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Static Fracture Mechanisms of SS316 Austenitic Stainless Steel Strip Liners in Reactor Pressure Vessel (RPV)

M.A.Khattak1, M.N.Tamin1, S.Kazi2, S. Badshah3, Rafiullah Khan3, Nida Iqbal4

1Department of Applied Mechanics and Design, 2UTM Centre for Low Carbon Transport,

1,2.3Faculty of Mechanical Engineering, 4Medical Devices & Technology Group, Faculty of Biosciences andMedical Engineering,

1,2,4UniveristiTeknologi Malaysia (UTM), 3International Islamic University, Pakistan 81310, Johor, Malaysia

MALAYSIA [email protected]*

Abstract:-The physical basis of fracture mechanisms of austenitic steel with different microstructures was introduced briefly. The samples (SS316 austenitic stainless steel) studied were supplied in the form of a 22 mm-diameter round bar, were heat treated (HT) at 500 oC, 800 oC and for 1000 oC followed by furnace cooling. This work examines the effect of different microstructures of austenitic stainless steel responses (static) of the alloy. It was found that yield strength decreases drastically along with hardness values with increase of HT temperature; however, tensile strength follows the same tendency above HT 800 oC. It was also observed that strain hardening coefficient, K, was mainly dependent on ferrite content, while n remain unchanged. Static fracture morphology reveals a normal progression in brittleness of material. Microstructure observations as well as SEM examinations of the fracture surface for statically fractured specimen were performed to support the above experimental results. Key-Words: - Static Fracture Mechanics, SS316 Austenitic Stainless Steel, Reactor Pressure Vessel 1 IntroductionReactor Pressure Vessels (RPV) and pipelines are commonly constructed using welded C-Mn (A516) steels and stainless steel liners. In oil refineries and chemical plants these steel vessels operate in corrosive environments where high concentration of hydrogen sulphide is present. The operating temperature typically ranges from -29 to 427 °C. C-Mn and Cr-Mo low-alloy ferritic steels are widely used in power and petrochemical industries because of its susceptibility to HIC and high toughness at lower operating temperatures respectively. Unfortunately, prolonged exposure of these steels to intermediate service temperatures (thermal aging), these conditions could lead to deleterious effects such as embrittlement, pitting corrosion, loss of mechanical, microstructural and creep rupture of the steel [1-7] and a shift in ductile to brittle transition temperature (DBTT) to higher temperatures. Previous research

showed that DBTT increases with increase in thermal aging temperature [8].

All these conditions could lead to failures of pressure vessels and pressure piping related accidents, which are often fatal and involved loss of capital investment [9,10]. Reactor pressure vessel failures have caused extensive damage to the plant, people and environment. The explosion of Union Oil amine absorber pressure vessel in 1984 has resulted in causing 17 fatalities and extensive property damage [10]. The explosion of boiler/pressure vessel on-board the Mississippi steamship ‘Sultana’ in 1865 have claimed 1238 lives, albeit more souls were lost when ship sank within 20 min after the explosion. In 1999, 23 percent of a total of 138 explosions and 82 percent of a total of 150 accidents involved failure of boilers, resulting in 21 fatalities [1] .The situation worsened in 2001 where 158 people died and 342 were injured in

Recent Advances in Applied and Theoretical Mechanics

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boilers, pressure vessels and pressure piping related accidents. Many of these reported mishaps were due to non-conforming design and fabrication of pressurized vessels and components and inadequate in-service inspection.

The practice of lining/wallpapering carbon steel vessels with stainless steel dates back to the late 1920s and was the first used by the chemical industry, the oil industry following. Today almost every process industry uses linings where corrosion protection of carbon steel vessels is needed.

Austenitic stainless steels are widely used in pressure vessel fabrication because of their low cost, versatile mechanical properties, excellent weldability, resistance to corrosion and availability in pre-fabricated forms. In heavy-wall vessels operating in similar service environment it is common to apply welded austenitic steel weld overlay to low alloy steels, thereby taking advantage of the high strength and relatively low cost of the base metal while retaining the superior corrosion resistance of the welded stainless steel overlay [11]. The stainless steel overlay vessel is primarily constructed by welding each exhibiting different microstructures. The application of heat for the fusion process greatly affects the microstructure, phase changes and mechanical properties of the steel in the vicinity of the welded region. These changes often lead to a decrease in toughness of the welded joints. Stress–strain model of austenitic stainless steel after exposure to elevated temperatures has also been studied by few researchers [12,13].

The method of the micro indentation test with Vickers diamond pyramid indenter is used to detect the deformation resistance changes in the surface layer of austenitic stainless steel. The micro hardness values (HV) of the base metal, HAZ and the solidified weld metal were reported to be in the ranges 159–173, 155–241 and 161–218, respectively [14].

Several commercially available steels have been studied for applications in RPV [11,15-18]. It is aimed at establishing the processing (heat treatment)-structure-properties relationship of the alloy. Although these studies have contributed to better understanding of the microstructures of these welded joints, little

information available on correlating the observed microstructures with mechanical responses of the alloy in a typical RPV loading and environment.

Fracture mechanism of welded austenitic stainless steel inlay is discussed in this research. Microstructures (with and without thermal treatment), chemical composition and properties of heat treated austenitic steel, normalized with respective values of as-received sample at room temperature are discussed in detail. These results will incorporate in this research, in getting better understanding of metallurgical and microstructural understanding of pressure vessel inlay steel. 2 Material and Experimental Procedures The material used in this study was Type-316 austenitic stainless steel, supplied in the form of a 22 mm-diameter round bar. The chemical composition of the as-received steel is (wt. %) 0.074C, 18Cr, 9.16Ni, 0.271Mo the remaining being Fe. This chromium-nickel (3xxx series) steel is austenitic, nonhardenable and nonmagnetic stainless steel with increased toughness and wear resistance. The low carbon content leads to improved weldability of the steel. The addition of 9.16(wt %) nickel along with 18% chromium offers excellent immunity of the steel from attack by nitric acid and other acids typically found in chemical processing plants. The structures of Chromium- Nickel steels (Schaeffler diagram) are primarily austenite, martensite and δ-ferrite.

Microstructure of the as-received Type-316 sample is shown in Fig.1. The structure consists of coarse-grained ferrite and pearlite phases. The as-received microstructure is considered as the referenced case. Microstructural study is performed using both optical and scanning electron microscopes (SEM). Samples were ground, polish using SiC papers of various grits from 230 to 4000, and then etched in a solution of 10 vol. % nitric acid, 30 vol. % glycerol and 30 vol. % hydrochloric acid at 303 K.


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Fig.1Microstructure of Type 316 stainless steel in the as-received condition (100 X)

Vickers hardness (Hv) values for the as-received sample were measured to be 166. Fig.2, shows the specimen geometry and dimensions used in both tension and fatigue test.

Mechanical properties of the Type-316 steel used in this study are derived from tensile tests on 6.25 mm-dia specimen with gauge length of 25 mm, as listed in Table 1. The strain-hardening behavior can be represented by Ramberg-Osgood equation as σ = K (ε p) n. The strain hardening coefficient K and exponent, n are listed in Table 1.

A series of heat-treatment procedures were performed on the Type-316 steel samples at three conditions of 500,800 and 1000 OC to generate different microstructures. In each treatment, the samples were soaked for 30 mins, followed by furnace-cooled to low temperature. The resulting microstructures are examined using optical and scanning electron microscopes (SEM). Tensile tests and high-cycle fatigue tests are performed on each modified sample. These tests are performed on a ±100 kN servo-hydraulic materials equipped with on-line data acquisition system. Fractographic analysis follows each test.

Table 1:Mechanical properties of the SS316 austenitic stainless steel Elastic Modulus (GPa) 195 Yield Strength,σy (0.2% offset,MPa) 318 Tensile Strength, σuts (MPa) 680 Elongation (%) >50 Hv 166 K 713 n 0.20

Micro vickers hardness measurements were also performed on pearlite and ferrite phases of heat-treated specimen (1000 OC) with applied load of 2.942 N. The micro hardness values (HV) for furnace cool, air cool and water quench were recorded as 126-160,137-154, and 145-166. It is obvious that water quench HV values are well in accordance with published values [14].

Fig.2Geometry of dimensions (mm) of tension test specimen (ASTM A-370) 3 Results and Discussions 3.1 Microstructures The micro structural observations for AR sample in Fig.3 show original structure, coarse-grained ferrite and pearlite. Micrograph for HT at 500 ºC shows little tendency of phase transformation in pearlite areas to small grains of austenite, moreover, the original large ferrite grains remain unchanged. Micrograph for HT at 800 OC shows that pearlite areas are transformed into small grains of austenite by means of eutectoid reaction, but the original large ferrite grains remain unchanged, moreover, cooling from this temperature does not refine the grain which is


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well shown in the Fig.3 (800 OC). Structure at

this phase is mainly Cm+γ .

Fig.3Scanning electron micrographs of HT specimen. (100 x)

Continued heating will allow the large ferrite grains to transform to small grain of austenite,

subsequent furnace cooling will result in small grains of proeutectiod ferrite and small areas of coarse lamellar pearlite. Heating at 1000 OC coarsen the austenitic grain, which, on cooling results in transformation to large pearlitic areas.

Fig.4Tentative cross-section diagram showing trend of reactions in steel alloyed with 18 percent chromium and 8 percent nickel [19]

The Fig.4 shows that the material remained

austenitic for a relatively long period of time in case of 1000oC. Transformation starts when the cooling curve crosses the beginning of transformation at lineat 875oC. The transformation product was Cm+α . Transformation continued until line at 580oC.

After crossing, this line the product is

completely Cm++ γα . Below the temperature line 580oCthe rate of cooling will have no effect on the microstructure or properties. The fractional austenite amount estimated is 38% - 56% at 1000 oC. At 800oC and less temperatures we don’t have a notable amount of austenite [19] 4 Mechanical Properties and Behavior It is seen that the micro hardness measurements determine the mean values of the resistance to deformation of the microstructure, within

500 OC

1000 OC

800 OC

Pearlite

Ferrite

Austenite

Ferrite

Ferrit

Pearlite

This Study (0.08% C)


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individual grains, in the surface layer, since the size of the indentation is smaller than the grain size by almost two times and the depth of the indentation is within 8µm [16]. Therefore hardness values must be in well accordance with yield strength of the material in use.

These indentation measurements can also be considered as non-destructiveness for the specimens. Increase in hardness is associated with the presence of martensite [17] favored by coarse grain. For instance, the values of the Vickers hardness at the surface of the specimens were measured periodically to analyze the effect of processing on microstructure. The results showed a gradual decrease in the hardness values due to heat treatment followed by furnace cooling. The hardness of the ASTM SS316 steel used in this study varies from an average value of 135.03 to 166.23.

From Fig.5, it is obvious for AR and HT samples for different hardness values, with increase in heat treatment temperature, hardness and strength of the material decreases accordingly. However, hardness profile follow the exact trend of yield strength proving the fact that hardness values always go in hand with yield strength only. The same can also be proved from the relative microstructures, as we observed coarse-grained ferrite and pearlite in AR sample, narrating more hardness and strength for the same microstructure. With heat inputs of 500 ºC, 800 ºC and 1000 ºC, the hardness values of HT specimen started to decrease sharply. This phenomenon is more profound at HT 500-800 ºC because of sudden

transformation from Cm++ γα to γα +phase.Properties of Heat treated 316 Steel, normalized with respective values of as-received sample (25 OC) are plotted and shown in Fig.5.

Fig.5Properties of Heat treated 316 Steel, normalized with respective values of as-received sample (25 0C)

Yield strength decreases sharply with increasing temperature. The change is more profound in between 500-1000 C as the

microstructure changes from Cm++ γα to pure γ phase. Ultimate tensile strength decreases rapidly with increasing temperature from 800 C to 1000 C. Moreover, variation of modulus of elasticity was recorded as 189 ± 6 GPa. 4.1 Fractrographic Analysis Fracture is a very complex phenomenon that involves the nucleation, growth of micro-voids or cracks and the propagation of these defects. The failure process of ductile materials can be divided into three stages. First, small micro voids from in the interior of the material as the tensile stress reaches the yield strength. Then with the increase of deformation, the micro voids enlarge to form a micro-crack. Fig.6 shows fracture surfaces of the as-received and heat treated specimens after tensile test. For the AR sample, it can withstand more deformation during the tensile testing and the obvious necking morphology can be clearly observed also it was observed that as-received specimen showed a certain surface roughness and dimple pattern, which are the characteristics of ductile fracture. At HT 500 °C, similar dimple pattern appeared on the fracture surface with increasing ductility. At HT 800 °C, a normal progression in brittleness of material is shown by the drastic shift in the fracture surface that changed from dimple pattern to mixed fracture pattern, which includes a certain amount of cleavage fracture


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pattern. Finally at HT 1000 °C, dimple pattern on fracture surface of the specimen are regained and becomes more profound. Final shearing of the specimen produces a cup type shape on one fracture surface and cone shape on the adjacent connecting fracture surface exhibiting a pure ductile fracture.

Table 2:Effect of strain hardening coefficient K with different heat treatment temperatures (microstructures)

Condition K navg AR 756

0.20

500 °C 747 800 °C 669

1000 °C 680

AR Sample

500 °C

800 °C

1000 °C Fig.6Fracture surfaces of as-received specimens tested at different heat treatments

The strain-hardening behavior can be represented by Romberg-Osgood equation as σ= K (ε p)n. The effect of strain hardening coefficient K with different heat treatment temperatures (microstructures) and exponent, n are listed in Table 2. As microstructure

transform from Cm++ γα (AR and HT 500 °C) to γα + (HT 800 °C) and pure γ (HT 100 °C) phases, there are no ferrite contents and due this lack of ferrite above HT 800 °C there is practically no effect on K. Moreover, exponent n, remain unchanged and it is proved that n is not a function of heat treatment for the studied material. 5 Conclusion As little information available on correlating the observed microstructures with mechanical responses of the alloy in a typical RPV loading


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and environment, this investigative work showed very important correlations in adding the life of stainless steel overlay in typical pressure vessels. The main conclusions of these preliminary studies are:

1. SEM studies reveal that, microstructures of

AR and HT 500 OC exhibit same properties consisting of coarse-grained ferrite and pearlite phases. HT at 800 OC shows that pearlite areas are transformed into small grains of austenite by means of eutectoid reaction and subsequent furnace cooling will result in small grains of ferrite and small areas of coarse lamellar pearlite. Heating at 1000 OC coarsen the austenitic grain, which, on cooling results in transformation to large pearlitic areas.

2. Mechanical properties showed that there is a drastic decrease in strength, esp. yield strength that goes hand in hand with hardness values. Tensile strength shows little tendency of decrease in values with increase in HT temperatures, mostly after 800 OC.

3. As microstructure transform from Cm++ γα (AR and HT 500 °C) to γα +

(HT 800 °C) and pure γ (HT 100 °C) phases, there are no ferrite contents and due this lack of ferrite above HT 800 °C there is practically no effect on K. Moreover, exponent n, which is a material property remain unchanged for the studied material.

4. Static fracture morphology reveals that at HT 800 °C, a normal progression in brittleness of material is shown by the drastic shift in the fracture surface that changed from dimple pattern to mixed fracture pattern, which includes a certain amount of cleavage fracture pattern. Finally at HT 1000 °C, dimple pattern on fracture surface of the specimen are regained and becomes more profound.

References: [1] J. Spence, D.H. Nash, “Milestones in

pressure vessel technology,” International Journal of Pressure Vessels and Piping 81 (2004) 89–118

[2] Mazmir Mat Noh, F.Mozafari,

Muhammad AdilKhattak and M.N.Tamin, Effect of pitting corrosion on strength of AISI 410 stainless steel compressor blades, Applied Mechanics and Materials, Vol. 606 (2014) pp 227-231, Trans Tech Publications, Switzerland.

[3] Jihwan Kim, GyuPil Lee and Do Young Moon, Evaluation of mechanical properties of steel-fibre-reinforced concrete exposed to high temperatures by double-punch test, Construction and Building Materials 79 (2015) 182–191

[4] Xizhang Chen, Yuming Huang and Yucheng Lei, Microstructure and properties of 700 MPa grade HSLA steel during high temperature deformation, Journal of Alloys and Compounds 631 (2015) 225–231

[5] MahsaMirmomeni, Amin Heidarpour, Xiao-Ling Zhao, Christopher R. Hutchinson, Jeffrey A. Packer, Chengqing Wu, Mechanical properties of partially damaged structural steel induced by high strain rate loading at elevated temperatures - An experimental investigation, International Journal of Impact Engineering 76 (2015) 178-188

[6] XiaoyunYuana, Liqing Chen, Yang Zhao, Hongshuang Di and Fuxian Zhu, Influence Of annealing temperature on mechanical properties and microstructures of a high manganese austenitic steel, Journal of Materials Processing Technology 217 (2015) 278–285

[7] Lin Zhang, Zhiyuan Li, Jinyang Zheng, Yongzhi Zhao, Chilou Zhou, Chengshuang Zhou, Xingyang Chen and Ping Xu, Dependence of hydrogen embrittlement on hydrogen in the surface layer in type 304 stainless steel, international journal of hydrogen energy, 39 (2014) 20578-20584

[8] S-H. Song, H.Zhuang, J.Wu, L.-Q.Weng, Z.-X.Yuan,T.-H.Xi, “Dependence of ductile-to-brittle transition temperature on phosphorous grain boundary segregation for a 2.25Cr1Mo steel”, Material Science and Engineering A 486 (2008) 433-438

[9] Tesman, A.B, “Materials of construction for process plants - I,” Chemical


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engineering, Reproduced by courtesy of Japan steel works (1973).

[10] N.V.Challenger, R.Phaal and S.J.Garwood. “Fracture mechanics assessment of industrial pressure vessel failures,” (1995).

[11] G.D.Nasman, “Some material consideration for heavy wall pressure vessels in hydrogen service”, Proceedings of the pressure vessels and piping conference, Orlando, Florida, June 27-July 2,1982, 1–23.

[12] Xing-Qiang Wang, ZhongTao,Tian-YiSong and Lin-Hai Han, Stress–strain model of austenitic stainless steel after exposure to elevated temperatures, Journal of Constructional Steel Research 99 (2014) 129–139

[13] Yuner Huang and Ben Young, Stress–strain relationship of cold-formed lean duplex stainless steel at elevated temperatures, Journal of Constructional Steel Research 92 (2014) 103–113

[14] R.F. Mehl, Atlas of Microstructures of Industrial Alloys, 8th ed.

[15] S.N. Krishnan , V. Toppo , A. Basak, K.K. Ray, “Wear behavior of a steel weld-joint”, Wear 260 (2006) 1285–1294.

[16] Duyi Ye, “Investigation of cyclic deformation behavior in the surface layer of 18Cr–8Ni austenitic stainless steel based on Vickers micro hardness measurement”, Materials Chemistry and Physics 93 (2005) 495–503

[17] M. Eroglu , M. Aksoy, Effect of initial grain size on microstructure and toughness of intercritical heat-affected zone of a low carbon steel, Materials Science and Engineering A286 (2000) 289–297.

[18] J.P. Strizak, L.K. Mansur, The effect of mean stress on the fatigue behavior of 316 LN stainless steel in air and mercury, Journal of Nuclear Materials 318 (2003) 151–156

[19] Sidney H. Avner, Introduction to physical metallurgy, 2nd ed.


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static fracture mechanisms of ss316 austenitic stainless ... … · static fracture mechanisms of...

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