Regeneration treatment on welding of nanostructured bainite
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Transcript: English(auto-generated)
00:29
The first person I would like to introduce to give his talk today is Kun Fang, from the Harbin Institute of Technology, and he's going to be giving a presentation about the welding of nanostrictured bainite. Thank you. First of all, I should thank Harry for his invitation.
00:51
I'm very glad to have this opportunity to give this presentation. Ladies and gentlemen, good afternoon. Hello. I'm a doctoral student from Harbin Institute of Technology,
01:03
the state laboratory of advanced welding and the journey. My research work is regeneration treatment on welding of nanostrictured bainite. The outlines of my presentation are as follows. First, I will briefly introduce the nanostrictured bainite,
01:25
its weldability, and the previous welding method. And then I will introduce the regeneration technique in detail. Nanostrictured bainite has been developed by Badeshi and his coworkers in the past decades.
01:44
This bainite steel with ultimate tensile strength is as high as 2,500 megapascal, and hardness in the range of 600 to 670 micron hardness.
02:08
Remarkable mechanical properties make the structure material show great potential for application, especially in the range of transition, weaponry, and aerospace.
02:27
Nanostrictured bainite, as mentioned, can be obtained by low temperature transformation in high carbon silicon rich steel.
02:41
However, high carbon concentration is also in very poor weldability of this material. During traditional welding, austenite will form in the fusion zone and the austen and high time pressure heat affected zone.
03:00
It will transform into brittle martensite. The cold cracks will form as well. On the other hand, a lot of cementite will precipitate in the low temperature heat affected zone.
03:26
It's leading to the decreasing of mechanical properties of the weld. As a result, the industrial application of this high carbon nanofructured bainite has been limited.
03:50
By far, only a career scholar attempt a post-weld rapid heat treatment for welding this nanofructured bainite. Cold cracks have been prevented.
04:02
But unfortunately, a lot of cementite precipitated in the weld and in the welded joint and ruined the mechanical properties. So new techniques should be prevented.
04:23
Using this method, the mechanical properties of the welded joint of nanofructured bainite should be similar to the original basemental. Considering the remarkable mechanical properties of the nanostructured bainite, the direct
04:43
way is to obtain this fine bainite again in the welded joint. This method is called regeneration treatment. Regeneration treatment is designed as the weld cools towards bainite start temperature.
05:07
It was transferred into a furnace set at temperature between Bs and Ms and held there for a long time to permit this fine bainite growth again in the weld. An experiment has been carried out. This is the chemical compositions and the microstructures of the nanostructured bainite.
05:34
During welding, the temperature is monitored by seven couples. The regeneration time is five days and the regeneration temperature is 250 degrees.
05:47
After welding, the structures in the fusion zone and austenitic zone are film-like structures. These film-like structures are proved to be nanoscaled bainite and retain the
06:03
austenite as a martensite, where some cementite is still precipitated in the temperate zone. Now let's have a look at the mechanical properties of the welded joint. From the hardness result, it can be seen that the hardness in the fusion zone and austenitic zone are almost the same as the base metal.
06:29
Only the hardness in the temperate zone is a little lower. To obtain the strength of the welded joint, the first tensile test sample is prepared.
06:43
The strength is 1,680 MPa, a little lower than base metal. However, this sample is fitted in the temperate zone, not in the fusion zone and austenitic zone. So to obtain the mechanical properties of the fusion zone, the second tensile test sample is prepared.
07:07
This time, the strength is almost 2,000 MPa, a little higher than base metal, and the elongation is about 40% of the base metal.
07:21
However, it confirms that the generation treatment has successfully realized the required bainite corrosion again in the fusion zone and austenitic zone. And the mechanical properties of the fusion zone and austenitic zone are almost the same as base metal.
07:45
However, there are still some differences of microstructure in the fusion zone and from the base metal. An interesting phenomenon is that a lot of cross structures distributed at the boundary of the columnar solidification structures.
08:07
This is because of the welding segregation. Theoretically, these cross structures may be martensite or retained austenite.
08:22
Fortunately, they are confirmed to be retained austenite by TEM. And the fraction of cross retained austenite is about 40% of the total retained austenite.
08:43
So it can be inferred that the cross structures keeping into retained austenite is an important factor for the weld to have such a good elongation. Now let's have a look at some effect factors.
09:05
The first is the regeneration temperature. From this diagram, the maximum fraction of retained austenite can be calculated by OA divided by AB.
09:23
It becomes higher when the regeneration temperature increases and becomes lower when the regeneration temperature decreases. For nanostructured bayonets, it's known that the elongation is controlled by the reflection of retained austenite.
09:45
And the strains always have the opposite changes of the elongation. So the regeneration temperature will have a strong effect on the microstructures and the mechanical properties of the welded joint.
10:07
By comparison, the two welded joints with different temperatures, 250 centigrade and 230 centigrade,
10:22
the austenite fraction increases with increasing regeneration temperature. And the elongation increases where the automated tensile strength decreases. The second parameter is regeneration time.
10:44
When regeneration time is short, the structure in the fusion zone and austenite zone are bayonet, martensite, and retained austenite. When the regeneration time increases, the fraction of bayonet increases, whereas the fraction of martensite decreases.
11:12
At last, the martensite disappears. The structures in the fusion zone and austenite zone only consist of bayonet and retained austenite.
11:24
The effect of regeneration time on the mechanical properties can be seen from the hardness result. When the regeneration time is six hours, the hardness is much higher than base metal.
11:43
This is because a lot of martensite forms here. When the regeneration time increases, the hardness decreases. At last, the hardness is almost the same as base metal.
12:02
The third parameter is welded heat input. From the research above, we know that by regeneration treatment, the mechanical properties of the fusion zone and austenite zone are almost the same as base metal. This is because the five bayonet grows again in the two zones, and the weaker part is the temporal zone.
12:27
This is because the original microstructures have been tempered in this area. For the tempering process, the most important factor is the heat input.
12:46
However, the evolution of nanostructures of bayonet during rapid and high temperature tempering and regeneration has not been clear.
13:00
First, the microstructures in the fusion zone and in the temporal zone have been obtained by using GOLIBO. The designed peak tempering temperature is 700 centigrade.
13:26
This is the same cycle curves. Different cooling rate means different heat input. By the XRD result, it can be seen that the fraction of the retentate austenite decreases with increasing heat input.
13:49
From the microstructures, it can be seen that by low heat input tempering, the long film like retentate austenite almost unchanged.
14:09
Only the shorter film like retentate austenite disappears between the long films disappears. Well, after high heat, the retentate austenite becomes discouraged.
14:28
It's beneficial to crack propagation. From the TM result, we can see that when the heat input increases, the dislocation density decreases and the size of cementite becomes larger.
14:53
All these results show that the high heat input is harmful to the mechanical properties of the welded joints.
15:07
The effect of welding heat input on the mechanical properties of the welded joint has been investigated by the two welders with TIG and LBW. LBW has much lower heat input than TIG.
15:30
Both welded joints are failure in the tempering zone. However, the strength of the welded joint with LBW is higher than that of TIG.
15:47
And almost the same as the basement. By far, the ideal welded joint has been obtained by the regeneration treatment.
16:03
However, there are still some problems to be considered. As I mentioned above, the very long regeneration time. The second is the hard cracks. The third is how to use this regeneration treatment in large scale structures.
16:26
For the first problem, it should be solved by promoting the bayonet transformation.
16:42
Maybe we will attempt several methods. Maybe we will refine the current size of the alternate or introduce some dislocations
17:06
because after welding and before regeneration treatment, we accumulated the regeneration treatment.
17:22
This is my conclusion. Thank you.
17:40
You showed data at 250 degrees centigrade compared with 230 degrees centigrade. What was the time of hold for those two comparisons or that comparison?
18:05
You showed 230 degrees centigrade. Also five days. Five days? In the regeneration time, maybe one day, the bayonet transformation may be completed.
18:38
Will it be possible to raise the temperature and lower the time?
18:43
Maybe the bayonet start temperature is limited. To what limit? Okay. Sorry. Thank you.
19:08
Just for everyone, I don't think the microphone is on, Martin. Martin was asking about the composition of the welding wire and the filler material. To avoid the welding defect, the welding process is processed on a whole plate.
19:33
I was just wondering whether you had a chance to check for cracking.
19:40
I saw your tensile properties, but I don't know whether you've had a chance to look for evidence of cold cracking in the heat-affected zone. This experiment I haven't done, but I think it can be done with SEM and tensile and see the result.
20:09
Okay. Well done. I've tried welding super bayonet myself, and I've seen other people make an absolute pig's ear of it.
20:21
So you've done a very good job in producing some very nice weld microstructures and mechanical properties. In response to the five-day time, some things are just worth waiting for. Professor Badesha, I believe, has come up with aluminium and cobalt-containing alloys that transform more quickly.
20:42
So if you use those at a slightly higher temperature, you could probably reduce the regeneration time to ten hours or something like that. And in terms of heating large structures, things like oil rigs and submarines are regularly made of steels that need preheating with thermal blankets.
21:05
So that technology of – and I think the preheat on some submarine steels are about 250 degrees C. So the technology is out there to protect or maintain large structures at the right temperatures of interest.
21:20
But reiterate, excellent job. Very enjoyable. Thank you. We've got last question from Tata Steel, Netherland. If the structure of the regeneration is the same as the base metal, why does the elongation is lower?
21:47
Maybe because of the segregation. And during welding, the loss of carbon may be another area.
22:03
Thank you. Did you have any preheat in your samples? I did. You did? Right? Okay. Okay. Okay. That's all I wanted to know. Well, good job done. Only one thing I didn't understand, because you are doing normal welding, and then you are trying to regenerate the welded zone.
22:29
So how do you avoid cold cracking? Because you are definitely going to experience cold cracking during welding. We avoid the temperature of the welded joint cool towards the martensite start temperature.
22:54
And then it was transferred into the furnace, and then the cold cracks avoid.
23:04
Your last slide, you mentioned about one of the worries of hot cracking. How did it come? I mean, why is that an issue there? This is, the symbol in my structure is very small. The hot crack may be formed when the thickness of the plant increases.
23:34
And the result of all the structure is from the symbols without hot cracks. But I find the hot cracks in some research.
23:52
Thank you. What is the thickness of your sample? That's another question that people want to ask. What's the thickness of the welded sample?
24:04
2 millimeter to 4 millimeter. May I add a comment to it? It's a beautiful work. I think you should be able to see the residual stresses also. It may be pretty low too, after welding. So if you measure the residual stresses, because it's cooling very slowly.
24:23
And those things could be looked into that also, to see what happened to that also. And if you have a large scale plate, probably Peter can give you. So you can weld it and see how the distortion plays a role too. So those would be very helpful if you're scaling up for a construction of this.
24:41
Okay, thank you. Oh, thank you very much. That was very entertaining. Very well done. Thank you.