Investigation of aging heat treatment on microstructure and mechanical properties of 316L austenitic stainless steel weld metal

Transcription

1 Computational Methods and Experiments in Material Characterisation II 63 Investigation of aging heat treatment on microstructure and mechanical properties of 316L austenitic stainless steel weld metal A. Amari Allahyari 1, H. Farhangi 1 & S. M. M. Hadavi 2 1 Department of Metallurgy and Materials Engineering, University of Tehran, Iran 2 School of Mining and Metallurgy, University of Amir Kabir, Iran Abstract Austenitic stainless steel alloys are used extensively in heat resistant structural components in power generating and chemical industries. Austenitic stainless steel weld metal exhibits a duplex microstructure consisting of a dispersion of high temperature delta-ferrite in austenite. It has been shown that aging between C results in progressive dissolution of the delta-ferrite and precipitation of M 23 C 6 carbides and intermetallic phases. In this paper, the effects of aging at temperatures between C for periods of up to hours on the microstructure and mechanical properties of 316L austenitic stainless steel weld metal have been investigated. Austenitic stainless steel 316L plates of 20 mm thickness were welded using the gas tungsten arc welding technique. Tensile and impact test specimens were machined from the weld sections and subjected to various aging heat treatments. Based on the results of mechnical testing it was found that whereas the strength properties show comparatively small variations due to aging, the tensile ductility and impact toughness are strongly affected by increasing aging temperature and time, such that aging at 850 C for 5 hour results in a 90% reduction in the value of impact energy of the weld metal. The loss of impact toughness and tensile ductility has been discussed based on the dissolution of delta-ferrite and precipitation of the intermetallic sigma phase. Keywords: aging, 316L weld metal, impact energy, ductility, delta-ferrite-sigma phase.

2 64 Computational Methods and Experiments in Material Characterisation II 1 Introduction Austenitic stainless steel alloys are used extensively in heat resistant structural components in power generating and chemical industries because of their excellent corrosion resistance and good fabricability and adequate high temperature mechanical properties [1]. During welding of austenitic stainless steels, high residual stresses can be induced in the weldment. These stresses arise from the low thermal conductivity and high coefficient of thermal expansion of these steels. The stresses can approach and may even exceed the proof stress and in thick sections of the weldments (> 12 mm) can extend up to 50 mm from the fusion line into the parent material [2]. Austenitic stainless steel weld metal exhibits a duplex microstructure consisting of a dispersion of high temperature delta-ferrite in austenite. Depending on the composition of the weld a small amount of delta-ferrite ranging from 3-9% is retained in the weld metal to overcome the problem of hot cracking. Delta-ferrite acts as a strengthener at room temperature, so that the weld metal tensile strength properties are equal to those of the parent metal in a 7-8% cold worked condition [3,4]. However, despite its beneficial effects, the presence of delta-ferrite leads to enhanced attack of the weld metal in some corrosive conditions. Furthermore, the deltaferrite is inherently unstable and can transform during high temperature exposure in service or during post weld heat treatment [5,6]. Therefore, the transformation kinetics of delta-ferrite and its subsequent effect on mechanical properties and corrosion resistance have been the subject of extensive studies [7]. It has been shown that aging between 500 and 700 C results in progressive dissolution of the delta-ferrite and precipitation of M 23 C 6 carbides and intermetallic phases. At lower aging temperatures (i.e. 500 C), transformation of delta-ferrite is very slow and the main transformation products following shortterm aging of up to hours are the M 23 C 6 carbides. At higher aging temperatures, upon full transformation of delta-ferrite, sigma phase is observed to grow rapidly at the expense of carbides and austenite [5, 8, and 9]. The effects of aging between C on room temperature tensile properties of 316L stainless steel weld metal has been extensively studied and reported. However, the impact properties of the aged 316L weld metal have not been thoroughly investigated. In the present work, the effects of aging between C on room temperature tensile and impact properties are studied. The changes in ductility and impact energy as a result of aging are correlated with dissolution of delta-ferrite and precipitation of sigma phase [9,10,11]. 2 Experimental procedure Using the gas tungsten arc welding process, type 316L stainless steel filler wire were deposited in a weld geometry and described by ISO2560. The steel plates used were 20 mm thick 316L stainless steel dimensioned 0x110 mm. the chemical composition of the weld metal and the welding parameters are given in Table 1 and Table 2, respectively. All welded sections were subjected to nondestructive examinations including ultrasonic and radiographic testing for quality control prior to sample preparation for mechanical testing.

3 Computational Methods and Experiments in Material Characterisation II 65 Round tensile specimens and standard charpy impact test specimens were machined from the welded sections. The weld metal tensile specimens and charpy impact specimens were aged at 550 C, 750 C and 850 C, for various durations ranging from 1 to hours. The ferrite content in the weld metal was measured before and after aging using a ferritoscope, which was calibrated with magnetic probe against National Bureau of Standard set of standard samples, in accordance with the procedure laid down in AWS A Micro structural observations were carried out using optical and scanning electron microscopy, after etching the weld metals in Murakami's reagent. Chemical composition of constituent intermetallic phases in aged samples was analyzed using EDS. Tensile tests were carried out at a nominal strain rate of 5x10-4 s -1 at room temperature on the weld metal and the aged weld metals. Values for yield stress (YS), ultimate tensile strength (UTS), and percent elongation were determined from the load-elongation curves. The average Impact energy of three specimens is reported for each weld metal condition. Charpy impact tests were performed at room temperature. Table 1: Chemical composition of 316L austenitic stainless steel weld metal. Element C Cr Ni Mo Mn Co Al P Cu Weight percent Table 2: Welding parameters. Welding Process Welding Wire Shielding Gas Tungsten inert gas AISI 316L Ar 99.99% Tungsten electrode diameter Welding Wire diameter Polarity 3.2 mm 2.4 mm DCEN Welding current Argon flow rate Welding speed 130 A 8.5 l min -1 1cm min -1 3 Results and discussions 3.1 Tensile properties The percentage changes in strength properties, UTS as a function of aging time at aging temperatures of 550, 750 and 850 C are shown in Fig. 1. Aging at 550 C results in a small increase of YS. However, the percentage changes in UTS with aging time for various aging temperatures can be evaluated from Fig.1. In general, aging causes a small increase in UTS at all aging temperatures

4 66 Computational Methods and Experiments in Material Characterisation II investigated. The increase in UTS reaches its maximum value after 5 hours of aging at 850 C. At longer aging times, the increase in UTS drops gradually to lower values. The percentage changes in reduction area, i.e., percent elongation, with aging time at various aging temperatures are shown in Fig. 2. The reduction area of the weld metal is considerably reduced at aging temperatures investigated. The reduction is normally larger at higher aging temperatures and times. However, the curves for changes in ductility at aging temperatures of 750 and 850 C both exhibit maximum points. The time required to reach the maximum point decreases from 25 to 5 hours with increasing aging temperature. Change in UTS (%) (%) Aging Time (h) Figure 1: Change in UTS as a function of aging time. 90 Change in reduction area (%) Aging time (h) Figure 2: Change in reduction area as a function of aging time.

5 Computational Methods and Experiments in Material Characterisation II Impact energy The percentage changes in impact toughness with aging time at different aging temperatures are plotted in Fig. 3. The impact toughness of the weld metal is also significantly reduced at test temperatures studied. The reduction in impact toughness is larger at higher aging temperatures and times. Change in impact energy and delta-ferrite transformation (%) TR to ferrite 750-TR to ferrite 850- TR to ferrite Aging Time (H) Figure 3: Change in impact energy and transformation of delta-ferrite as a function of aging time. At the aging temperature of 550 C, the impact energy of the weld metal drops by 20% after hours of aging. At 750 C, aging for 1 hour results in a 30% reduction in impact energy. The impact energy then decreases rapidly as aging time is increased to 5 hours. Further increase in aging time results in a gradual decrease of impact energy and shows a 90% reduction after hours of aging. The most adverse effect of aging on impact toughness is observed at the aging temperature of 850 C. The impact energy drops by 90% only after 1 hour of aging and remains constant at that level at longer aging times. 3.3 Microstructure and dissolution of delta-ferrite The microstructure of 316L austenitic stainless steel weld metal is shown in Fig. 4. Dual phase structure of the weld metal consists of white austenite matrix and dark delta-ferrite as a secondary phase. The average delta-ferrite content of the weld metal was found to be 7.5 FN, as measured by a ferritoscope. The ferrite content before and after aging at various temperatures was converted to the fraction of ferrite transformed. The percentage changes of ferrite transformed as a function of aging time for various aging temperatures are shown in Fig.4. It is clear that the dissolution of delta-ferrite at 550 C was sluggish and reached only about 20% after hours of aging. The transformation rate of delta-ferrite

6 68 Computational Methods and Experiments in Material Characterisation II 30 µm Figure 4: An optical micrograph showing the duplex microstructure of the weld metal. Figure 5: A SEM micrograph showing sigma phase particles in the microstructure of the weld metal aged at 750 o C for 25 hours. increases with temperature. The time required for 90% dissolution is about 1 hour and 5 hours, respectively, for 750 and 850 C. Optical and scanning electron microscopy observations showed no major modifications in the microstructure of the weld metal due to aging at 550 C. The only changes detected were limited to minor alterations in the morphology of the delta-ferrite. At higher aging temperatures of 750 and 850 C, a continuous network of sigma phase was found to form at lower aging times (Fig. 5). Prolonged aging at these temperatures led to spherodization of sigma phase as shown The progress of spherodization by necking of elongated, unstable sigma

7 Computational Methods and Experiments in Material Characterisation II 69 (a) (b) Figure 6: SEM fractograph of (a) weld metal showing a typical ductile mode of failure and (b) weld metal aged at 850 o C for hours revealing a brittle mode of failure. particles and the formation of smaller particles of equilibrium shapes can be observed in this micrograph. Energy dispersive x-ray spectrometry of spheroids white phases that are enriched from Fe, Cr and Mo and it shows that sigma [Fe-Cr-Mo] intermetallic phase has been appeared. 3.4 Fractography The fracture surface of the weld metal is shown in the scanning electron micrograph in Fig.6a. The fracture surface consists of microscopic dimples and

8 70 Computational Methods and Experiments in Material Characterisation II reveals a ductile mode of failure by micro void joining mechanism. A similar behavior was observed for the specimens aged at 550 C. The fracture surface of the weld metal aged at 850 C for hours is shown in Fig. 6b. This fractograph reveals a completely brittle mode of failure by cleavage. Cleaved sigma phase particles and extensive secondary cracking along sigma phase-matrix boundaries are also evident in this micrograph. All specimens aged at 850 C and specimens aged at 750 C for times between 5 to hours exhibited similar brittle fracture behavior. As mentioned in fig 8, it can be seen that with increasing aging temperature from 550 to 750 and 850 C, mode of failure alter from ductile fracture to completely brittle and in the other word, with increasing aging time up to hours, it was seen similar treatment. With comparison of figure 7 and 8, it is obviously observed that there is good correlation between the mode of fracture and delta-ferrite transformation Change of ductile fracture(%) Aging time (%) Aging temprature Figure 7: Change of ductile fracture as function of aging time and temperature. 4 Effects of aging Based upon the data presented it is clear that the effect of short term aging for periods of up to hours on tensile and impact properties become more significant with increasing aging temperature. At 550 C, where the rate of transformation of delta-ferrite is slow and the only transformation products are the M 23 C 6 carbides [4]. The change in tensile ductility and impact properties are

9 Computational Methods and Experiments in Material Characterisation II 71 comparatively smaller. However, at aging temperatures of 750 and 850 C, where the rate of transformation of delta-ferrite is much faster, and the main transformation product is the brittle intermetallic sigma phase, tensile ductility and impact properties are significantly reduced. Precipitation of sigma phase at these aging temperatures results in a corresponding change in the fracture mechanism from a ductile void coalescence mechanism to a brittle cleavage fracture mechanism. A good correlation between the amount of transformed delta-ferrite and the changes in impact energy can be observed for aging temperature of 750 and 850 C. The spherodization of sigma phase dose not appear to retard the steep fall in impact energy. However, it dose seem to temporarily arrest and even partly compensate the loss in tensile ductility Delta-ferrite transformation (%) Aging temprature 5 Aging time (h) Figure 8: Change of Delta-ferrite transformation as function of aging time and temperature. This effect can be deduced from the occurrence of maximum points on the change in ductility curves at higher aging temperatures. In as much as the progress of spherodization is faster at higher aging temperatures, the maximum point on the change in ductility curve is displaced to lower aging times, accordingly. 5 Conclusion 1- The rate of dissolution of delta-ferrite increases with increasing aging time and temperature. 2- Tensile ductility and impact properties are most adversely affected by aging at 750 and 850 C, as a result of precipitation of sigma phase.

10 72 Computational Methods and Experiments in Material Characterisation II 3- The changes in impact properties due to aging at 750 and 850 C can be correlated with the amount of transformed delta-ferrite. 4- It is obviously observed that there is good correlation between the mode of fracture and delta-ferrite transformation and with increasing delta-ferrite transformation to sigma phase, mode of fracture alter from ductile to completely brittle. 6 Nomenclature A YS UTS amperage, A yield stress, MPa ultimate tensile strength, MPa References [1] W.J. Mills, Fracture toughness of type 304 and 316 Stainless Steels and their welds, International Materials Reviews, 42, pp , [2] J.J. Smith, R.A. Farrar, Influence of microstructure and composition on mechanical properties of some AISI 300 series weld metals, International Materials Reviews, 33(1), pp.25 51, [3] R.A. Farrar, C. Huelin, Phase transformation and impact properties of type austenitic weld metals, Journal of Materials Science, 20, pp , [4] T.P.S. Gill, On Microstructure-property correlation of Thermally Aged type 316L Stainless Steel weld Metal, Met. Trans., 20A, pp , [5] H. Shaikh, T.V. Vinoy, Correlation of microstructure and tensile properties of 316 stainless steel weld metal solution annealed at high temperatures, Material Science and Technology, 14, pp , [6] G.F. Slattery, Microstructural transformations in stress relived type 316 Stainless Steel weld Metal, Metallography, 13, pp , [7] K. Yutaka,Preferential Precipitation site of sigma Phase in Duplex stainless steel weld metal, Scripta Materiala, 40(6), pp , [8] R. A. Farrar, Microstructure and phase transformations in duplex 316 submerged arc weld metal, an aging study at 700 C, Journal of Materials Science, 20, pp , [9] C.E. Lyman, Analytical Electron Microscopy of stainless steel weld metal, Welding research supplement, pp , [10] J.K. Lai, Delta-ferrite Transformations in a type 316 weld metal, Welding Research Supplement, January, pp. 1 6, [11] B. Weiss, Phase instabilities during high temperature exposure of 316 Austenitic Stainless Steel, Met. Trans, Vol.3, pp , 1972.