Q: When is it best to use a lap joint flange?

A:   Lap joint flanges are good for very expensive materials like tantalum and zirconium and other specialty alloys like Hastelloy or Inconel due to reduced cost of sleeve vs solid flange. Lap joint flanges are unique in that they
are made of two pieces, the flange itself and the stub end. This allows for the flange to be made of a lower cost steel, while only the stub end requires specialty materials.

Lap Joint Flange Uses
Economy
Because a lap joint flange has a two piece configuration, it offers a way to cut cost when piping systems requires a high cost alloy for all “wetted” parts to reduce corrosion. In this situation, it is only required for the stub-end to be can be made of the higher cost corrosion-resistant material, where the flange itself can be the produced from lower cost steel.

Ease of Work
By using lap joint flanges, work can be simplified in situations that require frequent and rapid disassemble and assembly during the operation of a plant. The ability to spin that backing flange compensates for misalignment of the bolt holes during assembly.

Physical Features
Flange
1.  The backside, has a slight shoulder that is square cut at the center or pipe hole
2.  The front side has a flat face with a filleted (rounded) center hole to match the filleted back face of the stub end. Here the stub end will wrap tightly around the center hole of the flange.

Stub End
1.  Shaped like a short piece of pipe with a weld bevel on one. This portion of the stub end is also called the sleeve.
2.  Narrow shoulder on the flange facing end called is the hub. The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the sleeve.

Q: What is the difference between Zirconia and Zirconium?

A:   Zirconia is a ceramic made from ZrO₂ and Zirconium (Zr) is a refractory metal element found on the periodic table. So while Zirconia is made from Zirconium they are VERY different.

Q: Are ceramics fragile? Do I have to worry about them breaking compared to metals?

A:   Compared to metals ceramic fasteners are indeed fragile. Unlike a metal, there is very little yield before it fractures. Additionally ceramic fasteners and susceptible to thermal shock.

Q: Why does Zirconia cost more than Alumina?

A:  1) Raw material cost: Aluminia is far more abundant, whereas Yttrium oxide which is a stabilizer for Zirconia ceramic, is a rare earth element.

2) Shaping costs: Due to its higher density it takes approximately 10x longer to grind down and shape Zirconia into parts.

3) Higher sintering temperatures:  The thermal shock resistance for Zirconia is poor so it requires a more costly sintering process.

Q: What advantages does Zirconia have over Alumina?

A:   Usually for most high temperature or electric insulator applications where ceramics are needed, alumina ceramic fasteners are the most popular choice due to their properties and relatively affordable price compared to zirconia fasteners. However sometimes the extreme properties of zirconia fasteners are needed which include:

• Higher material density provides greater wear and corrosion resistance
• Higher strength
• Higher temperature resistance

Q: Need to replace a standard steel washer with a non-conductive material.

A:   After speaking with this customer, I learned that he needed to replace a standard steel washer with a non-conductive material for a critical bolted joint. These washers needed to withstand 70°C temperatures while maintaining preload. He had been experiencing creep with non-metal washers  at elevated temperatures and needed to minimize this issue.

While Vespel (polyimide) fasteners may have superb creep properties, it may be overkill for your application.  Vespel is also very expensive. I think you may be better off considering PEEK fasteners.  It is much more affordable that Vespel fasteners and could hold up to the 70C temperatures easily. It is also very well known for excellent creep resistance (as well as chemical resistance and temperature resistance). To further improve the creep resistance, glass and carbon fibers can be added.  Glass fiber and carbon fiber will improve stiffness and creep resistance.

Q: Can you provide alternative for a nylon pan head screw?

A:   Here the customer needed an alternative to nylon. Resistance to electrical fields was a primary concern. In addition, he also needed a material that had a better dissipation factor than nylon. In this situation we would recommend PEEK. It has similar properties with regards to its electromagnetic properties, better dissipation.

Q: Need help with specific needs regarding Polyimide fasteners and their high temperature capabilities

A:   This Italian company was developing a new client oven.  They were interested in Polyimide fasteners and their high temperature capabilities. In particular they needed:

• Usability to 300°C
• Capability with metal connections
• Heat and electric insulation
• Used in a hot box of steel

Polyimide materials like Vespel fasteners are a good thermal and electrical insulator for this application. It is heat resistant to 300°C (572°F) continuous use and 500°C (932°F) intermittent. Vespel fasteners also has a 3.55 dielectric constant at 1MHz. The only drawback for using Vespel fasteners is that it is an expensive option. Here is a great video by DuPont on the heat resistance of polyimide compared to other polymers: http://www.dupont.com/products-and-services/plastics-polymers-resins/parts-shapes/videos/polyimide-heat-resistance-vespel.html

Q: We need advice on a material that will deliver a tensile strength of 210 to 250 ksi, as well as help us protect against galling. The attaching areas are stainless steel Grade 8.

A:   For this instance, we would suggest MP35N. This alloy was developed specifically for high strength and can provide an ultimate tensile strength of 250 ksi, even at 600°F.

In regards to the galling issue, MP35N is a nickel-cobalt alloy. Since MP35N is very different in composition to your base metal of stainless, and has a significantly different hardness it will have less change of galling.  In general, the more different the alloys, the less chance to gall however we would recommend a lubricant to reduce the chance of galling even more.

Q: I’m using a high temperature vacuum furnace at 2500F. What type of material do you suggest using and is there any advantage of using a metal over a ceramic?

A:   Under vacuum conditions with little to no oxygen present, there are some metallic options for fasteners which include molybdenum and tantalum. These metals are considered referactory metals and could be used up 2900F and 3600F for molybdenum and tantalum fasteners respectively. In addition to metals, alumina and zirconia ceramics could be used as well.

The advantages of each come from what is most important and the conditions of your application. For example, if quenching is involved, you might not want to use ceramic fasteners due to the thermal shock which will cause failure. However if having low thermal conductivity through the fastener is important, then ceramics are a good choice. If the fasteners are to hold higher loads, the molybdenum and tantalum metals might be more robust.