When it comes to mineral insulated cable (MI cable) we often talk about the thermocouple or RTD inside the cable. But what about the sheath materials used? Our MI cables are sheathed with materials varying from Inconel to different types of stainless steel. Also, the more exotic sheath materials, such as Pt10%Rh, Hastelloy X and Pyrosil® are available.
On this page we explain all common used sheath materials and their advantages compared to other sheath materials for MI cable.
All stainless steel materials are Iron based alloys. This is a general purpose stainless steel material. It shows good resistance to corrosion and has a melting point of 1370°C. However, this material is not well suited in low-oxygen environments and environments with poor air-circulation.
AISI 316 and AISI 304 are almost identical when looking at their chemical composition yet there are some significant differences. AISI 316 consists of approximately 2-2.5% of Molybdenum where AISI 304 does not. AISI 316 consists of approximately 13% nickel whereas AISI 304 consists of approximately 10% Nickel. These two materials make AISI 316 corrosion resistant in even the most aggressive environments like marine areas.
AISI 316Ti is a stainless, Titanium stabilized Chromium-Nickel-Molybdenum steel.
AISI 316Ti is very similar to AISI 316 and AISI 316L. Due to the presence of titanium the mechanical strength of this alloy is better. The chance of intracrystalline corrosion diminishes due to the presence of Titanium. AISI 316Ti can be used without heat treatment due to its low carbon content. The recommended maximum temperature of this material is 900°C.
The main difference between AISI 316L and AISI 316Ti is the addition of titanium with the purpose of diminishing the risk of intracrystalline corrosion when operating in temperatures around the 425°C and 815°C. Read here more about AISI 316 Ti.
AISI 321 has a chemical composition of approximately 18% chromium and approximately 11 % nickel. This material is very similar to AISI 304, yet AISI 321 is titanium stabilized for inter granular corrosion.
The maximum working temperature of this material is 870°C which is significantly lower than the other sheath material types. This material is widely used in chemical applications and the aerospace industry.
AISI 310 is an austenitic stainless steel typically used for elevated temperature applications. Its high chromium and nickel content provide comparable corrosion resistance than AISI 304, but superior resistance to oxidation.
This material has good heat resistance and a maximum temperature of 1150°C.
AISI 304 is one of the most applied stainless steel materials. This material has beneficial properties due to its resistance to corrosion and deformability. It is also perfectly weldable. The recommended maximum working temperature is 900 C.
AISI 304 is known for its quality. It consists of 18% chroom and 8% nickel.
It is excellent corrosion resistant, yet it is sensitive to chlorides and acids. By glowing the material, the deformability of the alloy increases and the hardness of the material is reduced, making the alloy easier to work with. The weldability of this material is also fine and it can be welded with all known welding techniques.
AISI 446 is a heat resistant ferritic chromium stainless steel and contains approximately 1.7% aluminum. AISI 446 is also a non-heat treatable stainless steel. This type of material is very well resistant to corrosion and oxidation at high temperatures while stress conditions are low.
This type is perfectly weldable using most of the traditional methods of welding. Yet the advice is to avoid gas welding and oxyacetylene welding.
This nickel-chromium material has good resistance in carburizing and chloride environments. The alloy is non magnetic and perfectly weldable. This material is also easily formed.
Inconel 600 has a melting point of 1400°C whilst the material is oxidation and corrosion resistant at these high temperatures. This material has been developed for cryogenic temperatures elevated up to temperatures of over 1000°C.
This alloy is a general-purpose nickel-chromium-Iron material. It is forged for applications where resistance to heat and corrosion is a must. Inconel 601 is particularly good resistant to high-temperature oxidation and aqueous corrosion. The nickel chromium and aluminium contents form a protective oxide film which ensures the alloy is not embrittled when exposed for long periods of time to high temperatures.
Inconel 601 is suitable for furnace operations such as annealing carburizing nitriding and other heat operations. It is also used in various chemical processing equipment and petrochemical processing equipment. Last but not least, this material is also used in nitric acid production.
Inconel 601 is weldable by all known commercial methods. The recommended maximum temperature is 1200°C.
Pyrosil® material has special characteristics which give it excellent oxidation and nitridation corrosion resistance. Pyrosil® is less prone to degradation in Carbon-based atmospheres. This alloy has proven to be better resistant to oxidation then stainless steels and alloys with higher amounts of Nickel at temperatures up to 1250°C. For Sulphur atmospheres with temperatures above 500°C Pyrosil® is not recommended.
The Pyrosil® material performs well in industrial processes like chemical and petrochemical processing. The same can be said for processes in the steel, ceramic and automotive industry.
Pyrosil® material has a superior oxidation resistance compared to AISI 310 and Inconel 600. And a superior high temperature strength compared to AISI 310s and Inconel 600.
Platinum rhodium alloys are used for applications at high temperatures. It is stronger than pure platinum, whilst being evenly resistant to oxidation as pure platinum. The chemical composition is 90% Platinum and 10% Rhodium. The melting point of this alloy is approximately 1850°C.
Alloys containing about 10% rhodium are used as the positive leg of thermocouples.
This material is also used as a heating element in resistance furnaces. Furthermore it is used in the production of glass and automotive catalysts.
The main characteristic of all platinum-rhodium alloy combinations used in thermocouple wire is that the operating temperature is increased by the rhodium content. This Pt20%Rh alloy can be used in oxidizing conditions up to 1800°C. The alloy can be exposed for a short time to temperatures up to 1850°C. This material is used in ultra variable atmospheres and process research.
More physical properties of Platinum sheath material, you can read here.
These are the main sheath materials which are generally used. We also offer more exotic materials for your application?
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