effect of aging on the toughness of austenitic and duplex stainless steel weldments

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J. Mater. Sci. Technol., 2010, 26(9), 810-816.

Effect of Aging on the Toughness of Austenitic

and Duplex Stainless Steel Weldments

Omyma Hassan Ibrahim1)†, Ibrahim Soliman Ibrahim2) and Tarek Ahmed Fouad Khalifa 3)

1) Metallurgy Dept., Nuclear Research Centre, Atomic Energy Authority, Egypt2) Nuclear Safety Centre, Atomic Energy Authority, Egypt

3) Shoubra Faculty of Engineering, Benha University, Egypt

[Manuscript received August 31, 2009, in revised form February 26, 2010]

In the present study, the effect of aging heat treatment at 650, 750, and 850◦C on the impact toughness of 316L austenitic stainless steel, 2205 duplex stainless steel and their weldments has been investigated. Weldingprocess was conducted using the TIG (tungsten inert gas) welding technique. Instrumented impact testing,at room temperature, was employed to determine the effect of aging treatment on the impact properties of investigated materials. Aging treatment resulted in degradation in the impact toughness as demonstrated bythe reduction in the impact fracture energy and deformation parameters (strain hardening capacity, fracturedeflection, and crack initiation and propagation energy). The degree of embrittlement was more noticeablein duplex stainless steel parent and weld-metal than in 316L stainless steel and became greater with theincrease of aging temperature. The degradation in impact toughness was discussed in relation to the observedprecipitation of the intermetallic sigma phase in the microstructure of the stainless steel weldments and the

corresponding fracture surface morphology.

KEY WORDS: Austenitic and duplex stainless steels; Aging; Instrumented impact; Toughness

1. Introduction

Austenitic and austenitic-ferritic (duplex) stain-less steels are widely used in many engineering appli-cations. These steels present excellent combinationof corrosion resistance, ductility, toughness and weld-ability. They are used as structural materials andcomponents of heat transfer equipment in the chem-ical and petrochemical industries. In the nuclear in-dustry, theses steels are employed as cladding materi-als in reactor pressure vessels and control rod assem-blies as well as cooling system piping.

A serious drawback to using such steels and theirweldments is the degradation of corrosion and me-chanical properties within certain high temperaturedue to the microstructural changes.

Austenitic stainless steels may be suscep-tible to intergranular corrosion (sensitization)caused by chromium depletion next to grain

† Corresponding author. Ph.D.; E-mail address:omyma−[email protected] (O.H. Ibrahim)

boundaries due to precipitation of M 23C6 carbidesduring usage in the temperature range 540 to860◦C[1]. In the case of molybdenum-containingaustenitic stainless steels, such as AISI 316 and 316Ltypes, long exposure time in 650 to 900◦C can leadto the precipitation of the intermetallic sigma ((Fe,

Ni)x (Cr, Mo)y) phase. During solidification or weld-ing, delta ferrite phase formation might occur, whichrenders the steel even more prone to precipitation of sigma phase[2].

In comparison to austenitic stainless steels, pre-cipitation of sigma phase in duplex stainless steels(DSS) occurs, within the ferrite phase at shortertime, at higher temperatures and with larger volumefractions[2]. For example, an aging treatment for 3 hin the temperature range 800 to 850◦C is sufficientfor the formation of 15% to 20% of sigma phase [2]. Incontrast to the carbides that form an almost contin-

uous network in the austenite regions, sigma phaseis finely distributed within the ferrite. The emer-gence of carbide precipitates and sigma phase in theaustenitic and duplex stainless steels can render



O.H. Ibrahim et al.: J. Mater. Sci. Technol., 2010, 26(9), 810–816 811

Table 1 Chemical composition of base metals

C Mn Si Ni Cr Mo Fe

316L 0.020 1.50 0.40 10.0 17.0 2.10 Bal.Duplex (2205) 0.020 1.50 0.40 6.0 23.0 3.00 Bal.

Table 2 Chemical composition of weld metal fillers

C Mn Si Ni Cr Mo Fe

AWS E 316L 0.020 1.80 0.07 11.7 18.70 2.70 Bal.ER 2209 Duplex 0.030 1.50 0.90 9.50 23.0 3.00 Bal.

them susceptible to embrittlement since such phasessuffer brittle fracture and offer a path for brittle crackpropagation, thus reducing ductility and toughness.It should be mentioned that the weld metal behaviorof these steels is not as good as the base metal byvirtue of the relatively higher ferrite content[2].

The present investigation is concerned with anevaluation of the effect of aging heat treatment at

650, 750, and 850◦

C on the impact toughness of 316Laustenitic stainless steel, 2205 duplex stainless steeland their weldments conducted through TIG (tung-sten inert gas) welding technique. The correlation be-tween the changes in the microstructure during agingtreatment, and impact toughness of the investigatedmaterials was assessed by instrumented impact test-ing.

2. Experimental

The chemical composition of the investigated ma-terials is presented in Tables 1 and 2.

The 316L austenitic and 2205 duplex stainlesssteel base were subjected to solution heat treatment at1050 and 1150◦C, respectively for one hour and thenwater quenched. Isothermal aging treatment was ap-plied to the above base and weld metals at 650, 750,and 850◦C for 3 h followed by furnace cooling.

Instrumented impact tests were conducted on un-aged and aged specimens of the investigated mate-rial with dimensions of 55 mm×10 mm×4.5 mm atroom temperature the total capacity of the impactmachine (Amsler RKP-300, Switzerland) was 300 Jwith a hammer velocity of 5.1 m/s.

3. Results and Discussion

3.1 Microstructure

The microstructure of the investigated austeniticand duplex stainless steel (DSS) weldments is pre-sented in Fig. 1. The 316L base metal (Fig. 1(a1))is characterized by equiaxed austenite grains with anaverage grain size of 50 µm. As can be seen that thestructure is free from annealing twins. Aging of the316L base metal, at 850◦C, developed grain bound-ary precipitation of M 23C6 carbides (Fig. 1(a2)) indi-

cating the occurrence of sensitization in this type of stainless steel.

Figure 1(b1) shows an optical micrograph of 316LTIG weld metal which displays a mixture of austenite

phase and vermicular ferrite morphology that devel-oped during the welding process. Aging of this 316LTIG weld metal at 850◦C results in the formation of small amounts of sigma phase which precipitate inthe ferrite regions (Fig. 1(b2)). The precipitation insigma phase resulted from the transformation of deltaferrite presented in the weld metal[1].

Figure 1(c1) shows a micrograph of 2205 duplex

stainless steel base metal and it presents very finegrained structure of 5 µm of austenite within ferritematrix. This structure changed after aging at 850◦Cinto austenite phase and ferrite masses with apparentwhite areas resembling sigma phase precipitates, asshown by scanning electron microscopy (SEM) exam-ination (Fig. 1(c2)). The 2205 duplex stainless steelweld metal microstructure is shown in (Fig. 1(d1))and it comprises acicular austenite structure within amatrix of ferrite. Aging at 850◦C of this weld metalleads to the precipitation of sigma phase as white ar-eas shown in (Fig. 1(d2)) through examination bySEM. In this figure, sigma phase is seen as massivemorphology of particles larger in size than that in theaged duplex stainless steel base metal.

3.2 Impact properties

Table 3 and Fig. 2 show the effect of aging temper-ature on the impact fracture energy of the austeniticand duplex stainless steels base metal and weld met-als tested at room temperature. In the un-aged con-dition, the impact fracture energy of the base metalis higher than that of the weld metal by 13% (63 vs55 J) and 30% (107 vs 75 J) for the austenitic and

duplex stainless steels, respectively. This can be as-cribed to the relatively higher content of delta ferritein the weld metal than that in the base metals. In ad-dition, the internal residual stresses produced duringthe welding process could give rise to the weld metalyield strength, leading to a reduction in the impactenergy[3].

It also can be seen that, the impact fracture en-ergy values of the duplex stainless steel, base and weldmetals, are higher than those of the austenitic stain-less steel by 41% (107 vs 63 J) and 27% (75 vs 55 J),respectively. This can be attributed to the much finergrain size of the duplex stainless steel base metal com-pared to that of the austenitic stainless steel (5 vs50 µm), which leads to a higher resistance to the prop-agation of brittle cracks and consequently a higher



812 O.H. Ibrahim et al.: J. Mater. Sci. Technol., 2010, 26(9), 810–816

Fig. 1 Microstructure of 316L and duplex stainless steels base and weld metals: (a1) 316L BM, (a2) 316L BM 850◦

C,(b1) 316L WM, (b2) 316L WM 850◦C, (c1) DSS BM, (c2) DSS BM 850◦C, (d1) DSS WM, (d2) DSS WM 850◦C




Table 3 Impact fracture energy of tested materials at room temperature

Base metals impact fracture energy/J Weld metals impact fracture energy/JAR 650◦C 750◦C 850◦C AW 650◦C 750◦C 850◦C

316L 63 63 55 36 55 47 22 21Duplex 107 38 16 7 75 7 3 3Notes: AR—as-received condition, AW—as-welded condition

Fig. 2 Variation of impact fracture energy with ageing temperature: (a) austenitic stainless steel, (b) duplexstainless steel

impact toughness[2].Table 3 shows that aging heat treatment leads to

a gradual decrease in the impact fracture energy forboth the austenitic and duplex stainless steels. A re-duction of 43% (63 vs 36 J) and 62% (55 vs 21 J) isobserved for the impact energy of the base and weld

metal of the austenitic stainless steel, respectively af-ter aging at 850◦C for 3 h. The corresponding reduc-tion in impact energy values of the duplex stainlesssteel base and weld metal is 93% (107 vs 7 J) and96% (75 vs 3 J), respectively. This shows that the de-gree of reduction in impact energy is much higher inthe duplex stainless steel than that in the austeniticstainless steels.

3.3 Load time traces

Figure 3 presents load-time traces of the investi-gated materials produced from instrumented impacttesting before and after aging treatment at 850◦C.These traces are a manifestation of the embrittlementfeatures produced by the aging process. These fea-tures are summarized in Table 4 and they comprisethe following: (1) an increase in both yield (P y) andmaximum (P max) loads after aging at 850◦C; (2) areduction in the strain hardening capacity, as indi-cated by the increase in the ratio between the yieldand the maximum loads (P y/P max); (3) a reduction inthe initiation (E i) and propagation (E p) energy val-ues due to the aging process; (4) a reduction in thetotal dynamic deflection (TDD) which represents the

total amount of strain exerted at each test condition.These observations could be associated with the

microstructural (Fig. 1) and fractural (Fig. 4) featuresas defined by optical microscopy and SEM, in addi-

tion to the measured ferrite content of the investigatedmaterials before and after aging processes.

3.4 Austenitic stainless steel weldments

The decrease in the impact fracture energy of

316L austenitic stainless steel base metal from 63 to36 J (Table 3) is accompanied by appearance of grainboundary precipitation (Fig. 1(a2)), which is shownby EDX analysis to contain increased content of Crelement indicating presence of M 23C6 carbides. Thefracture surface (Fig. 4(b)) reveals mixed mode of ductillity (dimples) and brittlement (cleavage facets).Grain boundary carbide precipitation has long beenknown to initiate cleavage fracture[1], which leads to areduction in the impact energy of austenitic stainlesssteels. Measurement of the ferrite content of the 316Lbase metal showed a value of about 1.65% which de-creased down to 0.25% after aging at 850◦C for 3 h.This indicates very little formation of sigma phasewhich might be transformed from delta ferrite andcontributed to the embrittlement processes[4].

On the other hand, the amount of reduction inthe impact fracture energy of the 316L weld metal,after aging at 850◦C, was much greater than thatof 316L base metal (62% vs 43%). The ferrite con-tent of the 316L TIG weld metal decreased, due toaging, from 6% to 0.75% which indicates the trans-formation of delta ferrite to sigma phase at this ag-ing temperature[4]. The corresponding fractographof this condition (Fig. 4(d)) reveals the mixture of

very fine shallow dimples and very small cleavagefacets which accounts for the observed embrittlement.Sigma phase is susceptible to brittle fracture and thusoffers a path for brittle crack propagation and hence




Fig. 3 Instrumented impact test traces for 316L and duplex stainless steels base and weld metals: (a) 316L BM(63 J), (b) 316L BM 850◦C (36 J), (c) 316L WM (55 J), (d) 316L WM 850◦C (21 J), (e) DSS BM (107J), (f) DSS BM 850◦C (7 J), (g) DSS WM (75 J), (h) DSS WM 850◦C (3 J)




Fig. 4 Fracture surface of 316L and duplex stainless steels base and weld metals as revealed by SEM: (a) 316LBM (63 J), (b) 316L BM 850◦C (36 J), (c) 316L WM (55 J), (d) 316L WM 850◦C (21 J), (e) DSS BM(107 J), (f) DSS BM 850◦C (7 J), (g) DSS WM (75 J), (h) DSS WM 850◦C (3 J)




Table 4 Instrumented impact test parameters for the tested materials at room temperature

P y P max P y/P max E i E p E t TDD/kN /kN /% /J /J /J /mm

316L BM 3.7 7.3 52 32 31 63 17.5316L BM-850◦C 21 35 60 13.5 22.5 36 1.5

316L WM 4.2 6.5 51 29 26 55 20316L WM-850◦C 4.8 6.5 65 13.5 7.5 21 7.5

Duplex BM 7 11 64 40 67 107 20Duplex BM-850◦C 13 15.5 84 5.7 1.3 7 0.75

Duplex WM 6 10.3 58 22 53 75 15Duplex WM-850◦C 13 13 100 3 0.2 3 0.35

Notes: P y—yield load, P max—maximum load, P y/P max—strain hardening capacity, E i—crack initiation energy,E p—crack propagation energy, E t—total fracture energy, TDD—total dynamic deflection

reducing toughness[5].

3.5 Duplex stainless steel weldments

The embrittlement behavior of the duplex stain-less steel weldments due to aging treatment was moreevident than that of 316L austenitic stainless steelweldments. The reduction in the impact fracture en-ergy of the duplex stainless steel base and weld metalwas 93% and 96%, respectively after aging at 850◦C.Load-time traces of both duplex stainless steel baseand weld metals display complete brittle behavior(Fig. 3(f) and (h)) with evidence of load drop. Thisbrittle aspect is represented by the change in the frac-ture surface features (Fig. 4(f) and (h)). Figure (4)shows variation from the well defined deep dimplesof the un-aged duplex stainless base and weld met-

als (Fig. 4(e) and (g)) to cleavage facets that appearlarger in the weld metal (Fig. 4(f) and (h)). The mi-crostructures of the aged duplex stainless steel baseand weld metals (Fig. 1(c2) and (d2)) display heavyprecipitation of sigma phase at the boundaries of theferrite phase[6]. This was supported by the EDXanalysis which gave high percent of molybdenum el-ement essential for the formation of sigma phase[7].The dense precipitation of sigma phase was also in-dicated by the large drop in the ferrite content, from49.5% to 7.5% for the base metal and from 45.5% to11.5% for the weld metal which is in agreement with

other comparative data[8,9]

.

4. Conclusions

In the present study, an evaluation of the effect of aging heat treatment at 650, 750, and 850◦C on theimpact toughness of 316L austenitic stainless steel,2205 duplex stainless steel and their weldments usingTIG (tungsten inert gas) welding technique has beengiven. The main conclusions are:

(1) In the un-aged condition, the duplex stain-less steel parent and weld metal specimens exhibithigher impact energy level than the corresponding316L austenitic stainless steel specimens.

(2) Aging treatment results in gradual embrittle-

ment as demonstrated by the reduction in the impactfracture energy and deformation parameters.

(3) The degree of embrittlement is more notice-able in duplex stainless steel parent and welds metalsthan in 316L stainless steel, becoming greater withthe increase of the aging temperature.

(4) The degradation in impact toughness is dis-cussed in relation to the observed precipitation of theintermetallic sigma phase which appears more intensein the microstructure of duplex stainless steel weld-ments.

(5) SEM examination reveals that the reduction inimpact energy due to aging is associated with the for-mation of shallow ductile dimples and cleavage facetson the fracture surface of 316L and duplex stainlesssteels, respectively.

Acknowledgement

The authors gratefully acknowledge the technical as-sistance and provision of the welding facilities by Prof. Dr.A.A. Sadek, Central Metallurgical Research and Develop-ment Institute (CMRDI).

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effect of aging on the toughness of austenitic and duplex stainless steel weldments

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