Effect of cold rolling on the recrystallization of 904L austenitic stainless steel.
Lyacine Rabahi (a)*, Brahim Mehdi (b), Idir Hadji (a), Riad Badji (a), Nabil Kherrouba (a) (a) Research Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga, 16014, Algeria
(b) Faculty of Physics, University of Sciences and Technology Houari Boumediene, BP 32 El-Alia, 16111 Algiers, Algeria
l.rabahi@crti.dz
Abstract
The aim of this work is to study the recrystallization in the 904L austenitic stainless steel, which underwent a cold rolling with two different deformation amounts (50 and 90%). DSC tests have been conducted to determine the temperature range of the recrystallization. It was found that reducing the deformation amount slightly delays the recrystallization. The microstructure of the as-received alloy consists of austenitic grains recognized by the twins they contain. The deformation induced a corrugated microstructure with the presence of ribs. The XRD analysis confirmed that only γ phase peaks were present in the as-received alloy and showed that the deformation induced the extinction of γ phase peaks and the apparition of α phase peaks.
Keywords: recrystallization, 904L, cold rolling, DSC.
1. Introduction
Austenitic stainless steels form an interesting family of alloys that combine greater corrosion resistance and higher mechanical properties from cryogenic to higher temperatures [1-2]. Compared to standard austenitic stainless steels, superaustenitic stainless steels, such as 904L alloy, have a similar microstructure, but a higher content of molybdenum that substantially improves its physical properties [1-2]. 904L is widely employed in marine, petrochemical and nuclear industries owing to its excellent strength and corrosion resistance even in very aggressive environment [2-3]. Its high Molybdenum content gives it greater resistance to chloride stress corrosion cracking, while its low carbon content makes it more resistant to sensitization by welding which prevents inter-granular corrosion [4].
It is well known that physical properties are very sensitive to the thermomechanical treatments underwent by materials. Among these treatments, the cold rolling results in hardened structure, accompanied with a noticeable increase in dislocation density. However, a subsequent heat treatment of such structure, leads to a recrystallization phenomenon, that improve the mechanical properties, through a microstructure refinement and particularly the
dislocation density reduction/ lowering. In the present work, effect of the deformation amount on the recrystallization is investigated particularly in terms of evolution of start temperature of this phenomenon and the microstructural evolutions.
2. Material and Experimental Procedure
Austenitic stainless steel 904L is provided as hot rolled and annealed sheets of 1.5 mm thickness, with the chemical composition presented in Table 1. The alloy underwent plastic deformations by cold rolling with 50% 90% thickness reduction. The microstructural analysis was carried out by a FEG-SEM ZEISS Gemini, operating at 15 KeV. The X-ray diffraction XRD patterns were recorded from the rolled and polished surface samples using BRUKERS D2 X-Ray Diffractometer operating at 30 kV, 30 mA with Co Kα radiation (λ=1.78A). All the diffraction patterns were obtained by varying 2θ from 20° to 120° with a scan step of 0.02.
The time spent for collecting the data per step was 5 s. The Rietveld refinement base software MAUD was used to perform the XRD analysis.
Table 1: Chemical composition of as received 904L sheets.
Fe P Pb S C Cr Ti Si Mn Mo
Min Max Moy
/ / 48.1
0.04 0.09 0.037
0.15 0.35 0.007
0.26 0.35 0.006
0.00 0.15
<0.005 / / 19.5
/ /
<0.005 / / 0.409
0.850 1.15 1.37
/ / 4.22
The DSC experiments were performed applying an empty reference crucible in a Setaram Setsys evolution DSC apparatus. During DSC test, the simples underwent the following thermal cycle:
• Heating to 1100°C with a heating rate of 20°C/min.
• Isothermal holding at 1100°C for 20 min.
• Cooling
3. Results and Discussion a) Microstructural analysis
The microstructure of the as received 904L alloy is shown in Figure 1-a. The microstructure consists of a matrix γ phase, with equiaxed grains randomly distributed, without any preferential orientation. The deformation therefore, as indicated in the figure 1-b induced twins disappearance and a preferential orientation of the grains along the rolling direction.
Figure 1: SEI Micrographs of the : a) as received 904L, b) and c) cold worked 904L with 50%, and 90% thickness reduction respectively.
b) X-ray Diffraction analysis
Figures 2 displays the XRD patterns obtained from the as received 904L austenitic (γ- Fcc) alloy with the presence of the characteristic diffraction peaks associated to this phase.
The phase identification by MAUD software, confirmed that, only the γ austenitic phase is present. In order to investigate the cold rolling effects, XRD patterns are also reported for the studied alloy after 50 and 90 % thickness reduction, as shown in Figures 3.
a) b)
c)
RL
Figure 2 : XRD patterns obtained from Rietveld profile fitting of : a) the as received, b) cold worked 90d L with 50 % and 90% thickness reduction
It should be noticed that, green and red colors refer to the 50% and 90% of thickness reduction respectively. As it is seen, the deformation induced peaks extinction of the γ phase and peaks apparition of the α phase of the 904L alloy. Moreover, the intensity/width of many existing peaks is also found to be increased by deformation. This fact reflects the strong correspondence between the deformation and texture of the studied alloys.
c) DSC measurements
The evolution of the processed heat flow curves with temperature for the 904L alloy, cold worked with 50 % and 90% thickness is presented in Figure 3 below.
Figure 3: DSC cooling curves of the 904L alloy cold worked with 50% and 90% thickness reduction.
1165 1170 1175 1180
0,0 0,2 0,4 0,6 0,8
Heat flow (W/g)
Temperature (°C)
50%
90%
a) b)
The picks appearing on the figure reflects the exothermic character of the recrystallization reaction. Temperatures Tr and Tf are corresponding to the start and the end of the recrystallization respectively, are estimated from the deviation of the DSC curve with the baseline.
As it can be seen, the temperature of the recrystallization becomes smaller with increasing deformation amount, also, the Tr and Tf temperatures are shifted toward lower temperatures, while the maximal intensity corresponding to the 90 % is found to be higher than that of 50 % deformation amount.
Conclusion
In this work, cold worked deformation effects on the recrystallization of the 904L stainless steel are investigated and the main results can be summarized as follow:
- Microstructural analysis indicated that the cold rolling causes twins disappearance preferential orientation of the grains along the rolling direction.
- X-ray diffraction analysis revealed a decrease in the size of the coherent domain of diffraction as a function of the deformation, as well as an increase of the dislocation density. Micro-hardness measurements performed on the two steels revealed a strong correlation between deformation and the local mechanical characteristics.
- Interestingly, 904L alloy is harder than the 316Ti one, owing to the relatively high concentration of copper and molybdenum present in the 904 L.
References
[1] C. W. Kovach, High-Performance stainless steels, Technical Marketing Ressources, Inc, Pittsburgh, PA.
[2] M. F. Mc Guire, Austenitic Stainless Steels, Encyclopedia of Materials : Science and Technology 2nd Edition (2001), 406-410, https://doi.org/10,1016/B0-08-043152-6/00081-4.
[3] F. Tehovnik, B. Zuzek, B. Arh, J. Burja, B. Podgornik, HOT ROLLING OF THE SUPERAUSTENITIC STAINLESS STEEL AISI 904L, MTAEC9, 48(1)137(2014).
[4] P.L. Andresen, Stress corrosion cracking (SCC) of austenitic stainless steels in high temperature light water reactor (LWR) environments, Understanding and Mitigating Ageing in Nuclear Power Plants, (2010), 236-307.
