Q: We have a pulp & paper digester that is used to cook wood chips in an alkaline solution. The fasteners will be exposed to high temperatures and abrasive conditions. Any suggestions?

A: Without knowing the specifics of your process, I can make two general material suggestions. Duplex 2205 fasteners are often used in pulp and paper digesters for their high resistance to chloride stress corrosion cracking, as well as pitting and crevice corrosion.  But keep in mind Duplex 2205 has a usable temperature limit of 572F. If you have harsher conditions or are are running at higher temperatures, Hastelloy C276 may be a better fit. C276 fasteners are also commonly used in digesters and are a good fit for more severe conditions. They can be used at operating temperatures up to 1900 F and are very resistant to chlorides, as well as hot, extremely caustic media.

Q: I have a robotic application where I need a fastener that is both lightweight and strong. Also, the style of the fastener needs to provide a wide clamping area in a low clearance area. Any suggestions?

A: A material you might consider is Titanium, grade 5. The most common fastener grade is Titanium grade 2 with an ultimate tensile strength of 70 ksi. However for higher strength Grade 5 is  about 2 x’s stronger at nearly 140ksi. This will have one of the best strength to weight ratios.

For the style of the fastener, many users with tight special applications use Titanium button head socket cap screws as they combine the low clearance installation capabilities of a socket driver with the wider clamping area of a pan shaped head.

Q: Is there any advantage of using a Hastelloy B3 fastener over a B2 fastener?

A: Both Hastelloy B2 and B3 fasteners provide good resistance to non-oxidizing acids, such as hydrochloric acid and sulfuric acid. The only advantages of Hastelloy B3 fasteners over B2, is that they offer enhanced thermal stability and improved fabrication characteristics. Remember, both B2 and B3 fasteners should not be used in oxidizing media or in the presence of ferric or cupric salts, as it will cause corrosion failure.

Q: I saw on your high temperature chart that Molybdenum is listed twice. I’m confused, which high temperature should I use?

A: Both high temperatures are accurate, depending on the situation where you are using Molybdenum fasteners. Molybdenum screws can be used up to 2912F only if it is used in a vacuum – meaning an air/oxygen free environment – otherwise it will rapidly oxidize immediately into a gas (always surprises people). If you are using Molybdenum in an air/oxygen environment, the maximum temperature you can use this metal is 1112F. For extremely high temperature environments (above 1500F) where air or oxygen is present it is best to use Waspaloy, Inconel or Alloy 330 fasteners.

Q: What is the difference between Incoloy 825 and 925 bolts?

A: Bolts made from Incoloy 925 are precipitation hardened version of Incoloy 825 bolts. What this means is it is put through a heat treatment process which hardens the alloy at the atomic level. Basically Incoloy 925 bolts offers similar corrosion resistance but are much stronger. Comparing tensile strength, Incoloy 925 has a tensile strength of 167 ksi where that of Incoloy 825 is only 100 ksi.

Q: I’m currently using Monel 400 hex head bolts in salt water. Should I be using K500? What’s the difference?

A: Basically, Monel K500 is a precipitation hardened version of Monel 400. This means the material goes through a heat treating process that changes the alloy’s structural matrix to increase its tensile and yield strength. Monel K500 bolts will give you similar corrosion resistance as Monel 400 bolts, but with added strength. Alloy K500 bolts will provide about 3 times the yield strength (110ksi vs 45ksi)  and double the tensile strength (160ksi vs 83ksi) when compared Monel 400 bolts. Overall, if the Monel 400 is failing from a strength perspective, then Monel K500 bolts will be a good choice.  However if the Monel 400 bolts are failing due to corrosion, a more corrosion resistant alloy may be needed like titanium bolts or a high performance nickel alloy fastener like Inconel.

Q: How does creep occur and what polymer is best to prevent this?

A: Creep is the permanent deformation of the material caused by constant load or stress, usually accompanied by constant high temperature. This can result in permanent changes of the fastener shape as well weakening and fracturing of the material. Depending on your conditions, it may not be possible to completely prevent this but it certainly could be minimized with creep resistant polymer fasteners like PEEK. PEEK screws are extremely creep resistant and could be further strengthened with glass or carbon fiber reinforcement if needed.

Q: I have a seawater application where the water can be stagnant or polluted. Is Monel bolts the best choice?

A: For stagnant seawater application we would usually suggest Titanium over Monel fasteners. Although Monel is very resistant to saltwater, it can experience pitting corrosion in stagnant water situations. There have been tests that shown Titanium to last almost two decades in tests of polluted seawater. So while Monel fasteners is excellent in moving water, titanium screws are usually the preferred material in stagnant water.

Q: What is the highest temperature I could use Inconel 718 bolts?

A: Inconel 718 fasteners could be used reliably up to about 1300F.  Because Inconel 718 is  heat treated or age hardened at 1325F, going beyond 1300F will significantly affect its strength properties. Furthermore, its strength may not fully return.  See the Inconel 718 strength vs temperature curves for more details.