The test environment effect on the mechanical properties of austenitic stainless steels

ELSEVIER

Materials Science and Engineering A234+236 (1997) 1083%1086

The test environment effect on the mechanical properties of austenitic stainless steels K.J. Kurzydlowski Department

of Materials

Science

and Engineering,

Warsaw

University

*

of Technology,

02-524

Warsaw,

Narbutta

85, Poland

Received 10 March 1997; received in revised form 1 April 1997

Abstract Properties of a 316 type austenitic stainless steel were investigated as a function of test temperature in two test environments: (a) vacuum and (b) air. It was observed that the number of serrations per unit strain is higher for specimens strained in air. On the other hand, the flow stress and ultimate tensile strength were systematically higher for the specimens strained in vacuum. Observations on the sections of the specimens revealed three areas: (a) a cracked layer of oxide scale (OS); (b) a near-surface zone (NSZ) characterized by chromium depletion, grain boundary oxidation and sensitization; and (c) core material. During tensile deformation, cracks in the OS and oxidized/sensitized grain boundaries act as stress concentrators. A hypothesis is put forward which links these concentrators with the environmental effect on the mechanical properties of the material. 0 1997 Elsevier Science S.A. Keywords:

Portevin-le Chatelier effect; Austenitic stainless steel; Oxide scale; Environmental effect

1. Introduction

ment,

this

opens

the possibility

for

an environmental

(DSA) [ 1- 131.Part of this temperature interval is characterized by the presence of serrations on stress-strain curves, known as the Portevin-le Chatelier effect (PLC). The model considerations which rationalize the PLC

effect on the mechanical properties of these materials. In the previous study an hypothesis has been put forward that the environment significantly influences the mechanical properties of austenitic stainless steels [S]. The purpose of the present study was to confirm the environment effect on the properties of 316 stainless steel over a wide range of temperatures both under

effect often simplify

static

In the temperature austenitic

stainless

range from steels exhibit

200 to 700°C type 316 dynamic strain aging

the microstructural

details

of mate-

rials. In particular, they disregard potential microstructural gradients related to the presence of a near-surface zone which develops during fabrication and/or in service conditions. The assumption of a negligible effect of the near-surface zone on the mechanical properties is justified for materials which deform in a uniform way. However, this is not true in the case of deformation controlled by propagation of defects which are generated or emerging on the outer surface of specimens. Examples of such a situation include fatigue and brittle fracture which are known to be influenced by surface phenomena. Since the outer surface, to a considerable degree, is created under the influence of the environfax:

+ 48 22 6607209;

e-mail:

0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PIISO921-5093(97)00406-l

dynamic

conditions

and

to provide

more

microstructural details of this phenomenon.

2. Materials and heat treatment The data reported here were collected for AISI 3 16 and 316L austenitic stainlesssteels. (The chemical compositions of the materials used are given in Table 1, see also [S]). The specimensin the present study have been subjected to the following types of heat-treatments: l annealing at 900°C in air (annealing time 1 h) followed by water quenching l

* Tel.: + 48 22 6604720; [email protected]

and

annealing

at 900°C

(water

quenching)

+ annealing

at

600°C (annealing time 1, 2 and 3 h) in air and vacuum (1.33*10W5 Pa)

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Kurq~diowski

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annealing at 900°C and at 600°C in air and vacuum combined with mechanical removing of the oxide scales (OS) and near-surface zone (NSZ) (surface cleaning operation resulting in the reduction of the specimen diameter by 0.025 mm). VACUUM

3. Mechanical

AIR

properties

Characteristic features of the stress-strain curves in the temperature range of PLC effect are exemplified by Fig. 1. It has been found that the flow stress, UTS and the serration frequencies are systematically lower in the case of specimens strained in air than in vacuum. Fig. 2 shows the results of tensile tests carried out at room temperature on specimens annealed at 900°C and 600°C and subjected to the cleaning operation. The following conclusions can be drawn from these results: 1. Specimens annealed at 900°C strained at room temperature exhibit continuous stress-strain curves with a relatively low apparent elastic stiffness (Fig. 24, 2. Additional annealing at 600°C in vacuum significantly increases the apparent elastic stiffness and brings about discontinuous yielding (Fig. 2a), 3. The effect of annealing at 600°C in vacuum is significantly reduced as a result of the surface cleaning operation (Fig. 2b), 4. Additional annealing at 600°C in air has a smaller impact on the flow stress and does not result in a discontinuous yielding, in fact the strengthening effect of the annealing in air is similar to the effect of annealing in vacuum combined with scale removing operation (Fig. 2~).

4. Studies of the microstructure The microstructure of the specimens have been examined using light and electron microscopy and X-ray microprobe imaging. The microstructural observations confirmed that the specimens show a considerable structural gradient related to the presence of a NSZ and layers of OS.

Table 1 Chemical composition present study (in wt.%)

of the austenitic

Steel

C

Cr

Ni

MO

Mn

Si

P

S

316L 316

0.02 0.04

16.5 17.0

10.9 15.9

2.0 2.7

1.4 2.0

0.77 0.19

0.031 0.023

0.044 0.005

stainless

steels used in the

0

Fig. 1. Stress-strain annealed at 900°C

0.1

0.2

curves recorded and subsequently

STRAIN

0.3

0.4

for 316 austenitic stainless steel strained in vacuum and air at

600°C.

5. Discussion The present results clearly demonstrate a measurable environmental effect on the DSA of 316 austenitic stainless steels (a lower frequency of serrations, higher values of the flow stress and higher UTS). Further, it has been shown that the environment in static annealing considerably influences yielding of the steel at room temperature (differences in the efficiency of aging and in the pattern of yielding). On the other hand the microstructural observations showed that the studied material should be considered as consisting of three structural constituents: OS, NSZ and ‘genuine’ material in the specimens core. Properties of the OS and the NSZ depend on the test environment with influences thickness and chemical composition of the OS and chemistry of the grain boundaries in the NSZ. Surface OS growing on metallic materials may have the beneficial effect of protecting the metal of corrosive attack. On the other hand, during plastic deformation, the surface scales are unable to accommodate plastic deformation of the substrate without cracking. Cracks of the OS and along the grain boundaries lead to a reduced strength of the material which can be noted both under dynamic conditions of straining at elevated temperatures or under static conditions during straining of pre-annealed specimens at room temperature. Cracking of the surface layers on metal substrates during their straining has been discussed by Riedel [14], Hu and Evans [15], Ashby and coworkers [16], Raj and coworkers [17,18] and Leyens et al. [19]. However, generally these papers, which focused on the cracking of the surface layers, give more attention to the properties of the surface film then to the metallic substrate.

K. J. Kurzydlowski

5.1. A model for the description surface effect

/Materials

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of the scale and near

%n order to explain the environmental effect observed in the present study an assumption has been made that

800 700 600

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A234-236

0.1

4

0.2 STRAIN

0.3

0.4

600 $00 8 400 2 $ 300 200 100 0

0.1 b)

0.2 STFCAIN

0.3

0.4

8oo I

700

cl

0,0

0.1

0.2 STRAIN

0.3

0.4

Filg. 2. The rest&s 05 ~ensi)e ‘tests cariled out a\ room temper&nTe on: (a) specimens recrystallized at 900°C and additionally annealed at 600°C in vacuum with and without prior cleaning of the specimen surface afnzter anneaYin~ a’l ‘XYPC: {bj stxsssstrain cum23 f0-r fne spcecimens ad&iona~~~ annealed at 600°C in vacuum with and without prior cleaning of the specimen surface after annealing at 900°C and with the final cleaning of the surface after annealing at 600°C; (c) the stresssstrin cnrves 507 Ike speeirnm addi\innaSy annea% at 600°C in air with and without prior cleaning after annealing at 900°C and with the final cleaning after annealing at 600°C. Discontinuous lines are from (a) and (b).

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it is related to the presence of cracks in the OS and stress concentration at the oxidized grain boundaries in the NSZ. Although the OS and the NSZ in the studied material occupy a negligibly small fraction of the specimens cross-sections it should be pointed out that during straining these defects act as stress concentrators. In the case of brittle materials at a sufficiently high level of applied stress these surface concentrators are frequently sufficient to cause fracture of strained specimen. In the present case, the stress concentrations at a specimen surface are relaxed by local plastic deformation. The size of this local deformation zone, R, can be estimated using the model proposed by Bilby, Cottrell and Swinden [20,21]. This model leads to the following relationship: R=,[sec(z)-

0

(I 997) 1083-

l]

where c is the size of the microcracks, ga is the applied stress and or is the flow stress of the matrix in the vicinity of the stress concentrator in question (set stands for secant or cos - ‘). It can be seen from the formula that the size of the plastic zone, R, is proportional to the size of the crack developed in the strained material. It should also be pointed out that the size of plastic zone strongly depends on the value of era/or ratio. For ga/gr = 0 the plastic zone R = 0, on the other hand for rsa/rrr + 1 the size of plastic zone approaches co. This means that microcracks formed under the conditions of ga/gr z 1 result in a large size of plastic zone at the crack tip which eventually may acquire the macroscopic size where its further growth is driven only by the loading system. It should be noted that in most cases the applied stress, ga, is smaller than the local flow stress in the vicinity of the microcrack, (ga < aJ. This is due to fact that the macroscopic flow takes place as a result of the activity of a large number of dislocation sources such as dislocation segments, grain boundaries, steps and cracks on the surface. The ratio of ~JB, is particularly small in well annealed polycrystals in the microplasticity range which are characterized by the abundance of the dislocation sources. Theoretical considerations show that surface steps and microcracks are potential source of dislocations. These microcracks and other stress concentrators exist iv 222DS32z.%?s$x2~a~d Jz!akLbs 0-f h3ikmkA kIqxxbnss prior to their deformation. However, these cracks usually develop a small size af the plastic deformation zone because they becwine relaxed at the applied stress aa much lower than the flow stress or, i.e. at a low values of the 0,/g, ratio. On the orher hand, Ihe cracks, or other Iocalized stress concentrators, which are formed in the materials subjected to strain aging are plastically relaxed at rela-

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tively higher values of the applied stress. As a result the plastic deformation zones, due to the higher values of the ga/gr ratio can reach macroscopic size leading to a drop on the stress-strain curves during DSA or plateau stress in the case of straining of pre-annealed specimens.

6. Conclusion

Studies carried out on 316 austenitic stainless steel demonstrated the environmental effect on the dynamic and static strain aging. This effect has been related to the relaxation of stress concentrations at the oxidized grain boundaries and cracks in the OS. References [I] E.O. Hall, Yield Point Phenomena in Metals and Alloy, Macmillan, London, 1970. [2] B.P. Kashyap, K. McTaggart, K. Tangri, Phil. Mag. A 57 (1988) 97. [3] R.A. Varin, Mater. Sci. Eng. 94 (1987) 93.

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[4] K.J. Kurzydlowski, W. Przetakiewicz, Bull. Pol. Acad. Sci. 37 (1989) 487. [5] S. Venkadesan, C. Phaniraj, P.V. Sivaprasaol, P. Rodriquez, Acta Metall. 40 (1992) 569. [6] L.H. de Almeida, I. LeMay, S.N. Monteira, Ser. Metall. 19 (1985) 1451. [7] J.T. Barnby, J. Iron Steel Inst. 4 (1965) 392. [8] A.A. Abduluyahed, K. Roiniatowski, K.J. Kurzydlowski, Ser. Metall. 33 (1995) 1489. [9] P. Rodriguez, Bull. Mater. Sci. 6 (1984) 653. [lo] S. Venkadesan, S. Venuqopal, P.V. Sivaprasad, P. Rodriquez, Mat. Trans. JIM 33 (1992) 1040. [Ill S.L. Mannan, K.G. Samuel, P. Rodriquez, Tran. Indian Inst. Met. 36 (1983) 313. [12] A.H. Cottrel, Phil. Mag. 44 (1953) 829. [13] P.G. McCormick, Acta Metall. 20 (1972) 352. [14] H. Riedel, Met. Sci. 16 (1982) 569. [15] M.S. Hu, A.G. Evans, Acta Metall. 37 (1988) 917. [16] M.F. Ashby, J. Blunt, M. Bannister, Acta metall. 37 (1989) 1847. [17] D.C. Agrawal, R. Raj, Acta Metall. 37 (1989) 1265. [18] V.C. Jobin, R. Raj, L. Phoenix, Acta Metall. 40 (1992) 2269. [19] C. Leyens, M. Peters, W.A. Kaysser, Ser. Mater. 35 (1996) 1423. [20] B.A. Bilby, A.H. Cottrell, K.H. Swinden, Proc. Roy. Sot. A272 (1963) 304. [21] E. Smith, Dislocations and cracks, in: F.R.N. Nabarro (Ed.), Dislocations in Solids, North-Holland, Amsterdam, 1979, p. 363-448