Selecting a Material for High Temperature Service

Having trouble selecting materials for applications in high temperature environments? The checklist below will help with the questions that you will need to ask for proper material selection.

It is important to first identify the temperature that the material will see. Knowing the temperature that the metal will see will give a great start as to what choices are applicable for that application. The temperature that we are concerned about here is not the operating temperature. Rather, it is the metal temperature that needs to be addressed. For example; when choosing a material for a radiant tube application it is important to know what the temperature of the tube will be. If the furnace is going to be set at 1800°F, it is likely that the radiant tube itself will reach temperatures up to 1900°F (and the inside temperatures could be higher) so choosing a material to handle temperatures only to 1800°F will result in premature oxidation and/or failure of that tube. In very general terms, the higher the chromium content of the alloy, the better the oxidation resistance.

While temperature is a great place to start, another critical factor is to know what type of environment the metal will see. This could include the type of gas that is surrounding the metal or how humid the environment will be. Typically for furnace applications most environments are either oxidizing air or some form of an inert atmosphere consisting of either nitrogen, hydrogen or some other similar gas; carburizing furnaces are also very common. The environment can also be influenced by contaminates, such as sulfur from off-gassing lubricants, or the fuel itself. Higher nickel alloys often offer advantages in reducing or carburizing environments.

Probably the third most critical consideration is that of thermal cycle.  Rapid heating and cooling (quenching) of components is more detrimental to service life than constant temperatures. Resistance to thermal fatigue or thermal shock is dependent on several metallurgical factors, but elevated temperature strength is a big consideration.

One of the most obvious considerations is the strength needed for a specific application. More often than not an application can use three or four different materials, each with varying levels of life expectancy. When this occurs, the tiebreaker often comes down to strength. However, increasing the load bearing capacity at elevated temperatures usually means compromising some other desired characteristic.

At the end of the day, the final decision comes down to cost. However, the decision should not be based simply on the initial capital cost of the material. The true evaluation of cost effective material selection relies on considering the total Design Life Cycle Costs. Here, factors such as maintenance, down time and product quality over the expected life of the unit are considered.

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