Al and Nb influence on low alloyed forged steels
Al and Nb influence on low alloyed forged steels
(OP)
Hello folks,
At the moment I am investigating surface defects of large forgings (up to 50 tonnes). The forgings are used in several types of applications. The common denominator is that they are rotor shafts, forged to cylindrical shape and then machined. What influence has Al and Nb in low alloyed steels? Al works as a desoxidation agent when added ni small amounts, but if you add more it refines the austenitic grain size... (AlN). More than that I do not know... Could Al cause the material to crack when it is deformed at hight temperatures? >1100 degrees C?
At the moment I am investigating surface defects of large forgings (up to 50 tonnes). The forgings are used in several types of applications. The common denominator is that they are rotor shafts, forged to cylindrical shape and then machined. What influence has Al and Nb in low alloyed steels? Al works as a desoxidation agent when added ni small amounts, but if you add more it refines the austenitic grain size... (AlN). More than that I do not know... Could Al cause the material to crack when it is deformed at hight temperatures? >1100 degrees C?





RE: Al and Nb influence on low alloyed forged steels
What is the specific aluminum content in the alloy you are dealing with because I am familiar with large steam turbine forgings? There can be many causes for surface defects in forgings.
RE: Al and Nb influence on low alloyed forged steels
RE: Al and Nb influence on low alloyed forged steels
There has also been some cracks already in the ingots, appearing before forging. But most of them appear at forging.
Im often down in the forgeshop to see at which surface temperatures these cracks occur, but always when im down there the product comes out perfectly fine ;)...
Next week ill be doing some metallografhic examination of material from a rotor with cracks... Some hints what to look for?
Most of our products have specified material compositions which we are not allowed to modify, due to customers demands. So a process change is the only way to solve this problem...
Thanks for your help
RE: Al and Nb influence on low alloyed forged steels
Factors to consider include chemical segregation (although I wouldn't expect Al & Nb to be a problem), residual stresses, actual metal temperature during forming, cooling rate, strain imparted during forging, and many others.
Regards,
Cory
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RE: Al and Nb influence on low alloyed forged steels
I believe it is wise to evaluate the cause of the cracks via metallographic examination. I would also perform some SEM-EDS analysis of the crack surfaces to check elemental segregation.
RE: Al and Nb influence on low alloyed forged steels
I won't mention any specific shops, but there is at least one US open die forge shop that specifically avoids aluminum in the steels they forge, reportedly because it leads to internal cracking. They deoxidize with Silicon and kill with Vanadium. I get concerned about grain size when I have to deal with their products because of the lack of Aluminum (these concerns would be much greater with marginal reductions). I have never experienced a problem (I've always had them normalized prior to Q&T), but I have heard of some. Good microstructure, good hardness, but poor impacts.
While I don't think Columbium (Nb?) is effective as a deoxidizer, it does a good job as a grain refiner. Maybe you could have some success by specifying vanadium killed, Columbium treated. If these elements are specified by your customers, you may not be able do this. If not, maybe you can work with your steel supplier.
Having worked in an Open Die shop, I was surprised to learn of what I said above since I had never experienced cracking issues that could not be attributed to other causes (too cold, too much reduction at one time, unsound centers of ingots, etc...). The nameless shop mentioned above would melt their own steel, while we would only re-heat ingots produced by commercial melt-shops, so maybe there was something in our reheating procedures that reduced the problem.
rp
RE: Al and Nb influence on low alloyed forged steels
A correction. The cracks are not longitudal but transverse.
The products made of this specific steel "cracks" in the surface. If we forge similar products in other materials there is no cracks, so the problem is specific for this material.
More investigation have led me closer to the solution of the problem though... Some time ago we lowered the forging temp and the forgings with these kind of problems seem to have increased after this. The cracks often start at the corners of the forged square, that later is worked to octagonal shapen and then roundforged. All this implies that the surface temperature is to low for this specific material... The investigation continues!
RE: Al and Nb influence on low alloyed forged steels