THE INFLUENCE OF VARIOUS SENSITIZING TEMPERATURES FOR STRESS CORROSION CRACKING OF AISI 304 IN 42 Wt% MgCl2 MEDIUM

THE INFLUENCE OF VARIOUS SENSITIZING
TEMPERATURES FOR STRESS CORROSION CRACKING OF AISI 304 IN 42 Wt% MgCl2 MEDIUM

Gadang Priyotomo
Research Center for Metallurgy – Indonesian Institute of Sciences Kawasan PUSPIPTEK Gd.470 Serpong Tangerang Banten Indonesia 15314

(This paper is still edited by editor board of National Journal of corrosion (majalah KOROSI) Vol.17 No.2 October 2008 from Research Center for Metallurgy -Indonesian Institute of Sciences )
(All figures and tables are not shown in this paper)



Abstract
The change in the mechanism for SCC on AISI 304 austenitic stainless steel was investigated in 42wt% saturated magnesium chloride solution at 106 0C by using a constant load method. The solution-annealed specimens were applied under constant load of 30.5 Kg/mm2 and 40 Kg/mm2. The non-solution annealed specimens were applied under constant load of 20 Kg/mm2 and 25 Kg/mm2. All experiments were done under an open circuit condition. The most of fracture shape in non-annealed type 304 at various sensitizing temperature (6000C, 7000C, 8000C) is intergranular. All fracture shape in annealed type 304 at various sensitizing temperature is trangranular. The deformation can be easy to move by growing grains in annealed type 304 so that it can be predicted by decreasing Vickers hardness value. In non-annealed type 304, the deformation can be sustained because of the small size of grains. The role of  martensite mechanism in non-annealed type 304 significantly contribute to suffer the forming of fracture in MgCl2 solution at 1060C especially in grain boundaries. In annealed type 304, mechanism of fracture can be associated by deformation through slip plane in grains.

Keyword : Stress corrosion cracking, a constant load, sensitizing temperature, intergranular, transgranular

Intisari
Perubahan mekanisme korosi retak tegang material baja tahan karat austenitik AISI 304 telah dilakukan larutan magnesium klorida 42Wt% pada suhu 1060C dengan menggunakan metode beban statis. Sampel hasil anil diberi beban 30,5 kg/mm2 dan 40 kg/mm2. Sampel non-anil diberi beban 20 Kg/mm2 dan 25 kg/mm2. Semua pengujian dilakukan pada kondisi sirkuit terbuka. Hampir semua bentuk patahan material 304 non-anil di berbagai variasi temperatur sensitasi(6000C, 7000C, 8000C) adalah intergranular. Semua bentuk patahan material 304 anil di berbagai variasi suhu sensitasi adalah transgranular. Deformasi dapat mudah bergerak melalui pertumbuhan butir pada material 304 anil sehingga dapat diprediksi melalui penurunan nilai kekerasan vickers. Pada material 304 non-anil, deformasi dapat ditahan karena ukuran butir-butir kecil. Peranan mekanisme  martensit material 304 non-anil secara signifikan berkontribusi to memperparah pembentukan patahan di dalam larutan MgCl2 di suhu 1060C khususnya di batas butir. Pada material 304 anil, mekanisme patahan dihubungkan dengan deformasi melalui bidang slip di dalam butiran.

Kata kunci : korosi retak tegang, beban konstan, temperatur sensitasi, antar butir, transgranular.



1. Introduction
Generally, austenitic stainless steels are susceptible for Stress Corrosion Cracking (SCC). The SCC of them (type 316 and type 304) was extensively investigated as functions of applied stress (σ), sensitizing temperature, sensitizing time, applied potential and environmental factors ( inhibitor, sensitizing time, pH, anion concentration, anion species and test temperature) by using a constant load method [1]. The change in the mechanism for SCC on AISI 304 austenitic stainless steel was investigated in 42wt% boiling saturated magnesium chloride solution by using a constant load method. Three parameters (time to failure; tf steady-state elongation rate ; iss and transition time at which a linear increase in elongation to deviate; tss) obtained from the corrosion elongation curve showed three regions ; stress – dominated, stress corrosion cracking dominated and corrosion – dominated regions [2]. AISI 304 are prone to microstructural changes when exposed to sensitization temperatures due to heat treatment. Precipitation of chromium carbide takes place along the grain boundary regions in the temperature range of 4800C to 8150C. These results in chromium depletion near the grain boundary, and the resultant grain –boundary region is susceptible to intergranular corrosion (IGC)[3]. The extent of which depends upon the degree of sensitization. The Cr depletion zone, while the Cr carbide would serve as an barrier of dislocation movement [4]. Nishimura demonstrated that the most severe SCC susceptibility took place at a sensitizing temperature of ~931 K (6600C) in hydrochloric acid solution[5]. Deformation of metastable austenite phase involves the formation of strain –induced ε α’-martensite[6]. Austenitic stainless steel can undergo phase transformation due to applied stress or hydrogen charging.
The objectives of this paper are: (1) to investigate the effect of sensitizing temperature on the susceptibility of austenitic stainless steels to stress corrosion cracking, (2) to evaluate tf and iss for type AISI stainless steels in 42wt% magnesium chloride solutions at temperature of 380K, and (3) to determine the cracking mechanism for austenitic stainless steels in 42wt% boiling magnesium chloride solutions at temperature of 380K.

2. Experimental
The specimens used were the type 304 austenitic stainless steel whose chemical compositions (wt%) are shown in Table 1. As shown in Fig. 1, the geometry for stress corrosion cracking experiments is as follows: the gauge length is 25.6 mm, the width 5 mm and the thickness 1 mm[7]. The specimens were solution-annealed at 1373 K (11000C) for 1 hour under an argon atmosphere and the water-quenched. The other specimens were conducted solution annealing process. Both of specimens (with or without solutions annealing process) were annealed with various sensitizing temperatures (6000C, 7000C, 8000C). Prior to main experiments, the specimens (with and without solutions annealing process) were polished to 1000 grit emery paper, degreased with acetone in a ultrasonic cleaner apparatus and washed with distilled water. After that the specimens were set into SCC cell.

Table 1.Chemical Compositions (wt%) of the type 304 austenitic stainless steel used
C Si Mn P S Ni Cr Mo
0.042 0.713 1.13 0.0297 0.008 7,92 18.309 --




Fig.1. Geometry of SCC Specimens (dimensions in mm)

SCC test were conducted in 42wt % Magnesium Chloride solution at 379K (1060C). The solution-annealed specimens were applied under constant load of 30.5 Kg/mm2 and 40 kg/mm2. The non-solution annealed specimens were applied under constant load of 20 kg/mm2 and 25 kg/mm2. All experiments were done under an open circuit condition.




Fig 2. a lever-type constant load apparatus

As Shown in Fig 2. A lever-type constant load apparatus (lever ratio 1:10) to which three specimens can be separately and simultaneously attached was used with a water cooling system on the top of a testing cell to avoid evaporation of the solution during the experiments. The specimens were insulated from rod and grip by alumina fibers. Elongation of the specimens under a constant load was measured by an inductive linear transducer with an accuracy of ±0.01 mm. The other supporting tests are Vickers hardness and oxalic acid etch test for classification of etch structures of austenitic stainless steel according to ASTM A262-02a.


3. Results & Discussion
3.1. Dependence of sensitizing temperatures and hardness property

Table 2 shows the Vickers hardness of type 304 at various sensitizing temperatures. Generally, Vickers hardness values of the type 304 without solution annealing process are higher than without solution annealing. Fig 3 shows the behavior of Vickers hardness at various sensitizing temperatures. The type 304 without sensitizing process that has the highest hardness values of 232.27 Hv. This material undergoes the decreasing of hardness value when heated from 6000C to 8000C. The same behavior is happen on the type 304 with sensitizing process.



Table 2. Vickers hardness of type 304 at various sensitizing temperatures
Sensitizing temperature
(celcius) Vickers Hardness (Hv)
Solution annealing Without Solution annealing
600 219.33 209.3
700 152.06 199.1
800 162.72 205.33


Fig.3. Curve of relationship between Vickers hardness and sensitizing temperatures

Both solution or without solution annealing process undergo a decadent hardness drastically at 7000C after that increase at 8000C. The lowest hardness value is 152.06 Hv on type 304 with solution annealing process. Susceptibility to intergranular attack associated with the precipitation of chromium carbides is readily detected in oxalic acid of electrolytic etches. Metallographic etching based on ASTM A-262 was conducted. Photomicrographs were obtained by using optical microscope. The microstructures that acquired were classified into three types: Ditch structure with one or more grains surrounded fully, step structure without ditches at grain boundaries and dual structure with several ditches at grain boundaries.









Fig. 4. microstructure appearances for type 304 steel after solution annealing 1373K (a) no annealing for sensitizing condition (b) annealing for sensitization at 6000C, (c) annealing for sensitization at 7000C, (d) annealing for sensitization at 8000C

Fig. 4 shows the kind of microstructures that depends on the influence of sensitizing temperature. Full ditch structures appear that surround grains at 7000C and 8000C. When associating the Vickers hardness of annealed 304, α’-martensite phase does not exist both grains and grain boundaries.





Fig. 5. microstructure appearances for original type 304 steel (a) no annealing for sensitizing condition (b) no annealing for sensitization at 6000C, (c) no annealing for sensitization at 7000C, (d) no annealing for sensitization at 8000C

Fig. 5 shows the difference of microstructures among the kinds of annealing temperature. The transformation of  (austenite) become the phase combination between  - ’martensite that was appeared. The formation of strain-induced alpha'-martensite significantly affects the mechanical behavior austenitic stainless steels by enhancing work hardening[6]. This statement can be associated with the difference of Vickers hardness value between non-annealed 304 and annealed 304. For overall reason, the number of Vickers hardness of non-annealed 304 is higher than annealed 304. These can be preliminary indicated that non-annealed 304 is susceptible for cracking.

3.2 A parameter for predicting time to failure from Corrosion Elongation curve

3.2.1. Non-annealed type 304

Fig.6 shows the corrosion elongation curves for non-annealed type 304 steel at 1060C under a constant applied stress condition (L=30,5 kg/mm2) in magnesium chloride solution at various annealing temperatures. From these curves the three parameters were obtained for the each specimen: the time to failure (tf), the steady-state elongation rate (Iss) for the straight part of corrosion elongation curve and ratio of transition time to time to failure (tss/tf). These parameters are shown in Table 2.

Table 2. The variation of sensitizing temperatures, failure time, and elongation rate

No Sensitizing temperatures (0C) iss (mm/s) tss (s) tf (s)
1 600 3,00E-08 60428 86400
2 700 2,00E-06 18674 25450
3 800 7,00E-07 51388 55880

The lowest failure time (tf) at 7000C is 25450 seconds. The highest failure time at 6000C is 86400 seconds. The fastest steady-state elongation rate at 7000C is 2 x 10-6 mm/s. The slowest elongation rate at 6000C is 3 x 10-8 mm/s. The lowest failure time at 7000C can be identified as the critical cracking time if this is compared with mechanical property especially hardness value.



Fig. 6. Corrosion elongation curves for type 304 steel at 1060C under a constant applied stress condition (L= 30,5 Kg/mm2 ) in MgCl2 solutions.

Generally, the increasing of applied load from 30, 5 Kg/mm2 to 40 Kg/mm2 significantly can reduce time to failure at various sensitizing temperatures that are shown in Table 3.

Table 3. The variation of sensitizing temperatures, failure time, and elongation rate
Sensitizing temperature (oC) tf (s) Iss (mm/s) Tss (s)
600 31387 3,00E-07 30564
700 8348 4,00E-05 5990
800 45484 2,00E-07 42198


Fig 7 shows the differences among tf, iss, and tss. The highest steady-state elongation rate at 7000C is 4.10-5 mm/s but the lowest elongation rate at 2.10-7 mm/s. The fastest time to failure at 7000C is 5990 seconds, on the contrary, the slowest time to failure at 42198 seconds.



Fig. 7. Corrosion elongation curves for type 304 steel at 1060C under a constant applied stress condition (L= 40 kg/mm2 ) in MgCl2 solutions.

Both 30, 5 kg/mm2 and 40 kg/mm2 almost have the same tendency at 7000C related to time to failure. These loads could contribute to decrease tf. Nishimura et al demonstrated that the most severe SCC susceptibility took place at a sensitizing temperature of ~931 K (6600C) in hydrochloric acid solution. In Magnesium chloride solution, this phenomenon seldom has been observed in all kinds of previous scientific papers.

3.2.2. Annealed type 304

The commercial types 304 commonly have undergone work hardening. The transformation sequence during work hardening has been found to be  (austenite)--’martensite. In order to change to be full austenite phase, the heat treatment was conducted at 1373 K. The other reason why it was conducted is to minimize ’martensite phase as the one of initial cracking site.The Table 4 shows how three parameters can influence the mechanical behavior of annealed type 304 under a constant load at 25 kg/mm2 in 42 wt% MgCl2 solution.

Table 4. The variation of sensitizing temperatures, failure time, and elongation rate
Sensitizing temperature (oC) tf (s) Iss (mm/s) Tss (s)
600 53128 2,00E-06 48326
700 3 4,80E+00 -
800 8459 5,00E-05 4763

Fig 8(a),(b) shows corrosion elongation curves which explain the prediction of time to failure. The highest elongation rate at 7000C is 4,8 mm/s but the lowest elongation rate at 6000C is 5.10-5 mm/s. The shortest time to failure at 7000C is 3 seconds; on the contrary, the longest time to failure at 6000C is 53128 seconds.





Fig. 8. Corrosion elongation curves for annealed type 304 steel at 1060C under a constant applied stress condition (L= 25 Kg/mm2 ) in MgCl2 solutions.

The critical sensitizing temperature of annealed type 304 is the same behavior as non-annealed type 304 at 7000C (973 K). Both of them have a tendency to decrease mechanical property and to be susceptible for cracking. The table 5 shows the relationship among three parameters in order to predict the behavior of annealed type 304 at 20 kg/mm2

Table 5. The variation of sensitizing temperatures, failure time, and elongation rate
Sensitizing temperature (oC) tf (s) Iss (mm/s) Tss (s)
600 45103 9,00E-07 39005
700 5 2,25E+00 0
800 42006 4,00E-07 36625

Fig 9(a),(b) show corrosion elongation curves which explain three the prediction of time to failure. The highest elongation rate at 7000C is 2,25 mm/s but the lowest elongation rate at 6000C is 4.10-5 mm/s. The shortest time to failure at 7000C is 5 seconds; on the contrary, the longest time to failure at 6000C is 45103 seconds.



Fig. 9. Corrosion elongation curves for annealed type 304 steel at 1060C under a constant applied stress condition (L= 20 Kg/ mm2 ) in MgCl2 solutions.

Although a constant load was increased from 20 kg/mm2 to 25 kg/mm2. The behavior of time to failure value at 7000C is the same. At 7000C, the reason why time to failure values at annealed type 304 are faster than non-annealed one is the existence of ’ martensite and chromium carbide.

3.3 Fracture appearance

The scanning electron microscopy photographs for the fracture surface appearance for the austenitic stainless steels in magnesium chloride solution at 1060C were investigated. Fig. 10 and 11 show the fracture surface appearances of type non-annealed 304. They showed that the cracking mode for non-annealed type 304 were transgranular and intergranular.




Fig. 10. The fracture surface on type 304 at L = 30,5 Kg/mm2 (a) Transgranular cracking for non-annealed type 304 at T = 6000C,(b) intergranular cracking for non-annealed type 304 at T = 7000C, and (c) intergranular for non-annealed type 304 at T = 8000C.








Fig. 11. The fracture surface on type 304 at L = 40 Kg/mm2 (a) Transgranular cracking for non-annealed type 304 at T = 6000C,(b) intergranular cracking for non-annealed type 304 at T = 7000C, and (c) intergranular for non-annealed type 304 at T = 8000C.


At 6000C (873 K) non-annealed type 304 appears transgranular crack. This cracking direction tears along grains. Many voids were distributed on the fracture surface appearance. These voids were an initial void coalescence to make dimples. At 7000C and 8000C both of them have the same cracking mode. The shape of fracture mode is intergranular crack. This fracture mode can be explained by the existence of carbide chromium approach and ’ martensite in grain boundaries. The existence of carbide chromium can create the weakness region in grain boundaries. The concentration of chromium near grain boundaries is lower than its concentration in grains. The theory of dissolution in depleted chromium zone can be associated by appearing many voids in near grain boundaries. The appearance of ’ martensite significantly could be considering to create these kinds of fracture. Nishimura et.al said that for metastable austenitic steels like type 304, the strain induced martensite along the grain boundaries will enhance the hydrogen permeation. Martensite structure has a very high diffusity coefficient and very small hydrogen content compared to those of austenite steel. This behavior can be explained by the material’s high content chromium like type 304 that is a natural inhabitant of martensite transformation. This reason can be reference why type 304 is susceptible to Stress Corrosion Cracking (SCC)
The scanning electron microscopy photographs for the fracture surface appearance for the austenitic stainless steels in magnesium chloride solution at 1060C were investigated in a constant load (20 kg/mm2, 25 kg/mm2). Fig. 12 and 13 show the fracture surface appearances of type non-annealed 304. They showed that the cracking mode for annealed type 304 was transgranular at various sensitizing temperatures. This type of fracture occurs through or across a crystal or grain. The approach of mechanism of trangranular fracture can be correlated to void coalescence Once the void forms, the connecting material continues to deform by slip, allowing the voids to expand until they begin to connect. This fracture mechanism develops a fracture topography consisting of cups an each fracture surface. a such surface is called dimple.






Fig. 12. The fracture surface on type 304 at L = 25 Kg/mm2 (a) Transgranular cracking for annealed type 304 at T = 6000C,(b) intergranular cracking for annealed type 304 at T = 7000C, and (c) transgranular for annealed type 304 at T = 8000C.






Fig. 13. The fracture surface on type 304 at L = 20 Kg/mm2 (a) Transgranular cracking for annealed type 304 at T = 6000C,(b) intergranular cracking for annealed type 304 at T = 7000C, and (c) transgranular for annealed type 304 at T = 8000C.

The type of dimple in all fracture surfaces at various sensitizing temperatures and a constant load (L = 20 Kg/mm2 & 25 Kg/mm2) is equiaxed dimple. in tension, equiaxed dimples are formed on both fracture surfaces. The size of grains on annealed type 304 is bigger than non-annealed one. This main reason can be associated to hardness value. Over 25 Kg/mm2 a constant load test was failed to hold annealed type 304 from rapid fracture because of above its yield stress. The deformation of materials will be easy to move if the size of their grains is large. The existence of void in grains makes material suffer to contribute the deformation by moving through slip plane.


4. Conclusion
The following conclusions can be drawn from this work:

1. Using a constant load method, the effect of sensitizing temperature on stress corrosion cracking of austenitic stainless steels can be evaluated. The most of fracture shape in non-annealed type 304 at various sensitizing temperature is intergranular. All fracture shape in annealed type 304 at various sensitizing temperature is trangranular.
2. The role of  martensite mechanism in non-annealed type 304 significantly contribute to suffer fracture in MgCl2 solution at 1060C especially in grain boundaries. In annealed type 304, mechanism of fracture can be associated by deformation through slip plane in grains.
3. The deformation can be easy to move by growing grains in annealed type 304 so that it can be predicted by decreasing Vickers hardness value. In non-annealed type 304, the deformation can be sustained because of the small size of grains.
4. The role of carbide metal forming can be considered to suffer the fracture of material.
5. At 7000C, the reason why time to failure (tf) at annealed type 304 are faster than non-annealed is the existence of ’ martensite and chromium carbide

5. References
[1] Rokuro Nishimura, Characterization and perspective of stress corrosion cracking of austenitic stainless steels (type 304 and type 316) in acid solutions using constant load method, Corrosion Science 49 (2007) 81–91
[2] O.Alyousif, R.Nishimura, The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions, Corrosion Science 49 (2007) 3040–3051
[3] Raghuvir Singh; B Ravikumar; A Kumar; P K Dey; I Chattoraj, The effects of cold working on sensitization and intergranular corrosion behaviour of AISI 304 Stainless Steel, Metallurgical and Materials Transactions; Nov 2003; 34A, 11; Academic Research Library pg. 2441
[4] G.E.Dieter, Mechanical Metallurgy. 2nd ed, McGraw-Hill 1976 p.195
[5] R Nishimura; I Katim; Y Maeda, Stress corrosion cracking of sensitized type 304 stainless steel in hydrochloric acid solutions- Predicting Time to failure and Effect of sensitizing Temperature, Corrosion; Oct 2001; 57, 10; ProQuest Science Journals.pg. 853
[6] Juho Talonen, Metallurgical and Materials Transactions; Feb 2005; 36A, 2; Academic Research Library pg. 421
[7] O,Alyousif,R.Nishimura, Corrosion Science 49 (2007) pg.3041