UDC/UDK 669.14.018.8:620.193 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 41(3)131(2007) THE EFFECT OF COLD WORK ON THE SENSITISATION OF AUSTENITIC STAINLESS STEELS VPLIV HLADNE DEFORMACIJE NA POVEČANJE OBČUTLJIVOSTI NERJAVNIH JEKEL Mįria Domįnkovį, Marek Peter, Moravčķk Roman Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Bottova 24, 917 24 Trnava, Slovakia maria.domankovaŽstuba.sk Prejem rokopisa – received: 2006-09-20; sprejem za objavo - accepted for publication: 2007-01-15 The sensitisation behaviour of austenitic stainless steel is greatly influenced by several metallurgical factors, such as the chemical composition, the degree of prior deformation, the grain size, and the ageing temperature and time. The precipitation behaviour of AISI 316 and 304 austenitic stainless steels has been investigated after ageing at various temperatures from 500 °C to 900 °C for 0.1 h to 1000 h. The TTS diagrams of the experimental steels after an oxalic-acid etch test ASTM A262 practice A were constructed. It was demonstrated that the C curves of the TTS diagrams were displaced towards shorter times by the increment of 20 % cold work (CW), since the sites inside the grain matrix have a high energy and the carbides can nucleate there easily. Cold work increases the number of dislocations/dislocation pipes along which the diffusion rate of chromium is very high. The sensitisation of the experimental steels accelerated the precipitation of M23C6. Besides M23C6, the (T-phase and M6C were detected at the grain boundaries and in the austenitic matrix in the case of the cold-worked samples. Key words: austenitic stainless steels, sensitisation, precipitation, cold working, intergranular corrosion Občutljivost nerjavnih jekel je odvisna od več metalurških dejavnikov, npr. od kemične sestave, predhodne deformacije, velikosti zrn, časa in temperature staranja. Izločilno vedenje jekel AISI 316 in 304 je bilo raziskano po različno dolgem staranju od 0,1 h do 1000 h pri temperaturah med 500 °C in 900 °C. TTS diagrami eksperimentalnih jekel so bili pripravljeni po oksalnem preizkusu po ASTM 262 - metoda A. Dokazano je, da so C-krivulje TTS-diagramov premaknjene h krajšim časom po 20-odstotni hladni deformaciji (CW), zato ker imajo tudi v zrnih mesta z veliko energijo, kjer nastanejo karbidni izločki. Hladna deformacija poveča število dislokacij/dislokacijskih cevk, kjer je difuzija kroma zelo hitra. Povečanje občutljivosti jekel je povečalo hitrost izločanja karbidov M23C6. (T-fazo in M6 karbide smo v hladno deformiranem jeklu opazili ob kristalnih mejah in v avstenitni matrici. Ključne besede: avstenitna nerjavna jekla, povečanje občutljivosti, hladna deformacija, interkristalna korozija 1 INTRODUCTION The intergranular corrosion (IGC) and stress-corrosion cracking (SCC) of austenitic stainless steels are the most important corrosion processes that affect the service behaviour of these materials. Exposure to temperatures in range 500-800 °C leads to the grain-boundary precipitation of chromium-rich carbides (Fe,Cr)23C6 and to the formation of chromium-depleted regions. If the mass fraction of chromium content near the grain boundaries drops under the passivity limit of 12 %, the steel becomes sensitised. The sensitisation temperature range is often encountered during isothermal heat treatment, slow cooling from the solution annealing temperature, the improper heat treatment in the heat-affected zone of the welds or welding joints or the hot working of the material. The degree of sensitisation (DOS) is influenced by factors such as the steel’s chemical composition, the grain size, the degree of strain or temperature and the time of isothermal annealing. The sensitisation involves both the nucleation and growth of carbides at the grain boundaries. Depending on the state or the energy of the grain boundaries they can provide preferential sites for carbide nucleation and act as a favoured diffusion path for the growth of carbides. Therefore, it has been suggested that the nature of grain boundaries could also influence the DOS and IGC 1–4. In this article we report on some preliminary comparisons of the combined effects of chemical composition, deformation, temperature and aging time on sensitisation in AISI 304 and 316 stainless steels. 2 MATERIALS AND EXPERIMENTAL PROCEDURES The chemical composition of the experimental steels is given in Table 1. The steels were mostly investigated in the as-received condition with some in the solution-annealed condition. The solution annealing was conducted on the as-received materials at 1050 °C for 60 min followed by water quenching. The steels were 20–40 % cold rolled by controlling the thickness of the plates. The cold-worked samples were heat treated at various temperatures in the range 500-900 °C for times of 0.1 to 1000 h. The samples were then water quenched after the heat treatment. The oxalic-acid etch test (ASTM A262 practice A) was used to determine the steels’ sensitivity to intergranular corrosion. The specimens were electrolytically etched in 10 % oxalic acid for 90 s at a current density of Materiali in tehnologije / Materials and technology 41 (2007) 3, 131-134 131 M. DOMĮNKOVĮ ET AL.: THE EFFECT OF COLD WORK ON THE SENSITISATION OF AUSTENITIC ... Table 1: Chemical composition (in mass fractions, w/%) of the austenitic stainless steels Tabela 1: Kemična sestava (v masnih deležih, w/%) avstenitnih nerjavnih jekel steel C N Si Mn P S Cr Ni Mo Fe AISI 304 0.04 0.012 0.54 1.08 0.0032 0.008 18.52 8.47 0.21 bal. AISI316 0.05 0.032 0.47 0.86 0.0026 0.001 17.55 11.56 2.10 bal. 1 A/cm2. The etched microstructure was then examined at 250x, and was characterised as a step, dual or ditch microstructure 5. For the individual secondary-phase identification transmission electron microscopy (TEM) of the carbon extraction replicas was applied. TEM observations were performed using a JEOL 200 CX operating at 200 kV. The carbon extraction replicas were obtained from mechanically polished and etched surfaces. The replicas were stripped from the specimens in the solution of CH3COOH : HClO4 = 4 : 1 at 20 °C and 20 V. 3 RESULTS The results of the light microscopy examination are summarised in Figure 1. The microstructure of AISI 304 after solution annealing consists of polyhedral austenitic grains with twinning typical of an fcc microstructure. The average austenitic grain size in this state is about 45 µm (Figure 1a). A small amount of ó-ferrite was also observed. No precipitates were detected at the grain boundaries (GBs) of the solution-annealed steels. Figure 1b shows the microstructure of the AISI 304 after 40 % of CW. The microstructures of the aged states are shown in Figure 1c and Figure 1d. Figure 1c shows the evolution of secondary phases precipitated at the GB in the isothermally aged specimen (650 °C/0.5 h) without cold work. The microstructure of the isothermally aged specimen (650 °C/0.5 h) and 40 % CW is shown in Figure 1d. The precipitation of secondary phases was observed at the GB and intragranularly and within the matrix. To compare the results of two austenitic stainless steels, time-temperature-sensitisation (TTS) diagrams for these steels for different degrees of CW ranging from 0 % to 40 % are presented in Figure 2. From the TTS diagrams it can be seen that the nose of the C curve corresponding to the maximum rate of sensitisation - m _ ""' -'1Š1 ';-£j : ASI* ¦- Figure 1: Microstructure of the AISI 304 a) after solution annealing - 0 % CW, b) after solution annealing - 40 % CW, c) after aging at 650 °C/0.5 h, 0 % CW, d) after aging at 650 °C/0.5 h, 40 % CW Slika 1: Mikrostruktura jekla AISI 304 a) po topilnem žarjenju - 0 % CW, b) po topilnem žarjenju - 40 % CW, c) po staranju 650 °C/0,5 h, 0 % CW, d) po staranju 650 °C/0,5 h, 40 % CW 132 Materiali in tehnologije / Materials and technology 41 (2007) 3, 131-134 M. DOMĮNKOVĮ ET AL.: THE EFFECT OF COLD WORK ON THE SENSITISATION OF AUSTENITIC ... 950 900 750 700 650 600 550 500 450 400 1000 950 900 550 BOO 750 700 650 __— -316(40%)l /7 — -316(0%) r / ( l VV vČs >«-. ȗ ČČČn: lg (thod./s) —304(40%)] čĢ-------304(0%) l V. V Č-«. ¦-ČČČČČ lg (thod./s) Figure 2: TTS diagrams for AISI type 316 and 304 stainless steels with various degrees of CW established as per ASTM A262 practice A test Slika 2: TTS diagrama za nerjavni jekli 316 in 304 pri različni stopnji deformacije, določeni z ASTM A262-preizkusom Figure 4: Microstructure of AISI 316 (650 °C/1000 h) - TEM Slika 4: Mikrostruktura jekla AISI 316 (650 °C/1000 h) – TEM AISI316 0%CW (650°C) ±Č, >¦ čČK y S V* Č * M23C6 J> Č ČČČČ sigma S Čv . X / r Č "»«-, » s— —tČ Č?—- ¦—A A V W occurs at 800 °C for the AISI 316 in the 0 % CW condition. As the degree of CW increases, the nose temperature remains almost that same, but tmin decreases with the increase in % CW up to 20 % and remains constant thereafter. The TTS diagram of AISI 304 is shifted towards shorter times than the 0 % CW material. The tendency of the shift of the AISI 304 C curve is similar to the case of AISI 316. To identify the type of secondary phases precipitated at the grain boundaries (GBs) during the isothermal Figure 5: Phase ratio in AISI 316 after aging at 650 °C Slika 5: Razmerje faz v jeklu AISI 316 po staranju pri temperaturi 650 °C AISI3040%CW(650°C) BS 60 2 S <° n c 0- 30 20 10 — M23C6 — sigma — M6C —? 30 (Al Figure 3: Microstructure of AISI 316 (650 °C/300 h) – TEM Slika 3: Mikrostruktura jekla AISI 316 (650 °C/300 h) – TEM Figure 6: Phase ratio in AISI 304 after aging at 650 °C Slika 6: Razmerje faz v jeklu AISI 304 po staranju pri temperaturi 650 °C treatment, TEM analysis was carried out. First, M23C6 was detected at the grain boundaries after aging. In addition to M23C6, the s-phase and M6C were detected at the grain boundaries (Figure 3 and Figure 4). Similar precipitation trends were detected using TEM analysis at Materiali in tehnologije / Materials and technology 41 (2007) 3, 131–134 133 M. DOMĮNKOVĮ ET AL.: THE EFFECT OF COLD WORK ON THE SENSITISATION OF AUSTENITIC ... the grain boundaries of the AISI 304 steel. The identified secondary phases in the experimental steels and the phase ratio are shown in Figures 5 and 6. 4 CONCLUSIONS The precipitation behaviour of AISI 316 and 304 austenitic stainless steels was investigated during aging at various temperatures in range from 500 °C to 900 °C for times from 0.1 to 1000 h. The following conclusions can be drawn: TTS diagrams of the experimental steels after the oxalic-acid etch test ASTM A262 practice A show that the C curves of the TTS diagrams are displaced towards shorter times with increasing amounts of CW. After ˜20 % cold working, even the sites inside the grain matrix have high energy and carbides can nucleate there also. Cold work increases the number of dislocations/dislocation pipes along which the diffusion rate of chromium is faster. Sensitisation of the experimental steels accelerated the precipitation of M23C6. In addition to this carbide, s-phase and M6C were detected at the grain boundaries and in the austenitic matrix in the cold-worked samples. Acknowledgement The authors wish to thank the Scientific Grant Agency of the Slovak Republic (VEGA) for their financial support of grant No. 1/2113/05. 5 REFERENCES 1 Murr L. E. Advani A., Shakar S., Atteridge D. G.: Effects of deformation and heat treatment on grain boundary sensitisation and precipitation in austenitic stainless steels. Material Characterization, 24 (1990), 135–158 2 Trillo E. A., Murr L. E.: Effects of carbon content, deformation and interfacial energetics on carbide precipitation and corrosion sensitisation in 304 stainless steel. 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