Mechanical property enhancement of superalloys
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There are many applications that can benefit from grain refinement. Grain refinement is a result of specific manufacturing techniques which allow for a finer grain. In almost every case, a finer grain material will outperform a coarser grain item of the same material. The basis of finer grains being stronger is that while there are more dislocations in the material, it is more difficult to make all these dislocations line up. Finer grains also typically have less precipitates in the boundaries. Precipitates are oxides, carbon, and interstitial elements that are not in solution of the grain. Precipitates are normally not as strong as the material itself - tending to make a precipitated material more brittle.
Microphotograph above shows ASTM Grain size No. 5.5, ALA ASTM No. 1
Some grain refinement comes from the material itself, through additions of elements such as Columbium (Cb), Zirconium (Zr) and Boron (B). In small amounts, known as trace or residual amount, these elements tend to be grain creators or form grains in localized sites around them. While this type of grain refinement is good because it enhances the properties of the materials, it doesn't have to be the end of the grain refinement process.
Materials can receive further grain structure enhancement through specific manufacturing techniques of the material while it is being forged, rolled, or drawn. The steps immediately preceding the final processing and anneal can be essential to having the best product possible. This means during the last major heating cycle, the temperatures attained during the processing, as well as the final anneal it is given. Proper manufacturing techniques and methods of processing are essential, and rarely will it make up for abuses the material could have been subjected to in prior processing during a more raw stage.
You want to ensure that the billet a product is going to be made from is of good quality. Heating metals to temperatures outside its normal limits can cause issues like burning out Carbon, incipient melting, or cause oxides or precipitates to collect at grain boundaries. A simple billet test can reveal many of these by performing a macro view of the ends of the material. ASTM A604 is a common specification for preparation and macroetching of samples. You may see spots, pins, rings or nodules in this very simple test. It is pretty commonly used for higher melt materials, such as VIM VAR and ESR melt methods, as it ensures the quality of the billet at a fairly early stage. It could even be used by lower quality melted materials such as EF and AOD.
Now that we have covered the basis of good melting, processing and checks along the way we can now cover the final steps that can really have an impact on your material and its resultant properties. Mechanical strengthening of materials can be achieved by several methods. The most commonly used is through heat treatment, sometime known as aging or precipitation hardening. We briefly touched on these earlier, but we failed to mention the impact of such an easy method of raising the strength. Not only do precipitation hardening treatments lower ductility (more brittle), it can also cause the material to be subject to corrosion issues that the annealed material was not.
How can this be, it is the same material - and the composition didn't change? The mechanics of precipitation are basically the elements contained in the material change. During the temperature cycle(s), various phases in the material will be developed. The most common is gamma, or gamma prime - used in Inconel 718. This is the effect of Titanium & Vanadium additions. In Titanium Grade 5, the strengtheners are Aluminum and Vanadium to make Alpha-Beta phase alloy. Many elements can have phases associated with them, some are good and some are bad. The good ones are typically not as brittle as the bad ones, and when I say good typically it is enhancing the strength of the material rather than detrimental.
I know what question you are asking: If precipitation hardening is sometimes too brittle or it affects the corrosion resistance too much, what options are left? There are still a few ways to strengthen the material without having to use precipitation hardening. One way is through the use of hot rolling. Hot rolling material does elevate the mechanical properties, but the effects are normally reversed when it is annealed after processing. It is hot rolled and left it in the unannealed condition. I say hot roll, but it could also be hot forged, warm forged, warm worked or warm rolled. Do these methods attain the same strength as precipitation hardened material? Well, no they don't. Normally not enough strength is imparted during warm or hot processing to reach near 180 KSI UTS. It might get between 115 to 155 KSI UTS though, which helps in many instances.
I might add at this point that hot or warm processing can leave precipitates in the grain boundary, so refer back to the quality issues already identified, and you may need to look at the microstructure as well to tell if the issues could be detrimental. Typically if the outside of the grains are completely enclosed with precipitation or carbides, you have found what is called sensitized. Sensitized is a condition where precipitates are left at the grain boundary, rather than being in solution or homogeneous of the grain. The etchant would eat away the precipitates around the grain and leave a trench, known as a ditch structure. The etchant solution is your friend, because it will tell you what could happen in the field. If a corrosive media can dissolve these areas with a short treatment, how well will they hold up in real life? Pick an etchant solution and technique that is commonly used for the alloy, has good coloring or dye properties and is aggressive enough to show the detail you need without lots of prep.
The problem of attaining 200 KSI UTS isn't over yet. We still have a few processing methods to review that can attain the strengths of a precipitation hardened material. We have cold processing, which at the basic level, means that there is no heat added to the material before forming. It's basically processed at room temperatures, and it might be called cold rolling, cold drawing, cold forging, or cold swaging. Cold processing is, what I feel, the best way to mechanically strengthen a material. Let me clarify with - it's the best way if it has good quality going into production (a minor caveat). I'm sure you have heard the term garbage in equals garbage out. It definitely applies, and it applies on every type of metal processing. With that said, can you mess up good material? As long as you stay within known strain rate limits of the material, it is difficult to process material cold and end up with something bad. Bad is kind of relative, so let's look at it compared to precipitation hardened material and make some comparisons. Depending on the level of strength needed the elongation is going to be at least twice that of precipitation material. Elongation (Elon. %) is typically showing the ductility a material has. When a material only has 3 or 5% Elongation, we think of it as not very ductile. Most applications want 10-15% as a minimum Elongation. It should also be noted that the reduction of area is normally much higher in sold processed then precipitated material. A materials reduction of area property (R/A %) of a material shows its tendency to be brittle, or have low impact resistance. Lower numbers means it is more brittle, or that the material has been sensitized at some point.
Superalloy and Super-stainless Alloy Summary
So what alloys and strengths are available with this mechanical strengthening? High Performance Alloys supplies several alloys with properties capable of 180 to 200 KSI UTS. Many superalloys are austenitic, which means it has low magnetic permeability. Superalloys are either a Nickel-base or Cobalt-base material and will normally have a UNS that starts with N or R. So if you wanted a high strength C-276, the UNS would be N10276 and you simply state the properties that you need. High Performance Alloys also works with C-22 or Alloy 622, which is UNS N06022. 686 is UNS N06686.
There is also a group of material called super-stainless, which means that they normally outperform a stainless counterpart. High Performance Alloys can attain these properties with super-stainless NITRONIC grades; such as NITRONIC 60 UNS S21800 and NITRONIC 50 UNS S20910.
If you look at the alloys mentioned that can be work strengthened so far, you will notice that these are all corrosion materials, alloys that are used to solve corrosion issues. There is not much point in cold processing alloys that will be used for high heat. For medium temperatures to 1600F, work strengthening a cobalt base high temperature material that can be aged can enhance its properties at temperature as well. Examples of these alloys are L605 (Haynes 25), as well as MP35N. These two grades actually also have medical applications for their corrosion resistance.
While many Superalloys and super-stainless material can be strain hardened, there are only a few that will reach these ultra high strengths. A few were mentioned, the common ones, but there are more. There are many more that can be strengthened through grain refinement that will only take 10 to 30 percent reduction, and will benefit from an enhanced grain structure as well as enhanced properties.
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