ATIG welding of ferritic stainless steels.docx

1、ATIG welding of ferritic stainless steelsA-TIG welding of ferritic stainless steelsA-TIG WELDING OF FERRITIC STAINLESS STEELS 1 2T. Sndor, J. Dobrnszky 1ESAB Kft., Hungary, 2 Research Group for Metals Technology of the Hungarian Academy of Sciences Abstract As a consequence of the enormous changing

2、of the alloy surcharges of austenitic corrosion resistant steels in the last years more and more attention was focused on the cheaper ferritic stainless steels. Knowing the weldability problems of the ferritic stainless steels (sensitivity of grain growth during heat input, low ductility, etc.) it s

3、eemed the application of activated TIG (ATIG) welding with its lower heat input and focused arc may will provide advantageous results. This paper sums up the experiments that were done on ferritic steels with different thicknesses, different welding speeds and heat inputs. To strengthen our observat

4、ions microscopic examinations also were done to compare the conventional welding methods and the ATIG welding. Introduction In the last some years an enormous market demand appeared concerning to vagy: in connection with the stainless steels. Here and usual when we say “stainless” we mean “austeniti

5、c stainless steel” as the 80-90% of the total stainless steel production and consumption is austenitic. The advantages of austenitic stainless types are very well known: easy forming, very good weldability, good corrosion resistance, decorative outlook, and so on. An additional benefit is the very w

6、ide literature of the previously mentioned properties, which helps the user to find solutions for any problems that may occur during production. In opposite with austenitic types the ferritic and martensitic types have several problems which did not help them to spread in the industry, but on the ot

7、her hand in some specific areas these steels may be the unique solutions. As the well sellable austenitic stainless steels contains 8, 12, 13, etc percent nickel, parallel with the increasing demand of austenitic stainless steels the demand of nickel also increased. This leaded to the substantial in

8、creasing of alloy surcharges and nickel prices. The reaction of market did not wait too long time and soon began the developing of the low nickel content stainless steels. Among the duplex steels these types are called “lean duplex”. Parallel with these developments the focus of the markets attentio

9、n turned to the direction of elder and new ferritic stainless steels (FSSs). This work would like to represent the results of welding of the grain coarsening and intergranular corrosion sensitive FSSs by the Activated Tungsten Inert Gas (ATIG) welding. The ATIG welding The ATIG welding is the high p

10、roductivity variation of the conventional TIG welding. When applying this hardly known welding process, the welding may be executed with substantially lower welding current and higher welding speed while the penetration is 2-3 times deeper as it is with conventional TIG welding. When ATIG welding is

11、 applied for welding of stainless steels the following rules must be kept: , ATIG welding is applicable without bevelling. This decreases the cost and time of production and let the end-user easier fitting; , Gap is not recommended when fitting the pieces, because it increases the possibility of por

12、osity; , One size bigger tungstan electrode should be used to resist the higher reflected heat; , Electrode sharpening should be around 45 for longer life expectancy; , Consistent active flux portioning is indispensable; , Welding consumable is not added to the weld pool. Generally about ferritic st

13、ainless steels In spite of the advantageous properties of FSSs (similar mechanical properties to austenitic types, very good corrosion resistance to Cl-containing medias, high stress corrosion cracking resistance, low thermal coefficient and consequently low thermal fatigue tendency, strongly adhere

14、 oxide layer which is really prosperous at elevated temperatures and quite stable and low price) unfortunately the user should also be aware of some handicaps: , In case of presence of carbon in the FSS the -loop opens which leads to formation of austenite and martensite (with appropriate cooling ra

15、te) (Figure 1); , The presence of even a very small carbon content in FSSs also tend to form carbides in really wide variety (the most regular forms are MC and MC where M stands for 23673Fe+Cr in FSSs) which finally results the high risk of intergranular corrosion (This is the so called sensitisatio

16、n-effect); , Aptitude to 475C embrittlement (especially when Cr-content is above 18%); , -phase formation in the temperature range 500 800C (aptitude is increasing with Cr content); , In the case of Nb or Ti stabilization knife-edge corrosion may occur in the heat affected zone (HAZ); , Grain coarse

17、ning in HAZ; , In pure ferritic types because of the lack of ? transformation heat treatment is not possible to repair the grain-coarsened structure. Figure 1. Fe-Cr binary system with 0,01%C content (left figure); Fe-Cr-C ternary system with 0,05%C content (middle figure); Fe-Cr-C ternary system wi

18、th 0,1%C content (right figure). 1, 6 After solidification the FSSs keeps their body centered cubic lattice (bcc) until the room temperature. This explains why the regenerating heat treatment is not possible in FSSs, so consequently it may be stated that the biggest disadvantage of FSSs is the grain

19、 coarsening of HAZ. This phenomenon substantially decreases the mechanical and corrosion resistance properties. Moreover if C is present in the FSS the formation of carbides is almost unavoidable which finally leads to worse mechanical properties and decreased corrosion resistance. In the following

20、this paper would like to represent the effect of ATIG welding to grain structure of FSSs compared to TIG welding. 6, 8, 3 Experiments The ATIG welding experiments were done in flat butt weld (PA) position without bevelling and fitted with no gap. Both shielding and backing gas were pure argon (T4.5)

21、. The arc length (the gap between tungsten electrode and the plate) was kept at 2 mm. The consistent arc length and welding speed was ensured with mechanised TIG torch moving table. Here the constant arc length parallel with analogue setting of welding speed with a potentiometer was also possible. T

22、he base material was 430 (X6Cr17) type (W.Nr. 1.4016; UNS S43000) with the plate thickness of 8 mm (Table 1). Table 1. Chemical composition of investigated 430 type ferritic stainless steel according to inspection certificate 3.1. C Mn Si Cr P S 0,046 0,67 0,46 16,36 0,02 0,003 The cut edges were gr

23、inded manually and cleaned with alcohol to remove the possible grease or oil residuals from the proximate surrounding of the welding. The welding parameters were optimised for ATIG welding to obtain absolutely perfect root penetration. The same parameters were applied for TIG welding afterward to en

24、sure the same heat input. Thus the comparison of TIG and ATIG welding was possible from the point of view of heat input. The measured average grain size of base material was in the range of 30 80 m. After parameter optimisation the following welding parameters were applied (Table 2). Table 2. Weldin

25、g parameters of TIG and ATIG welding of 8 mm thick 430 type ferritic stainless steel. Arc Welding Welding Heat input Voltage (V) Power (kW) efficiency speed current (A) (kJ/mm) (%) (mm/min) 240 20,8 5,00 75 70 ATIG welding 3,214 240 20,8 5,00 75 70 TIG welding 3,214 Results of TIG welding As the wel

26、ding parameters of TIG welding were set to ATIG naturally perfect penetration was not expected. Thus only simple bead on plate welds were examined (Figure 2). Figure 2. Cross section of bead on plate weld with TIG process. As in the examined 430 type FSS the carbon content was 0,046% according to Fi

27、gure 1. (middle figure) austenite and optionally martensite formation was expectable. Consequently the most interesting questions were how large mutation of grain size occurred due to the counted 3,214 kJ/mm heat input in the HAZ and how much austenite/martensite formed in the welded joint and in th

28、e HAZ. Naturally the grain coarsening was befallen (Figure 3) in the HAZ. The average grain size increased to the range of 60 120 m in the HAZ. Figure 3. Macro- (A) and microstructure (B) of the HAZ of TIG-welded joint and microstructure of the base material (C). The tremendous grain size increasing

29、 in the weld metal may have been improved with lower heat input but it was not the aim of this experiment. 7 Results of ATIG welding The joints made with ATIG welding with the parameters stated in Table 2 resulted total root penetration (Figure 4). Figure 4.Cross section of ATIG welded joint; plate

30、thickness is 8 mm. The cross section of the joints shows that the welding current could have been lower slightly. The little bit higher current was necessary to evade the root penetration faults that may origin from the not absolutely perfect fitting. The most interesting observation was between HAZ

31、 and welded joint of TIG and ATIG was that in case of ATIG welding the grain coarsening was a little bit moderate and the martensite formation was bigger. This origins from that the weld pool of TIG is more shallow (thus its volume is less) so the arc energy heats up to higher temperature the weld p

32、ool thus grains have more time to become larger in the HAZ as they are in the critical temperature range for longer time. In case of ATIG welding the weld pool is deeper and the volume of molten metal is bigger which flows faster. By this the arc energy does not heats up the weld pool overly. So the HAZ (with the weld pool) can cool down faster through the critical temperature interval, which results finer grain structure there 9. If the previous statement is appropriate the faster cooling rate must be observed in the weld pool