and TiC with0.5wt.%Cr–3.6wt.%Fe–2.9wt.%Cr–5.8wt.%Ni–78.6wt.%Ti.In addition, M23C6particles in grain boundaries and numbers of NbC particles in grains are observed on the347 base materials detected by EDS detector shown in Table3,which is near to the results of Bogdan (2001).

Fig.7shows the microstructures observed in sam-ples aged for6000h.For347base material,discon-tinuous strip type precipitates are mainly distributed at grain boundaries.A slightly larger number of coars-ened strip precipitates distributed at grain boundaries are found on347weld sample compared with the base material.Fine gridding shape precipitate is observed in grains except for coarsened particle and strip pre-cipitates at grain boundaries in321base samples.In 321weld,the precipitate becomes more coarsened and more numerously.

3.3.Fractography

Fig.8shows the SEM fractography of impact test samples performed at room temperature for original materials.Dimple characteristics are obvious for the fracture surface of original material.Fig.9shows the SEM fracture surface of impact test samples aged up to6000h.The fractographs showed,in general,brittle fracture mode in321weld impact-fractured specimens. Large amount of strip precipitates is present and the fracture occurs along these precipitates.These precipi-tates are rather brittle and present numerous secondary cracks on fracture surface.

However,347weld sample shows ductile fracture mode in general.Although fracture occurs also along these precipitates,dimples at the periphery of these precipitates are observed on the347fracture surface. Although large amounts of precipitates are observed on both base samples,the surface presents ductile dimples in nature.

3.4.Precipitates

It is important to know when and what kind of phases have precipitated during aging at700?C.Table4 gives the chemical composition of precipitates using SEM/EDS for weld and base metal detected on the impact fractures surface.From the ratio of[Fe]/[Cr] we can deduce that the precipitates are probably sigma phase which is close to the results reported by Minami

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Fig.7.Micrographs of(a)347base,(b)321base,(c)347weld and(d)321weld samples aged at700?C for6000h.

Fig.8.SEM micrographs of fracture surfaces of original material impact specimens performed at room temperature of(a)321base,(b)321 weld,(c)347base and(d)347weld.

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Fig.9.Fractographs of impact specimens aging for6000h for(a)321base material,(b)347base material,(c)321weld and(d)347weld.

et al.(1985)and Kallqvist and Andren(1999).The chemical composition of sigma phase can vary in a rel-atively large range.This is possibly the reason that in the solution-annealed and aged specimens,the sigma-phase chemistries were generally not of equilibrium values,and a wide variation in sigma composition was seen(Ritter,1988).In Fe base alloy,sigma phase is Fe rich.

No?-ferrite was detected in all the weld metals after aging6000h at700?C.The result shows that?-ferrite, distributing mainly at the grain boundaries both for321 weld and347weld materials,is transformed to?-phase during high-temperature aging.

Table4

Chemical composition of precipitates using SEM/EDS on weld and base metal(wt.%)

Distribution Cr Ni Fe Ti Si Mn 321weld metal26.04 3.5866.7 1.68

321base metal38.59 2.9556.390.380.710.98 347weld metal29.02 6.7760.670.52 2.78 347base metal23.92 5.4468.73 1.924.Discussion

The following results can be inferred from the exper-iments:large amount of M23C6carbide particles pre-cipitates on321weld material.After aging at700?C for6000h,sigma phase is present in all the samples, and the ductility and toughness have signi?cantly been reduced in the material,especially in321weld sample.

4.1.Effect of number of precipitates

The precipitation reactions are considered to be of primary importance in controlling the mechanical properties of the weld metals,and can result in signif-icant reduction in weld metal ductility and toughness. Much of the deterioration in properties can be directly attributed to the formation of secondary phases such as M23C6and the intermetallic sigma and chi phases (Smith and Farrar,1993).Sigma phase is a common precipitate in austenitic steel which is a hard and brit-tle intermetallic phase of normal composition FeCr. The crystal structure has a tetragonal unit cell(space group P42/mnm),containing30atoms per unit cell

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with lattice parameters a=0.88nm and c=0.4544nm (Kallqvist and Andren,1999).From Figs.3,4and7we can derive that the reduction of toughness is increas-ing with the amount of precipitates.321weld mate-rial precipitates the maximum secondary phase,and its toughness reduction is also the maximum.Con-trarily,347base material precipitates the minimum secondary phase,and its toughness reduction is also the minimum.

4.2.Precipitate phases and sequence

Precipitation in austenitic stainless steel has been widely studied in the literature.For AISI300series austenitic SS with or without Nb/Ti addition,precipi-tation of M23C6and sigma phase can take place during heat treatments over the temperature range from650to 850?C(Ayer et al.,1992;Schweinsberg et al.,1994; Farrar,1995).However,which is a preferential pre-cipitation,carbide or?-phase,is not well understood (Wasnik et al.,2003).Following paragraphs will dis-cuss the precipitation sequence of carbides and sigma phase.

4.2.1.M23C6carbide

It is well known that carbide precipitation takes place in HAZ during welding.In this case,a large amount of carbides precipitation is observed on321 weld immediately after welding,while a relatively lit-tle amount of precipitates is found on347weld due to Nb effect.This indicated that carbide is a prefer-ential precipitation phase compared to sigma phase.It is likely that carbide precipitation is easier than sigma phase,because carbide precipitation requires only car-bon diffusion to the sites with highest concentration of strong carbide formers like Cr,Mo,etc.(Grover and Wickens,1982;Briant,1982).This is in agree-ment with the results reported by Gill’s that carbide precipitated in316stainless steel rapidly(in less than 1h)whereas the chi phase was detected only after ageing for500h.The presence of sigma phase was not detected up to2000h(Gill and Gnanamoorthy, 1981).

4.2.2.Sigma phase

The specimens,after aging at700?C for6000h, precipitate mainly sigma phase,which can be derived from three ways:(1)transformation of austenite,(2)transformation of delta ferrite and(3)transformation of carbide M23C6.

4.2.2.1.Effect of austenite.In fully austenitic weld metal sigma phase precipitation can start at austenite–austenite grain boundaries(Borst and Pohl, 1990)and needs a long time(Tanaka et al.,1997). We do not think that sigma phase precipitates directly from austenite during aging at700?C.Precipitate is signi?cantly lower in both base materials than in both weld materials.If sigma phase precipitates directly from the austenite,base metals should have the same amount of precipitates in the light of following reasons.347base metal has almost the same chemical composition as that of347weld(containing Nb), while the chemical composition for321weld is different from321base material.321base materials contain stabilized titanium,however,its weld metal does not contain stabilized titanium.Moreover, titanium promotes sigma phase precipitation,if sigma precipitates directly from austenite,321base materials should have the same or more number of sigma phase with its weld material.

4.2.2.2.Effect ofδ-ferrite.Now we can further dis-cuss the effect of ferrite on the transformation of sigma phase.Fully austenitic steels are prone to hot crack-ing which occurs during weld meal solidi?cation.This cracking takes place due to segregation of impurity ele-ments such as S,P and other elements leading to low melting point phases.It is well known that the delta fer-rite in weld in austenitic steel increases the resistance to hot cracking.A certain amount of delta ferrite(5–8%) will scavenge the impurities and take care of a cer-tain amount of thermal strain(Chowdhury et al.,1988). However,during aging at temperatures over the range 550–850?C,rapid transformation of the ferrite phase to the embrittling sigma phase occurs,because delta fer-rite is not in a state of equilibrium at these temperatures (Schweinsberg et al.,1994).Carbide precipitation takes place at the austenite–ferrite grain boundaries.After carbon has been used up,the remaining delta ferrite changes into sigma phase,if its alloying element con-tent is high enough(Chowdhury et al.,1988).Because ?-ferrite is enriched in bcc stabilizing elements,notably Cr and Mo,the reaction,especially to intermetallic phases,takes place more rapidly than the formation of the similar phases.

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The delta ferrite converts to sigma phase and alloy carbides by two-step eutectoid reaction.The?rst step leads to the formation of austenite and a Cr-rich carbide(M23C6)by a lamellar eutectoid reaction. Sigma phase forms as a result of a second eutec-toid reaction(Tseng et al.,1994;Sasikala et al., 2003).

4.2.2.3.Effect of carbide of M23C6.The kinetics of precipitation in Fe–Cr–Ni stainless steel is a competi-tive process between intermetallic phases and M23C6 carbides.Carbide precipitation along grain boundaries preceded the formation of chi phase which occurred at grain boundaries,triple grain junctions,and in grain interiors(Gill and Gnanamoorthy,1981).In this case, M23C6is formed immediately after welding.Then these carbides transform to sigma phase after long-term aging at elevated temperatures.Generally,the precipitation of sigma phase upon heat treatments in the temperature range from650to850?C can be strongly accelerated by the pre-existence of carbides in the steel,particularly those of the Cr23C6and Cr7C3 type(Garin and Mannheim,2003;Chen and Yang, 2001).

Based on the above analysis,the possible precipita-tion sequence was deduced as following.During weld-ing,delta ferrite and large amount of carbide M23C6are formed in321weld material.During aging at700?C, M23C6and ferrite transform to sigma phase.So a large quantity of sigma phase is observed in321weld metal. Contrarily,321base metal has little original delta ferrite and no M23C6,so the precipitation is little compared with its weld material.As far as347material is con-cerned,NbC is formed at the beginning of aging,so347 base material precipitates relatively little carbide com-pared to321base steel.347weld steel precipitates little carbides during welding,so little carbide can trans-form to sigma phase during aging at700?C and rela-tively little sigma is observed compared with321weld steel.

4.3.Effect of Nb/Ti in welds

Unstabilized elements in321weld material are responsible for the existence of a more abundant precip-itation compared to347weld material.Therefore sev-eral approaches should be employed to control sigma phase:decreasing content of ferrite and M23C6carbide in the weld.For the former,selecting proper chem-ical composition of weld wire can get rid of ferrite presence.However,this gives rise to the dangerous effect of hot cracking phenomenon during welding. This can be avoided by proper welding procedures. For the latter,in selecting Nb added(since Ti addi-tion in weld can cause Ti-inclusion defect)weld wire, M23C6carbide precipitation can be decreased during welding which can decrease the sigma phase precipi-tation.Generally,Nb is susceptible to the sigma phase precipitate.However,in this case,sigma phase precip-itate is converted directly or indirectly from the ferrite and carbide,so Nb present in weld can decrease car-bide precipitation which can equally decrease sigma phase.

5.Conclusions

The relationships between the microstructure,pre-cipitation,impact energy have been investigated for two kinds of materials,one is AISI321base with unsta-bilized weld material,another is AISI347base with Nb-stabilized weld material.The main results are as follows.

Both materials present increased precipitation and decreased impact energy as the aging time is increased. The decrease of impact energy is almost the same for both base materials.However,it presents difference for AISI347and AISI321.The latter shows a more drastic decrease of impact energy than the former due to the difference of precipitated amount.

Total amount of precipitation in type347weld sam-ples is smaller than that in type321weld sample. Dimple type fracture is dominant at room temperature impact test for321base,347base and347weld sam-ples,while brittle fracture is dominant for321weld sample.

Experimental results show that carbide is a pref-erential precipitate than sigma phase.These carbides transform into sigma phase during long-term aging at 700?C.Unstabilized element in321weld sample is responsible for the more abundant precipitation as com-pared to the other materials.Several approaches should be employed to control sigma phase precipitation in weld.These approaches include:decreasing content of ferrite and M23C6carbide in weld and selecting Nb added weld wire during welding.

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