1、EFFECT OF COOLING RATE ON THE MICROSTRUCTURE AND PROPERTIES OF DUPLEX STAINLESS STEEL WELDSSTP-PT-088STP-PT-088 EFFECT OF COOLING RATE ON THE MICROSTRUCTURE AND PROPERTIES OF DUPLEX STAINLESS STEEL WELDS Prepared by: M. Boring Kiefner however, there was no analysis of the microstructure included in
2、the report. Cao and Hertzman also performed limited research on samples that were subjected to a second thermal cycle to simulate the reheating effect from a subsequent weld pass on the HAZ of the original pass in a multipass weld. The samples were subjected to an original thermal cycle including he
3、ating the sample to 1350 C, holding for 5 seconds and then cooling to room temperature at either 160 C/sec or 430 C/sec followed by a second thermal cycle with a peak temperature of either 1050, 900, 800, or 700 C and allowed to cool after no hold time at a cooling rate much slower than the original
4、 thermal cycle. The results of the reheating study showed toughness properties could be recovered as a result of exposing the original HAZ to a second heating cycle. The extent of the recovery was related to the peak temperature of the second thermal cycle. The toughness improvement was credited to
5、the formation of additional austenite during the reheating cycle. Menon et al. studied two different stainless steel alloys of Ferralium 255 which both had a PREN of 37.9.3 The samples were subjected to a peak temperature of 1300 C, two different peak temperature exposure times (1 second and 10 seco
6、nds) and four different cooling rates (2 C/sec, 20 C/sec, 50 C/sec, and 75 C/sec). The range of cooling rates were considered representative of the range of cooling rates that would be experienced in the HAZ of low heat input welds deposited using the shielded metal arc welding (SMAW) process and a
7、high heat input weld deposited using the submerged arc welding (SAW) process. The resulting Ferrite Number (FN) ranged from 91 for a cooling rate of 2 C/sec up to 112 C/sec for a cooling rate of 75 C/sec. All the samples were full sized CVN samples (10 x 10 mm) and tested at a six different test tem
8、peratures (-100, -60 C, -40 C, -20 C, 20 C, and 100 C). Both duplex stainless steels exhibited a similar decrease in toughness as the cooling rates increased for samples tested at the same test temperature. The study related the toughness over the range of cooling rates as a function of the ferrite
9、to austenite balance, prior grain size and degree of precipitation. The results showed that the faster cooling rates resulted in higher ferrite content and smaller grains. Lippold et al. evaluated the microstructure and toughness of the HAZ of Uranus 52N which had a PREN of 41.3.4 The samples were s
10、ubjected to a peak temperature of 1350 C, a peak temperature exposure time of 1 second and two different cooling rates (20 C/sec and 50 C/sec). The full size CVN samples (10 mm by 10 mm) were tested at -20 C. The results of the toughness tests did not show a large variation over the relative small r
11、ange of cooling rates evaluated. The authors suggested that this response would be expected given the small variation between the two microstructures which were 78 FN and 85 FN for the corresponding cooling rates of 20 C/sec and 50 C/sec, respectively. The toughness values of the simulated HAZs (141
12、 Joules and 159 Joules) were much less than the recorded toughness of the unaffected base metal which was recorded to be 243 Joules. Two papers compared cooling rates to toughness data relevant to SAF 2205.3,5 The two SAF 2205 duplex stainless steels tested had a PREN of 32.8 and 33.5. The samples w
13、ere subjected to a peak temperature of 1300 C or 1370 C and, where reported, peak temperature exposure times were either 1 second or 10 seconds. The cooling conditions were reported as t12-8, t8-5 or actual cooling rate. The CVN sample sizes were either full size samples (10 mm by 10 mm) or near two
14、-thirds size samples (6 mm by 10 mm). The range of eight testing temperatures was from -110 C to 20 C. STP-PT-088: Effect of Cooling Rate on the Microstructure and Properties of Duplex Stainless Steel Welds 4 Menon et al. reported no significant difference in transition temperature of SAF 2205 betwe
15、en the two different cooling rates of 20 C/sec and 50 C/sec at the test temperatures evaluated.3 These cooling rates resulted in a HAZ microstructure with 97 and 104 FN, respectively. The toughness was found to be a function of ferrite to austenite balance and prior ferrite grain size. In agreement
16、with Menon et al., Kivineva et al. showed that the highest cooling rates resulted in the lowest toughness values at all test temperatures.5 Kivineva et al. suggested, based on their results, that 30 seconds was an optimum t12-8 for SAF 2205 which represents a cooling rate of 13.3 C/sec. Three papers
17、 discussed the cooling rate effect on SAF 2507.4,5,6 The duplex stainless steels evaluated had a range of PREN from 40.8 to 42.3. The samples were subjected to a peak temperature between of 1350 C and 1400 C and, where reported, the peak temperature exposure time was 1 second. The cooling conditions
18、 were reported as t12-8, t8-5, or actual cooling rate. The CVN samples sizes were either near two-thirds size samples (6 mm by 10 mm) or near half size samples (4.5 mm by 10 mm). There was a large range of test temperatures from -196 C to 20 C. For the SAF 2507 duplex stainless steels, two papers sh
19、owed that the t12-8 did not have a significant effect on the resulting toughness.4,6 Kivineva et al. showed that the highest cooling rates resulted in the lowest toughness values at all test temperatures and the metallurgical analysis of the highest cooling rate samples showed the highest ferrite co
20、ntent and some chromium nitride precipitation in the ferrite. Slower cooling rates resulted in poorer toughness at low test temperatures but the toughness values increased to be comparable to base metal toughness values at higher test temperatures. The toughness reduction at the lower test temperatu
21、res for the slower cooling rate samples was related to the larger grain size present in the slowly cooled samples. Kivineva et al. proposed that 15 seconds was an optimum t12-8 for SAF 2205 which represents a cooling rate of 26.7 C/sec. The data reported from the cooling rate effect on toughness por
22、tion of the literature review are summarized in Table 2-1 through Table 2-3. Table 2-1 provides the chemical composition of the different duplex stainless steels organized by increasing PREN. Table 2-2 lists the thermal simulation conditions that were used to simulate the HAZ microstructure. The met
23、allurgical analysis of the simulated HAZ microstructures is included in Table 2-3. STP-PT-088: Effect of Cooling Rate on the Microstructure and Properties of Duplex Stainless Steel Welds 5 Table 2-1: Duplex Stainless Steel Chemical Compositions from Previous Research Duplex Stainless Steel Chemistry
24、 Alloy Thickness Cr Ni Mo Mn Si Cu C N S P PREN SAF 2205 10 mm 21.75 5.82 2.73 1.74 0.48 - 0.019 0.13 0.002 0.021 32.8 NR 22.12 5.73 2.8 1.78 0.2 - 0.022 0.13 0.002 0.028 33.4 6 mm 22.4 5.6 2.9 1.49 0.48 0.23 0.019 0.155 0.001 0.021 33.5 22Cr12N 5 mm 22.01 5.63 3.05 1.51 0.42 - 0.024 0.12 0.001 0.02
25、4 34.0 22Cr18N 5 mm 22.03 5.71 3.16 1.5 0.4 - 0.021 0.18 0.001 0.024 35.3 25Cr17N 5 mm 25.1 5.8 3 1.87 0.45 - 0.026 0.17 0.003 0.01 37.7 Ferralium 255 10 mm 24.9 5.39 3.13 1.05 0.51 1.72 0.027 0.17 0.001 0.023 37.9 SAF 2507 4.5 mm 24.6 6.9 3.8 0.31 0.3 0.15 0.024 0.227 0.002 0.02 40.8 Uranus 52N NR
26、25.19 6.37 3.67 1.18 0.25 - 0.018 0.25 0.001 0.018 41.3 SAF 2507 NR 24.7 7.1 3.82 0.62 0.26 - 0.06 0.28 0.005 0.023 41.8 NR 25.4 6.7 3.8 - - - - 0.27 - - 42.3 NR = None Recorded Table 2-2: Duplex Stainless Steel Thermal Simulation Conditions from Previous Research Duplex Stainless Steel Thermal Simu
27、lation Conditions Alloy Thickness, mm PREN Peak Temperature, C Hold Time, sec t12-8, sec t8-5, sec Cooling Rate, C/sec SAF 2205 10 32.8 1300 1 and 10 2 1 and 10 20 1 and 10 50 1 and 10 75 SAF 2205 NR 33.4 1300 1 2 20 50 75 SAF 2205 6 33.5 1370 NR 93 256 23 64 6.4 16 4.15 9 22Cr12N 5 34.0 1350 0, 5,
28、and 10 160 0, 5, and 10 300 0, 5, and 10 430 22Cr18N 5 35.3 1350 0, 5, and 10 160 0, 5, and 10 300 0, 5, and 10 430 25Cr17N 5 37.7 1350 0, 5, and 10 160 0, 5, and 10 300 0, 5, and 10 430 Ferralium 255 10 37.9 1300 1 2 1 and 10 20 1 and 10 50 1 and 10 75 SAF 2507 4.5 40.8 1355 NR 93 256 23 64 6.4 16
29、2.8 6 Uranus 52N NR 41.3 1350 1 20 50 SAF 2507 NR 41.8 1350 1 20 50 SAF 2507 NR 42.3 1400 NR 114 6 NR = None Recorded STP-PT-088: Effect of Cooling Rate on the Microstructure and Properties of Duplex Stainless Steel Welds 6 Table 2-3: Duplex Stainless Steel Microstructure and Toughness Test Results
30、from Previous Research Duplex Stainless Steel Microstructure CVN Test Temperature, C CVN Impact Energy, Joules Alloy Thickness, mm PREN Ferrite Content FN Area Percent Ferrite Austenite Volume Fraction SAF 2205 10 32.8 88 -20 128 90 -20 91 97 -100/-60/-40/-20/20/100 15/30/120/194/208/250 107 -20 178
31、 104 -100/-60/-40/-20/20/100 15/30/100/166/175/205 108 -20 150 110 -20 125 121 -20 134 SAF 2205 NR 33.4 88 -20 127 97 -20 195 104 -20 161 110 -20 130 SAF 2205 6 33.5 67 -110/-100/-80/-60/-40/ -20/0/20 10/13/27/57/95/122/ 128/128 78 -110/-100/-80/-60/-40/ -20/0/20 25/40/60/62/86/124/ 135/135 80 -110/
32、-100/-80/-60/-40/ -20/0/20 21/23/40/85/112/115/ 127/140 90 -110/-100/-80/-60/-40/ -20/0/20 16/26/40/50/67/85/92/ 95 22Cr12N 5 34.0 NR RT (1) 22Cr18N 5 35.3 NR RT (1) 25Cr17N 5 37.7 NR RT (1) Ferralium 255 10 37.9 91 -20 66 93 -100/-60/-40/-20/20/100 13/25/156/213/222/256 99 -20 (2) 137/66 101 -100/-
33、60/-40/-20/20/100 13/42/100/131/220/255 108 -20 63 109 -20 194 112 -20 78 SAF 2507 4.5 40.8 46 -195/-150/-100/-50/0/20 5/10/40/72/91/93 51 -195/-150/-100/-50/0/20 5/16/68/81/87/87 56 -195/-150/-100/-50/0/20 5/14/61/70/78/79 62 -195/-150/-100/-50/0/20 5/10/58/70/78/79 Uranus 52N NR 41.3 78 -20 141 85
34、 -20 159 SAF 2507 NR 41.8 57 -20 216 61 -20 194 SAF 2507 NR 42.3 67 -196/-117/-79/-54/0 (2) 7/17/41/129/229/200 35 -196/-117 (2)/-79/-54/0 (2) 9/30/19/75/150/250/196 RT = Room Temperature (1) CVN test results were recorded but there was no microstructure description so the results were not included
35、(2) There were two tests performed at the specified temperature STP-PT-088: Effect of Cooling Rate on the Microstructure and Properties of Duplex Stainless Steel Welds 7 2.2 Duplex Stainless Steel Welding Industry Practice The second topic of the literature review consisted of summarizing current in
36、dustry practices when welding duplex stainless steels. A paper by Noble and Gunn from The Welding Institute (TWI), an internationally recognized welding research institute located in England, provides a good understanding of duplex stainless steel welding considerations and the issues that can arise
37、.7 They discuss the effect of welding process on expected weld toughness and relate higher toughness with gas-shielded processes e.g., gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) relative to flux-shielded processes e.g., SMAW and flux-cored arc welding (FCAW). The reduction in t
38、oughness with the flux-shielded welding processes was primarily attributed to the presence of non-metallic inclusions with other contributing factors such as heat input and process efficiency. The paper states that the preferred welding procedure should use a low arc energy, i.e., low welding heat i
39、nput, no preheat temperature and to limit the maximum interpass temperature to between 100 and 250 C. It goes on to state that although this may be the preferred welding technique for thin duplex stainless steels, welding on thicker duplex stainless steels following these recommendations could resul
40、t in an excessively fast cooling rate resulting in high ferrite content in the weld metal and HAZ. The paper indicates that a heat input of 0.5 to 2.5 kJ/mm could be considered an acceptable heat input range for 22% Cr steels, such as 2205, but indicates that the use of recommended arc energy ranges
41、 for duplex stainless steels is becoming less important. The reduced concern associated with heat input is mainly due to the change in filler metal and base metal compositions that make duplex stainless steels more cooling rate insensitive. In most cases, this is accomplished with higher nitrogen co
42、ntents that promote the formation of the austenite phase during weld cooling. Karlsson also discussed the recommended welding procedure variables for a wide range of duplex stainless steels which were based on a survey identifying industry recommended practices.1 Based on the survey responses, Karls
43、son suggested welding lean and duplex stainless steels with arc energies in the range from 0.5 to 3.5 kJ/mm with a recommended maximum arc energy of 2.5 kJ/mm along with a maximum interpass temperature of 250 C. For super duplex stainless steels, the recommended arc energy range is 0.2 to 1.8 kJ/mm
44、with a maximum interpass temperature of 150 C. Hyper duplex stainless steels should be welded with similar arc energies as the super duplex stainless steels but the maximum interpass temperature should be further limited to 100 C. The reduction in maximum interpass temperature for the super duplex a
45、nd hyper duplex stainless steels addresses the potential for the formation of alpha-prime and sigma which are embrittling phases which form at slow cooling rates in high-Cr duplex stainless steels. It is important to note that the suggested welding conditions are general recommendations and more spe
46、cific limitations may be required based on the specific duplex stainless steel alloy and its service environment. Another source of common duplex stainless steel welding practice came from the International Molybdenum Association (IMOA).8 IMOA recommendations mirror the suggestions from the industry
47、 survey performed by Karlsson, however, IMOA does discuss the use of postweld heat treatment (PWHT). Generally, preheating is not recommended while welding duplex stainless steels because it could increase the time at the temperature during which detrimental phases may develop. However, preheating c
48、ould be used to remove moisture from the surface prior to welding or when welding heavy sections where a very high cooling rate is expected. IMOA suggests lean duplex stainless steel welding heat inputs should be 0.5 to 1.5 kJ/mm, 0.5 to 2.5 kJ/mm for duplex stainless steel, and 0.3 to 1.5 kJ/mm for
49、 super duplex stainless steel. As a general rule the interpass temperature when welding lean and duplex stainless steels should be limited to 150 C and 100 C for super duplex stainless steels. The recommendations state however that the interpass temperature used during construction should not exceed the interpass temperature that was recorded during qualification. PWHT is likely not required when welding duplex stainless steels since the heat treatment may result in detrimental intermetallic phases or alpha prime formation. However if PWHT is used the t
